WHAT IS TRISMUS? Trismus is derived from the Greek term meaning 'grinding together.' Taber's Cyclopedic Medical Dictionary defines it as the tonic contraction of muscles of mastication. True trismus is a muscular, involuntary protective reflex. Localized muscle irritation or inflammation can cause afferent signals of pain and input from reset muscle spindles to the central nervous system, resulting in foreshortening of the muscles of mastication. This is different from voluntary muscular guarding, which can occur in patients with TMJ internal derangement. These patients resist normal opening due to joint pain and dysfunction.