Patent Publication Number: US-10758367-B2

Title: Expandable fusion device and method of installation thereof

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This patent application is a continuation-in-part of U.S. Ser. No. 15/635,267 filed Jun. 28, 2017, which is a continuation-in-part of U.S. Ser. No. 15/189,188, filed Jun. 22, 2016, which is a continuation-in-part of U.S. Ser. No. 15/014,189, filed Feb. 3, 2016, which is a continuation-in-part of U.S. patent Ser. No. 14/109,429 filed on Dec. 17, 2013, which is a divisional application of U.S. patent application Ser. No. 12/875,818 filed on Sep. 3, 2010, now U.S. Pat. No. 8,632,595, the entire disclosures of which are incorporated by reference herein. 
    
    
     BACKGROUND 
     A common procedure for handling pain associated with intervertebral discs that have become degenerated due to various factors such as trauma or aging is the use of intervertebral fusion devices for fusing one or more adjacent vertebral bodies. Generally, to fuse the adjacent vertebral bodies, the intervertebral disc is first partially or fully removed. An intervertebral fusion device is then typically inserted between neighboring vertebrae to maintain normal disc spacing and restore spinal stability, thereby facilitating an intervertebral fusion. 
     There are a number of known conventional fusion devices and methodologies in the art for accomplishing the intervertebral fusion. These include screw and rod arrangements, solid bone implants, and fusion devices which include a cage or other implant mechanism which, typically, is packed with bone and/or bone growth inducing substances. These devices are implanted between adjacent vertebral bodies in order to fuse the vertebral bodies together, alleviating the associated pain. 
     However, there are drawbacks associated with the known conventional fusion devices and methodologies. For example, present methods for installing a conventional fusion device often require that the adjacent vertebral bodies be distracted to restore a diseased disc space to its normal or healthy height prior to implantation of the fusion device. In order to maintain this height once the fusion device is inserted, the fusion device is usually dimensioned larger in height than the initial distraction height. This difference in height can make it difficult for a surgeon to install the fusion device in the distracted intervertebral space. 
     As such, there exists a need for a fusion device capable of being installed inside an intervertebral disc space at a minimum to no distraction height and for a fusion device that can maintain a normal distance between adjacent vertebral bodies when implanted. 
     SUMMARY 
     In an exemplary embodiment, the present invention provides an expandable fusion device capable of being installed inside an intervertebral disc space to maintain normal disc spacing and restore spinal stability, thereby facilitating an intervertebral fusion. In one embodiment, the fusion device includes a central ramp, a first endplate, and a second endplate. The central ramp may be capable of moving in a first direction to push the first and second endplates outwardly and into an unexpanded configuration. The expandable fusion device may be capable of being placed into the disc space down an endoscopic tube and then expanded into an expanded configuration. In an exemplary embodiment, an apparatus may be provided comprising: a first endplate for an intervertebral implant, wherein the first endplate may comprise a first plate portion having a first upper surface and a first lower surface, wherein the first endplate further comprises first front ramped portions extending away from the first lower surface and first rear ramped portions extending away from first lower surface. The apparatus may further comprise a second endplate for an intervertebral implant, wherein the second endplate may comprise a second plate portion having a second upper surface and a second lower surface, wherein the second endplate further comprises second front ramped portions extending away from the second lower surface and second rear ramped portions extending away from second lower surface. The apparatus may further comprise a body positioned between the first endplate and the second endplate, wherein the body may comprise rear endplate engaging ramps. The apparatus may further comprise a driving ramp positioned at a front end of the apparatus, wherein the driving ramp comprises front endplate engaging ramps. When the apparatus is in an unexpanded configuration, the rear endplate engaging ramps and the front endplate engaging ramps may have ramp angles with respect to a longitudinal axis of the apparatus that differ from ramp angles of the first rear ramped portions and first front ramped portions of the first endplate with respect to the longitudinal axis. The apparatus may be configured such that movement of the driving ramp in one direction causes the first and second endplates to move apart and a movement of the driving ramp in a second direction causes the first and second endplates to move towards one another. 
     In an exemplary embodiment, an apparatus may include a fastening device including a head, wherein the head includes an opening and a portion of the opening extends to an outer diameter of the head, a first ring including a protuberance extending from a first side, wherein the protuberance comprises a first portion and a second portion, and wherein the first ring is operatively connected to the head by positioning the first portion within the portion of the opening of the head that extends to the outer diameter of the head; and a second ring comprising a plurality of recesses, wherein the second portion of the protuberance is selectively engageable with at least one of the plurality of recesses. 
     Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred or exemplary embodiments of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will become more fully understood from the detailed description and the accompanying drawings, wherein: 
         FIG. 1  is a side view of an embodiment of an expandable fusion device shown between adjacent vertebrae according to the present invention; 
         FIG. 2  is a front perspective view of the expandable fusion device of  FIG. 1  shown in an unexpanded position in accordance with one embodiment of the present invention; 
         FIG. 3  is a front perspective view of the expandable fusion device of  FIG. 1  shown in an expanded position in accordance with one embodiment of the present invention; 
         FIG. 4  is a rear perspective view of the expandable fusion device of  FIG. 1  shown in an unexpanded position in accordance with one embodiment of the present invention; 
         FIG. 5  is a rear perspective view of the expandable fusion device of  FIG. 1  shown in an expanded position in accordance with one embodiment of the present invention; 
         FIG. 6  is a side view of the expandable fusion device of  FIG. 1  shown in an unexpanded position in accordance with one embodiment of the present invention; 
         FIG. 7  is a side view of the expandable fusion device of  FIG. 1  shown in an expanded position in accordance with one embodiment of the present invention; 
         FIG. 8  is a perspective view of the central ramp of the expandable fusion device of  FIG. 1  in accordance with one embodiment of the present invention; 
         FIG. 9  is a perspective view of the driving ramp of the expandable fusion device of  FIG. 1  in accordance with one embodiment of the present invention; 
         FIG. 10  is a perspective of an endplate of the expandable fusion device of  FIG. 1  in accordance with one embodiment of the present invention; 
         FIG. 11  a perspective view showing placement of the first endplate of an embodiment of an expandable fusion device down an endoscopic tube and into the disc space in accordance with one embodiment of the present invention; 
         FIG. 12  is a perspective view showing placement of the second endplate of the expandable fusion device down an endoscopic tube and into the disc space in accordance with one embodiment of the present invention; 
         FIG. 13  is a perspective view showing placement of the central ramp of the expandable fusion device down an endoscopic tube and into the disc space in accordance with one embodiment of the present invention; 
         FIG. 14  is a perspective view showing expansion of the expandable fusion device in accordance with one embodiment of the present invention; 
         FIG. 15  is a side schematic view of the expandable fusion device of  FIG. 1  having different endplates; 
         FIG. 16  is a partial side schematic view of the expandable fusion device of  FIG. 1  showing different modes of endplate expansion; 
         FIG. 17  is a side schematic view of the expandable fusion device of  FIG. 1  with artificial endplates shown between adjacent vertebrae; 
         FIG. 18  is a front perspective view of an alternative embodiment of an expandable fusion device shown in an unexpanded position in accordance with one embodiment of the present invention; 
         FIG. 19  is a front perspective view of the expandable fusion device of  FIG. 18  shown in an expanded position in accordance with one embodiment of the present invention; 
         FIG. 20  is a rear perspective view of the expandable fusion device of  FIG. 18  shown in an unexpanded position in accordance with one embodiment of the present invention; 
         FIG. 21  is a rear perspective view of the expandable fusion device of  FIG. 18  shown in an expanded position in accordance with one embodiment of the present invention; 
         FIG. 22  is a side view of the expandable fusion device of  FIG. 18  shown in an unexpanded position in accordance with one embodiment of the present invention; 
         FIG. 23  is a side view of the expandable fusion device of  FIG. 18  shown in an expanded position in accordance with one embodiment of the present invention; 
         FIG. 24  is a perspective of an endplate of the expandable fusion device of  FIG. 18  in accordance with one embodiment of the present invention; 
         FIG. 25  is a perspective view of the central ramp of the expandable fusion device of  FIG. 18  in accordance with one embodiment of the present invention; 
         FIG. 26  is a side view of the central ramp of the expandable fusion device of  FIG. 18  in accordance with one embodiment of the present invention; 
         FIG. 27  is a top view of the central ramp of the expandable fusion device of  FIG. 18  in accordance with one embodiment of the present invention; 
         FIG. 28  a perspective view showing placement of the central ramp of the expandable fusion device of  FIG. 18  in accordance with one embodiment of the present invention; 
         FIG. 29  is a perspective view showing placement of the first endplate of the expandable fusion device of  FIG. 18  in accordance with one embodiment of the present invention; 
         FIG. 30  is a perspective view showing placement of the second endplate of the expandable fusion device of  FIG. 18  in accordance with one embodiment of the present invention; 
         FIG. 31  is a perspective view showing placement of the actuation member of the expandable fusion device of  FIG. 18  in accordance with one embodiment of the present invention; 
         FIG. 32  is a perspective view showing expansion of the expandable fusion device of  FIG. 18  in accordance with one embodiment of the present invention; 
         FIG. 33  is a front perspective view of an alternative embodiment of an expandable fusion device shown in an unexpanded position in accordance with one embodiment of the present invention; 
         FIG. 34  is a front perspective view of the expandable fusion device of  FIG. 33  shown in an expanded position in accordance with one embodiment of the present invention; 
         FIG. 35  is a rear perspective view of the expandable fusion device of  FIG. 33  shown in an unexpanded position in accordance with one embodiment of the present invention; 
         FIG. 36  is a rear perspective view of the expandable fusion device of  FIG. 33  shown in an expanded position in accordance with one embodiment of the present invention; 
         FIG. 37  is a side cross-sectional view of the expandable fusion device of  FIG. 33  shown in an unexpanded position in accordance with one embodiment of the present invention; 
         FIG. 38  is a side cross-sectional view of the expandable fusion device of  FIG. 33  shown in an expanded position in accordance with one embodiment of the present invention; 
         FIG. 39  is a perspective of an endplate of the expandable fusion device of  FIG. 33  in accordance with one embodiment of the present invention; 
         FIG. 40  is a rear perspective view of an alternative embodiment of an expandable fusion device shown in an unexpanded position in accordance with one embodiment of the present invention; 
         FIG. 41  is a rear perspective view of the expandable fusion device of  FIG. 40  shown in a partially expanded position in accordance with one embodiment of the present invention; 
         FIG. 42  is a rear perspective view of the expandable fusion device of  FIG. 40  shown in an expanded position in accordance with one embodiment of the present invention; 
         FIG. 43  is a side exploded view of the expandable fusion device of  FIG. 40  in accordance with one embodiment of the present invention; 
         FIG. 44  is a side cross-sectional view of the expandable fusion device of  FIG. 40  shown in an unexpanded position in accordance with one embodiment of the present invention; 
         FIG. 45  is a perspective view of an endplate of the expandable fusion device of  FIG. 40  in accordance with one embodiment of the present invention; 
         FIG. 46  is a perspective view of the central ramp of the expandable fusion device of  FIG. 40  in accordance with one embodiment of the present invention; 
         FIGS. 47-49  are perspective views of the driving ramp of the expandable fusion device of  FIG. 40  in accordance with one embodiment of the present invention; 
         FIG. 50  is a rear perspective view of an alternative embodiment of an expandable fusion device shown in an expanded position in accordance with one embodiment of the present invention; 
         FIG. 51  is a side cross-sectional view of the expandable fusion device of  FIG. 50  shown in an expanded position in accordance with one embodiment of the present invention; 
         FIG. 52  is an exploded view of the expandable fusion device of  FIG. 50  in accordance with one embodiment of the present invention; 
         FIG. 53  is a top view of the expandable fusion device of  FIG. 50  shown in an unexpanded position in accordance with one embodiment of the present invention; 
         FIG. 54  is a read end view of the expandable fusion device of  FIG. 50  shown in an expanded position in accordance with one embodiment of the present invention; 
         FIG. 55  is a perspective view of an endplate of the expandable fusion device of  FIG. 50  in accordance with one embodiment of the present invention; 
         FIG. 56  is a perspective of a central ramp of the expandable fusion device of  FIG. 50  in accordance with one embodiment of the present invention; and 
         FIG. 57  is a perspective view of a driving ramp of the expandable fusion device of  FIG. 50  in accordance with one embodiment of the present invention. 
         FIG. 58  is an exploded view of an alternative embodiment of an expandable fusion device in accordance with one embodiment of the present invention. 
         FIG. 59  is a side view of the expandable fusion device of  FIG. 58  shown in partial cross-section in an unexpanded configuration in accordance with one embodiment of the present invention. 
         FIG. 60  is a side view of the expandable fusion device of  FIG. 58  shown in an unexpanded configuration in accordance with one embodiment of the present invention. 
         FIG. 61  is a perspective view of the expandable fusion device of  FIG. 58  shown in an unexpanded configuration in accordance with one embodiment of the present invention. 
         FIG. 62  is a side view of the expandable fusion device of  FIG. 58  shown in partial cross-section in a lordoctic expanded configuration in accordance with one embodiment of the present invention. 
         FIG. 63  is a side view of the expandable fusion device of  FIG. 58  shown in a lordoctic expanded configuration in accordance with one embodiment of the present invention. 
         FIG. 64  is a perspective view of the expandable fusion device of  FIG. 58  shown in a lordoctic expanded configuration in accordance with one embodiment of the present invention. 
         FIG. 65  is a side view of the expandable fusion device of  FIG. 58  shown in partial cross-section in a fully expanded configuration in accordance with one embodiment of the present invention. 
         FIG. 66  is a side view of the expandable fusion device of  FIG. 58  shown in a fully expanded configuration in accordance with one embodiment of the present invention. 
         FIG. 67  is a perspective view of the expandable fusion device of  FIG. 58  shown in a fully expanded configuration in accordance with one embodiment of the present invention. 
         FIG. 68  is an exploded view of an expandable fusion device having a ratcheting mechanism in accordance with some embodiments. 
         FIGS. 69A, 69B, and 69C  are side views of the expandable fusion device of  FIG. 68  in the process of expansion in accordance with some embodiments. 
         FIGS. 70A, 70B, and 70C  are different views of the expandable fusion device of  FIG. 68  in a contracted state in accordance with some embodiments. 
         FIGS. 71A, 71B, and 71C  are different views of the expandable fusion device of  FIG. 68  in a tipped state without full expansion in accordance with some embodiments. 
         FIGS. 72A, 72B, and 72C  are different views of the expandable fusion device of  FIG. 68  in a fully expanded state in accordance with some embodiments. 
         FIG. 73  is an upper view of the expandable fusion device of  FIG. 68  in accordance with some embodiments. 
         FIG. 74  is an upper cross-sectional view of the expandable fusion device of  FIG. 68  in accordance with some embodiments. 
         FIG. 75  is a close up view of the ratcheting mechanism of the expandable fusion device of  FIG. 68  in accordance with some embodiments. 
         FIG. 76  is a close up view of the ratchet teeth of the expandable fusion device of  FIG. 68  in accordance with some embodiments. 
         FIG. 77  is a top perspective view of the expandable fusion device of  FIG. 68  in accordance with some embodiments. 
         FIGS. 78A, 78B, 78C, 78D, 78E, 78F, and 78G  are top perspective views of the expandable fusion device of  FIG. 68  transitioning from a locked configuration to a disengaged configuration in accordance with some embodiments. 
         FIG. 79  is an exploded view of an expandable fusion device having a threading mechanism in accordance with some embodiments. 
         FIGS. 80A, 80B, and 80C  are side views of the expandable fusion device of  FIG. 79  in the process of expansion in accordance with some embodiments. 
         FIGS. 81A-81B  are different views of the expandable fusion device of  FIG. 79  in a contracted state in accordance with some embodiments. 
         FIGS. 82A-82B  are different views of the expandable fusion device of  FIG. 79  in a tipped state without full expansion in accordance with some embodiments. 
         FIGS. 83A-83B  are different views of the expandable fusion device of  FIG. 79  in a fully expanded state in accordance with some embodiments. 
         FIGS. 84A, 84B, 84C, and 84D  are different views of a TLIF device having threaded expansion in accordance with embodiments of the present application. 
         FIGS. 85A, 85B, 85C, and 85D  are different views of a PLIF device having threaded expansion in accordance with embodiments of the present application. 
         FIG. 86  is an exemplary surface of a device according to any of the embodiments of the present application. 
         FIG. 87  is an exemplary locking mechanism according to any of the embodiments of the present application. 
         FIG. 88  is a diagram showing a close up view of one embodiment of the screw lock ring and the drive screw head. 
         FIG. 89  is a diagram showing a side view of the embodiment of the screw lock ring and the drive screw head shown in  FIG. 88 . 
         FIG. 90  is a diagram showing a close up view of another embodiment of the screw lock ring and the drive screw head. 
         FIG. 91  is a diagram showing a side view of the screw lock ring and the drive screw head shown in  FIG. 90 . 
         FIG. 92  is a diagram showing a side view of one embodiment of the housing lock ring and the screw lock ring. 
         FIG. 93  is a diagram showing a side view of another embodiment of the housing lock ring and the screw lock ring. 
         FIG. 94  is a diagram showing a close up view of one embodiment of the engagement between the housing lock ring and the screw lock ring. 
         FIG. 95  is a diagram showing a close up view of one embodiment of the housing lock ring and the screw lock ring in a disengaged position. 
         FIG. 96  is a diagram showing one embodiment of a spring component and drive screw head. 
         FIG. 97  is a diagram showing one embodiment of the spring component and drive screw head of  FIG. 96  in the engaged position. 
         FIG. 98  is a diagram showing one embodiment of the spring component and drive screw head in the disengaged position. 
         FIG. 99  is a diagram showing one embodiment of a locking mechanism according to the present invention. 
         FIG. 100  is a diagram showing a side view of one embodiment of the locking mechanism. 
         FIG. 101  is a diagram showing a side view of one embodiment of the collar and the drive screw head. 
         FIG. 102  is a diagram showing one embodiment of the lock tab and the collar in the engaged position. 
         FIG. 103  is a diagram showing one embodiment of the lock tab, collar, and drive screw in the disengaged position. 
         FIG. 104  is a diagram showing one embodiment of the lock tab, collar, and drive screw in the engaged position. 
         FIG. 105  is a diagram showing one embodiment of a locking mechanism according to the present invention. 
         FIG. 106  is a diagram showing a close up view of one embodiment of the spring and drive screw head. 
         FIG. 107  is a diagram showing a close up view the spring operatively connected to the drive screw head in accordance with one embodiment of the present invention. 
         FIG. 108  is a diagram showing a close up view of one embodiment of the spring and housing lock ring in the engaged position. 
         FIG. 109  is a diagram showing a close up view of one embodiment of the spring and housing lock ring in the disengaged position. 
         FIG. 110  is a diagram showing one embodiment of a locking mechanism according to the present invention. 
         FIG. 111  is a diagram showing a close up view of components of one embodiment of the present invention. 
         FIG. 112  is a diagram showing a close up view of components of one embodiment of the present invention. 
         FIG. 113  is a diagram showing a close up view of components of one embodiment of the present invention. 
         FIG. 114  is a diagram showing components of one embodiment of the present invention in the disengaged position. 
         FIG. 115  is a diagram showing components of one embodiment of the present invention in the engaged position. 
         FIG. 116  is a diagram showing another embodiment of teeth that may be used in combination with embodiments of the present invention. 
         FIG. 117  is a diagram showing one embodiment of a locking mechanism according to the present invention. 
         FIG. 118  is a diagram showing a close up view of one embodiment of components shown in  FIG. 117 . 
         FIG. 119  is a diagram showing a top view of one embodiment of components shown in  FIG. 117 . 
         FIG. 120  is a diagram showing exemplary components shown in  FIG. 117  in the disengaged position. 
         FIG. 121  is a diagram showing exemplary components shown in  FIG. 117  in the engaged position. 
         FIG. 122  is a diagram showing exemplary tapered notches according to one embodiment of a drive screw head. 
         FIG. 123  is a diagram showing an exemplary locking mechanism according to one embodiment of the present invention. 
         FIG. 124  is a diagram showing an exemplary snap-ring according to one embodiment of the present invention. 
         FIG. 125  is a diagram showing exemplary teeth according to one embodiment of the present invention. 
         FIG. 126  is a diagram showing another exemplary locking mechanism according to one embodiment of the present invention. 
         FIGS. 127A, 127B and 127C  are diagrams showing an exemplary locking mechanism according to one embodiment of the present invention. 
         FIG. 128  is a diagram showing a close up view of one embodiment of a housing. 
         FIG. 129A  is a diagram showing one embodiment of a screw lock ring in a closed position. 
         FIG. 129B  is a diagram showing one embodiment of the screw lock ring of  129 A in an opened position. 
         FIG. 130  is a diagram showing another embodiment of a screw lock ring. 
         FIG. 131  is a diagram showing an exemplary embodiment of a screw lock ring. 
         FIG. 132  is a diagram showing an exemplary locking mechanism according to one embodiment of the present invention. 
         FIG. 133  is a diagram showing a close up view of one embodiment of a screw lock ring shown in  FIG. 132 . 
         FIG. 134  is a diagram showing a close up view of one embodiment of a housing. 
         FIG. 135  is a diagram showing a close up view of an exemplary locking mechanism according to one embodiment of the present invention. 
         FIG. 136  is a diagram showing a side view of an exemplary locking mechanism according to one embodiment of the present invention. 
         FIG. 137  is a diagram showing an exemplary locking mechanism according to one embodiment of the present invention. 
         FIGS. 138A, 138B, and 138C  are diagrams showing different aspects of a screw according to one embodiment of the present invention. 
         FIG. 139  is a diagram showing an exemplary locking mechanism according to one embodiment of the present invention. 
         FIG. 140  is a diagram showing an exemplary locking mechanism according to one embodiment of the present invention. 
         FIG. 141  is a close up view of the exemplary locking mechanism shown in  FIG. 140 . 
         FIG. 142  is a diagram showing an exemplary locking mechanism according to one embodiment of the present invention. 
         FIG. 143  is a diagram showing a top view of the exemplary locking mechanism shown in  FIG. 142 . 
     
    
    
     DETAILED DESCRIPTION 
     The following description of the preferred embodiment(s) is merely exemplary in nature and is in no way intended to limit the invention, its application, or uses. 
     A spinal fusion is typically employed to eliminate pain caused by the motion of degenerated disk material. Upon successful fusion, a fusion device becomes permanently fixed within the intervertebral disc space. Looking at  FIG. 1 , an exemplary embodiment of an expandable fusion device  10  is shown between adjacent vertebral bodies  2  and  3 . The fusion device  10  engages the endplates  4  and  5  of the adjacent vertebral bodies  2  and  3  and, in the installed position, maintains normal intervertebral disc spacing and restores spinal stability, thereby facilitating an intervertebral fusion. The expandable fusion device  10  can be manufactured from a number of materials including titanium, stainless steel, titanium alloys, non-titanium metallic alloys, polymeric materials, plastics, plastic composites, PEEK, ceramic, and elastic materials. In an embodiment, the expandable fusion device  10  can be configured to be placed down an endoscopic tube and into the disc space between the adjacent vertebral bodies  2  and  3 . 
     In an exemplary embodiment, bone graft or similar bone growth inducing material can be introduced around and within the fusion device  10  to further promote and facilitate the intervertebral fusion. The fusion device  10 , in one embodiment, is preferably packed with bone graft or similar bone growth inducing material to promote the growth of bone through and around the fusion device. Such bone graft may be packed between the endplates of the adjacent vertebral bodies prior to, subsequent to, or during implantation of the fusion device. 
     With reference to  FIGS. 2-7 , an embodiment of the fusion device  10  is shown. In an exemplary embodiment, the fusion device  10  includes a first endplate  14 , a second endplate  16 , a central ramp  18 , and a driving ramp  260 . In an embodiment, the expandable fusion device  10  can be configured to be placed down an endoscopic tube and into the disc space between the adjacent vertebral bodies  2  and  3 . One or more components of the fusion device  10  may contain features, such as through bores, that facilitate placement down an endoscopic tube. In an embodiment, components of the fusion device  10  are placed down the endoscopic tube with assembly of the fusion device  10  in the disc space. 
