Jaw Mechanics and Behavior in Dolicofacial versus Brachyfacial Adolescents

Craniofacial morphology is commonly assumed to influence orofacial functions and vice versa. Objectives:   This pilot study aimed to evaluate jaw mechanics and muscle behavior in adolescents with two different craniofacial morphologies.  Methods: Ten dolichofacial and ten brachyfacial  (SN-GoGn angles ≥37° and ≤27°, respectively) subjects consented to participate in protocols with Institutional Review Board approval.  For jaw mechanics, each subject’s anatomical geometry was obtained from lateral and posteroanterior cephalograms and jaw tracking. Geometries were employed in numerical models which used minimization of muscle effort to predict TMJ loads for a comprehensive range of first molar biting angles. TMJ loads were compared via response surface regressions and F tests (α=0.05) determined between-group differences. Anterior temporalis and masseter muscle behaviors were recorded unilaterally in subjects over 2 days and nights using portable surface electromyography (EMG) devices and bilaterally during measured static and dynamic bites at two separate laboratory EMG sessions. As an initial investigation, right masseter EMG:bite-force regressions were calculated for all subjects to determine activity for a 20 N bite force (EMG_20N) from laboratory data and then used to calibrate field recordings. Duty factors (DF=duration of EMG_20N/duration of recording period, %) for a range of magnitude (5-80% of EMG_20N) and duration (2-20 sec) thresholds were determined. Between-group DF differences were evaluated using repeated measures ANOVA and Tukey-Kramer post hoc tests (α=0.05). Results: Ipsilateral TMJ loads were ≥9% larger in dolichofacial than brachyfacial subjects about seventeen-fold more frequently for the biting angles investigated.  Masseter muscles were significantly more active in dolichofacial compared to brachyfacial subjects by three-fold at thresholds of 5% (P<0.01) and 10% (P<0.05). Conclusion: Dolichofacial individuals had larger predicted ipsilateral TMJ loads more frequently and produced significantly more low-level masseter muscle activity compared to brachyfacial individuals.