Prosthesis (i.e. – artificial body members) – parts thereof – or ai – Implantable prosthesis – Bone
Reexamination Certificate
2000-03-13
2001-10-02
Snow, Bruce (Department: 3738)
Prosthesis (i.e., artificial body members), parts thereof, or ai
Implantable prosthesis
Bone
C623S020330
Reexamination Certificate
active
06296666
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to knee prostheses, and more particularly to a mobile knee prosthesis allowing substantial A/P movement, insubstantial M/L movement, and substantial rotation.
BACKGROUND OF THE INVENTION
Total knee protheses or knees can be divided into two general categories: fixed bearing, where the tibial insert is firmly attached to the baseplate with movement only between the femoral and tibial condylar surface; and mobile bearing, where the insert moves in relation to the baseplate as well as the femoral component. Mobile bearing knees can be further divided into two groups: a rotating platform type, in which the movement between the insert and baseplate is purely internal/external rotation; and the meniscal bearing type, which incorporates gliding in the A/P and M/L directions in addition to rotation.
After total knee replacement, failure is usually caused by loose or worn components. Loose components are caused by excessive loads being transferred to the implant/bone interface that causes interface bonding failure and the implants to loosen. With knee implants, these failures are commonly seen on the tibial side because the implant is susceptible to high shear loading. In a normal intact knee, these shear loads are resisted by soft tissue support and joint congruency.
In order to minimize the transfer of shear stresses to the implant/bone interface with a fixed bearing total knee replacement, the femoral component is allowed to freely slide on the tibia component, using soft tissue to provide constraint. In order to freely slide, these designs require low congruency between the femur and tibia resulting in low contact area. The low contact area causes high contact stresses and contact stresses shorten component life due to material fatigue.
The congruency between the femoral and tibial articulating surfaces of fixed bearing knees must be carefully balanced in order to provide maximum contact area, which lowers the stresses in the polyethylene, yet not be so constrained that normal movement of the femur on the tibia is hampered resulting in high shear stresses.
Mobile bearing knees were developed in an effort to replicate the normal biomechanics of the knee, maximizing the congruency between the tibio-femoral articulation and minimizing shear loading on the tibia. This is accomplished by allowing the tibial insert to be mobile with respect to the baseplate and configured to provide maximum contact area with the femur. These designs are indicated for patients who have adequate collateral ligamentous stability.
The Oxford Knee (Biomet) was an uni-compartmental knee that was the first to use the mobile bearing concept. The plastic “menisci” were totally unconstrained and were prone to dislocation. The LCS Knee (DePuy) was introduced shortly after the Oxford Knee and is the most widely used mobile bearing knee in the world and, having just marked its 20
th
year in clinical use, has the longest clinical experience with large numbers. The first LCS design as of the meniscal bearing type with two poly bearings that allowed retention of the ACL and/or PCL. These bearings allowed internal/external rotation and some A/P movement. This design has been fairly successful, although bearing dislocation and fractures have been problems. The second version of the LCS was the rotating only type that sacrificed both cruciate ligaments. This design has also enjoyed a good clinical track record, and because of fewer complications, is used more that the meniscal version of the LCS. A more recent version of the LCS has added A/P gliding.
Other mobile bearing knees in clinical use are: the Rotaglide Knee (Corin), which incorporates a single tibial insert that allows rotation and A/P movement and is approaching ten years of good clinical experience; and the TACK Knee (Link) which is a rotating platform. The more recent SAL (Sulzer) and MBK (Zimmer) Knees are single piece poly designs that allow A/P movement and rotation about a post centered mediolaterally on the baseplate. These two knees have good results at eight and five years, respectively. The Interax ISA (Howmedica) offers A/P movement with asymmetric rotation about a medialized post.
All of the newer designs have added A/P movement in an effort to improve on the rotating platform design or embrace the concept that mobile bearing total knee replacement should replicate, as much as possible, the natural biomechanics and kinematics of the knee while secondarily increasing the contact area of articulating surfaces.
BRIEF SUMMARY OF THE INVENTION
In accordance with the present invention, a mobile bearing knee prosthesis is provided having a femoral component having a femoral condylar surface, a tibial component, and a meniscal insert. The tibial component has an upper baseplate which includes an upstanding post, and a cap provided at an upper end of the post. The cap includes an extending lip extending laterally outward from the post whereby the cap has a generally oval shape in plan view with a major axis in the A/P direction and a minor axis in the M/L direction. The meniscal insert is disposed between the femoral condylar surface and the baseplate. The insert includes an undercut cavity which is elongated in the A/P direction and in which the post is received. The undercut cavity has a length in the A/P direction allowing substantial movement of the post therealong, and a width in the M/L direction allowing only a minor movement of the post in the M/L direction. The insert also includes an upper cutout extending around an upper portion of the cavity which is sized to receive therein an adjacent portion of the lip of the cap. With this construction, the insert and the baseplate are free to slide relative to one another along a plane of contact therebetween, except when constrained by (i) a contact of an adjacent portion of the post with a wall of the undercut cavity as movement occurs in the A/P and M/L directions of the plane of contact, whereby substantial relative movement of the insert and baseplate in the A/P direction is allowed and only a minor relative movement in the M/L direction is allowed, and (ii) an engagement of portions of the lip of the cap located between the minor axis and the major axis with a wall of the upper cutout as rotation occurs about an axis perpendicular to the plane of contact, whereby only some predetermined rotational relative movement is allowed.
In a preferred embodiment of the invention, the upper cutout includes straight portions provided between anterior and posterior ends thereof. The lip is then provided laterally with opposed pairs of flats. Respective flats of a pair are located in opposite quadrants and adjacent the minor axis. Thus, when the insert and baseplate are constrained by engagement of portions of the lip and the wall of the upper cutout, one pair of opposed flats engages adjacent straight portions of the upper cutout to provide a strong surface contact therebetween.
In the preferred embodiment, the cavity includes an anterior end and a posterior end. These ends are shaped congruently with respective anterior and posterior portions of the post. Thus, when extremes of anterior or posterior relative movement of the baseplate and the insert are experienced, about one half of the lip is received in the adjacent part of the upper cutout providing a large locking engagement to prevent separation of the insert from the baseplate.
Also in the preferred embodiment, the width of the cavity is slightly smaller than a minor diameter the lip such that the post is received in the cavity by a snap-fit of the lip past the width of the cavity. In addition, a lower edge of the undercut cavity is chamfered to provide an easy entrance of the lip of the cap upwards to the upper cutout.
Further in the preferred embodiment, the post is round in plan cross section. In addition, a difference between the minor diameter of the cap and a width of the cavity is less than 2 mm. Further, a height of the post is less than a depth of the cavity so that a space in the cavit
Encore Medical Corporation
Larson & Taylor PLC
Pellegrino Brian
Snow Bruce
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