Background and Summary of the Invention

The present invention relates generally to prosthetic implants for replacing natural joints. More particularly, the present invention relates to prosthetic implants which include a load bearing member which dampens and distributes forces within the implant.

The treatment of damaged or traumatized joints, such as hip joints and knee joints, may be repaired through surgical procedures. However, when the damaged tissue cannot be surgically repaired, total replacement of the joint with a prosthesis may be required. Both knee joints and hip joints can be replaced with a prosthetic implant. In conventional prosthetic knee assemblies, a femoral component is attached to a resected femur, and a tibial component is attached to the resected tibia. The femoral component articulates with the tibial component to replicate the motion provided by the natural knee. Typically, the femoral component includes a pair of spaced apart bearing surfaces or condyles which represent the condylar surfaces found on the femur. The tibial component typically includes a platform having a pair of spaced apart bearing surfaces which respectively engage the condylar surfaces of the femoral implant. In conventional hip prosthetic assemblies, portions of the innominate, or hip, bone are resected, and an acetabular cup is attached thereto. The acetabular cup includes a generally concave surface, which engages a second component of the implant, a femoral implant. The superior portion of the femur is resected and the femoral implant is attached thereto. The femoral implant typically includes a head or ball thereon which engages the concave surface of the acetabular cup which is implanted in the pelvic bone. The relationship of the femoral implant

within the acetabular cup imitates the natural motion provided by the hip joint.

One problem associated with known joint prostheses is that they are unable to replicate the ability of a natural joint to dampen and distribute forces. When a joint is replaced with a prosthetic implant, much of the surrounding tissue is also often removed. For example, when a damaged knee is replaced with a knee prosthesis, much of the lateral and medial menisci and anterior and posterior cruciate ligaments are incised and removed. In a natural knee, these tissues not only bond the femur to the tibia, they also dampen and distribute forces between the two bones. As these tissues have been removed during the implantation procedure, a knee prosthesis typically lacks the natural force dampening and redistribution capabilities of a natural knee.

Similarly, when a hip joint is prepared for a prosthetic implant, much of the surrounding tissue is removed. Portions of the iliofemoral and pubofemoral ligaments are removed. The replacement prosthesis typically is unable to replicate the force dampening and redistribution qualities of these tissues.

Accordingly, one object of the present invention is to provide a prosthetic joint implant which closely replicates the functions of the natural joint.

Another object of the present invention is to provide a prosthetic joint implant which replicates the force dampening and redistribution capabilities of the natural joint. According to the present invention, a prosthetic joint implant is provided for replacing a natural joint. While the present invention may be used in conjunction with various prosthetic joint implants, the most preferred embodiments are for use in conjunction with a prosthetic knee implant and a prosthetic hip implant. The prosthetic

knee implant includes a femoral implant, a tibial implant and a load bearing member positioned therebetween. The femoral implant is inserted onto the femur, and has opposing condylar surfaces representing the lateral and medial condyles of the femur. The tibial implant includes a stem, or a stemless device, which is implanted onto the tibia, and a platform surface exposed adjacent the femoral implant.

The load bearing member is positioned between the tibial implant and femoral implant. The load bearing member includes a first shell member, a second shell member, and a filler material disposed therebetween. The first shell member, positioned adjacent the condylar surfaces of the femoral implant, includes opposing condylar bearing surfaces thereon. The condylar bearing surfaces mate with the condyles on the femoral implant, and replicate the motion provided by the natural knee joint. The first shell member also includes a raised ledge or plateau between the opposing condylar bearing surfaces. The plateau is positioned between the opposing condyles on the femoral implant, and provides medial and lateral support for the femoral implant.

The load bearing member also includes a second shell member, positioned adjacent the platform on the tibial implant. The second shell member is generally planar and rests substantially on the resected boney platform, or into a structural platform part of the implant system. The first shell member and second shell member contact one another along their respective perimeters, to form a gap therebetween. The filler material is disposed within the gap, and may substantially fill the entire gap area. The filler material is contained within the gap, and restrained from expanding outside of the first and second shell members. The filler material is of hydrostatic composition, and thus displays fluid properties. When a

force is applied to one of the two shell members, the force is transmitted through the filler and is dampened and redistributed. The resultant force on the opposing shell member is thus reduced and redistributed, so that the force transmitted to the adjacent bone provides less stress and pressure to the bone and surrounding tissue. By distributing the load at the femoral/tibial interface, the reduced force will afford improved wear properties. A similar load bearing member is used in conjunction with a prosthetic hip implant. The prosthetic hip implant of the present invention generally includes an acetabular cup which is implanted into the innominate bone, a femoral implant attached to the femur, and a load bearing member positioned therebetween. The acetabular cup replaces the natural acetabular socket located in the pelvic bone. The acetabular cup includes a generally concave, semi-spherical surface which receives a portion of the femoral implant.

