WO1995015132A1

WO1995015132A1 - Orthopedic implant

Info

Publication number: WO1995015132A1
Application number: PCT/US1994/013793
Authority: WO
Inventors: John Egan
Original assignee: Minnesota Mining And Manufacturing Company
Priority date: 1993-12-03
Filing date: 1994-12-02
Publication date: 1995-06-08
Also published as: GB9324864D0

Abstract

An orthopedic prosthetic implant, such as an acetabular cup or femoral hip stem, having a flange (20; 42) with an undulating or scalloped edge (22). The implant is designed to be implanted without bone cement, and the undulating or scalloped edges (22) allow bony overgrowth to lock the implant in position and prevent rotation. Mechanical fasteners, such as bone screws (25) or threads (44) along the outer surface of an acetabular cup, may also be employed.

Description

ORTHOPEDIC IMPIANT This invention relates to an orthopedic prosthetic joint implant used to replace damaged or worn out joints.

Background of the invention

It is well known to replace hip, knee and other joints by artificial implants when the original joint is no longer serviceable and indeed many designs of suitable implants are known. They are generally fixed to the remaining bone by shaping the bone to accept the implant and then providing some form of fixing means to retain the implant in place. Generally this is some form of screw, whether integral with the implant outer surface or in the form of separate screws, or a cement which glues the implant to the bone. Usually some secondary fixing is used to provide long term stability and this can be provided in a number of ways including mechanical fixing using supplementary screws or barbs, cementing to the bone or providing a porous surface on the implant into which the bone can grow to unite the implant and bone.

In the latter case the living bone is encouraged to grow into specific regions of the implant to provide a biological grouting between the implant and the bone. The effect of this bony ingrowth is to augment other methods of fixing and this provides a secondary and longer term stabilization of the joint between the implant and the bone. For example, co-assigned U.S. Patent Nos. 4,608,052 and 4,673,409 (Van Kampen et al.) disclose laser-cut regions that are adapted to encourage bone ingrowth. British Patent No. 2,181,354 discloses EDM machined regions that are designed to encourage bone ingrowth.

Co-assigned U.S. Patent No. 5,217,499 (Shelley) discloses a rim-bearing acetabular component of a hip joint prosthesis. The component includes a continuous, circumferential rim that is adapted to contact the prepared rim of the acetabulum of the patient. That prosthesis is held in position by bone screws or cement. Co-assigned U.S. Patent No. 4,997,447 (Shelley) discloses a screw-threaded acetabular component of a hip joint prosthesis. The metal cup shell of that prosthesis includes an external self-tapping thread extending around the shell to hold the prosthesis in position in bone.

Co-assigned U.S. Patent No. 5,080,677 discloses an acetabular component of a hip joint prosthesis. That acetabular component includes a metal cup shell and plastic insert for receiving the ball of a femoral hip stem.

Examples of cemented acetabular cups include the plastic acetabular cups shown in British Patent Application No. 2,134,360; British Patent No. 1,563,334 and U.S. Patent No. 4,327,449. British Patent No. 2,134,360 shows a polyethylene acetabular cup having a gapped perimeter that would be cemented into position in bone. Such plastic acetabular cups are cemented in place, and are not designed for cementless fixation to bone.

Summary of the Invention

What we have now discovered is that bony overgrowth occurs at the interface of the bone and a flange resting on the surface of the bone. Thus an acetabular implant into the pelvis for a hip replacement may comprise a cup surrounded by a flange which rests on the exposed surface of the prepared bone whilst the cup sits in or is screwed into a prepared recess in the bone. See, e.g., co-assigned U.S. Patent No. 5,217,499 (Shelley) . It is over the flange of such acetabular cups that the bony overgrowth has been found to occur.

Accordingly, this invention involves a novel orthopedic prosthetic implant and method in which such bony overgrowth is used to further assist in the fixing and stabilization of the implant. The invention provides an implant that is adapted for cementless affixation in a recess prepared in bone, with the recess in the bone being prepared with a generally flat bone surface surrounding the recess. It is now believed that a relatively thin flange (e.g., less than 3mm thick) having an undulating or scalloped edge facilitates such bony overgrowth. It is also believed that cement should be avoided adjacent such a flange. Generally, a novel orthopedic prosthetic implant according to the invention comprises a support body adapted to be affixed in the recess prepared in the bone without cement, and a flange integral with the support body and extending outwardly from the support body. The flange has a generally flat surface arranged to rest on the surface of the bone surrounding the recess. The flange has an undulating or scalloped edge that defines a width of the flange as varying in a direction around the support body, whereby bony overgrowth over the edge of the flange will trap the flange and so prevent its removal and will also prevent rotation of the flange and implant. The support body and flange are integrally formed of material selected from the group comprising metal, graphite and ceramic material. For example, the support body may comprise a metal cup shell of an acetabular cup, with the flange extending radially or laterally outwardly from the opening of the acetabular cup.

