Acute Effects of Neck Muscle Fatigue on Masticatory Muscles in Probable Bruxists: A Sonoelastography and Perfusion Imaging Approach.

Objective: The interplay between cervical and masticatory muscles remains unclear, despite evidence suggesting co-contraction. This cross-sectional study aims to investigate the acute effects of induced neck muscle fatigue on the mechanical and hemodynamic properties of masticatory muscles using sonoelastography and microvascular imaging.

Methods: Thirty-two individuals with probable bruxism were included based on positive responses to a standardized questionnaire and the presence of clinical signs (e.g., tooth wear, linea alba). Sonoelastography assessed the elasticity index (EI) of the masseter and temporalis muscles, while microvascular imaging evaluated the velocity index (VI) of the masseter muscle. Pain-free maximum mouth opening (PMMO), pain pressure thresholds (PPTs) of the masseter, temporalis, sternocleidomastoid, and trapezius muscles, as well as masseter and temporalis muscle thickness at rest and during clenching, were measured. All clinical and ultrasound measurements were repeated after a fatigue protocol consisting of repetitive flexion and extension movements.

Results: PPTs significantly decreased in all muscle groups (p < 0.05). Temporalis EI showed a significant reduction (right: 1.29-1.05, p = 0.044; left: 0.94-0.75, p = 0.027), while masseter EI decreased non-significantly. VI of the masseter muscle significantly increased on both sides (right: 49.26%-54.18%, p = 0.027; left: 34.09%-47.08%, p < 0.001). Post-fatigue, PMMO significantly decreased from 4.11 ± 0.64 to 3.78 ± 0.70 mm (p < 0.001). Muscle thickness remained unchanged.

Conclusions: Neck muscle fatigue acutely alters the mechanical and hemodynamic properties of masticatory muscles. These findings support the need for an integrated assessment of cervical and masticatory muscle function in bruxism.