Patent Publication Number: US-2022233326-A1

Title: Expandable interbody spacer

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation of co-pending U.S. patent application Ser. No. 16/894,308 filed on Jun. 5, 2020, entitled “Expandable interbody spacer” now issued as U.S. Pat. No. 11,304,817 incorporated herein by reference in its entirety. 
    
    
     FIELD OF THE INVENTION 
     This application relates generally to spinal implants, and in particular, expandable intervertebral spacers and fusion cages. 
     BACKGROUND OF THE INVENTION 
     Back pain can be caused by a variety of factors including but not limited to the rupture or degeneration of one or more intervertebral discs due to degenerative disc disease, spondylolisthesis, deformative disorders, trauma, tumors and the like. In such cases, pain typically results from compression or irritation of spinal nerve roots arising from reduced spacing between adjacent vertebrae, a damaged disc and or misalignment of the spine resulting from the injury or degeneration. 
     Common forms of treating such pain include various types of surgical procedures in which a damaged disc may be partially or totally excised. After the disc space is prepared, one or more implants are inserted between the adjacent vertebrae in an effort to restore the natural spacing and alignment between the vertebrae, so as to relieve the compression, irritation or pressure on the spinal nerve or nerves and, thereby, eliminate or significantly reduce the pain that the patient is experiencing. Typically, one or more implants are used together with substances that encourage bone ingrowth to facilitate fusion between adjacent vertebrae and achieve immobilization of adjacent bones. Surgeons insert these intervertebral devices to adjunctively facilitate bone fusion in between and into the contiguous involved vertebrae. This fusion creates a new solid bone mass and provides weight bearing support between adjacent vertebral bodies which acts to hold the spinal segment at an appropriate biomechanically restored height as well as to stop motion in a segment of the spine and alleviate pain. 
     In a posterior lumbar interbody fusion (PLIF) surgery, spinal fusion is achieved in the lower back by inserting an implant such as a cage and typically graft material to encourage bone ingrowth directly into the disc space between adjacent vertebrae. The surgical approach for PLIF is from the back of the patient, posterior to the spinal column. An anterior lumbar interbody fusion (ALIF) surgical procedure is similar to the PLIF procedure except that in the ALIF procedure, the disc space is fused by approaching the spine through the abdomen from an anterior approach instead of from a posterior approach. Another fusion procedure is called a transforaminal lumbar interbody fusion (TLIF) which involves a posterior and lateral approach to the disc space. To gain access to the disc space, the facet joint may be removed whereby access is gained via the nerve foramen. In an extreme lateral interbody fusion (XLIF), the disc space is accessed from small incisions on the patient&#39;s side. 
     In the typical procedures described above, the adjacent vertebrae must be distracted apart by a substantial amount in order to allow the surgeon to advance the implant with relatively little resistance along the delivery path. Also, the surgeon must typically release the implant at least once as the implant is being delivered along the delivery path and align and position the implant at the target position of implantation, typically in the anterior aspect of the disc space. If static spacers having a fixed height are employed, the right-sized spacer is selected from a plurality of spacers. Sometimes the selected static spacer must be interchanged for one of a different height during the procedure. Expandable spacers provide several advantages over static spacers. For example, expandable spacers may be more easily inserted in their low-profile configuration and then mechanically expanded into their high-profile configuration when in the right position. Another advantage of some expandable spacers is that the degree of expansion easily can be adjusted in-situ according to the specific anatomy of the patient. Generally, expandable spacers avoid the need to stock multiple sizes, and to remove and replace spacers during the procedure. 
     There is a need to provide an expandable spacer that is capable of customized expansion given a wide variability in patient anatomy at each vertebral level that meets the surgeon&#39;s demands for providing the best stabilization solutions. Sometimes uniform parallel expansion of the spacer is required. Sometimes only distal or proximal angulation of the spacer is required and sometimes a combination of distal or proximal angulation together with parallel expansion is required. Therefore, there is a need to provide a new and improved expandable interbody spacer that is versatile in both angulation and parallel expansion, easy to position, deploy from a low-profile to a high-profile configuration, angulate both proximally and distally as well as expand uniformly. This invention, as described in the detailed description, sets forth an improved interbody spacer that meets these needs. 
     SUMMARY OF THE INVENTION 
     According to one aspect of the invention, an expandable interbody spacer for the spine is provided. The expandable interbody spacer includes a housing having two sides interconnected by a distal endwall and a proximal endwall defining a hollow interior. The distal endwall includes a threaded distal opening and the proximal endwall having a threaded proximal opening. The spacer includes an upper endplate and a lower endplate each having a posterior end and an anterior end, a bone-engaging surface and an interior surface opposite to the bone-engaging surface. The interior surface includes an anterior ramp surface extending at an angle with respect to the interior surface and a posterior ramp surface extending at an angle with respect to the interior surface. The spacer includes an anterior actuator located between the interior surfaces of the upper endplate and lower endplate near the distal end of the spacer. The anterior actuator includes an upper anterior actuator segment and a lower anterior actuator segment. The upper anterior actuator segment has a curved inner surface for contact with the anterior drive screw and an angled leading surface for contact with the anterior ramp surface of the upper endplate. The lower anterior actuator segment has a curved inner surface for contact with the anterior drive screw and an angled leading surface for contact with the anterior ramp surface of the lower endplate. The spacer includes a posterior actuator located between the interior surfaces of the upper endplate and lower endplate near the proximal end of the spacer. The posterior actuator includes an upper posterior actuator segment and a lower posterior actuator segment. The upper posterior actuator segment has a curved inner surface for contact with the posterior drive screw and an angled leading surface for contact with the posterior ramp surface of the upper endplate. The lower posterior actuator segment has a curved inner surface for contact with the posterior drive screw and an angled leading surface for contact with the posterior ramp surface of the lower endplate. The anterior drive screw includes a proximal ball head connected to a distal threaded shank. The ball head of the anterior drive screw is located between the curved inner surfaces of the upper and lower anterior actuator segments. The distal threaded shank is threadingly connected to the threaded distal opening. The anterior drive screw has an anterior drive bore extending from a proximal opening along a longitudinal drive axis. The spacer includes a posterior drive screw including a proximal threaded shank connected to a distal ball head. The ball head of the posterior drive screw is located between the curved inner surfaces of the upper and lower posterior actuator segments. The proximal threaded shank is threadingly connected to the threaded proximal opening. The posterior drive screw has a posterior drive bore extending along the longitudinal drive axis between a proximal opening in the threaded shank and a distal opening in the ball head. Rotation of the posterior drive screw in a first direction relative to the proximal end of the spacer around the drive axis translates the posterior drive screw distally to wedge apart and expand the distance between the posterior ends of the upper and lower endplates. Rotation of the anterior drive screw in the first direction relative to the proximal end of the spacer around the drive axis translates the anterior drive screw proximally to wedge apart the anterior ends of the upper and lower endplates. Rotation of the posterior drive screw in a second direction relative to the proximal end of the spacer around the drive axis translates the posterior drive screw proximally to reduce the distance between the posterior ends of the upper and lower endplates. Rotation of the anterior drive screw in the second direction relative to the proximal end of the spacer around the drive axis translates the anterior drive screw distally to reduce the distance between the anterior ends of the upper and lower endplates. 
     According to another aspect of the invention, a driver for an expandable interbody spacer having a proximal end and a distal end is provided. The driver includes a first drive portion having a first length extending along a longitudinal axis of the driver. The first drive portion has a first diameter and a non-circular cross-sectional first shape taken perpendicular to the longitudinal axis extending along the first length. The driver includes a second drive portion having a second length extending along the longitudinal axis. The second drive portion has a second diameter and a non-circular cross-sectional second shape taken perpendicular to the longitudinal axis extending along the second length. The driver includes a middle portion located between the first drive portion and the second drive portion. The middle portion has a middle length extending along the longitudinal axis. The middle portion has a middle diameter and a cross-sectional middle shape taken perpendicular to the longitudinal axis extending along the middle length. The driver includes a handle located at the proximal end. The handle has a handle length extending along the longitudinal axis and a handle diameter. The first drive portion extends from the distal end of the spacer to a distal end of the middle portion. The middle portion extends from a proximal end of the first drive portion to a distal end of the second drive portion. The handle extends from a proximal end of the second drive portion to the proximal end of the spacer. 
