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