Anteroposterior drawer measurements in the knee using an instrumented test device

A newly developed instrumented knee laxity tester was used to measure anteroposterior (AP) drawer parameters (Lachmann test) in populations of normal subjects and patients with anterior cruciate ligament (ACL) and posterior cruciate ligament (PCL) insufficiencies. The AP drawer tester features a differential electronic measuring procedure of displacements between patella and tuberosity over a continuous range between 180 and -180 N of AP forces. In the statistical evaluation, shift and compliance (or stiffness) parameters at various forces are considered. In a representative normal population, all relevant parameters were established, thus creating a normal data base for future reference. The average normal AP drawer shift between maximal anterior and posterior forces (180 N) was 6.4 +/- 1.7 mm. Right-left differences and male-female differences were found not to be statistically significant in any of the parameters. The validity of the instrumented drawer tester was confirmed relative to a group of patients with acute documented ligament insufficiencies. Measurements in two groups of patients with chronic postoperative knee instabilities revealed that although the diagnosis of instability could be confirmed, the results could hardly discriminate between ACL and PCL insufficiencies, probably due to unrecognized associated lesions at the time of operation and/or gradual postoperative developments of insufficiencies and deformities. An unexpected finding was that the mean laxities of the uninjured contralateral knees were significantly extended relative to the normal reference group, suggesting that these patients could have been classified as a high-risk group even before the time of injury.