The clinical relevance of instrumented testing for ACL insufficiency. Experience with the UCLA clinical knee testing apparatus.

An instrumented clinical testing device developed at UCLA records a continuous anteroposterior force versus displacement curve of the tibia with respect to the femur at 20 degrees of flexion. Laxity and stiffness are calculated from the response curve. With this device, 95% of normal knees have an anterior laxity less than 7.5 mm and a side-to-side difference less than 2 mm. In contrast, an anterior cruciate ligament (ACL) absent knee has a mean anterior laxity of 10 mm and a mean side-to-side difference of 5 mm. In a small group of patients with an intraarticular ACL substitution using the medial or lateral one-third of the patellar tendon, laxity and stiffness of the injured knees were returned to within the normal range and remained constant three years after surgery. In a group of 76 patients treated with ACL substitution using the torn meniscus, 51% of the patients still had an anterior laxity outside the normal range 3.5 years after surgery. In a preliminary study of 19 patients receiving a Gore-Tex synthetic ACL substitution, 55% of the patients still had a side-to-side difference greater than 2 mm two years after the procedure. These studies illustrate the advantages of impartial, objective measurements of knee stability. Laxity and stiffness values can supplement, but never replace, a thorough patient examination and patient history. As sports medicine matures as a scientific discipline, improved instrumented test devices may ultimately provide a standardized means for reporting knee stability parameters.