Acetabular Cup Bearing Liner

The present invention relates to an improved bearing liner for a ball-and-socket prosthesis that resists dislocation of the ball from the socket by providing a generally spherical cavity that encloses more than half of the ball. More particularly, the present invention relates to a single bearing liner that lacks breaks or cuts in the bearing surface and encloses more than half of the prosthetic femoral ball upon insertion of the ball into the cavity of the bearing liner.

Luxation can cause problems with ball-and-socket prostheses. It is therefore desirable to design a bearing liner that inhibits dislocation of the ball from its socket in prosthetic ball-and-socket joints. One design that decreases the chance of ball dislocation is a bearing liner that encloses more than a half of the ball within a hemisphere shaped cavity of the bearing liner. Bearing designs that enclose more than half the ball within the bearing liner have been previously described and typically either use multiple bearing liner components that can assemble around the ball, or employ a single bearing liner component with breaks or cuts in its surface to allow expansion of the opening to the bearing liner cavity while the ball is forced into the liner cavity. Such conventional designs suffer from several disadvantages. For example, bearing designs that require multiple components add to the cost of manufacture (due to the need for specific tolerances between multiple sections of the bearing surface). Therefore, a continuous bearing liner that encloses more than half of the ball is highly desirable.

According to the present invention, the bearing component comprises a bearing liner and a locking component configured to inhibit the elastic deformation of an opening to a bearing liner cavity. The bearing liner includes a rim that defines the opening to the bearing liner cavity. In one embodiment, the bearing liner cavity is concave in shape and of sufficient size that over half of a femoral ball fits within the bearing liner cavity. The bearing liner is further provided with means for permitting elastic deformation of the bearing surface around the rim of the bearing liner, wherein the deformation enlarges the opening to the concave bearing surface. In accordance with one embodiment, the locking component is formed to fit around the rim of the bearing liner to inhibit elastic deformation of the bearing surface around the rim.

In accordance with one embodiment of the bearing liner, an outer face is located at the opening to the bearing liner cavity and is formed to include a concentric channel defining a first radially inner and a second radially outer concentric extension. The inner surface of the first extension forms an integral portion of the inner articular surface, and is provided with a rim. The rim defines the opening to the bearing liner cavity. The first extension is designed to be flexible toward the second extension so that a biasing force will deform the first extension allowing the enlargement of the opening to the bearing liner cavity. Upon removal of the biasing force the first extension returns to its original position. A ball having a diameter greater than the diameter of the opening of the bearing liner can be inserted into the bearing liner cavity of that bearing liner. As the ball is pushed through the opening of the bearing liner, the first extension will be moved toward the second extension, decreasing the width of the channel formed between the first and second extensions. After complete insertion of the ball into the bearing liner cavity, the first extension will return to its original position to hold the ball within the bearing liner cavity.

In one embodiment, the bearing liner further comprises means for securing the ball within the bearing liner cavity. Once the ball has been inserted in the bearing liner cavity, preventing the first extension from being moved toward the second extension will prevent enlargement of the opening and will effectively lock the ball within the bearing liner cavity. In one embodiment, a securing ring having similar dimensions as the channel is placed into the channel to frictionally engage the outer surface of the first extension and the inner surface of the second extension. In this manner the opening to the bearing liner cavity cannot be expanded to release the ball.

In an alternative embodiment, the securing ring can be formed to have a first end having a width equal or smaller than the width of the channel and a second end having a width greater than the width of the channel. In this embodiment, insertion of the securing ring into the channel biases the first extension away from the second extension, thus narrowing the opening to the bearing liner cavity.

In one preferred embodiment of the present invention, the bearing component is formed with an outer surface that is generally concentric with the bearing liner cavity. This embodiment is suitable for use with a socket housing with an opening such as the acetabular cup of U.S. Pat. No. 5,049,158. The bearing component includes a groove formed near a meridian in the outer surface that is axially aligned with the opening to the cavity of the bearing. The groove of the bearing aligns with a groove in the opening of the socket housing upon full insertion of the bearing into the socket housing. A lock wire engages the grooves in both the bearing and socket housing when the bearing is fully inserted into the socket housing. Anti-rotation lugs can be formed on the inner surface of the socket housing, where the lugs interfere or machine into the bearing as the bearing is inserted into the socket housing.