     Although the following discussion relates to the second endplate  16 , it should be understood that it also equally applies to the first endplate  14  as the second endplate  16  is substantially identical to the first endplate  14  in embodiments of the present invention. Turning now to  FIGS. 2-7 and 10 , in an exemplary embodiment, the second endplate  16  has a first end  39  and a second end  41 . In the illustrated embodiment, the second endplate  16  further comprise an upper surface  40  connecting the first end  39  and the second end  41 , and a lower surface  42  connecting the first end  39  and the second end  41 . In an embodiment, the second endplate  16  further comprises a through opening  44 , as seen on  FIG. 11 . The through opening  44 , in an exemplary embodiment, is sized to receive bone graft or similar bone growth inducing material and further allow the bone graft or similar bone growth inducing material to be packed in the central opening in the central ramp  18 . 
     As best seen in  FIGS. 7 and 10 , the lower surface  42  includes at least one extension  46  extending along at least a portion of the lower surface  42 , in an embodiment. In an exemplary embodiment, the extension  46  can extend along a substantial portion of the lower surface  42 , including, along the center of the lower surface  42 . In the illustrated embodiment, the extension  46  includes a generally concave surface  47 . The concave surface  47  can form a through bore with the corresponding concave surface  47  (not illustrated) of the first endplate  14 , for example, when the device  10  is in an unexpanded configuration. In another exemplary embodiment, the extension  46  includes at least one ramped surface  48 . In another exemplary embodiment, there are two ramped surfaces  48 ,  50  with the first ramped surface  48  facing the first end  39  and the second ramped surface facing the second end  41 . In an embodiment, the first ramped surface  48  can be proximate the first end  39 , and the second ramped surface  50  can be proximate the second end  41 . It is contemplated that the slope of the ramped surfaces  48 ,  50  can be equal or can differ from each other. The effect of varying the slopes of the ramped surfaces  48 ,  50  is discussed below. 
     In one embodiment, the extension  46  can include features for securing the endplate  16  when the expandable fusion device  10  is in an expanded position. In an embodiment, the extension  46  includes one or more protuberances  49  extending from the lateral sides  51  of the extension. In the illustrated embodiment, there are two protuberances  49  extending from each of the lateral sides  51  with each of the sides  53  having one of the protuberances  49  extending from a lower portion of either end. As will be discussed in more detail below, the protuberances  49  can be figured to engage the central ramp  18  preventing and/or restricting longitudinal movement of the endplate  16  when the device  10  is in an expanded position. 
     As illustrated in  FIGS. 2-5 , in one embodiment, the upper surface  40  of the second endplate  16  is flat and generally planar to allow the upper surface  40  of the endplate  16  to engage with the adjacent vertebral body  2 . Alternatively, as shown in  FIG. 15 , the upper surface  40  can be curved convexly or concavely to allow for a greater or lesser degree of engagement with the adjacent vertebral body  2 . It is also contemplated that the upper surface  40  can be generally planar but includes a generally straight ramped surface or a curved ramped surface. The ramped surface allows for engagement with the adjacent vertebral body  2  in a lordotic fashion. While not illustrated, in an exemplary embodiment, the upper surface  40  includes texturing to aid in gripping the adjacent vertebral bodies. Although not limited to the following, the texturing can include teeth, ridges, friction increasing elements, keels, or gripping or purchasing projections. 
     Referring now to  FIGS. 2-8 , in an exemplary embodiment, the central ramp  18  has a first end  20 , a second end  22 , a first side portion  24  connecting the first end  20  and the second end  22 , and a second side portion  26  (best seen on  FIG. 5 ) on the opposing side of the central ramp  12  connecting the first end  20  and the second end  22 . The first side portion  24  and the second side portion  26  may be curved, in an exemplary embodiment. The central ramp  18  further includes a lower end  28 , which is sized to receive at least a portion of the first endplate  14 , and an upper end  30 , which is sized to receive at least a portion of the second endplate  16 . 
     The first end  20  of the central ramp  18 , in an exemplary embodiment, includes an opening  32 . The opening  32  can be configured to receive an endoscopic tube in accordance with one or more embodiments. The first end  20  of the central ramp  18 , in an exemplary embodiment, includes at least one angled surface  33 , but can include multiple angled surfaces. The angled surface  33  can serve to distract the adjacent vertebral bodies when the fusion device  10  is inserted into an intervertebral space. 
     The second end  22  of the central ramp  18 , in an exemplary embodiment, includes an opening  36 . The opening  36  extends from the second end  22  of the central ramp  18  into a central guide  37  in the central ramp  18 . 
     In an embodiment, the central ramp  18  further includes one or more ramped surfaces  33 . As best seen in  FIG. 8 , the one or more ramped surfaces  33  positioned between the first side portion  24  and the second side portion  26  and between the central guide  37  and the second end  22 . In an embodiment, the one or more ramped surfaces  33  face the second end  22  of the central ramp  18 . In one embodiment, the central ramp  18  includes two ramped surfaces  33  with one of the ramped surfaces  33  being sloped upwardly and the other of the ramped surfaces  33  being sloped downwardly. The ramped surfaces  33  of the central ramp can be configured and dimensioned to engage the ramped surface  48  in each of the first and second endplates  14 ,  16 . 
     Although the following discussion relates to the second side portion  26  of the central ramp  18 , it should be understood that it also equally applies to the first side portion  24  in embodiments of the present invention. In the illustrated embodiment, the second side portion  26  includes an inner surface  27 . In an embodiment, the second side portion  26  further includes a lower guide  35 , a central guide  37 , and an upper guide  38 . In the illustrated embodiment, the lower guide  35 , central guide  37 , and the upper guide  38  extend out from the inner surface  27  from the second end  22  to the one or more ramped surfaces  31 . In the illustrated embodiment, the second end  22  of the central ramp  18  further includes one or more guides  38 . The guides  38  can serve to guide the translational movement of the first and second endplates  14 ,  16  with respect to the central ramp  18 . For example, protuberances  49  on the second endplate  16  may be sized to be received between the central guide  37  and the upper guide  38 . Protuberances  49  of the first endplate  16  may be sized to be received between the central guide  37  and the lower guide  35 . A first slot  29  may be formed proximate the middle of the upper guide  38 . A second slot  31  may be formed between end of the upper guide  38  and the one or more ramped surfaces  33 . The protuberances  49  may be sized to be received within the first slot  29  and/or the second slot  31  when the device  10  is in the expanded position. 
     Referring now to  FIGS. 4-7 and 9 , the driving ramp  260  has a through bore  262 . In an embodiment, the driving ramp  260  is generally wedge-shaped. As illustrated, the driving ramp  260  may comprise a wide end  56 , a narrow end  58 , a first side portion  60  connecting the wide end  56  and the narrow end  58 , and a second side portion  62  connecting the wide end  56  and the narrow end  58 . The driving ramp  260  further may comprise ramped surfaces, including an upper ramped surface  64  and an opposing lower ramped surface  66 . The upper ramped surface  64  and the lower ramped surface  66  may be configured and dimensioned to engage the ramped surface  50  proximate the second end  41  in of the first and the second endplates  14 ,  16 . The first and second side portions  60 ,  62  may each include grooves  68  that extend, for example, in a direction parallel to the longitudinal axis of the through bore  262 . The grooves  68  may be sized to receive the central guide  37  on the interior surface  27  of each of the side portions  24 ,  26  of the central ramp  18 . In this manner, the grooves  68  together with the central guide  37  can surface to guide the translational movement of the driving ramp  260  in the central ramp  18 . 
     A method of installing the expandable fusion device  10  of  FIG. 1  is now discussed in accordance with one embodiment of the present invention. Prior to insertion of the fusion device  10 , the intervertebral space is prepared. In one method of installation, a discectomy is performed where the intervertebral disc, in its entirety, is removed. Alternatively, only a portion of the intervertebral disc can be removed. The endplates of the adjacent vertebral bodies  2 ,  3  are then scraped to create an exposed end surface for facilitating bone growth across the intervertebral space. One or more endoscopic tubes can then be inserted into the disc space. The expandable fusion device  10  can then be introduced into the intervertebral space down an endoscopic tube and seated in an appropriate position in the intervertebral disc space. 
     After the fusion device  10  has been inserted into the appropriate position in the intervertebral disc space, the fusion device  10  can then be expanded into the expanded position. To expand the fusion device  10 , the driving ramp  260  may be moved in a first direction with respect to the central ramp  18 . Translational movement of the driving ramp  260  through the central ramp  18  may be guided by the central guide  37  on each of the first and second side portions  24 ,  26  of the central ramp  18 . As the driving ramp  260  moves, the upper ramped surface  64  pushes against the ramped surface  50  proximate the second end  41  of the second endplate  16 , and the lower ramped surface  66  pushes against the ramped surface  50  proximate the second end  41  of the first endplate  14 . In addition, the ramped surfaces  33  in the central ramp  18  push against the ramped surface  48  proximate the first end  41  of the first and second endplates  14 ,  16 . In this manner, the first and second endplates  14 ,  16  are pushed outwardly into an expanded configuration. As discussed above, the central ramp  16  includes locking features for securing the endplates  14 ,  16 . 
     It should also be noted that the expansion of the endplates  14 ,  16  can be varied based on the differences in the dimensions of the ramped surfaces  48 ,  50  and the angled surfaces  62 ,  64 . As best seen in  FIG. 16 , the endplates  14 ,  16  can be expanded in any of the following ways: straight rise expansion, straight rise expansion followed by a toggle into a lordotic expanded configuration, or a phase off straight rise into a lordotic expanded configuration. 
     Turning back to  FIGS. 2-7 , in the event the fusion device  10  needs to be repositioned or revised after being installed and expanded, the fusion device  10  can be contracted back to the unexpanded configuration, repositioned, and expanded again once the desired positioning is achieved. To contract the fusion device  10 , the central ramp  18  is moved with respect to the central ramp  260  away from the central ramp  260 . As the central ramp  18  moves, the ramped surfaces  33  in the central ramp  18  ride along the ramped surfaces  48  of the first and second endplates  14 ,  16  with the endplates  14 ,  16  moving inwardly into the unexpanded position. 
     With reference now to  FIG. 17 , fusion device  10  is shown with an exemplary embodiment of artificial endplates  100 . Artificial endplates  100  allows the introduction of lordosis even when the endplates  14  and  16  of the fusion device  10  are generally planar. In one embodiment, the artificial endplates  100  have an upper surface  102  and a lower surface  104 . The upper surfaces  102  of the artificial endplates  100  have at least one spike  106  to engage the adjacent vertebral bodies. The lower surfaces  104  have complementary texturing or engagement features on their surfaces to engage with the texturing or engagement features on the upper endplate  14  and the lower endplate  16  of the fusion device  10 . In an exemplary embodiment, the upper surface  102  of the artificial endplates  100  have a generally convex profile and the lower surfaces  104  have a generally parallel profile to achieve lordosis. In another exemplary embodiment, fusion device  10  can be used with only one artificial endplate  100  to introduce lordosis even when the endplates  14  and  16  of the fusion device  10  are generally planar. The artificial endplate  100  can either engage endplate  14  or engage endplate  16  and function in the same manner as described above with respect to two artificial endplates  100 . 
     With reference to  FIGS. 11-14 , an embodiment for placing an expandable fusion device  10  into an intervertebral disc space is illustrated. The expandable fusion device  10  can be introduced into the intervertebral space down an endoscopic tube utilizing a tool  70  that is attached to endplate  16 , with the second endplate  16  being first placed down the tube with tool  70  and into the disc space, as seen in  FIG. 11 . After insertion of the second endplate  16 , the first endplate  14  can be placed down the same endoscopic tube with tool  72  and into the disc space, as shown on  FIG. 12 . Following the first endplate  14 , the central ramp  12  can be placed down the same endoscopic tube and into the disc space guided by tools  70  and  72 , as shown on  FIGS. 13 and 14 . 
     Referring now to  FIGS. 18-23 , an alternative embodiment of the expandable fusion device  10  is shown. In an exemplary embodiment, the fusion device  10  includes a first endplate  14 , a second endplate  16 , a central ramp  18 , and an actuator assembly  200 . As will be discussed in more detail below, the actuator assembly  200  drives the central ramp  18  which forces apart the first and second endplates  14 ,  16  to place the expandable fusion device in an expanded position. One or more components of the fusion device  10  may contain features, such as through bores, that facilitate placement down an endoscopic tube. In an embodiment, components of the fusion device  10  are placed down the endoscopic tube with assembly of the fusion device  10  in the disc space. 
     Although the following discussion relates to the second endplate  16 , it should be understood that it also equally applies to the first endplate  14  as the second endplate  16  is substantially identical to the first endplate  14  in embodiments of the present invention. With additional reference to  FIG. 24 , in an exemplary embodiment, the second endplate  16  has a first end  39  and a second end  41 . In the illustrated embodiment, the second endplate  16  further comprise an upper surface  40  connecting the first end  39  and the second end  41 , and a lower surface  42  connecting the first end  39  and the second end  41 . While not illustrated, in an embodiment, the second endplate  16  further comprises a through opening. The through opening, in an exemplary embodiment, is sized to receive bone graft or similar bone growth inducing material. 
     In one embodiment, the upper surface  40  of the second endplate  16  is flat and generally planar to allow the upper surface  40  of the endplate  16  to engage with the adjacent vertebral body  2 . Alternatively, as shown in  FIG. 15 , the upper surface  40  can be curved convexly or concavely to allow for a greater or lesser degree of engagement with the adjacent vertebral body  2 . It is also contemplated that the upper surface  40  can be generally planar but includes a generally straight ramped surface or a curved ramped surface. The ramped surface allows for engagement with the adjacent vertebral body  2  in a lordotic fashion. While not illustrated, in an exemplary embodiment, the upper surface  40  includes texturing to aid in gripping the adjacent vertebral bodies. Although not limited to the following, the texturing can include teeth, ridges, friction increasing elements, keels, or gripping or purchasing projections. 
     In one embodiment, the second endplate  16  further comprises a first side portion  202  connecting the first end  39  and the second end  41 , and a second side portion  204  connecting the first end  39  and the second end  41 . In the illustrated embodiment, the first and second side portions  202 ,  204  are extensions from the lower surface  42 . In an exemplary embodiment, the first and second side portions  202 ,  204  each include ramped surfaces  206 ,  208 . In the illustrated embodiment, the ramped surfaces  206 ,  208  extend from the first end  39  of the second endplate  16  to bottom surfaces  210 ,  212  of each of the side portions  202 ,  204 . In one embodiment, the ramped surfaces  206 ,  208  are forward facing in that the ramped surfaces  206 ,  208  face the first end  39  of the second endplate. As previously discussed, the slope of the ramped surfaces  206 ,  208  may be varied as desired for a particular application. 
     In an embodiment, the first and second side portions  202 ,  204  each comprise at least one protuberance  214 . In an exemplary embodiment, the first and second side portions  202 ,  204  each comprise a first protuberance  214 , a second protuberance  216 , and a third protuberance  218 . In one embodiment, the protuberances  214 ,  216 ,  218  extend from the interior surface  220  of the first and second side portions  202 ,  204 . In an exemplary embodiment, the protuberances  214 ,  216 ,  218  extend at the lower side of the interior surface  220 . As best seen in  FIG. 24 , the first and the second protuberances  214 ,  216  form a first slot  222 , and the second and third protuberances  216 ,  218  form a second slot  224 . 
     As best seen in  FIG. 24 , the lower surface  42  of the second endplate  16 , in an embodiment, includes a central extension  224  extending along at least a portion of the lower surface. In the illustrated embodiment, the central extension  224  extends between the first and second side portions  202  and  204 . In an exemplary embodiment, the central extension  224  can extend from the second end  41  of the endplate  16  to the central portion of the endplate. In one embodiment, the central extension  224  includes a generally concave surface  226  configured and dimensioned to form a through bore with the corresponding concave surface  226  (not illustrated) of the first endplate  14 . The central extension  224  can further include, in an exemplary embodiment, a ramped surface  228 . In the illustrated embodiment, the ramped surface  228  faces the first end  39  of the endplate  16 . The ramped surface  228  can be at one end of the central extension  224 . In an embodiment, the other end of the central extension  224  forms a stop  230 . In the illustrated embodiment, the stop  230  is recessed from the second end  41  of the second endplate  16 . 
     Referring to  FIGS. 25-27 , in an exemplary embodiment, the central ramp  18  includes a body portion  232  having a first end  234  and a second end  236 . In an embodiment, the body portion  232  includes at least a first expansion portion  238 . In an exemplary embodiment, the body portion  232  includes a first expansion portion  238  and a second expansion portion  240  extending from opposing sides of the body portion with each of the first and second expansion portions  238 ,  240  having a generally triangular cross-section. In one embodiment, the expansion portions  238 ,  240  each have angled surfaces  242 ,  244  configured and dimensioned to engage the ramped surfaces  206 ,  208  of the first and second endplates  14 ,  16  and force apart the first and second endplates  14 ,  16 . In an embodiment, the engagement between the angled surfaces  242 ,  244  of the expansion portions  238 ,  240  with the ramped surfaces  206 ,  208  of the first and second endplates  14 ,  16  may be described as a dovetail connection. 
     The second end  236  of the central ramp  18 , in an exemplary embodiment, includes opposing angled surfaces  246 . The angled surfaces  246  can be configured and dimensioned to engage the ramped surface  228  in the central extension  224  in each of the first and second endplates  14 ,  16 . In other words, one of the angled surfaces  246  can be upwardly facing and configured, in one embodiment, to engage the ramped surface  228  in the central extension  224  in the second endplate  16 . In an embodiment, the engagement between the angled surfaces  246  of the second end  236  of the central ramp  18  with the ramped surface  228  in the first and second endplates  14 ,  16  may be described as a dovetail connection. 
     The second end  236 , in an exemplary embodiment, can further include an extension  252 . In the illustrated embodiment, the extension  252  is generally cylindrical in shape with a through bore  254  extending longitudinally therethrough. In one embodiment, the extension  252  can include a beveled end  256 . While not illustrated, at least a portion of the extension  252  can be threaded. 
     Referring still to  FIGS. 25-27 , the central ramp  18  can further include features for securing the first and second endplates  14 ,  16  when the expandable fusion device  10  is in an expanded position. In an embodiment, the body portion  232  of the central ramp  18  includes one or more protuberances  248 ,  250  extending from opposing sides of the body portion  232 . As illustrated, the protuberances  248 ,  250 , in one embodiment, can be spaced along the body portion  232 . In an exemplary embodiment, the protuberances  248 ,  250  can be configured and dimensioned for insertion into the corresponding slots  222 ,  224  in the first and second endplates  14 ,  16  when the device  10  is in an expanded position, as best seen in  FIGS. 19 and 21 . The protuberances  248 ,  250  can engage the endplates  14 ,  16  preventing and/or restricting movement of the endplates  14 ,  16  with respect to the central ramp  18  after expansion of the device  10 . 
     With reference to  FIGS. 20-23 , in an exemplary embodiment, the actuator assembly  200  has a flanged end  253  configured and dimensioned to engage the stop  232  in the central extension  224  of the first and the second endplates  14 ,  16 . In an embodiment, the actuator assembly  200  further includes an extension  254  that extends from the flanged end  253 . In a further embodiment, the actuator assembly  200  includes a threaded hole  256  that extends through the actuator assembly  200 . It should be understood that, while the threaded hole  256  in the actuator assembly  200  is referred to as threaded, the threaded hole  256  may only be partially threaded in accordance with one embodiment. In an exemplary embodiment, the threaded hole  256  is configured and dimensioned to threadingly receive the extension  252  of the central ramp  18 . 
     With additional reference to  FIGS. 28-32 , a method of installing the expandable fusion device  10  of  FIGS. 18-27  is now discussed in accordance with one embodiment of the present invention. Prior to insertion of the fusion device, the disc space may be prepared as described above and then one or more endoscopic tubes may then inserted into the disc space. The expandable fusion device  10  can then be inserted into and seated in the appropriate position in the intervertebral disc space, as best seen in  FIGS. 28-32 . The expandable fusion device  10  can be introduced into the intervertebral space down an endoscopic tube (not illustrated), with the central ramp  18  being first placed down the tube and into the disc space, as seen in  FIG. 28 . After insertion of the central ramp, the first endplate  14  can be placed down an endoscopic tube, as shown on  FIG. 29 , followed by insertion of the second endplate  16 , as shown on  FIG. 30 . After the second endplate  16 , the actuator assembly  200  can then be inserted to complete assembly of the device  10 , as best seen in  FIG. 31 . 
     After the fusion device  10  has been inserted into and assembled in the appropriate position in the intervertebral disc space, the fusion device  10  can then be expanded into the expanded position. To expand the fusion device  10 , the actuator assembly  200  can be rotated. As discussed above, the actuator assembly  200  is in threaded engagement with the extension  250  of the central ramp  18 . Thus, as the actuator assembly  200  is rotated in a first direction, the central ramp  18  moves toward the flanged end  253  of the actuator assembly  200 . In another exemplary embodiment, the actuator assembly  200  can be moved in a linear direction with the ratchet teeth as means for controlling the movement of the central ramp  18 . As the central ramp  18  moves, the angled surfaces  242 ,  244  in the expansion portions  238 ,  240  of the central ramp  18  push against the ramped surfaces  206 ,  208  in the first and second side portions  202 ,  204  of the first and second endplates  14 ,  16 . In addition, the angled surfaces  246  in the second end  236  of the central ramp  18  also push against the ramped surfaces  228  in the central extension  224  of each of the endplates  14 ,  16 . This is best seen in  FIGS. 22-23 . 
     Since the expansion of the fusion device  10  is actuated by a rotational input, the expansion of the fusion device  10  is infinite. In other words, the endplates  14 ,  16  can be expanded to an infinite number of heights dependent on the rotational advancement of the actuator assembly  200 . As discussed above, the central ramp  16  includes locking features for securing the endplates  14 ,  16 . 
     In the event the fusion device  10  needs to be repositioned or revised after being installed and expanded, the fusion device  10  can be contracted back to the unexpanded configuration, repositioned, and expanded again once the desired positioning is achieved. To contract the fusion device  10 , the actuator assembly  200  can be rotated in a second direction. As discussed above, actuator assembly  200  is in threaded engagement with the extension  250  of the central ramp  18 ; thus, as the actuator assembly  200  is rotated in a second direction, opposite the first direction, the central ramp  18  moves with respect to the actuator assembly  200  and the first and second endplates  14 ,  16  away from the flanged end  253 . As the central ramp  18  moves, the first and second endplates are pulled inwardly into the unexpanded position. 
     Referring now to  FIGS. 33-38 , an alternative embodiment of the expandable fusion device  10  is shown. In the illustrated embodiment, the fusion device includes a first endplate  14 , a second endplate  16 , a central ramp  18 , and an actuator assembly  200 . The fusion device  10  of  FIGS. 33-38  and its individual components are similar to the device  10  illustrated on  FIGS. 18-23  with several modifications. The modifications to the device  10  will be described in turn below. 
     Although the following discussion relates to the second endplate  16 , it should be understood that it also equally applies to the first endplate  14  as the second endplate  16  is substantially identical to the first endplate  14  in embodiments of the present invention. With additional reference to  FIG. 39 , in an exemplary embodiment, the lower surface  42  of the second endplate  16  has been modified. In one embodiment, the central extension  224  extending from the lower surface  42  has been modified to include a second ramped surface  258  rather than a stop. In an exemplary embodiment, the second ramped surface  258  faces the second end  41  of the second endplate  16 . In contrast, ramped surface  228  on the central extension  228  faces the first end  39  of the second endplate. The concave surface  228  connects the ramped surface  228  and the second ramped surface  258 . 
     With reference to  FIGS. 35-38 , in an exemplary embodiment, the actuator assembly  200  has been modified to further include a driving ramp  260 . In the illustrated embodiment, the driving ramp  260  has a through bore  262  through which the extension  254  extends. In an embodiment, the driving ramp  260  is generally wedge-shaped. As illustrated, the driving ramp  260  may comprise a blunt end  264  in engagement with the flanged end  253 . In an exemplary embodiment, the driving ramp  260  further comprises angled surfaces  266  configured and dimensioned to engage the second ramped surface  258  of each of the endplates  14 ,  16  and force apart the first and second endplates  14 ,  16 . 
