Effect of stabilization-type splints on the asymmetry of masseter muscle sites during maximal clenching

Patients with temporomandibular disorders 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 maximal clenching-related asymmetry patterns of masseter muscle sites associated with the immediate 'application of splint therapy' using the diagnostic approach of high-resolution gray-scale ultrasonography. The study included 24 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 maximal clenching-related muscle asymmetry patterns and to evaluate the respective effect of occluding splints, the 'absolute asymmetry index' was used, with the mean maximum muscle diameter of the respective right and left sides calculated from three consecutive measurements before and after splint insertion. Comparing the values assessed before splint insertion with those after splint insertion revealed a significant decrease in local maximal clenching-related muscle asymmetry values for the antero-inferior masseter muscle site (P < 0.05). The results of this study suggest stabilization-type splints to have a site-specific effect in the immediate reduction of local maximal clenching-related 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.