Flexion after total knee replacement. A comparison between the Medial Pivot knee and a posterior stabilised implant.

A comparison was made between 261 knees replaced with the Medial Pivot arthroplasty and 288 replaced with the 913 posterior stabilised arthroplasty (PS knee). There was no significant difference in the flexion obtained at 12 months after surgery (111 degrees and 109 degrees , respectively). When the knees were grouped into preoperative flexion ranges, there was no significant difference between the two implants. Those knees with preoperative flexion up to 90 degrees gained most (mean 22.6 degrees and 19 degrees for the PS knee and Medial Pivot, respectively). Knees with a preoperative flexion of 125 degrees or greater lost flexion. Regression analysis of individual knees revealed a small (average 2.9 degrees ), but significant greater loss of flexion at 12 months after surgery in the Medial Pivot group, with increasing preoperative flexion (beta coefficient = 2.923, P = 0.007). Some knees in both groups which had less than average preoperative flexion lost rather than gained flexion. We attributed this to patient factors such as pain, swelling and poor compliance with rehabilitation. Although the Medial Pivot knee may have advantages in terms of contact area and kinematics we found no advantage in terms of postoperative flexion over our posterior stabilised knee.