Artifical Implant Component and Method for Securing Same

This invention relates generally to a prosthetic total knee replacement system, and more specifically to an artificial patella component of a prosthetic total knee replacement system.

The hyaline anticular cartilage of the knee joint may undergo degenerative changes due to multiple etiologies. When these degenerative changes are advanced, irreversible and unresponsive to non operative management, it may ultimately become necessary to replace the natural knee joint with an artificial knee joint prosthetic system. On many of these occasions, it is necessary to surgically implant within the natural patella an artificial patella component which then articulates with the patella femoral groove of the femoral component of the total knee joint replacement system.

When such an artificial patella component is implanted, the anticular portions of the natural patella are initially surgically removed and the subchondral bone is cut by appropriate surgical instrumentation so as to form a "patella bed" into which the artificial patella component is securely implanted. When the artificial patella component is surgically implanted in its proper location in the natural patella, the articular surface of the artificial patella component is able to articulate with the patello femoral groove of the femoral component of the knee joint prosthesis in a manner similar to the way in which the natural patella would articulate with the femur of the natural knee. As one would expect, the means for securing the artificial patella component to the natural patella should be relatively strong in order to prevent loosening of artificial patella component from the natural patella during normal usage of the knee joint prosthesis.

Different methods are known in the art for securing an artificial patella component to the natural patella. In one of these methods, the natural patella is surgically altered so as to form two cylindrical cavities proximate to the center of the natural patella. The artificial patella component, which includes a flat undersurface and a cylindrical stem portion extending anteriorly from the undersurface, is secured to the natural patella by placing the stem portion of the artificial patella component into the smaller cylindrical cavity of the natural patella. The stem portion of the artificial patella component may also include a circumferential ridge which is able to cause mechanical interference with the sides of the cylindrical cavity of the natural patella so as to attach the artificial patella component to the natural patella more securely and at times without bone cement. Bone cement is usually applied to the contact area between the artificial patella component and the natural patella in order to further secure the artificial patella component to the natural patella. Some patella designs may have more than one solid small cylindrical peg in conjunction with an undersurface groove.

The present invention provides design features of an artificial patella component which ultimately provides rigid fixation to a natural patella when used in conjunction with an artificial knee joint prosthesis. The invention also encompasses the method of securing an artificial patella component to a natural patella as part of an artificial knee joint replacement system.

An advantage of the present invention is that an artificial patella component as described herein results in a more secure engagement between the artificial patella component and the natural patella.

Another advantage of the present invention is to provide an artificial patella component in which bone cement is able to attach more securely the artificial patella component to the natural patella, through means of macrointer digitation with a plurality of angled holes.

Yet another advantage of the present invention is to provide an artificial patella component which can be attached to a natural patella with relative ease.

A further advantage of the present invention is to provide an artificial patella component having a metal backing which can incorporate a modular threadable divergent hollow slit flexible peg.

The invention, in one form thereof, provides an artificial patella component which includes a tapered peg which is able to engage an undersized cavity which has been formed in the natural patella. The tapered peg of the artificial patella component includes two or more flexible lobes which diverge from each other in the anterior direction from the undersurface of the artificial patella component. The flexible lobes are compressible and thus engage the undersized cavity in the natural patella as the artificial patella component is clamped into position in the natural patella. Accordingly the flexible lobes of the artificial patella component are able to engage the undersized cavity of the natural patella in a press fit manner without permanent deformation so as to secure the artificial patella component to the natural patella.

The invention further provides, in one form thereof, a plurality of closed ended canals which are located on the undersurface of the dome-shaped portion of the artificial patella component. One set of these canals are angularly displaced from the antero-posterior centerline of the artificial patella component and to each other. The other set of canals are perpendicular to the undersurface. Accordingly, after bone cement is applied to the natural patella, the bone cement will become lodged in these undersurface canals of the artificial patella component as the artificial patella component engages the natural patella causing macro interdigitation so as to further secure the artificial patella component to the natural patella. These undersurface canals are effectively angled to each other

Additional objects, advantages, and features of the present invention will become apparent from the following description and appended claims, taken in conjunction with the accompanying drawings.

