Effects of loading on the biomechanical [correction of biochemical] behavior of molars of Homo, Pan, and Pongo.

In a previous study, we found systematic differences in the biomechanical behavior of modern human molars using finite element stress analyses (FESA), which led us to propose that molars are adapted to differently-directed loads depending on their position within the mouth (Spears and Macho [1998] Am. J. Phys. Anthropol. 106:467-482). While the FESA results thus derived have not been verified experimentally, such an interpretation seemed reasonable. To refine the model previously presented, this study assessed the effects of 1) food particle size on the biomechanical behavior of molars, and those of 2) differences in morphology, particularly enamel thickness, on stress distribution. In order to appraise the evolutionary significance of the findings, the FESA results for modern humans were subsequently compared with those obtained for molars of one individual of Pan and Pongo, respectively. Bearing in mind limitations imposed by the FESA models created and analyzed in this study, constant cleavage-type loads and cuspal tip loads at different directions were employed on all teeth: this facilitated comparisons of patterns of stress distribution across molars and species. In Pan and Homo, cleavage-type loads exerted by big food particles tended to be better dissipated anteriorly than posteriorly, although trends in Pongo were less clear-cut. Furthermore, similar to modern humans, the buccal cusps of mandibular molars appeared to be able to dissipate the loads associated with a pestle-type action, while maxillary molars were better designed to dissipate the loads which would result if they acted as mortars against which the food is crushed/ground. While increases in enamel thickness lowered the overall stress values in teeth only slightly, changes in outer morphology could have a more profound effect on these stress levels. Overall, Pan appeared to be most generalized, while Homo and Pongo showed a number of unique specializations, which are in accordance with what is currently understood about their respective masticatory apparatus and dietary niche.