     According to another aspect of the invention, a method for an interbody spacer for the spine is provided. The method includes the step of providing an expandable interbody spacer having a longitudinal axis, a proximal end and a distal end. The spacer includes a housing having a threaded proximal opening and a threaded distal opening. The spacer includes an upper endplate having an anterior end and a posterior end. The upper endplate has an anterior angled surface and a posterior angled surface. The spacer includes a lower endplate having an anterior end and a posterior end. The lower endplate has an anterior angled surface and a posterior angled surface. The spacer includes an anterior drive screw threadingly connected to the distal opening. The anterior drive screw includes an anterior ball head connected to a threaded anterior shaft. The anterior drive screw includes an anterior drive bore having a bore diameter and a cross-sectional shape taken perpendicular to and extending along a longitudinal drive axis. The spacer includes an anterior actuator coupled to the anterior drive screw. The anterior actuator includes an upper drive surface for mating with the anterior angled surface of the upper endplate and a lower drive surface for mating with the anterior angled surface of the lower endplate. The spacer includes a posterior drive screw threadingly connected to the proximal opening. The posterior drive screw includes a posterior ball head connected to a threaded posterior shaft. The posterior drive screw includes a posterior drive bore having a bore diameter and cross-sectional shape taken perpendicular to and extending along the drive axis. The posterior drive bore is coaxially aligned with the anterior drive bore along the drive axis. The spacer includes a posterior actuator coupled to the posterior drive screw. The posterior actuator includes an upper drive surface for mating with the posterior angled surface of the upper endplate and a lower drive surface for mating the posterior angled surface of the lower endplate. The method includes the step of providing a driver having a longitudinal axis, a proximal end and a distal end. The driver includes a first drive portion having a first length extending along a longitudinal axis of the driver. The first drive portion has a first diameter and a non-circular cross-sectional first shape taken perpendicular to the longitudinal axis extending along the first length. The first shape is sized and configured to matingly engage the anterior drive bore and the posterior drive bore to rotate the anterior drive screw or posterior drive screw. The driver includes a second drive portion having a second length extending along the longitudinal axis; the second drive portion having a second diameter and a non-circular cross-sectional second shape taken perpendicular to the longitudinal axis extending along the second length. The second shape is sized and configured to matingly engage the posterior drive bore to rotate the posterior drive screw. The driver includes a middle portion located between the first drive portion and the second drive portion. The middle portion has a middle length extending along the longitudinal axis. The middle portion has a middle diameter and a cross-sectional middle shape taken perpendicular to the longitudinal axis extending along the middle length. The driver includes a handle located at the proximal end. The handle has a handle length extending along the longitudinal axis and a handle diameter. The first drive portion extends from the distal end of the spacer to a distal end of the middle portion. The middle portion extends from a proximal end of the first drive portion to a distal end of the second drive portion. The handle extends from a proximal end of the second drive portion to the proximal end of the driver. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a front top perspective view of an expandable interbody spacer in its low-profile configuration according to the present invention. 
         FIG. 2  is a rear top perspective view of the expandable interbody spacer of  FIG. 1  in its low-profile configuration. 
         FIG. 3  is a side elevational view of the expandable interbody spacer of  FIG. 1 . 
         FIG. 4  is a cross-sectional view of the expandable interbody spacer of  FIG. 1 . 
         FIG. 5  is an exploded view of the expandable interbody spacer of  FIG. 1 . 
         FIG. 6  is a top perspective view of a housing of the expandable interbody spacer according to the present invention. 
         FIG. 7  is a side elevational view of the housing of  FIG. 6 . 
         FIG. 8  is a top perspective view of an endplate of the expandable interbody spacer according to the present invention. 
         FIG. 9A  is a side elevational view of the endplate of  FIG. 8 . 
         FIG. 9B  is a front elevational end view of the endplate of  FIG. 8 . 
         FIG. 10  is a bottom view of the endplate of  FIG. 8 . 
         FIG. 11  is a cross-sectional view of the endplate taken along line  11 - 11  of  FIG. 10 . 
         FIG. 12A  is a top perspective view of an anterior actuator segment of the expandable interbody spacer of  FIG. 1 . 
         FIG. 12B  is a bottom perspective view of the anterior actuator segment of  FIG. 12A . 
         FIG. 12C  is a bottom view of the anterior actuator segment of  FIG. 12A . 
         FIG. 12D  is a side view of the anterior actuator segment of  FIG. 12A . 
         FIG. 12E  is an end view of the anterior actuator segment of  FIG. 12A . 
         FIG. 12F  is a side view of the anterior actuator segment of  FIG. 12A . 
         FIG. 12G  is a cross-sectional view of the anterior actuator segment taken along line  12 G- 12 G of  FIG. 12E . 
         FIG. 12H  is a cross-sectional view of the anterior actuator segment taken along line  12 H- 12 H of  FIG. 12F . 
         FIG. 13A  is a top perspective view of a posterior actuator segment of the expandable interbody spacer of  FIG. 1 . 
         FIG. 13B  is a bottom perspective view of the posterior actuator segment of  FIG. 13A . 
         FIG. 13C  is a bottom view of the posterior actuator segment of  FIG. 13A . 
         FIG. 13D  is a side view of the posterior actuator segment of  FIG. 13A . 
         FIG. 13E  is an end view of the posterior actuator segment of  FIG. 13A . 
         FIG. 13F  is a side view of the posterior actuator segment of  FIG. 13A . 
         FIG. 13G  is a cross-sectional view of the posterior actuator segment taken along line  13 G- 13 G of  FIG. 13E . 
         FIG. 13H  is a cross-sectional view of the posterior actuator segment taken along line  13 H- 13 H of  FIG. 13F . 
         FIG. 14A  is side elevational view of an anterior threaded actuator is of an expandable interbody spacer according to the present invention. 
         FIG. 14B  is a cross-sectional view of the anterior threaded actuator taken along line  14 B- 14 B of  FIG. 14A . 
         FIG. 15A  is a side elevational view of a posterior threaded actuator of an expandable interbody spacer according to the present invention. 
         FIG. 15B  is a cross-sectional view of the posterior threaded actuator taken along line  15 B- 15 B of  FIG. 15A . 
         FIG. 16  is a side elevational view of a driver for the expandable interbody spacer according to the present invention. 
         FIG. 17A  is a top view of a driver engaged with an expandable spacer for parallel expansion according to the present invention. 
         FIG. 17B  is a cross-sectional view of a driver engaged with an expandable spacer taken along line  17 B- 17 B of  FIG. 17A . 
         FIG. 18  is a top perspective view of an expandable interbody spacer in its high-profile configuration according to the present invention. 
         FIG. 19  is a top perspective view of an expandable interbody spacer in its high-profile configuration according to the present invention. 
         FIG. 20  is a side elevational view of the expandable interbody spacer of  FIG. 18 . 
         FIG. 21  is a cross-sectional view of the expandable interbody spacer of  FIG. 18 . 
         FIG. 22  is an anterior end view of an expandable interbody spacer in its low-profile configuration according to the present invention. 
         FIG. 23  is an anterior end view of an expandable interbody spacer in its high-profile configuration according to the present invention. 
         FIG. 24  is a posterior end view of an expandable interbody spacer in its low-profile configuration according to the present invention. 
         FIG. 25  is a posterior end view of an expandable interbody spacer in its high-profile configuration according to the present invention. 
         FIG. 26A  is a top view of a driver engaged with an expandable spacer for anterior angular expansion according to the present invention. 
         FIG. 26B  is cross-sectional view of a driver engaged with an expandable interbody spacer taken along line  26 B- 26 B of  FIG. 26A . 
         FIG. 27  is a side elevational view of an expandable interbody spacer in its anterior angulated configuration according to the present invention. 
         FIG. 28  is a cross-sectional view of the expandable interbody spacer of  FIG. 27 . 
         FIG. 29  is a side elevational view of an expandable interbody spacer in its combined configuration of anterior angulation and parallel expansion according to the present invention. 
         FIG. 30  is a cross-sectional view of the expandable interbody spacer of  FIG. 29 . 
         FIG. 31A  is a top view of a driver engaged with an expandable spacer for posterior angular expansion according to the present invention. 
         FIG. 31B  is cross-sectional view of a driver engaged with an expandable interbody spacer taken along line  31 B- 31 B of  FIG. 31A . 
         FIG. 32  is a side elevational view of an expandable interbody spacer in its posterior angulated configuration according to the present invention. 
         FIG. 33  is a cross-sectional view of the expandable interbody spacer of  FIG. 32 . 
         FIG. 34  is a side elevational view of an expandable interbody spacer in its combined configuration of posterior angulation and parallel expansion according to the present invention. 
