Enamel thickness of human maxillary molars reconsidered

Forty-four modern human maxillary molars (M1 = 21, M2 = 12, and M3 = 11) were sectioned through the mesial cusps in a plane perpendicular to the cervical margin of the crown. Eight measurements of enamel thickness as well as bucco-lingual (BL) and mesio-distal (MD) diameters were recorded for each tooth in order to investigate differences in these dimensions between tooth categories. Uni- and multi-variate analyses revealed first maxillary molars to have generally thinner enamel than second or third upper molars, especially with regard to the occlusal basin. Furthermore, the decrease of MD diameters from anterior to posterior is greater than that of BL diameters. Principal Component Analysis using enamel thickness measurements resulted in complete separation of first molars, while second and third maxillary molars showed a certain amount of overlap. This finding casts doubt on using an overall measure of "molar enamel thickness" derived from mixed samples of molars for taxonomic purposes. There appears to be a relationship between bite force and enamel thickness such that posterior molars, where masticatory forces are stronger, have thicker enamel than anterior teeth. It is suggested that the gradient of enamel thickness between (and within) teeth in extant and extinct species may thus provide further information about relative wear resistance as well as the biomechanical constraints of the orofacial skeleton.