Prosthesis (i.e. – artificial body members) – parts thereof – or ai – Implantable prosthesis – Bone
Reexamination Certificate
2002-08-02
2004-09-28
Philogene, Pedro (Department: 3732)
Prosthesis (i.e., artificial body members), parts thereof, or ai
Implantable prosthesis
Bone
C623S020290
Reexamination Certificate
active
06797005
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The subject invention relates to a knee joint prosthesis, and particularly a posterior stabilized replacement knee joint prosthesis.
2. Description of the Related Art
A natural knee joint includes the distal end of the femur with articular cartilage, the proximal end of the tibia with articular cartilage and a meniscus between the femur and tibia. The femur and the tibia are held in a proper relationship to the bearing by ligaments. These stabilizing ligaments include the posterior cruciate ligament, the anterior cruciate ligament and collateral ligaments.
Flexion of the knee causes the tibia to rotate relative to the femur about an axis that extends generally in a medial-to-lateral direction and simultaneously causes the contact area of the femur to roll back relative to the tibia. Flexion also generates rotation of the tibia about its own axis. The amount of rotation of the tibia during flexion of the knee is controlled and limited by the ligaments.
The natural knee joint can become damaged or diseased. For example, damage or disease to the knee can deteriorate the articular surfaces of the femur or tibia and can damage the articular cartilage between the bones. The prior art includes prosthetic knee joints to replace a damaged or diseased natural knee. A prosthetic knee joint typically includes a femoral component that is mounted to the distal end of a resected femur, a tibial component mounted to the proximal end of a resected tibia and a bearing between the femoral and tibial components. The inferior face of the femoral component of a prosthetic knee joint typically defines a pair of arcuate convex condyles. The superior face of the bearing has a corresponding pair of arcuately concave regions for articular bearing engagement with the condyles of the femoral component. The superior face of the tibial component may be substantially planar and is disposed in engagement with the inferior face of the bearing.
Prior art prosthetic knee joints have taken many different forms, depending upon the preferences of the orthopedic surgeon, the condition of the natural knee and the health, age and mobility of the patient. Some prior art knee joint prostheses fixedly secure the inferior surface of the bearing to the superior surface of the tibial component. Other prior art knee joint prostheses permit rotational movement between the bearing and the tibial component. Still other prior art knee joint prosthesis permit a controlled amount of anterior-posterior sliding movement between the bearing and a tibial component. Movement of the bearing relative to the tibial component achieves many functional advantages, as described in the prior art. Prior art knee joint prostheses that incorporate certain of the structural and functional features referred to above are disclosed in U.S. Pat. Nos. 4,470,158 and 4,309,778.
As noted above, the inferior bearing surface of the femoral component on most prosthetic knee joints comprises a pair of convexly arcuately condyles. The condyles of the femoral component are in articular bearing engagement with arcuately concave regions on the superior face of the bearing. Thus, the superior face of the bearing typically includes a pair of dished regions each of which has a relatively depressed center portion and a relatively elevated peripheral lip. As explained above, flexion of the knee joint causes the tibia to rotate about a medial-lateral axis relative to the femur. Flexion also causes the tibia to rotate around its own axis. These combined movements cause the condyles of the femur to ride up or climb the concavities on the superior surface of the bearing and to approach the peripheral lips of the bearing. Thus, flexion tends to move the relative components of the prosthetic knee toward dislocation. The degree to which dislocation is possible depends on several factors, most significantly, the presence or absence of ligaments. The likelihood of dislocation also depends upon the degree of flexion and on the degree of congruency between the inferior articular bearing surface of the femoral component and the superior surface of the bearing. For example, climbing of the femoral component on the bearing is not a significant problem in prosthetic knees that have a substantially flat superior surface on the bearing. However, the relatively great incongruency between the inferior bearing surface of the femoral component and the superior surface of the bearing on these prosthetic knees results in a very high contact stress that can damage the bearing. Prosthetic knees that have greater congruency between the femoral component and the bearing provide desirably low contact stress. However, the greater congruency when combined with a bearing that is slidable on the tibial component creates the problem of the tibial component climbing on the bearing, and hence creates the potential of dislocation. Climbing of the femoral component on the bearing also is a particular problem for prosthetic knee joints that employ a posterior stabilization post. In particular, the climbing of the femoral component on the bearing substantially increases sheer forces on the post and can lead to traumatic failure of the prosthesis.
Valgus-varus stability of a knee joint refers to the ability of the joint to resist the lateral forces or rotary forces that would cause rotation of the tibia relative to the femur in the frontal plane. Lateral forces or rotary movements that cause rotation of the tibia relative to the femur in the frontal plane tend to create a dislocation. Such dislocation is particularly likely to occur on either the medial or lateral side of the prosthesis, depending upon the direction of the lateral forces. Such a dislocation in a prior art prosthesis is shown in
FIG. 18
hereto.
The prosthetic knee joint is under a compressive loading during normal activities. As a result, valgus-varus moments typically are resisted adequately by the articulating surfaces of the prosthetic components and by the ligaments. However, there are instances where additional valgus-varus stability may be desired, such as those instances where ligaments are deficient.
Some prior art prosthetic knee joints enhance valgus-varus stability by providing a stabilization post that extends into a posterior region between the femoral condyles. This region would be occupied by the posterior cruciate ligament if that ligament were present. Prosthetic knee joints that permit anterior-posterior sliding movement of the bearing on the tibial component provide superior roll back. In this regard, the term “roll back” refers to a posterior movement of the contact point of the femur relative to the tibia during flexion. Roll back, however, causes the femoral component to climb on the bearing, and thus increases the probability of dislocation. Additionally, this greater roll back and increases of climbing of the femoral component on the bearing substantially reduce shear forces on the posterior stabilizing post for those prosthetic joints that have such a posterior stabilizing post. A prosthetic bearing that can slide posteriorly during flexion avoids impingement between the bearing and anterior soft tissue of the knee. Thus, a prosthetic knee joint with a bearing capable of anterior-posterior sliding movement can avoid discomfort during deep flexion.
A prior art prosthetic knee joint with a stabilizing post and a bearing capable of anterior-posterior sliding movement is shown in U.S. Pat. No. 5,395,401 which issued to Bahler. In particular, U.S. Pat. No. 5,395,401 shows a prosthetic knee having a tibial component and a bearing slidably disposed on the superior face of the tibial component. The inferior surface of the bearing is provided with a dovetailed groove that extends along an anterior-posterior direction and at a location between the two concave condyles formed on the superior surface of the bearing. The bearing shown in U.S. Pat. No. 5,395,401 also includes a notch extending into the posterior portion of the bearing at a location between the two concave co
Biomedical Engineering Trust
Casella Anthony J.
Hespos Gerald E.
Philogene Pedro
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