REFLEX JAW MOTIONS AND JAW STIFFNESS PERTAINING TO WHIPLASH INJURY OFTHE NECK

Because a so-called mandibular whiplash injury requires the absence of short-latency jaw-closing reflexes in order to explain the postulated mechanism of injury (excessive jaw opening), the authors studied the presence and absence and, more importantly, the kinematics (duration, displacement, velocity, acceleration) of monosynaptic and, possibly, p olysynaptic myotatic (stretch) reflexes in the jaw elevator muscles. I n six healthy adults jaw jerk maneuvers were elicited through a brisk tap on the chin, and surface electromyography identified elevator refl exes while translational electrognathography identified the kinematics of the reflexes, The maneuvers were done while maintaining the rest p osition (3% MVC) and moderate clenching of the teeth (30% MVC). Electr omyography was also used to identify phasic elevator excitations durin g a passive brisk neck extension maneuver, A sudden and unexpected elo ngation of the jaw elevators released autogenic reflex responses that, in conjunction with augmented tissue elasticity (stiffness), elevated the mandible into centric occlusion within approximately 150 millisec onds. In 86% of trials, the responses occurred regardless of the preva iling resting and clenching contractile activities. There was no evide nce of a depressor force that consistently would and could anchor the mandible in a position of extreme or moderate depression, the theoreti cal linchpin of the mandibular whiplash injury. It was concluded that the mandibular locomotor system is very efficient in maintaining the r est and intercuspal positions of the mandible. This study found no evi dence corroborating the mechanism claimed to release a so-called mandi bular whiplash injury.