Enamel thickness in Asian human canines and premolars

Abstract

Dental enamel thickness continues to feature prominently in anthropological studies of ape and human evolution, as well as studies of preventative oral care and treatment. Traditional studies of enamel thickness require physical sectioning of teeth for linear and scaled measurements. Recent applications of microtomographic imaging allow scientists to employ larger and more diverse samples, including global samples of recent humans as well as fossil hominin teeth. Unfortunately, little is known about the degree of enamel thickness variation among human populations, particularly across the dentition. This study employed microtomography to virtually image, section, and quantify the average enamel thickness of a sample of clinically extracted Indonesian canine and premolar teeth. This virtual sample was compared to physically sectioned African and European teeth. The results demonstrate that average enamel thickness is similar among human dentitions; no significant differences were detected within tooth positions, which is surprising given developmental differences between European and African canines and premolars. When populations were combined, differences were found in average enamel thickness between maxillary and mandibular premolars, and between canines and premolars within both dental arcades. This finding is potentially due to differences in premolar morphology and a trend of increasing enamel thickness distally throughout the dentition. The finding of limited population variation within tooth positions and significant variation between tooth positions is consistent with previous two-dimensional and three-dimensional studies of human molar enamel thickness. Average enamel thickness in canines and premolars does not differ between the sexes in our sample, although male teeth tend to have larger enamel and dentine cross-sectional areas, enamel-dentine junction lengths, and bi-cervical diameters. Males have significantly greater dentine area and enamel-dentine junction length than females for maxillary canines and premolars. The results of this study suggest that enamel thickness values in mixed-populations of humans are appropriate for comparisons with fossil hominins.

Technical Note: Guidelines for the digital computation of 2D and 3D enamel thickness in hominoid teeth

“…Several methods have been proposed for the digital computation of 2D and 3D enamel thickness (Kono, 2004;Tafforeau, 2004;Olejniczak, 2006;Olejniczak et al, 2008;Feeney et al, 2010). All these methods include either operator-dependent procedures or other technical shortcomings that might bias the final outcome and constrain the analysis to specific tooth classes.…”

“…Therefore, for 2D enamel thickness, we have modified previous methods suggested by Feeney et al (2010) for modern human premolars and canines both to include hominoid teeth in the analysis and to remove manual (subjective) interactions with the micro-CT image stack (method 2D-b). With regard to the molars, our results suggest using planes of section perpendicular to the cervical plane (method 2D-a, which follows Martin's (1985) (2006) protocol (see Table 1 for more information); Mesial section (M); Distal section (D); AET 5 average enamel thickness; RET 5 relative enamel thickness Recent Homo sapiens (HS); Neandertal (N); Method 3D-a 5 the crown was separated from the root based on the cervical plane; Method 3D-b 5 the coronal dentine was separated from the root dentine based on the cervical line; Method 3D-c 5 Olejniczak's (2006) protocol (see Table 1 for more information); AET 5 average enamel thickness; RET 5 relative enamel thickness.…”

“…We slightly modified Feeney et al (2010) approach (method 2D-b; see Table 1). The virtual section passes through two points digitized on the widest labiolingual bi-cervical diameter and the dentine horn tip (or central mamelon in incisors) (Fig.…”

“…Feeney et al (2010) addressed this issue, but the digital approach they developed presents several limitations. First of all, it has been developed for modern human canines and premolars, but it does not apply to other hominoid teeth with more complex morphology; second, it is still limited to a plane of section (2D analysis); third, it implies a manual (and therefore subjective) handling (or translation and rotation) of the micro-CT image stack both to orient the tooth and to identify a ring of enamel with uniform width near the cervical margin.…”

Is the deciduous/permanent molar enamel thickness ratio a taxon-specific indicator in extant and extinct hominids?

“…However, even if molar enamel thickness does not seem to behave as sexually dimorphic (e.g., Hlusko, 2016;Hlusko et al, 2004;Rossi et al, 1999), a growing body of evidence indicates a considerable amount of interspecific temporal and geographic variation (e.g., Kato et al, 2014;Smith et al, 2011Smith et al, , 2012. Conversely, the extent of intraspecific variation ranges in most cases from poorly reported to simply unknown, and even in extant humans enamel thickness chrono-geographic variation is far from being appropriately documented and, with very few exceptions (e.g., Feeney et al, 2010;Grine, 2005), most currently available information is limited to European or European-derived population samples (rev. in Le Luyer, 2016;see Zanolli et al, 2017).…”