FASTENING SYSTEM WITH DEFORMABLE SET SCREW

Abstract: An implanted anchoring assembly (12) includes an anchor (13) with a hook (18) for fixing the anchor (13) to a vertebra or other osseous body, and a recess (19) with a cylindrical profile for receiving an elongate cylindrical stabilizing rod (11). The anchoring assembly (12) further includes a set screw (14) with external threads corresponding to internal threads (22) in the anchor (13). Rotating the set screw (14) within the anchor (13) while the rod (11) is nested within the recess advances the screw (14) toward the rod (11). A leading edge region of the set screw includes a thin wall (27) that extends generally transversely across the set screw (14) and forms a convex/concave dome (27) convex in the direction toward the stabilizing rod (11). After the dome (27) contacts the rod (11), further turning of the set screw (14) deforms the dome (27), whereby the outer surface of the dome (27) tends to conform to the rod (11) to more securely retain the rod (11) within the anchor recess (19).

Description: FASTENING SYSTEM WITH DEFORMABLE SET SCREW Background of the Invention The present invention relates to orthopedic implant components, and more particularly to anchors for securing a stabilization rod relative to bone material. An increasingly accepted surgical technique for vertebral fusion, and for treating spinal disorders such as scoliosis, involves the use of hardware for the internal fixation or stabilization of osseous components with respect to one another. Such internal fixation systems typically include an elongate rod and several anchors. Each anchor includes a hook or other structure to facilitate fixation to bony material, and a coupling structure to form a releasable engagement with the elongate rod. Thus, a rod in combination with two of the anchors can be used to secure two vertebrae or other bone components integrally with respect to one another. One example of this approach is a spinal implant assembly shown in U.S. Patent No. 5,496,321 (Puno, et al.). The implant assembly includes an elongate rod and a top loading anchor with a hook for fixation to a spinal member. The anchor has a seat that forms a recess to receive the rod, and opposite side walls with channels that extend longitudinally, i.e., parallel to the length of the rod. A closure member also has a recess to accommodate the rod, and further has longitudinally extending shoulders on opposite sides. The shoulders are accommodated by the channels, to allow a longitudinal sliding of the closure member into engagement with the seat. An internally threaded opening runs transversely with respect to the rod through the closure member, and accepts a set screw that has external threads. A counter sink in the set screw receives a driving tool, and a beveled tip with serrations improves a grip of the set screw on the rod. While this arrangement is workable in a variety of situations, it requires a special instrument with a pronged head housing a boss and a spring-loaded plunger ball, shown in Figure 11 of the patent. Aside from requiring this tool for its insertion, the closure member does not contribute to the gripping force on the rod, but rather is urged away from the rod as it resolves forces between the seat and the set screw. More generally, the number of components required adds to the cost and complexity of the system. Therefore, it is an object of the present invention to provide a fixation system that more reliably secures vertebrae or other osseous components with respect to one another. Another object is to provide an anchor assembly for securing an elongate stabilizing rod to bony material, that minimizes the number of moving parts and does not require complex tooling in use. A further object is to provide an anchoring assembly that more readily conforms to the shape of a stabilizing rod or other stabilizing member, to provide a more secure frictional hold on the stabilizing member. Yet another object is to provide implantable orthopedic fixation systems and anchoring assemblies that are more reliable, less complex, and less costly to manufacture. Summary of the Invention To achieve these and other objects, there is provided a system for fixing two or more osseous bodies with respect to one another. The system includes a stabilizing member, and an anchor adapted for a fixation with respect to a first osseous body. The anchor has a recess for receiving the stabilizing member. The system also includes a retainer adapted for a coupling to the anchor in a manner that allows advancing the retainer toward the stabilizing member when the stabilizing member is disposed in the recess. The retainer includes a leading edge region with a leading edge surface. The leading edge surface engages the stabilizing member when the retainer is advanced to a predetermined location relative to the anchor. The leading edge region is adapted to undergo a deformation responsive to advancing the retainer beyond the predetermined location. As a result of this deformation, the leading edge surface tends to conform to the stabilizing member. In addition, the system includes a coupling structure for coupling the stabilizing member integrally to a second osseous body. Thus, the coupling structure cooperates with the anchor, the retainer