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.

FIG. 1 is an exploded perspective view of a preferred embodiment of the present invention illustrating a hip prosthesis assembly comprising a lock wire positioned between a socket housing and a bearing liner, and a securing ring positioned between the bearing liner and a femur stem with a ball;

FIG. 2 is a sectional view illustrating insertion of the ball into the bearing liner, the bearing liner situated within the socket housing and the lock wire situated inside arcuate grooves formed in both the bearing liner and the socket housing;

FIG. 3 is a sectional view showing the installation of the securing ring after full insertion of the ball into the bearing liner; and

FIG. 4 is a sectional view illustrating the fully assembled prosthesis.

Referring to the drawings and particularly to FIG. 1, it will be seen that assembly 10 provides a single piece bearing liner that encloses more than half of a ball, wherein the diameter of the entrance to the cavity of the bearing liner is smaller than the diameter of the prosthetic ball, without requiring any cuts or breaks in the bearing surface of the liner. The desired effect is obtained through the use of an extension that forms the entrance to the cavity wherein the extension is elastically deformable to allow insertion of the ball into the cavity. The bearing component is further provided with inhibiting means that prevent elastic enlargement of the opening to the cavity of the liner after the ball is inserted into the liner cavity. The bearing component of the present application can be used in combination with an acetabular cup assembly for replacement of natural ball-and-socket joints. In particular the present bearing component is compatible with the invention in U.S. Pat. No. 5,049,158 entitled "Acetabular Cup Assembly", by inventors John A. Englehardt et al., the disclosure of which is expressly incorporated herein by reference.

In particular, assembly 10 comprises an acetabular cup or socket housing 12 designed to be affixed in the acetabulum to replace the natural hip socket of a patient and a bearing liner 14 designed to be inserted into the acetabular cup 12 as taught in U.S. Pat. No. 5,049,158, the disclosure of which is expressly incorporated herein by reference. A lock wire 16 is provided to retain the bearing liner 14 within the acetabular cup 12. The acetabular cup 12 includes an outer surface 18 and a generally hemispherically shaped inner surface 20. The inner surface 20 of acetabular cup 12 is provided with a plurality of anti-rotation lugs 22 which engage an outer surface 42 of the bearing liner 14 upon insertion of the bearing liner 14 into acetabular cup 12. The acetabular cup 12 has a lip or rim 36 through which the bearing liner 14 enters a cavity 38 formed by inner surface 20. Arcuate groove 40 extends around the periphery of cavity 38 spaced apart from rim 36 by a predetermined distance.

Bearing liner 14 includes the generally hemispherical convex outer surface 42 which is congruent or complimentary to inner surface 20. Bearing liner 14 further includes an annular flange 44 which abuts the rim 36 when the bearing liner 14 is fully inserted into the acetabular cup 12 and an annular groove 52 formed in the convex surface outer surface 42 such that the annular groove 52 aligns with the arcuate groove 40 of the acetabular cup 12 when the bearing liner 14 is fully inserted in the acetabular cup 12. (See FIG. 2) Lock wire 16 is inserted into the arcuate groove 40 of acetabular cup 12 prior to insertion of the bearing liner 14 into the acetabular cup 12 and engages both the annular groove 52 and the arcuate groove 40 when the annular groove 52 aligns with the arcuate groove 40. Essentially, as taught by U.S. Pat. No. 5,049,158, wire 16 has radially outwardly extending lobes which engage into groove 40 and radially inwardly extending lobes which engage into groove 52. The spring-like nature of the wire 16 provides a snap-ring which holds the bearing liner 14 in the cup 12. While the lock wire 16 is illustrated and described, it will be understood that other locking elements may be used to retain the bearing liner 14 within the acetabular cup 12.

