Acetabular cup assembly

Abstract: A prosthetic acetabular cup assembly (10) includes a single piece bearing component (14) having an inner bearing surface (34) for receiving a ball attached to a femoral prosthesis and an outer surface (32). Assembly (10) also includes a shell component (12) for attachment to an acetabulum to replace a natural hip socket. Shell component (12) includes an inner surface (20) defining a cavity (26) for receiving bearing component (14) therein. Inner surface (20) of shell component (12) is formed to include an arcuate groove (28) therein. A formed wire (16) is situated in arcuate groove (28) of shell component (12). Wire (16) is configured so that a portion of wire (16) extends radially inwardly from arcuate groove (28) of shell component (12) to engage an arcuate groove (40) formed in bearing component (14) to retain bearing component (14) inside shell component (12).

Description: Background and Summary of the Invention The present invention relates to hip prosthesis assemblies for replacing a natural hip socket. More particularly, the present invention relates to an acetabular cup assembly designed to retain a polymeric bearing inside an acetabular metal shell without the use of attachment screws. It is known to provide an acetabular cup assembly including a metal shell component for attachment to an acetabulum to replace the natural socket and a polymer bearing component which is inserted into the shell to provide a hemispherical bearing surface for receiving a femur ball prosthesis element. Often, the polymer bearing component is nonsymmetrical and includes a built-up lip around a portion of the hemispherical bearing surface to help prevent dislocation of an installed femur ball from the hemispherical bearing surface. During installation of the acetabular cup assembly, the shell component is first secured to the acetabulum. When a surgeon installs the bearing component, the surgeon selects an orientation of the bearing with respect to the shell component to align the lip of the nonsymmetrical bearing component in the most advantageous position to reduce the likelihood of dislocation of the femur ball. Therefore, it is desirable to produce an acetabular cup assembly in which the bearing component can be attached to the shell component in a large number of selected orientations to provide the maximum degree of flexibility for the surgeon. An installed bearing component must be secured to a shell component by a retention force strong enough to prevent dislocation of the bearing component from the shell component. Several different approaches for fixing the bearing component inside the shell component are known. These include screw shaped profiles in both components, conical-fit-fixation, snap-fit-designs using the elasticity of the plastic bearing component and lock rings which cooperate with axially aligned retaining grooves. In the latter case an outer portion of the lock ring runs in a groove of the shell component and an inner portion runs in the groove of the bearing component (see e.g. DE-A-2301810). It is an object of the present invention to provide a locking mechanism which reliably retains the bearing component inside the shell component. According to the present invention, a prosthetic acetabular cup assembly is provided for receiving a ball attached to a femoral prosthesis. The assembly includes a bearing component having an inner bearing surface for receiving the ball and an outer surface. The assembly also includes a shell component for attachement to an acetabulum to replace a natural hip socket. The shell component includes an inner surface defining a cavity for receiving the bearing component therein. The inner surface of the shell component is formed to include an arcuate groove therein. The assembly further includes a ring shaped retaining means located in the arcuate groove of the shell component for engaging the bearing component to retain the bearing component inside the shell component upon insertion of the bearing component to the shell component. The bearing component includes an arcuate groove formed in the outer surface. The arcuate groove of the bearing component is positioned on the outer surface of the bearing component in a position axially aligned with the arcuate groove of the shell component upon full insertion of the bearing component into the shell component. The retaining means comprises a serpentine shaped or polygon shaped wire so that a plurality of sections of the wire extend radially inwardly from the arcuate groove of the shell component to lie within the arcuate groove of the bearing component. Therefore, the wire engages the bearing component to retain the bearing component inside the shell component. The shell component is typically made from metal such as titanium. Therefore, the position of the wire is accurately maintained during insertion of the bearing component into the shell component. This prevents shifting of the axial position of the wire relative to the shell component and ensures that the wire will be properly aligned with the arcuate groove formed in the outer surface of the bearing component to retain or "lock" the bearing