In a second aspect of the invention, a novel prosthetic acetabular cup is provided. Generally, the prosthetic acetabular cup of the invention comprises a metal cup shell having a generally convex outer surface adapted to be affixed in the recess in the bone without cement, a generally concave inner surface, and a rim defining an opening to the inner surface. A metal flange is provided that is integral with the metal cup shell and extends outwardly from adjacent the rim of the metal cup shell. The metal flange has a generally flat surface arranged to rest on the surface of the bone surrounding the recess. The flange has an undulating or scalloped edge that defines a width of the flange as varying in a direction around the metal cup shell, whereby bony overgrowth over the edge of the flange will trap the flange and so prevent its removal and will also prevent rotation of the flange and thus the acetabular cup. Cementless means is provided in addition to the flange for affixing the acetabular cup to bone. Preferably, the undulating or scalloped edge of the flange defines a plurality of spaced-apart, teeth-like extensions extending laterally outwardly with respect to the cup shell. Such spaced-apart, teeth-like extensions each may conveniently have a through hole for receiving a bone screw to affix the acetabular cup to bone, in which case the cementless means may comprise a plurality of bone screws received in the through holes in the teeth-like extensions.

Most preferably, the spaced-apart, teeth-like extensions are separated from each other by generally rounded, concave edge portions defining the edge of the flange between the teeth-like extensions, the arrangement tending to encourage bony overgrowth adjacent the concave edge portions that tends to prevent rotation of the acetabular cup.

Most preferably, the plurality of spaced-apart, teeth-like extensions comprise at least six teeth-like extensions, with all but two of the teeth-like extensions being equally spaced apart, with the spacing between said two teeth-like extensions being substantially greater than the spacing between the other teeth-like extensions.

Also, preferably, the flange has a thickness no greater than 3mm.

Preferably, the cup shell and flange are formed of material selected from the group comprising metal, graphite and ceramic material. Most preferably, the cup shell is formed of metal, particularly titanium or titanium alloy.

Also, preferably, a bearing insert formed of synthetic resin material is provided for use with a metal cup shell. The bearing insert is closely received in the cup shell adjacent the inner surface of the cup shell. The bearing insert has a generally concave, generally hemispherical cavity adapted to receive a head of a femoral component of a hip prosthesis.

The cementless means may conveniently comprise a plurality of bone screws adapted to be screwed through the acetabular cup into bone. For example, the bone screws may be received in through holes through the cup shell or through the flange. Alternately, other suitable cementless means may be employed, such as other mechanical fastening means. For example, the cementless means may comprise threaded means formed along the outer surface of the cup shell.

A third aspect of the invention involves a method of using an implant. The method generally comprises the following steps: (a) providing an orthopedic prosthetic implant comprising a support body adapted to be affixed in the recess prepared in the bone without cement; and a flange integral with the support body and extending outwardly from the support body, the flange having a generally flat . surface arranged to rest on the surface of the bone surrounding the recess, the flange having an undulating or scalloped edge that defines a width of the flange as varying in a direction around the support body;

(b) preparing a recess in bone to receive the support body of the implant;

(c) preparing a flat surface in the bone around the recess for face-to-face engagement with a surface of the flange to support the flange on the flat surface;

(d) inserting the support body in the recess prepared in the bone with a surface of the flange in face- to-face engagement with the flat surface of the bone without cement; and

(e) allowing bony overgrowth over the undulating or scalloped edge of the flange to thereby trap the flange and so prevent its removal, and to also thereby prevent rotation of the orthopedic prosthetic implant relative to the bone. In this way one can use the bony overgrowth which occurs to assist in fixing the implant to the bone. Also importantly one can use the overgrowth to prevent rotation of the implant in the recess which does otherwise occur sometimes and which is highly undesirable because such movement is liable to lead to failure of the joint between the implant and the bone and also damage to or failure of the implant.

The invention does extend to other implants, in addition to acetabular cups, where there is potentially a thin and relatively flattish surface interfacing closely with either cut surface of the bone or healthy cortical bone surface. Such interfaces may be achieved on the collars of femoral or humeral components against the cut calcar of the femoral or humeral neck or on bone plates in close apposition to the cortical bone. The bone/implant interfaces of distal femoral knee implants and tibial implants may also be provided with suitable flanges to bring them into accordance with the invention. Further details of the invention are defined in the features of the claims.

