Exercise therapy in self-care for temporomandibular disorders (TMD) is a conservative treatment method that does not interfere with the natural course, and is considered to be the first choice of treatment along with disease education for patients. Muscle stretching exercises, jaw mobilization exercises, muscle strengthening exercises, etc. are prescribed according to the pathophysiology of temporomandibular disorders. Evidence of the mechanism and effects of jaw mobilization exercises for anterior disc displacement without reduction have been shown, and the exercises are recommended in clinical practice guidelines in Japan. International studies by experts in the treatment of TMD have shown that muscle stretching exercises for myalgia of masticatory muscles are recommended along with jaw mobilization exercises. When applying the exercises, doses such as the method, frequency, intensity, and time will be prescribed for each patient, but future studies are expected to determine the optimal formulation. In addition, multidisciplinary collaboration and treatment will contribute to improving the health of patients.

Objectives: The aim of this study was to evaluate the number of masseter muscle tendons (TMM) and the thickness of the tendons on the anterior border of the masseter muscle (TABMM) in a masticatory muscle tendon-aponeurosis hyperplasia (MMTAH) group and a control group using reproducible 3DCT images.

Study Design: The disease group contained 20 MMTAH patients and the control group contained 20 patients without temporomandibular disorders who visited our hospital for orthodontic treatment. CT and MRI were performed for all patients. Data were extracted, and 80 (left and right) 3DCT images were used for visual evaluation. The evaluators were four dentists and the evaluation items were TMM number and TABMM thickness. The Kappa coefficient was used for intra-rater reliability, Chronbach's alpha was used for inter-rater reliability for each item, and the Mann-Whitney U test was used to determine the differences between groups for the TMM number and TABMM thickness.

Results: The lowest value of the κ coefficient was 0.871 (TMM number) and 0.742 (TABMM thickness), Chronbach's alpha of the TMM number was 0.917, and that of the TABMM thickness was 0.939. The number of TMMs was 2.0 in the control group and 3.0 in the MMTAH group; in addition, the TABMM thickness was 3.0 in the MMTAH group and 1.5 in the control group. A significant difference between the groups was confirmed for both TMM number and TABMM thickness (p<0.01).

Conclusions: We confirmed a high intra-rater and inter-rater reliability of visual evaluation using 3DCT images of MMTAH patients. MMTAH patients had a higher TMM number and a thicker TABMM compared with the patients in the control group.