and the stabilizing member to integrally couple the first and second osseous bodies. In a preferred internal fixation system, the stabilizing member is an elongate circular-cylindrical rod used in combination with several two-component anchor assemblies. Each anchor assembly includes a unitary anchor member providing a recess to accept the rod. Each retainer engages the rod and urges the rod securely against the recess, to establish a frictional engagement that integrally couples the implant anchor assembly and the rod. To ensure a more secure coupling, a lead end region of the retainer is deformable. The leading edge region is the portion of the retainer that first encounters the rod and establishes the surface engagement as the retainer is moved toward the rod during coupling. The leading edge region preferably can be plastically deformed, whereby the leading edge of the retainer tends to conform to the profile of the rod, increasing the surface area over which the retainer and rod are contiguous. This increases the capacity of the anchor assembly to maintain the integral coupling, despite longitudinal, rotational and tortional forces acting along the rod/anchor interface. The preferred retaining member is a set screw with external buttress form threads adapted to engage internal threads formed along the side walls of the anchor. The set screw has a hexagonal or other non-circular depression to accept a drive tool. Because of the deformable end region, the set screw can be turned inwardly beyond the point at which it engages the rod. In a preferred version, the deformable portion of the set screw includes a thin wall extending transversely across the set screw. The exposed, outer surface of the wall provides the leading edge surface of the screw. Before the set screw contacts the rod, the thin wall is convex in the forward direction to form a dome. The rod, responsive to forward turning of the set screw after engagement, exerts a force on the wall, eventually exceeding the elastic limit of the wall. The result is a permanent deformation of the wall, such that the leading edge surface not only engages the rod, but substantially conforms to the surface profile of the rod. The thin wall exerts a clamping force against the rod, cooperating with the recess of the anchor to provide a firm, frictional coupling. Further, because of its conformity to the rod, the thin wall tends to resist any tendency of the set screw to work loose. The implant anchor assemblies are easy to use, and each can consist entirely of the anchor and the set screw. No further components are required. The set screw is secured with a relatively simple tool on the order of the screw driver, typically with a hexagonal blade corresponding to the drive opening in the set screw. Thus in accordance with the present invention, a spinal implant assembly employs anchor assemblies with deformable set screws, eliminating the need for auxiliary components and specialized installation tools. The surface of each set screw that confronts the rod assumes a shape to substantially conform to the profile of the rod as the screw is turned inwardly against the rod, establishing an effective integral coupling of the anchor and rod. The Drawings Figure 1 is a prospective view of an elongate stabilizing rod and an implant anchor assembly constructed in accordance with the present invention, before the assembly is coupled to the rod; Figure 2 is an end view of the rod and anchor assembly, again prior to coupling; Figure 3 is a prospective, cutaway view of a set screw of the anchor assembly; Figures 4-7 are schematic representations of set screw/rod interaction during coupling; Figure 8 is a prospective view of the rod and the assembly, after coupling; Figure 9 is a sectional view of the set screw, after coupling; Figure 10 illustrates a smear path along the interface of the set screw leading edge and the rod; Figure 11 is a top view of an alternative set screw, showing an alternative non- circular drive opening; Figure 12 illustrates a blade of a drive tool used with the set screw shown in Figure 11 ; Figure 13 illustrates a hexagonal blade of a drive tool for use with the set screw having a hexagonal drive opening; Figure 14 illustrates an alternative embodiment fixation system including three anchor assemblies coupled to three vertebrae through respective brackets and bone screws, in combination with a single stabilizing rod; and Figure 15 is a sectional elevation of an alternative embodiment set screw. Detailed Description of the Preferred Embodiments Turning now to the drawings, Figures 1 and 2 illustrate a spinal implant system 10 including an elongate stabilizing rod 11 and an implant anchor assembly 12 adapted for releasable fixation to the rod. Typically at least two such assemblies are coupled to the rod, and further are joined to respective vertebrae or other bone components to fix the vertebrae with respect to one another. The anchor assembly includes two unitary components: an implant anchor 13 and a set screw 14. The implant anchor includes a base 15, side walls 16 and 17 extending upwardly from the base, and a hook 18 extending below the base. Hook 18 is used to secure anchor 13 to bony material. As an alternative, a bracket with an opening to accommodate a bone screw could