Knee instability after injury to the anterior cruciate ligament - Quantification of the Lachman test

We have measured anterior and posterior displacement in 563 normal knees an d 487 knees with chronic deficiency of the anterior cruciate ligament (ACL) . We performed stress radiography using a simple apparatus which maintained the knee at 20 degrees of flexion while a 9 kg load was applied, There was no significant difference in posterior translation dependent on the condit ion of the ACL. Measurement of anterior translation in the medial compartme nt proved to be more reliable than in the lateral compartment for the diagn osis of rupture of the ACL, with better specificity, sensitivity and predic tive values. We have classified anterior laxity based on the differential anterior trans lation of the medial compartment and identified four grades in each of whic h we can further distinguish four subgrades for laxity of the lateral compa rtment. Within each of these subgroups, either internal or external rotatio n may dominate and sometimes there is a major translation of both compartme nts, Radiological evaluation of displacement of the knee in 20 degrees of flexio n provides conclusive evidence of rupture of the ACL. A detailed study of p athological displacement is the basis for a classification of laxity, It is then possible to decide for each type of laxity, the surgical treatment wh ich is specifically adapted to the lesion, and to define a reference value for judging outcome.