Mechanobehavior distinguishes mandibular growth differences in two facial types

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In the United States, >9% of children are orthodontic patients who are treated for crooked teeth and jaw discrepancies known as malocclusions (AAO, 2017). Costs for this treatment are >$9.5 billion annually (Guay et al. 2008; Laniado et al. 2017). Management of jaw growth via dentofacial orthopedics is indicated in >30% of orthodontic patients but 13-36% of mandibular growth enhancement therapies fail to correct the malocclusion (Hedlund and Feldmann 2016; O'Brien et al. 2003). Hence, the costs of poor results are considerable. Orthopedic therapies for mandibular enhancement try to promote growth of the load-sensitive secondary cartilages of the jaw through changes in temporomandibular joint (TMJ) loading mechanics and behaviors. Mandibular growth is expected to be less in amount and expressed more vertically (Buschang et al. 2017; Karlsen 1997) with poorer prognoses for orthopedic therapies in children with dolichofacial (long-narrow) compared to brachyfacial (short-wide) phenotypes (Deen and Woods 2015; Pancherz and Michailidou 2004; Rogers et al. 2018). However, differences in the TMJ loading mechanics and behaviors (mechanobehavior) in these phenotypes during growth and treatment are unknown. The broad, long-term goals of this research are to provide evidence-informed dentofacial orthopedic therapies with predictable results. Thus, this application proposes a prospective observational study in selected populations of dolichofacial and brachyfacial children before, during and after orthodontic treatment. This study will contribute to the mission of improving dental, oral, and craniofacial health by addressing three Specific Aims: 1. Test the hypothesis that dolichofacial compared to brachyfacial children have energy densities (energy input per volume of cartilage, mJ/mm3) that are significantly larger for the same jaw tasks and, thus, limit mandibular growth earlier. 2. Test the hypothesis that dolichofacial compared to brachyfacial children have duty factors (% time of jaw muscle activity/total recording time) that are significantly smaller and, thus, promote less mandibular growth per year. 3. Test the hypothesis that phenotypic dentofacial subgroups exist, differentiated by mechanobehavior scores ((energy densities)2 X duty factors) that correspond with significant differences in mandibular growth. Longitudinal clinical treatment records and data collected via validated techniques, including numerical modeling, dynamic stereometry, and laboratory and in-field electromyography, will be used to compare mechanobehavior variables between phenotypes and test if these variables predict mandibular growth as indicated by ramus height. The novel outcomes of this research will be critical foundations for future clinical approaches that modify mechanobehavior to achieve more successful and predictable orthopedic therapies in children with jaw discrepancies.