extend from base 15. The side walls and base cooperate to provide a recess 19 with a circular profile conforming to the circular profile of rod 11, slightly larger to facilitate a nesting of the rod within the recess. The side walls have respective interior surfaces 20 and 21 that are arcuate and shaped to provide internal threads 22. as best seen in Figure 1. Set screw 14 is provided with external threads 23 adapted for threaded engagement with internal threads 22 of the anchor. The set screw has a bottom or leading edge 24 that is convex in the downward direction as viewed in Figures 1 and 2. As seen in Figure 1, the set screw includes a centrally positioned hexagonal drive opening or recess 25, open to the top of the screw, adapted to receive a drive tool with a hexagonal blade. The drive tool can be similar to a screwdriver. From Figure 3, it is seen that hexagonal recess 25 extends inwardly (downwardly as viewed in the figure) to an annular rim 26. Immediately below rim 26 is a thin, disk-like wall 27. Wall 27 is concave/convex and substantially uniform in thickness, to form an inverted dome that protrudes below the rest of the set screw. Thus, the lowest, or most outward portion of the set screw is the central region of wall or dome 27. The anchor and set screw can be constructed of titanium or other suitable body compatible metals. Preferably the leading edge of the screw is plastically deformable as described herein. However, a screw that is elastically deformable, at least along its leading edge, can be used as an alternative. The interaction of set screw 14 and rod 11, as the set screw is turned into anchor 13 to couple the rod and the anchor assembly, is illustrated schematically in Figures 4-7. Figure 4 is a partial top view showing dome 27 positioned above rod 11. Clockwise rotation of the set screw, indicated by the arrow, turns the set screw toward the rod, in a forward direction or downward as viewed in Figures 1 and 2. Figure 5 is a sectional view taken along the line 5-5 in Figure 4. As seen in Figure 5, the outside or bottom surface of dome 27 provides leading edge 24. As the set screw is turned downward, an axially centered, forward-most region 29 of the leading edge encounters rod 11. This can be conveniently thought of as an initial contact point, or predetermined location of the set screw relative to the anchor and rod. Figure 6, a sectional view similar to Figure 5, shows the deformation of dome 27 resulting from downward travel of the set screw beyond the initial contact point illustrated in Figure 5. With the section taken through the axial center, the dome deformation appears substantially symmetrical. The deformation of dome 27 illustrated in Figure 6 is at least in part a plastic deformation, the result of forces applied by rod 1 1 above the elastic limit of the material forming dome 27. Figure 7 shows a section taken along a line spaced apart from the axial center of the set screw. Travel of dome 27 relative to rod 11, at least over a short distance, can be considered linear, in the direction of the arrow in Figure 7. Thus, the figure illustrates a region 30 of the dome that has encountered rod 11 and has undergone plastic deformation, and a region 31 of the dome that has not yet encountered the rod. When deformed, dome 27 undergoes a permanent set, yet retains elastic properties. Accordingly, once set screw 14 is turned sufficiently to carry region 30 beyond rod 11, there is a slight elastic recovery of the dome that moves leading edge 24 downward, to a point below the top of rod 11 as shown. Region 30, however, remains above region 31, reflecting the permanent set in that part of the dome that has traveled past the rod. The permanent set, in combination with at least a slight elastic recovery, affords several advantages. First, because of the retained elasticity, dome 27 exerts a radially inward force against rod 11, cooperating with the radially inward force due to anchor 13 along recess 19 to provide a secure frictional coupling. Second, because of the partial elastic recovery, dome 27 exerts radially inward force on both sides of a vertical line 32 bisecting rod 11. As a result, the horizontal components of these radial forces act both leftward and rightward as viewed in Figure 7. The counterbalancing horizontal force components of the rod upon the dome likewise act in both directions, counteracting any tendency of dome 27 to move relative to rod 11. In particular, the horizontal force components exerted by rod 11 against region 30 of the dome effectively counteract any tendency of set screw 14 to work loose, which would require movement of dome 27 to the right of rod 11 as viewed in Figure 7. Figure 8 shows the completed coupling of implant anchor assembly 12 and rod 1 1. The rod is captured between recess 19 of the anchor and leading edge 24 of the set screw, two surfaces that substantially conform to the cylindrical surface of the rod and thus cooperate to establish a firm frictional coupling. In Figure 9, set screw 14 is shown in sectioned elevation, following its removal from anchor 13 and the rod. A dimple 33 is formed in dome 27, representing the permanent set due to plastic deformation of the set screw material. Figure 10 shows leading edge 24 of the set screw, again upon removal after