Relationship between the pivot shift and Lachman tests: a cadaver study.

BACKGROUND: While the Lachman and pivot shift tests have been used clinically for decades to assess the anterior cruciate ligament, the relationship between the two has undergone limited experimental study. The goal of this study was to evaluate biomechanical relationships between the Lachman and pivot shift tests in anterior cruciate ligament-deficient and reconstructed knees.
METHODS: Knee kinematics during simulated pivot-shift testing and anteroposterior knee laxities were measured in seventeen knees in the intact condition, in the anterior cruciate ligament-deficient condition, and after anterior cruciate ligament reconstruction. Pivot shift magnitude was plotted against laxity for all knees with the grafts unfixed (the cruciate ligament-deficient condition). The grafts were then tensioned to match the laxities of the intact knees, and the change in pivot shift magnitude was plotted versus the change in laxity for all knees. In a separate series of tests for individual specimens, pivot shift magnitude was plotted versus laxity for each knee by incrementally loosening the anterior cruciate ligament graft.
RESULTS: Linear correlations between pivot shift magnitude and absolute laxity for anterior cruciate ligament-deficient knees were weak. When the unfixed grafts were tensioned to match anteroposterior laxities of the intact knees, changes in pivot shift were better correlated with corresponding changes in anteroposterior laxity (r(2) = 0.53 for tibial rotation and 0.73 for lateral tibial plateau displacement). When graft fixation was progressively loosened for each reconstructed knee, pivot shift magnitude increased linearly from the laxity of the intact knee up to an end point of the linear range, at which point the slope decreased abruptly. Between this end point and the anterior cruciate ligament-deficient condition, further increases in pivot shift were relatively small.
CONCLUSIONS: Our findings suggest that the magnitude of laxity of an injured knee, when considered alone, may not accurately predict the magnitude of the pivot shift, but the difference in laxity between the injured knee and the normal knee (the injured-normal difference) could be a good clinical predictor of the injured-normal difference in the pivot shift. We demonstrated that an insufficiently tensioned anterior cruciate ligament graft could substantially reduce anterior laxity, while leaving the pivot shift evaluation virtually unchanged.