     Referring now to  FIGS. 40-44 , an alternative embodiment of the expandable fusion device  10  is shown. In the illustrated embodiment, the fusion device  10  includes a first endplate  14 , a second endplate  16 , a central ramp  18 , an actuator assembly  200 , and a driving ramp  300 . As will be discussed in more detail below, the actuator assembly  200  functions, in an embodiment, to pull the central ramp  18  and the driving ramp  300  together, which forces apart the first and second endplates  14 ,  16 . 
     Although the following discussion relates to the first endplate  14 , it should be understood that it also equally applies to the second endplate  16  as the second endplate  16  is substantially identical to the first endplate  14  in embodiments of the present invention. With reference to  FIGS. 40-45 , in an exemplary embodiment, the first endplate  14  has a first end  39  and a second end  41 . In the illustrated embodiment, the first endplate  14  further comprises an upper surface  40  connecting the first end  39  and the second end  41 , and a lower surface  42  connecting the first end  39  and the second end  41 . While not illustrated, in an embodiment, the first endplate  14  may comprise further comprises a through opening. The through opening, in an exemplary embodiment, is sized to receive bone graft or similar bone growth inducing material. 
     In one embodiment, the upper surface  40  of the first endplate  14  is flat and generally planar to allow the upper surface  40  of the endplate  14  to engage with the adjacent vertebral body  2 . Alternatively, as shown in  FIG. 15 , the upper surface  40  can be curved convexly or concavely to allow for a greater or lesser degree of engagement with the adjacent vertebral body  2 . It is also contemplated that the upper surface  40  can be generally planar but includes a generally straight ramped surface or a curved ramped surface. The ramped surface allows for engagement with the adjacent vertebral body  2  in a lordotic fashion. While not illustrated, in an exemplary embodiment, the upper surface  40  includes texturing to aid in gripping the adjacent vertebral bodies. Although not limited to the following, the texturing can include teeth, ridges, friction increasing elements, keels, or gripping or purchasing projections. 
     In one embodiment, the first endplate  14  further comprises a first side portion  202  connecting the first end  39  and the second end  41 , and a second side portion  204  connecting the first end  39  and the second end  41 . In the illustrated embodiment, the first and second side portions  202 ,  204  are extensions from the lower surface  42 . In an embodiment, the first and second side portions each have an interior surface  302  and an exterior surface  304 . In an exemplary embodiment, the first and second side portions  202 ,  204  each include one or more ramped portions. In the illustrated embodiment, the first and second side portions  202 ,  204  include first ramped portions  306 ,  308  at the first end  39  of the endplate  14  and second ramped portions  310 ,  312  at the second end  41  of the endplate. The first and second side portions  202 ,  204  each can include a bridge portion  314  connecting the first ramped portions  306 ,  308  and the second ramped portions  310 ,  312 . In an embodiment, the first ramped portions  306 ,  308  abut the exterior surface  304  of the respective side portions  202 ,  204 , and the second ramped portions  310 ,  312  abut the interior surface  302  of the respective side portions  202 ,  204 . As illustrated, the first ramped portions  306 ,  308  may include tongue portions  316 ,  318  with the tongue portions  316 ,  318  extending in an oblique direction with respect to the upper surface  40  of the endplate  14 . As further illustrated, the second ramped portions  310 ,  312  may include tongue portions  320 ,  322  that extend in an oblique direction with respect to the upper surface  40  of the endplate  14 . 
     As best seen in  FIG. 45 , the lower surface  42  of the second endplate  16 , in an embodiment, includes a central extension  224  extending along at least a portion of the lower surface. In the illustrated embodiment, the central extension  224  extends between the first and second side portions  202  and  204 . In an exemplary embodiment, the central extension  224  can extend generally between the first ramped portions  306 ,  308  and the second ramped portions  310 ,  312 . In one embodiment, the central extension  224  includes a generally concave surface  226  configured and dimensioned to form a through bore with the corresponding concave surface  226  (not illustrated) of the second endplate  16 . 
     With reference to  FIGS. 43 and 44 , the actuator assembly  200  includes a head portion  324 , a rod receiving extension  326 , and a connecting portion  328  that connecting portions that connects the head portion  324  and the rod receiving extension  326 . As illustrated, the head portion  324  may include one or more instrument gripping features  330  that can allow it to be turned by a suitable instrument. In addition, the head portion  324  has a larger diameter than the other components of the actuator assembly  200  to provide a contact surface with the driving ramp  300 . In the illustrated embodiment, the head portion  324  includes a rim  332  that provides a surface for contacting the driving ramp  300 . As can be seen in  FIG. 44 , in an exemplary embodiment, the rod receiving extension  326  includes an opening sized and dimensioned to receive the extension  336  of the central ramp  18 . In an embodiment, the rod receiving extension  326  includes threading for threadingly engaging the extension  336 . In another embodiment, the rod receiving extension  326  includes ratchet teeth for engaging the extension  336 . In the illustrated embodiment, the head portion  324  and the rod receiving extension  326  are connected by connecting portion  328  which can be generally cylindrical in shape. 
     With reference to  FIGS. 43, 44, and 46 , the central ramp  18  includes expansion portion  334  and extension  336 . As best seen in  FIG. 46 , the expansion portion  334  may include an upper portion  338  and side portions  340 ,  342  that extend down from the upper portion  338 . In an embodiment, each of the side portions  340 ,  342  include dual, overlapping ramped portions. For example, side portions  340 ,  342  each include a first ramped portion  344  that overlaps a second ramped portion  346 . In the illustrated embodiment, the first ramped portion  344  faces the extension  336  while the second ramped portion  344  faces away from the extension  336 . In one embodiment, angled grooves  348 ,  350  are formed in each of the first and second ramped portions  344 ,  346 . In another embodiment, the angled grooves  348 ,  350  are sized to receive the corresponding tongues  316 ,  318 ,  320 ,  322  in the first and second endplates with angled grooves  348  receiving tongues  320 ,  322  in the second endplate  16  and angled grooves  350  receiving tongues  316 ,  318  in the first endplate  14 . Although the device  10  is described with tongues  316 ,  318 ,  320 ,  322  on the endplates  14 ,  16  and angled grooves  348 ,  350  on the central ramp  18 , it should be understood that that device  10  can also be configured with grooves on the endplates  14 ,  16  and tongues on the central ramp  18 , in accordance with one embodiment of the present invention. 
     In an exemplary embodiment, the extension  336  is sized to be received within the rod receiving extension  326  of the actuator assembly  200 . In one embodiment, the extension  336  has threading with the extension  336  being threadingly received within the rod receiving extension  326 . In another embodiment, the extension  336  has ratchet teeth with the extension  336  being ratcheted into the rod receiving extension  336 . In an embodiment, the extension  336  include nose  352  at the end of the extension  336 . 
     With reference to  FIGS. 47-49 , in an exemplary embodiment, the driving ramp  300  includes an upper portion  354  having an upper surface  356  and an oblique surface  358 . In an embodiment, the driving ramp  300  further includes side portions  360 ,  362  that extend from the upper portion  354  connecting the upper portion  354  with the lower portion  364  of the driving ramp  300 . As best seen in  FIGS. 48-49 , the driving ramp  300  further includes a bore  366 , in an exemplary embodiment, sized to receive the connection portion  328  of the actuator assembly  200 . In one embodiment, the driving ramp  300  moves along the connection portion  328  when the actuator assembly  200  is pushing the driving ramp  300 . In an exemplary embodiment, the driving ramp  300  further includes contact surface  368  that engages the rim  332  of the head portion  324  of the actuator assembly  200 . In the illustrated embodiment, the contact surface  368  has a generally annular shape. 
     In an exemplary embodiment, the side portions  360 ,  362  of the driving ramp  300  each include overlapping ramped portions. For example, the side portions  360 ,  362  each include first ramped portions  370  that overlap second ramped portions  372 . In the illustrated embodiment, the first ramped portions  370  face central ramp  18  while the second ramped portions  372  face the opposite direction. In one embodiment, angled grooves  374 ,  376  are formed in each of the first and second ramped portions  370 ,  372 .  FIG. 48  is a perspective view of the driving ramp  300  that shows the top ends of the angled grooves  374  in ramped portions  370 .  FIG. 49  is a perspective view of the driving ramp  300  that shows the top ends of the angled grooves  376  in ramped portions  372 . In an exemplary embodiment, the angled grooves  374 ,  376  are sized to receive corresponding tongues  316 ,  318 ,  320 ,  322  in the first and second endplates  14 ,  16  with angled grooves  370  receiving tongues  316 ,  318  in the second endplate  16  and angled grooves  372  receiving tongues  320 ,  322  in the first endplate  14 . Although the device  10  is described with tongues  316 ,  318 ,  320 ,  322  in the first and second endplates  14 ,  16  and angled grooves  370 ,  372 ,  374 ,  376  on the driving ramp  300 , it should be understood that that device  10  can also be configured with grooves on the second endplate  16  and tongues on the driving ramp  300 , in accordance with one embodiment of the present invention. 
     Turning now to  FIGS. 40-42 , a method of installing the expandable fusion device  10  of  FIGS. 40-49  is now discussed in accordance with one embodiment of the present invention. Prior to insertion of the fusion device, the disc space may be prepared as described above. The expandable fusion device  10  can then be inserted into and seated in the appropriate position in the intervertebral disc space. The expandable fusion device  10  is then introduced into the intervertebral space, with the end having the expansion portion  334  of the central ramp  18  being inserted. In an exemplary method, the fusion device  10  is in the unexpanded position when introduced into the intervertebral space. In an exemplary method, the intervertebral space may be distracted prior to insertion of the fusion device  10 . The distraction provides some benefits by providing greater access to the surgical site making removal of the intervertebral disc easier and making scraping of the endplates of the vertebral bodies  2 ,  3  easier. 
     With the fusion device  10  inserted into and seated in the appropriate position in the intervertebral disc space, the fusion device can then be expanded into the expanded position, as best seen in  FIG. 42 . To expand the fusion device  10 , an instrument is engaged with the head portion  324  of the actuator assembly  200 . The instrument is used to rotate actuator assembly  200 . As discussed above, actuator assembly  200  is threadingly engaged with the extension  336  of the central ramp  18 ; thus, as the actuator assembly  200  is rotated in a first direction, the central ramp  18  is pulled toward the actuator assembly  200 . In an exemplary embodiment, the actuator assembly  200  is moved in a linear direction with the ratchet teeth engaging as means for controlling the movement of the actuator assembly  200  and the central ramp  18 . As the central ramp  18  is pulled towards the actuator assembly  200 , the first ramped portions  344  of the central ramp  18  push against the second ramped portions  310 ,  312  of the second endplate  16  and the second ramped portions  346  of the central ramp  18  push against first ramped portions  306 ,  308  of the first endplate  14 . In this manner, the central ramp  18  acts to push the endplates  14 ,  16  outwardly into the expanded position. This can best be seen in  FIGS. 40-42 . As the endplates  14 ,  16  move outwardly the tongues  316 ,  318 ,  320 ,  322  in the endplates  14 ,  16  ride in the angled grooves  348 ,  350  with the tongues  320 ,  322  in the second endplate  16  riding in angled grooves  348  and the tongues  316 ,  318  in the first endplate  14  riding in angled grooves  350 . 
     As discussed above, the actuator assembly  200  also engages driving ramp  300 ; thus, as the actuator assembly  200  is rotated in a first direction, the actuator assembly  200  pushes the driving ramp  300  towards the central ramp  18  in a linear direction. As the driving ramp  300  is pushed towards the central ramp  18 , the first ramped portions  370  of the driving ramp  300  push against the first ramped portions  306 ,  308  of the second endplate  16  and the second ramped portions  372  of the driving ramp  300  push against the second ramped portions  310 ,  312  of the first endplate  14 . In this manner, the driving ramp  300  also acts to push the endplates  14 ,  16  outwardly into the expanded position. This can best be seen in  FIGS. 40-42 . As the endplates  14 ,  16  move outwardly the tongues  316 ,  318 ,  320 ,  322  in the endplates  14 ,  16  ride in the angled grooves  370 ,  372  with the tongues  316 ,  318  in the second endplate  16  riding in angled grooves  370  and the tongues  320 ,  322  in the first endplate  14  riding in angled grooves  372 . 
     Since the expansion of the fusion device  10  is actuated by a rotational input, the expansion of the fusion device  10  is infinite. In other words, the endplates  14 ,  16  can be expanded to an infinite number of heights dependent on the rotational advancement of the actuator assembly  200 . 
     Referring now to  FIGS. 50-54 , an alternative embodiment of the expandable fusion device  10  is shown. In the illustrated embodiment, the fusion device  10  includes a first endplate  14 , a second endplate  16 , a central ramp  18 , an actuator assembly  200 , and a driving ramp  300 . As will be discussed in more detail below, the actuator assembly  200  functions, in an embodiment, to pull the central ramp  18  and the driving ramp  300  together, which forces apart the first and second endplates  14 ,  16 . In an embodiment, the expandable fusion device may contain features, such as a through bore, that facilitate placement down an endoscopic tube. In an embodiment, the assembled fusion device  10  may be placed down the endoscopic tube and then expanded. 
     Although the following discussion relates to the first endplate  14 , it should be understood that it also equally applies to the second endplate  16  as the second endplate  16  is substantially identical to the first endplate  14  in embodiments of the present invention. It should be understood that, in an embodiment, the first endplate  14  is configured to interlock with the second endplate  16 . With additional reference to  FIG. 55 , in an exemplary embodiment, the first endplate  14  has a first end  39  and a second end  41 . As illustrated, the first end  39  may be wider than the second end  41 . In the illustrated embodiment, the first endplate  14  further comprises an upper surface  40  connecting the first end  39  and the second end  41 , and a lower surface  42  connecting the first end  39  and the second end  41 . As best seen in  FIG. 54 , the lower surface  42  can be curved concavely such that the first and second endplates  14 ,  16  form a through bore when the device  10  is in a closed position. In an embodiment, the first endplate  14  may comprise a through opening  44 . The through opening  44 , in an exemplary embodiment, is sized to receive bone graft or similar bone growth inducing material. 
     In one embodiment, the upper surface  40  of the first endplate  14  is flat and generally planar to allow the upper surface  40  of the endplate  14  to engage with the adjacent vertebral body  2 . Alternatively, as shown in  FIG. 15 , the upper surface  40  can be curved convexly or concavely to allow for a greater or lesser degree of engagement with the adjacent vertebral body  2 . It is also contemplated that the upper surface  40  can be generally planar but includes a generally straight ramped surface or a curved ramped surface. The ramped surface allows for engagement with the adjacent vertebral body  2  in a lordotic fashion. As illustrated, in an exemplary embodiment, the upper surface  40  includes texturing to aid in gripping the adjacent vertebral bodies. For example, the upper surface  40  may further comprise texturing  400  to engage the adjacent vertebral bodies. Although not limited to the following, the texturing can include teeth, ridges, friction increasing elements, keels, or gripping or purchasing projections. 
     In one embodiment, the first endplate  14  further comprises a first side portion  202  connecting the first end  39  and the second end  41 , and a second side portion  204  connecting the first end  39  and the second end  41 . In the illustrated embodiment, the first and second side portions  202 ,  204  are extensions from the lower surface  42 . In an embodiment, the first and second side portions  202 ,  204  each include an interior surface  302  and an exterior surface  304 . In an embodiment, the first end  39  of the first endplate  14  is generally designed and configured to fit over the second end  41  of the second endplate  16  when the device  10  is in a closed position. As illustrated, the first and second side portions  202 ,  204  each may include first ramped portions  306 ,  308 , second ramped portions  310 ,  312 , and/or central ramped portion  402 . 
     In an embodiment, the first ramped portions  306 ,  308  are proximate the first end  39  of the endplate  14 . In accordance with embodiment of the present invention, the first ramped portions  306 ,  308  of the first endplate  14  are generally designed and configured to fit over the second ramped portions  310 ,  312  of the second endplate  16  when the device  10  is in a closed position. In an exemplary embodiment, the first ramped portions  306 ,  308  generally face the first end  39  and can extend in an oblique direction with respect to the upper surface  40 , for example. As illustrated, the first ramped portions  306 ,  308  may include tongue portions  316 ,  318  extending in an oblique direction with respect to the upper surface  40  of the endplate  14 . 
     In an embodiment, the second ramped portions  310 ,  312  are proximate the second end  41  of the endplate  14 . In an exemplary embodiment, the second ramped portions  310 ,  312  can extend in an oblique direction with respect to the upper surface  40  and generally face the second end  41 . The first and second side portions  202 ,  204 , in an embodiment, each can include a bridge portion  314  connecting the first ramped portions  306 ,  308  and the second ramped portions  310 ,  312 . As further illustrated, the second ramped portions  310 ,  312  may include tongue portions  320 ,  322  that extend in an oblique direction with respect to the upper surface  40  of the endplate  14 . 
     In an embodiment, the endplate  14  further may include a central ramped portion  402  proximate the bridge portion  314 . In the illustrated embodiment, the endplate  14  includes a central ramped portion  402  proximate the bridge portion  314  of the second side portion  204 . In an exemplary embodiment, the central ramped portion  402  can extend in an oblique direction with respect to the upper surface  40  and face the first end  39  of the endplate  14 . As illustrated, the first ramped portions  306 ,  308  may include tongue portions  316 ,  318  with the tongue portions  316 ,  318  extending in an oblique direction with respect to the upper surface  40  of the endplate  14 . 
     With reference to  FIGS. 50-52 and 54 , in an embodiment, the actuator assembly  200  includes a head portion  324 , an extension  404 , and a through bore  406  that extends longitudinally through the actuator assembly  200 . As illustrated, the head portion  324  may include one or more instrument gripping features  330  that can allow it to be turned by a suitable instrument. In addition, the head portion  324  has a larger diameter than the other components of the actuator assembly  200  to provide a contact surface with the driving ramp  300 . In the illustrated embodiment, the head portion  324  includes a rim  332  that provides a surface for contacting the driving ramp  300 . In an embodiment, the extension  404  is a generally rod-like extension. In another embodiment, the extension  404  includes ratchet teeth for engaging the extension  336 . 
     With reference to  FIGS. 51, 52, and 56 , the central ramp  18  has a first end  408  and a second end  410 . In an embodiment, the central ramp  18  includes a first expansion portion  412 , a second expansion portion  414 , a rod-receiving extension  416 , and a through bore  418  that extends longitudinally through the central ramp  18 . In an exemplary embodiment, first expansion portion  412  can be proximate the first end  408  of the central ramp  18 . As best seen in  FIG. 56 , the first expansion portion  412  may include side portions  420 ,  422 . In an embodiment, each of the side portions  420 ,  422  includes dual, overlapping ramped portions that extend in oblique directions with respect to the through bore  418 . For example, side portions  420 ,  422  each include a first ramped portion  424  that overlaps a second ramped portion  426 . In the illustrated embodiment, the first ramped portion  424  faces the rod-receiving extension  416  while the second ramped portion  426  faces the opposite direction. In one embodiment, angled grooves  428 ,  430  are formed in each of the first and second ramped portions  424 ,  426 . In an exemplary embodiment, the angled grooves  428 ,  430  are sized to receive the corresponding tongues  316 ,  318 ,  320 ,  322  in the first and second endplates  14 ,  16  with angled grooves  428  receiving tongues  320 ,  322  in the second endplate  16  and angled grooves  430  receiving tongues  316 ,  318  in the first endplate  14 . Although the device  10  is described with tongues  316 ,  318 ,  320 ,  322  on the endplates  14 ,  16  and angled grooves  428 ,  430  on the central ramp  18 , it should be understood that that device  10  can also be configured with grooves on the endplates  14 ,  16  and tongues on the central ramp  18 , in accordance with one embodiment of the present invention. 
     In an embodiment, the second expansion portion  414  is located on the rod-receiving extension  416  between the first end  408  and the second end  410  of the central ramp  18 . In an exemplary embodiment, the second expansion portion  414  includes central ramped portions  432 . In one embodiment, the second expansion portion  414  includes two central ramped portions  432  on opposite sides of the rod-receiving extension  416 . In an exemplary embodiment, the central ramped portions  424  extend in an oblique direction with respect to the through bore  418  and face the second end  410  of the central ramp  18 . 
     The rod-receiving extension  416  extends from the first expansion portion  412  and has an opening  434  at the second end of the central ramp  18 . In an embodiment, the rod-receiving extension  416  is sized and configured to receive the extension  404  of the actuator assembly  200 . In an embodiment, the rod-receiving extension  416  has threading with the rod-receiving extension  416  threadingly receiving extension  404  of the actuator assembly  200 . In another embodiment, the rod-receiving extension  416  has ratchet teeth with the extension  404  being ratcheted into the rod-receiving extension  416 . 
     With reference to  FIGS. 50-52 and 57 , in an exemplary embodiment, the driving ramp  300  includes an upper portion  354  having an upper surface  356  and an oblique surface  358 . In an embodiment, the driving ramp  300  further includes a bore  366 , in an exemplary embodiment, sized to receive the extension  404  of the actuator assembly  200 . In the illustrated, embodiment, the upper portion  354  has a hole  436  that extends through the upper surface  356  to the bore  366 . Set screw  438  may be inserted through the hole  436  to secure the driving ramp  300  to the actuator assembly  200 . In one embodiment, the driving ramp  300  further includes contact surface  368  that engages the rim  332  of the head portion  324  of the actuator assembly  200 . In the illustrated embodiment, the contact surface  368  has a generally annular shape. 
     In an embodiment, the driving ramp  300  further includes side portions  360 ,  362  that extend from the upper portion  354  connecting the upper portion  354  with the lower portion  364  of the driving ramp  300 . In an exemplary embodiment, the side portions  360 ,  362  of the driving ramp  300  each include a ramped portion  438 . In the illustrated embodiment, the ramped portion  438  faces central ramp  300 . In an embodiment, the ramped portion  438  is configured and dimensioned to engage the ramped portions  306 ,  308  at the first end  39  of the second endplate  16 . In one embodiment, angled grooves  440  are formed in the ramped portions  316 ,  318 . In an exemplary embodiment, the angled grooves  440  are sized to receive the corresponding tongues  316 ,  318  in the second endplate  16 . Although the device  10  is described with tongues  316 ,  318  on the second endplate  16  and angled grooves  440  on the driving ramp  300 , it should be understood that that device  10  can also be configured with grooves on the second endplate  16  and tongues on the driving ramp  300 , in accordance with one embodiment of the present invention. 
     A method of installing the expandable fusion device  10  of  FIGS. 50-57  is now discussed in accordance with one embodiment of the present invention. Prior to insertion of the fusion device, the disc space may be prepared as described above. The expandable fusion device  10  can then be inserted into and seated in the appropriate position in the intervertebral disc space. In an embodiment, the device  10  is assembled prior to insertion. The expandable fusion device  10  can be introduced into the intervertebral space, with the end having the first end  408  of the central ramp  18  being inserted. In an exemplary method, the fusion device  10  is in the unexpanded position when introduced into the intervertebral space. In an exemplary method, the intervertebral space may be distracted prior to insertion of the fusion device  10 . The distraction provides some benefits by providing greater access to the surgical site making removal of the intervertebral disc easier and making scraping of the endplates of the vertebral bodies  2 ,  3  easier. 
     With the fusion device  10  inserted into and seated in the appropriate position in the intervertebral disc space, the fusion device can then expand into the expanded position. To expand the fusion device  10 , an instrument is engaged with the head portion  324  of the actuator assembly  200 . The instrument is used to rotate actuator assembly  200 . As discussed above, actuator assembly  200  is threadingly engaged with the rod receiving extension  416  of the central ramp  18 ; thus, as the actuator assembly  200  is rotated in a first direction, the central ramp  18  is pulled toward the actuator assembly  200 . In an exemplary embodiment, the actuator assembly  200  is moved in a linear direction with the ratchet teeth engaging as means for controlling the movement of the actuator assembly  200  and the central ramp  18 . 
