Enamel thickness and the helicoidal occlusal plane

In the present study 38 unworn maxillary molars (M1 = 16, M2 = 12, M3 = 10) of modern humans from a Slavic necropolis were sectioned through the mesial cusps in a plane perpendicular to the cervical margin of the crown. Five slightly worn M1s and one slightly worn M3 were also used thus increasing the total sample to 44, but measurements made on the worn areas were coded as missing values. Seven measurements of enamel thickness as well as the heights of the protocone and the paracone dentine horns were recorded in order to analyze whether changes 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 maxillary molars corresponds to a helicoidal occlusal wear pattern. Enamel thickness along the occlusal basin increases from anterior to posterior, which may lead to rapid development of a reverse curve of Monson in first molars when compared to posterior teeth. However, although these overall differences together with the serial, especially delayed eruption pattern of human molars, contribute to the marked expression of the helicoidal occlusal plane in Homo, differences in enamel patterning between 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 the 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 buccal slope of that cusp. These proportional differences are most pronounced in M3, as evidenced by uni- and multivariate statistics. It thus appears that the pattern of enamel thickness distribution from M1 to M3 follows a trend towards providing additional tooth material in areas that are under greater functional demands, that is, corresponding 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 anteroposterior direction change in a way that lends support to the hypothesis that the axial inclination of teeth could be one of the most important factors for the development of the helicoidal occlusal plane. Finally, the changes in morphology and enamel thickness distribution from first to third upper molars found in this study suggest that molars could be "specialized" in their function, i.e., from performing proportionally more shearing anteriorly to increased crushing and grinding activities posteriorly.