Orthopaedic surgeons routinely use passive tests, in which the
displacing force is applied externally, to evaluate the integrity of the
ligaments of the knee. Using a quadriceps active test, in which the muscle
contractures of the subject served as the displacing force, tibial
displacement was measured with an arthrometer in ninety-two subjects:
sixty-seven who had an acute or chronic rupture of the posterior or
anterior cruciate ligament and twenty-five who had normal knees. With the
knee joint in 90 degrees of flexion, contraction of the quadriceps resulted
in anterior translation of the tibia in forty-one of forty-two knees that
had a documented disruption of the posterior cruciate ligament. This
anterior translation did not occur in the contralateral, normal knee of the
same subjects; in the knees of the twenty-five normal subjects; or in
twenty-five knees that had a known unilateral anterior cruciate-ligament
disruption.