Brief Description of the Drawing

The invention will be further described with reference to the drawing wherein corresponding reference characters indicate corresponding parts throughout the several views of the drawing, and wherein:

Figure 1 is a plan view of an orthopedic prosthetic implant of the invention, represented by way of example, as a metal cup shell of a prosthetic acetabular cup.

Figure 2 is a cross-sectional view of the prosthetic acetabular cup of figure 1 also including a bearing insert of synthetic resin material; and Figure 3 is a side elevational view of a second embodiment of the invention, in which screw threads are provided on the outer surface of the acetabular cup. Detailed Description of Preferred Embodiments

Referring to the drawing, figure 1 shows an outer metal cup shell 10 forming part of an acetabular cup component of a hip joint prosthesis. An implant according to this invention could take the form of other orthopedic prosthetic joint implants, such as for example a femoral hip stem or the humoral component of a shoulder prosthesis.

The implant 10 according to this invention preferably is made from metal, and in particular titanium or titanium based alloys. However, an implant according to the invention can be made of other metals, graphite or ceramics, so long as they are suitable for cementless affixation to bone. Inside cup shell 10 a bearing insert 32 will sit. The bearing insert 32 is formed of synthetic resin material ("plastics material") , such as thermoplastic or thermoset material, and preferably ultra high molecular weight polyethylene. Co-assigned U.S. Patent No. 5,080,677 (Shelley) discloses a technique for locking such a bearing insert in a metal cup shell.

As shown in figure 2, the shell 10 has a cavity 12 for the bearing insert 32. Near the bottom of the cavity 12 are provided holes 14, e.g., three as shown, through which suitable screws 25 can pass to fix the implant 10 into a prepared cavity in the bone. In addition a small circular recess 16 is provided in the base of the cavity 12 to receive a corresponding shaped projecting spigot on the bearing inset 32 to locate this positively in place. Co-assigned U.S. Patent No. 5,217,499 (Shelley) discloses a rim-bearing acetabular component including suitable bone screws.

The shell 12 has around its upper edge a thin integral flange 20. As can be seen from the drawing, the flange 20 has a width which is not constant and in fact is of scalloped shape as viewed in the direction of the open face of the shell 12. In the example shown there are six scallops 22, although the flange could be provided with a different number of scallops 22. Between each of the scallops in the widest part of the flange 20 are holes 24 for small screws for initially fixing the flange 20 to bone. The widest parts of the flange 20 may take the form of generally spaced-apart, teeth-like extensions as shown in figure 1, and the scallops 22 may take the form of generally concave arcuate or rounded edge portions 22.

Further it will be noted that there is a region 30 where the flange 20 is very narrow and of substantially constant width. This is a region which in practice will correspond to a lack of underlying bone.

We have found that to promote bony overgrowth the flange 20 and the bone surface against which it is to rest should be as flat as possible so that the two can be in close and firm contact. In this way the flange 20 will press on the bone surface and this pressure will promote the bony overgrowth. In addition the flange 20 should be as thin as possible commensurate with maintaining satisfactory mechanical properties. Generally, therefore, the flange 20 should be no thicker than 3mm and preferably from 1 to 2mm.

As shown in figure 1, the flange 20 will normally be substantially circular with the edge of the flange forming a generally an undulating or scalloped shape. The undulations or scalloping provides the required variation in the width of the flange 20, so preventing rotation of the implant 10 once the bone has grown over the edge of the flange 20.

Once the implant 10 has been in place for some time, bone will overgrow the outer edge of the flange 20. This bony overgrowth will fix the implant 10 in place since it will not be possible for the implant 10 to move outwardly from the bone, or to rotate because of the bone which will have grown into the shallower parts of the scallops 22.

Figure 3 illustrates an alternate embodiment of the acetabular cup, herein designated 40, which is similar in most respects to acetabular cup 10. Acetabular cup 40, however, includes an external, self-tapping screw thread 44. Acetabular cup 40 includes a flange 42 that is substantially identical to the flange 20 of metal cup shell 10. Co-assigned U.S. Patent No. 4,997,447 (Shelley) discloses a suitable external, self-tapping screw thread on an acetabular cup.

