Lifespan Morphologic Trajectories of Masticatory Muscles: A Cross-Sectional High-Resolution MRI Study

BACKGROUND: The muscles of mastication (MMTs) are essential for mandibular function and have recently emerged as potential biomarkers for age-related conditions. However, lifespan morphologic data remain limited, particularly for muscles beyond the temporalis. We aimed to characterize the developmental trajectories of MMTs across the human lifespan using high-resolution MRI and to evaluate bilateral symmetry, sex differences, and comparative morphology. METHODS: A cross-sectional cohort of 211 subjects (36 pediatric and 175 adults; age: 1-87 y) underwent 3.0 T MRI. Length, width, and thickness of the temporalis, masseter, medial pterygoid, and lateral pterygoid (superior and inferior heads) were measured bilaterally. LOESS regression with bootstrap CIs characterized age-related trends; bilateral symmetry was assessed using Bland-Altman analysis. RESULTS: All muscles demonstrated rapid growth during childhood and adolescence, reaching peak dimensions by age 25 to 30 years. Length and width remained stable throughout adulthood, whereas thickness (particularly of the temporalis and masseter) showed a progressive decline beginning in the fourth decade, with acceleration after the age of 60 years. High bilateral symmetry was observed across all parameters. Sexual dimorphism emerged during adolescence and persisted throughout adulthood, most pronounced in the temporalis and masseter. CONCLUSIONS: MMT morphology follows distinct lifespan trajectories, with thickness being the most age-sensitive parameter. The temporalis and masseter undergo significant age-related thickness decline, while the pterygoid muscles remain more stable. These findings provide essential reference data for interpreting MMT measurements as clinical biomarkers and suggest that parameters beyond temporalis thickness warrant consideration.