its coupling to rod 1 1. The smear pattern illustrates the nature of the plastic deformation. Use of elongate rod 11 and two or more anchor assemblies 12 in an internal fixation procedure requires an initial determination of fixation locations, e.g., vertebrae, where two or more of the anchor assemblies will be fixed to osseous material. Typically the elongate rod is linear, requiring in-line positioning of the anchor assemblies whenever three or more are used. Each anchor 13 is either integrally secured with its recess aligned in the direction of longitudinal rod extension, or mounted in a manner that allows orienting the anchor to properly align its recess. After mounting the anchors, the surgeon places elongate rod 11 into the recesses of the anchors, then, if necessary, slides the rod longitudinally relative to the anchors to a desired axial position. With rod 11 thus positioned, the set screw of each anchor assembly is turned into its associated anchor member to frictionally and integrally secure rod 1 1 within the anchor assembly in the manner previously described. Each set screw is turned into its associated anchor member by a relatively simple drive tool that resembles a conventional screwdriver with a hexagonal head, or alternatively another head with a non-circular profile to match a non-circular recess in the set screw. With the anchor member already secured, the set screw is the only part requiring movement during closure of the anchoring system. There are no intermediate parts to align during closure. As previously noted, a leading edge region of the set screw plastically deforms during closure, and thus assumes a profile substantially the same as that of the rod. This considerably increases the area of set screw/rod surface contact, for a more reliable coupling. A further benefit of the deformable set screw is that the desired surface conformity and contiguous engagement are achieved without the need for any auxiliary parts or components. The internal anchoring assembly can consist entirely of anchor member 13 and set screw 14. Free of the concern for additional components and their proper positioning and installation, the surgeon can direct closer attention to the procedure at hand. Figure 11 is a top view of an alternative embodiment set screw 34 with a recess or drive opening 35 having a non-circular profile as an alternative to the hexagonal profile of recess 25. Set screw 34 is frequently referred to as a "torx" set screw. Figure 12 illustrates a blade 36 of a drive tool suitable for rotating set screw 34. Figure 13 shows a hexagonal blade 37 of a drive tool used in conjunction with set screw 14. A variety of other non-circular profiles also are suitable. Figure 14 shows an alternative embodiment fixation system 38 employing three anchoring assemblies 39, 40 and 41 in combination with a single stabilizing rod 42. Each of the anchoring assemblies includes an anchor 43 and a set screw 44 threadedly engaged with the anchor and having a leading edge contiguous with rod 42 and plastically deformed as previously described. Anchors 43 include brackets 45 and bone screws 46. Each bone screw passes through an opening in its associated bracket. The screws and openings can be configured, for example, as described in U.S. Patent Application Serial No. 09/314,617 entitled "Anterior Cervical Plate and Fixation System," filed May 19, 1999 and assigned to the assignee of the present application. Anchor assemblies 39-41, coupled respectively to vertebrae 47, 48 and 49, cooperate with rods 42 to integrally couple the vertebrae relative to one another. Figure 15 is a sectional elevation showing an alternative embodiment set screw 50 engaged with a stabilizing rod 51. Rod 51 is nested within the recess of an anchor not shown in the figure. A leading edge 52 of the set screw is elastically deformed along a region of contact with the rod, forming a dimple indicated at 53. Set screw 50, formed for example of a body compatible, elastically compressible polymer, includes external threads 54 that interact with corresponding internal threads (now shown) of the anchor. The set screw incorporates a hexagonal recess or drive opening 55. It should be understood that while the present invention has been described in connection with fixing vertebrae relative to one another, other bony material or osseous bodies can be fixed with respect to one another using the fixation systems and components described. Certain applications may require non-linear stabilizing members in lieu of the linear stabilizing rods, or may require stabilizing rods having non-circular profiles. Also, fixation systems may employ the anchoring assemblies described, in combination with other anchoring components that may not involve set screw deformation. Thus, the present invention provides an internal fixation system that is easier to use. Each anchor assembly requires just two mating parts, and assembly does not require any complex, specialized tools. The deformable set screw, with its leading edge conforming substantially to the profile of the rod, more reliably cooperates with the anchor member to capture the rod in a sound, integral frictional engagement. The simpler, more reliable anchor assemblies reduce both the cost and the complexity of the internal fixation system.

