Detrimental kinematics of a flat on flat total condylar knee arthroplasty.

Fourteen subjects having a flat on flat condylar posterior cruciate retaining total knee arthroplasty were evaluated under different in vivo weightbearing conditions, with six performing a deep knee bend and eight walking at normal gait. An interactive model fitting algorithm was used to convert two-dimensional fluoroscopic images into three-dimensional computer aided design solid model images. The femorotibial contact positions for the medial and lateral condyle started posterior at full extension. With a deep knee bend the lateral condyle acted as a pivot, and the medial condyle slid in the anterior direction. Five of six had lateral condyle liftoff (maximum 1.6 mm) and abnormal positive screw home motion was seen from 0 degree to 90 degrees flexion. During gait, all femorotibial contact positions were posterior in extension and throughout the cycle. Six of eight patients experienced lateral condyle liftoff (maximum 3.5 mm), but minimal screw home motion was seen. Abnormal medial condyle posteroanterior sliding, lateral condyle liftoff, and erratic screw home motion may be related to abnormal wear characteristics of this flat on flat condylar design.