Bearing Surface Design in Total Knee Replacement

Surfaces for condylar total knee replacement are designed using computergraphics techniques. An average anatomical femoral surface is represented mathematically. Mathematical equations are written to describe normal knee motion and normal laxity. Tibial surfaces are generated by placing the femur stepwise in multiple sequential positions, through a defined three-dimensional motion or laxity path. In addition, a flat tibial surface is defined, to represent the least amount of femoral-tibial conformity in currently-used knee replacements. Elasticity theory is used to calculate the maximum contact stresses at the femoral-tibial contact points. The least stresses are produced with a fixed axis cylindrical motion, while the highest are with a flat tibial surface. A surface based on laxity produces lower stresses than for normal knee motion, and is thought to be acceptable in terms of both freedom of motion and stability. Such a laxity surface is proposed as being suitable for total knee design.