Claims: What is Claimed is: 1. A system for fixing two or more osseous bodies with respect to one another; including: a stabilizing member; a first anchor adapted for a fixation with respect to a first osseous body and having a recess for receiving the stabilizing member; a first retainer, adapted for a coupling to the first anchor in a manner that allows advancing the first retainer toward the stabilizing member when the stabilizing member is disposed in the recess, said first retainer including a leading edge region having a leading edge surface that engages the stabilizing member when the first retainer is advanced to a predetermined location relative to the first anchor, wherein said leading edge region is adapted to undergo a deformation responsive to advancing of the first retainer beyond the predetermined location, and the leading edge surface, due to said deformation, tends to conform to the stabilizing member; and a coupling structure for coupling the stabilizing member integrally to a second osseous body, thereby cooperating with the first anchor, first retainer and stabilizing member to integrally couple the first and second osseous bodies. 2. The system of claim 1 wherein: the stabilizing member comprises an elongate rod. 3. The system of claim 2 wherein: the elongate rod has a circular-cylindrical shape. 4. The system of claim 1 wherein: the first anchor includes a hook for securing the anchor to the first osseous body. 5. The system of claim 1 wherein: the recess is shaped to conform to the stabilizing member. 6. The system of claim 1 wherein: said leading edge region of the retainer is plastically deformable. 7. The system of claim 1 wherein: said leading edge region of the retainer is elastically deformable. 8. The system of claim 7 wherein: said leading edge region further is plastically deformable in response to an external force that exceeds an elastic limit of the leading edge region. 9. The system of claim 1 wherein: the retainer is substantially symmetrical about a retainer axis, and the leading edge region comprises a thin wall extending generally transversely across the retainer. 10. The system of claim 9 wherein: said thin wall is substantially uniform in thickness in the direction of the retainer axis, and has a concave/convex configuration to define the leading edge surface as convex in the direction of retainer advancement. 11. The system of claim 1 wherein: the first retainer is removably coupled to the first anchor. 12. The system of claim 11 wherein: the first retainer comprises a set screw having external threads, and the first anchor includes at least one anchor wall having internal threads adapted for engagement with the external threads of the set screw. 13. The system of claim 12 wherein : the first retainer includes a non-circular drive opening adapted to receive a drive tool for rotating the first retainer about a longitudinal axis thereof. 14. The system of claim 13 further including: a drive tool having a non-circular blade shaped to conform to the non-circular drive opening. 15. The system of claim 1 wherein: the coupling structure comprises a second anchor similar in construction to the first anchor and adapted for a fixation with respect to the second osseous body; and a second retainer similar in construction to the first retainer, adapted for a coupling to the second anchor that allows advancing the second retainer toward the stabilizing member when the stabilizing member is disposed in a recess of the second anchor, wherein a leading edge region of the second retainer is deformable responsive to further advancement of the retainer after a leading edge surface of the second retainer contacts the stabilizing member, whereby the leading edge surface tends to conform to the retaining member. 16. An anchoring assembly for securing a stabilizing rod integrally with respect to osseous material, including: an anchoring component adapted for an integral fixation with respect to osseous material and having a recess shaped for receiving a stabilizing rod in a nesting engagement within the recess; and a retainer adapted for a coupling to the anchor in a manner that allows advancing the retainer toward a stabilizing rod in said nesting engagement, said retainer including a leading edge region with a leading edge surface positioned to engage the stabilizing rod upon advancing of the retainer to a predetermined location relative to the anchor; and wherein the leading edge region is adapted to undergo deformation responsive to advancing of the retainer beyond the predetermined location, whereby the leading edge surface tends to conform to the stabilizing rod, and thereby tends to retain the stabilizing rod in said nesting engagement. SUBSTrTUTE SHEET (RULE 26) 17. The assembly of claim 16 wherein: the first anchor includes a hook for securing the anchor to the first osseous body. 18. The assembly of claim 16 wherein: the recess is shaped to conform to the stabilizing member. 