WO1995014176A1

WO1995014176A1 - Self-locking set screw for spinal fixation system

Info

Publication number: WO1995014176A1
Application number: PCT/US1994/013316
Authority: WO
Inventors: Randall R. Theken; Philip A. Mellnger; J. Abbott Byrd, Iii; Rolando M. Puno
Original assignee: Cross Medical Products, Inc.
Priority date: 1993-11-19
Filing date: 1994-11-17
Publication date: 1995-05-26
Also published as: AU1290895A

Abstract

A set screw (10) for exerting compressive forces between a rod-holding member and a rod (51) of a spinal fixation system to hold the components thereof together in fixed relationship to each other. The screw (10) includes an externally threaded cylindrical portion (25) having at one end an internal hex (20) for allowing the set screw (10) to be screwed into contact with a rod (51) contained within a rod-receiving channel (47) of a rod-holding member, and a frustoconical portion (30) having a large radius base which is coextensive with and integrally attached to the other end of the cylindrical portion (25) of the set screw (10). The frustoconical portion (30) has a multi-faceted external surface (35) and has an internal recess (36) at its small radius base. The internal surface of the recess (36), together with the external surfaces of adjacent facets (35), forms a plurality of point contacts (39) disposed radially about the axis of the set screw (10) in a plane perpendicular to the axis of the set screw (10). The set screw (10) is harder than the rod (51).

Description

SELF-LOCKING SET SCREW FOR SPINAL FIXATION SYSTEM

FIELD OF INVENTION The invention pertains generally to orthopedic implants and more particularly to a low-abrasion set screw for use with a spinal fixation system.

BACKGROUND As surgical techniques have advanced, it has become increasingly common for surgeons to use specially designed hardware for the internal fixation of bones. A particular area of concentration for the recent develop¬ ment of this technology has been the spine. Internal fixation is used most frequently in the spine in conjunc- tion with vertebral fusion, and also for the manipulation of the spine to correct spinal deformities, such as scoliosis.

There are several important criteria for a hardware system which is used internally for spinal fixation:

1. . The implant should provide rigidity as indicated, generally along the long axis of the patient's spine.

2. The system should be able to accommodate a broad variation in the size of the spinal member with which it is used; for example, a surgeon may wish to use the implant on a variety of individuals. In addition, the difference in the area and size of the point of fixation is compounded by the change in the shape of the vertebrae over the length of the full spinal column. Since it is an advantage to allow the surgeon to master implanting a particular type of assembly, it is preferable if the same or similar anchoring means can be used for a variety of locations. This advantage results in cost efficiency of inventory as well as efficiencies with respect to minimiz¬ ing operating time.

3. The hardware must be able to apply and oppose considerable stresses and strains. Thus,, the anchor means must be securely fixed to the bone, and the stabilizer must be securely fixed with regard to the anchor means. Moreover, it is desirable to provide the hardware with the integrity to resist to breaking.

4. The system should be designed for ease of implantation and removal. Implant hardware is relatively small and therefore somewhat difficult to manipulate. Any difficulty with assembly is compounded by the fact that the assembly occurs during surgery and in a living being. Therefore, it is critical that the hardware is designed with the surgeon's convenience in mind, i.e., to limit the time required and the stress required to implant an assembly. Consequently, a fixation system should be designed, to the extent possible, for easy assembly while maintaining the option of removal when necessary.

SUMMARY OF THE INVENTION The invention is directed toward a set screw for orthopedic implant components which is self-locking and exhibits improved holding power under cyclic loading. The set screw generally includes an externally threaded cylindrical portion and an integrally attached multi- faceted, tapered portion having a generally frustoconical shape. The large-diameter base of the multi-faceted frustoconical portion is coextensive with and integrally attached to one end of the cylindrical portion of the set screw. The small base of the multi-faceted frustoconical portion of the set screw includes a conical recess having an axis which is generally coincident with the axes of the cylindrical and frustoconical portions of the set screw, and having a base which is substantially coincident with the small-diameter base of the frustoconical portion such that the internal surface of the conical recess, together with the external surfaces of adjacent facets of the frustoconical portion, define a plurality of projecting point contacts which are substantially uniformly disposed about the axis of the set screw in a plane perpendicular to the axis of the set screw. In accordance with the principles of the invention the set screw has a hardness which is greater than that of the rod which it engages. The set screw of the invention is generally used in association with a rod-holding member having a rod- receiving channel and a threaded bore perpendicular to and in communication with the rod-receiving channel. The rod- holding members can be rod connectors or can be anchored to the bones which are to be fixed relative to one another by the spinal fixation system. In many implant systems, spinal fixation is achieved by retaining one or more rods in fixed relationship to at least a pair of rod-holding members which are anchored to the bones which are to be held in a relative position. Fixation of the rods rela¬ tive to the rod-holding members can be achieved by screw- ing a set screw into the threaded bore of the rod-holding member and engaging the outer circumferential surface of a rod received within the rod-receiving channel of the rod-holding member.

