Knee rotation after unicompartmental arthroplasty

Knee rotation was tested in twenty patients before and 6 months after unicompartmental replacement. Rotation during active extension was measured using a Genucom Knee Analysis System. From 90° of flexion the tibia normally rotates internally, followed by external rotation until full extension. Compared to 14 healthy control knees the internal rotation was about the same but the external rotation was less (P < 0.01). There was no difference between the pre- and postoperative measurements. Ten patients had replacement with a constrained prosthesis (PCA) and ten patients replacement with a non-constrained prosthesis (Oxford). There was no difference in rotation between the two prostheses. A possible explanation is that ligaments are more important in the restriction of rotation than is the shape of condyles. The extension lag in several knees was not found to be crucial.