The habitual chewing side syndrome

Background. Temporomandibular joint (TMJ) disorders (TMD) are defined as a subgroup of craniofacial pain problems that involve the masticatory musculature, the temporomandibular joints (TMJs), and associated structures. Pain and/or limited mouth opening are the main symptoms. The prevalence of treatment need for TMD in adults was estimated to be 15.6%. The cause of TMD remains unknown; however, TMJ overloads are a recognized factor. Aims. The main goal of this work is to assess an association between functional, dynamic and anatomical characteristics of the masticatory system in patients suffering TMD-pain condition and in healthy groups. With this in mind, our team conducted a series of studies to explain this situation, using strictly the scientific method. Method. The masticatory function of 21 completely dentate participants with chronic temporomandibular disorders (all but one with unilateral symptoms) was assessed by observing them eat almonds, inspecting the lateral horizontal movement of the jaw, with kinesiography, and by means of interview. The condylar path in the sagittal plane and the lateral anterior guidance angles with respect to the Frankfort horizontal plane in the frontal plane were measured on both sides in each individual. Results. Sixteen of 20 participants with unilateral symptoms chewed on the affected side; the concordance (Fisher? s exact test, P = .003) and the concordance-symmetry level (Kappa coefficient k = 0.689; 95% confidence interval [CI], 0.38 to 0.99; P=.002) were significant. The mean condylar path angle was steeper (53.47(10.88) degrees versus 46.16(7.25) degrees; P = . 001), and the mean lateral anterior guidance angle was flatter (41.63(13.35) degrees versus 48.32(9.53) degrees P = . 036) on the symptomatic side. Study power: with a sample size of 19, and assuming a two-sided test at the 0.05 level, the study had power of 0.8 to detect a value of Kappa of 0.6 or larger when testing the null hypothesis that Kappa = 0. Interpretation. The results of this study support the use of a new term based on etiology, ``habitual chewing side syndrome? , instead of the nonspecific symptom-based ``temporomandibular joint disorders? ; this denomination is characterized in adults by a steeper condylar path, flatter lateral anterior guidance, and habitual chewing on the symptomatic side. Additionally, because muscular activity is responsible for TMJ-loads, muscular coactivation was assessed using surface electromyography (sEMG); although limited, sEMG could inform about some subclinical condition and about the type of muscular co-activation and their asymmetry in TMD patients, that could be of interest to elucidate the physiology of this conditionand showing moderate discriminatory capacity between TMD or healthy subjects. Lastly, this study evaluated incisal forces and muscular coactivation simultaneously. It is suggested that mean incisal forces promote equilibrated muscular coactivation; however, submaximal incisal effective forces required the activation mainly of masseter muscles; due that masseter muscles are responsibles for TMJ-loads, this study allow infer TMJ loads increases during submaximal incisal bitting; probably this task should be avoided in TMD patients. Funding. This study was financed with a grant PI11/02507 from the Institute of Health Carlos III of the Ministry of Science and Innovation of the Government of Spain. It was completed with the equipment granted by the General Directorate of Universities and Research; Department of Education and University. Xunta de Galicia. Official Journal No. 143 Galicia, July 22, 1996. And the Department of Innovation, Industry and Commerce. Xunta de Galicia, April 30, 2004. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.