         FIG. 35  is a cross-sectional view of the expandable interbody spacer of  FIG. 34 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     An expandable interbody spacer that is movable from an unexpanded configuration into a variety of expanded configurations including uniform parallel expansion, anterior angulation, posterior angulation and a combination of parallel expansion and anterior or posterior angulation is described below.  FIGS. 1-4  depict an expandable interbody spacer  10  in an unexpanded configuration. The spacer  10  is typically used to stabilize or fuse vertebral bodies in the lumbar or other region of the spine. With particular reference to the exploded view of  FIG. 5 , the expandable interbody spacer  10  includes a housing  12 , upper and lower endplates  14 , an anterior actuator  16 , a posterior actuator  18 , an anterior drive screw  20 , and a posterior drive screw  22 . The expandable interbody spacer  10  is insertable into the disc space between two adjacent vertebral bodies from a posterior approach while in an unexpanded state illustrated in  FIGS. 1-4 . Generally, the unexpanded state is characterized by a low-profile configuration in which the height of the spacer  10  is the lowest and the endplates  14  are parallel to each other. Once inserted and properly positioned inside the disc space, both upper and lower endplates  14  are expanded in height on both sides of the housing  12  into an expanded state. The spacer  10  has a number of possible expanded states. The expanded states include parallel expansion, angular expansion, or a combination of both angular and parallel expansion and, furthermore, the spacer  10  has two types of angular expansion—anterior angular expansion and posterior angular expansion. In the expanded state characterized by parallel expansion, the endplates  14  are moved away from the housing  12  to increase the distance between the endplates  14  in a uniform manner such that the endplates  14  remain parallel to each other in the expanded state. In anterior angular expansion, the height of the spacer  10  at the anterior end, also called the distal end, is greater than the height of the spacer  10  at the posterior end, also called the proximal end. In posterior angular expansion, the height of the spacer  10  at the posterior end is greater than the height of the spacer  10  at the anterior end. The expanded states are effected by a unique driver  23  that selectively engages with the anterior drive screw  20 , posterior drive screw  22  or both. As one or both of the anterior drive screw  20  and posterior drive screw  22  are engaged by the driver  23  and rotated, the anterior actuator  16 , posterior actuator  18  or both are moved to wedge the endplates  14  into one of the expanded states. 
     Turning now to the  FIGS. 6-7 , the housing  12  will now be described in greater detail. The housing  12  includes two opposite sidewalls  24  interconnected by two opposite endwalls  26  that together define an open interior of the housing  12 . A guidepost  28  is formed on the inner surface of each of the sidewalls  24  and opposite from each other at approximately the midpoint. The guideposts  28  are sized and configured to be engaged with vertical slots  62  formed on the upper and lower endplates  14  to align and guide the endplates  14  with respect to the housing  12 . The sidewalls  24  are parallel to each other and of equal length. The endwalls  26  are parallel to each other and approximately of equal length. Both the sidewalls and endwalls  26  define a rectangular shaped housing  12  having a top end and bottom end that open to the interior. The top end and the bottom end are parallel to each other and the sidewalls  24  have a constant height. The front distal endwall  26  is slightly curved and defines a threaded distal opening  30  that is sized and configured to threadingly engage with the anterior drive screw  20 . The height of the housing  12  is slightly greater in the location of the distal threaded opening  30 . The rear proximal endwall  26  includes a cylindrical-like collar  34  extending proximally and defining a threaded rear opening  32  that opens to the interior of the housing  12 . The threaded rear opening  32  is sized and configured to threadingly engage with the posterior drive screw  22 . The collar  34  includes external threads for connecting with a driver instrument  23  and notches  36  for aligning the connection with the driver  23 . The height of the housing  12  is greater in the location of the proximal threaded opening  32 . The top and bottom of the proximal endwall  26  includes top and bottom flats to give the collar  34  a low-profile height. 
     Turning to  FIGS. 8-11 , the upper and lower endplates  14  will now be described. The upper and lower endplates  14  are identical and are connected to the housing  12  via the anterior and posterior actuators  16 ,  18  and the anterior and posterior drive screws  20 ,  22  threaded into the housing  12 . Each endplate  14  has a bone-engaging surface  46  and an interior surface  48  opposite from the bone-engaging surface  46 . The bone-engaging surface  46  includes a plurality of tooth-like ridges. The ridges have pointed peaks to engage and increase the purchase on the adjacent vertebra between which the spacer  12  is located. The ridges may further be angled to help hold and directionally prevent migration of the spacer  10  relative to the adjacent vertebrae when implanted within the intervertebral space. The endplate  14  further includes a leading surface  55  that does not have tooth-like projections. The leading surface  55  is slightly angled to form a leading ramp-like surface at the distal end for easier penetration and distraction of the disc space as the spacer  10  is inserted. Each endplate  14  includes at least one endplate opening  52  extending between the bone-engaging surface  46  and the interior surface  48  and opening to the interior of the housing  12 . The endplate opening  52  reduces the weight of the spacer  10  and permits bone ingrowth to take place into the endplate  14 . A family of bone graft materials, such as autograft, bone morphogenic protein (BMP), bone marrow aspirate, concentrate, stem cells and the like, may be placed inside the endplate openings  52  and into the interior of the housing  12  to promote bone growth into the spacer  10 . Also, small holes may be formed in the bone-engaging surface  46  to promote osseointegration. The bone-engaging surface  46  of the endplates  14  are substantially flat and parallel to each other when in the collapsed, low-profile unexpanded state. The endplates  14  have a width that is approximately equal to the overall width of the spacer  10  and approximately equal to the width of the housing  12 . Each endplate  14  includes two oppositely-disposed and parallel side rails, a first side rail  50   a  and a second side rail  50   b , extending perpendicularly from the interior surface  48 . The first side rail  50   a  includes an inner surface facing the longitudinal axis of the endplate  14  and an outer surface facing outwardly. The first side rail  50   a  is offset inwardly from the adjacent side edge of the bone-engaging surface  46  by a first distance  47 . The first side rail  50   a  includes a U-shaped slot  62   a  that is perpendicular to the horizontal plane. The slot  62   a  is sized and configured to receive a guidepost  28  of the housing  12 . The second side rail  50   b  includes an inner surface facing the longitudinal axis of the endplate  14  and an outer surface facing outwardly. The second side rail  50   b  is offset inwardly from the adjacent side edge by a second distance  49  wherein the second distance  49  is greater than the first distance  47 . The second side rail  50   b  includes a U-shaped slot  62   b  that is perpendicular to the horizontal plane. The slot  62   b  is sized and configured to receive a guidepost  28  of the housing  12 . The slots  62   a ,  62   b  are oppositely disposed and aligned with each other. The endplate  14  further includes an anterior ramp  58  and a posterior ramp  60 . The anterior ramp  58  is located proximal to the anterior end of the endplate  14 . The anterior ramp  58  extends at an angle from the interior surface  48  of the endplate  14 . The angle of the anterior ramp  58  is such that the height of the anterior ramp  58  increases toward the posterior end of the endplate  14  as clearly shown in  FIG. 11 . The anterior ramp  58  is U-like in shape with the opening of U-shape facing the center of the endplate  14 . The posterior ramp  60  is located proximal to the posterior end of the endplate  14 . The angle of the posterior ramp  60  is such that the height of the posterior ramp  60  increases toward the anterior end of the endplate  14  as clearly shown in  FIGS. 5 and 11 . The posterior ramp  60  is also U-like in shape with the opening of the U-shape facing the center of the endplate  14 . The top ends of the U-shaped ramps  58 ,  60  meet and are aligned with the slots  62   a ,  62   b . A first gap or channel  64  is defined and formed between the first side rail  50   a  and the anterior and posterior ramps  58 ,  60  as can be seen in  FIGS. 9B and 10 . The first channel  64  is sized and configured to receive the second side rail  50   b  of the lower endplate  14 . As previously mentioned, the upper and lower endplates  14  are identical. Due to the offset distances  47 ,  49  of the side rails  50   a ,  50   b , the second side rail  50   b  of the lower endplate  14  will be received inside the first channel  64  adjacent to the inner surface of the first rail  50   a  of the upper endplate  14  to interlock the upper and lower endplates  14 . A second gap or channel  66  is defined and formed between the second side rail  50   b  and the adjacent side edge of the endplate  14 . The second channel  66  can be seen in  FIG. 10 . The second channel  66  is located within the second distance  49 . The second channel  66  is sized and configured to receive first side rail  50   a  of the lower endplate  14 . Due to the offset distances  47 ,  49  of the side rails  50   a ,  50   b , the first side rail  50   a  of the lower endplate  14  will be received inside the second channel  66  adjacent to the outer surface of the second rail  50   b  of the upper endplate  14  to interlock the upper and lower endplates  14 . With particular reference to  FIGS. 5 and 10 , the first and second channels  64 ,  66  are grooves formed into the interior surface  48  of the endplate  14  and are slightly longer in length than the length of the side rails  50   a ,  50   b  in order to accommodate the siderails  50   a ,  50   b  during angulation. Formed within the first channel  64  is an anterior indent  68  and a posterior indent  70  sized and configured to receive a portion of the anterior and posterior actuators  16 ,  18 , respectively. There is no gap or channel between the anterior and posterior ramps  58 ,  60  and the second side rail  50   b . The inner surface of the second side rail  50   b  includes an anterior projection  72  and a posterior projection  74  shown in  FIGS. 5, 9B, 10 and 11 . The anterior projection  72  extends outwardly from the inner surface of the second side rail  50   b  toward the longitudinal axis of the endplate  14 . The anterior projection  72  is angled parallel to the angle of the anterior ramp  58 . The anterior projection  72  is spaced apart from the anterior ramp  58  surface defining an anterior recess  76  therebetween. The anterior recess  76  is sized and configured to receive and guide part of the anterior actuator  16  which will be described in greater detail below. The posterior projection  74  extends outwardly from the inner surface of the second side rail  50   b  toward the longitudinal axis of the endplate  14 . The posterior projection  74  is angled parallel to the angle of the posterior ramp  58 . The posterior projection  74  is spaced apart from the posterior ramp  60  surface defining a posterior recess  78  therebetween. The posterior recess  78  is sized and configured to receive and guide part of the posterior actuator  18  which will be described in greater detail below. The interior surface  48  of the endplate  14  includes a concave area  82  that corresponds to and accommodates the convex surface of the distal endwall  26  of the housing  12  in the location of the distal threaded opening  30 . The concave area  82  provides the spacer  10  with a low-profile configuration while allowing for a larger anterior drive screw to be utilized. The posterior end of the endplate  14  includes a cutout  80  that is sized and configured to clear the proximal collar  34  of the housing  12  during angulation of the anterior end and to provide the spacer  10  with the lowest profile, largest bone-engaging surface  46 , largest posterior drive screw  22  for strength and a greater range of angulation. With particular reference to  FIGS. 5, 10 and 11 , the endplate  14  further includes a rectangular-shaped anterior well  84  that is sized and configured to receive part of the anterior actuator  16  when the spacer  10  is in the unexpanded state. The endplate  14  also includes a rectangular-shaped posterior well  86  that is sized and configured to receive part of the posterior actuator  18  when the spacer  10  is in the unexpanded state. The wells  84 ,  86  are depressions formed into the interior surface  48  of the endplate  14  that permits the actuators  16 ,  18  a larger range of translation providing a lower profile in the unexpanded state compared to the absence of such wells  84 ,  86 . 
