Abstract:
An implant for insertion on or into a bone of a patient and for receiving a rod member in an open channel formed between two arms of a head thereof. A closure for closing the channel and capturing the rod in the open channel. The closure including an axially projecting dome that is positioned to engage and abut against the rod in use. The closure also including guide and advancement structure that joins with mating guide and advancement structure in the bone screw head. The guide and advancement structure preferably resists splaying of the arms during installing of the closure. Most preferably, the guide and advancement structure is helically wound mating and interlocking flange form structure or reverse angle thread form structure.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     The present application is directed to a closure for closing a channel in a rod receiving open spinal implant, especially a bone screw, so as to capture and fixedly position the rod relative to the spinal implant, that is used to anchor the rod to the spine.  
         [0002]     Bone screws are utilized in many different medical procedures to secure implants to patient&#39;s bones. Typically, the bone screw is fixedly positioned within a bone and a second implant such as a rod or a rod-like structure is secured to the bone screw. A common use of such implants is to strengthen or replace one or more vertebral bones or to correct curvature of vertebrae within the spine. In such processes, the bone screws are conventionally implanted in various selected vertebrae along the spine and connected by rods or other implants which provide support to the spine where the spine has been damaged or weakened due to accident, disease or genetic predisposition.  
         [0003]     Bone screws utilized for this purpose may be of two types. In a fixed or monoaxial type of bone screw, the head of the bone screw that receives the second implant is permanently fixed relative to a threaded shank that screws into the bone. A second type of bone screw is a polyaxial bone screw where the shank rotates relative to the head during initial installation stages to allow the head to be positioned and thereafter locked in position. Both types of bone screws are utilized in spinal surgery and may be used in combination or separately. However, the occurrence of use of the polyaxial bone screw is much greater than the monoaxial bone screw. Nevertheless, an overall system of the implants should be able to take advantage of either type of bone screw.  
         [0004]     Each of the types of bone screws typically has a head which receives a rod-like implant in an open channel. It is possible to have bone screws where the channel is closed, but those type of bone screws are not the subject of the present application and are difficult to use since a rod must be threaded through some kind of opening for the rod, which makes them relatively difficult to use and, consequently, the number of such bone screws is significantly lower than the type having an open channel. The open channel allows the rod-like implants to be placed within the channel from above by hand or be urged there by some type of tooling. In such bone screws having an open head, there must be some type of closure for closing the head so as to capture the rod within the head and, also preferably to urge the rod into a seated and locked position relative to the head.  
         [0005]     Certain types of prior art closures for this purpose have included nuts which essentially go about the outside of the bone screw head and abut against the rod as the nuts are tightened down on the head. Furthermore, closures have been utilized that slide in from the side and are thereafter locked in place by a set screw or some similar structure. A third type of closure is a plug-like structure having a closure body that lies between opposite or opposed arms of the head and abuts against the rod. The closures of the present application are generally of the later type of closures.  
         [0006]     Certain prior art plug type closures have been threadably received between the opposed arms of the bone screw head using conventional V-shaped thread forms which has resulted in a significant amount of radially outward pressure or force being applied to the arms of the bone screw head. Such outward force may result in splaying of the arms after which the closure becomes loose which may either result in a failure of the implant by allowing the rod to slip relative to the bone screw or the closure may even come completely out of the head of the bone screw for total failure of the implant. In either case, the results are entirely unsatisfactory. Nevertheless, a substantial amount of torque is required to seat the plug against the rod which in turn seats the rod in the head channel so as to prevent relative motion between the rod and the bone screw. Consequently, the need to highly torque the plug functions counter to the need to prevent the arms from splaying. In order to help relieve this problem, certain of the prior art has added structure to the rod engaging lower surface of the closure. In particular, the most common structure has been a central or axial point which has been designed to penetrate into the rod and help lock the rod into place. The second type of structure for this purpose has been a ring-like structure having a sharpened lower edge that is also centered axially with respect to the closure and which cuts into the rod at a radius spaced from the point to give additional penetration into the rod. A third attempt at holding the rod in place at lower torquing pressure has been the use of knurling on the bottom surface of the closure which is designed to cut into and help secure the rod in place.  
         [0007]     Absent the use of an external nut (which undesirably increases the profile of the implant) to prevent splaying in combination with an internal closure, it has been difficult in the prior art to assure that sufficient torque can be applied to the closure to lock the rod in place relative to the head and yet also be sure that no splaying of the arms occurs. Consequently, a need exists for a closure having a rod engaging surface that is suitable for engaging the rod and that can be torqued to a sufficient torque to ensure that no slippage occurs between the rod and the head of the bone screw or other spinal implant while simultaneously ensuring that the opposed arms of the head do not splay.  
