Intraoperative direct mechanical stimulation of the anterior cruciate ligament elicits short- and medium-latency hamstring reflexes.

The anterior cruciate ligament (ACL) has not only a mechanical but also a sensorimotor function. Patients with injuries of the ACL frequently complain of knee instability despite good mechanical stabilization after surgical reconstruction. Compared with healthy subjects, their latencies of hamstring reflexes after anterior tibia translation are considerably increased. There is evidence for the existence of a reflex arc between the ACL and the hamstrings. The aim of this study was to determine if there is a direct reflex response after an isolated mechanical stimulation of the ACL in humans. In 10 patients who underwent arthroscopy, hamstring electromyographic (EMG) responses were assessed intraoperatively after applying an isolated load on the ACL. Latencies, amplitudes, and integrals of the EMG responses were analyzed. In four patients, the measurements were repeated after injection of local anesthetics into the ACL. In all subjects, responses with mean latencies of 42 +/- 4.4 (SD) ms corresponding to a medium latency response (MLR) were found. In seven subjects, they were preceded by responses with a short-latency (SLR) of 24 +/- 2.7 ms. The maximum amplitude was 8.6 +/- 7 mV, the integral 0.064 +/- 0.05 mV*s. The injection of local anesthetics reduced the amplitude by 34 +/- 12% and the integral by 50 +/- 20%. Direct mechanical stimulation of the ACL evokes considerably smaller SLRs and MLRs than anterior tibia translation during standing. It is argued that latency changes observed in patients with ACL ruptures may be rather due to changes in the sensorimotor integration of the afferent input from the knee joint than to the absence of the direct ACL reflex.