component in place upon insertion of the bearing component into the shell component. Anti-rotation means may be provided to prevent rotation of the bearing component relative to the shell component after insertion of the bearing component into the shell component in a desired orientation. Preferably anti-rotation lugs are formed on the inner surface of the shell component. The lugs are situated below the arcuate groove formed in the inner surface of the shell component. The lugs interfere or machine into the outer surface of the bearing component as the bearing component is inserted into the shell component to prevent rotation of the bearing component relative to the shell component. There are no preformed slots in the bearing component for receiving the lugs. This feature advantageously provides no clearance or tolerance between the lugs and the outer surface of the bearing. Torsional backlash can occur if there is any clearance or tolerance between the lug and a preformed notch in the bearing component. Torsional backlash occurs when a force is exerted on the bearing component after installation of the assembly into the patient which causes slight movement of the bearing component. Therefore, this embodiment of the present invention reduces torsional backlash by providing no clearance or tolerance between the lugs and notches formed in the bearing. A favorable feature of the present invention is that the locking mechanism permits "infinite dialability" of the bearing component with respect to the shell component while maintaining its ability to lock the bearing component inside the shell component. Bearing components are often nonsymmetrical and include a lip portion which aids in the retention of a femur ball within the bearing component. The locking mechanism of the present invention advantageously permits the surgeon to align the bearing component at any one of a continuum of positions relative to the shell component and then lock the bearing component in that position. This gives the surgeon greater flexibility when installing the assembly and allows the surgeon to select the precise orientation of the bearing component which is most advantageous to the patient. Preferably the shell component has an inner surface which is congruent with an outer surface of the bearing component. Advantageously, this congruency prevents loading on the lip or flange surrounding the bearing component. Such lip loading can occur when the femur ball exerts forces on the bearing component and the bearing component is not congruent with the shell component. The present invention provides a locking mechanism which is capable of adjusting itself to compensate for changes which can occur in the bearing component. Because the bearing component is made from a polymeric material, its size can change over time. Often, bearing components show a propensity to shrink slightly over time after installation. Advantageously, the lock ring or wire of the present invention directs a retaining force radially inwardly into the cavity from the inner surface of the shell component and toward the bearing component. Therefore, if the arcuate groove in the bearing gets deeper due to wear over time or the bearing component shrinks after installation, the lock ring or wire can move radially inwardly to compensate for such changes in the bearing component. Thus the bearing component is retained reliably despite possible shrinkage of the bearing component after installation. Additional objects, features, and advantages of the invention will become apparent to those skilled in the art upon consideration of the following detailed description of a preferred embodiment exemplifying the best mode of carrying out the invention as presently perceived. Brief Description of the Drawings The detailed description particularly refers to the accompanying figures in which. Fig. 1 is an exploded perspective view of a preferred embodiment of the present invention illustrating an acetabular cup assembly including a lock wire positioned between a shell component and a bearing component; Fig. 2 is a plan view illustrating the configuration of a preferred embodiment of the lock wire; Fig. 3 is a plan view with portions broken away illustrating the lock wire situated inside an arcuate groove formed in the shell component; Fig. 4 is an enlarged perspective view of a portion of the shell component illustrating the position of an anti-rotation lug relative to the arcuate groove formed in the inner surface of the shell component; Fig. 5 is a plan view illustrating the configuration of the outer surface and flange of the bearing component; Fig. 6 is a sectional view of the assembled acetabular cup illustrating the bearing component situated within the shell component and the lock wire inside both the arcuate groove formed in the shell component and the arcuate groove formed in the bearing component to lock the bearing component in place within the shell component; Fig. 7 is an enlarged sectional view illustrating the position of the lock wire and an anti-rotation lug upon partial insertion of the bearing component into the shell component; Fig. 8 is an enlarged sectional view