       SUMMARY OF THE INVENTION  
       [0008]     The present invention is directed to a spinal implant, especially a bone screw implant for use in humans and especially for use in surgical procedures wherein it is required to secure rods or rod-like implants in vertebrae or other boney structure of a human.  
         [0009]     The bone screws of the present invention include a threaded shank which is adapted to be fixedly secured in a bone and an upstanding head which has an open channel that is adapted to receive a rod or rod-like member. The bone screw may be of either a fixed, monoaxially type or an initially rotatable polyaxially type wherein the shank is rotatable relative to the head until fixed at some point during surgery. The head includes two spaced apart arms which form the open channel therebetween and receive a closure between the arms to close the upper end of the channel. The closure captures the rod or rod-like member and preferably also locks the rod in a fixed relationship to the bone screw.  
         [0010]     The closure comprises a cylindrical body having a helical wound guide and advancement structure on the radially external surface thereof. The guide and advancement structure may be any helical type structure that allows the closure to be rotated and advanced toward the rod and urged against the rod under torque. The guide and advancement structure has a mating guide and advancement structure that comprises two sets of receivers that are located in opposed or facing relationship in the arms. Preferably, the guide and advancement structure either urges the arms inwardly, as in the case of a reverse angle thread form, or alternatively, radially locks the arms in place relative to the closure during positioning of the closure within the channel, as in the case of a flange form, so as to lock the arms in place and prevent splaying thereof. Preferred guide and advancement structure of this type is a helical wound reverse angle thread form or alternatively, a helically wound flange form. The closure also includes a domed lower rod engaging surface. The dome may be radiused with a radius of generation that is larger or smaller than the radius of the cylinder forming the body of the closure, but preferably larger. The dome may also have a curved surface other than a spherical surface forming a complex curve with a central apex or may have a spherical surface along the axis and feather out toward the edges or the like. The dome on the bottom of the closure is in any event centered so that the furthest extension of the dome or apex is along the central axis of the body of the closure and also at the front or leading end of advancement as the closure is inserted into the head of the bone screw.  
         [0011]     The closure may include various types of structure for driving the closure initially and/or removing the closure should removal be necessary. For example, the closure may include a break off head that has a socket type exterior for use in driving and torquing the closure to a preselected torque at which point the break off head breaks away from the body leaving the body of the closure between the arms of the head. The body may also be driven by other types of structures such as a centrally located aperture having, for example, a hexagonal pattern commonly referred to as an Allen type drive or one commonly referred to as a Torx type drive. The closure body may also have alternatively, spaced apertures designed for receiving a tool to be utilized to drive the device or a slot for receiving a screw driver type tool. Certain of the previously described structure may also be utilized to remove the closure, such as the Allen, Torx and spaced apart apertures or additional structure such as a step down bore may be utilized to be engaged with an easy out or the like to remove the closure should removal be necessary to reposition the implant in some manner.  
       OBJECTS OF THE INVENTION  
       [0012]     Therefore, the objects of the present invention are: to provide a closure for use in conjunction with an open channel spinal implant, particularly a bone screw, and especially a vertebral bone screw, for closing a channel in the head of the bone screw and for securing a rod or rod-like member in the head and further lockably positioning the rod relative to the head; to provide such a closure having a domed rod engaging surface; to provide such a closure having guide and advancement structure that resists splaying of upstanding and opposed arms of the head of the bone screw; to provide such a closure having a reverse angle thread as guide and advancement structure; to provide such a closure having an interlocking helical flange as a guide and advancing structure; to provide such a closure adaptable for use with a variety of driving and removal structures; to provide such a closure that can be used in a comparatively reduced volume or low profile implant and torqued to a comparatively high torque relative to the prior art; and to provide such a closure which is easy to use, comparatively inexpensive to produce and especially well adapted for the intended usage thereof.  
         [0013]     Other objects and advantages of this invention will become apparent from the following description taken in conjunction with the accompanying drawings wherein are set forth, by way of illustration and example, certain embodiments of this invention.  
         [0014]     The drawings constitute a part of this specification and include exemplary embodiments of the present invention and illustrate various objects and features thereof. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]      FIG. 1  is a perspective view of a first bone screw closure in accordance with the present invention.  