In accordance with the principles of the inven- tion, the plurality of circularly disposed sharp point contacts engage the outer radial surface of the rod as the screw is tightened thereby cutting or biting into the outer surface of the rod with the plurality of points to lock the screw into position relative to the rod and rod- holding member while reducing the amount of abrasion, thereby also reducing the amount of wear debris generated during the fixation procedure. Moreover, the screw is harder than the rod to facilitate the compressive grip on the rod to protect the screw threads and to minimize wear debris from the screw. The set screw of the present invention also has application for rod connectors and rod linking members.

BRIEF DESCRIPTION OF DRAWINGS Figure 1 is a side view of the set screw in accordance with the invention;

Figure 2 is a top view of the set screw; Figure 3 is a cross section of the set screw taken along line 3-3 shown in Figure 2;

Figure 4 is a bottom view of the set screw;

Figure 5 is an exploded side view of a spinal implant assembly utilizing the set screw in accordance with the invention;

Figure 6 is a front exploded view of the spinal implant assembly;

Figure 7 is a cross section of the spinal implant assembly taken along line 6-6 of Figure 5;

Figure 8 is a cross section of the spinal implant assembly taken along line 7-7 of Figure 4;

Figure 9 is a top view of a rod connector utilizing the set screw of the present invention; Figure 10 is a cross section of a rod connector shown in Figure 8, and

Figure 11 is a front view of the rod connector shown in Figure 8.

DETAILED DESCRIPTION OF INVENTION

Figures 1-3 illustrate the set screw member 49 in accordance with the invention. The screw 10 has an internal hex 20 which allows the set screw to be screwed with the seat 12. The set screw includes an externally threaded cylindrical portion 25 and a tapered or frustoconical-shaped portion 30 integrally attached there¬ to. The large base of the frustoconical portion is substantially coextensive with and integrally attached to one end of the cylindrical portion. The dimensions of the small-diameter base of the frustoconical portion of the set screw is not critical but is typically about one-half of the diameter of the large base of the frustoconical portion. The distance from the large-diameter base to the small-diameter base of the frustoconical portion is not critical, but is generally from about 20 percent to about

30 percent of the axial length of the cylindrical portion.

The frustoconical portion of the set screw has a plurality of facets 35 or surfaces connecting the large- diameter base with the small-diameter base. Each of the facets is preferably a trapezoidal shape which together form a repeating pattern of peaks and valleys about the peripheral frustoconical surface.

A generally conically shaped hollow 38 or recess having an axis coincident with the axes of the cylindrical and frustoconical portions of the set screw is provided at the small-diameter base of the frustoconical portion. The internal surface of the recess, together with the external surfaces of adjacent facets of the frustoconical portion, provide a plurality of pointed contacts 39 which are rela¬ tively uniformly radially disposed about the central axis of the set screw in a circular pattern perpendicular to the axis of the set screw.

A spinal implant assembly suitable for use with the set screw of the invention is illustrated in FIGS. 4- 7.

The assembly is shown in general at 40, and comprises anchor member 41 which includes seat means 42 and fixation means 44 which, in the present case, comprise a screw. It is envisioned that a vertebral hook could be used instead of a screw.

The seat means 42 includes an upper bevel 43 to eliminate edges which interface with the patient's muscle. Additionally, the seat 42 has a rod channel 47 which receives a rod 51. The seat means 42 has a tapered area 41 which forms a bone interface area. The seat means 42 further includes a bore 48, which is substantially perpen- dicular to the rod-receiving channel 47. The bore 48 includes internal threads 52 which mate with the external threads 56 of a set screw member 49.

In accordance with the principles of the inven¬ tion, the set screw has a hardness greater than that of the rod so that when the set screw is screwed into the internally threaded bore 48, the point contacts cut or bite into the rod 51 thereby applying compressive forces to the rod to hold the rod in fixed relation to the seat means 12 and to lock the set screw into position. The plurality of point contacts of the set screw reduce the amount of contact between the set screw and the rod thereby reducing abrasion and .consequently reducing the amount of wear debris generated during the fixation procedure.

It is a particular advantage that the set screw has a hardness differential from the rod of between 5 to 20 Rockwell points on the C-Scale. Spinal implant systems must be made of a material which has been approved for implantation and has the characteristics which are desir¬ able for an implant. Stainless steel is traditionally used for all parts of the implant system. Customarily, the rod is made of 316L stainless steel with a Rockwell Hardness on the C-Scale of from about 34 to about 36. The components which are used in conjunction with the rod must be compatible with the rod. An acceptable material for the screw is 22-13-5-SS (stainless steel) . This material is subsequently cold worked in order to achieve a Rockwell Hardness on the C-Scale of at least 40. Thus a more desirable range of differentiation is from about 12 to about 15 points RHC between the rod and the screw.