     Turning now to  FIGS. 12A-12H , the anterior actuator  16  will now be described in greater detail. The anterior actuator  16  comprises of two identical anterior actuator segments  88  wherein one anterior actuator segment  88  is inverted with respect to the other anterior actuator segment  88  and mated therewith. With reference back to  FIG. 5 , an upper anterior actuator segment  88  is adjacent to the upper endplate  14  and a lower anterior actuator segment  88  is adjacent to the lower endplate  14 . The upper anterior actuator segment  88  together with the lower anterior actuator segment  88  form the anterior actuator  16 . 
     With continued reference to  FIGS. 12A-12H , the anterior actuator segment  88  includes a leading surface  90 , a trailing surface  92 , a landing surface  94 , a front wall  96 , a rear wall  98 , and an inner surface  100  all integrally interconnected by a first sidewall  102  and a second sidewall  104 . The leading surface  90  includes a scallop  106  sized and configured to accommodate part of the anterior drive screw  20 . The scallop  106  corresponds to the U-shaped anterior ramp  58  and defines a contact area of the leading surface  90  that corresponds substantially to the contact area of the anterior ramp  58 . The leading surface  90  is angled. The angle of the leading surface  90  corresponds to the angle of the anterior ramp  58  such that ideal mating contact is maintained during expansion of the spacer  10  for a uniform distribution of forces when loaded in situ. The angled leading surface  90  is sized and configured to contact the anterior ramp  58  of the endplate  14  and slide along the anterior ramp  58  to move the endplate  14  into expansion or reduction as the anterior drive screw  20  is threadingly translated. The trailing surface  92  is angled toward the vertical rear wall  98  to provide a tapered actuator segment  88 . The landing surface  94  is substantially rectangular in shape and includes a notch in the location of the side channel  108 . The landing surface  94  is flat, horizontally orientated and sized and configured to fit inside the anterior well  84  of the endplate  14 . The front wall  96  is vertical and substantially parallel to the rear wall  98 . The first sidewall  102  and second sidewall  104  are parallel to each other and vertical in orientation. The first sidewall  102  includes a side channel  108  having an angle equal to the angle of the leading surface  90 . The side channel  108  is sized and configured to receive the anterior projection  72  of the endplate  14  within the side channel  108 . The anterior projection  72  when mated with the side channel  108  serves to hold the endplates  14  and anterior actuator  16  together and in position and also serves to guide the movement of the anterior actuator  16  along the anterior ramp  58 . The first sidewall  102  includes a curved projection  110  that is sized and configured to mate with a curved indentation  112  on the second sidewall  104  of an adjacent and up-side down-oriented anterior actuator segment  88  comprising the anterior actuator  16 . The inner surface  100  is curved to match the curvature of the ball head of the anterior drive screw  20 . The inner surface  100  is spherical in shape and, in particular, it is semi-spherical in shape and further it is truncated and semi-spherical in shape. 
     As mentioned previously, the upper anterior actuator segment  88  is identical to the lower anterior actuator segment  88  and that together they are joined to form the anterior actuator  16 . One of the two identical anterior actuator segments  88  of the anterior actuator  16  is turned upside down or inverted such that the inner surfaces  100  face each other. The lower anterior actuator segment  88  is adjacent to the lower endplate  14  and the upper anterior actuator segment  88  is adjacent to the upper endplate  14 . As can be seen in  FIG. 5 , the first sidewall  102  of the lower anterior actuator segment  88  faces the second side rail  50   b  of the lower endplate  14  and, as such, the side channel  108  of the lower actuator segment  88  engages with the anterior projection  72  of the lower endplate  14  whereas the first sidewall  102  of the upper anterior actuator segment  88  is on the opposite side and faces the second side rail  50   b  of the upper endplate  14  and, as such, the side channel  108  of the upper actuator segment  88  engages with the anterior projection  72  of the upper endplate  14 . The curved projection  110  of the upper anterior actuator segment  88  mates with the curved indentation  112  of the lower anterior actuator segment  88  on one side and the curved projection  110  of the lower anterior actuator segment  88  mates with the curved indentation  112  of the upper anterior actuator segment  88 . The mated upper and lower anterior actuator segments  88  form a clamshell-like chamber that captures the anterior drive screw  20 . The ball head  118  of the anterior drive screw  20  is located between the upper anterior actuator segment  88  and the lower anterior actuator segment  88  and captured inside the clamshell-like enclosure such that the curved, spherical ball head  118  of the anterior drive screw  20  may make contact with the truncated spherical ball shape chamber comprised of the inner surfaces  100  of the adjacent upper and lower anterior actuator segments  88  as needed for the transmission of load from the endplates  14  to the actuator  16  to the drive screw  20  and, in turn, to the housing  12  while permitting the drive screw  20  to rotate about its axis relative to the anterior actuator  16 . The leading surfaces  90  of the upper and lower anterior actuator segments  88  face toward the proximal end and are angled such that the distance between the leading surfaces  90  increases towards the distal end. With particular reference to  FIG. 12H , the first sidewall  102  depends downwardly or otherwise extends in the location of the curved projection  110  such that the curvature of the spherical inner surface  100  is longer than a semi-circle to define an overhang  114  wherein the slope of a plane tangential to the cross-sectional curve goes from a negative value to a positive value in the location of the curved projection  110 . This overhang  114  advantageously causes the ball head  118  of the drive screw  20  to snap in position past the overhangs  114  of both anterior actuator segments  88  and to be held in place by the overhangs  114 . 
     Turning now to  FIGS. 13A-13H , the posterior actuator  18  will now be described in greater detail wherein like numbers are used to describe like parts. The posterior actuator  18  is comprised of two identical posterior actuator segments  116  wherein one posterior actuator segment  116  is inverted with respect to the other posterior actuator segment  116  and mated therewith. With reference back to  FIG. 5 , an upper posterior actuator segment  116  is adjacent to the upper endplate  14  and a lower anterior actuator segment  116  is adjacent to the lower endplate  14 . The upper anterior actuator segment  116  together with the lower anterior actuator segment  116  form the anterior actuator  18 . 
