Human Craniofacial Form and Jaw Mechanics: A longitudinal study

Differences in facial form are expected to be linked to function; however, evidence for these relations is equivocal. Objectives: This study determined if young adults (T3) with dolichofacial (Frankfort-Horizontal Mandibular Plane Angle (FHMPA)≥30°) versus brachyfacial (FHMPA≤22°) features showed differences in FHMPA and temporomandibular joint (TMJ) loads during canine biting at three time-points (T1, T2, T3). Methods: Ten dolichofacial and ten brachyfacial individuals were identified using lateral and posteroanterior cephalometric images made at 16-21 years (T3) from the American Association of Orthodontists Foundation Legacy Collection. Three-dimensional anatomical data and FHMPA for each subject at T1-T3 were derived from these images using established methods. Anatomical data were then applied in a numerical model based on minimization of muscle effort to predict TMJ loads (% bite-force) for a range of 324 canine biting angles. Results: Groups had similar mean ages at each time-point but significantly different FHMPA (Table).

Group	T1		T2		T3
	Age(years)	FHMPA(°)	Age(years)	FHMPA(°)	Age(years)	FHMPA(°)
Dolichofacial	6.6±1.1	30.1±2.6	12.2±0.3	30.8±4.2	18.8±1.8	32.5±3.0
Brachyfacial	6.0±0.0	23.1±3.2	12.0±0.0	23.0±3.8	18.8±1.7	18.0±2.4
P	0.094	0.0007	0.049	0.0028	0.993	<0.001

Between T1-T3, average FHMPA increased in dolichofacial subjects by 2.4° and decreased in brachyfacial subjects by 5.1°. Generally, patterns of predicted TMJ loads with respect to biting angles were similar for both groups at T1-T3. TMJ loads, except for ipsilateral at T1, were higher in dolichofacial versus brachyfacial subjects by >20% and significantly different for both TMJs at T2 and T3 for biting angles that directed the jaw anteriorly. Overall, largest between-group differences (mean 21%) were found at T2 for contralateral TMJ loads. Conclusions:  FHMPA was significantly different between groups at T1-T3 and differences increased with time. For canine biting, TMJ loads were higher for dolichofacial compared to brachyfacial subjects at T2 and T3 and significantly different for bite-forces that directed the jaw anteriorly.