The femoral implant includes a stem which is implanted into the femur. The femoral implant also includes a head having a femur ball thereon. The femur ball has a generally convex bearing surface which is positionable within the acetabular cup. Positioned between the bearing surface of the femoral implant and the acetabular cup is a load bearing member.

The load bearing member includes a first shell member, second shell member and filler material. The first shell member is positioned on the bearing surface of the femoral implant, and includes a shoulder and lip thereon, which engage with the periphery of the second shell member. The second shell member is generally semi-spherical in shape, and is positioned within the acetabular cup. Thus, the two shell members combine to form a generally semi- spherical gap therebetween. The filler is disposed within the gap, substantially filling the entire space. The

filler material is restrained within the two shell members, and constrained from expanding outside of the gap. As with the prosthetic knee device, the filler is preferably a hydrostatic material which allows for forces being transmitted through the filler to be dampened and redistributed.

The prosthetic joint implant of the present invention thus provides the ability to replicate the dampening and redistribution capabilities of a natural joint. As a force is transmitted through the filler material of the load bearing member, the force is dampened and redistributed, such that the resultant force produces less stress and strain at the implant interface and on the surrounding tissue. Additional objects, features, and advantages of the invention will be apparent to those skilled in the art upon consideration of the following detailed description of preferred embodiments exemplifying the best mode of carrying out the invention as presently perceived.

Brief Description of the Drawings

Fig. 1 is a cross-sectional view of a prosthetic knee implant according to the present invention, shown in its implanted position with respect to the adjacent tibia and femur bones;

Fig. 2 is a cross-sectional view of a prosthetic hip implant of the present invention, showing the device in an implanted position with respect to the hip bone and the femur; Fig. 3 is a cross-sectional view of a second illustrative embodiment of a load bearing member of a prosthetic hip implant of the present invention;

Fig. 4 is a cross-sectional view of a third illustrative embodiment of a load bearing member of a prosthetic hip implant of the present invention;

Fig. 5 is a cross-sectional view of a fourth illustrative embodiment of a load bearing member of a prosthetic hip implant of the present invention;

Fig. 6 is a cross-sectional view of a fifth illustrative embodiment of a load bearing member of a prosthetic hip implant of the present invention;

Fig. 7 is a cross-sectional view of a second illustrative embodiment of a tibial implant of a prosthetic knee implant of the present invention; Fig. 8 is a cross-sectional view of a third illustrative embodiment of a tibial implant of a prosthetic knee implant of the present invention.

Detailed Description of the Preferred Embodiments The present invention is directed to a load bearing member used in conjunction with an orthopedic implant device. One illustrative embodiment is for a prosthetic knee device, shown in Fig. 1 and indicated by the reference numeral 10. Prosthetic knee device 10 is designed to be surgically implanted as a replacement for a damaged knee joint.

When a knee joint has become damaged or otherwise impaired, it often may be replaced with a prosthetic knee. Portions of the traumatized knee joint and surrounding tissue are removed in a surgical procedure preparatory to implanting the prosthetic device. Inferior portions of femur 12, including the medial and lateral condylar surfaces of the femur, are resected. The inferior surface of the femur is further prepared, as is well understood, so that a prosthetic device may be attached. Similarly, superior portions of tibia 14 are resected and the tibia is prepared to receive an implant. Some ligaments and other connective tissue may also be removed from areas adjacent the femur and the tibia. The implant preparatory surgical