An advantage of an implant 10 or 40 according to the invention over one having a porous surface or coating to promote bony ingrowth is that the expensive porous surface can be avoided. However, it is contemplated that a porous surface of coating (not shown) could be combined with this invention. For example, the porous surface disclosed in U.S. Patent Nos. 4,608,052 and 4,673,409 (Van Kampen et al.) could be employed in an implant having a flange with a scalloped edge. Another example of a suitable porous surface is shown in British Patent No. 2,181,354.

In addition, the implant 10 or 40 can be machined to provide the required flange 20 or 42 as part of the normal manufacturing procedure for the implant 10 or 40 so reducing the cost of the implant 10 or 40 whilst still retaining the advantages of non-cemented stabilization.

As various changes could be made in the above constructions and methods without departing from the scope of the invention as defined in the claims, it is intended that all matter contained in the above description or shown in the accompanying drawing be interpreted as illustrative and not in a limiting sense.

Claims

CLAIMS :

1. An orthopedic prosthetic implant adapted for cementless affixation in a recess prepared in bone, the recess being prepared with a generally flat bone surface surrounding the recess, the implant comprising: a support body (10) adapted to be affixed in the recess prepared in the bone without cement; a flange (20; 42) integral with the support body (10) and extending outwardly from the support body (10) , the flange (20; 42) having a generally flat surface arranged to rest on the surface of the bone surrounding the recess, the flange (20; 42) having an undulating or scalloped edge (22) that defines a width of the flange (20; 42) as varying in a direction around the support body (10) , whereby bony overgrowth over the edge (22) of the flange (20; 42) will trap the flange (20; 42) and so prevent its removal and will also prevent rotation of the flange (20; 42) and implant; the support body (10) and flange (20; 42) being integrally formed of material selected from the group comprising metal, graphite and ceramic material.

2. A prosthetic acetabular cup (40) adapted for cementless affixation in a recess prepared in bone, the recess being prepared with a generally flat bone surface surrounding the recess, the acetabular cup (40) comprising: a cup shell (10) having a generally convex outer surface adapted to be affixed in the recess in the bone without cement, a generally concave inner surface, and a rim defining an opening to the inner surface; a flange (20; 42) integral with the cup shell (10) and extending outwardly from adjacent the rim of the cup shell (10) , the flange (20; 42) having a generally flat surface arranged to rest on the surface of the bone surrounding the recess, the flange (20; 42) having an undulating or scalloped edge (22) that defines a width of -li¬ the flange (20; 42) as varying in a direction around the cup shell (10) , whereby bony overgrowth over the edge (22) of the flange (20; 42) will trap the flange (20; 42) and so prevent its removal and will also prevent rotation of the flange (20; 42) and thus the acetabular cup; and cementless means (14, 24, 25 or 44), in addition to the flange (20; 42) , for affixing the acetabular cup to bone.

3. An implant or acetabular cup according to claims 1 or 2 further characterized in that the undulating or scalloped edge (22) of the flange (20; 42) defines a plurality of spaced-apart, teeth-like extensions extending laterally outwardly with respect to the cup shell or support body (10) .

4. An implant or acetabular cup according to claim 3 further characterized in that the spaced-apart, teeth-like extensions are separated from each other by generally rounded, concave edge portions (22) defining the edge (22) of the flange (20; 42) between the teeth-like extensions, the arrangement tending to encourage bony overgrowth adjacent the concave edge portions (22) that tends to prevent rotation of the acetabular cup or implant.

5. An implant or prosthetic acetabular cup according to claim 3 or 4 further characterized in that the plurality of spaced-apart, teeth-like extensions comprise at least six teeth-like extensions, all but two of the teeth-like extensions being equally spaced apart with the spacing between said two teeth-like extensions being substantially greater than the spacing between the other teeth-like extensions.

6. An implant or acetabular cup according to any of claims 1-5 further characterized in that the flange (20; 42) has a thickness no greater than 3mm.

7. A prosthetic acetabular cup (40) according to any of claims 2-6 further characterized in that the cup shell (10) and flange (20; 42) are formed of metal, the prosthetic acetabular cup (40) further comprising a bearing insert (32) formed of synthetic resin material, the bearing insert (32) being closely received in the cup shell (10) adjacent the inner surface of the cup shell (10) , the bearing insert (32) having a generally concave, generally hemispherical cavity adapted to receive a head of a femoral component of a hip prosthesis.

8. An implant according to claim 1 further characterized in that cementless means (14, 24, 25 or 44), in addition to the flange (20; 42) , is provided for affixing the implant to bone.

9. An implant or acetabular cup according to any of claims 2-8 further characterized in that the cementless means (14, 24 or 25) comprises a plurality of bone screws (25) adapted to be screwed through the implant or acetabular cup into bone.

