Controlling the motion of total knee replacements using intercondylar guide surfaces

Total knee replacements using intercondylar cams, such as posterior stabili zed types, have been in use for many years. In a previous study, software w as written to analyze an alternative shape of the intercondylar cams. The g oal of the current study was to investigate in a more general way the poten tial of intercondylar cams, or guide surfaces, for reproducing the anterior -posterior motion of the natural knee throughout the flexion range. Typical sagittal outlines for the femoral and tibial hearing surfaces were defined , and a parametrized shape for the femoral guide surface was defined to pro duce a wide range of shapes. Software was written in which the femoral comp onent was flexed in increments, with the posterior translation defined as a function of the flexion angle. The shape of the tibial guide surface was d erived from the locus of the femoral guide surface at its multiple flexion positions. By iterating methodically through possible shapes of femoral gui de surfaces, several types of total knee replacement components in common u se today were identified, as well as other configurations of potential inte rest. For quantification of a given design, the software calculated the ant erior and posterior laxity at each flexion angle. Laxity was defined as the motion before the femoral guide surface impacted the tibial guide surface or until the contact point of the bearing surfaces reached a specified slop e. Convex femoral and concave saddle-shaped tibial guide surfaces produced small laxities in both directions over most of the flexion range. A saddle design with small laxities in the first half of flexion, combined with a po sterior stabilized feature. was an interesting combination. potential impro vements to the currently used designs were shown in this study, and new sha pes of intercondylar guide surfaces were derived that could be considered f or application.