Incidence and mechanism of the pivot shift - An in vitro study

The aim of this study was to determine the incidence and mechanism of the p ivot shift phenomenon in the normal and anterior cruciate ligament transect ed knee in vitro. Fifteen knees were tested under a range of valgus moments and iliotibial tract tensions when intact and after anterior cruciate liga ment transection. Knee kinematics were measured and described in terms of t ibial rotation as the knee flexed. Eight knees pivoted after anterior cruci ate ligament transection. The mean pivot shift motion was an external tibia l rotation of 17 degrees (+/- 11 degrees standard deviation) over a range o f 27 degrees (+/- 24 degrees) knee flexion, at a mean flexion angle of 56 d egrees (+/- 27 degrees), Clinically, this corresponds to a reduction of an anteriorly subluxed lateral tibial plateau as the knee flexes, When intact, pivoting and nonpivoting knees had similar anteroposterior laxity, but aft er anterior cruciate ligament transection, the pivoting group had significa ntly greater laxity. The loading required to elicit the pivot shift was cri tical and variable between knees, which raises questions about comparing cl inicians' techniques and results in assessing the buckling instability attr ibutable to anterior cruciate ligament injury.