19. The assembly of claim 16 wherein: said leading edge region of the retainer is plastically deformable. 20. The assembly of claim 16 wherein: said leading edge region of the retainer is elastically deformable. 21. The assembly of claim 20 wherein: said leading edge region further is plastically deformable in response to an external force that exceeds an elastic limit of the leading edge region. 22. The assembly of claim 16 wherein: the retainer is substantially symmetrical about a retainer axis, and the leading edge region comprises a thin wall extending generally transversely across the retainer. 23. The assembly of claim 22 wherein: said thin wall is substantially uniform in thickness in the direction of the retainer axis, and has a concave/convex configuration to define the leading edge surface as convex in the direction of retainer advancement. 24. The assembly of claim 16 wherein: the retainer is removably coupled to the anchor. 25. The assembly of claim 24 wherein: the retainer comprises a set screw having external threads, and the anchor includes at least one anchor wall having internal threads adapted for engagement with the external threads of the set screw. 26. The assembly of claim 16 wherein: the retainer includes a non-circular opening adapted to receive a drive tool for rotating the retainer about a longitudinal axis thereof. 27. A body implantable combination including: an elongate stabilizing member having a longitudinal axis and a substantially uniform transverse profile; an anchoring component adapted for an integral fixation with respect to an osseous body, and having a recess adapted to receive the stabilizing member; a retainer adapted for a coupling to the anchoring component in a manner that allows advancing the retainer toward the stabilizing member when the stabilizing member is disposed within the recess, said retainer including a leading edge region with a leading edge surface positioned to engage the stabilizing member upon advancing the retainer to a predetermined location relative to the anchoring component; and wherein the leading edge region is adapted to undergo a deformation responsive to advancing the retainer beyond the predetermined location, whereby the leading edge surface tends to conform to the profile of the elongate stabilizing member. 28. The implantable combination of claim 27 wherein: the retainer includes external threads, and the anchoring component includes at least one anchor wall having internal threads adapted to receive the external threads, whereby the retainer is advanced by rotating the retainer about a retainer axis. 29. The combination of claim 28 wherein: SUBSTTΓUTE SHEET (RULE 26) the retainer axis is substantially perpendicular to the longitudinal axis of the stabilizing member during said advancement. 30. The combination of claim 28 wherein: the retainer includes a non-circular drive opening adapted to receive a drive tool for rotating the retainer. 31. The combination of claim 30 further including: a drive tool having a non-circular blade shaped to conform to the non-circular drive opening. 32. The combination of claim 27 wherein: the elongate stabilizing member comprises a cylindrical rod, and said profile is circular. 33. The combination of claim 27 wherein: said leading edge region of the retainer is plastically deformable. 34. The combination of claim 27 wherein: said leading edge region of the retainer is elastically deformable. 35. The combination of claim 34 wherein: said leading edge region further is plastically deformable in response to an external force that exceeds an elastic limit of the leading edge region. 36. The combination of claim 27 wherein: the retainer is substantially symmetrical about a retainer axis, and the leading edge region comprises a thin wall extending generally transversely across the retainer. 37. The combination of claim 36 wherein: said thin wall is substantially uniform in thickness in the direction of the retainer axis, and has a concave/convex configuration to define the leading edge surface as convex in the direction of retainer advancement.

Inventors: CAMPBELL CHRISTOPHER M, TOKISH LEONARD J JR, CAMPBELL, CHRISTOPHER, M., TOKISH, LEONARD, J., JR.
Applicants: ALPHATEC MFG INC, ALPHATEC MANUFACTURING, INC.
Application year: 1999
Main classification: A61B17
IPC codes: A61B17