     As the central ramp space  18  is pulled towards the actuator assembly  200 , the central ramp  18  acts to push endplates  14 ,  16  outwardly into the expanded position. By way of example, the first ramped portions  424 , second ramped portions  426 , and central ramped portions  432  push against the corresponding ramped portions in the first and second endplates  14 ,  16 . The first ramped portions  424  in the first expansion portion  412  of the central ramp  18  push against the second ramped portions  310 ,  312  of the second endplate  16  with the corresponding tongues  320 ,  322  in the second ramped portions  310 ,  312  of the second endplate  16  riding in angled grooves  428  in the first ramped portions  424  in the first expansion portion  412 . The second ramped portions  426  in the first expansion portion  412  push against the first ramped portions  316 ,  318  of the first endplate  14  with the corresponding tongues  316 ,  318  in first ramped portions  316 ,  318  of the first endplate  14  riding in angled grooves  430  in the second ramped portions  426  in the first expansion portion  412 . The central ramped portions  432  in the second expansion portion  414  push against the central ramped portion  402  in the first and second endplates  14 ,  16 . 
     As discussed above, the actuator assembly  200  also engages driving ramp  300 ; thus, as the actuator assembly  200  is rotated in a first direction, the actuator assembly  200  pushes the driving ramp  300  towards the central ramp  18  in a linear direction. As the driving ramp  300  is pushed towards the central ramp  18 , the driving ramp  300  also acts to push the endplates  14 ,  16  outwardly into the expanded position. By way of example, the ramped portions  438  of the driving ramp  300  push against ramped portions  306 ,  308  at the first end  39  of the second endplate  16 . As the endplates  14 ,  16  move outwardly, the tongues  316 ,  318  in the ramped portions  306 ,  308  of the second endplate  16  ride in the angled grooves  440  in the ramped portions  438  of the driving ramp  300 . 
     It should also be noted that the expansion of the endplates  14 ,  16  can be varied based on the differences in the dimensions of the various ramped portions in the central ramp  18 , the driving ramp  300 , and the first and second endplates  14 ,  16 . As best seen in  FIG. 16 , the endplates  14 ,  16  can be expanded in any of the following ways: straight rise expansion, straight rise expansion followed by a toggle into a lordotic expanded configuration, or a phase off straight rise into a lordotic expanded configuration. 
     Referring now to  FIG. 58 , an alternative embodiment of the expandable fusion device  10  is shown in which the expandable fusion device  10  expands into a lordotic expanded configuration. In the illustrated embodiment, the expandable fusion device  10  includes a first endplate  14 , a second endplate  16 , an actuator assembly  200 , a driving ramp  300 , and a body  500 . As will be discussed in more detail below, the actuator assembly  200  functions, in an embodiment, to pull the driving ramp  300  and the body  500  together, which forces apart the first and second endplates  14 ,  16 . For example, the actuator assembly  200  may be rotated to pull the driving ramp  300  toward the body  500 . When this occurs, the expandable fusion device  10  first expands into a lordotic expanded configuration ( FIGS. 62-64 ) and then expands in height until it is fully expanded ( FIGS. 65-67 ). In embodiments, expandable fusion device  10  may have two stages of expansion, generally referred to as lordotic stage and parallel stage. In lordotic stage, the expandable fusion device  10  may expand at one end to achieve a lordotic angle. The expandable fusion device  10  may then expand in parallel sage wherein the lordotic expansion may be maintained at both ends of the expandable fusion device  10  may expand at generally constant rates. In an embodiment, the expandable fusion device  10  may contain features, such as a through bore, that facilitate placement down an endoscopic tube. In an embodiment, the assembled fusion device  10  may be placed down the endoscopic tube and then expanded. 
     Although the following discussion relates to the first endplate  14 , it should be understood that it also equally applies to the second endplate  16  as the second endplate  16  is substantially identical to the first endplate  14  in embodiments of the present invention. It should be understood that, in an embodiment, the first endplate  14  is configured to interlock with the second endplate  16 . In an exemplary embodiment, the first endplate  14  has a first end  39  and a second end  41 . In the illustrated embodiment, the first endplate  14  further comprises a plate portion  502  that may extend between first end  39  and the second end  41 . Plate portion  502  may comprise an upper surface  40  and a lower surface  42 . In an embodiment, the first endplate  14  may comprise a through opening  44 . The through opening  44 , in an exemplary embodiment, may be sized to receive bone graft or similar bone growth inducing material. 
     In one embodiment, the upper surface  40  of the plate portion  502  is flat and generally planar to allow the upper surface  40  of the plate portion  502  to engage with the adjacent vertebral body  2 . Alternatively, as shown in  FIG. 15 , the upper surface  40  can be curved convexly or concavely to allow for a greater or lesser degree of engagement with the adjacent vertebral body  2 . It is also contemplated that the upper surface  40  can be generally planar but includes a generally straight ramped surface or a curved ramped surface. The ramped surface allows for engagement with the adjacent vertebral body  2  in a lordotic fashion. As illustrated, in an exemplary embodiment, the upper surface  40  includes texturing to aid in gripping the adjacent vertebral bodies. For example, the upper surface  40  may further comprise texturing  400  to engage the adjacent vertebral bodies. Although not limited to the following, the texturing can include teeth, ridges, friction increasing elements, keels, or gripping or purchasing projections. 
     In one embodiment, the first endplate  14  further comprises front side extensions  504  that extend from plate portion  502 . As illustrated, the front side extensions  504  may extend from either side of plate portion  502  proximate to second end  41  of first endplate  14 . The front side extensions  504  may extend opposite from the upper surface  40  of plate portion  502 . In one embodiment, the first endplate  14  may further comprise rear side extensions  506  that extend from plate portion  502 . As illustrated, the rear side extensions  506  may extend from either side of plate portion  502  proximate to first end  39  of first endplate  14 . The rear side extensions  506  may extend opposite from the upper surface  40  of plate portion  502 . As illustrated, the front side extensions  504  and the rear side extensions  506  may each include ramped portions. For example, the front side extension  504  may include front ramped portions  508  and the rear side extensions  506  may include rear ramped portions  510 . The front ramped portions  508  and the rear ramped portions  510  may be considered ramped as they may be at an oblique angle with respect to longitudinal axis  512  of expandable fusion device  10 . In an exemplary embodiment, the front ramped portions  508  may generally face the second end  41 , and the rear ramped portions  510  may generally face the first end  39 . 
     Embodiments of actuator assembly  200  will now be described in more detail with reference to  FIG. 58 . In the illustrated embodiment, the actuator assembly  200  is in the form of a drive screw. As illustrated, the actuator assembly  200  may include a head portion  324  and an extension  404 . As illustrated, the head portion  324  may include one or more instrument gripping features  330  that can allow it to be turned by a suitable instrument. In addition, the head portion  324  may have a larger diameter than the other components of the actuator assembly  200  to provide a contact surface with the body  500 . In the illustrated embodiment, ring  514  may ride in groove  516  on head portion  324 . In some embodiments, ring  514  may be a compressible ring, such as a c-ring as shown on  FIG. 58 , that is configured to retain head portion  324  in rear throughbore  536  of body  500 . In an embodiment, the extension  404  is a generally rod-like extension that may be threaded for engaging a corresponding opening  522  in driving ramp  300 . In another embodiment, the extension  404  may include ratchet teeth (not shown) for engaging opening  522  in driving ramp  300 . 
     Embodiments of driving ramp  300  will now be described in more detail with respect to  FIG. 58 . As illustrated, the driving ramp  300  may include a ramped body portion  518  and an extension  520 . In the illustrated embodiment, extension  520  may extend from ramped body portion  518  toward first end  39  of expandable fusion device  10 . Extension  520  may include an opening  522  that may engage extension  404  of actuator assembly  200 . In embodiments, extension  520  may threadingly engage the extension  404  of actuator assembly  200 . Rotation of driving ramp  300  may be limited so that when actuator assembly  200  may be rotated, driving ramp  300  may be pulled toward body  500 . Driving ramp  300  may be secured to actuator assembly  200  at a front end of expandable fusion device  10 . In embodiments, the front end of expandable fusion device  10  may be the front of the expandable fusion device  10  so that the driving ramp  300  may be considered the nose of the expandable fusion device  10 . In embodiments, the front end  524  of driving ramp  300  may be angled, rounded, or otherwise tapered so that the driving ramp may serve to distract the adjacent vertebral bodies when the expandable fusion device  10  is inserted into an intervertebral space. 
     As illustrated, driving ramp  300  may include front endplate engaging ramps  526 . Front endplate engaging ramps  526  may be at an oblique angle with respect to longitudinal axis  512  of the expandable fusion device  10 . As illustrated, a pair of front endplate engaging ramps  526  that engage second endplate  16  may be on one side of driving ramp while another pair of front endplate engaging ramps  526  that engage first endplate  14  may be on an opposite side of driving ramp  300 . In operation, front endplate engaging ramps  526  may engage front ramped portions  508  of the first and second endplates  14 ,  16 . The first and second endplates  14 ,  16  may ride up the front endplate engaging ramps  526  as the driving ramp  300  may be pulled towards the body  300  causing the first and second endplates  14 ,  16  to be pushed relatively apart such that a height of expandable fusion device  10  may be increased. 
     Embodiments of body  500  will now be described in more detail with respect to  FIG. 58 . As illustrated, the body  500  may have a first body end  528  and a second body end  530 . Lateral sides  532  may connect the first body end  528  and the second body end  530 . In the illustrated embodiment, the body  500  may have a central opening  534  that may extend through the body  500  transverse to longitudinal axis  512  of expandable fusion device. As illustrated, first body end  528 , second body end  530 , and lateral sides  532  may define central opening  534 . Rear throughbore  536  may be formed through second body end  530 . Rear throughbore  536  may be centrally positioned and generally aligned with longitudinal axis  512  of expandable fusion device  10 . As previously described, head portion  324  of actuator assembly  200  may be retained in rear throughbore  536 , for example, using ring  514 . Washer  515  may also be retained on corresponding grooves of head portion  324 . Rear throughbore  506  may also be threaded, for example, to facilitate engagement with an insertion device. Second body end  530  may also include tool engaging features, such as side recesses  538 , which may facilitate use of a device for insertion of expandable fusion device  10  into a desired position in a patient. First body end  528  may include a corresponding front throughbore  540 . As illustrated, front throughbore  540  may be centrally positioned and generally aligned with longitudinal axis  512  of expandable fusion device. Extension  404  of actuator assembly  200  may extend through front throughbore  540  to engage driving ramp  300 . 
     As illustrated, second body end  530  may include rear endplate engaging ramps  542 . Rear endplate engaging ramps  542  may be at an oblique angle with respect to longitudinal axis  512  of the expandable fusion device  10 . In operation, rear endplate engaging ramps  542  may engage rear ramped portions  510  of the first and second endplates  14 ,  16 . As illustrated, a pair of rear endplate engaging ramps  542  that engage second endplate  16  may be on one side of second body end  530  while another pair of rear endplate engaging ramps  542  (not seen on  FIG. 58 ) that engage first endplate  14  may be on an opposite side of second body  530 . The first and second endplates  14 ,  16  may ride up the rear endplate engaging ramps  542  as the driving ramp  300  may be pulled towards the body  300  causing the first and second endplates  14 ,  16  to be pushed relatively apart such that a height of expandable fusion device  10  may be increased. 
     As previously described, the expandable fusion device  10  shown on  FIG. 58  may first expand lordotically and then expand in parallel until full expansion of the expandable fusion device  10  may be reached. To achieve this lordotic expansion, the front ramped portions  508  and rear ramped portions  510  of the first and second endplates  14 ,  16  may be at a different angle with respect to longitudinal axis  512  than the front endplate engaging ramps  526  of the driving ramp  300  and the rear endplate engaging ramps  542  of the body  500 . This difference in angles may be present when the expandable fusion device  10  is in the unexpanded configuration. As the driving ramp  300  may be pulled back towards the body  500 , the position of the first and second endplates  14 ,  16  and/or the driving ramp  300  and the body  500  with respect to body  500  may change so that the difference in angles may be reduced and potentially approach zero as the first and second endplates  14 ,  16  are pushed outward. As this angle is being reduced, the rear portion of the expandable fusion device may be expanding causing a lordotic angle. When this angle is reduced (or reaches approximately zero), the first and second endplates  14 ,  16  may then expand in parallel with the first end  39  and second end  41  expanding at approximately the same height until the expandable fusion device  10  may reach its full height. The lordotic angle may be maintained while the first and second endplates  14 ,  16  expand in parallel. 
       FIGS. 59 to 61  illustrate the expandable fusion device  10  in the unexpanded configuration in accordance with present embodiments. As seen on  FIG. 60 , the expandable fusion device  10  may have a lordotic angle θ LA  of approximately 0° when unexpanded. By way of example, the first and second endplates  14 ,  16  may be generally aligned with longitudinal axis  512  of expandable fusion device  10 . In accordance with present embodiments, lordotic expansion of expandable fusion device  10  may be achieved by use of different in ramp angles with respect to longitudinal axis  512 . As best seen on  FIG. 59 , rear endplate engaging ramps  542  of the body  500  may have an angle α body  and rear ramped portions  510  of first and second endplates  14 ,  16  may have an angle α rearendplate . The front endplate engaging ramps  526  of the driving ramp  300  may have an angle α driving  ramp and the front ramped portions  508  of the first and second endplates  14 ,  16  may have an angle α frontendplate . These angles may be selected, for example, to provide a desired rate of height increase during expansion of expandable fusion device  10 . By way of example, the angles may each individually by selected, for example, from about 5° to about 85° and alternatively from about 35° to about 65°. However, as described above, embodiments may provide differences in these angles, for example, to drive the lordotic expansion. As best seen on  FIG. 59 , the difference between the angles α rearendplate  and α body  may be provided by Δ rear , and the difference between the angles α frontendplate  and α driving  ramped may be provided by Δ front . Δ rear  and Δ front  may be the same or different. By way of example, Δ rear  and Δ front  may each range from 1° to about 20° and, alternatively, from about 2° to about 5°. 
       FIGS. 62 to 64  illustrate the expandable fusion device  10  in a lordotic expanded configuration in accordance present embodiments. The expandable fusion device  10  may be expanded to provide a lordotic angle θ LA  of up to about 15° and, more particularly, of about 4° to about 10°. Lordotic angles θ LA  of up to 120 may be desired in certain applications, such as cervical, but other lordotic angles θ LA  may be desired in alternative applications. 
     To expand the expandable fusion device  10 , driving ramp  300  may be moved in a first direction with respect to body  500 . By way of example, driving ramp  300  may be pulled towards body  500 . In some embodiments, actuator assembly  200  (best seen on  FIG. 58 ) may be rotated to pull driving ramp  300  towards body  500 . As driving ramp  300  may be pulled towards body  500 , the driving ramp  300  and body  500  may engage the first and second endplates  14 ,  16 . By way of example, the front ramped portions  508  of the first and second endplates  14 ,  16  may engage the front endplate engaging ramps  526  of the driving ramp  300  and the rear ramped portions  510  of the first and second endplates  14 ,  16  may engage the rear endplate engaging ramps  542  of the body  500 . However, because of the difference in ramp angles (shown as Δ rear  and Δ front  on  FIG. 59 ), the first and second endplates  14 ,  16  may not ride up the front endplate engaging ramps  526  and the rear endplate engaging ramps  542  to increase the height of the expandable fusion device. Instead, in some embodiments, the first and second endplates  14 ,  16  may pivot at the contact point between the first and second endplates  14 ,  16  and the body  500  causing expansion of the endplates  14 ,  16  at the opposite end. As seen in  FIGS. 62-64 , this pivoting may result in expansion of the first and second endplates  14 ,  16  into an expanded lordotic configuration. As will be appreciated, pivoting of the first and second endplates  14 ,  16  may cause the angles α rearendplate  and α frontendplate  with respect to longitudinal axis  512  to change, thus reducing the difference in ramp angles Δ rear , Δfront. When the difference in ramp angles Δ rear , Δ front  approaches 0° (e.g., within 0.5°, 0.1°, or less), lordotic expansion may stop, and expandable fusion device  10  may be in its lordotic expanded configuration. 
       FIGS. 65 to 67  illustrate expandable fusion device  10  in a fully expanded configuration, in accordance with present embodiments. In some embodiments, it may be desired to further expand the expandable fusion device  10  from the lordotic expanded configuration of  FIGS. 62-64 . By way of example, continued movement of driving ramp  300 , for example, translational movement towards body  500 , may cause further expansion of expandable fusion device  10 . This further expansion may be considered parallel expansion as both ends of the expandable fusion device  10  may expand at the same rate. Expansion may be continued, for example, until the expandable fusion device  10  has reached its fully expanded configuration or until a desired height of expandable fusion device  10  has been achieved. Expansion of expandable fusion device  10  may be limited by engagement of driving ramp  300  with body  500 . 
     In the event the fusion device  10  needs to be repositioned or revised after being installed and expanded, the fusion device  10  can be contracted back to the unexpanded configuration, repositioned, and expanded again once the desired positioning is achieved. To contract the fusion device  10 , an instrument can be used to rotate the actuator assembly  200  in a second direction that is opposite the first direction. Rotation of the actuator assembly  200  in the opposite direction may result in movement of the body  500  and the driving ramp  300  away from one another. As the body  500  and driving ramp  300  move away from one another, the endplates  14 ,  16  move inwardly into the unexpanded position. 
     Expanded heights of expandable fusion device  10  may typically range from 7 mm to 12 mm, but may be larger or smaller, including as small as 5 mm, and as large as 16 mm, although the size is dependent on the patient, and the joint into which the expandable fusion device  10  may be implanted. Expandable fusion device  10  may be implanted within any level of the spine, and may also be implanted in other joints of the body, including joints of the hand, wrist, elbow, shoulder, hip, knee, ankle, or foot. 
     Although the preceding discussion only discussed having a single fusion device  10  in the intervertebral space, it is contemplated that more than one fusion device  10  can be inserted in the intervertebral space. It is further contemplated that each fusion device  10  does not have to be finally installed in the fully expanded state. Rather, depending on the location of the fusion device  10  in the intervertebral disc space, the height of the fusion device  10  may vary from unexpanded to fully expanded. It should be noted that, as well as the height being varied from an unexpanded state to an expanded state, the fusion  10  may be positioned permanently anywhere between the expanded state and the unexpanded state. 
     In some embodiments, an expandable fusion device can be provided whereby expansion is performed via a ratcheting mechanism. By providing a ratcheting mechanism, this advantageously provides for rapid, convenient, non-continuous expansion of the fusion device. 
       FIG. 68  is an exploded view of an expandable fusion device having a ratcheting mechanism in accordance with some embodiments. The expandable fusion device  600  comprises a first endplate  620 , a second endplate  630 , a body  610  positioned between the first endplate  620  and the second endplate  630 , a stem  660  and associated collar  670 , and a nose  680 . The stem  660  and associated collar  670  advantageously provide a non-continuous ratcheting mechanism to the expandable fusion device, whereby the expandable fusion device can alternatingly incrementally increase and then stop, until a desired expansion occurs. 
     The first endplate  620  comprises a lower endplate having a first end  622  and a second end  624 . The first end  622  comprises a pair of first end ramped portions  626   a ,  626   b . Each of these ramped portions  626   a ,  626   b  is configured to engage corresponding lower nose ramps  682   a ,  682   b  on the nose  680  to aid with expansion of the expandable fusion device. The second end  624  comprises a pair of second end ramped portions  628   a ,  628   b . Each of these ramped portions  628   a ,  628   b  is configured to engage corresponding rear lower ramps  616   a ,  616   b  on the body  610  to aid with expansion of the expandable fusion device. A first side portion  623  having a central ramp  627   a  and a second side portion  625  having a central ramp  627   b  are positioned between the first end  622  and the second end  624  of the first endplate  620 . Each of the central ramps  627   a ,  627   b  is configured to engage corresponding front lower ramps  615   a ,  615   b  (not visible) of the base  610  to aid with expansion of the expandable fusion device. The ramps of the first endplate  620  are formed along a perimeter that surrounds a central opening  629 . 
     The second endplate  630  comprises an upper endplate having a first end  632  and a second end  634 . The first end  632  comprises a pair of first end ramped portions  636   a ,  636   b . Each of these ramped portions  636   a ,  636   b  is configured to engage corresponding upper nose ramps  684   a ,  684   b  on the nose  680  to aid with expansion of the expandable fusion device. The second end  634  comprises a pair of second end ramped portions  638   a ,  638   b . Each of these ramped portions  638   a ,  638   b  is configured to engage corresponding rear upper ramps  618   a ,  618   b  on the body  610  to aid with expansion of the expandable fusion device. A first side portion  633  having a central ramp  637   a  and a second side portion  635  having a central ramp  637   b  are positioned between the first end  632  and the second end  634  of the second endplate  630 . Each of the central ramps  637   a ,  637   b  is configured to engage corresponding front upper ramps  617   a ,  617   b  of the base  610  to aid with expansion of the expandable fusion device. The ramps of the second endplate  630  are formed along a perimeter that surrounds a central opening  639 . 
     The body  610  comprises a front throughbore  612  and a rear throughbore  614 . The front throughbore  614  comprises an opening for receiving the collar  670 , and hence the stem  660 , therethrough. The rear throughbore  614  comprises an opening through which one or more tools (e.g., an expansion tool and a disengagement tool) can pass through, as shown in  FIGS. 78B and 78D . In some embodiments, the rear throughbore  614  is threaded to allow engagement by an insertion tool. In addition, the body  610  comprises one or more tool recesses  611  that can be engaged by an insertion tool to provide easy delivery of the implant into a surgical site. As shown in  FIG. 68  and discussed above, the body  610  comprises a number of angled surfaces or ramps that are configured to engage corresponding ramps on the first endplate  620  or second endplate  630 . As the ramps slide against one another, this causes expansion of the expandable fusion device. 
     The stem  660  and associated collar  670  form a ratcheting mechanism for causing expansion of the expandable fusion device. The stem  660  comprises a head  662  and a shaft  664 . The stem  660  (via its head  662 ) is receivable within the nose  680  of the implant, whereby it is capable of rotation. In some embodiments, rotation of the stem  660  causes the implant to be changed from a “locked” ratcheting configuration into a “disengaged” non-ratcheting configuration, as will be discussed further below. The head  662  of the stem  660  comprises one or more grooves or slots  668  for receiving one or more nose pins  690   a ,  690   b  that extend through the nose  680 . The shaft  664  of the stem  660  comprises an elongate body having an opening  663  for receiving an expansion tool  710  (shown in  FIG. 78C ) therethrough. The stem  660  further comprises ratchet teeth  665  that extend along a length of the shaft  664 . In addition, the stem  660  comprises one or more flat areas  667  that are positioned adjacent to the ratchet teeth  665 . In some embodiments, the stem  660  comprises a pair of flat areas  667  that are positioned 180 degrees apart from one another. In some embodiments, the stem  660  comprises a half ring portion  664  that is advantageously designed to hit against the body  610  at full expansion in order to prevent over expansion of the device. 
     The stem  660  is capable of two configurations. In a first “locked” configuration (shown in  FIG. 78D ), the ratchet teeth  665  of the stem  660  are engaged with corresponding ratchet recesses  675  of the collar  670 , thereby creating a ratcheting mechanism that provides for expansion of the implant  600 . In a second “disengaged” configuration (shown in  FIG. 78E ), the stem  660  is rotated such that the one or more flat areas  667  are positioned adjacent the ratchet recesses  675 , such that the ratcheting mechanism is not operable. In this second disengaged configuration, the stem  660  is capable of being pulled back, thereby causing contraction of the implant  600 . 
     The stem  660  is insertable through the collar  670 , whereby it is placed in either the “locked” ratcheting configuration or the “disengaged” non-ratcheting configuration. In some embodiments, the collar  670  comprises a C-shaped ring having inner ratchet recesses  675  formed along an inner wall. In some embodiments, the collar  670  is housed within the front throughbore  616  of the body  610 . In some embodiments, the collar  670  comprises a compressible C-ring type body that is capable of compression within the front throughbore  616 . In some embodiments, the collar  670  is not rotatable, and can be keyed into place to prevent rotation. Advantageously, the collar  670  can comprise a tab  679  that prevents rotation of the collar  670  within the body  610 . With the stem  660  attached to the collar  670 , a ratcheting mechanism is formed whereby an expansion tool  710  (shown in  FIG. 78C ) can extend through the collar  670  and into the stem  660  via the shaft opening  663 . The expansion tool  710  is capable of pulling or ratcheting the stem  660  in a direction towards the second ends of the first endplate  620  and second endplate  630 . As the stem  660  is operably connected to the nose  680 , the nose  680  is also drawn, thereby causing ramps of the first endplate  620  and second endplate  630  to slide up corresponding ramps of the body  610  and nose  680 . 
