The Biomechanics of Medial Patellofemoral Ligament Repair Followed by Lateral Retinacular Release

Background: Repair of the medial patellofemoral ligament (MPFL) for acute patellar instability has recently become popular, with good clinical success rates reported in the literature. Usually, a lateral retinacular release (LRR) is added to the medial repair in an effort to "rebalance" the patella. In the native knee, however, isolated LRR reduces the force required to displace the patella laterally and may be an undesirable component of instability surgery. Hypothesis: The authors' hypothesis was that LRR, when performed after MPFL repair, would reduce the force required to displace the patella laterally. Study Design: Controlled laboratory study. Methods: Eight fresh-frozen human cadaveric knees were prepared as a model for acute patellar dislocation by transecting the MPFL at its patellar attachment. The knees were sequentially tested in the native (control), cut MPFL, repaired MPFL, and repaired MPFL with LRR conditions. Each knee was mounted and tested on an MTS machine that measured the amount of force required to displace the patella 1 cm laterally. Testing was done at 0 degrees, 15 degrees, 30 degrees, 45 degrees, and 60 degrees of knee flexion. Results: Cutting the MPFL reduced the force required to displace the patella 1 cm laterally by 14% to 22% compared with the native knee. Repair of the ligament restored the ability of the patella to resist lateral force. Adding a lateral release to the repair reduced the force required to displace the patella 1 cm by 7% to 11% compared with the MPFL-repaired knee. Conclusion: After repair of the MPFL, adding an LRR lowered the ability of the patella to resist lateral displacement.