     With continued reference to  FIGS. 13A-13H , the posterior actuator segment  116  includes a leading surface  90 , a trailing surface  92 , a landing surface  94 , a front wall  96 , a rear wall  98 , and an inner surface  100  all integrally interconnected by a first sidewall  102  and a second sidewall  104 . The leading surface  90  includes a scallop  106  sized and configured to accommodate part of the posterior drive screw  22 . The scallop  106  corresponds to the U-shaped posterior ramp  60  and defines a contact area of the leading surface  90  that corresponds substantially to the contact area of the posterior ramp  60 . The leading surface  90  is angled. The angle of the leading surface  90  corresponds to the angle of the posterior ramp  60  such that ideal mating contact is maintained during expansion of the spacer  10  for a uniform distribution of forces when loaded in situ. The angled leading surface  90  is sized and configured to contact the posterior ramp  60  of the endplate  14  and slide along the posterior ramp  60  to move the endplate  14  into expansion or reduction as the posterior drive screw  22  is threadingly translated. The trailing surface  92  is angled toward the vertical rear wall  98  to provide a tapered actuator segment  116 . The landing surface  94  is substantially rectangular in shape and includes a notch in the location of the side channel  108 . The landing surface  94  is flat, horizontally orientated and sized and configured to fit inside the posterior well  86  of the endplate  14 . The front wall  96  is vertical and substantially parallel to the rear wall  98 . The first sidewall  102  and second sidewall  104  are parallel to each other and vertical in orientation. Unlike the anterior actuator segment  88 , the second sidewall  104  of the posterior actuator segment  116  includes a side channel  108  having an angle equal to the angle of the leading surface  90 . The side channel  108  is sized and configured to receive the posterior projection  74  of the endplate  14  within the side channel  108  having the same angle. The posterior projection  74  when mated with the side channel  108  serves to hold the endplates  14  and posterior actuator  18  together and in position and also serves to guide the movement of the posterior actuator  18  along the posterior ramp  60 . The first sidewall  102  includes a curved projection  110  that is sized and configured to mate with a curved indentation  112  on the second sidewall  104  of an adjacent and up-side down-oriented posterior actuator segment  116  comprising the posterior actuator  18 . The inner surface  100  is curved to match the curvature of the ball head of the posterior drive screw  22 . The inner surface  100  is spherical in shape and, in particular, it is semi-spherical in shape and further it is truncated and semi-spherical in shape. 
     As mentioned previously, the upper posterior actuator segment  116  is identical to the lower posterior actuator segment  116  and that together they are joined to form the posterior actuator  18 . One of the two identical posterior actuator segments  116  comprising the posterior actuator  18  is turned upside down or inverted such that the inner surfaces  100  face each other. The lower posterior actuator segment  116  is adjacent to the lower endplate  14  and the upper posterior actuator segment  116  is adjacent to the upper endplate  14 . As can be seen in  FIG. 5 , the second sidewall  104  of the lower posterior actuator segment  116  faces the second side rail  50   b  of the lower endplate  14  and as such the side channel  108  of the lower actuator segment  116  is oriented to engage with the posterior projection  74  of the lower endplate  14 ; whereas, the second sidewall  104  of the upper posterior actuator segment  116  is on the opposite side and faces the second side rail  50   b  of the upper endplate  14  and as such the side channel  108  of the upper actuator segment  88  engages with the posterior projection  74  of the upper endplate  14 . The curved projection  110  of the upper posterior actuator segment  116  mates with the curved indentation  112  of the lower posterior actuator segment  116  on one side and the curved projection  110  of the lower posterior actuator segment  116  mates with the curved indentation  112  of the upper posterior actuator segment  116  on the other side to form a clamshell-like chamber that captures the posterior drive screw  22  therebetween. The spherical-shaped ball head  118  of the posterior drive screw  22  is located between the upper posterior actuator segment  116  and the lower posterior actuator segment  116  and captured inside the clamshell-like enclosure such that the curved, spherical ball shape of the posterior drive screw  20  may polyaxially make contact with the truncated spherical ball shape chamber comprised of the inner surfaces  100  of the adjacent upper and lower posterior actuator segments  116  for the transmission of load from the endplates  14  to the actuator  18  to the drive screw  20  and to the housing  12  while permitting the drive screw  22  to rotate about its axis relative to posterior actuator  18 . The leading surfaces  90  of the upper and lower posterior actuator segments  116  face toward the distal end and are angled such that the distance between the leading surfaces  90  increases towards the proximal end. With particular reference to  FIG. 13H , the first sidewall  102  depends downwardly or otherwise extends at the curved projection  110  such that the curvature of the spherical inner surface  100  is longer than a semi-circle to define an overhang  114  wherein the slope of a plane tangential to the cross-sectional curve goes from a negative value to a positive value in the location of the curved projection  110 . This overhang  114  advantageously causes the ball head  118  of the drive screw  22  to snap in position past the overhangs  114  of both posterior actuator segments  116  and to be held in place by the overhangs  114 . 
     Turning now to  FIGS. 14A-14B , the anterior drive screw  20  will now be described. The anterior drive screw  20  includes a ball head  118  at a proximal end connected to a threaded shank portion  120  that extends toward a distal end. The ball head  118  has a spherical shape that is truncated at the shaft  120 . The diameter of ball head  118  is larger than the diameter of the threaded shank  120 . A neck portion  122  without threads is located between the ball head  118  and the shank  120 . As can be seen in  FIG. 14B , the anterior drive screw  20  includes a drive bore  124  that extends between the proximal end and the distal end of the drive screw  20 . The drive bore  124  extends along the entire length of the drive screw  20  and has a proximal opening in the ball head  118  at the proximal end and a distal opening in the threaded shank  120  at the distal end. In one variation, the drive bore  124  does not have a distal opening in the threaded shank  120 . The drive bore  124  has a hexalobe shape (visible in  FIG. 5 ) or hexagonal shape in cross-section along the entire length of the bore  124 . The drive bore  124  is sized and configured to be engaged to rotate the drive screw  20  by the driver instrument  23 . The bore  124  may have any non-circular cross-sectional shape that is corresponds to and is sized and configured to mate with to the cross-sectional shape of distal drive portion of the driver instrument  23 . 
     Turning now to  FIGS. 15A-15B , the posterior drive screw  22  will now be described wherein like reference numbers are used to describe like parts. The posterior drive screw  22  includes a ball head  118  at the distal end connected to a threaded shank portion  120  that extends toward the proximal end. The ball head  118  has a shape that is truncated at the shaft  120 . The diameter of ball head  118  is larger than the diameter of the threaded shank  120 . A neck portion  122  without threads is located between the ball head  118  and the shank  120 . As can be seen in  FIG. 15B , the posterior drive screw  22  includes a drive bore  124  that extends between the proximal end and the distal end of the drive screw  22 . The drive bore  124  extends along the entire length of the drive screw  22  and has a proximal opening in the threaded shank  120  at the proximal end and a distal opening in the ball head  118  at the distal end. The drive bore  124  has a hexalobe shape (visible in  FIG. 5 ) or hexagonal shape in cross-section along the entire length of the bore  124 . The drive bore  124  is sized and configured to be matingly engaged for rotation by the driver instrument  23 . The bore  124  may have any non-circular cross-sectional shape that is corresponds to and is sized and configured to mate with to the cross-sectional shape of the proximal drive portion and distal drive portion of the driver instrument  23 . A non-circular cross-section will have a major diameter and minor diameter. 
     With reference to both  FIGS. 14A-14B  and  FIGS. 15A-15B , the threaded shanks  120  of the anterior and posterior drive screws  20 ,  22  have the same length, the same size thread and the same number of threads per inch. The helical threaded shank  120  of the posterior drive screw  22  has a right-handed thread; whereas, the helical threaded shank  120  of the anterior drive screw  20  has a left-handed thread. This difference in handedness of the threads is clearly visible in  FIGS. 14A and 15A . In another variation, the helical threaded shank  120  of the posterior drive screw  22  has a left-handed thread; whereas, the helical threaded shank  120  of the anterior drive screw  20  has a right-handed thread. In essence, the helical direction of one of the two drive screws  20 ,  22  is opposite in direction from the other. Viewing from one of the proximal end or distal end, the direction of translation with respect to the housing  12  of one of the anterior drive screw  20  and posterior drive screw  22  is positive and the direction of translation of the other one of the anterior drive screw  20  and posterior drive screw  22  is negative. The advantage of this difference will be described in greater detail below. 