     The nose  680  comprises a throughhole  685  through which the head  662  of the stem  660  can extend therethrough. A pair of nose pins  682   a ,  682   b  can then extend through the nose  680  and into the head  662 , thereby retaining the stem  660  in the nose  680 . As noted above, the nose  680  comprises one or more upper nose ramps  684   a ,  684   b , which are configured to mate and engage corresponding ramps on the second endplate  630 . In addition, the nose  680  comprises one or more lower nose ramps  682   a ,  682   b , which are configured to mate and engage corresponding ramps on the first endplate  620 . 
       FIGS. 69A-69C  are side views of the expandable fusion device of  FIG. 68  in the process of expansion in accordance with some embodiments. In some embodiments, the expandable fusion device  600  is advantageously capable of expansion, and in particular, lordotic expansion. In some embodiments, the device  600  can begin in a contracted state, as shown in  FIG. 69A . Afterwards, by pulling the nose  680  via a ratcheting mechanism, the device  600  can expand and tip into lordosis, as shown in  FIG. 69B . Once the device  600  has achieved maximum lordosis, the device  600  can continue to expand in height in a parallel fashion, whereby both the anterior and posterior aspects expand at the same rate, until the implant  600  reaches a maximum expansion, as shown in  FIG. 69C . In other words, once the device  600  reaches a particular lordotic angle (as shown in  FIG. 69B ), the device  600  will maintain the lordotic angle throughout the expansion range until maximum expansion has been achieved, as shown in  FIG. 69C . More details on the expansion of the device  600  are provided with respect to  FIGS. 70A-72C . 
       FIGS. 70A-70C  are different views of the expandable fusion device of  FIG. 68  in a contracted state in accordance with some embodiments. From the contracted state, the device  600  is capable of first expanding and tipping into lordosis, and then expanding in a parallel fashion. The angle tipping is driven by a difference in ramp angle x that is seen between the first end ramped portions  636   a ,  636   b  of the second endplate  630  and the upper nose ramps  684   a ,  684   b  of the nose  680 . Similarly, the same difference in ramp angle x is also seen between the second end ramped portions  638   a ,  638   b  of the second endplate  630  and the rear upper ramps  618   a ,  618   b  of the body  610 . In other words, at the contracted height, the difference in angle x between the different ramps causes a gap  702  between the ramps, with a first end gap  702   a  formed closer to the first end of the second endplate  630  and a second end gap  702   b  formed closer to the second end of the second endplate  630 . The degree of the gap  702  will determine what lordosis the device will tip into upon expansion. For example, if the degree of the gap  702  is 4 degrees (e.g., x=4), the second endplate  630  will tip into 4 degrees of lordosis. As the same mechanism is provided for the first endplate  620 , the first endplate  620  will also tip into 4 degrees of lordosis, thereby providing an overall lordosis of 8 degrees once both endplates  620 ,  630  have been tipped. In some embodiments, the endplates  620 ,  630  themselves can have built-in lordosis. For example, if the built in lordosis of both endplates  620 ,  630  was 7 degrees inclusive, then the overall lordosis following expansion wherein x=4 is 15 degrees of lordosis. While the present embodiment shows an angle x difference of 4 degrees, the angle can be less or more, thereby resulting in less or more lordosis. 
       FIGS. 71A-71C  are different views of the expandable fusion device of  FIG. 68  in a tipped state without full expansion in accordance with some embodiments. To tip the expandable fusion device  600  into lordosis, the nose  680  is initially ratcheted or pulled back towards the body  610 , thereby causing the gaps x to close and the corresponding ramps to mate. The amount of lordosis will be pre-determined based on the initial ramp gap x. In the present embodiment, the expandable fusion device  600  has been tipped into a lordotic angle of 4 degrees for the second endplate  630  and 4 degrees for the first endplate  620 , thereby resulting in a total of 8 degrees of lordosis (as shown in  FIG. 71B ). One skilled in the art can appreciate that the total degree of lordosis can be less than or greater than 8 degrees, and that 8 degrees in just a representative example. 
       FIGS. 72A-72C  are different views of the expandable fusion device of  FIG. 68  in a fully expanded state in accordance with some embodiments. As the nose  680  is pulled back further the corresponding ramps of the device  600  are fully mated, the implant then begins to expand in overall height in a parallel fashion. In other words, the anterior and posterior aspects of the device  600  expand at the same rate. As this happens, the device maintains the same lordosis allowing the lordotic angle to be seen throughout the expansion range. For example, the degree of lordosis of the device  600  in the fully expanded state (as shown in  FIG. 72B ) is the same as the degree of lordosis of the device  600  after the endplates have been tipped (as shown in  FIG. 71B ). However, due to further parallel expansion, the height of the device  600  in the fully expanded state (as shown in  FIG. 72B ) is greater than the height of the device  600  after the endplates have been tipped (as shown in  FIG. 71B ). 
     The expandable fusion device  600  can advantageously be expanded via a ratcheting mechanism. More details regarding the ratcheting mechanism—in particular, the stem  660  and the collar  670 —will be provided with respect to  FIGS. 73-76 . 
       FIG. 73  is an upper view of the expandable fusion device of  FIG. 68  in accordance with some embodiments. From this view, one can see how collar  670  is housed in the body  610 , and how the stem  660  is received in the collar  670 . The stem  660  is further received in the nose  680 , such that as the stem is pulled back, the nose  680  can also be pulled back thereby causing ratcheted expansion of the device  600 . 
       FIG. 74  is an upper cross-sectional view of the expandable fusion device of  FIG. 68  in accordance with some embodiments. In this view, one can see how the stem  660  having ratchet teeth  665  is engaged with the collar  670  to create an expandable ratcheting mechanism. In some embodiments, the stem  660  comprises the “male” ratcheting feature, while the collar  670  comprises the “female” ratcheting feature. 
       FIG. 75  is a close up view of the ratcheting mechanism of the expandable fusion device of  FIG. 68  in accordance with some embodiments. This view shows the male ratchet of the stem  660  and the female ratchet of the collar  670  in more detail. As the stem  660  is pulled back, the collar  670  springs open like a C-ring and allows the ratchet teeth  665  of the stem  660  to advance to the next slot or recess  675  formed in the collar  670 . The stem  660  advantageously moves in increments through the collar  670 . These non-continuous increments drive height increases. In some embodiments, the height increases can increase in increments greater than 0.2 mm and 0.8 mm. In some embodiments, the height increases are in increments of approximately 0.5 mm. 
       FIG. 76  is a close up view of the ratchet teeth of the expandable fusion device of  FIG. 68  in accordance with some embodiments. Each of the ratchet teeth  665  comprises an inclusive angle  668   a  and a back angle  668   b . In some embodiments, the ratchet teeth  665  comprise an inclusive angle  668   a  of between 30 and 60 degrees, and in particular about 45 degrees. In some embodiments, the back angle  668   b  comprises between 2 and 8 degrees, and in particular about 5 degrees. Under load, the ratchet connection is pulled in the direction of disengagement. Advantageously, the purpose of the back angle  668   b  is to keep the stem  660  more engaged, especially in the back area when the device  600  is under load by pulling the collar  670  closer to the ratchet teeth  665  when pulled in the direction of disengagement. 
       FIG. 77  is a top perspective view of the expandable fusion device of  FIG. 68  in accordance with some embodiments. In this configuration, the fusion device  600  is capable of ratcheted expansion. In addition to providing ratcheted expansion, the device is also capable of collapse and contraction. To accommodate contraction, the device  600  advantageously provides ratchet teeth  665  on only a portion of the stem  660 , whereby the ratchet teeth  665  are separated by one or more flat areas  667 . In the particular embodiment, the device  600  includes two sets of ratchet teeth  665  each of which is adjacent two sets of flat areas  667 . These features allow a device to be converted between a “locked” configuration whereby ratcheting is enabled and a “disengaged” configuration whereby ratcheting is disabled. These features are discussed below with respect to  FIGS. 78A-78G . 
       FIGS. 78A-78G  are top perspective views of the expandable fusion device of  FIG. 68  transitioning from a locked configuration to a disengaged configuration in accordance with some embodiments.  FIG. 78A  shows an expandable fusion device in a “locked” configuration whereby the device is capable of ratcheted expansion. As shown in  FIG. 78A , the ratchet teeth  665  of the stem  660  are aligned and engaged with the ratchet recesses  675  of the collar  670 , thereby enabling ratcheted expansion. 
       FIG. 78B  shows the expandable fusion device with an expansion tool inserted therein. The expansion tool  710  is capable of engaging the stem  660  in the “locked” configuration, whereby the stem  660  (and hence the nose  680 ) is capable of being pulled back. As the stem  660  and nose  680  are drawn back, this causes incremental ratcheting expansion of the device  600  based on the design of the ratchet teeth. 
       FIG. 78C  shows the expandable fusion device when fully expanded. As shown in the figure, the stem  660  has been pulled further into the body  610 , thereby causing greater height expansion of the device. The fusion device  600  has a relatively higher height in  FIG. 78C  than in  FIG. 78B . Advantageously, the fusion device  600  can also be contracted by a surgeon if desired. 
       FIG. 78D  shows the expandable fusion device prior to contraction with the device still in a “locked” ratcheting configuration. To contract the device  600 , a disengagement tool  720  (separate from the expansion tool  710 ) is provided. The disengagement tool  720  comprises a shaft having a distal nub  730 . The disengagement tool  720  is advantageously designed to rotate the stem  660 , such that the device is changed from a “locked” ratchetable configuration to a “disengaged” unratchetable configuration, as discussed above. To rotate the stem  660 , the distal nub  730  of the disengagement tool  720  mates with a correspondingly shaped recess  669  in the stem  660 . With the disengagement tool  720  engaged with the stem  660 , the stem  660  can be rotated (e.g., 90 degrees), thereby converting the device into a disengaged configuration, as shown in  FIG. 78E . 
       FIG. 78E  shows the expandable fusion device in a “disengaged” non-ratchetable configuration. The stem  660  has been rotated such that its pair of flat areas  667  align and face the collar  670 . As such, the ratchet teeth  665  of the stem are no longer engaged with ratchet slots of the collar  670 , thereby allowing the stem  660  to be pushed forward to contract the device. 
       FIG. 78F  shows the expandable fusion device in a “disengaged” configuration whereby the device has been fully contracted. At this stage, the device  600  is the same height as it was prior to expansion. The device  600  is fully capable of expansion again. A surgeon simply needs to rotate the stem  660  in an opposite direction 90 degrees, such that the device is brought back into a “locked” ratcheting configuration. 
       FIG. 78G  shows the expandable fusion device whereby the device is brought back to a “locked” ratcheting configuration. By rotating the disengagement tool  720  in a reverse direction 90 degrees, this rotates the stem  660  whereby the ratchet teeth  665  are once again engaged with ratchet slots of the collar  670 . The fusion device  600  can once again be expanded via a ratcheting mechanism if desired. Advantageously, the expandable fusion devices described above are each capable of being inserted through a minimal incision, as the devices can maintain a minimal profile prior to expansion. 
     In some embodiments, an expandable fusion device can be provided whereby expansion is performed via a threading mechanism. By providing a threading mechanism, this advantageously provides for controlled expansion and/or controlled of the fusion device. 
       FIG. 79  is an exploded view of an expandable fusion device having a threaded mechanism in accordance with some embodiments. The expandable fusion device  800  comprises a first endplate  820 , a second endplate  830 , a body  810  positioned between the first endplate  820  and the second endplate  830 , a drive screw  860 , a washer  870 , a retaining ring  890 , and a nose  880 . The drive screw  860  advantageously provides a threaded mechanism for expanding and contracting the expandable fusion device. 
     The first endplate  820  comprises a lower endplate having a first end  822  and a second end  824 . The first end  822  comprises a pair of first end ramped portions  826   a ,  826   b . Each of these ramped portions  826   a ,  826   b  is configured to engage corresponding lower nose ramps  882   a ,  882   b  on the nose  880  to aid with expansion of the expandable fusion device. The second end  824  comprises a pair of second end ramped portions  828   a ,  828   b . Each of these ramped portions  828   a ,  828   b  is configured to engage corresponding rear lower ramps  816   a ,  816   b  on the body  810  to aid with expansion of the expandable fusion device. A first side portion  823  having a central ramp  827   a  and a second side portion  825  having a central ramp  827   b  are positioned between the first end  822  and the second end  824  of the first endplate  820 . Each of the central ramps  827   a ,  827   b  is configured to engage corresponding front lower ramps  815   a ,  815   b  of the base  810  to aid with expansion of the expandable fusion device. The ramps of the first endplate  820  are formed along a perimeter that surrounds a central opening  829  (shown in  FIG. 84A ). 
     The second endplate  830  comprises an upper endplate having a first end  832  and a second end  834 . The first end  832  comprises a pair of first end ramped portions  836   a ,  836   b . Each of these ramped portions  836   a ,  836   b  is configured to engage corresponding upper nose ramps  884   a ,  884   b  on the nose  880  to aid with expansion of the expandable fusion device. The second end  834  comprises a pair of second end ramped portions  838   a ,  838   b . Each of these ramped portions  838   a ,  838   b  is configured to engage corresponding rear upper ramps  818   a ,  818   b  on the body  810  to aid with expansion of the expandable fusion device. A first side portion  833  having a central ramp  837   a  and a second side portion  835  having a central ramp  837   b  are positioned between the first end  832  and the second end  834  of the second endplate  830 . Each of the central ramps  837   a ,  837   b  (not visible) is configured to engage corresponding front upper ramps  817   a ,  817   b  of the base  810  to aid with expansion of the expandable fusion device. The ramps of the second endplate  830  are formed along a perimeter that surrounds a central opening  839  (shown overlapping with central opening  829  in  FIG. 84A ). 
     The body  810  comprises a front throughbore  812  and a rear throughbore  817 . The front throughbore  812  comprises an opening through which the threaded shaft  864  of the drive screw  860  extends therethrough. The rear throughbore  817  comprises an opening through which the head  862  of the drive screw  860  extends therethrough. The rear throughbore  817  also receives the retaining ring  890  and washer  870  therethrough. The retaining ring  890  is received in a recess  863  of the head  862 , which is then received in the rear throughbore  817 . In some embodiments, the retaining ring  890  comprises a c-shaped ring. 
     The drive screw  860  comprises a head portion  862  and a shaft portion  864 . The head portion  862  comprises a recess  863  for receiving a retaining ring  890  therethrough. The head portion  862  can be received in the rear throughbore  817  of the body  810 . The shaft portion  864  comprises a threaded portion that extends through the nose  880 . The threaded portion mates with threads  886  found within the nose  880 . Rotation of the drive screw  860  thereby causes movement or translation of the nose  880 . 
     In some embodiments, one or more tools (e.g., an expansion tool) can engage the head of the drive screw  860 . Rotation of the drive screw  860  in a first direction translates and draws the nose  880  inwardly, thereby causing expansion between the first endplate  820  and the second endplate  830 . As the nose  880  is drawn inwardly, upper nose ramps  884   a ,  884   b  engage first end ramped portions  836   a ,  836   b  of the second endplate  830 , while rear upper ramps  818   a ,  818   b  of the body  810  engage second end ramped portions  838   a ,  838   b  of the second endplate  830 . Likewise, lower nose ramps  882   a ,  882   b  engage first end ramped portions  826   a ,  826   b  of the first endplate  820 , while rear lower ramps  816   a ,  816   b  engage second end ramped portions  828   a ,  828   b  of the first endplate  820 . The engagement of these ramps causes outward expansion between the first endplate  820  and the second endplate  830 . Rotation of the drive screw  860  in a second direction opposite to the first direction translates the nose  880  outwardly, thereby causing contraction between the first endplate  820  and the second endplate  830 . 
     The nose  880  comprises a throughhole  885  through which the shaft portion  864  of the drive screw  860  can extend. The throughhole  885  of the nose  880  comprises nose threads  886  that engage and mate with the threads of the shaft portion  864 . As noted above, the nose  880  comprises one or more upper nose ramps  884   a ,  884   b , which are configured to mate and engage corresponding ramps on the second endplate  830 . In addition, the nose  880  comprises one or more lower nose ramps  882   a ,  882   b , which are configured to mate and engage corresponding ramps on the first endplate  820 . 
       FIGS. 80A-80C  are side views of the expandable fusion device of  FIG. 79  in the process of expansion in accordance with some embodiments. In some embodiments, the expandable fusion device  800  is advantageously capable of expansion, and in particular, lordotic expansion. In some embodiments, the device  800  can begin in a contracted state, as shown in  FIG. 80A . Afterwards, by pulling the nose  880  via rotation of the drive screw  860 , the device  800  can expand and tip into lordosis, as shown in  FIG. 80B . Once the device  800  has achieved maximum lordosis, the device  800  can continue to expand in height in a parallel fashion, whereby both the anterior and posterior aspects expand at the same rate, until the implant  800  reaches a maximum expansion, as shown in  FIG. 80C . In other words, once the device  800  reaches a particular lordotic angle (as shown in  FIG. 80B ), the device  800  will maintain the lordotic angle throughout the expansion range until maximum expansion has been achieved, as shown in  FIG. 80C . More details on the expansion of the device  800  are provided with respect to  FIGS. 81A-83B . 
       FIGS. 81A-81B  are different views of the expandable fusion device of  FIG. 79  in a contracted state in accordance with some embodiments. From the contracted state, the device  800  is capable of first expanding and tipping into lordosis, and then expanding in a parallel fashion. The angle tipping is driven by a difference in ramp angle x that is seen between the first end ramped portions  836   a ,  836   b  of the second endplate  830  and the upper nose ramps  884   a ,  884   b  of the nose  880 . Similarly, the same difference in ramp angle x is also seen between the second end ramped portions  838   a ,  838   b  of the second endplate  830  and the rear upper ramps  818   a ,  818   b  of the body  810 . In other words, at the contracted height, the difference in angle x between the different ramps causes a gap  802  between the ramps, with a first end gap  802   a  formed closer to the first end of the second endplate  830  and a second end gap  802   b  formed closer to the second end of the second endplate  830 . The degree of the gap  802  will determine what lordosis the device will tip into upon expansion. For example, if the degree of the gap  802  is 4 degrees (e.g., x=4), the second endplate  830  will tip into 4 degrees of lordosis. As the same mechanism is provided for the first endplate  820 , the first endplate  820  will also tip into 4 degrees of lordosis, thereby providing an overall lordosis of 8 degrees once both endplates  820 ,  830  have been tipped. In some embodiments, the endplates  820 ,  830  themselves can have built-in lordosis. For example, if the built in lordosis of both endplates  820 ,  830  was 7 degrees inclusive, then the overall lordosis following expansion wherein x=4 is 15 degrees of lordosis. While the present embodiment shows an angle x difference of 4 degrees, the angle can be less or more, thereby resulting in less or more lordosis. 
       FIGS. 82A-82B  are different views of the expandable fusion device of  FIG. 79  in a tipped state without full expansion in accordance with some embodiments. To tip the expandable fusion device  800  into lordosis, the nose  880  is initially ratcheted or pulled back towards the body  810 , thereby causing the gaps x to close and the corresponding ramps to mate. The amount of lordosis will be pre-determined based on the initial ramp gap x. In the present embodiment, the expandable fusion device  800  has been tipped into a lordotic angle of 8 degrees for the second endplate  830  and 8 degrees for the first endplate  820 , thereby resulting in a total of 8 degrees of lordosis (as shown in  FIG. 82B ). One skilled in the art can appreciate that the total degree of lordosis can be less than or greater than 8 degrees, and that 8 degrees in just a representative example. 
       FIGS. 83A-83B  are different views of the expandable fusion device of  FIG. 79  in a fully expanded state in accordance with some embodiments. As the nose  880  is pulled back further the corresponding ramps of the device  800  are fully mated, the implant then begins to expand in overall height in a parallel fashion. In other words, the anterior and posterior aspects of the device  800  expand at the same rate. As this happens, the device maintains the same lordosis allowing the lordotic angle to be seen throughout the expansion range. For example, the degree of lordosis of the device  800  in the fully expanded state (as shown in  FIG. 83B ) is the same as the degree of lordosis of the device  800  after the endplates have been tipped (as shown in  FIG. 82B ). However, due to further parallel expansion, the height of the device  800  in the fully expanded state (as shown in  FIG. 83B ) is greater than the height of the device  800  after the endplates have been tipped (as shown in  FIG. 82B ). 
     In some embodiments, the device  800  can be used via different approaches. For example, in some embodiments, the device  800  can be a TLIF device that enters a disc space via a transforaminal approach, while in other embodiments, the device  800  can be a PLIF device that enters a disc space via a posterior approach. In other embodiments, the device  800  can be an ALIF device that enters via an anterior approach. One skilled in the art will appreciate that the device  800  is not limited to any particular approach. In some embodiments, depending on the approach, the device  800  can have distinct features, as will be discussed below. 
       FIGS. 84A-84D  are different views of a TLIF device having threaded expansion in accordance with embodiments of the present application.  FIG. 84A  shows the device  800  from a top view.  FIG. 84B  shows the device  800  from a side perspective view.  FIG. 84C  shows the device  800  from an anterior view.  FIG. 84D  shows the device  800  from a posterior view. The TLIF device  800  has a specific curvature as shown in the figures. In particular, the TLIF device  800  has a curvature cut at a 30 degree angle from the sagittal plane of the device. This advantageously allows for the lordosis of the TLIF device to be in the same plane as the lordosis of the spine. In some embodiments, the curvature will provide a convex surface to the device. The curved surface can be particularly seen in  FIGS. 84C and 84D . 
       FIG. 84A  depicts a TLIF device. The dark line  7  represents the midline of the sagittal plane in a vertebral body, as well as the plane of the curvature of the device  800 . The dotted line  9  represents the midline of the device itself. The angle between the midline of the sagittal plane and the midline of the device (e.g., 30 degrees) represents the orientation of the curvature cut in the device  800 . While in some embodiments, the curvature cut is generally at a 30 degree angle from the sagittal plane of the device, in other embodiments, the curvature cut can be between 15 and 45 degrees, or 15 and 60 degrees. 
       FIGS. 85A-85D  are different views of a PLIF device having threaded expansion in accordance with embodiments of the present application.  FIG. 85A  shows the device  800  from a top view.  FIG. 85B  shows the device  800  from a side perspective view.  FIG. 85C  shows the device  800  from an anterior view.  FIG. 85D  shows the device  800  from a posterior view. The PLIF device  800  has a specific curvature as shown in the figures. In particular, the PLIF device  800  has a curvature that is offset from its midline. This advantageously allows for the lordosis of the PLIF device to be in the same plane as the lordosis of the spine. In some embodiments, the curvature will provide a convex surface to the device. The curved surface can be particularly seen in  FIGS. 85C and 85D . 
       FIG. 85A  depicts a PLIF device. The dark line  7  represents the midline of the sagittal plane in a vertebral body, as well as the plane of the curvature of the device  800 . The dotted line  9  represents the midline of the device itself. The curvature of the device  800  is offset from its midline to accommodate its offset placement relative to the midline of the sagittal plane. In some embodiments, the offset distance is 10 mm, while in other embodiments, the offset distance is between 8 and 12 mm, or between 5 and 15 mm. 
     In some embodiments, the devices above can have a novel surface treatment. In some embodiments, the treatment is a roughened and/or porous surface that can be achieved through several manufacturing processes.  FIG. 86  is an exemplary surface  890  of a device having an exemplary roughened and/or porous surface. Various surface treatments can be provided to the devices above, including sinker EDM, chemical etching, laser etching, and blasting. A sinker EDM is used to burn a roughened profile into any surface of the implant. The roughness of a surface can be controlled by varying the power setting of the EDM machine. Sinker electrodes are customized for each surface profile for each part instance or family. In chemical etching, a surface of a device is introduced to a corrosive chemical which subtracts material, thereby leaving pores and pits. The etching chemical may be applied in a random or non-random arrangement. A mask may be used prior to the application of the etching chemical to better control the outcome of the texture. In laser etching, laser pulses are used to deform the surfaces of the devices. Multiple laser pulses create pores, pits, and peaks of varying dimensions based upon the laser raster rate, peak power, travel pattern and frequency. In blasting, treated surfaces are sprayed with an abrasive media, such as aluminum oxide, at high pressure to create a porous, pitted surface. 
