The effect of a proximal tibial medial opening wedge osteotomy on posterolateral knee instability: a biomechanical study.

BACKGROUND Increased stability of posterolateral corner knee injuries has been observed clinically after proximal tibial medial opening wedge osteotomies. HYPOTHESIS Static varus and external rotatory stability will be significantly improved in a knee with a grade 3 posterolateral knee injury after a proximal tibial medial opening wedge osteotomy. STUDY DESIGN Controlled laboratory study. METHODS Biomechanical testing of 10 nonpaired, cadaveric knees was performed in the intact state, after transection of the posterolateral corner (fibular collateral ligament, popliteus tendon, and popliteofibular ligament), and after a 10-mm proximal tibial medial opening wedge osteotomy. Loading conditions consisted of 12 N.m varus moments and 6 N.m external rotation torques. Six degrees of freedom motion analysis was used to assess motion changes, and a buckle transducer was used to measure the force on the superficial medial collateral ligament during applied loads. RESULTS After transection of the posterolateral corner structures, a significant increase in varus rotation was found to applied varus moments with a mean increased opening of 5.9 degrees to varus stress at 30 degrees and 5.8 degrees at 90 degrees of knee flexion. After proximal tibial medial opening wedge osteotomy, varus rotation was increased by a mean of 1.6 degrees at 30 degrees and 1.7 degrees at 90 degrees of knee flexion compared with the intact state. There was a significant decrease in varus rotation to a varus moment after osteotomy compared with the posterolateral sectioned state at both 30 degrees and 90 degrees . External rotation of the knee increased by 4.7 degrees at 30 degrees and 4.8 degrees at 90 degrees after posterolateral structure sectioning compared with the intact state. After the osteotomy, there was a significant decrease in external rotation compared with the posterolateral sectioned state, and there was no significant difference in external rotation compared with the intact state. There was a significant increase in force on the superficial medial collateral ligament after the osteotomy compared with both the intact and posterolateral corner cut state for both an applied varus moment and external rotation torque at both 30 degrees and 90 degrees . CONCLUSION Our results demonstrate that a proximal tibial medial opening wedge osteotomy decreased varus and external rotation laxity for posterolateral corner-deficient knees. Concurrently, an increase in force was observed on the superficial medial collateral ligament compared with the native state. CLINICAL SIGNIFICANCE The improved stability observed in some patients with grade 3 posterolateral knee injuries after a proximal tibial medial opening wedge osteotomy appears to at least in part be due to tightening of the superficial medial collateral ligament. The long-term consequences of the increased force on the superficial medial collateral ligament on the medial compartment, and whether it elongates with time, merit further investigation.