Intraoperative patellar tendon strain: predicting the range of knee flexion after total knee arthroplasty

Background  The preoperative range of motion is an important factor that influences the range of motion after total knee arthroplasty. Because the length and tightness of the extensor mechanism are extracapsular elements with an influence on knee flexion, it is reasonable to assume that the tension of the knee extensor mechanism during surgery has a considerable impact on the postoperative range of motion. The purpose of this study was to determine the influence of the tightness of knee extensor mechanism on postoperative knee flexion. Methods  In 18 knees undergoing posterior-stabilized type total knee arthroplasty, we measured the longitudinal strain on the patellar tendon with all the components in position during passive knee flexion up to 135°. The patellar tendon strains measured during surgery were compared with the preoperative maximum knee flexion angle and postoperative maximum knee flexion angle at 1 year. Results  There was a significant inverse correlation between the patellar tendon strain during surgery at 60° (r = -0.54, P < 0.05), 90° (r = -0.55, P < 0.05), or 135° of flexion (r = -0.65, P < 0.05) and postoperative knee flexion. Conclusions  The results indicated that subjects with high intraoperative patellar tendon strain during passive flexion of the knee had more restricted postoperative knee flexion. Therefore, the tightness of the knee extensor mechanism measured at total knee arthroplasty is a good predictor of maximum postoperative range of flexion.