     As discussed above, the actuator assembly  200  may comprise a fastening device, such as drive screw, in certain embodiments of the present invention. Various features of embodiments of an exemplary drive screw are discussed with reference to  FIGS. 58 and 79  above. Those skilled in the art will understand that screws are used in multiple applications ranging from securing two items together to translating one item with respect to another. When enough torque is applied or the screw is continuously under a high amount of load, the screw is less likely to loosen over time to due to increased friction forces. However, if the screw is not held under sufficient load, known as low-load mode, it may loosen over time due to vibration or other outside forces. 
     To prevent loosening of a screw, one embodiment of the present invention may include a locking mechanism that substantially prevents rotation of the actuating device, such as the drive screw described with respect to  FIGS. 58 and 79 . Various embodiments of locking mechanisms are described individually below to facilitate their description. Those skilled in the art will understand that the different embodiments of locking mechanisms described below with respect to  FIGS. 87-143  may be used separately, or in combination with one another. Certain embodiments, for example, may be combined with one another to ensure the locking mechanism can withstand vibration or other outside forces that are present in a particular application. 
     According to one embodiment of the present invention, the locking mechanism includes a screw ring that is operatively connected to a drive screw. When the screw ring is operatively connected to the drive screw, the two elements are rotationally engaged. A housing ring is also included that is operatively connected to the housing into which the drive screw is inserted. When the housing ring is operatively connected to the housing, these two elements are also rotationally engaged. The housing ring and screw ring include complementary mating surfaces that selectively engage with one another. When engaged, the housing ring (which is rotationally locked to the housing) prevents rotational movement of the screw ring, and therefore the drive screw, because the two are also rotationally engaged. 
       FIG. 87  illustrates one embodiment of a locking mechanism  900  that may be used in accordance with any of the embodiments described herein. In the illustrated embodiment, the locking mechanism  900  is described with respect to an actuator assembly  200  that comprises a drive screw  860  described with reference to  FIGS. 58 and 79 . This embodiment of the locking mechanism  900  includes a housing  902 , a drive screw  860 , a retaining ring  890 , a screw lock ring  904 , and a housing lock ring  906 . As will be discussed in more detail below, the drive screw  860  sits inside of the housing  902  and is axially restrained by the retaining ring  890  but can rotate freely inside the housing  902 . The screw lock ring  904  is then assembled inside the housing  902  and is radially keyed to the screw head  862  with the use of a tab or slot feature. The screw lock ring  904  is capable of flexing up and down but remains radially keyed to the drive screw  860  to rotate when the drive screw  860  rotates. The housing lock ring  906  is assembled next and is retained and radially keyed with a tab to the housing  902 . The housing lock ring  906  cannot rotate once assembled. 
     With reference to  FIGS. 87-89 , one embodiment of the screw lock ring  904  is described in more detail. The screw lock ring  904  can be manufactured from a number of materials including titanium, stainless steel, titanium alloys, non-titanium metallic alloys, polymeric materials, plastics, plastic composites, PEEK, ceramic, and elastic materials. In an exemplary embodiment, the screw lock ring  904  comprises a substantially circular inner diameter  912 . The screw lock ring  904  also includes a spring tail  914  that allows the lock ring  904  to flex when a force is applied, for example, to its inner surface  912  or its outer surface  932 . 
     A first side  916  of the screw lock ring  904  includes a substantially flat surface that is operable to sit flush with a corresponding face  918  of the drive screw head  862 . The screw lock ring  904  includes a first protuberance  920  that extends from at least a portion of the first side  916  that is operable to sit inside a corresponding recess  922  of the drive screw head  862 . The first protuberance  920  can be selectively positioned such that it is located between the inner surface  912  and outer surface  932  of the screw lock ring  904 . In alternate embodiments the protuberance  920  may extend to the inner surface  912 , outer surface  932 , or both surfaces. The protuberance  920  may comprise any shape and dimensions. In some embodiments, for example, the protuberance  920  may comprise a rectangular tab with flat surfaces, as shown in  FIGS. 88-89 . In other embodiments, however, the protuberance  920  may be configured and dimensioned to include at least one pointed surface, a ratcheted surface, or the like. 
     In one embodiment, the screw lock ring  904  also includes a second protuberance  926  that extends from at least a portion of the second side  924 , as shown in  FIGS. 87 and 89 . The second protuberance  926  may extend from only a predetermined portion of the second side  924 . For example, in one embodiment the second protuberance  926  extends from the second side  924  of the screw lock ring  904  and is configured and dimensioned to engage with the housing lock ring  906  described in more detail below. The second protuberance  926  is also configured and dimensioned such that the upper portion  928  does not extend to the top edge  930  of the outer surface  932  of the screw lock ring  904 . In other words, the top edge  930  extends beyond the upper portion  928 . Other portions of the second protuberance  926  may extend along the second side  924  from the inner surface  912  to the outer surface  932 . For instance, as best illustrated in  FIGS. 87 and 89 , the second protuberance  926  may extend from the inner surface  912  to the outer surface  932  along a bottom portion  934  of the screw lock ring  904 . As a result, at least a portion of the second side  924  may not include any protuberance  926 . 
     When configured and dimensioned as described above, and operatively connected to one another, the screw lock ring  904  and the drive screw head  862  remain radially keyed to one another. The operative connection is accomplished when the first protuberance  920  sits inside the recess  922  on the drive screw head  862 , as illustrated by the arrow in  FIGS. 88 and 89 . When assembled in this manner, the screw lock ring  904  and the drive screw head  862  are rotationally engaged. 
     With reference to  FIGS. 90 and 91 , in an exemplary embodiment, the screw locking ring  904  and the screw head  862  have been modified so that the drive screw head  862  includes at least one protuberance  934  and the screw lock ring  904  includes at least one recess  936 . In other words, the recess  936  may be included on the screw lock ring  904  while the protuberance  934  may be included on the drive screw head  862 , as shown in  FIGS. 90-91 . In the illustrated embodiment, the drive screw head  862  includes two protuberances  934 . Those skilled in the art will understand that the number of protuberances and recesses can be modified as desired as long as they are able to operatively connect or otherwise engage to allow the drive screw head  862  and screw lock ring  904  to be radially keyed to one another. 
     In the embodiment illustrated in  FIGS. 90-91 , the outer surface  932  of the body  934  includes at least one recess  936  that is configured and dimensioned to matingly engage with the protuberances  934  on the screw head  862 . The recesses  936  may be formed by configuring and dimensioning a protuberance  938  on the outer surface  932  of the screw lock ring  904 . As shown best in  FIG. 90 , the protuberance  938  creates two recesses  936 . The protuberances  934  on the screw head  862  are then able to engage with the recesses  936  to lock the screw head  862  to the screw lock ring  904 . The screw lock ring  904  in the embodiment illustrated in  FIGS. 90-91  also includes a second protuberance  926  that extends from at least a portion of the second side  924 , in a manner similar to that described above with respect to the embodiment illustrated in  FIGS. 87-89 . 
     In any of the embodiments, for example as described with respect to  FIG. 87-91  above, the screw lock ring  904  may be configured and dimensioned to include a body  934  and a spring tail  914 . One advantage of the spring tail  914  is that it allows the screw lock ring  904  to translate, or flex, up and down inside the elongated groove in the housing  902 . The spring tail  914  may be configured and dimensioned to allow the body  934  to flex when an external force is applied. When the screw lock ring  904  flexes, the protuberance  920 ,  944  can move up and down inside the mating recess  922 ,  936  of the screw head  862 , while remaining engaged inside the recesses  922 ,  936 . 
     The body  934  of the screw lock ring  904 , according to one embodiment, includes a center opening  952 , i.e., a center hole. The central opening  952  may be configured and dimensioned so that its center is offset from the center of the opening  970  in the drive screw head  862 . One advantage of offsetting the center of the central opening with respect to the center of the opening  970  is that it allows the screw lock ring  904  to be displaced when a tool, such as a driver, is inserted into the screw lock ring  904  and the opening  970 , as described in more detail below. 
       FIG. 92  illustrates one exemplary embodiment of a housing lock ring  906 . The housing lock ring  906  includes a first side  940  that faces away from the screw head  862 . The second side, opposite the first side  940  of the housing lock ring  906 , comprises a castle feature that faces the second protuberance  926  of the screw lock ring  904 . Those skilled in the art will understand that the castle feature  942  of the housing lock ring includes slots, also known as notches, cut into one end, similar to the castle feature of a castellated or slotted nut. The housing lock ring  906  also includes at least one protuberance  944  that is configured and dimensioned to extend from its first side  940 , as shown in  FIGS. 87 and 92 . It may also be desirable for the protuberance  944  to be configured and dimensioned to extend outwardly from the center of the housing lock ring  906 , as shown in  FIG. 92 . One or more additional protuberances  946  may be included that extend from the outer diameter  948  of the housing lock ring  906 . One side  950  of one or more of the additional protuberances  946  may also sit substantially flush with the first side  940  of the housing lock ring  906  such that the side  950  of the additional protuberance  946  shares a common surface with the first side  940  of the housing lock ring  906 . Each of the protuberances  944 ,  946  can selectively engage with corresponding recesses in the housing  902  in order to prevent rotational movement of the housing lock ring  906 . 
     In the embodiments discussed here, the housing  902  may comprise any device that is operable to receive a drive screw  862 . For example, the housing  902  may comprise at least a portion of the actuator assembly  200  described above, such as first endplate  14  and second endplate  16  described above. The housing  902  may have any shape or dimensions known to those skilled in the art. In certain embodiments described herein, the housing  902  may be configured and dimensioned to include recesses, notches, protuberances, or other features that allow for the elements described herein to engage one of its inner or outer surfaces, as described in more detail below. 
     When configured as discussed above with respect to exemplary embodiments illustrated in  FIGS. 87 and 92 , the housing lock ring  906  sits directly behind the screw lock ring  904 , with the castle feature  942  facing, and operatively connecting to, the second protuberance  926  of the screw lock ring  904 . When the screw lock ring  904  does not have external forces applied to its inner surface  912 , at least the upper portion  928  of the second protuberance  926  (shown with an arrow in  FIG. 92 ) will sit inside the grooves of the castle feature  942 , as illustrated in  FIG. 94  (circled in the figure). This prevents the screw lock ring  904 , and subsequently the drive screw  860 , from rotating because the housing lock ring  906  is keyed to the housing  902 . 
     Referring to  FIG. 93 , an alternative embodiment of the housing lock ring  906  is shown in combination with the embodiment of the screw lock ring  904  described with respect to  FIGS. 90-91 . In the illustrated embodiment, the housing lock ring  906  includes a first side  940 , castle feature  942 , a first protuberance  944 , and additional protuberances  946  along an outer diameter  948  of the housing lock ring  906 . The housing lock ring  906  illustrated in  FIG. 93  and its individual components are similar to the housing lock ring  906  described with respect to  FIG. 92 , with slight modifications. The modifications to the housing lock ring  906  will be described in turn below. 
     With reference to  FIG. 93 , the housing lock ring  906  has been modified to include a first protuberance  944  that extends from the outer diameter  948  of the housing lock ring  906 . However, the first protuberance  944  in this embodiment is substantially flush with the first side  940  of the housing lock ring  906 . In this embodiment, the additional protuberances  946  are selectively positioned between the first side  940  and the castle feature  942 , as shown in  FIG. 93 . In this embodiment, the additional protuberances  946  extend from the outer diameter  948 , and a first side  950  of the additional protuberance  946  does not share a common surface with the first side  940  of the housing lock ring  906 . 
     When configured and dimensioned as discussed above with respect to  FIG. 93 , the housing lock ring  906  sits directly behind the screw lock ring  904 , with the castle feature  942  facing the protuberance  938  of the screw lock ring  904 . When the screw lock ring  904  does not have external forces applied to its inner surface  912 , at least a portion of the protuberance  938  (shown with an arrow in  FIG. 93 ) will sit inside the grooves of the castle feature  942 , as illustrated in  FIG. 94  (circled in the figure). This prevents the screw lock ring  904 , and subsequently the drive screw  860 , from rotating because the housing lock ring  906  is keyed to the housing  902 . 
     According to one embodiment, when the housing  902 , drive screw  860 , retaining ring  890 , screw lock ring  904 , and housing lock ring  906 , the drive screw  860  is prevented from rotating on its own. To rotate the drive screw  860 , the screw lock ring  904  must be flexed such that the protuberance  920 ,  938  disengages from the housing lock ring  906 . In one embodiment, this may be accomplished by inserting one or more tools, such as a driver, into an opening  970  in the drive screw head  862 . The center hole  952  of the screw lock ring  904  is offset such that when a tool, e.g., a driver is inserted into the drive screw head  862 , it displaces that center hole  952  and aligns it with the drive screw  860  and driver. 
     The displacement of the center hole  952  is sufficient to flex the screw lock ring  904  and disengage it from the castle feature  942  of the housing lock ring  906 , as shown in  FIG. 95  (the circled portion). In this manner, the drive screw  860  and the screw lock ring  904  can rotate freely using a driver. In the disengaged state, the screw lock ring  904  and the housing lock ring  906  are oriented as shown in  FIG. 95  and indicated by the arrow. The protuberance  928 ,  938  is no longer engaged with the castle feature  942  and can freely rotate underneath, allowing the drive screw  860  to rotate as well. 
     Once the driver is removed from the center hole  952  of the screw lock ring  904 , the tail spring  914  of the screw lock ring  904  flexes the protuberance  928 ,  938  back into engagement with the castle feature  942  of the housing lock ring  906 , once again preventing the drive screw  860  from rotating. Thus, the screw lock ring  904  is always rotationally engaged with the drive screw  860 , and the housing lock ring  906  is rotationally engaged with the housing  902 . When a driver is not engaged with the drive screw  860 , the screw lock ring  904  engages with the housing lock ring  906  preventing the drive screw  860  from rotating. Conversely, when a driver is engaged with the center hole  952  and the opening  970  in the drive screw head  862 , the screw lock ring  904  translates as the driver pushes it down and disengages from the housing lock ring  906 , allowing the drive screw  860  to be rotated by the driver. Once the driver is removed, the screw lock ring  904  is reengaged inside the castle feature  942  of the housing lock ring  906 , thereby locking the rotation of the drive screw  860  again. 
     Referring now to  FIGS. 96-98 , an alternate embodiment of a locking mechanism is shown. In this embodiment, the locking mechanism comprises a drive screw  860 , a spring component  954 , and a housing  902 . The housing  902  and drive screw  860  of  FIGS. 96-98  and their individual components are similar to the those described with respect to  FIGS. 87-95 , with several modifications. The modifications and components that differ from the locking mechanism illustrated in  FIGS. 87-95  will be described in turn below. 
     As shown in  FIG. 96 , the drive screw head  862  includes at least one recess  956  selectively positioned around the outer diameter of the drive screw head  862 . Each recess  956  may be configured and dimensioned to extend to the front face  958  of the drive screw head  862 . The recesses  956  may comprise any shape or dimensions known to those skilled in the art. 
     In one embodiment, the spring component  954  includes a protuberance  960  that is configured and dimensioned to be operable to engage with at least one recess  956 . The spring component  954  may comprise a flat disc having an extruded geometry that allows a central portion  962  to move, e.g., translate or flex. The protuberance  960  may extend away from a back side (not shown) of the central portion  962  of the spring component  954 , as shown in  FIG. 96 . The central portion  962  may be supported by an arm  964  that maintains a spacing, or gap, between the central portion  962  and the outer frame  966  of the spring component  954 . One advantage of including a spacing between the central portion  962  and the outer frame  966  is that the central portion  962  is operable to translate when a force is applied to its inner surface  968 . 
     One embodiment of the protuberance  960  extends from the back side (not shown) of the central portion  962  and sits flush on the face  958  of the screw, with the protuberance  960  engaged with a recess  956 , as shown in  FIG. 97 . The spring component  954  may then be installed into a housing  902  and fixed thereto to prevent rotational motion of the spring component  954 . Those skilled in the art will understand that the spring component  954  may be fixed to the housing  902  in any manner known to those skilled in the art including, but not limited to, geometry, weld, or a pin. When the spring component  954  is rotationally locked to the housing  902 , and the protuberance  960  is engaged with the recess  956  of the drive screw head  862 , the drive screw  860  is rotationally locked with respect to the housing  902 . 
     An opening in the central portion  962  of the spring component  954  may be offset from the center of the opening  970  in the drive screw head  862 , as described with respect to the screw lock ring  904  described with respect to  FIGS. 87-95 . When a tool, such as driver, is inserted into this opening  970 , the central portion  962  moves, e.g., translates or flexes, to be in line with the driver. When this occurs, the protuberance  960  disengages from the recess  956  on the screw head  862 , as shown in  FIG. 98 . When the protuberance  960  is disengaged, the drive screw  860  can rotate with respect to the housing  902 . By removing the driver, the central portion  962  moves back into position and reengages the protuberance  960  with one of the recesses  956 , once again preventing rotational movement of the drive screw  860 . 
     Referring now to  FIGS. 99-103 , an alternative embodiment of the locking mechanism is shown. In the illustrated embodiment, the locking mechanism comprises a housing  902 , a drive screw  860 , a collar  972 , a spring  974 , and a lock tab  976 . The housing  902  and drive screw  860  of  FIGS. 99-103  and their individual components are similar to the locking mechanism  900  illustrated in  FIGS. 87-95  with several modifications. The modifications and components that differ from the locking mechanism  900  illustrated in  FIGS. 87-95  will be described in turn below. 
     According to one embodiment shown in  FIG. 99 , the drive screw  860  includes a bore  978  configured and dimensioned into the side  980  of the drive screw head  862 . The bore  978  may include any dimensions, such as depth, shape, width, that is desirable according to a particular application. In some applications, the dimensions of the bore  978  may be selected based on the dimensions of at least one of the drive screw  860 , drive screw head  862 , or both. Alternately, the dimensions of the bore  978  may also be determined based on the dimensions of at least one of the spring  974  and/or lock tab  976 . If desired, more than one bore  978  may be selectively positioned along the side  980  of the drive screw head  862 . 
     The lock tab  976  and spring  974  are configured and dimensioned to sit inside the drive screw head  862  through the bore  978 , according to one embodiment of the present invention. Thus, the dimensions of the lock tab  976  and the spring  974  may be selected such that they are operable to fit inside the bore  978 . As shown in  FIGS. 99-100 , one embodiment of the spring  974  may comprise a v-spring. The v-spring may include two longitudinal walls  974   a  and  974   b  provided with an angle therebetween. The angle between the two longitudinal walls  974   a ,  974   b  of the v-spring may vary, and can be selected based on a number of factors including, but not limited to, the dimensions of the bore  978 . 
     Although a v-shaped spring is exemplified in this embodiment, the spring  974  may be formed in any suitable shape or configuration not limited to the v-shape, and may include, for example, U-shape, S-shape, coiled, square, rectangular, sinusoidal, corrugated, and accordion pleated. In addition, the shape of the spring features  974  may be symmetrical or non-symmetrical. For example, the longitudinal walls  974   a ,  974   b  may be symmetrical or non-symmetrical with respect to one another. 
     One embodiment of the collar  972  may include an opening, resulting in a C-shaped ring, as shown in  FIG. 99 , or it may comprise a closed ring (not shown). The collar  972  may be configured and dimensioned such that it can slide axially over the drive screw head  862 . One advantage of using a C-shaped collar  972  is that it includes an opening that can facilitate insertion over the drive screw head  862 . As shown in  FIGS. 99 and 101 , the collar  972  includes at least one opening  982 . The at least one opening  982  is configured and dimensioned to receive a top protrusion  986  of the lock tab  976 , discussed below. As such, it may be desirable for the dimensions of the lock tab to be configured to allow the top protrusion  986  to engage with the opening  982  while preventing the lock tab  976  from exiting through the opening  982 . In embodiments where more than one opening  982  is included, the openings  982  may be selectively positioned to have spaces between the openings  982 , as shown in  FIGS. 99 and 101 . 
     In an exemplary embodiment, the lock tab  976  includes a body  984  and an upper protuberance  986  that extends from the upper surface of the body  984 . The lock tab  976  may also include a lower protuberance  988  that extends from a lower surface of the body  984  that is opposite the upper surface of the body  984  from which the upper protuberance  986  extends. As shown in  FIG. 100 , the lower protuberance  988  may extend away further from the body  984  than the upper protuberance  986 . 
     The protuberances  986 ,  988  may comprise any suitable configuration and dimensions known to those skilled in the art. In embodiments where the protuberances  986 ,  988  comprise a substantially rectangular shape, it may be desirable for the upper protuberance  986  to be formed such that it is substantially perpendicular to the lower protuberance  988 . One advantage of forming the protuberances  986  and  988  at a substantially perpendicular angle is to promote stability of the lock tab  976  when it sits inside the bore  978 . The lower protuberance  988  can also include a stabilizing projection  990 . One advantage of the stabilizing projection  990  is that it minimizes movement of the lower protuberance  988 , and therefore the lock tab  976 , within the bore  978 . 
     In this embodiment, the lock tab  976  and the spring  974  may be installed within the bore  978  in the drive screw head  862 . The spring  974 , e.g., the v-spring, pushes up against a shoulder  992  of the body  984  of the lock tab  976 . At the same time, the spring  974  pushes down against the bottom surface of the bore  978 , which allows the lock tab  976  to flex up and down within the bore  978 . The collar  972  may then be positioned over the side  980  of the drive screw head  862  in any suitable manner known to those skilled in the art. This may include, for example, flexing the collar to expand it such that it can fit over the drive screw head  862 , or by sliding it over the threads of the drive screw  860 . 
     When the lock tab  976  is in its natural position, the upper protuberance  986  sits inside one of the openings  982  of the collar  972 , preventing the drive screw  860  from rotating relative to the collar  972 . The drive screw  860 , collar  972 , spring  974 , and lock tab  976  may then sit inside the housing  902 , according to one embodiment. A retaining ring  890  may also be fit over the drive screw head  862  to retain the drive screw  860  within the housing  902 . The collar  972  may be fastened to the inside of the housing  902  in any manner known to those skilled in the art including, but not limited to, press fit, pin, or welding to prevent the collar  972  from moving, e.g., rotating. 
     When the elements described with respect to  FIGS. 99-101  are assembled and in their natural position, the lock tab  976  is selectively positioned inside the bore  978  and engaged with the collar  972 , as illustrated in  FIG. 102 . Since the collar  972  is rotationally locked to the housing  902 , the lock tab  976  and therefore the drive screw  860  is prevented from rotating within the housing  902  due to the lock tab  976  and collar  972  being engaged. 
     According to one embodiment, an instrument is required to allow rotation of the drive screw  860 . The instrument can be any device known to those skilled in the art, such as a driver or the like. When the driver is engaged with the drive screw  860 , it grabs the lock tab  976  and flexes it downwards into the bore  978 , disengaging it from the collar  972 , as illustrated in  FIG. 103 . When the diver is removed from the drive screw  860 , the spring  974  pushes the lock tab back into its steady state position, illustrated in  FIG. 104 , locking the rotation of the drive screw  860 . 
     With reference to  FIGS. 105-109 , an alternative embodiment of the locking mechanism is described. In the illustrated embodiment, the locking mechanism comprises a housing  902 , a drive screw  860 , a spring  994 , a retaining ring  890 , and a housing lock ring  906 . The housing  902 , drive screw  860 , retaining ring  890 , and drive screw  860  of  FIGS. 105-109  and their individual components are similar to the components described with respect to the embodiments illustrated in  FIGS. 87-95 , with several modifications. The modifications and components that differ from the locking mechanism  900  illustrated in  FIGS. 87-95  will be described in turn below. 
     According to one aspect of this embodiment, the drive screw  860  includes a drive screw head  862  that includes a first, back portion  862   a  and a second, front portion  862   b . The back portion  862   a  comprises an outer diameter that is greater than the outer diameter of the front portion  862   b . In one embodiment, the front portion  862   b  extends beyond the back portion  862   a  in a direction away from the threads of the drive screw  860 , as shown in  FIGS. 105-107 . The front portion  862   b  may include an opening  970  through which an instrument such as a driver may be inserted. As shown in the  FIG. 105 , part of the front portion  862   b  may include a second opening  996  selectively positioned along its outer diameter, such that the circumference of the front portion  862   b  is non-contiguous. In other words, the second opening  996  extends to a side of the front portion  862   b . The second opening  996  may pass completely through to the opening  970 , as shown in  FIG. 105 . In one embodiment, the second opening  996  is positioned at one of the trilobe nodules shown in  FIG. 106 , for example. In other embodiments, however, the second opening may be separate, and non-contiguous with, the first opening  970 . 