     Turning now to  FIG. 16 , there is shown the driver instrument  23  according to the present invention. The driver  23  includes a handle  126  at the proximal end. The driver  23  further includes a proximal drive portion  128  having a length and distal drive portion  130  having a length separated by a middle portion  132  having a length. The handle  126  may include a neck portion  134  between the handle  126  and the proximal drive portion  128 . The distal drive portion  130  is located at the distal end of the driver  23  and extends proximally toward the middle portion  132 . The proximal drive portion  128  is located near the handle  126  and extends proximally from the middle portion  132  towards the handle  126  or neck portion  134  if a neck portion  134  is defined. The proximal and distal drive portions  128 ,  130  have a mating cross-section that is sized and configured to engage with drive bores  124  of the anterior and posterior drive screws  20 ,  22 , respectively, in order to rotate the anterior and posterior drive screws  20 ,  22 . The cross-sectional shape of the proximal and distal drive portions  128 ,  130  are uniform and extend along their entire respective lengths. The distal drive portion  130  has a cross-section that is sized and configured to also engage with the drive bore  124  of the posterior drive screw  22 . In one variation, all of the drive bores  124  have the same cross-sectional shape and the drive portions  128 ,  130  have the same corresponding cross-sectional shape and, in another variation, have the same diameter. For example, in one variation, the drive bores  124  have a hexalobe shape as shown in  FIGS. 14-15  and the driver  23  has proximal and distal drive portion  128 ,  130  that also have a hexalobe shape sized and configured to engage the drive screws  20 ,  22  for rotation. The diameters of the proximal and distal drive portions  128 ,  130  are the same to match the diameters of the drive bores  124  and are greater in diameter than the diameter of the middle portion  132 . The middle portion  132  does not have an outer surface sized and configured to engage any drive bores  124  into rotation. In another variation, the middle portion has a smooth circular cross-section and middle diameter that is the same as the diameter or inner diameter of the drive bores  124 . The diameter of the handle  126  is greater than the diameter of the drive portions  128 ,  130  and, if a neck portion  134  is provided, the neck portion  134  has a diameter greater than the drive portions  128 ,  130 . The neck portion  134  does not have an outer surface or cross-sectional shape configured to engage any of the drive screws  20 ,  22 . The handle  126  or neck portion  134  serves as an abutment or stop for the insertion of the driver  23 . If there is no neck portion  134 , the handle  126  will serve as an abutment. The distal drive portion  130  of the driver  23  is sized and configured such that it can be inserted first into the posterior drive screw  22  and passed distally into the spacer  10  into the anterior drive screw  20  until the neck portion  134  or handle  126 , because of its larger diameter, abuts the proximal end of the posterior drive screw  20  or spacer  10  and, thereby, the driver  23  is prevented from further insertion into the spacer  10 . The proximal drive portion  128 , the middle portion  132  and the distal drive portion  130  altogether define the active portion  136  of the driver  23  and their combined lengths or the length of the active portion  136  is not longer than the length of the spacer  10 . If the active length  136  of the driver  23  is longer than the spacer  10 , the distal end of the driver  23  would extend beyond the length of the spacer  10  when the handle  126  is abutted at the proximal end and potentially impinge on surrounding tissue. Hence, the neck portion  134  serves as an abutment that simplifies the insertion of the driver  23  by allowing the user to insert the driver  23  until abutment is made with the neck portion without fear of the driver  23  extending beyond the distal end of spacer  10 . 
     The driver  23  is configured such that the distal drive portion  130  engages the anterior drive screw  20  and the proximal drive portion  128  engages the posterior drive screw  22  simultaneously when the spacer  10  is in the collapsed, low-profile configuration in order to rotate both of the drive screws  20 ,  22  simultaneously. The driver  23  is also configured to be pulled back in the proximal direction such that the proximal drive portion  128  is disengaged from the posterior drive screw  22  while the distal drive portion  130  remains engaged with the anterior drive screw  20  to effect variable angulation of the anterior end of the spacer  10 . When the spacer  10  is in the collapsed, low-profile configuration, the drive screws  20 ,  22  will be at the farthest distance apart from each other. Hence, the middle portion  132  is longer than the distance between the drive screws  20 ,  22  when in the collapsed low-profile configuration so that the driver  23  may be pulled back in the proximal direction to disengage the proximal drive portion  128  from the posterior drive screw  22  while the distal drive portion  130  still engages with the anterior drive screw  20 . The length of the active portion  136  or the combined length of the distal drive portion  130 , proximal drive portion  128  and middle portion  132  is approximately equal to the length of the spacer  10 . The length of the proximal drive portion  128  is shorter than the length of the distal drive portion  130 . Given these parameters and to reduce the torque required to rotate the drive screws  20 ,  22 , the length of the distal drive portion  130  is approximately equal to the length of the anterior drive screw  20  and the length of the proximal drive portion  22  is shorter than the length of the posterior drive screw  22 . In one variation, the distal drive portion  130  is equal to the length of the anterior drive screw  20 , the proximal drive portion  128  is ⅘ths the length of the posterior drive screw  22  and the middle portion  132  is 5/4ths longer than the length of the distance between the two drive screws  20 ,  22  when the spacer  10  is in the collapsed low-profile configuration. The drive portions  128  and  130  are coaxial. 
     The expandable interbody spacer  10  is assembled by placing one endplate  14  such that the interior surface  48  faces upwardly defining a lower endplate  14 . One anterior actuator segment  88  is placed into the anterior well  84  of the lower endplate  14  such that the anterior projection  72  of the lower endplate  14  is received inside the side channel  108  of the anterior actuator segment  88 . One posterior actuator segment  116  is placed into the posterior well  86  of the lower endplate  14  such that the posterior projection  74  of the endplate  14  is received inside the side channel  108  of the posterior actuator segment  116 . The anterior drive screw  20  is threaded into the distal threaded opening  30  of the housing  12  and the posterior drive screw  22  is threaded into the rear threaded opening  32 . The guideposts  28  of the housing  12  are aligned with the slots  62  of the lower endplate  14 . The drive screws  20 ,  22  may be threaded to adjust their alignment such that the ball heads  118  are received inside the inner surface  100  of the anterior and posterior actuator segments  88 ,  116 . A second anterior actuator segment  88  is connected to the upper endplate  14  by inserting the anterior projection  72  into side channel  108  of the second anterior actuator segment  88 . A second posterior actuator segment  116  is connected to the upper endplate  14  by inserting the posterior projection  74  into the side channel  108  of the second posterior actuator segment  116 . The upper endplate  14  is aligned so that the slots  62  of the upper endplate  14  receive the guideposts  28  of the housing  12  and that the actuator segments  88 ,  116  connected to the upper endplate  14  cover the ball heads  118  of the drive screw  20 ,  22 . Pressure is applied such that the overhangs  114  of the actuator segments  88 ,  116  snap over the ball heads  118 . 
     In use, the present expandable interbody spacer  10  is inserted into the disc space between adjacent vertebral bodies. The spacers  10  of  FIGS. 1-29  are generally configured for use as a PLIF cage in spinal surgical procedures. It is understood that novel features of the present invention can find application in different types of spacers including but not limited to interbody spacers for PLIF, TLIF, XLIF surgical procedures as well as other types of orthopedic implants. 
     Implanting the interbody spacer  10  involves removal, in whole or in part, of the disc material from the intervertebral space at the target vertebral level where the interbody spacer  10  will be implanted. The patient is oriented to provide some distraction of the disc space and to provide access to the spine. Additional distraction of the disc space and surrounding tissues may be needed to decompress the nerve roots, realign the anatomical axis of the spine, and restore disc space height at the particular target level. After disc material is removed, a clean space is achieved in which to place the device. The vertebral endplates may be further prepared using burrs, curettes and the like to abrade and clean the endplates to encourage bone regeneration. 
     A surgeon will then connect the spacer  10  for to an insertion instrument (not shown). The insertion instrument is aligned with the spacer  10  via the notches  36  and connected at the proximal end of the spacer  10  such that it is secured to the collar  34  by threadingly engaging the insertion instrument around the collar  34 . The driver  23  is configured to be inserted into one or more of the anterior drive screw  20  and posterior drive screw  22  by aligning the distal and proximal drive portions within the selected one or more drive bores  124  as will be described in greater detail below. The surgeon uses the insertion instrument to grasp the spacer  10  and place it at the mouth of the intervertebral space in its low-profile configuration. The spacer  10  is moved and orientated into its proper location within the intervertebral space. Bone graft or other material may be placed inside the interior of the spacer  10  through the endplate openings  52  prior the insertion of the spacer  10  into the disc space. The bone graft material promotes ingrowth and improves blood supply in order to grow active and live bone from the adjacent spinal vertebrae to inter-knit with the spacer  10  and, thereby, eventually immobilize and fuse the adjunct spinal vertebrae. 
     The spacer  10  is placed such that the upper endplate  14  contacts the lower endplate of the upper vertebral body and the lower endplate  14  of the spacer  10  contacts the upper endplate of the lower vertebral body on either side of the target intervertebral space. The geometry of the teeth on the bone-engaging surface  46  provides resistance to migration of the spacer  10  while inside the target space. Other coatings and surface textures may also be provided on the spacer  10 . When the spacer  10  is in position, the driver  23  is connected to the spacer  10  to deploy the spacer  10  into its expanded or high-profile configuration. The insertion instrument may be disconnected and removed when needed. 
