The short-term effect of stabilization-type splints on the local asymmetry of masseter muscle sites

Patients with temporomandibular disorders (TMD) may present with a cluster of joint and muscle disorders characterized primarily by pain, joint sounds and irregular or deviating jaw function. Maxillary stabilization-type splints represent the best standard therapy, so the purpose of this study was to evaluate changes in local cross-sectional dimensions (LCSD) of masseter muscle sites associated with short-term application of 'splint therapy' using the diagnostic approach of high-resolution gray-scale ultrasonography. The study included 29 patients who had signs and symptoms of temporomandibular disorders. Ultrasonographic investigation was performed with a linear (B-scan) 7.5 MHz small-part transducer to visualize the antero-superior, antero-inferior, medio-superior, medio-inferior, postero-superior and postero-inferior sites of the masseter muscle. To assess local muscle asymmetry patterns and to evaluate the respective effect of occluding splints, the 'absolute asymmetry index (AAI)' was used, with the mean maximum muscle diameter of the respective right and left sides calculated from three consecutive measurements before and after splint therapy. Comparing the pre-treatment with the 2-month follow-up values revealed a significant decrease in the overall mean asymmetry indices at the anterior (P < 0.01), medial (P < 0.05) and posterior (P < 0.05) muscle sites. There was no significant change in LCSDs at the various muscle sites (P > 0.05). The results of this study suggest stabilization-type splints to be effective in reducing local muscle asymmetries. Further studies are warranted to evaluate muscle-site specific effects in patient and non-patient groups and to relate these effects to pre-treatment variables like bite force, preferred chewing side, facial morphology and occlusion.