Neural and muscular electric activity in the cat's knee: Changes when the anterior cruciate ligament is transected

We studied the response of the normal and unstable knee to passive motion and anterior tibial displacement in the cat. 6 cats were anesthetized and the deep level of anesthesia was controlled by electroencephalograms. We recorded electric activity in the articular nerves (posterior PAN and medial MAN) and periarticular muscles (quadriceps and hamstring), while performing passive flexion, extension, internal and external rotation. We then produced anterior displacement of the tibia at 30° and 90° of flexion, as in the Lachman and the anterior drawer maneuvers. The anterior cruciate ligament was surgically sectioned and the same series of passive displacements was performed. We observed statistically significant increased activity in the MAN, the PAN and the quadriceps muscle during knee flexion, in the MAN during extension, and in the PAN and hamstring during external rotation with the knee 90° flexed. Anterior cruciate transection caused anterior displacement of the tibia during stress. This produced a significant increase in the MAN activity and a significant decrease in the hamstring electric activity at 30° and 90° of flexion, as in Lachman and anterior drawer maneuvers. We conclude that electric activity in the articular nerves and periarticular muscles, in response to passive motion and anterior tibial displacement, is altered in the cat's knee after anterior cruciate transection. This suggests that various patterns of periarticular muscle reaction in the anterior cruciate-deficient knee may be related to the unconscious perception of abnormal motion.