     Turning now to  FIGS. 17-25 , uniform parallel expansion of the spacer  10  will now be described. The spacer  10  is inserted into the disc space while it is in an unexpanded, collapsed state. The unexpanded state is illustrated in  FIGS. 1-4, 22 and 24 . The distal end of the driver  23  is inserted into the drive bore  124  of the posterior drive screw  22  and moved in a distal direction relative to the spacer  10  until the distal drive portion  130  of the driver  23  is completely inserted into the drive bore  124  of the anterior drive screw  20  while the spacer  10  is in an unexpanded, low profile configuration as shown in  FIGS. 17A-17B . When the distal drive portion  130  is aligned with the anterior drive screw  20 , such that the length of the distal drive portion  130  is resident within the length of the anterior drive screw  20 , the proximal drive portion  128  will be advantageously aligned with the posterior drive screw  22  such that the hexalobe cross-section of the posterior drive screw  22  engages the hexlobe cross-section of the drive bore  124 . The drive bore  124  is formed along the entire length of each drive screw  20 ,  22  and each drive screw  20 ,  22  has proximal and distal openings. The insertion of the driver  23  into the spacer  10  is facilitated advantageously by the drive screws  20 ,  22  and their respective drive bores  124  being aligned with each other coaxially along a drive axis. Because of the corresponding hexalobe cross-sectional shape of the drive bores  124  and drive portions, the driver  23  is easily inserted into both the anterior and posterior drive screws  20 ,  22  with a minimum of rotation of the driver  23  for alignment purposes before the distal drive portion  128  enters the anterior drive screw  20 . Furthermore, insertion of the driver  23  into a position for parallel expansion is facilitated by the neck portion  134  of the driver  23  which has a diameter larger than the diameter of the drive bore  124 . The user simply inserts the driver  23  until the neck portion  134  abuts against the posterior drive screw  22 . If a driver  23  variation without a neck portion  134  is used, the handle  126 , having a larger diameter than the posterior drive screw  22 , will abut the drive screw  22 . The neck portion  134  advantageously allows the driver  23  to have a longer in length while at the same time providing a low-profile and a substantial handle  126  of increased diameter to the driver  23  to fit a surgeon&#39;s hand. The active portion  136  of the driver  23  is approximately equal to the length of the spacer  10 . The neck portion  134  or handle  126 , if an intermediate diameter neck portion  134  is not employed, advantageously serves as a stop preventing insertion of the driver  23  beyond a distance of approximately the length of the spacer  10 . This stop advantageously prevents the distal end of the driver  23  from protruding beyond the approximate distal end of the spacer  10 . Furthermore, the neck portion  134  allows the surgeon to easily and quickly insert the driver  23  into the spacer  10  all the way until abutment is made with the enlarged diameter of the neck portion  134  or handle  126 . Also, advantageously, the interior of the spacer  10  provides a clear pathway for the passage of the driver  23  between the two drive screws  20 ,  22  as the spacer  10  is configured so that there are no impeding mechanical or anatomical structures that would interfere with clear passage of the driver  23 . When the driver  23  is inserted for parallel expansion as shown in  FIG. 17B , the distal drive portion  130  will be automatically aligned within the drive bore  124  of the anterior drive screw  20  and the proximal drive portion  128  will be automatically aligned within the drive bore  124  of the posterior drive screw  22  and the middle portion  132  will be located between the two drive screws  20 ,  22 . As mentioned previously, the length of the distal drive portion  130  is approximately the same length as the length of the anterior drive screw  20  to provide the user with maximum torqueing advantage. The length of the proximal drive portion  128  is shorter than the length of the posterior drive screw  22  and the middle portion  132  is longer than the distance between the drive screws  20 ,  22  when the spacer  10  is in the unexpanded. When the driver  23  is in position for parallel expansion as shown in  FIG. 17B , the driver  23  is rotated in one of a clockwise direction or counterclockwise direction to bring the spacer  10  into an expanded state. When the driver  23  is rotated, the posterior drive screw  22  is rotated. As the posterior drive screw  22  is rotated, the threads on the threaded shank  120  engage the complementary threads on the rear threaded opening  32  of the housing  12  allowing the posterior drive screw  22  to translate distally with respect to the housing  12  due to the right-handedness of the threads of the posterior drive screw  22  as viewed from the proximal end of the spacer  10 . As the posterior drive screw  22  moves distally, it moves the posterior actuator  18  distally along with it. The leading surfaces  90  of upper and lower posterior actuator segments  116  will contact the posterior ramps  60  and slide along the posterior ramps  60  to wedge the upper and lower endplates  14  apart causing the endplates  14  at the proximal end to separate and increase in height. Simultaneously, when the driver  23  is rotated in the same direction, the threads on the threaded shank  120  of the anterior drive screw  20  engage the complementary threads on the distal threaded opening  30  of the housing  12  causing the anterior drive screw  20  to move in a proximal direction with respect to the housing  12  due to the left-handedness of the threads of the anterior drive screw  20 . As the anterior drive screw  20  moves proximally, it moves the anterior actuator  16  proximally along with it. The leading surfaces  90  of upper and lower anterior actuator segments  88  contact the anterior ramps  58  and slide along the anterior ramps  58  to wedge the upper and lower endplates  14  apart causing the endplates  14  at the distal end of the spacer  10  to separate and increase in height. Hence, the drive screws  20 ,  22  advantageously move in opposite directions from each other, in particular, towards each other to effect expansion of the endplates  14  increasing the distance of the spacer  10  uniformly on both sides simultaneously as both the upper and lower endplates  14  move away from the housing  12  when the driver  23  is rotated in the same direction. The spacer  10  in a condition of uniform parallel expansion is shown in  FIGS. 18-21, 23 and 25 . The degree of expansion is variable with rotation of the driver  23  and the surgeon may advantageously select the desired height of the spacer  10  according to patient anatomy by rotating the driver  23  to expand the spacer  10  as much as needed. Also, rotation of the driver  23  in the opposite direction reduces the height of spacer  10  at both ends simultaneously. The forces exerted onto the endplates  14  from the weight of the spinal column are distributed along two drive screws  20 ,  22  and, hence, there is less friction on the threads of one drive screw requiring less torque to increase or decrease the height of the spacer  10 . Incremental rotation in either direction increases or decreases the height as needed. Both the upper and the lower endplates  14  are wedged apart by both actuators  16 ,  18  and the endplates  14  move away from the longitudinal axis of the spacer  10  uniformly. The ball heads  118  of the drive screws  20 ,  22  face the center of the spacer  10  and their threaded shafts  120  face the distal and proximal ends of the spacer  10 , respectively. Advantageously, both drive screws  20 ,  22  can be rotated from the insertion end which is the proximal end of the spacer  10 . 
     Turning now to  FIGS. 26-28 , anterior angular expansion of the spacer  10  will now be described. When the spacer  10  is inserted into the anatomical disc space while it is in an unexpanded, collapsed state, typically with the use of an insertion instrument (not shown) aligned with the notches  36  and threaded to the collar  32  of the housing  12 . The spacer  10  is inserted in its unexpanded state in order to provide the least invasive approach. Of course, according to surgeon preference, the disc space may be distracted prior to insertion of the spacer  10  and the spacer  10  may be in a semi-expanded configuration, either in angled or parallel expansion. The spacer  10  may be inserted into the disc space while spacer  10  is in a posterior angled configuration in order to help distract the disc space during insertion of the spacer  10 . The posterior angled configuration will be described in greater detail below. The unexpanded state is illustrated in  FIGS. 1-4, 22 and 24 . While the spacer  10  is, preferably in an unexpanded, low profile configuration, the distal end of the driver  23  is mated and inserted into the drive bore  124  of the posterior drive screw  22  and moved in a distal direction relative to the spacer  10  until the distal drive portion  130  of the driver  23  is inserted into the drive bore  124  of the anterior drive screw  20 . The driver  23  is not inserted all the way until abutment with the neck portion  134  is achieved. Instead, insertion of the driver  23  is arrested at a position prior to the proximal drive portion  128  entering the posterior drive screw  22  so that the hexalobe-shaped, bore-engaging cross-section of the proximal drive portion  128  is not engaged with the hexalobe-shaped, driver-engaging cross-section of the drive bore  124  of the posterior drive screw  22  as shown in  FIGS. 26A-26B . This partial insertion leaves the posterior drive screw  22  completely disengaged from the driver  23  and only part of the length of the distal drive portion  130  engaged with the anterior drive screw  20 . As a result, when the driver  23  is rotated in one of a clockwise direction or counterclockwise direction to bring the spacer  10  into an expanded state, the anterior drive screw  20  will only be rotated and the posterior drive screw  22  will not be rotated because in the anterior expansion position, the middle portion  132  having a smaller diameter or a smooth, non-engaging, circular cross-section will be resident along the entire length of the posterior drive screw  20 . Hence, when the driver  23  is rotated, the posterior drive screw  22  will not be rotated and, thereby, remain stationary with respect to the housing  12 . However, when the driver  23  is rotated, the anterior drive screw  20  will move in a proximal direction with respect to the housing  12  due to the left-handedness of the threads of the anterior drive screw  20 . As the anterior drive screw  20  moves proximally, it moves the anterior actuator  16  proximally along with it. The leading surfaces  90  of the upper and lower anterior actuator segments  88  will contact the anterior ramps  58  and slide along the anterior ramps  58  to wedge the upper and lower endplates  14  apart bringing the anterior/distal end of the spacer  10  into an expanded condition forming an angle relative to the un-expanded height of the posterior/proximal end. In anterior angular expansion, the anterior drive screw  20  moves proximally. Only the distal end of the spacer  10  will increase in height as both the upper and lower endplates  14  are wedged apart uniformly oppositely from the longitudinal axis of the spacer  10 ; whereas, the posterior/proximal end of the spacer  10  will remain in an unexpanded state creating an angle of the upper endplate  14  and lower endplate  14  with respect to the housing  12 . The spacer  10  in anterior angular expansion is shown in  FIGS. 27 and 28 . The degree of angulation or angular expansion is variable and incremental with incremental rotation of the driver  23  and the surgeon may advantageously select the desired height of the anterior end of the spacer  10  according to patient anatomy by rotating the driver  23  only as much as is needed to expand and angulate the spacer  10  as desired by the surgeon. The range of angulation of each endplate  14  is approximately between 0 and 15 degrees from the horizontal. To collapse or readjust the spacer  10 , the driver  23  can be rotated in the opposite direction to reduce the height and angle of the spacer  10 . If needed the driver  23  can then again be rotated to increase the height again and repeated as needed for surgeon satisfaction. Variable and incremental rotation reduces the height as needed. Both the upper and the lower endplates  14  are wedged apart by the anterior actuator  16  and both of the upper and lower endplates  14  move away from the longitudinal axis of the spacer  10  uniformly at the anterior end for anterior angular expansion. The driver  23  may be color coded with a color band around the driver  23  at a location to denote the distance to insert the driver  23  for anterior angular expansion. The driver  23  may also be marked with an arrow, a line or other indicia to denote the insertion limit for anterior angular expansion. 