         [0016]      FIG. 2  is a side elevational view of the first closure.  
         [0017]      FIG. 3  is a top plan view of the first closure.  
         [0018]      FIG. 4  is a bottom plan view of the first closure.  
         [0019]      FIG. 5  is a fragmentary, exploded and perspective view of the first closure in conjunction with a bone screw and rod on a reduced scale and prior to capture of the rod with the closure.  
         [0020]      FIG. 6  is a fragmentary side elevational view of the bone screw and first closure with the closure capturing the rod.  
         [0021]      FIG. 7  is a fragmentary side elevational view of the bone screw and first closure subsequent to break away of a driving head of the closure.  
         [0022]      FIG. 8  is an enlarged cross sectional view of the first closure lockably securing a rod within the bone screw, taken along like  8 - 8  of  FIG. 5 .  
         [0023]      FIG. 9  is a perspective view of a second closure for a bone screw in accordance with the present invention.  
         [0024]      FIG. 10  is a side elevational view of the second closure.  
         [0025]      FIG. 11  is a top plan view of the second closure.  
         [0026]      FIG. 12  is a bottom plan view of the second closure.  
         [0027]      FIG. 13  is a fragmentary, exploded and perspective view on a decreased scale of the second closure utilized in conjunction with the second bone screw prior to capture of a rod in the second bone screw.  
         [0028]      FIG. 14  is a side elevational view of the second bone screw and second closure showing the second closure just prior to placement in the second bone screw and a tool for driving and rotating the second closure.  
         [0029]      FIG. 15  is a fragmentary side elevational view of the second bone screw showing the second closure positioned within the second bone screw and with a driving tool removed therefrom.  
         [0030]      FIG. 16  is a fragmentary cross sectional view on an enlarged scale of the second bone screw and second closure, taken along line  16 - 16  of  FIG. 13 . 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0031]     As required, detailed embodiments of the present invention are disclosed herein; however, it is to be understood that the disclosed embodiments are merely exemplary of the invention, which may be embodied in various forms. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a basis for the claims and as a representative basis for teaching one skilled in the art to variously employ the present invention in virtually any appropriately detailed structure.  
         [0032]     The reference number  1  generally designates a bone screw closure that is illustrated in  FIGS. 1 through 8  that is used within a bone screw  2  to capture and secure a rod or rodlike member  3 . The closure  1  includes a cylindrical body  4  having a radially outer surface  5 , a top or trailing surface  6  and a lower or lead surface  7 .  
         [0033]     Helically wound about the body outer surface  5  is a first guide and advancement structure  10 . The guide and advancement structure  10  includes a flange form  11  which operably joins with a mating structure, discussed below. The flange form  11  includes a root  12  and a crest  13 . Furthermore, the flange form  11  also has a trailing surface  14  and a leading surface  15  which are relative to the forward movement of the closure  1  as it is rotated clockwise about a central axis A and joined with the bone screw  2 . Located on the trailing surface  14  or the leading surface  15  or both is a projection which protrudes rearwardly or frontwardly with respect to the width of the flange form  11  at the root  12  and which interlocks with a guide and advancement mating structure, described below.  
         [0034]     In the illustrated embodiment, the flange form  11  has a protrusion  18  that projects rearwardly from the trailing surface  14 . The flange form  11  utilized in accordance with the present invention may be any structure which effectively locks the closure  1  to the structure within which it is set so as to prevent splaying of the structure upon which mating guide and advancement structure is mounted. Various flange form structures which can be used alternatively are illustrated in applicant&#39;s U.S. Pat. No. 6,726,689, which is incorporated herein by reference.  
         [0035]     The closure lower surface  7  has a dome  19  located thereon. The dome  19  extends greatest from the body  4  along the axis A. The dome  19  in the present embodiment is spherical in shape and, in particular, is a partial sphere that has a uniform or constant radius of generation.  
         [0036]     However, it is foreseen that in certain embodiments the radius may vary depending upon the needs and desires of the particular structure and the dome  19  may have shape that is only partly a spherical curved surface or some other shape. The dome  19  may be simple a curved surface that allows greatest projection along the axis. That is, the dome surface  7  could be radiused at the location of greatest projection and feathered along the periphery so as to not have a continuous uniform radius of generation throughout, but rather a continually changing radius of generation along at least the length thereof. Preferably, the dome  19  is smoothly curved where the dome  19  intersects with axis A.  