The outer surface 42 of the bearing liner 14 is provided with an annular channel 62 generally concentric with the bearing liner cavity 72 and defining a first concentric annular extension 64 and a second concentric annular extension 66. The inner surface of first annular extension 64 is provided with an annular rim 70 that defines the opening to the bearing liner cavity 72 of the bearing liner 14. The first annular extension has a ramp portion 65 leading to its outer lip 67. Channel 62 is formed for receiving a securing ring 68 that frictionally engages a radially inner surface of second annular extension 66 and a radially outer surface of first annular extension 64 and prevents the first annular extension 64 from being biased toward the second annular extension 66. Prior to insertion of securing ring 68, a ball 78 on a femur stem 80 will move the first annular extension 64 toward the second annular extension 66 during insertion of the ball 78 into bearing liner cavity 72 of the bearing liner 14, as shown in FIG. 2. Insertion of the securing ring 68 into the annular channel 62 fixes the ball 78 in place in the bearing liner cavity 72 of the bearing 14. It will be seen that the bearing liner cavity 72 has a shape that is greater than hemispherical, such that the ball 78 will be captured in the bearing liner cavity 72 when the first annular extension 64 is secured against radially outward movement by the insertion of the securing ring 68 into the channel 62.

Bearing liner 14 is formed from a conventional plastic suitable for implantation and having sufficient flexibility to allow elastic deformation of the first annular extension 64 to allow entry of the inelastic ball 78 into the bearing liner cavity 72 prior to insertion of the securing ring 68. The bearing liner cavity 72 encloses more than half of the ball 78 to restrain dislocation of the ball 78 from the bearing liner cavity 72. The securing ring 68 prevents elastic deformation of the first annular extension 64 and thereby prevents dislocation of the ball 78. While a securing ring 68 is illustrated and described, it will be understood that other expansion elements may be used to restrict radially outward movement of the first extension 64 and thus secure the ball 78 within the bearing liner cavity 72.

FIGS. 2-4 illustrate the assembly of the prosthetic joint incorporating a preferred embodiment of the present invention. The ball 78 is first inserted through the securing ring 68 prior to insertion into the bearing liner cavity 72. The securing ring 68 has a diameter greater than that of the ball 78 such that the securing ring 68 hangs from the femur stem 80 during insertion of the ball 78 into the bearing liner cavity 72, as best shown in FIGS. 2 and 3. FIG. 2 shows the inelastic prosthetic ball 78 being inserted into the bearing liner cavity 72. In a preferred embodiment, the bearing liner 14 is manufactured from a material such as ultra high molecular weight polyethylene, allowing radially outward elastic deformation of the first annular extension 64 as the inelastic ball 78 is inserted into the bearing liner 72. It is understood that another suitable material or combination of materials can be used in the place of ultra high molecular weight polyethylene to achieve a similar effect in the present invention. As the ball 78 is forced against the annular rim 70 that defines the opening to the bearing liner cavity 72, the first annular extension 64 is moved towards the second annular extension 66 enlarging the opening defined by the annular rim 70 until it is approximately the diameter of the ball 78, at which time the ball 78 can fully enter the bearing liner cavity 72 as shown in FIG. 2. After full entry of the ball 78 into the bearing liner cavity 72, the first annular extension 64 elastically returns to its original position, as best shown in FIG. 3.

As shown in FIGS. 3-4, the ball 78 is retained in the bearing liner cavity 72 by insertion of the securing ring 68 into the annular channel 62. After the securing ring 68 is fully inserted in the annular channel 62, as shown in FIG. 4, the first annular extension 64 is prevented from moving towards the second annular extension 66 and the annular rim 70 resists dislocation of the ball 78 from the bearing liner cavity 72.

One feature of the present invention is the provision for a single bearing liner with a bearing surface without joints between separate pieces or breaks in the bearing surface. A unitary, unbroken design provides the benefit of simplified manufacturing. The bearing liner is formed to allow elastic deformation of the liner sufficient to allow an inelastic ball with a diameter generally the same as the bearing liner cavity to pass through the opening and into the bearing liner cavity.

Another feature of the present invention is means for inhibiting elastic deformation of the first annular extension (defining the opening to the bearing liner cavity) after the ball is placed in the bearing liner cavity. By restraining the elastic deformation of the first annular extension forming the opening to the bearing liner cavity, the prosthetic joint thereby resists dislocation of the ball from the bearing liner cavity.

Although the present invention has been described in detail with reference to a preferred embodiment as shown in FIGS. 1-4, variations and modifications exist within the scope and spirit of the invention as described and defined in the following claims.