similar to Fig. 7 illustrating the position of the lug and the lock wire after the bearing component is fully inserted into the shell component; Fig. 9 is an elevational view of a symmetrical bearing component that can be used with the present invention; and Fig. 10 is a plan view of another embodiment of the lock wire which can be used with the acetabular cup assembly of the present invention. Detailed Description of the Drawings Referring now to the drawings, Fig. 1 illustrates an acetabular cup assembly 10 of the present invention. The assembly 10 includes a shell component 12 designed to be affixed to the acetabulum to replace the natural hip socket and a bearing component 14 designed to be inserted into shell component 12. A lock wire 16 is also provided to retain the bearing component 14 within the shell component 12. The shell component 12 includes an outer surface 18 which can be textured to facilitate securing the shell component 12 in place within an appropriately prepared acetabulum. Shell component 12 is preferably made from titanium, but may be made from a cobalt chrome material. Shell component 12 also includes a generally hemispherically shaped inner surface 20. A threaded aperture 22 is also formed in the shell component 12 for receiving an appropriate tool (not shown) to remove the shell component 12 from the acetabulum. Shell component 12 further includes a lip or rim 24. The rim 24 defines a plane through which bearing component 14 enters cavity 26 of shell component 12 formed by inner surface 20. Inner surface 20 of shell component 12 is formed to include an arcuate groove 28 therein. In a preferred embodiment of the present invention, arcuate groove 28 extends around the entire periphery of cavity 26 spaced apart from rim 24 by a predetermined distance. It is possible, however, that arcuate groove 28 could be formed to extend only partially around the periphery and at varying depths in inner surface 20. More than one of these arcuate grooves which extend only partially around the periphery can be provided. Shell component 12 is also formed to include anti-rotation lugs 30 on inner surface 20 of shell component 12. In selecting the number of lugs 30 to be used, there is a trade off between the insertion force required to insert bearing component 14 into shell component 12 and the rotational torque resistance that the lugs 30 provide. Each lug 30 added to shell component 12 increases the insertion force required but also increases the rotational torque resistance. Additionally, the depth of lugs 30 engagement into the bearing component 14 can be varied to adjust inversely the insertional force and rotational torque resistance. The preferred number of lugs 30, determined by balancing these factors, is four lugs 30 for each shell component 12. It is understood, however, that any number of lugs 30 can be used. Bearing component 14 includes a generally hemispherically shaped outer surface 32 which is congruent or complimentary to inner surface 20 of shell component 12. Bearing component 14 also includes an inner bearing surface 34 for receiving a prosthetic femoral ball (not shown) and a radially outwardly projecting lip or flange 36 extending circumferentially around the bearing component 14. The bearing component 14 illustrated in the Figs. 1-8 is a nonsymmetrical bearing component. It is understood, however, that the bearing component of the present invention may be a symmetrical component such as the bearing component 114 illustrated in Fig. 11. Bearing component 14 shown in Figs. 1-8 includes a built-up lip portion 38 extending away from the flange 36 to aid in the retention of the femur ball inside bearing surface 34. Bearing component 14 is also formed to include radially outwardly opening arcuate groove 40 spaced apart from flange 36 by the same predetermined distance that arcuate groove 28 is spaced apart from rim 24. Bearing component 14 is preferably made from a polymeric material such as ultra high molecular weight polyethylene (UHMWPE). Of course, the bearing component 14 could be made of other types of implantable bearing material. A preferred embodiment for lock wire 16 is illustrated in Fig. 2. Lock wire 16 is a serpentine shaped wire which is preferably made from cobalt chrome material and shaped by conventional wire forming techniques. Titanium may also be used to make wire 16. The lock wire 16 shown in Figs. 1 and 2 has a somewhat hexagonal shape and includes six engaging sections 42. The wire 16 can be either serpentine shaped as shown in Figs. 1 and 2 or polygon shaped having any number of sides. A gap 44 is provided between two adjacent sections 42 of lock wire 16 to permit lock wire 16 to expand radially outwardly. Corner portions 46 are situated between the side sections 42. The wire 16 may be conventionally heat treated to increase its strength. Lock wire 16 is inserted into the arcuate groove 28 of shell component 12 as illustrated in Fig. 3. Corner portions 46 remain inside arcuate groove 28 to retain lock wire 16 inside arcuate groove 28. The configuration of the arcuate groove 28 is best shown in Fig. 4. The arcuate groove 28 includes an upper edge 50 spaced apart from