procedure readies the inferior surface of femur 12 and the superior surface of tibia 14 for the prosthetic implant. As with known knee prostheses, prosthetic knee device 10 includes femoral implant 20 and tibial implant 30. Femoral implant 20 is surgically implanted into the inferior surface of femur 12. Femoral implant 20 can include one or more posts 22 which are inserted directly into the femur bone. A cement or grout (not shown) may also use to bond femoral implant 20 to femur 12. As can be seen in Fig. 1, femoral implant 20 includes two condylar surfaces 24, representing the medial and lateral condyles of the knee joint. Condylar surfaces 24 of femoral implant 20 allow femur 12 to translate relative to tibia 14, thus replicating the motion provided by the natural knee joint. In a similar fashion, tibial implant 30 is surgically implanted to tibia 14, superior to the tibial tuberosity. Tibial implant 30 includes stem 32 which is implanted directly into tibia 14. Opposite stem 32, tibial implant 30 includes platform 34. Platform 34 is a generally horizontal, planar surface which replicates the superior surface of tibia 14.

Prosthetic knee device 10 of the present invention also includes load bearing member 40. Load bearing member 40 is positioned between femoral implant 20 and tibial implant 30. As much of the ligament tissue connecting the femur to the tibia may be removed during the implant preparatory surgery, including much of the lateral and medial menisci, load bearing member 40 acts as a stabilizing and load bearing medium between the femur and the tibia.

Load bearing member 40 generally includes first shell member 50, second shell member 60 and filler 80. First shell member 50 includes opposing condylar bearing surfaces 52, interrupted by plateau 54. Opposing condylar bearing surfaces 52 are designed to engage with condylar

surfaces 24 on femoral implant 20. The engagement of condylar surfaces 24 on femoral implant 20 with condylar bearing surfaces 52 on first shell member 50 imitate the motion provided by a knee joint. Plateau 54 projects superiorally from first shell 50 between the opposing condyles on femoral implant 20. This configuration limits both the lateral and medial movement of femur 12 with respect to load bearing member 40, and thus also with respect to tibial implant 30 and tibia 14. First shell member 50 also includes downwardly extending flange 56, formed around the periphery of the shell. As the shell transitions from condylar bearing surfaces 52, flange 56 is formed substantially orthogonally to condylar bearing surface 52. Thus, interior surface 58 of first shell 50, together with surrounding peripheral flange 56, generally forms a cavity therebetween.

Load bearing member 40 also includes second shell member 60. The second shell member is a substantially planar body, which includes contact surface 62 designed to mate with the superior surface of tibial implant 30, and engagement surface 64.

Peripheral flange 56 on first shell member 50 contacts and abuts the perimeter of second shell member 60, such that the cavity formed within the first shell member is substantially closed by the second shell member. Thus, a gap or cavity is formed substantially between the first and second shell members. First shell member 50 and second shell member 60 are connected such that this gap is enclosed. The two shell members may be connected in any of a number of ways. For example, first shell member 50 and second shell member 60 can be connected about their peripheries by ultrasonically welding the two components together. The two shell members could also be bonded together using any well known bonding agent, adhesive, or molding technique.

First shell member 50 and second shell member 60 can also be connected to one another in a mechanical fashion. For example, one of the shell members could be compression or injected molded, forming the inner member of the connecting pair. The remaining shell member would be machined or molded as the outer or encapsulating body. The two would fit together in a pressure, snap-fit arrangement. A further manner in which the two shell components could be mechanically connected together to form the enclosed gap is by a mechanical locking fixture. For example, first shell member 50 may include a substantially V-shaped groove on its perimeter, which engages a tapered portion of the peripheral edge of second member 60. Further, in some cases, first shell member 50 may be connected directly to tibial implant 30 such that platform 34 serves as the second shell member.

The composition of first shell member 50 and second shell member 60 may also be a factor in determining the method of connecting the two bodies together. It is preferred that the two shell members be constructed of a durable, generally rigid material. Known bone implants are constructed of ultra-high molecular weight polyethylene (UHMWPE) . This material is effective because its homogeneity allows it to be accepted by the living tissue, and because it will resist distortion and creep when loaded. For these reasons, first shell member 50 and second shell member 60 may be constructed of UHMWPE. However, other materials, including metals, ceramics or plastics can be used to form the two shell members. Once connected together, first shell member 50 and second shell member 60 form the gap therebetween. Load bearing member 40 also includes filler 80 which is disposed within the gap. Filler 80 is preferably of a hydrostatic composition, having a low modulus. Thus, filler 80 displays rubber-like properties similar to a fluid.

