Effect of anesthesia of the sinus tarsi on peroneal reaction time in patients with functional instability of the ankle.

We investigated peroneal reaction time (PRT) of 18 patients (21 ankles) with functional instability of the ankle and 8 healthy volunteer controls (9 ankles) before and after injecting local anesthetic into the sinus tarsi. The median PRT in patients before the injection was 82.0 ms, a significant delay from 71.0 ms in controls. After the injection, PRT in controls did not change, but PRT in patients significantly shortened to 69.3 ms. The sense of instability and the functional instability improved simultaneously. A disorder of the gamma-muscle-spindle system, induced by proprioceptive deficit after damage to a ligament, has been advocated as a cause of prolonged PRT. We suggest that irritability of mechanoreceptors or nociceptors or both, induced by inflammation at the sinus tarsi, may suppress the activities of gamma motor neurons of peroneal muscles, which in turn might cause the symptoms of functional instability and prolonged PRT.