Detrimental kinematics of a flat on flat total condylar knee arthroplasty

Fourteen subjects having a flat on flat condylar posterior cruciate retaini ng total knee arthroplasty were evaluated under different in vivo weightbea ring 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 desi gn solid model images. The femorotibial contact positions for the medial an d lateral condyle started posterior at full extension. With a deep knee ben d 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 degrees to 90 deg rees flexion, During gait, all femorotibial contact positions were posterio r 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 lif toff, and erratic screw home motion may be related to abnormal wear charact eristics of this flat on flat condylar design.