Because the gap is enclosed, filler 80 cannot escape. Thus the filler is retained within the gap and provides the dampening and redistributing feature of the present invention. When a force is transmitted through load bearing 40, filler 80 dampens and redistributes the force. As can been seen in Fig. 1, filler 80 cannot escape either laterally or medially outside the confines of the prosthetic joint. Filler 80 substantially fills the gap formed between first shell member 50 and second shell member 60.

Filler 80 can comprise a low modulus material which dampens and redistributes forces between the two shell members. As examples, the following materials may be used to construct filler 80: injection molded or poured polyurethane; silicon; latex; or poly butadien rubber.

Depending upon the composition used, filler 80 may be pre¬ formed or molded, injection molded or poured into a pre¬ formed shell.

When a force is applied in a substantially downward direction, as is shown in Fig. 1 by the letter F, the force is transmitted from the tibia through condylar surfaces 24 of femoral implant 20 and to load bearing member 40. If force F is distributed in an even manner from the femoral implant to first shell member 50 of load bearing member 40, it will be similarly distributed throughout filler 80, second shell 60 and tibia 14. If force F is not uniform, this uneven force will be transmitted by first shell member 50 to filler 80. The hydrostatic nature of the filler causes force F to be distributed such that the resultant force F* transmitted from filler 80 to second shell 60 is a more uniform force than force F transmitted from femur 12 to first shell member 50. Filler 80 dampens and distributes the force such that less stress or strain is applied by the resultant force F' . This distribution application works in a similar

fashion when a force is initially applied to second shell member 60 and is transmitted through load bearing member 40 to femur 12. Load bearing member 40 also dampens and redistributes forces which are applied to prosthetic knee device 10 at substantially non-direct or non-vertical angles.

Another illustrative embodiment of the present invention is shown in Fig. 2, showing hip prosthetic device 100. Hip prosthetic device 100, much like knee prosthetic device 10, is a replacement prosthesis for a damaged or traumatized hip joint.

During the implant preparatory surgery, the acetabulum on the innominate, or hip, bone is prepared to receive an implant. Similarly, the superior portion of the femur is prepared for an implant. The femoral head and much of the greater trochanter are resected from the femur. Once the traumatized joint and surrounding tissue are removed, hip prosthetic device 100 may be implanted.

Hip prosthetic device 100 includes acetabular cup 110 and femoral implant 120. Acetabular cup 110 can be secured to innominate bone 108 using any of a number of commonly known methods. Acetabular cup 110 includes surface 112 which is left exposed. Surface 112 is generally concave and semi-spherical, and is dimensioned to receive a portion of femoral implant 120.

Femoral implant 120 includes stem 122 which is implanted into the femur, and a projecting head 124. Head 124 replaces the head of the femur removed during the preparatory surgery. The head includes a ball, or bearing surface 126 which is generally convex and semi-spherical in shape. Bearing surface 126 is dimensioned to be received within concave surface 112 of acetabular cup 110. The engagement of the ball within acetabular cup 110 provides for relative motion between femur 106 and innominate bone 108 which replicates the motion provided by the hip joint.

As with knee prosthetic device 10, hip prosthetic device 100 also includes a load bearing member, indicated by the reference numeral 140. Load bearing member 140 is positioned between the interface of bearing surface 126 of femoral implant 120, and surface 112 of acetabular cup 110. Load bearing member 140 includes first shell 150, second shell 160, and filler 180.

First shell member 150 is generally semi- spherical in design and includes interior surface 152 which is dimensioned to snugly receive head 124 of femoral implant 120. First shell member 150 also includes shoulder 154 along the periphery of the shell, extending substantially orthogonally to the periphery of the shell. The distal edge of shoulder 154 includes lip 156 thereon extending substantially orthogonally to the shoulder.

Second shell 160 is also generally semi-spherical in design and is designed to fit snugly adjacent the surface 112 of acetabular cup 110.

First shell member 150 and second shell member 160 fit together such that the distal peripheral edge of the second shell fits within lip 156 on shoulder 154 of the first shell. Thus, a substantially semi-spherical gap or cavity is formed between the first and second shell members. Filler 180 is disposed within this gap, and substantially fills the gap. As with the gap in the prosthetic knee joint, this gap is substantially closed. Thus, filler 180 is retained within the gap and cannot escape.