     It may be desirable for the outer diameter of the front portion substantially near the face of the front portion  862   b  to include a flanged opening, such as a ledge  998 . One advantage of including a ledge  998  is that it requires the spring  994  to be splayed open to get past it. Once the spring  994  returns to its natural, steady state behind the ledge  998 , it is prevented from disassembling from the front portion  862   b , and thus also the drive screw head  862 . 
     The spring  994 , according to one embodiment, comprises a C-shaped ring that includes a protuberance  1000 . The protuberance  1000  may be selectively positioned substantially opposite the opening in the C-shaped ring, and may comprise a first portion  1000   a  and a second portion  1000   b . The first portion  1000   b  may extend from a surface of the C-shaped ring, and the second portion  1000   b  may extend from a surface of the first portion  1000   b . In the illustrated embodiment, the first portion  1000   a  may be operatively connected to the drive screw head  862  by slidingly engaging with flats on the interior of the second opening  996 . When the spring  994  is splayed open, i.e., expanded, to get past the ledge  998 , the first portion  1000   a  can be aligned with the flats and engaged to fit within the second opening  996 . 
     It is desirable for the first portion  1000   a  to be configured and dimensioned such that it substantially fills the opening  996  without extending substantially beyond the ledge  998  of the front portion  862   b . The second portion  1000   b  may be configured and dimensioned to extend beyond the front portion  862   b , and may have smaller dimensions than the first portion  1000   a . The second portion  1000   b  may comprise at least one tab that may be straight sided, as shown in  FIGS. 105-107 , or angled, as shown in  FIG. 108 . Alternately, the second portion  1000   b  may comprise at least one straight tab and one angled tab. One advantage of including an angled tab is that it prevents the tab from springing into an unlocked position when rotational forces are present, as discussed in more detail below. 
     One embodiment of the housing lock ring  906  is substantially similar to the housing lock ring  906  described with reference to  FIGS. 92-95  above. For instance, the housing lock ring  906  of this embodiment includes a castle feature  942  that includes slots into which the second portion  1000   b , e.g., the tab, can align. This embodiment of the housing lock ring  906  also includes at least one protuberance  1002  that is configured and dimensioned to align with slots included in the housing  902  to prevent rotation. 
     As described above, the protuberance  1000  on the spring  994  may be aligned with the second opening  996  of the drive screw head  862 . When the spring  994  is splayed open and positioned over the front portion  862   b , according to this embodiment, the C-shaped ring may sit substantially flush with the back portion  862   a , as shown in  FIG. 107 . The ledge  998  prevents the spring  994  from disengaging with the drive screw head  862 . When configured and installed in the manner shown in  FIG. 107 , the spring  994  is rotationally locked to the drive screw  860 . The drive screw  860  and spring  994  may then be inserted into the housing  902  and are free to rotate. 
     The retaining ring  890  may be placed on the housing lock ring  906 , which can then be inserted into the housing  902 . The at least one protuberance  1002  of the housing lock ring  906  may be aligned with the slots on the housing  902  to prevent rotation. The slots in the castle feature  942  of the housing lock ring  906  may be aligned with the second portion  1000   b  of the protuberance  1000  on the spring  994 . 
     In the steady state position, the second portion  1000   b  of the protuberance  1000  aligns with a slot in the castle feature  942  on the housing lock ring  906 . Since the housing lock ring  906  is rotationally aligned with the housing  902 , and the second portion  1000   b  of the protuberance  1000  is rotationally aligned with the drive screw  860 , the drive screw  860  is now rotationally locked relative to the housing  902 , as shown in  FIG. 108 . 
     To unlock the second portion  1000   b  of the protuberance  1000  from the housing lock ring  906  to allow the drive screw  860  to rotate, a tool, e.g. a driver may be inserted into the opening  970  of the drive screw head  862 . An interference between the second portion  1000   b  of the protuberance  1000  and the driver will translate the second portion  1000   b  down when the driver is present, disengaging the second portion  1000   b  from the slots in the castle feature  942  on the housing lock ring  906 , as shown  FIG. 109 , and allowing the drive screw  860  to rotate. Once the driver is removed, the two arms of the C-shaped spring  994  act as springs, pulling the tab back into its steady state position and reengaging the second portion  1000   b  with the slots on the castle feature  942  of the housing lock ring  906 . This, once again, prevents rotation of the drive screw  860 . 
     With reference to  FIGS. 110-116 , an alternative embodiment of the locking mechanism is described. In the illustrated embodiment, the locking mechanism comprises a housing  902 , a drive screw  860 , an actuating tab  1004 , and a retaining ring  890 . The housing  902 , drive screw  860 , and retaining ring  890  of  FIGS. 110-116  and their individual components are similar to the elements described with respect to the locking mechanism  900  illustrated in  FIGS. 87-95 , with several modifications. The modifications and components that differ from the locking mechanism  900  illustrated in  FIGS. 87-95  will be described in turn below. 
     Similar to the embodiment described with respect to  FIGS. 105-109 , the drive screw  860  includes a drive screw head  862  that includes a first, back portion  862   a , and a second, front portion  862   b . In the embodiment illustrated in  FIG. 110 , the front portion  862   b  includes notches  1006  that are selectively positioned around its outer diameter. The notches  1006  are configured and dimensioned to extend to the back portion  862   a  as well as the front face  1008  of the front portion  862   b , as shown in  FIG. 111 . The notches  1006  may have any shape and dimensions known to those skilled in the art. In some embodiments, the shape and dimensions of the notches  1006  may be selected such that they are operable to receive and engage with correspondingly shaped protrusions included on the actuating tab  1004 , as described in more detail below. 
     The actuating tab  1004 , according to one embodiment, comprises a ring that includes at least one protuberance. In the embodiment illustrated in  FIG. 110 , for example, two protuberances  1010  may be included. The protuberances  1010  may be located on a first side  1014  of the ring and can operatively connect with the housing  902  to prevent rotational movement. In order to maximize stability and prevent rotation, the protuberances may be selectively positioned opposite one another, at about a 180° angle, as shown in  FIG. 110 . In other embodiments, four protuberances  1010  may be included that are selectively spaced apart from one another by about 90°. The number of protuberances  1010  may be selected, for example, depending on the structural integrity required to prevent rotation of the actuating tab  1004 . In applications where the rotational forces are greater, a larger number of protuberances  1010  may be desirable. Conversely, when the rotational forces are smaller, fewer protuberances  1010  may be used. 
     The shape and dimensions of the protuberances  1010  may also be varied as desired. In one embodiment, the shape and dimensions of the protuberances  1010  may be selected so that they are operable to engage with recesses  1012  in the housing  902 . The shape and dimensions of the protuberances  1010  may also be selected based on the rotational forces that are present in a particular application. For instance, when the rotational forces are greater, the protuberances  1010  may be configured and dimensioned to be larger to maintain their structural integrity. If, however, the rotational forces are not as large, the protuberances  1010  may be configured and dimensioned to minimize dimensions in order to reduce the size and shape of the overall locking mechanism. 
     The back side (not shown) of the actuating tab  1004  opposite the first side  1014 , includes at least one protuberance  1016 , e.g., a tab, that faces towards and is operatively connectable to the front portion  862   b  of the drive screw head  862 , as shown in  FIG. 110 . The protuberance  1016  may be configured and dimensioned to comprise any desirable shape and dimensions, as described with respect to the embodiments shown in  FIGS. 87-95, 96-98, and 105-109 , for example. The actuating tab  1004  may be positioned opposite at least one of the protuberances  1010   
     In one embodiment, the actuating tab  1004  is positioned between about 175° and about 185° from at least one of the protuberances  1010 . In another embodiment, the actuating tab is positioned about 180° degrees from at least one of the protuberances  1010 . One advantage of positioning the actuating tab  1004  in this manner is that it allows the actuating tab  1004  to be forced into one of the notches  1006  when pressure is applied to a recess formed by the protuberance  1010 . The pressure may be applied to the recess using a spring component, such as the retaining ring  890 , as described in more detail below. 
     The protuberance  1016 , e.g., the tab, is configured and dimensioned to engage with the notches  1006  in the front portion  862   b , shown in  FIGS. 110-112 , so that the drive screw  860  and the actuating tab  1004  are rotationally constrained to one another. The retaining ring  890  may comprise any ring shaped component that substantially prevents the actuating tab  1004  and the drive screw  860  from moving axially out of the housing  902 , as described with respect to the embodiments illustrated in  FIGS. 87-95, 96-96, and 105-109 , for instance. The retaining ring  890  may also act as a spring to translate the actuating tab  1004  from the unlocked to the locked position, as described in more detail below. In some embodiments, a secondary retaining ring (not shown) may also be used directly between the drive screw head  862  and the housing  902  for increased drive screw retention. 
     With respect to  FIGS. 111-116 , the exemplary operation of the locking mechanism illustrated in  FIG. 110  is described. In one embodiment, the actuating tab  1004  is operatively connected, i.e., engaged, with one of the notches  1006 , as indicated by the arrow in  FIG. 111 . The drive screw  860  and the actuating tab  1004  may then be positioned inside the housing  902  so that the protuberances  1010  engage with the recesses  1012 . In this manner, the actuating tab  1004  may be secured to the housing  902  to substantially prevent rotation of the actuating tab  1004 , as illustrated in  FIG. 112 . 
     The retaining ring  890  may then be collapsed and inserted into the housing  902 , as shown in  FIG. 113 . The housing  902  includes a groove to capture the retaining ring  890  once it springs back out to its natural shape. In one embodiment, the retaining ring  890  is selectively positioned so that its opening is in the same location as the protuberance  1016 , e.g., tab on the actuating tab  1004 . The actuating ring  1004  may be configured and dimensioned so that its inner diameter contacts the outer diameter of the retaining ring  890  on the side of the retaining ring  890  that is opposite its opening. When the retaining ring  890  is in its natural state, the actuating tab  1004  may be forced into a locked position because the protuberance  1016 , e.g., tab, engages with one of the notches  1006 . In this manner, the actuating tab  1004  may be rotationally locked to the housing  902  based on the two protuberances  1010 , and the drive screw  860  is rotationally locked to the actuating tab  1004  with the engaged protuberance  1016 . In this exemplary configuration, therefore, the drive screw  860  is also rotationally locked with respect to the housing  902 . 
     According to one embodiment, a central opening in the actuating tab  1004  is configured and dimensioned such that it is offset from the opening  970  in the drive screw head  862 , as described with respect to the screw lock ring  904  illustrated in  FIGS. 87-95 . When a tool, e.g., a driver is introduced and engaged with the opening  970 , the actuating tab  1004  is pulled into alignment with the opening  970 . The retaining ring  890  may act as a spring, so that when the driver displaces the actuating tab  1004 , its contact with the retaining ring  890  pushes it against the outer wall of the housing  902 , collapsing the retaining ring  890 . 
     As shown in  FIG. 114 , with the translation of the actuating tab  1004 , the protuberance  1016  moves as well and disengages from the notches  1006 . The drive screw  860  is then free to rotationally move independent of the actuating tab  1004  and housing  902 . As shown in  FIG. 115 , when the tool, e.g., the driver is removed from the opening  970 , the retaining ring  890  pushes back to its natural, open position, contacting the actuating tab  1004  and returning the protuberance  1016  to one of the notches  1006  in the drive screw head  862 . The drive screw  860  is then rotationally locked with respect to the actuating tab  1004  and the housing  902 . 
     As described above, the at least one protuberance  1016  may be configured and dimensioned as desired. For instance, in some embodiments the at least one protuberance  1016  may be tapered to provide a ratcheting design that substantially resists fracturing under excessive loads. One advantage of this design is that the at least one protuberance  1016  may ratchet into the next notch  1006  under excessive loads, as shown in  FIG. 116 . 
     With reference to  FIGS. 117-121 , an alternative embodiment of the locking mechanism is described. In the illustrated embodiment, the locking mechanism comprises a housing  902 , a drive screw  860 , an actuating tab  1004 , retaining ring  890 , and spring bar  1018 . The housing  902 , drive screw  860 , actuating tab  1004 , and retaining ring  890  of  FIGS. 117-121  and their individual components are similar to the locking mechanism described with respect to  FIGS. 110-116 , with several modifications. The modifications and components that differ from the locking mechanism illustrated in  FIGS. 110-116  will be described in turn below. 
     The embodiment illustrated in  FIGS. 117-121  is substantially similar to the embodiment described with respect to  FIGS. 110-116 , except that the spring mechanism comprises a separate element. In this embodiment, the spring mechanism comprises a spring bar  1018 . The spring bar  1018  may be preassembled to the actuating tab  1004  prior to installation into the housing  902  using any means known to those skilled in the art. The spring bar  1018  may be configured and dimensioned so that it may be keyed to the actuating tab  1004  to maintain rotational position, and then operatively connected together. 
     In this embodiment, the retaining ring  890  may configured and dimensioned such that it is spaced from the spring bar  1018  when positioned within the housing, as described in more detail below. The retaining ring  890 , for example, may comprise a C-shaped ring that extends around about 180° or less of the outer diameter of the drive screw head  862 . Alternately, the retaining ring  890  may comprise a C-shaped ring that extends around about 200° or less of the outer diameter of the drive screw head  862 . In still another embodiment, the retaining ring  890  may comprise a C-shaped ring that extends around about 250° or less of the outer diameter of the drive screw head  862 . 
     The spring bar  1018 , according to one embodiment, may be configured and dimensioned according to any method known to those skilled in the art. In one embodiment illustrated in  FIG. 117 , the spring bar  1018  comprises two arms that flex around a central point. One advantage of configuring the spring bar  1018  in this manner is that it allows a load to be created when the arms of the spring bar  1018  push against the inner diameter of the housing  902  and the actuating tab  1004 . The two arms may be configured and dimensioned to engage with at least one of the actuating tab  1004 , the housing  902 , and/or the drive screw  860 . In the  FIG. 117  embodiment, the spring bar  1018  may also include a groove, notch, recess, depression, or the like that may be selectively positioned, for example, substantially near a central point between the two arms of the spring bar  1018 . One advantage of including a groove, notch, recess, or depression is that it allows the spring bar  1018  to operatively connect with the protuberance  1010  on the front face  1008  of the actuating tab  1004 , as shown in  FIG. 118 . 
     After the drive screw  860  is inserted into the housing  902 , the spring bar  1018  is deflected inward and it, along with the actuating tab  1004  is selectively positioned within the housing  902 , as shown in  FIG. 118 . Once inserted into the housing  902 , the spring bar  1018  may return to its natural, expanded position and engage with a groove of the housing  902  that is configured and dimensioned to receive the spring bar  1018 , as shown in  FIG. 119 . The spring bar  1018  operatively connects or otherwise engages with a portion of the actuating tab  1004  that is substantially near the protuberance  1016 , along with the inner diameter of the housing  902 , creating a load that pushes the actuating tab  1004  into a locked position with the notches  1006 . In an exemplary embodiment, the spring bar  1018  acts as a partial retaining ring to hold its side of the actuating tab  1004  and drive screw  860  in the housing  902 . The retaining ring  890  may be inserted to provide additional retaining strength on the substantially opposite side of the spring bar  1018 . 
     As shown in  FIG. 120 , when a tool, e.g., a driver or the like, is inserted into the opening  970  in the drive screw head  860 , the actuating tab  1004  translates against the spring bar  1018 , disengaging the protuberance  1016  from the notches  1006  and allowing it to rotate independently. When the driver is removed from the opening  970 , as illustrated in  FIG. 121 , the spring bar  1018  pushes against the actuating tab  1004 , reengaging the protuberance  1016  with one of the notches  1006  of the drive screw head  862 , substantially preventing rotation of the drive screw  860 . 
     As described above, the notches  1006  in the front portion  862   b  of the drive screw head  862  described with respect to  FIGS. 110-121  may comprise varied dimensions. For instance, in an embodiment shown in  FIG. 122 , the notches  1006  may be configured and dimensioned to comprise tapered sides in the vertical direction. Although the tapered notches  1006  are illustrated as external protuberances on the drive screw head  862  (indicated by the arrow in the diagram), they may also be removed material around the outer diameter of the front portion  862   b  of the drive screw head  862 . In this embodiment, the actuating tab  1004  similarly includes at least one tapered protuberance  1016 . The tapered protuberance  1016  includes two edges that are configured and dimensioned to operatively connect to, or engage with, the tapered edges of the tapered notches  1006 . 
     In this embodiment, a central opening in the actuating tab  1004  is configured and dimensioned such that its center is offset from the center of the opening  970  in the drive screw head  862 , as described with respect to the screw lock ring  904  illustrated in  FIGS. 87-95 . When a tool, such a driver or the like, is introduced and engaged with the opening  970 , the actuating tab  1004  is pulled into alignment with the opening  970 . The retaining ring  890 , or spring bar  1018 , may act as a spring so that when the driver displaces the actuating tab  1004 , its contact with the retaining ring  890  or spring bar  1018  pushes it against the far wall of the housing  902 , collapsing the retaining ring  890  or spring bar  1018 . With the translation of the actuating tab  1004 , the protuberance  1016  moves as well and disengages from the tapered notches  1006 . The drive screw  860  is then free to rotationally move independently of the actuating tab  1004  and the housing  902 . 
     When the tool, such as a driver or the like, is removed from the opening  970 , the retaining ring  890  or spring bar  1018  pushes back to its natural, open, position, contacting the actuating tab  1004  and returning the protuberance  1016  to one of the tapered notches  1006  in the drive screw head  862 . In this position, the drive screw  860  is rotationally locked with respect to the actuating tab  1004  and the housing  902 . When an external force attempts to rotate the drive screw  860 , the tapers on the notches  1006  will ramp the drive screw  860  and actuator tab  1004  apart from one another. Since the drive screw  860  is axially contained within the housing  902 , and the actuating tab  1004  is axially retained by the retaining ring  890  and/or the spring bar  1018 , the tapered notches  1006  and tapered protuberance  1016  will engage and prevent rotation of the drive screw  860 . 
     With reference to  FIGS. 123-125 , an alternative embodiment of the locking mechanism is described. In the illustrated embodiment, the locking mechanism comprises a housing  902 , a drive screw  860 , a screw lock ring  904 , a pivot pin  1020 , and a snap ring  1022 . The housing  902 , drive screw  860 , and screw lock ring  904  of  FIGS. 123-125  and their individual components are similar to the elements described with respect to locking mechanism illustrated in  FIGS. 87-95 , with several modifications. The modifications and components that differ from the locking mechanism illustrated in  FIGS. 87-95  will be described in turn below. 
     In the embodiment illustrated in  FIGS. 123-125 , the screw lock ring  904  comprises an e-shape including a spring tail  914 . The screw lock ring  904  of this embodiment includes at least one locking tooth  1024  (or teeth), as shown in  FIG. 123 . The locking tooth  1024  may be selectively positioned around the central hole  952 . The positioning of the locking tooth  1024  may be selected such that it can be disengaged from corresponding mating teeth on the snap ring  1022 , discussed in more detail below, when the spring tail  914  is compressed. 
     In one embodiment, the screw lock ring  904  also includes a pivot pin  1020 . The pivot pin  1020  may be machined as part of the screw lock ring  904 , may be a separate element, or it may be part of the screw head  862 . In embodiments where the pivot pin  1020  is a separate element from the screw lock ring  904 , the screw lock ring  904  may include an opening that is configured and dimensioned to receive the pivot pin  1020 . The pivot pin  1020  (or opening to receive the pivot pin  1020 ) may be selectively positioned around the center hole  952  as part of the “e” piece of the screw lock ring  904 , away from the spring tail  914 . 
     The screw lock ring  904  may be rotationally restricted using a snap ring  1022 , according to one embodiment illustrated in  FIG. 124 . The snap ring  1022  may comprise a variety of shapes including, but not limited to, a circular ring shape or a C-shaped ring. The snap ring  1022  may include mating teeth  1028  machined on one side with which the at least one tooth  1024  of the screw lock ring  904  is operable to engage. It may desirable for the mating teeth  1028  teeth on the snap ring  1022  to be manufactured along its inner diameter, for instance, as shown in  FIG. 124 . The snap ring  1022  may also include a wavy outer diameter. The advantage of including a wavy outer diameter is that relief can be provided for larger interference fits. 
     The snap ring  1022  may be prevented from rotating within the housing  902  by including a pin  1026  that occupies the space between its opening, in embodiments where it comprises a C-shaped ring. The pin  1026  and the pivot pin  1020  may comprise a single element in some embodiments or, alternately, they may comprise separate elements that are operatively connected to one another. In other embodiments, however, the pin  1026  and the pivot pin  1020  may comprise separate elements in substantially different locations within the housing  902 . In other embodiments, the snap ring  1022  may be welded into place within the housing  902 . 
     Similar to the embodiments described with respect to  FIGS. 87-95 , the center of center hole  952  of the screw lock ring  904  is offset from the center of the opening  970  in the drive screw head  862 . When a tool, such a driver or the like, is engaged with the opening  970 , the center hole  952  (and thus the e-shaped piece) will be pushed into alignment with the driver and will pivot on the pivot pin  1020 , compressing the spring tail  914 . When the spring tail  914  is compressed, the lock tooth  1024  (or teeth) disengage from the mating teeth  1028  on the snap ring  1022 . One advantage of the pivot pin  1020  is that the relative motion unlocking the lock tooth  1024  can be controlled to a greater degree when the driver causes the spring tail  914  to compress. 
     Those skilled in the art will understand that the magnitude of the unlocking motion or translation of the screw lock ring  904  may be obtained with different geometries separating the pivot pin  1020  and the lock tooth  1024 . Additional manipulation of the translation of the screw lock ring  904  may also be generated based on geometries separating the pivot pin  1020 , locking tooth  1024 , and spring tail  914 . Another advantage of the screw lock ring  904  of this embodiment is that the geometry between the pivot pin  1020 , lock tooth  1024 , and spring tail  914  enables a strong locking motion to be created when the drive screw  860  is rotated in the unlocking, or collapsing, direction. The tendency of the spring tail  914  to compress when a counter-clockwise motion is applied may be reduced by the positioning of the pivot pin  1020 , lock tooth  1024 , and spring tail  914  when compared to a conventional up/down spring action of a screw lock ring  904  without a pivot pin  1020   
     As discussed above, the screw lock ring  904  is rotationally restricted based on the snap ring  1022 . The locking tooth  1024  of the screw lock ring  904  operatively connects, or otherwise engages with the mating teeth  1028  of the snap ring  1022  to prevent rotation of the drive screw  860 . Because the screw lock ring  904  is rotationally locked to the drive screw  860 , and the snap ring  1022  is rotationally locked to the housing  902 , the drive screw  860  is also rotationally locked until a driver is inserted into the opening  970 . 
     With respect to  FIG. 126 , another embodiment of the present invention is described. In this embodiment, the locking mechanism comprises at least two locking elements  1030 , at least two pins  1032 , at least one spring  1034 , and a drive screw  860 . The drive screw  860  is similar to the drive screw  860  described with respect to  FIGS. 87-125  with slight modifications, which will be described below. 
     In this embodiment, the drive screw  860  includes at least one tooth  1036  cut around the drive screw head  862 . It certain embodiments, however, it may be desirable for the drive screw head  862  to include a plurality of teeth  1036 . According to one embodiment, the drive screw head  862  may be rotationally restricted based on at least two locking elements  1030 . Each of the locking elements  1030  includes a recess  1038 , facing the drive screw head  862 , that is configured and dimensioned to engage with the shape of the drive screw head  862 . In the embodiment shown in  FIG. 126 , for example, each recess  1038  is semi-circular so that locking elements  1030  together form a recess that is operable to engage with the circular drive screw head  862 . Those skilled in the art will understand that the shape of the recesses  1038  can be varied as desired based on the shape of the drive screw head  862 . 
