The facial projection angle (FPA) is formed between the functional occlusal plane (FOP) and the facial plane, depicting the displacement of prosthion (Pr) anterior to nasion (N). The functional occlusal plane (FOP is defined by biting points of the second molar (MBP) and the third premolar (PBP) on the lateral view of the skull (Thayers, 1990). The facial plane (FP) is defined by anthropometric landmarks nasion (N) and prosthion (Pr) on the lateral view of the skull. Monkeys have more projected faces than humans. However, the modeling of human states of facial projection could be simplified through the tilt of the monkey skull during experiments.
The midfacial skeleton in the human lineage demonstrates a wide spectrum of variation that may be the consequence of different environ- mental and mechanical selective pressures. However, different facial con- figurations may develop under comparable selective regimes. For example, the Neanderthal high and projected face and the Inuit broad and flat face are hypothesized to be the consequence of (1) life in a cold climate, and (2) excessive paramasticatory stresses focused on the ante- rior dentition. In this study, the second of these two hypotheses is tested using finite element analyses of a monkey skull. Results indicate that in- cisor loading induces heavy stress in the anterior midface of macaques. Additional analyses using incremental increases in the anteroinferior tilt of the skull to simulate different magnitudes of facial projection revealed that comparable muscular force generates less stress in a less-projected face. However, the findings of our final analyses, which attempted to com- bine biting with the incisors and pulling with the hands, differed from the analyses that mimicked only incisor loading (without any sort of ante- rior pulling component). These findings suggest that shortening the face may be the most effective way to compensate for anterior dental loading but not necessarily offset the forces incurred when using the anterior dentition as a vice for various paramasticatory behaviors. Although Nean- derthals may have frequently loaded their anterior dentition, countervail- ing selection pressures, such as the inclusion of tough foods in the diet that demanded molar grinding, may have selected for a longer face with a lower load- to lever-arm ratio.
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