Variation in enamel mechanical properties throughout the crown in catarrhine primates.

Enamel mechanical properties vary across molar crowns, but the relationship among mechanical properties, tooth function, and phylogeny are not well understood. Fifteen primate lower molars representing fourteen taxa (catarrhine, n = 13; platyrrhine, n = 1) were sectioned in the lingual-buccal plane through the mesial cusps. Gradients of enamel mechanical properties, specifically hardness and elastic modulus, were quantified using nanoindentation from inner (near the enamel-dentine junction), through middle, to outer enamel (near the outer enamel surface) at five positions (buccal lateral, buccal cuspal, occlusal middle, lingual cuspal, lingual lateral). Cuspal positions had higher mechanical property values than lateral positions. Middle enamel had higher mean hardness and elastic modulus values than inner and outer locations in all five crown positions. Functionally, the thicker-enameled buccal cusps of lower molars did not show evidence of increased resistance to failure; instead, lingual cusps-which show higher rates of fracture-had higher average mechanical property values, with no significant differences observed between sides. Preliminary phylogenetic results suggest there is relatively little phylogenetic signal in gradients of mechanical properties through the enamel or across the crown. There appears to be common mechanical property patterns across molar crowns in Catarrhini and potentially among primates more broadly. These results may allow more precise interpretations of dental biomechanics and processes resulting in mechanical failure of enamel in primates, such as wear and fracture.

Variation in enamel and dentine mineral concentration and density in primate molars.

OBJECTIVE: Variation in enamel and dentine mineral concentration and total effective density can be reliably collected using Micro-CT scans. Both variables are suggested to reflect mechanical properties such as hardness and elastic modulus in dental tissues, meaning Micro-CT methods allow relative composition and mechanical properties to be collected non-destructively. DESIGN: 16 lower molars from 16 Catarrhine primates were Micro-CT scanned alongside hydroxyapatite phantoms using standardized settings and methods to calculate mineral concentration and total effective density. Mineral concentration, total effective density and thickness of dentine and enamel were calculated for four cusps, representing each 'corner' of the tooth and four lateral crown positions (i.e., mesial, buccal, lingual and distal). RESULTS: The results show mean mineral concentration and total effective density values were higher in areas of thicker enamel, while the opposite was observed for dentine. Buccal positions had significantly higher mineral concentration and total effective density values than lingual areas. Cuspal positions had higher mean values than lateral enamel, for both dentine (mineral concentration cuspal: 1.26 g/cm3; lateral: 1.20 g/cm3) and enamel (mineral concentration cuspal: 2.31 g/cm3; lateral: 2.25 g/cm3). Mesial enamel had significantly lower values than other locations. CONCLUSIONS: These common patterns across Catarrhine taxa may be linked to functional adaptations related to optimization of mastication and tooth protection. Variation in mineral concentration and total effective density may also be associated with wear and fracture patterns, and can be used as baseline information to investigate the effect of diet, pathological changes and aging on teeth through time.

Alkaline Tolerance and Biofilm Formation of Root Canal Isolates of Enterococcus faecalis: An In Vitro Study.

INTRODUCTION: The aim of this study was to compare the effects of glucose and glycerol (provided as principal fermentable supplements) on alkaline tolerance and biofilm-forming capabilities of root canal-derived strains of Enterococcus faecalis and those from other environments. METHODS: The planktonic growth kinetics and the biofilm-forming capabilities of E. faecalis isolates (identified by 16S ribosomal DNA sequencing) were compared when supplied with glucose and glycerol at pH levels of 8 and 11 in a microtiter plate. The metabolic activity of the biofilms plate that formed at a neutral pH level (supplied with either glucose or glycerol) was measured after subsequent adjustment to a pH level of 11. RESULTS: Ten isolates (7 from root canals and 3 from other sources) were examined. The lag phase and the doubling time increased under elevated alkalinity irrespective of either the fermentable supplement (glucose or glycerol) or the origin of the isolate. Biofilm formation and metabolic activity varied among strains, but neither was related to the source of isolation. In general, biofilm formation was enhanced when grown in glucose compared with glycerol and at a pH of 8 compared with a pH of 11 (irrespective of the fermentable supplement). The provision of glycerol did not increase either the planktonic growth rate or biofilm development compared with glucose but significantly increased the metabolic activity of biofilms, especially at a pH of 11 compared with a pH of 8. CONCLUSIONS: In the nutrient-deprived environment of a necrotic or root-filled root canal, glycerol may be an alternative energy source that can promote increased metabolic activity of E. faecalis under alkaline treatment conditions.