     At least one opening may be included to allow a pin  1032  to pass through the body of the locking elements  1030 . In the embodiment illustrated in  FIG. 126 , each of the locking elements  1030  include at least two openings. When the recesses  1038  of each of the locking elements  1030  are facing one another, the openings on each side can overlap one another, allowing a pin  1032  to pass through each set of openings. In this embodiment, the pins  1032  placed between pairs of locking elements  1030  prevents them from coming too close together and disassembling within the housing  902 . The openings may be configured and dimensioned such that they are larger than the pins  1032 . One advantage of including openings that are larger than the pins  1032  is that it allows for motion of the locking elements  1030  while also preventing disassembly. The recesses  1038  may also include teeth  1040  that are operable to engage with the one or more teeth  1036  included in the drive screw head  862 . The teeth  1036  and teeth  1040  may be configured and dimensioned such that they are operable to matingly engage with one another. 
     Each of the locking elements  1030 , according to one embodiment, may be further held in place by at least one spring element  1034 . One advantage of including at least one spring element  1034  is that it forces the locking elements  1030  to be held close to the center of the drive screw head  862 . In combination with the pins  1032 , which prevents the locking elements  1030  from getting too close to one another and disassembling, the spring element  1034  maintains the engagement of the teeth  1040  of the locking element  1030  and the teeth  1036  of the drive screw head  862 . 
     In some embodiments, one of the locking elements  1030  may be locked in place within the housing  902 , while the other may be operable to move when impacted by an outside force, such as driver the like. In other embodiments where the locking elements  1030  are both configured and dimensioned to be selectively movable, a spring element  1034  may be used with each of the locking elements  1030 . In the  FIG. 126  embodiment, for instance, a spring element  1034  may be included on the side of the locking element  1030  opposite the recesses  1038  to provide a load that forces each of the locking elements  1030  towards the drive screw head  862 . Those skilled in the art will understand that, although a curved leaf spring is shown in the  FIG. 126  embodiment, any spring known to those skilled in the art (including those discussed above with respect to  FIGS. 87-125 ) may be used as desired for a particular application. In addition, skilled artisans will understand that more than two locking elements  1030  may be used to engage the drive screw head  862  as long as the locking elements  1030  are operable to engage the drive screw head  862  with their teeth  1040 . 
     In this embodiment, the locking elements  1030  are fastened to the housing  902  (not shown) such that they are rotationally restricted with respect to the housing  902 . The locking elements  1030  may be fasted to the housing using, for example, pins  1032 . Each of the locking elements  1030  are then are pushed towards the drive screw head  862  by the at least one spring  1034 . The teeth from the locking elements  1030  engage the teeth  1036  on the drive screw head to prevent motion. Because the locking elements  1030  are rotationally locked with respect to the housing, and the teeth  1040  and teeth  1036  on the drive screw head  862  are engaged, rotation of the drive screw  860  is prevented. In one embodiment, a tool, such as a driver or the like, is inserted and overlaps the drive screw head  862 , disengaging the teeth  1036  from teeth  1040  and allowing the drive screw  860  to rotate. 
     In an alternate embodiment, a single locking element  1030  may be used instead of the two or more locking elements  1030  described with respect to  FIG. 126 . As shown in  FIG. 127 , another embodiment comprises a drive screw  860  that includes one or more teeth, as described with respect to  FIG. 126 . However, in this embodiment, the locking element  1030  comprises a single locking element  1030  that comprises a ring shape with an opening, e.g., a c-shaped ring. The locking element  1030  may comprise a snap ring that is spring loaded to force its recess  1038  into engagement with the drive screw head  862 . Alternately, a spring  1034  may be included to force the locking element  1030  into engagement with the drive screw head  862 , as described with respect to  FIG. 126 . In other embodiments, a retaining ring  890  that is spring loaded may be selectively positioned around the c-shaped locking element  1030  to force the locking element  1030  into contact with the drive screw  860 . 
     Similar to the embodiment of  FIG. 126 , the locking element  1030  includes a plurality of teeth  1040  along a surface of the recess  1038 . The teeth  1040  engage the teeth  1036  around the drive screw head  862  to prevent rotational motion. In this embodiment, a pin  1032  may be selectively positioned within the opening of the c-shaped locking element  1030  to prevent it from rotating when installed in the housing  902 . When a driver is inserted into the opening  970  of the drive screw head  862 , it displaces the locking element  1030  which disengages the teeth  1040  and  1036 , allowing the drive screw  860  to rotate. 
     With reference to  FIGS. 128-131 , an alternative embodiment of the locking mechanism is described. In the illustrated embodiment, the locking mechanism comprises a housing  902 , a drive screw  860 , and a screw lock ring  904 . The housing  902 , drive screw  860 , and screw lock ring  904  of  FIGS. 128-131  and their individual components are similar to the elements described with respect to the locking mechanism illustrated in  FIGS. 87-95 , with several modifications. The modifications and components that differ from the locking mechanism illustrated in  FIGS. 87-95  will be described in turn below. 
     In this embodiment, the housing  902  includes a groove with lateral access openings  1042 . The groove may be configured and dimensioned to receive at least a portion of the screw lock ring  904 , a portion of which is similarly configured and dimensioned to fit within the groove. In this embodiment, the groove houses the screw lock ring  904  which includes an interference fit with a corresponding groove in the drive screw  860 . The interference fit or engagement may be via fit or via discrete features, as will be appreciate by those skilled in the art. One advantage of the interference is that it serves to prevent free rotation of the drive screw  860 . 
     In this embodiment, the screw lock ring  904  may comprise one or more elements. For example, the screw lock ring  904  may comprise either a solid or split configuration, as illustrated in  FIGS. 129-131 . The embodiment shown in  FIGS. 129 a - b   , for instance, illustrates a screw lock ring  904  that comprises a single solid element with an opening at one end. The portion of the screw lock ring  904  near the opening may include protuberances  1046  along its inner diameter. In the closed configuration shown in  FIG. 129 a   , the two protuberances  1046  join together to create an interference that operatively connects with a groove in the drive screw  860  to prevents rotation of the drive screw  860 . 
     Other aspects of the screw lock ring  904  may also be varied. The thickness of the screw lock ring  904  may be configured and dimensioned as desired. Varying the thickness of the screw lock ring  904  and/or and the shape of the opening in the screw lock ring  904 , as shown in  FIG. 130 , allows the amount of friction exerted on the drive screw  860  to be selected as desired. For instance, the screw lock ring  904  may be configured and dimensioned to comprise an oval opening. If the protuberances  1046  are selectively positioned along a portion of the oval opening that is in closer contact with the drive screw  860 , the amount of friction exerted on the drive screw  860  may be increased, which also increases the ability of the locking mechanism to resist rotational movement of the drive screw  860 . In some embodiments, the screw lock ring  904  may also comprise a uniform thickness. Alternately, other embodiments may include a tapered thickness to coerce deflection only in certain regions. 
     In embodiments where the screw lock ring  904  comprises a split configuration, as shown in  FIG. 131 , the thicker portions may be positioned towards the protuberances  1044  that engage with the lateral access openings  1042  and the thinner portions may be positioned towards the interference area that engages with the groove in the drive screw  860 . For instance, each part of the split configuration may have one end that is thick and one that is thinner, as shown in  FIG. 131 . The thicker portion may be configured substantially near the protuberances  1046 , for example, while the thinner portions may be configured and dimensioned to join together, or overlap, to form one or more protuberances  1046 , as shown in  FIG. 131 . Configuring each of part of the split configuration of the screw lock ring  904  in this manner creates at least one ratcheted protuberance  1046  that can then engage with a groove in the drive screw  860 . Of course, the thickness may be varied in different configurations in other embodiments. 
     To configure the screw lock ring  904  to prevent rotation of the drive screw  860 , it can be compressed from its open form, e.g. as shown in  FIG. 129 b   , to its compressed form, e.g., as shown in  FIG. 129 a   , and inserted into the housing  902 . In this embodiment, the protuberances  1044  on the outer diameter can be operatively connected with the lateral access openings  1042 . The protuberances  1046  on the inner diameter may selectively engage with the one or more grooves on the drive screw head  862  to prevent rotational movement. The protuberances  1046  can be contacted and deflected by using a tool that attaches to the housing  902 . This action causes elastic deformation of the screw lock ring  904  such that the interference is removed, i.e., the protuberances  1046  are disengaged from the one or more grooves in the drive screw head  862 , permitting the drive screw  860  to rotate. Removing the tool, e.g., driver or lateral jaw style holder, permits the screw lock ring  904  to relax back to its original shape, reengaging the interference and preventing free rotation of the drive screw  860 . 
     With reference to  FIGS. 132-134 , an alternative embodiment of the locking mechanism is described. In the illustrated embodiment, the locking mechanism comprises a housing  902 , a drive screw  860 , and a screw lock ring  904 . The housing  902 , drive screw  860 , and screw lock ring  904  of  FIGS. 132-134  and their individual components are similar to the elements described with respect to the locking mechanism illustrated in  FIGS. 105-109 , with several modifications. The modifications and components that differ from the locking mechanism illustrated in  FIGS. 105-109  will be described in turn below. 
     The screw lock ring  904  according to this embodiment includes at least one protuberance  1048 , or tab, on the inner diameter of the ring  904 , as shown in  FIGS. 132-133 . The protuberance  1048  may be configured and dimensioned such that it can be inserted into the side of the opening  970  in the drive screw head  862  to prevent it from rotating. In this embodiment, the opening  970  is configured and dimensioned to extend to the outer diameter of at least one portion of the drive screw head  862 . The screw lock ring  904  may also include vertical protrusions  1052  that are configured and dimensioned to operatively connect with at least a portion of the housing  902 . 
     The housing  902 , according to one embodiment, includes openings  1050 , e.g., blind pockets, that are configured and dimensioned to receive the vertical protrusions  1052  from the screw lock ring  904 , as shown in  FIG. 134 . Additionally, the housing  902  includes an expansion space  1054  that allows the screw lock ring  904  to expand when impacted by a tool, such as a driver. The openings  1050  in the housing  902  operatively connect with the vertical protrusions  1052  to rotationally constrain the screw lock ring  904  within the housing  902 . In this embodiment, the insertion of a driver into the opening  970  translates the protuberance  1048  outside of the outer diameter of the drive screw  860 , allowing rotation within the housing  902 . 
     An alternative embodiment of the locking mechanism is described with reference to  FIGS. 135-136 . In the illustrated embodiment, the locking mechanism comprises a housing  902  and a drive screw  860 . The illustrated embodiment may optionally include one or more washers  1056 - 1060 . The housing  902  and the drive screw  860  of  FIGS. 135-136  and their individual components are similar to the elements described with respect to the locking mechanism illustrated in  FIGS. 87-95 , with several modifications. The modifications and components that differ from the locking mechanism illustrated in  FIGS. 87-95  will be described in turn below. 
     According to this embodiment, the drive screw head  862  includes one or more protuberances, such as teeth or the like, on either the top face  1062  or bottom face  1064  of the drive screw head  862 , or both. In the embodiment shown in  FIG. 135 , for example, teeth are included on the bottom face  1064  of the drive screw head  862 . Alternately, the protuberances, e.g., teeth, can be included on the top face  1062  of the drive screw head  862 . 
     The housing  902  in this embodiment also includes one or more recesses that are operable to receive the protuberances on the top face  1062  or bottom face  1064  of the drive screw head  862 . In the embodiment shown in  FIG. 135 , a locking washer  1056  may be included in combination with the housing  902  and drive screw head  862 . The locking washer  1056  also includes protuberances, such as teeth or the like, that are configured and dimensioned to engage with the protuberances on the bottom face  1064  of the drive screw head  862 . The locking washer  1056  may comprise any washer known to those skilled in the art, such as a Belleville washer, and may optionally be spring loaded such that it is operable to operatively connect, or otherwise engage with, the bottom face  1064  of the drive screw head  862 . 
     The teeth on the bottom face  1064  of the drive screw head  862 , according to the embodiment shown in  FIG. 135 , engage with teeth that are configured and dimensioned to be included on a locking washer  1056  that sits between the housing  902  and the drive screw head  862 . When a tool, such as a driver or the like, applies a force on the locking washer  1056  axially away from the protuberances on the bottom face  1064 , the drive screw head  862  is free to rotate. When the driver is removed, the spring load of the locking washer  1056  pushes it against the bottom face  1064 , reengaging the protrusions and rotationally locking the drive screw head  862 . 
     In an alternate embodiment, drive screw head  862  may include protuberances on both the top face  1062  and the bottom face  1064 . In this embodiment, the housing  902  also includes protuberances, such as teeth or the like, that are operable to engage with the protuberances on the top face  1062  of the drive screw head  862 . The combination of the locking washer  1056 , teeth on the bottom face  1064  and top face  1062 , and teeth included in the housing  902  may prevent undesirable rotational movement of the drive screw head  862 . 
     In other embodiments, other washers may be included to further prevent rotation of the drive screw  860  when it is engaged within the housing  902 . For instance, as shown in  FIG. 136 , at least one of a thrust washer  1058  and bent washer  1060  may be included either alone or in combination with the locking washer  1056  described above. In an exemplary embodiment, the thrust washer  1058  is positioned between the drive screw  860  and the bent washer  1060 . The bent washer  1060 , in turn, is positioned between the housing  902  and the thrust washer  1058 , as shown in  FIG. 136 . One advantage of the bent washer  1060  is that it provides a force on the thrust washer  1058 , and thereby the bottom face  1064  of the drive screw head  862 , when the drive screw  860  is not being driven into the housing  902 . In such an embodiment, the drive screw head  862  may include protuberances on the top face  1062  that are then forced into engagement with protuberances, e.g., teeth included in the housing  902 , as described above. 
     Those skilled in the art will appreciate that the thrust washer  1058  may include protuberances in some embodiments that are operable to engage with corresponding protuberances on the bottom face  1064  of the drive screw head  862 . Alternately, the bent washer  1060  may be used in combination with the locking washer  1056  to provide additional force that drives the protuberances on the locking washer  1056  into engagement with protuberances on the bottom face  1064 . The locking washer  1056 , thrust washer  1058 , and bent washer  1060  may optionally be rotationally locked with respect to the housing  902  using pins, welding, or any other method known to those skilled in the art. Rotationally locking these elements may assist with preventing the rotation of the drive screw  860  when the protuberances included on the surface of these elements and the drive screw head  862  are forced into engagement. Any combination of elements described with respect to  FIGS. 135-136  may be used to prevent rotational movement of the drive screw  860 , as will be appreciated by those skilled in the art. 
     In embodiments described above with respect to  FIGS. 87-136 , it may be desirable to include a set screw to provide an additional mechanism to prevent rotation of the drive screw  860 . However, in other embodiments, a set screw alone may provide sufficient force to prevent rotation of the drive screw  860 . As described above with respect to  FIGS. 50-52 and 70 , for example, a set screw  438  may be inserted through the hole  436  to secure the driving ramp  300  to the actuator assembly  200 . In an exemplary embodiment, a spring element  1066  may be selectively positioned within the housing  902  to interfaces, engage, and/or operatively connect to the set screw  438 . 
     For example, in one embodiment the spring element  1066  may be selectively positioned within the housing  902  that interfaces with the set screw  438 , as illustrated in  FIG. 137 . In such an embodiment, the set screw  438  may be inserted into the hole  436  configured and dimensioned in a portion of the housing  902  and operatively connected to the drive screw head  862 . The set screw  438  may include a tip point  1068  that that is configured and dimensioned to engage with protuberances, e.g., teeth, included on a portion of the drive screw head  862 . The set screw  438  may comprise any tip point  1068  known to those skilled in the art, such as conical tip point, as shown in  FIG. 138 a   . The tip point  1068  can be configured and dimensioned so that it may engage with the protuberances on the drive screw head  862  to prevent rotational movement within the housing  902 . 
     The top end  1070  of the set screw  438 , opposite the tip point  1068 , may be configured and dimensioned to include an interface that can selectively engage with the spring element  1066 . For instance, the top end  1070  may include protuberances, such as teeth, that include angles that prevent the set screw  438  from turning counterclockwise, as shown in  FIG. 138 b   . Alternately, the top end  1070  may comprise a recess  1072 , such as a groove or a notch, that relies on friction to resist rotational movement, e.g. counterclockwise or loosening movement. 
     The spring element  1066 , according to an exemplary embodiment, is configured and dimensioned to fit within the hole  436 . One end of the spring element  1066  may include one or more protuberances that are configured and dimensioned to engage with the corresponding protuberances on the top end  1070  of the set screw  438 . The protuberances may include, for example, teeth that are operable to engage with teeth on the top end  1070  of the set screw  438 . Alternately, the protuberance may include a tab that is operable to engage with the recess  1072  in the top end  1070  of the set screw  438 . When the spring element  1066  is inserted into the hole  436  after the set screw  438  is in place, the protuberances on the spring element  1066  engage with the protuberances or recess  1072  on the top end  1070  of the set screw  438 . Because the spring element  1066  applies a constant force on the set screw  438 , the engagement of the protuberances and/or recess  1072  prevents rotation of the set screw  438 . In turn, the tip point  1068  of the set screw  438  engages with protuberances on the drive screw head  862 , thereby preventing rotational movement of the drive screw  860 . 
     An alternative embodiment of the locking mechanism is described with reference to  FIG. 139 . In the illustrated embodiment, the locking mechanism comprises a housing  902 , drive screw  860 , retaining ring  890 , ring  514 , deflectable arm  1074 , spring  1076 , and spring retention device  1078 . The housing  902 , drive screw  860 , retaining ring  890 , and ring  514  of  FIG. 139  and their individual components are similar to the those described with respect to  FIGS. 58 and 87-136 , with several modifications. The modifications and components that differ from the locking mechanism illustrated in  FIGS. 58 and 87-136  will be described in turn below. 
     The spring  1076  may include any spring known to those skilled in the art. It is desirable for the spring  1076  to be configured and dimensioned so that it can fit within a recess  1080  within the drive screw head  862 . The recess  1080  may be configured and dimensioned as part of opening  970  in the drive screw head  862 . While it may comprise any shape and/or dimensions, the recess  1080  may be large enough to receive the spring  1076  and/or spring retention device  1078  while also not compromising the structural integrity of the drive screw head  862 . 
     The drive screw head  862  in this embodiment includes a deflectable arm  1074 , and a recess through which the deflectable arm  1074  can pass through the side of the drive screw head  862 . The recess may extend through the side of the drive screw head  862  in at least one portion, or alternately, it may extend through the side of the drive screw head  862  in at least two places. The deflectable arm  1074  may comprise a separate element that is installed within the drive screw head  862  prior to installation in the housing  902 . Alternately, at least a portion of the deflectable arm  1074  may be formed as part of the drive screw head  862 . The deflectable arm  1074  may comprise a single arm as shown in  FIG. 139 , or it may extend from at least two sides of the drive screw head  862  (not shown) in order to increase the ability to resist rotational movement. The end of the deflectable arm  1074  that protrudes from the drive screw head  862  may comprise a ratcheting interface in order to prevent rotational movement, and may face upwards towards the face of the drive screw head  862  or downwards, away from the face of the drive screw head. In alternate embodiments, one end of the deflectable arm  1074  may face towards the face of the drive screw head  862  while the other may be face towards the body of the drive screw  860 . One advantage of configuring the deflectable arm  1074  in this manner is that at least one side of the deflectable arm  1074  may be engaged with the housing  902  regardless of the direction the drive screw  860  rotates. 
     The spring retention device  1078  illustrated in  FIG. 139  may comprise a button or other type of retention device known to those skilled in the art. The spring retention device  1078  may be a separate element, may comprise a part of the spring  1076 , or it may comprise a separate element that is operatively connected to the spring  1076 . Alternately, the spring retention device  1078  may comprise a part of the deflectable arm  1074  or be operatively connected to the deflectable arm  1074 . The spring retention device  1078  may be configured and dimensioned to fit within the recess  1080  in the drive screw head  862 . 
     The housing  902  of this embodiment includes protuberances, such as teeth, that can engage with the deflectable arm  1074 . In order to assemble this embodiment, the spring  1076  is inserted into the recess  1080 . If the deflectable arm  1074  is a separate element, it may be inserted into the side recess of the drive screw head  862  before or after the spring  1076  is inserted. The spring retention device  1078  may then be inserted into the recess  1080 . When installed, the spring retention device  1078  forces the spring  1076  to be compressed and also exerts a downward (towards the spring  1076 ) force on at least a portion of the deflectable arm  1074  that is within the recess  1080 , forcing the end of the deflectable arm into engagement with the protuberances included in the housing  902 . The spring retention device  1078  may be secured in place using any method or device known to those skilled in the art. In this manner, rotation of the drive screw head  862  may be rotationally limited. When a tool, such as a driver, is inserted into the opening  970  of the drive screw head  862 , the spring  1076  pushes the spring retention device  1078  out of the recess  1080 , which also pushes the deflectable arm  1074  downwards (away from the face of the drive screw head  862 ) so that it disengages from the protuberances included in the housing  902 , permitting rotational movement of the drive screw  860 . 
     An alternative embodiment of the locking mechanism is described with reference to  FIGS. 140-141 . In the illustrated embodiment, the locking mechanism comprises a housing  902 , drive screw  860 , retaining ring  890 , ball bearing  1082 , spring  1076 , and shuttle ramp  1084 . The housing  902 , drive screw  860 , and retaining ring  890  of  FIGS. 140-141  and their individual components are similar to the those described with respect to  FIGS. 87-136 , with several modifications. The modifications and components that differ from the locking mechanism illustrated in  FIGS. 87-136  will be described in turn below. 
     In one embodiment, one or more ball bearings  1082  are included. The ball bearings  1082  may comprise any dimensions known to those skilled in the art. It is desirable, however, for the ball bearings  1082  to be configured and dimensioned such that they can fit into a space between the drive screw head  862  and the housing  902 , as described below. This embodiment also includes a spring  1076 , which is operatively connected to a shuttle ramp  1084 . The shuttle ramp  1084  and the spring  1076  may fit into a recess that is included in the housing  902 , as shown in  FIGS. 140-141 . It is desirable for the recess to be selectively positioned such that it is substantially near the drive screw head  862 . The recess may be configured and dimensioned to allow the spring  1076 , shuttle ramp  1084 , and ball bearing  1082  to fit within and be retained when the drive screw  860  is positioned within the housing  902 , as shown in  FIG. 141 . 
     As shown in  FIGS. 140-141 , when the drive screw  860  is inserted into the housing  902 , the one or more ball bearings  1082  push against the wall of the drive screw head  862 . The ball bearings  1082  are pushed against the drive screw head  862  by the force of the spring  1076  pushing against the shuttle ramp  1084 , which is operatively connected to the ball bearing  1082 . When a tool, such as a driver or the like, is inserted into the space between the drive screw head  862  and the housing  902 , the ball bearing  1082  may be displaced, allowing the drive screw  860  to turn. Those skilled in the art will appreciate that the locking mechanism described with respect to  FIGS. 140-141  may be used in combination with any of the locking mechanisms described with respect to  FIGS. 87-139 . 
     An alternative embodiment of the locking mechanism is described with reference to  FIGS. 142-143 . In the illustrated embodiment, the locking mechanism comprises a housing  902 , drive screw  860 , retaining ring  890 , secondary set screw  1086 , and deflectable arm  1074 . The housing  902 , drive screw  860 , retaining ring  890 , and deflectable arm  1074  of  FIGS. 142-143  and their individual components are similar to the those described with respect to  FIGS. 87-136 , with several modifications. The modifications and components that differ from the locking mechanism illustrated in  FIGS. 87-136  will be described in turn below. 
     In this embodiment, the deflectable arm  1074  faces downwards, towards the body of the drive screw  860 . The deflectable arm  1074  passes through the side of the drive screw head  862  in two places, 180° apart, as shown in  FIG. 143 . A secondary set screw  1086  can be configured and dimensioned to lock the deflectable arm  1074  in place with respect to the drive screw head  862 . The drive screw head  862  may include a recess that is configured and dimensioned to engage with the secondary set screw  1086 . The recess may be a part of the opening  970  in the drive screw head  862 . The housing  902  may include a friction surface, such as protuberances or teeth, that operatively engage with the ends of the deflectable arms  1074  that protrude from the side of the drive screw head  862 . In this manner, the drive screw head  862  is rotationally locked to the housing  902 . When a device, such as a driver or the like, is inserted into the opening  972 , it forces the ends of the deflectable arm  1074  away from the friction surface included in the housing  902 , allowing rotation of the drive screw  860 . 
     While the invention is described herein according to the above embodiments, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.