Two-dimensional patterns of human enamel thickness on deciduous (dm1, dm2) and permanent first (M1) mandibular molars

To characterise patterns of enamel thickness on deciduous (dm1, dm2) and permanent first (M1) mandibular molars and evaluate these against functional and morphological interpretative models.

Histological sections of mesial and distal cusps from 69 unworn molars were produced and examined using transmitted light microscopy. Enamel cap area, dentine area, as well as average and linear measurements of enamel thickness were recorded from digital images of the sections using image analysis software. Comparisons were made along the molar row, and between the mesial and distal sections of each tooth, using univariate and multivariate inferential statistics.

The enamel cap area, dentine area, and average enamel thickness increased from the anterior to the posterior molars. The greatest proportional increase in linear enamel thickness occurred between the outside surface of the lingual cusps when dm1 was compared to dm2, and between the outside surface of the buccal cusps when dm2 was compared to M1. The enamel cap area increased from the mesial to the distal sections in M1. Dentine area decreased from the mesial to distal sections in dm1. Enamel cap and dentine areas did not change across dm2.

Results for the deciduous molars are interpreted within a functional model of mastication, in which the dm2 dissipates less laterally orientated loads compared to dm1. Differences in enamel thickness between dm2 and M1 support previous functional interpretations for this permanent molar. Some mesial-distal results are not easily explained from either a functional or a morphological perspective and suggest an underlying developmental constraint.