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LATERAL PTERYGOID AND OTHER MASTICATORY MUSCLES ACTIVITY IN TEMPOROMANDIBULAR JOINT INTERNAL DERANGEMENT AND CONTROL SUBJECTS DURING DYNAMIC CONDITIONS C.M. Lafrenière1 PT, M. Lamontagne1,2 PhD & R. Elsawy M.D. School of Human Kinetics1 and Dept. of Anatomy and Neurobiology2, University of Ottawa, Ottawa, Ontario Canada INTRODUCTION TMJ practice is in a state of confusion because there is no consensus as to the nature and the cause of ID. The leading voices proposed a muscular/active cause on top of the list whether primary or secondary to posture, malocclusion, trauma, stress and parafunction in opposition to a possible passive or nonmuscular origin. The most popular hypothesis is that muscle hyperactivity of the masseter and temporalis is a cause or a consequence of TMJ disorders (Dahlstrom, 1990; Jankelson, 1990; Sheikholeslam et al.,1982). On the other hand, the Lateral Pterygoid Muscles (LPMs) were described as being hyperactive, hypoactive and exhibiting altered activity definitely suggesting a close relationship between the functions of the LPMs and TMJ ID (Mahan et al., 1983; Isberg et al., 1985; Zijun et al., 1989; Juniper, 1984). Probably because they are anatomically and functionally related to the intra-articular joint mechanics. Various limitations of previous studies are as follow: inexplicit samples, uncontrolled head and neck posture which may lead to hyperactivity, conclusions based on theoretical reports, simply just done on normal subjects, on the masseter and temporalis or during static tasks analysis only. Therefore there is a necessity to bridge the gap in further understanding the TMJ muscle mechanics in regards to ID. Hence, masticatory muscular activity measurements need to be examined during dynamic jaw motion between TMJ ID and control subjects. Consequently, the purpose of this study was to record and analyze the myoelectric activity of four masticatory muscles with a special attention to the two heads of the LPMs during functional dynamic jaw motion in subjects with and without TMJ ID disorders. The LPMs were investigated because they are believed to be abnormally recruited in TMJ ID. The masseter and the temporalis muscles were recorded to correlate their activity with the LPMs and to assess their recruitment pattern and contribution in TMJ ID in order to have a complete masticatory picture. METHODS Intramuscular EMG of the two LPMs, surface EMG of the temporalis and masseter muscles, two custom-made force transducers, electrogoniometry and joint sound recordings were used in synchronisation to investigate the muscular mechanics associated with TMJ ID compare to controls during dynamic openclose-clench (OCC) cycles and functional gum chewing motion. Four measures were computed: 1-the integrated linear envelope EMG (ILEEMG) normalized by 100% Maximum Voluntary Contraction (MVC) per OCC phase; 2- the normalized ILEEMG by MVC by primary function; 3- the normalized ILEEMG by peak; and 4- a descriptive analysis of the EMG signals. All of these were computed for the 4 muscles and the 2 groups. RESULTS AND DISCUSSION Results showed that, overall, there was no strong reason to believe that the masseter (figure 1) and the temporalis are associated with TMJ ID in a hyperactive manner as previously reported (Dahlstrom, 1990; Jankelson, 1990; Sheikholeslam et al.,1982). In general, the TMJ ID subjects muscles functioned closer to their MVC or were firing more compare to controls per chewing phase, per primary function and by peak (figure 2). This Figure 1. Normalized ILEEMG of the masseter muscle by 100% MVC per phase of the OCC cycle for both groups. The open phase was normalized by the MVC in opening, the close and clench phase were normalized by the MVC in molar clench. could probably be secondary to a lack of strength, lack of stability, lower muscular endurance, inefficiency and fatigue widely found in TMJ ID subjects, hence they are using greater relative forces during chewing activities (Mahan et al., 1983; Carlsson, 1974). Normal function of the ILP and SLP were refined: the ILP seemed to be active only in forced opening and as a synergist or a co-contractor in clenching probably to gain stability in order to provide protective splinting and coordinate fine movements (Gibbs et al., 1983). Even if the results were variable per muscle and per subjects, dysfunctions of the ILP and SLP could definitely be detected in ID: for the SLP results varied per phases but showed hypercontraction in dynamic phases and hypocontraction during clenching. In TMJ ID cases, the two muscles seems to become dependant and somewhat synchronous as well as exhibiting uncoordinated and altered muscular activities likely Figure 2. Normalized ILEEMG by peak of a gum to compensate for the lost of inner-joint stability. chewing cycle for the four muscles for both groups. The results also demonstrated the complexity of the LPMs function during dynamic motions as controllers of the joint and discal units. It is suggested that TMJ clicking might be a consequence of muscular incoordination, as reported previously and that a neuromuscular adaptation is happening in ID of the TMJ to permit stability for effective chewing and survival. This plasticity of the masticatory muscular mechanics has been well named “internal rearrangement” by Ogus (1987). Finally, causes, pathogenesis, consequences and treatment principles of TMJ ID are discussed. REFERENCES Dahlstrom, L. (1990) J Oral Rehab 16,1-6. Carlsson GE: Front Oral Physiol 1974;1:265-292. Gibbs Hertling D, Kessler M: (1983) New York: Isberg, A., Wildmalm, S.V. and Ivarsson, R. (1985) Am J Orthod 88(6), 453-460. Jankelson, R.R. (1990) Pathophysio.Head & Neck Musculosketal disorders 7, 83-98. Juniper, R.P. (1984) Br J Oral Maxillofac Surg 22,1-8. Mahan, P. E., Wilkinson, T.M., Gibbs, C.H., Mauderli, A., Brannon, L.S. (1983) J Prosthet Dent 50(5),710-718. Ogus H: (1987) Brit Ass Oral MaxilloSurg 218-226. Sheikholeslam, A., Moller, E. & Lous, I. (1982). Scand J Dent Res 90,37-46. Zijun, L., Huiyun, W. and Weiya, P. (1989) J Prosthet Dent 62(2),229-233.