rim 24 and a lower edge 52. Upper edge 50 and lower edge 52 define the arcuate groove 28 therebetween. Each of the four anti-rotation lugs 30 includes a top face 54 and edges 55 which interfere or machine into the outer surface 32 of bearing component 14 upon insertion of bearing component 14 into shell component 12. Lugs 30 are spaced apart from lower edge 52 by a predetermined distance as shown in Fig. 4. This gap between top face 54 of lug 30 and lower edge 52 of arcuate groove 28 provides a region to accommodate pieces of the bearing component 14 which are cut from outer surface 32 by lugs 30. After shell component 12 is affixed to an appropriately prepared acetabulum, outer surface 32 of bearing component 14, as best illustrated in Fig. 5, is inserted into cavity 26 of shell component 12. The assembled acetabular cup assembly 10 is illustrated in Fig. 6. Shell component 12 includes an axis of symmetry 56 and the cavity defined by inner surface 34 of bearing component 14 includes an axis of symmetry 58. When the bearing component 14 is fully inserted into the shell component 12, axis of symmetry 58 is aligned at an angle with respect to axis of symmetry 56 of shell component 12 to aid in the retention of a femur ball (not shown) within a cavity defined by inner surface 34 of bearing component 14. Figs. 7 and 8 illustrate the operation of lock ring 16 and anti-rotation lugs 30 during insertion of bearing component 14 into shell component 12. Fig. 7 illustrates the bearing component 14 in a partially inserted position. Outer surface 32 of bearing component 14 engages the sections 42 of lock wire 16. As bearing component 14 is inserted in the direction of arrow 60, outer surface 32 of bearing component 14 applies a radially outwardly directed force on sections 42 of lock wire 16 to move sections 42 into arcuate groove 28 in the direction of arrow 62. In addition, when bearing component 14 is inserted to the position shown in Fig. 7, anti-rotation lugs 30 begin to interfere or machine into outer surface 32 of bearing component 14. Fig. 8 illustrates the bearing component 14 in its fully inserted position. In the fully inserted position, arcuate groove 40 of bearing component 14 is axially aligned with arcuate groove 28 formed in shell component 12. When arcuate groove 40 of bearing component 14 is aligned with arcuate groove 28 of shell component 12, engaging sections 42 move in the direction of arrow 64 to lie within arcuate groove 40 of bearing component 14. Each section 42 applies a radially inwardly directed force to the bearing component 14 to retain sections 42 in arcuate groove 40 to hold the bearing component 14 in place with shell component 12. When bearing component 14 is fully inserted, lugs 30 have machined notches in the outer surface 32 of bearing component 14 to engage the bearing component 14 to prevent rotation of bearing component 14 with respect to shell component 12 about axis 56. Therefore, no clearance or tolerance exists between lugs 30 and the notches cut in the outer surface 32 of bearing component 14. This reduces torsional backlash forces on the bearing component 14 after installation. Once side sections 42 of lock wire 16 snap into place within arcuate groove 40 of bearing component 14, bearing component 14 is locked within shell component 12. Since there are no preformed notches in outer surface 32 of bearing component 14 to receive lugs 30, the bearing component 14 can be inserted into shell component 12 at any desired orientation not limited by such preformed notches. Lock wire 16 therefore retains bearing component 14 inside shell component 12 regardless of the position of bearing component 14 relative to shell component 12. This maximizes a surgeon's flexibility to position the bearing component 14 in the most advantageous position relative to shell component 12 to reduce the likelihood that a femur ball (not shown) will become dislodged from the cavity defined by inner surface 34. As shown in Fig. 6, outer surface 32 of bearing component 14 is congruent with inner surface 20 of shell component 12. There are no gaps or spaces between outer surface 32 and inner surface 20. By eliminating these gaps, the present assembly 10 reduces lip loading which can occur after the assembly 10 is installed in a patient if gaps remain between the outer surface 32 of bearing component 14 in the inner surface 20 of shell component 12. Fig. 10 illustrates an alternate embodiment of a lock wire 116 which may be used with the present invention. Lock wire 116 includes relatively straight side sections 142, a gap 144, and corner sections 146. Corner sections 146 are configured to be situated inside arcuate groove 28 of shell component 12. Side sections 142 enter arcuate groove 40 formed in bearing component 14 to retain bearing component 14 inside shell component 12 upon insertion of bearing component 14 into shell component 12 in a manner similar to lock wire 16 shown in Figs. 1-8. Although the invention has been described in detail with reference to several preferred embodiments, variations and modifications exist within the scope of the invention as described and defined in the following claims.

