ENAMEL THICKNESS AND THE HELICOIDAL OCCLUSAL PLANE

In the present study 38 unworn maxillary molars (M(1) = 16, M(2) = 12, M(3) = 10) Of modern humans from a Slavic necropolis were sectioned t hrough the mesial cusps in a plane perpendicular to the cervical margi n of the crown. Five slightly worn M(1)s and one slightly worn M(3) we re also used thus increasing the total sample to 44, but measurements made on the worn areas were coded as missing values. Seven measurement s of enamel thickness as well as the heights of the protocone and the paracone dentine horns were recorded in order to analyze whether chang es in these dimensions in anteroposterior direction can be related to the helicoidal occlusal plane. Uni- and multivariate analyses revealed that the distribution of enamel thickness within and between maxillar y molars corresponds to a helicoidal occlusal wear pattern. Enamel thi ckness along the occlusal basin increases from anterior to posterior, which may lead to rapid development of a reverse curve of Monson in fi rst molars when compared to posterior teeth. However, although these o verall differences together with the serial, especially delayed erupti on pattern of human molars, contribute to the marked expression of the helicoidal occlusal plane in Home, differences in enamel patterning b etween molars indicate that a helicoidal plane is a structural feature of the orofacial skeleton. In contrast to first upper molars, second and third molars show absolutely and relatively thicker enamel under t he Phase I wear facet of the paracone, i.e., the lingual slope of the paracone, than under the Phase II facet of the protocone, i.e., the bu ccal slope of that cusp. These proportional differences are most prono unced in M(3), as evidenced by uni- and multivariate statistics. It th us appears that the pattern of enamel thickness distribution from M(1) to M(3) follows a trend towards providing additional tooth material i n areas that are under greater functional demands, that is, correspond ing to a lingual slope of wear anteriorly and to a flat or even buccal one posteriorly. In addition, the heights of the dentine horns in ant eroposterior direction change in a way that lends support to the hypot hesis that the axial inclination of teeth could be one of the most imp ortant factors for the development of the helicoidal occlusal plane. F inally, the changes in morphology and enamel thickness distribution fr om first to third upper molars found in this study suggest that molars could be ''specialized'' in their function, i.e., from performing pro portionally more shearing anteriorly to increased crushing and grindin g activities posteriorly. (C) 1994 Wiley-Liss, Inc.