Prosthesis (i.e. – artificial body members) – parts thereof – or ai – Implantable prosthesis – Bone
Reexamination Certificate
1999-10-14
2001-07-24
Philogene, Pedro (Department: 3732)
Prosthesis (i.e., artificial body members), parts thereof, or ai
Implantable prosthesis
Bone
C623S020210, C623S020140
Reexamination Certificate
active
06264697
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a total knee replacement prosthesis (TKR). Total knee replacement involves the surgical removal of the entire natural knee bearing surfaces and their replacement with artificial femoral and tibial components.
2. Description of Related Art
The invention is concerned with a type of TKR which includes femoral condylar surfaces which, to some extent, mimic the shape of the natural condyles and an interposed bearing component supported on a tibial platform. Condylar TKRs generally comprise (a) a femoral component having a pair of condylar surfaces, (b) a tibial component having a tibial platform fixed to the resected tibia, and (c) a bearing component usually of low friction plastic material interposed between the condylar surfaces and the tibial platform. The bearing component generally has dished surfaces for receiving the condylar surfaces of the femoral component. The bearing component can be made to be fixed onto the tibial platform or be rotatable and/or slidable in the anterior/posterior direction.
Stability of the artificial knee joint is provided by the dishing of the bearing surfaces and by the ligaments. In all cases, the collateral ligaments are required. In a fixed bearing design with dished bearing surfaces, the stability is sufficient particularly when there is a compressive force acting across the joint. In this situation, the cruciate ligaments are not necessary. For shallow bearing surfaces, which have the advantage of allowing extra freedom of motion, the posterior cruciate ligament is required. The above also applies to a mobile bearing design which only allows rotation. However, when anterior-posterior translation is allowed, the posterior cruciate ligament is required, no matter how dished are the bearing surfaces.
In the natural knee in extension, contact area is central on the tibial bearing surface or even anterior to the centre. As the knee is flexed, the contact area moves progressively posteriorly. This is important in that it provides an increasing lever arm to the quadriceps muscle in activities such as stair climbing and rising from a low chair, when there are high flexing moments acting on the knee. In addition, it is considered to be highly desirable for the femur to be displaced posteriorly with respect to the tibia at the higher flexion angles, since this will generally avoid posterior impingement of bone and soft tissues, allowing for a high range of flexion.
While proposals have been made in total knee design to provide for posterior displacement of the contact point with flexion, the corresponding problem of deliberately causing the contact point to be displaced anteriorly on extension, and the maintenance of stability during these movements has not been explored in prior proposals. The advantage of anterior displacement towards extension is an increase in lever arm of the hamstrings and gastrocnemius, thus helping to prevent hyperextension. It is, therefore, to a solution of these problems that the present inventions directed.
OBJECT AND SUMMARY
It is, therefore, an object of the present invention to provide a TKR in which the femoral component provides for roll-back posteriorly with flexion and for a corresponding compensatory displacement anteriorly with extension, while maintaining stability of the joint during such movements.
According to one aspect of the present invention there is provided a total knee replacement prosthesis which comprises:
(a) a femoral component having a pair of condylar surfaces
(b) a tibial component adapted to be attached to the tibia and having a tibial platform fixed to the resected tibia,
(c) a mobile bearing component which is interposed between each condylar surface and the tibial platform and which has dished bearing surfaces adapted to support and conform with the corresponding femoral condylar surfaces, the mobile bearing component being slidable on the tibial platform in the anterior/posterior direction; said femoral component having an intercondylar guide surface which is adapted to engage a corresponding tibial guide surface which is fixed on the tibial platform or is integral therewith, the intercondylar surface being round and having a center of curvature when viewed sagittally, which is offset from a major center of curvature of the femoral condylar surfaces.
Displacement of the femoral component with respect to the tibial component can be effected by ‘rigid’ body motion or by ‘contact’ point motion or by a combination of both. In rigid body motion, the femoral component moves bodily with respect to a tibial platform which is fixed to the tibia. Such movement can be effected by permitting a tibial bearing component to be mobile on the tibial platform. On the other hand, the contact point (or centre point of a contact area) between the femoral-tibial bearing surfaces as viewed in a sagittal plane moves during flexion. These relative movements will be discussed in more detail subsequently in this specification in connection with various figures of the drawings. In most cases, there will be a mixture of rigid body and contact point motions.
Preferably, the tibial guide surface has an anterior and posterior upward sweep which engages in recesses in the femoral component to contributive to the stability of the prosthesis at or close to maximum flexion and extension, while anterior-posterior stability is also afforded during the mid-range of flexion due to similar engagements. The tibial guide surface may also include lateral surfaces which engage with corresponding lateral surfaces adjacent to the condylar surfaces of the femoral component.
In one embodiment, the tibial guide surface may be an integral part of the tibial platform, or alternatively be a plastic component fixed relatively to the anterior/posterior direction on the tibial platform. The guide surface may be rotatably mounted on the tibial platform in order to provide internal/external rotation of the knee joint to a desirable degree, e.g. ±12 to 15 degrees. This kind of arrangement is illustrated in
FIGS. 1A and 1B
. Alternatively, the guide surface may be fixed relatively to the tibial platform and the tibial component mounted for rotation within or on a member fixed to the resected tibia. For example, the tibial component may incorporate a stem which is rotatbly received within a tubular member fixed into the tibial bone canal.
In cases where the tibial guide surface is a part of or fixed in the anterior/posterior direction to the tibial platform, the bearing component may take the form of two plastic bearing components each moveable within limits on the tibial platform. In this case, the bearing components may be guided on the tibial platform in such a way as to leave clearance between the tibial guide surface and the bearing components, to permit combinations of rotational and anterior/posterior movements. Again, this is illustrated in FIG.
1
A.
In an alternative embodiment of the invention the tibial guide surface is an integral part of the bearing component. This is illustrated in FIG.
17
. In this case, the bearing component can be fixed to the tibial platform. Sufficient laxity is then required between the femoral and tibial bearing surfaces to allow both rotational and anterior/posterior (A-P) motions to occur. If the bearing component is allowed to rotate about a vertical axis in the tibial platform, femoral-tibial laxity is only required to accommodate the A-P translation.
Posterior displacement of the femoral-tibial contact point is achieved in accordance with the invention by providing a rounded femoral guide surface, (typically as an intercondylar femoral guide surface), having a centre of curvature, when viewed sagittally, which is offset from a major centre of curvature of the femoral condylar surfaces.
By ‘major centre of curvature’ in this context is meant the centre of the major part of the arc from the lowest point (at 0° flexion) of the femoral condylar surface to its posterior-most point.
A degree of posterior displacement of the co
Burns Doane Swecker & Mathis L.L.P.
Philogene Pedro
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