Masticatory Muscle Duty Factors Larger in Brachyfacial Than Dolicofacial Adolescents

Objectives: Functional differences may contribute to growth differences between individuals with markedly different facial types. Jaw function requires muscle use which can be measured by duty factor (DF=duration of muscle activities/total recording time, %). Hence, this study tested for day- and night-time differences in masticatory muscle DFs between brachyfacial and dolicofacial adolescents.
Methods: According to Institutional Review Board-approved protocols, ten brachyfacial and ten dolichofacial adolescents participated (Table). Surface masseter and anterior temporalis muscle activities (EMG) were recorded bilaterally during biting tasks repeated in two laboratory sessions and plotted as EMG (root-mean-square, µV)/bite-force (N). These plots were used to determine subject- and muscle-specific average EMG per 20 N bite-force (EMG_20N). Over two days and nights, subjects self-recorded ambulatory unilateral masseter and anterior temporalis EMG using portable recorders at home. Day- and night-time DFs were determined based on thresholds of ≥5-≥80% EMG_20N. ANOVA and Tukey-Kramer post-hoc tests were used to determine group differences in DFs.
Results: Average total duration of subjects’ self-recordings for day- and night-time were 11.1±3.5 and 12.9±5.5 hours, respectively. DFs for both muscles were highest at the lowest threshold (≥5% EMG_20N) and on average were ≤12% of a recording period. DFs were larger in brachyfacial compared to dolicofacial adolescents, where average day- and night-time masseter DFs were 7.3- and 2.4-times larger, respectively, and significantly different between groups (all P<0.05). Similarly, average night-time anterior temporalis DFs in brachyfacial compared to dolicofacial adolescents were significantly larger (all P<0.005) but by 13.8-times.
Conclusions: More masticatory muscle use was shown by brachyfacial than dolicofacial adolescents in their natural environments, where the largest differences occurred at night in anterior temporalis muscles.