     Turning now to  FIGS. 29-30 , anterior angular expansion may also be combined with uniform parallel expansion in which the posterior drive screw  22  is rotated to increase the height of the proximal end prior to or subsequent to anterior angular expansion in which the driver  23  is positioned such that the distal drive portion  130  engages only with the posterior drive screw  22  as shown in  FIGS. 31A-31B . Alternatively, the driver  23  may be positioned as shown in  FIGS. 17A-17B  for uniform parallel expansion prior to or subsequent to being positioned for anterior angular expansion as shown in  FIGS. 26A-26B . The combination of anterior angular expansion with parallel expansion results in the distal/anterior end having an overall greater height than the proximal/posterior end of the spacer  10  resulting in an expanded and angulated condition of expansion. In essence, customized as well as variable uniform parallel and angular expansion is made possible by positioning the driver  23  to rotate one or both of the anterior and posterior drive screws  20 ,  22  providing the greatest flexibility in angulation and expansion. 
     Turning now to  FIGS. 31-33 , posterior angular expansion of the spacer  10  will now be described. The spacer  10  is inserted into the anatomical disc space while it is in an unexpanded, collapsed state. The unexpanded state is illustrated in  FIGS. 1-4, 22 and 24 . While the spacer  10  is in an unexpanded, low profile configuration, the distal end of the driver  23  is inserted into the drive bore  124  of the posterior drive screw  22  and moved in a distal direction relative to the spacer  10  until, preferably, the entire length of the distal drive portion  130  is inserted into the drive bore  124  of the posterior drive screw  22 . A color-coded marker or other indicia may be provided on the driver  23  to indicate to the user where to stop insertion of the driver  23  for posterior angular expansion. The driver  23  is not inserted all the way until abutment with the neck portion  134  is achieved. Instead, insertion of the driver  23  is arrested when the distal drive portion  130  is engaged with the posterior drive screw  22 , in particular, when the hexalobe-shaped, bore-engaging cross-section of the distal drive portion  130  is engaged with the hexalobe-shaped, driver-engaging cross-section of the drive bore  124  of the posterior drive screw  22  as shown in  FIGS. 31A-31B . This partial insertion of the driver  23  leaves the anterior drive screw  20  completely disengaged from the driver  23 . As a result, when the driver  23  is rotated in one of a clockwise direction or counterclockwise direction to bring the spacer  10  into an expanded state, the posterior drive screw  22  will only be rotated and the anterior drive screw  22  will not be rotated When the driver  23  is rotated in this position, the posterior drive screw  22  moves in a distal direction with respect to the housing  12  due to the right-handedness of the threads of the posterior drive screw  20 . As the posterior drive screw  22  moves distally, it moves the posterior actuator  18  distally along with it. The leading surfaces  90  of the upper and lower posterior actuator segments  116  will contact the posterior ramps  60  and slide along the posterior ramps  60  to wedge the upper and lower endplates  14  apart increasing the distance between the endplates  12  at the posterior end bringing the posterior/distal end of the spacer  10  into an expanded angular condition. In posterior angular expansion, the posterior drive screw  22  moves distally. Only the proximal end of the spacer  10  will increase in height as both the upper and lower endplates  14  are wedged apart uniformly oppositely from the longitudinal axis of the spacer  10 ; whereas, the anterior/distal end of the spacer  10  will remain in an unexpanded state creating an angle of the upper endplate  14  and lower endplate  14  with respect to the horizontal housing  12 . The spacer  10  in a condition of posterior angular expansion is shown in  FIGS. 32 and 33 . The degree of angulation or angular expansion is variable with rotation of the driver  23  and the surgeon may advantageously select the desired height of the posterior end of the spacer  10  according to patient anatomy by rotating the driver  23  only as much as is needed to expand and angulate the spacer  10  as desired by the surgeon. The range of angulation of each endplate  14  is approximately between 0 and 15 degrees from the horizontal. To collapse or readjust the spacer  10 , the driver  23  can be rotated in the opposite direction to reduce the height and angle of the spacer  10 . If needed the driver  23  can then again be rotated to increase the height again and repeated as needed for surgeon satisfaction. Variable rotation increases or reduces the height as needed. Both the upper and the lower endplates  14  are wedged apart by the posterior actuator  18  and both of the upper and lower endplates  14  move away from the longitudinal axis of the spacer  10  uniformly at the posterior end for posterior angular expansion. 
     Turning now to  FIGS. 34-35 , posterior angular expansion may also be combined with uniform parallel expansion in which the anterior drive screw  20  is rotated to increase the height of the distal end prior to or subsequent to posterior angular expansion in which the driver  23  is positioned such that the distal drive portion  130  engages only with the anterior drive screw  22  as shown in  FIGS. 26A-26B . Alternatively, the driver  23  may be positioned as shown in  FIGS. 17A-17B  for uniform parallel expansion prior to or subsequent to being positioned for posterior angular expansion as shown in  FIGS. 31A-31B . The combination of posterior angular expansion with parallel expansion results in the proximal/posterior end having an overall greater height than the distal/anterior end of the spacer  10  resulting in an expanded and angulated condition of expansion. In essence, customized as well as variable uniform parallel and angular expansion is made possible by positioning the driver  23  to rotate one or both of the anterior and posterior drive screw  20 ,  22  providing the greatest flexibility in angulation and expansion. Each of the posterior and anterior ends may be expanded and/or angled independently to a height or angle as desired with incremental rotation in either direction to increase or decrease the angle and/or height with the use of one driver that is positioned variably along the longitudinal axis to effect the different states of expansion/angulation. 
     The expandable interbody spacer  10  is made of any suitable biocompatible material. The expandable interbody spacer  10  may be made from any one or combination of one or more metal such as titanium, ceramic, polymer such as polyether ether ketone (PEEK), carbon fiber reinforced polymer, biomaterial including but not limited to any of a number of biocompatible implantable polymers including PEKK, PEKEK, polyetheretherketone (PEEK) being preferred, titanium ceramic, bone or other material etc. The present invention can be employed and is suitable for use anywhere along the spine including but not limited to cervical, thoracic, lumbar or sacral or between other bony structures outside of the spinal region. Embodiments of the present invention are standalone interbody devices which may be designed in the general style of a TLIF device, PLIF device, ALIF or other device. In addition, the size and/or shape of the basic embodiments disclosed herein may be adapted by one skilled in the art for use in various levels of the spine, namely the cervical spine, thoracic spine and the lumbar spine. Thus, while various embodiments herein may be described by way of example with respect to the lumbar spine such disclosures apply with equal weight to the other levels of the spine. 
     It is understood that various modifications may be made to the embodiments of the interbody spacer disclosed herein. Therefore, the above description should not be construed as limiting, but merely as exemplifications of preferred embodiments. Those skilled in the art will envision other modifications within the scope and spirit of the present disclosure.