10. An acetabular cup according to claim 9 further characterized in that the cementless means (14, 25) comprises through holes (14) through the cup shell (10) for receiving the bone screws (25) .

11. An implant or acetabular cup according to claim 9 wherein the cementless means (24) comprises through holes (24) through the flange (20) for receiving the bone screws to affix the implant or acetabular cup to bone.

12. An implant or acetabular cup according to claim 11 further characterized in that the through holes

(24) are provided in the spaced-apart, teeth-like extensions, the cementless means further comprising a plurality of bone screws received in the through holes (24) in the teeth-like extensions.

13. An acetabular cup (40) according to any of claims 2-7 wherein the cementless means comprises threaded means (44) formed along the outer surface of the cup shell.

14. A prosthetic acetabular cup according to any of claims 2-13 further characterized in that the cup shell (10) and flange (20; 42) are formed of material selected from the group comprising metal, graphite and ceramic material.

15. An implant or acetabular cup according to any of claims 1-14 further characterized in that the support body or cup shell (10) and flange (20; 42) are formed of metal.

16. An implant or acetabular cup according to claim 15 further characterized in that the support body or cup shell (10) are formed of titanium or titanium alloy.

17. A method of fixing an implant (10; 40) to a bone, the method comprising the following steps:

(a) providing an orthopedic prosthetic implant (10; 40) comprising: a support body (10; 40) adapted to be affixed in the recess prepared in the bone without cement; and a flange (20; 42) integral with the support body (10; 40) and extending outwardly from the support body (10; 40) , the flange (20; 42) having a generally flat surface arranged to rest on the surface of the bone surrounding the recess, the flange (20; 42) having an undulating or scalloped edge (22) that defines a width of the flange (20; 42) as varying in a direction around the support body (10; 40) ; (b) preparing a recess in bone to receive the support body (10; 40) of the implant; (c) preparing a flat surface in the bone around the recess for face-to-face engagement with a surface of the flange (20; 42) to support the flange (20; 42) on the flat surface;

(d) inserting the support body (10; 42) in the recess prepared in the bone with a surface of the flange

(20; 42) in face-to-face engagement with the flat surface of the bone without cement; and

(e) allowing bony overgrowth over the undulating or scalloped edge (22) of the flange (20; 42) to thereby trap the flange (20; 42) and so prevent its removal, and to also thereby prevent rotation of the orthopedic prosthetic implant (10; 40) relative to the bone.

18. A method according to claim 17 wherein the undulating or scalloped edge (22) of the flange (20; 42) defines a plurality of spaced-apart, teeth-like extensions extending laterally outwardly with respect to the support body (10; 40) ; the step of allowing bony overgrowth over the undulating or scalloped edge (22) of the flange (20; 42) further comprising: allowing bony overgrowth through open areas defined between the teeth-like extensions and thereby tending to prevent rotation of the implant.

19. A method according to claim 18 wherein the spaced-apart, teeth-like extensions each have a through hole (24) for receiving a bone screw to affix the orthopedic prosthetic implant to bone; the method further comprising the following step: affixing the implant to bone with a plurality of bone screws received in the through holes (24) in the teeth-like extensions in threaded engagement with bone.

20. A method according to claims 18 or 19 wherein the spaced-apart, teeth-like extensions are separated from each other by generally rounded, concave edge portions (22) defining the edge (22) of the flange (20; 42) between the teeth-like extensions; the step of allowing bony overgrowth over the undulating or scalloped edge (22) of the flange (20; 42) further comprising: allowing bony overgrowth over the generally rounded, concave edge portions (22) defining the edge of the flange (20; 42) between the teeth-like extensions and thereby tending to prevent rotation of the implant.

21. A method according to any of claims 18-20 wherein the flange (20; 42) has a thickness no greater than 3mm; the step of allowing bony overgrowth over the undulating or scalloped edge (22) of the flange (20; 42) further comprising: allowing bony overgrowth over a flange (20; 42) having a thickness no greater than 3mm and thereby tending to prevent rotation of the implant.

22. A method according to any of claims 17-21 wherein the orthopedic prosthetic implant comprises an acetabular cup, and the support body comprises a metal cup shell (10; 40), the method further comprising the following step: inserting a bearing insert (32) formed of synthetic resin material into the metal cup shell (10;

40) , the bearing insert (32) having a generally concave, generally hemispherical cavity adapted to receive a head of a femoral component of a hip prosthesis.

23. A method according to any of claims 17-22 further comprising the following step: affixing the implant to the bone with bone screws (25) received in through holes (14 or 24) in the implant.