The effect of release of the popliteus and quadriceps force on rotation of the knee.

The current study was done to determine whether an isolated, partial, or complete injury to the popliteus at the femur increases rotational knee laxity. The other aim was to determine how quadriceps loading affects internal and external rotation. Ten cadaver knee specimens with an intact posterolateral complex were held in a biomechanical testing rig at 0 degrees, 30 degrees, 60 degrees, and 90 degrees flexion. Movement of the tibia relative to the femur was measured while internal and external moments of 3 N-m were applied about the long axis of the tibia. Laxity was assessed for an intact specimen, and with partial and complete detachment of the popliteus femoral insertion. In five of the 10 specimens laxity additionally was assessed with sufficient quadriceps loading to resist 100 N vertical force at the hip. The results showed that partial and total release of the popliteus increased external laxity of the knee by as much as 6.6 degrees (90 degrees flexion) and by as much as 3.5 degrees (90 degrees flexion). Quadriceps loading reduced internal and external knee laxity significantly. Injury of the popliteus at the femoral insertion may be associated with increased rotational laxity of the knee. An increase in quadriceps force may be necessary to control increased external rotation of the tibia.