~ Pergamon Archs oral Biol. Vol. 41, No. 6, pp. 553-563, 1996 Copyright © 1996 Elsevier ScienceLtd, All rights reserved Printed in Great Britain PII: S0003-9969(96)00018-0 0003-9969/96 $15.00 + 0.00 SENSITIVITY OF THE JAW-JERK REFLEX IN PATIENTS WITH MYOGENOUS TEMPOROMANDIBULAR DISORDER F. LOBBEZOO, 1'2 H. W. VAN DER GLAS, 2'* A. VAN DER BILT, 2 R. BUCHNER 2 and F. BOSMAN 2 ~Department of Oral Function, Academic Centre for Dentistry, ACTA, University of Amsterdam, Louwejweg 1, 1066 EA Amsterdam, The Netherlands and 2Laboratory of Oral Pathophysiology, Department of Oral-Maxillofacial Surgery, Prosthodontics and Special Dental Care, Utrecht University, P.O. Box 80.037, 3508 TA, Utrecht, The Netherlands (Accepted 5 February 1996) Summary--Changes in the activity of human jaw-elevator muscles related to the mandibular stretch (jaw-jerk) reflex could be involved in the aetiology of temporomandibular disorders (TMD). In order to investigate whether there are differences in the sensitivity of the jaw-jerk reflex between myogenous TMD patients (n = 10) and gender- and age-matched controls (n = 10), jaw-jerk reflexes were elicited under standardized conditions. By measuring the reflex with bipolar surface electromyography (EMG), reflex sensitivity was determined from relations between reflex amplitude and jaw displacement from the masseter and the anterior temporalis muscles. Reflex amplitude and background EMG activity were normalized with respect to the maximal voluntary contraction (MVC) to correct for differences in the thickness of soft tissues overlying the muscle or in electrode placement. In addition to normalization with respect to MVC, for the patients, normalization was also applied with respect to a MVC that was scaled by multiplying values by the ratio of the mean MVC of controls to the mean MVC of patients. At a constant level of background EMG activity, the reflex sensitivity can be determined from the slope (reflex gain) and x-intercept (reflex threshold) of the reflex amplitude-jaw displacement relation. No sig- nificant differences between patients and controls were found for the gain or threshold values of either the masseter or the anterior temporalis muscles with a univariate analysis of variance. It is concluded that jaw-jerk reflex sensitivity is not significantly changed in myogenous TMD patients. Therefore, the fusimotor system probably does not play a part in the perpetuating myogenous TMD. Copyright © 1996 Elsevier Science Ltd. Key words: temporomandibular disorders, jaw-jerk reflex, electromyography. INTRODUCTION The principal symptoms of patients suffering from temporomandibular disorders are pain and tender- ness of the masticatory muscles and the temporo- mandibular joint. In addition, many such patients display temporomandibular joint sounds and suffer from a restricted range of mandibular movements (for a review, see De Boever, 1979). The aetiology of temporomandibular joint disorder is stated to be multifactorial: many factors of neuromuscular, psychological, occlusal and anatomical origin (for a review, see Yemm, 1985) may predispose, initiate or perpetuate the disorder (McNeill, 1993). To isolate neuromuscular factors, EMG measures related to activity of jaw-elevator muscles may be examined in patients with a type of myogenous tem- *To whom correspondence should be addressed at Laboratory of Oral Pathophysiology, Department of Oral-Maxillofacial Surgery, Prosthodontics and Special Dental Care, Utrecht University, P.O. Box 80.037, 3508 TA Utrecht, The Netherlands. Abbreviations." EMG, electromyogram, -graphic, -graphy. poromandibular disorder in which signs and symp- toms are selectively related to muscular dysfunction, viz. by excluding as far as possible arthrogenous and occlusal factors (Buchner et al., 1992). During submaximal isometric contraction, the jaw-jerk reflex, which is the trigeminal equivalent of the spinal stretch reflex (Lund et al., 1983), can be used to reveal the function of the fusimotor system in jaw-elevator muscles and of the influence of pre- and postsynaptic mechanisms (Sessle, 1981). In order to compare the reflex sensitivity between patients with temporomandibular disorders and controls, several factors must be taken into account. The EMG amplitude of the short-latency stretch reflex is influenced by: (1) the amount and velocity of the mandibular displacement after loading (Lobbezoo et al., 1993a); (2) the reflex sensitivity, which, in turn, is determined by the amount of gamma drive, the properties of the intrafusal muscle fibres, and presynaptic mechanisms, as mediated by cutaneous, periodontal, mucosal, and temporoman- dibular joint receptors, and higher centres (for reviews, see Dubner et al., 1978 for masticatory 553