Jaw mechanics in dolichofacial and brachyfacial phenotypes: A longitudinal cephalometric‐based study

Abstract

To determine whether dolichofacial (Frankfort horizontal mandibular plane angle (FHMPA) ≥30°) vs brachyfacial (FHMPA ≤22°) phenotypes differ in temporomandibular joint (TMJ) loads and whether these differences correlate longitudinally with mandibular ramus height (Condylion-Gonion, Co-Go). Lateral and posteroanterior cephalographs from ten dolichofacial and ten brachyfacial individuals made at average ages of 6 (T1), 12 (T2) and 18 (T3) years and available online (http://www.aaoflegacycollection.org/aaof_home.html) were used. Three-dimensional anatomical data were derived from cephalographs and used in numerical models to predict TMJ loads for a range of biting angles on incisors, canines and first molars. Two criteria were used to define clinically important between-group TMJ load differences: statistical significance was defined with a two-group t-test, and where differences were also ≥20%. A statistical approach called response surface analysis was used to assess correlation between TMJ loads and its predictors considered in this study. The two phenotypes had significantly different FHMPA at all ages (P<.05). No differences in TMJ loads were found at T1. Ipsilateral and contralateral TMJ loads at T2 and T3 were significant and ≥20% larger in dolichofacial than brachyfacial phenotypes for specific biting angles (all adjusted P<.05). Regression analysis indicated age and ramus height contribute 53% of the variability in normalized values of TMJ loads. At higher ages, dolichofacial phenotypes had significantly higher TMJ loads which were correlated with shorter ramus heights compared to brachyfacial phenotypes. Craniofacial mechanics may explain, in part, mandibular growth differences between dolichofacial and brachyfacial phenotypes.