First shell member 150 and second shell member 160 can be connected about their respective peripheries in any manner as previously discussed with respect to prosthetic knee device 10. As with the prosthetic knee device, first shell member 150 and second shell member 160 may be constructed of any of a number of metallic, ceramic or plastic materials. For example, first shell member 150

and second shell member 160 may be constructed of UHMWPE. Furthermore, as discussed earlier in regard to the prosthetic knee device, filler 180 may comprise a low modulus material. Force R represents a force applied by the femur through head 124 of femoral implant 120 and to the hip bone. As force R contacts first shell member 150 of load bearing member 140, it is distributed and more evenly directed through filler 180. Thus, resultant force R' is more evenly distributed along second shell member 160, and thus along the acetabular surface of innominate bone 108. Likewise, a force S directed from the hip through acetabular cup 110 is dampened and redistributed as it passes through filler 180. The resultant force S' is more evenly distributed and causes less stress and strain on the femur and reduces the resultant force at the interface, thus improving the wear qualities of the device.

In both prosthetic knee device 10 and prosthetic hip device 100, the load bearing member serves to dampen, dissipate, and to evenly distribute forces through the prosthesis from one adjacent bone to another. The plastic or resilient property of the filler evenly distributes the force to the adjacent shell member.

Fig. 3 shows another illustrative embodiment of the present invention. Load bearing member 240 is designed for use in a prosthetic hip implant, and is positioned between an acetabular cup and a femoral implant. Load bearing member 240 includes a single, unitary shell 210 having a hollow cavity or gap therein. This gap or cavity is filled with filler 280. In this embodiment, when a force is exerted on any portion of shell 210, the force is distributed and dampened as it travels through filler 280.

Another illustrative embodiment of the present invention is shown in Fig. 4. Load bearing member 340 is designed for use in a prosthetic hip implant, to be

positioned between a femoral implant and an acetabular cup. Load bearing member 340 includes first shell member 350 and second shell member 360, which fit together to form a cavity therebetween. This cavity is filled with filler 380. As with the embodiment shown in Fig. 2, first shell member 350 includes shoulder 354 about the periphery of the shell. The peripheral edge of second shell member 360 overlaps shoulder 354, to form interface 390 between the two shell members. In this manner, filler 380 is closed within the gap or cavity formed between first shell member 350 and second shell member 360. Second shell member 360 may be formed of UHMWPE, or a rigid material, such as metal or ceramic, while first shell member 350 may be formed of UHMWPE. Another illustrative embodiment of the present invention is shown in Fig. 5. This embodiment illustrates load bearing member 440 designed for use in a prosthetic hip implant. In this embodiment, first shell member 450 and second shell member 460 mate along an arcuate interface 490. First shell member 450 and second shell member 460 are joined using an adhesive or other commonly known bonding arrangement. The two shell members form a gap or cavity therebetween, in which filler 480 is disposed.

Fig. 6 shows another illustrative embodiment of a load bearing member for a prosthetic hip implant. Load bearing member 540 includes a unitary shell 510 having a hollow cavity or gap therein. Discrete bodies, such as strips or balls, of filler 580 are disposed at predetermined locations within the cavity. As a force is exerted on shell 510, the force is distributed and dissipated by filler 580. Filler 580, displaying rubber¬ like properties, deforms in response to the force applied to shell 510, and is allowed to expand within the cavity. Fig. 7 shows another illustrative embodiment of the present invention. In this embodiment, tibial implant

630 is designed for use in a prosthetic knee implant. Tibial implant 630 is of a single, unitary design. In this embodiment, no separate load bearing member is required. A gap or cavity is formed within tibial implant 630 which is filled with filler 680. Thus, forces exerted on tibial implant 630 are reduced and dampened as they transition through filler 680.

Fig. 8 shows tibial implant 730 and load bearing member 740 of another illustrative embodiment of the present invention. Load bearing member 740 includes a single shell member 770. Shell member 770 includes a pair of gaps or cavities cut or otherwise formed therein. Load bearing member 740 is disposed adjacent tibial implant 730 such that the gaps or cavities in the load bearing member are enclosed by tibial implant 730. In this manner, two gaps or cavities which are enclosed are formed. Filler 780 is disposed within these two cavities and acts as a dampening and redistributing mechanism for forces exerted on the implant. Although the invention has been described in detail with reference to certain embodiments, variations and modifications exist within the scope and spirit of the invention as described and defined in the following claims.