Morphological and mechanical properties of masticatory muscles in patients with temporomandibular disorders: a systematic review and meta-analysis

BACKGROUND: Temporomandibular disorders (TMDs) are defined by pain and dysfunction of the masticatory system, often accompanied by alterations in muscle activity and structures. The masticatory muscles play a critical role in jaw function and their morphological and mechanical characteristics may reflect underlying pathophysiological processes. However, alterations reported in muscle thickness, stiffness, in individuals with TMDs remain inconsistent across studies. This study aimed to investigate alterations in masticatory muscle morphology and muscle biomechanical characteristics in individuals with TMDs relative to healthy controls. METHODS: We performed a comprehensive literature search using the PubMed, Web of Science, and Scopus platforms. We included studies that evaluated morphological or biomechanical features of the masticatory muscles in participants with TMDs and healthy groups, provided that the articles were published in English. We evaluated the methodological rigor of each study using a modified Newcastle-Ottawa Scale tailored for case-control research. Group comparisons were summarized using standardized mean differences (SMDs) along with 95% confidence intervals (CIs). RESULTS: Thirteen studies met the inclusion criteria and were included in the review, of which 12 were suitable for meta-analysis. Seven studies were rated as high quality and six as moderate quality. The meta-analysis revealed that masseter muscle stiffness was significantly higher in the TMDs group (SMD = 0.78; 95% CI = 0.34-1.22; p < 0.001). CONCLUSIONS: The findings indicate increased masseter muscle stiffness in individuals with TMDs, suggesting possible neuromuscular hyperactivity or altered muscle tone. Further high-quality studies integrating morphological and mechanical parameters are required to clarify the mechanisms underlying these changes.