         [0037]     The closure  1  also includes a break off head  20  that is secured to the body  4  by a break off region  21  that is designed to allow the head  20  to break from the body  4  at a preselected torque, for example, 150 inch pounds. The break off head  20  has an external radial outward surface  23  with six planar facets  24  so as to form a structure designed to be received within a socket of a driving type tool  30  with a similar receiving shape. The break off head  20  has a central tool receiving bore  31 . At the bottom of the bore  31  is a step down region  33  having two separate steps  34  and  35  that are sized and shaped to provide evenly spaced shoulders  36  and  37  that provide edges are suitable for engagement by an easy out tool (not shown) for purposes of removal.  
         [0038]     The closure  1  is received in the bone screw  2 . The illustrated bone screw  2  has a shank  41  and a head  42 . The bone screw shank  41  has a helical wound thread  44  thereon and is designed to be threadedly received within a bone  45  of a patient. The shank  41  is received and captured in a capture ring  46  such that once joined, the shank  41  and ring  46  become preferably permanently secured to one another. The head  42  includes a body  48  and a pair of upstanding arms  49  and  50 . The head  42  also has an internal generally hemispherically shaped chamber  55  and a lower aperture  56 . An upper end of the shank  41  is received through the aperture  56  while the capture ring  46  is received within the chamber  55  and initially is slidably and rotatably received in the chamber  55 , so that the shank  41  is initially rotatably relative to the head  42 . The shank  41  has an upstanding projection  60  that protrudes axially upward therefrom. When received in the chamber  55 , the projection extends upwardly through the chamber  55 . The head  42  has a channel  62  that is located between the arms  49  and  50 . The projection  60  extends into the channel  62 . During use, the rod  3  is located or positioned within the channel  62  and secured in place therein by the closure  1 . In particular, the projection  60  engages the rod  3 , as seen in  FIG. 8  when the rod  3  is in the channel  62 . The closure  1  is rotatably advanced into a pair of mating guide and advancement structures  72  and  73  on the arms  49  and  50  until the dome  19  engages the rod  3 , especially in line with the axis A. The mating guide and advancement structures  72  and  73  include a mating flange form  75  having a counter extending and locking projection  79  and also having a foot  80  and a crest  81 . The dome  19  especially engages the rod  3  at an apex  74  as seen in  FIG. 8 . The closure  1  is torqued until a preselected pressure is reached at which point the closure  1  at the apex  74  abuts the rod  3  which in turn is urged toward but not completely to the bottom of the channel  62 . In turn, the rod  3  braces against the shank projection  60  which urges the capture ring  46  to fixedly seat in the chamber  55 . Thereafter, the head  42  is no longer rotatable relative to the shank  41 , but rather is locked in position.  
         [0039]     Tool gripping apertures  69  are located on opposite sides of the head  48  for gripping by tools (not shown) for various purposes.  
         [0040]     The reference numeral  100  generally designates a second embodiment of a closure in accordance with the present invention which is illustrated in FIGS.  9  to  16 . The closure  100  is utilized with a bone screw  102  and a rod  103 .  
         [0041]     The closure  100  has a generally cylindrical or plug shaped body  110  with a cylindrically shaped radial outer surface  111  and a central axis of rotation B. The closure  100  has an upper, top or trailing surface  112  and a lower, bottom or lead end  113 .  
         [0042]     Located on the lead end  113  is a convex shaped region or dome  115  that projects outwardly from the body  110  along the axis B (downwardly in  FIG. 10 ) and has an apex  116  whereat the dome  115  intersects the axis B.  
         [0043]     In this embodiment, dome  115  covers the entire bottom end  113  of the closure  100 . In contrast, the dome of the prior embodiment covered only a portion of the lower surface of the closure  1 . It is foreseen that domes in accordance with the invention may cover more or less of the bottom surface and may vary in radius of generation or curvature. However, in a spherical dome having a radius of generation that is substantially greater than the radius of the closure surface  111  and which is located to project in the range of 5% to 15% beyond the length of the cylindrical body  110 . It is preferred that the dome  115  be smooth and convex whereat the axis B intersects with the dome  115  and not pointed. However, in certain embodiments, it is foreseen that the dome could be at least partially covered with knurling or the like to provide additional gripping during usage.  