FIG. 1 is an illustration of a mid-longitudinal section of a human leg having an artificial knee joint prosthesis with an artificial patella component according to the first preferred embodiment of the present invention;

FIG. 2 is an enlarged cross-sectional longitudinal view of the artificial patella component;

FIG. 3 is an undersurface view of the artificial patella component taken along the line 3--3 in FIG. 2;

FIG. 4(A) is a partial cross-sectional view showing the artificial patella component and the surgically prepared natural patella of FIG. 1 prior to engagement;

FIG. 4(B) is a partial cross-sectional view of the artificial patella component and the natural patella shown in FIG. 1 as the artificial patella component is being inserted into the surgically prepared natural patella;

FIG. 4(C) is a partial cross-sectional view of the artificial patella component and the natural patella of FIG. 1 showing final engagement of the artificial patella component within the surgically prepared natural patella;

FIG. 5 is an undersurface view of an artificial patella component according to the second preferred embodiment of the present invention that is with the divergent flexible peg having four lobes; and

FIG. 6 is a longitudinal cross sectional view of yet another preferred embodiment of the present invention that is a modular threaded slit flexible peg with a transverse channel between the lobes, in a metal backed porous coated patella.

The following discussion of the preferred embodiments of the artificial patella component of the present invention is merely exemplary in nature and is not intended to limit the invention or its application or uses.

Referring to FIG. 1, there is shown a total knee joint prosthesis 10 depicted as being functionally secured to a tibia 12 and a femur 14 of a human leg 16. Knee joint prosthesis 10 includes a femoral component 18 which is rigidly connected to the distal end of the femur 14 after the femur 14 has been resected in a manner which is well known in the art. The femoral component 18 includes a condylar portion 20 which engages a tibial component which is more fully described below. Superiorly adjacent to the condylar portion 20 on the femoral component 18 is an intercondylar portion 22 is used to operatively engage a superiorly extending peg (not shown) on the tibial component so as to limit antero-posterior movement of the femoral component 18.

Connected superiorly to the intercondylar portion 22 of the femoral component 18 is a femoral stem boss 24. The femoral stem boss 24 is used to receive a support member (not shown) which is operable to secure the femoral component 18 to the femur 14. Positioned at a superior end of the femoral stem boss 24 is an anti-rotation member 26. The anti-rotation member 26 engages a recess in the support member so as to prevent rotation of the support member with respect to the femoral component 18. While the femoral component 18 is generally unitary in nature and is comprised of a biocompatible high strength alloy such as Ti.sub.4 Al.sub.6 V, other suitable materials may be used. Other distal femoral designs may also be used.

The knee joint prosthesis 10 further comprises a tibial component 30 having a tibial insert liner 32 which is able to articulate with the condylar portion 20 of the femoral component 18. The tibial component 30 is connected to the tibia 12 by an inferiorly extending tibial boss stem 34 which is able to receive a support member (not shown) which is secured to the tibia 12. The tibial component 30 is generally a unitary structure and is preferably formed from Ti.sub.4 Al.sub.6 V, while the tibial insert liner 32 is preferably made from ultra-high molecular weight polyethylene (UHMWP). However, other suitable materials and designs may be used.

The knee joint prosthesis 10 further comprises a patella 40 which includes a natural patella 42 and an artificial patella component 44. The artificial patella component 44 is attached to the natural patella 42 so as to provide an articulating surface between the patella 40 and the knee joint prosthesis 10. The artificial patella component 44 includes a dome-shaped portion 46 having an articular surface 48 shaped in a general spherical manner which is operable to articulate with the patello femoral groove (not shown) in the anterior condylar portion 20 of the femoral component 18. The spherical shape of the articular surface 48 of the artificial patella component 44 permits the patella 40 to remain relatively stationary as flexion of the knee joint prosthesis 10 occurs.