Claims: Prosthetic assembly for receiving a ball attached to a bone, the assembly comprising a bearing component (14) including an inner bearing surface (34) for receiving the ball and an outer surface (32), a shell component (12) for attachment to an acetabulum to replace a natural hip socket, the shell component (12) including an inner surface (20) defining a cavity (26) for receiving the bearing component (14) therein, the inner surface (20) of the shell component (12) being formed to include an arcuate groove (28) therein, an arcuate groove (40) formed in the outer surface (32) of the bearing component (14) at a location that is axially aligned with the arcuate groove (28) formed in the shell component (12) upon insertion of the bearing component (14) into the shell component (12) and a ring shaped retaining means located after insertion of the bearing component (14) into the shell component (12) in the arcuate groove (28) of the shell component (12) and in the arcuate groove (40) of the bearing component (14) for retaining the bearing component (14) inside the shell component, characterized in that the retaining means comprises a serpentine shaped or polygon shaped wire (16,116) including a plurality of first sections (42,142) which extend from the arcuate groove (28) of the shell component (12) to lie inside the cavity (26) of the shell component (12) and a plurality of second sections (46,146) located inside the arcuate groove (28) of the shell component (12) to hold the wire (16,116) therein, the first sections (42) being configured to enter the arcuate groove (40) formed in the bearing component (14) to engage the bearing component (14). Assembly according to claim 1, characterized in that the arcuate groove (28) of the shell component (12) extends around the entire periphery of the cavity (26) of the shell component (12). Assembly according to one of the preceding claims, characterized in that the wire (16,116) is made from cobalt chrome material. Assembly according to one of the preceding claims, characterized in that the wire (16,116) has a round cross section. Assembly according to one of the preceding claims, characterized in that the wire is provided with a gap (44,144) to permit radial expansion. Assembly according to one of the preceding claims, characterized in that the wire (16,116) directs a retaining force radially inwardly into the cavity (26) from the inner surface (20) of the shell component (12) toward the bearing component (14), whereby the wire can move radially inwardly to compensate for changes of the size of the bearing component over time. Assembly according to one of the preceding claims, characterized in that the bearing component (14) has a generally hemispherically shaped outer surface (32) and the cavity of the shell component (12) is generally hemispherically shaped complementary to the outer surface (32) of the bearing component (14).

Inventors: ALLEN WAYNE C, DINELLO ALEX M, ENGELHARDT JOHN A, ONDRLA JEFF M, SERBOUSEK JON C, SNYDER DUANE G, ALLEN, WAYNE C., DINELLO, ALEX M., ENGELHARDT, JOHN A., ONDRLA, JEFF M., SERBOUSEK, JON C., SNYDER, DUANE G.
Applicants: DEPUY ORTHOPAEDICS INC, DEPUY ORTHOPAEDICS, INC.
Application year: 1991
Main classification: A61F2
IPC codes: F16B21, A61F2