Important to the invention is that the contact between the set screw and the rod be provided by a plural¬ ity of points rather than a continuous circular ridge.

For convenience of machining, the recess at the small-radius base of the frustoconical portion of the set screw is preferably conical, as shown. However, various other recesses can be provided such as a frustoconical recess, so long as a plurality of pointed contacts are provided.

The angle of the facets defining the frustoconi¬ cal surface relative to the axis of the cylindrical portion is generally in the range from about 30° to about 60° and preferably in the range from about 45° to about 55°. The angle of the conical recess relative to the longitudinal axis of the set screw is generally in the range from about 45° to about 75° and preferably from about 55° to about 65°. The rod channel 47 can include one or preferably a plurality of serrations 58 which extend from about 60° to about 120° about the channel. In addition, the serrations include an interruption or cross-channel 59 which provides lateral edge areas 60 which hold the rod 51 against translational rotation about its longitudinal axis.

In accordance with another embodiment of the invention as shown in FIG. 8, the set screw can be used with an axial rod connector. The axial connector 70 includes connector seat means 72 having an internal bore 76 and including rod-receiving channels 77. The bore 76 includes an intermediate neck portion which forms rod abutments 78 which form a positive stop for the end of the rod. In addition, the axial connector includes the set screws of the invention which can be advantageously aligned along the longitudinal axis of the rod. The axial connector can include a plurality of serrations 88 which are interrupted as set forth hereinabove so as to inhibit both rotational and axial misalignment or movement. In accordance with an additional embodiment, the spinal fixation system can as shown in FIGS. 14-16, comprise a parallel connector 100. The parallel connector includes connector seat means 102 having parallel rod channels 107. The rod channels include substantially perpendicular set screws 109 which receive set screws as previously illustrated which apply a compressive force to a rod, to bias the rod into contact with interrupted serrations 118, as previously discussed. Additionally, the parallel connector includes indentations 103 for a connector holder for ease of insertion.

While in accordance with the patent statutes the best mode and preferred embodiment has been set forth, the scope of the invention is not limited thereto, but rather by the scope of the attached claims.

Claims

WHAT IS CLAIMED ISt

1. A set screw for use with orthopedic im¬ plants, said set screw comprising: an externally threaded cylindrical portion having means at one end for driving said set screw, a multi-faceted generally frustoconical shaped portion having a larger base which is coincident with and integrally attached to the other end of said cylindrical portion, said frustoconical portion having a small radius base provided with a recess having an internal surface which, together with adjacent pairs of the surfaces of said facets of said frustoconical portion, define a plurality of sharp point contacts.

2. The set screw of Claim 1, wherein the means for driving the set screw is an internal hexagonal shaped recess.

3. The set screw of Claim 1, wherein the multi- faceted frustoconical portion of said set screw is com¬ prised of a plurality of facets extending from the large- diameter base to the small-diameter base of said frustoconical portion.

4. The set screw of Claim 1, wherein the facets of said multi-faceted frustoconical portion each form an angle of from about 30° to about 60° with the axis of said set screw.

5. The set screw of Claim 1, wherein the facets of said multi-faceted frustoconical portion each form an angle of from about 45° to about 55° with the axis of said set screw.

6. The set screw of Claim 4, wherein the conical recess in the vicinity of the point contacts forms - lo ¬

an angle with the longitudinal axis of said set screw of from about 45° to about 75°.

7. The set screw of Claim 4, wherein the conical recess in the vicinity of the point contacts forms an angle with the longitudinal axis of said set screw of from about 55° to about 65°.

8. The set screw of Claim 1, wherein said set screw has a hardness of at least 40 on the Rockwell

Hardness C-Scale.

9. The set screw of Claim 8, wherein said set screw is made from 22-13-5-SS.

10. A set screw as set forth in Claim 1, wherein said set screw is cold-worked.

11. A spinal implant comprising a rod and at least two rod holding means, said rod holding means comprising a rod seat and a set screw having an externally threaded cylindrical portion having means at one end for driving said set screw and a multi-faceted generally frustoconical shaped portion having a larger base which is coincident with and integrally attached to the other end of said cylindrical portion, said frustoconical portion having a small radius base provided with a recess having an internal surface which together with adjacent pairs of the surfaces of said facets of said frustoconical portion, define a plurality of sharp point contacts.

12. A spinal implant as set forth in claim 11, wherein said set screw is from 12 to 15 units on the Rockwell Hardness C-Scale harder than said rod. lS. A spinal implant as set forth in claim 12, wherein said rod is made from 316L stainless steel and said screw is made from 22-13-5-SS.

14. A spinal implant as set forth in claim 13, wherein said screw is cold-worked.

15. A spinal implant as set forth in claim 14, wherein said rod holding means is an anchor which is secured to a bone.

16. A spinal implant as set forth in claim 11, wherein said rod holding means is a connector.