Enamel thickness of deciduous and permanent molars in modern Homo sapiens

Abstract

This study presents data on the enamel thickness of deciduous (dm2) and permanent (M1-M3) molars for a geographically diverse sample of modern humans. Measurements were recorded from sections through the mesial cusps of unworn teeth. Enamel is significantly thinner on deciduous than on permanent molars, and there is a distinct trend for enamel to increase in relative thickness from M1 to M3. The relatively thicker enamel of M2s and especially M3s can be related to the overall reduction in size of more distal molar crowns, which has been attained through a differential loss of the dentine component. Enamel tends to be thicker on the protocone than on the paracone, and thicker on the protoconid than on the metaconid, but its distribution is not wholly concordant with models that predict increased thickness as a means by which to counter heavier attritional loss on these "functional" cusps. Indeed, the thickness of enamel tends to be more variable on cusp tips and occlusal surfaces than over the lateral aspects of cusps. The proportionately thicker enamel over the lateral aspects of the protocone and protoconid more likely serves as a means to prolong functional crown life by preventing cusp fracture, rather than being an adaptation to increase the attritional longevity of wear facets. The present data suggest that the human dentition is not predisposed to develop a helicoidal wear plane through the disposition of molar enamel thickness.

Variation in hominoid molar enamel thickness

“…However, Schwartz (2000a) suggested that the majority of published data on enamel thickness in modern humans are derived from European populations, which may not be representative of the species mean and may underestimate variation. A more recent investigation of enamel thickness in disparate modern human regional populations found values similar to those reported for European populations, with low levels of variation within and among populations (Grine, 2002(Grine, , 2005. Differences in enamel thickness between populations or subspecies of non-human primates have not previously been studied.…”

“…Finally, data on relative enamel thickness in enlarged samples of Pan and Pongo were compared to data from a smaller sample of Gorilla. These results were considered in the context of recent studies of crosssectional enamel thickness in modern human molars (Grine, 2002(Grine, , 2005 and enamel volume in hominoid molars (Kono, 2004).…”

“…Macho's (1994) study of ''relative'' enamel thickness (partial cuspal enamel area divided by the mean M1 occlusal crosssectional area) of human maxillary molars showed that human first molars have significantly thinner enamel than second and third molars, which was attributed to differences in the functional demands on anterior and posterior molars. Grine (2002Grine ( , 2005 reported a similar, sometimes significant pattern in human permanent molars, but suggested that increases in ''relative'' enamel thickness (enamel cap area divided by dentine area) relate to decreases in the area of the underlying dentine due to tooth size reduction, rather than functional demands.…”

“…While a number of studies have examined differences in absolute or relative enamel thickness within and between buccal and lingual cusps of mesial sections (e.g., Martin, 1983;Macho andBerner, 1993, 1994;Macho, 1994;Schwartz, 2000a,b;Grine, 2005), few studies have compared enamel thickness between mesial and distal section planes within a tooth. Little is known about the distribution of enamel and dentine throughout a tooth, and it is not clear if cross-sectional planes of mesial and distal cusps are directly comparable.…”

“…Reported hominoid values from a controlled (physical) plane of section have necessarily been based on small sample sizes (Gantt, 1977;Martin, 1983Martin, , 1985Grine and Martin, 1988;Andrews and Martin, 1991;Macho, 1994;Beynon et al, 1998;Shellis et al, 1998;Grine, 2002;Olejniczak and Martin, 2002;Schwartz et al, 2003;Smith et al, 2003bSmith et al, , 2004Grine, 2005). Previous studies of extant ape molars have produced a maximum reported sample of 17 teeth from seven individuals of a single species (Martin, 1983).…”