         [0044]     Located on the closure cylindrical surface  111  is a guide and advancement structure  118  which in the present embodiment is a helical wound reverse angle thread form  119 . The guide and advancement structure  118  acts cooperatively, as described below with the bone screw  102  to allow the closure  100  to be inserted into and rotated relative to the bone screw  102  and to guide and advance the closure  100  along the axis B as the closure  100  is rotated clockwise or to reverse direction when rotated counterclockwise. The guide and advancement structure  118  resists splaying in the bone screw  102  as forces applied to the closure  100  are conveyed by the reverse angle thread form  119  during application of clockwise rotational torque into a downward axial force and inwardly directed radial force. It is foreseen that other types of guide and advancement structure could be utilized. For example, a buttress thread form or a square thread form may be effectively used which theoretically has little or no radially outward directed forces, especially if the arms are thickened to resist splaying. Other thread forms may also be used with the dome  115 .  
         [0045]     The thread form  119  has a root  121  and a crest  122 . Further, the thread form  119  has a lead surface  123  and a trailing surface  124  (described relative to the position thereof during insertion of the closure  100  into the bone screw  102 ). In a reverse angle thread, the trailing surface  124  from the root  121  to the crest  122  extends at an angle rearwardly from a perpendicular line relative to the axis B. Normally, the trailing surface  124  is at an angle between 1 and 20 degrees relative to such a perpendicular.  
         [0046]     Located in the closure top surface  112  is an aperture  130  that is centrally located and axially extending. The aperture  120  is faceted with six equal walls to form an inner faceted wall  131  sized and shaped to receive an Allen style driving tool  134  with a mating surface  135  thereon.  
         [0047]     The bone screw  102  includes a head  140  and a threaded shank  141 . The bone screw  102  is of a fixed headed bone screw type as opposed to the polyaxial type of the first embodiment wherein the head can rotate relative to the shank prior to being locked in place by torquing of the closure. The shank  141  of the bone screw  102  is operably screwed into a bone  142 , such as a vertebral body in the spine of a patient.  
         [0048]     The head  140  has a body  144  with a pair of upstanding spaced arms  145  and  146 . The body  144  in conjunction with the arms  145  and  146  form between and define a rod receiving channel  150  having a width approximately equal to the diameter of the rod  103  and a seat  151  which snugly receives the rod  103  during installation, as seen in  FIG. 16 .  
         [0049]     Located on inwardly facing surfaces of each of the arms  145  and  146  are mating guide and advancement thread forms  154  and  155  respectively that are sized and shaped to receive the closure thread form  119  during installation and rotation of the closure  100  between the arms  145  and  146 . Each of the thread forms  154  and  155  have a root  158  and a crest  159 , as well as a first mating surface  160  that mates with the closure lead surface  123  and a second mating surface  161  which mates with the closure trailing surface  124 .  
         [0050]     It is foreseen that a closure of the present invention may have a wide range of structures for installing, driving and removing the closure. In the first embodiment, the break off head  20  is utilized for driving and torquing the closure  1  in the bone screw  2 , while the step down bore region  33  may be effectively used with an easy out (not shown) for removal. In the second embodiment of the closure  100 , the Allen tool receiving aperture  130  receives the tool  134  for both installation and removal. It is foreseen that structures including spaced apertures or other structure may effectively be used to aid in installing or removing the closure from the head of the bone screw.  
         [0051]     In use, the bone screw  102  is screwed into and secured to a bone  142 , such as is shown in  FIG. 15 . The rod  103  is placed in the channel  150  and the closure  100  is inserted into the channel  150  by aligning the thread form  119  with the mating thread forms  154  and  155  on the head  140  and rotating clockwise using the tool  134  to rotate the closure  100  and drive the apex  116  against the rod  103 , as shown in  FIG. 15 , until a desired torque is achieved. The tool  134  is then removed. If it is later desired to remove the closure  100 , the tool  134  is reinserted into the aperture  130  and the process is reversed.  
         [0052]     The apex  116  of the dome  115  abuts against and secures the rod  3  in a fixed position both relative to the bone screw  2  and the closure  1 . Because the guide and advancement structure of both this embodiment and the first illustrated embodiment resist splaying of the arms  145  and  146  by directing forces inward in the case of the present embodiment due to the reverse angle thread and by radially locking together the closure and arms in the first embodiment using the flange form, greater torque can be applied to the closure  1  and  100  in comparison to the prior art which allows the smooth surfaced domes  19  and  115  to be effective in fixing the rods relative to the respective heads and closures.  
         [0053]     It is to be understood that while certain forms of the present invention have been illustrated and described herein, it is not to be limited to the specific forms or arrangement of parts described and shown.