As shown in FIGS. 2 and 3, artificial patella component 44 further includes relatively flat undersurface 50. The undersurface 50 of the artificial patella component 44 is operable to engage a surgically prepared patella bed 52 formed in the natural patella 42 as discussed below. Extending anteriorly from the undersurface 50 of the artificial patella component 44 is a tapered peg 53 having two flexible divergent lobes 54 and 56 which are separated by a slot 58. "Tapered" is hereby considered to be in a direction toward the dome-shaped portion 46. The flexible lobes 54 and 56 are generally semicircular in shape and diverge from each other as they extend from the undersurface 50 of artificial patella component 44. Each of the flexible lobes 54 and 56 includes a lip portion 60 and 62 opposite the undersurface 50 which are used to facilitate engagement of the artificial patella component 44 with the natural patella 42 in the manner described below. In addition, each of the flexible divergent lobes 54 and 56 may have a centrally disposed transverse channel or cross hole 63. The channels 63 are used to receive bone cement when the artificial patella component 44 is attached to the natural patella 42 as more fully described below. In addition, the transverse hole 63 is used to screw in the modular threaded patella peg (FIG. 6) into the threaded undersurface of the patella.

The artificial patella component 44 further comprises a first plurality of closed ended canals 64 and a second plurality of closed ended canals 66. The first and second plurality of canals 64 and 66 extend into the artificial patella component 44 from the undersurface 50 of the artificial patella component 44. The first plurality of canals 64 are oriented at an angle of approximately 45.degree. from the antero-posterior centerline of the artificial patella component 44 and are generally directed toward the center of the dome of artificial patella component 44. The second plurality of canals 66 extend into the artificial patella component 44 in a direction perpendicular to the anterior surface 50 of the artificial patella component 44. The first plurality of canals 64 and the second plurality of canals 66 are oriented in an alternating configuration around the region of the undersurface 52 where the flexible divergent lobes 54 and 56 engage the undersurface 50. The first and second plurality of canals 64 and 66 improve the fixation between the natural patella 42 and the artificial patella component 44 as will be discussed in greater detail below. In a preferred embodiment, artificial patella component 44 is a unitary structure formed of UHMWP, though other suitable materials may be used. The patella implant can also be metal backed and coated with a porous surface for cementless fixation (FIG. 6).

The method of securing the artificial patella component 44 to the natural patella 42 will now be described with reference to FIGS. 4(A)-(C). The natural patella 42 is initially prepared to receive the artificial patella component 44 by reaming a first cavity 68 in the natural patella 42. The diameter of the first cavity 68 is such that it is slightly smaller than the largest distance separating the external surfaces of the flexible divergent lobes 54 and 56. In a preferred embodiment, the first cavity 68 is approximately 1 mm less than the greatest distance separating the external surfaces of the flexible divergent lobes 54 and 56. After the first cavity 68 has been reamed, a second larger cavity 70 is reamed concentric with the first cavity 68 such that the edge of the dome-shaped portion 46 of the artificial patella component 44 will seat inside of the natural patella 42. The formation of the patella bed 52 is completed after the first and second cavities 68 and 70 have been formed in the natural patella 42.

Turning to FIG. 4(B), the flexible divergent lobes 54 and 56 of the artificial patella component 44 are shown partially inserted in the first cavity 68. When the lips 60 and 62 of the flexible divergent lobes 54 and 56 engage the superior rim of the first cavity 68, the flexible divergent lobes 54 and 56 will be compressed inwardly so that the flexible divergent lobes 54 and 56 can be received into the first cavity 68. As the artificial patella component 44 is progressively inserted into the first cavity 68, the undersurface 50 of the artificial patella component 44 will eventually engage the bottom surface of the larger cavity 70 of the natural patella 42 as shown in FIG. 4(C). Since the flexible divergent lobes 54 and 56 remain in a flexed configuration, they will be continually applying a force by virtue of design and material characteristics against the walls of the first cavity 68 and secure the artificial patella component 44 to the natural patella 42, in a press fit manner.

