Specific Diurnal EMG Activity Pattern Observed in Occlusal Collapse Patients: Relationship between Diurnal Bruxism and Tooth Loss Progression

Aim: The role of parafunctional masticatory muscle activity in tooth loss has not been fully clarified. This study aimed to reveal the characteristic activity of masseter muscles in bite collapse patients while awake and asleep. Materials and Methods: Six progressive bite collapse patients (PBC group), six age-and gender-matched control subjects (MC group), and six young control subjects (YC group) were enrolled. Electromyograms (EMG) of the masseter muscles were continuously recorded with an ambulatory EMG recorder while patients were awake and asleep. Diurnal and nocturnal parafunctional EMG activity was classified as phasic, tonic, or mixed using an EMG threshold of 20% maximal voluntary clenching. Results: Highly extended diurnal phasic activity was observed only in the PBC group. The three groups had significantly different mean diurnal phasic episodes per hour, with 13.29 +/- 7.18 per hour in the PBC group, 0.95 +/- 0.97 per hour in the MC group, and 0.87 +/- 0.98 per hour in the YC group (p<0.01). ROC curve analysis suggested that the number of diurnal phasic episodes might be used to predict bite collapsing tooth loss. Conclusion: Extensive bite loss might be related to diurnal masticatory muscle parafunction but not to parafunction during sleep. Clinical Relevance: Scientific rationale for study: Although mandibular parafunction has been implicated in stomatognathic system breakdown, a causal relationship has not been established because scientific modalities to evaluate parafunctional activity have been lacking. Principal findings: This study used a newly developed EMG recording system that evaluates masseter muscle activity throughout the day. Our results challenge the stereotypical idea of nocturnal bruxism as a strong destructive force. We found that diurnal phasic masticatory muscle activity was most characteristic in patients with progressive bite collapse. Practical implications: The incidence of diurnal phasic contractions could be used for the prognostic evaluation of stomatognathic system stability.