Relative role changing of lateral collateral ligament on the posterolateral rotatory instability according to the knee flexion angles: a biomechanical comparative study of role of lateral collateral ligament and popliteofibular ligament

Purpose This cadaveric study assessed the relative role of the lateral collateral ligament (LCL) and popliteofibular ligament (PFL) in limiting tibia external rotation. Methods Eight paired cadaveric knees were divided into two groups. The specimens were mounted on a rotational wheel and 5 Nm external rotation torque was applied before and after cutting the ligaments at 0°–30°–60°–90° knee flexion. Three cutting steps were applied: (1) PT (popliteus tendon)-, (2) LCL-, (3) PFL in group I, and (1) PT-, (2) PFL-, (3)LCL in group II. Increased external rotation at each step was taken as the ratio of final external rotation at the end of step 3. Repeated measure ANOVA and a Mann–Whitney U test were used for statistical analysis. Results At step 2, the ratio of increased external rotation after cutting the LCL (group I) was similar to the ratio after cutting the PFL (group II) at 0° and 30° flexion, but that of group I was lower than group II at 60° and 90° flexion ( p = 0.029 and p = 0.029). At step-3, the ratio after cutting the LCL (group II) was less than the ratio after cutting the PFL (group I) at 90° flexion ( p = 0.029). Conclusion The PFL and LCL play equally important roles in limiting external rotation at the knee extended position (0°, 30°) but the LCL contribution becomes smaller than PFL at the flexed position (60°, 90°).