In this experiment we induced posterolateral knee instability to cadaveric knees in vitro. We observed the changes in these knees to posterior displacement (PD), varus-valgus (V-V) rotation, external rotation (ER) and coupled external rotation (CER) and compared the effect of reconstructing, the popliteo-fibular ligament and the popliteus muscle in correcting posterolateral rotatory instability.

Method: Fourteen intact freshly frozen cadaveric knees were damaged using a specially constructed jig that pivoted at two points simulating the hip and ankle joints while an anteromedial force was applied to the proximal tibia causing combined varus, and hyperextension. Eight of these knees were suitable for reconstruction and the modified Larson popliteo-fibular and the Muller popliteal reconstruction were tested on each knee. A hamstring graft was used for the popliteo-fibular Larson techniques and the biceps tendon for Muller technique. Pd and CER were measured at 0, 30, 60 and 90o, ER was measured at 0, 30 and 90o and Varus rotation measured at 0 and 20o. Measurements were recorded for intact, damaged and reconstructed knees.

Discussion: Both reconstructions repaired Pd at 0 flexion with no significant difference between intact and reconstructed states. However both failed to do so at 30o of flexion (P< 0.05). Both significantly over constraint the knees at 90o of flexion (P< 0.05). Varus displacement was significantly corrected by both procedures at 0o flexion, at 200 of flexion the popliteal reconstruction failed to significantly restore varus displacement while the P-F reconstruction significantly did so P< 0.002. At 0,300 flexion ER & CER was significantly corrected by both reconstructions and over-constraint at 90o of flexion.

Conclusions: Both reconstructions failed to correct posterior displacement at 300 of flexion, this will manifest clinically in giving way in low angles of knee flexion e.g. negotiating staircases. Of clinical significance is the ability of the P-F reconstruction to correct varus rotation at low angles of flexion. Surprisingly there was no significant difference between both reconstructions in correcting external rotation and coupled external rotation. The popliteal reconstruction fails to correct both posterior displacement and varus rotation at low angles of flexion.