The press fit engagement between flexible divergent lobes 54 and 56 and the cavity 68 provides only part of the possible attachment mechanism of the artificial patella component 44. Bone cement (not shown) may also be applied to both of the first and second cavities 68 and 70 as well as patella bed 52 prior to engagement between the artificial patella component 44 and the natural patella 42. Consequently, not only is the artificial patella component 44 secured in a press fit manner to the natural patella 42, but the artificial patella component 44 is also cemented to the natural patella 42. It is stressed, however, that this design is able to provide a sufficient attachment between the artificial patella component 44 and the natural patella 42 without the need of bone cement, although cemented fixation is generally used more than cementless fixation with non-metal backed patellae.

As will be appreciated by those skilled in the art, the first and second cavities 68 and 70 provide a recessed or inlaid area into which the artificial patella component 44 can be cemented to the natural patella 42 or from which bone ingrowth may occur. Furthermore, the slot 58 provides a canal for accepting bone cement to provide additional fixation. In addition, bone cement will also flow into the first and second plurality of canals 64 and 66 as discussed above, thereby further increasing the surface area upon which the bone cement may act. As will be appreciated by those skilled in the art, the macro interdigitation associated with the first and second plurality of canals 64 and 66 provides a greater degree of security against loosening of the artificial patella component 44 as compared to other designs. The bone cement in the first and second plurality of angled holes 64 and 66 finally locks the patella compound 44 onto the patella bed 52. If the non-metal backed patella component 44 is implanted in a cementless fashion, then the plurality of canals 64 and 66 and slot 58 can be filled with cancellous bone acting as bone graft to enhance boney ingrowth and bone graft incorporation eventually.

Now turning to FIG. 5, a second preferred embodiment of the artificial patella component 44' is shown. Each of the primed reference numerals correspond exactly to the unprimed reference numerals of FIG. 3 except as indicated. The preferred embodiment of FIG. 5 comprises four flexible divergent lobes 74-80 instead of the two flexible divergent lobes 54 and 56 of the first embodiment. The flexible divergent lobes 74-80 are about one half the arcuate size of the flexible divergent lobes 54 and 56 of the first embodiment and provide for four independent areas of engagement between the artificial patella component 44' and the first cavity 68' of the natural patella 42'. In addition, the slots 82 and 84 formed by the flexible divergent lobes 74-80 provides greater area for accepting bone cement to enable a more rigid fixation of the artificial patella component 44' to the natural patella 42'.

Now turning to FIG. 6, yet another preferred embodiment is shown as a longitudinal cross-sectional view of an artificial patella component 44" similar to that of FIG. 3. Each of the double primed reference numerals correspond exactly to the unprimed reference numerals of FIG. 3, except where indicated. The artificial patella component 44" is shown as having the same geometric configuration as the artificial patella component 44 of FIG. 2, but in which the flexible divergent lobes 54" and 56" are part of a modular threadable member 86". The threadable member 86" has a stem 88" for threadable engagement within a threaded cavity 90" located in the dome-shaped portion 46". This embodiment may be preferred when the dome-shaped portion 46" includes a porous metal backing 92 secured to the undersurface 52" of the artificial component 44". In this manner, the flexible divergent lobes 54" and 56" can be rigidly secured to the dome-shaped portion 46" despite the presence of the metal backing 92. As will be appreciated by those skilled in the art, the metal backing 92 may be porous coated to facilitate bone ingrowth so as to provide additional stability.

The foregoing discussion discloses and describes merely exemplary embodiments of the present invention. As those skilled in the art will appreciate, the present invention may be used with resurfacing patellae as well as inlaid or recessed patellae. One skilled in the art will readily recognize from such discussion, and from the accompanying drawings and claims, that various changes, modifications and variations can be made therein without departing from the spirit and scope of the invention as defined in the following claims.