Fixed flexion deformity and flexion after knee arthroplasty. What happens in the first 12 months after surgery and can a poor outcome be predicted?

Fixed flexion deformity and flexion of 284 knee replacements were recorded pre-operatively at 6 weeks and 12 months after surgery. Eighteen knees (6.3%) achieved unsatisfactory movements at 12 months. Seven knees (2.4%) had a fixed flexion greater than 10°, six (2.1%) had flexion less than 90° and five (1.8%) had both. The only pre-operative variable of predictive value for a poor outcome was a high fixed flexion deformity (10 out of 52 knees P<0.001). Only one out of 15 knees with limited pre-operative flexion was restricted afterwards. Seven knees (2.4%) with good pre-operative movements developed either limited flexion or a fixed flexion greater than 10°, none had both. There were no post-operative events which contributed to this. Eight knees (2.8%) achieved very poor movements. In each case there was a significant post-operative event which contributed to this. Overall, the flexion achieved at 12 months was directly related to the pre-operative flexion. Those knees with <90° of flexion gained 29.3° (S.D. 18). Those with 130° or greater flexion lost 15.2° (S.D. 13.6). The mean pre-operative fixed flexion was reduced by 50% at 6 weeks and again at 12 months. Knees with a high fixed flexion deformity alone pre-operatively have an increased risk of developing stiffness afterwards even if the deformity is corrected at surgery. In most cases significant post-operative events play a major role in this outcome. A small percentage of knees develop stiffness without any obvious explanation.