Effects of loading on the biochemical behavior of molars of Homo, Pan, andPongo

In a previous study, we found systematic differences in the biomechanical b ehavior of modern human molars using finite element stress analyses (FESA), which led us to propose that molars are adapted to differently-directed lo ads depending on their position within the mouth (Spears and Macho [1998] A m. J. Phys. Anthropol. 106:467-482). While the FESA results thus derived ha ve not been verified experimentally, such an interpretation seemed reasonab le. To refine the model previously presented, this study assessed the effec ts of 1) food particle size on the biomechanical behavior of molars, and th ose of 2) differences in morphology, particularly enamel thickness, on stre ss distribution. In order to appraise the evolutionary significance of the findings, the FESA results for modern humans were subsequently compared wit h those obtained for molars of one individual of Pan and Pongo, respectivel y. Bearing in mind limitations imposed by the FESA models created and analy zed in this study, constant cleavage-type loads and cuspal tip loads at dif ferent directions were employed on all teeth: this facilitated comparisons of patterns of stress distribution across molars and species. In Pan and Ho mo, cleavage-type loads exerted by big food particles tended to be better d issipated anteriorly than posteriorly, although trends in Pongo were less c lear-cut. Furthermore, similar to modern humans, the buccal cusps of mandib ular molars appeared to be able to dissipate the loads associated with a pe stle-type action, while maxillary molars were better designed to dissipate the loads which would result if they acted as mortars against which the foo d is crushed/ground, While increases in enamel thickness lowered the overal l stress values in teeth only slightly, changes in outer morphology could h ave a more profound effect on these stress levels. Overall, Pan appeared to be most generalized, while Homo and Pongo showed a number of unique specia lizations, which are in accordance with what is currently understood about their respective masticatory apparatus and dietary niche. (C) 1999 Wiley-Li ss, Inc.