Possible uses of Hunter-Schreger bands of dental enamel for automated personal identification.

BACKGROUND: Hunter-Schreger bands (HSB) are optical phenomena observed on tooth surfaces under polarized light, resulting from the intersection of enamel prisms. Anthropological studies demonstrate the prevalence of HSB in large mammals, contributing to enamel resistance. Historically, John Hunter and Schreger depicted HSB in dental literature. In dentistry, HSB play a role in non-carious cervical lesions (NCCL) and internal dental perikymata, suggesting their potential for personal identification. Personal identification, crucial in both daily and professional life, involves biometric characteristics, such as fingerprints or facial recognition. The need for non-invasive, rapid, and user-friendly methods has prompted the exploration of using HSB dental images for personal identification. The review aimed to consolidate studies employing HSB for personal identification. METHODS: The scoping review was carried out strictly following the PRISMA-ScR checklist; the search was carried out on tree databases (PubMed, Scopus, Science Direct,) and a register (Cochrane library). RESULTS: The research produced a number of bibliographic sources totaling 410. With the removal of duplicates, 334 were obtained; potentially eligible articles amounted to 14, of which only 4 fully complied with the criteria of eligibility. CONCLUSIONS: From the data in the literature, we can assert that HSB could be used in personal identification, as they are characteristics that are difficult to change and easily detectable.

A novel image processing procedure for the quantitative evaluation of dental enamel prism arrangement.

Enamel prism is the main microstructural unit of mammalian enamel which composed of hundreds of bioapatite nanocrystals. Prism structure plays a key role in the excellent mechanical performance of dental enamel during millions of chewing cycles without significant remodeling. Thus, quantitative understanding of prism architecture is of utmost importance for biomechanical materials design. To characterize enamel prism orientation quantitatively, a novel image processing method has been developed. Our method is based on scanning electron microscopy images of etched enamel surface and consists of an ellipse fitting procedure, which provides a numerical approximation of prism shape and orientation in the studied cross section. The obtained analytical data allow to construct color coded orientation maps, which provide quick and useful insight into the microstructure of enamel. Besides striking visualization, orientation maps allow to extract and plot the rich information on the azimuthal and inclination angles of the prisms as function of location. Numerical data on prism arrangement can be analyzed using statistical tools over large areas, which paves the way towards quantifying comparative investigation of prism arrangement either in dentistry research or evolution biology. The application of the method is demonstrated for a distal-mesial cross-section of sound human tooth enamel. HIGHLIGHTS: Scanning electron microscopy images of etched enamel surface are analyzed using ellipse fitting. Geometrical parameters of the fitted ellipses provide numerical data of thousands of prisms. Prism arrangement is visualized on color coded orientation maps and analyzed using statistical tools.

Microstructurally driven self-sharpening mechanism in beaver incisor enamel facilitates their capacity to fell trees.

Beavers (Castor) stand out among mammals for their unique capacity to fell trees using their large, ever-growing incisors. This routine consumption of resistant fodder induces prodigious wear in the lower incisors, despite this blunting effect the incisors maintain a remarkably sharp cutting edge. Notably, the enamel edges of their incisors show a highly complex two-part microstructure of which the biomechanical import is unknown. Here, using fracture analysis, nanoindentation, and wear testing on North American beaver (C. canadensis) incisors we test the microstructure's possible contribution to maintaining incisal sharpness. Although comparable in hardness, the inner enamel preferentially fails and readily wears at 2.5 times the rate of the outer enamel. The outer microstructure redirects all fractures in parallel, decreasing fracture coalescence. Conversely, the inner microstructure facilitates crack coalescence increasing the wear rate by isolating layers of enamel prisms that readily fragment. Together these two architectures form a microstructurally driven self-sharpening mechanism contained entirely within the thin enamel shell. Our results demonstrate that enamel microstructures exposed at the occlusal surface can markedly influence both enamel crest shape and surface texture in wearing dentitions. The methods introduced here open the door to exploring the biomechanical functionality and evolution of enamel microstructures throughout Mammalia. STATEMENT OF SIGNIFICANCE: Enamel microstructure varies significantly with the diversity of diets, bite forces, and tooth shapes exhibited by mammals. However, minimal micromechanical exploration of microstructures outside of humans, leaves our understanding of biomechanical functions in a nascent stage. Using biologically informed mechanical testing, we demonstrate that the complex two-part microstructure that comprises the cutting edge of beaver incisors facilitates self-sharpening of the enamel edge. This previously unrecognized mechanism provides critical maintenance to the shape of the incisal edge ensuring continued functionality despite extreme wear incurred during feeding. More broadly, we show how the architecture of prisms and the surrounding interprismatic matrix dictate the propagation of fractures through enamel fabrics and how the pairing of enamel fabrics can result in biologically advantageous functions.

Microstructure, elemental composition and mechanical properties of enamel and dentine in the polar bear Ursus maritimus.

OBJECTIVE: To investigate the microstructure, elemental composition and mechanical properties of polar bear teeth. DESIGN: Incisors, canines and fourth premolar teeth of two subadult male museum specimens were analysed. Teeth were measured, photographed, embedded in Epoxy resin, sectioned, polished and etched for scanning electron microscope (SEM) imaging, elemental composition and nanomechanical testing analyses. RESULTS: The thickness of enamel ranged from 350-430 µm in canines, 220-330 µm in incisors and 320-510 µm in premolars. SEM images showed distinct transversely-oriented undulating Hunter Schreger bands from the enamel-dentine junction (EDJ) to the outer enamel surface. Enamel prisms had a hexagonal shape, with open prism sheaths. Prisms measured 6-8 µm in diameter. The EDJ was straight with no evidence of scalloping. Larger tubules adjacent to the EDJ were observed in the mantle dentine zone. Enamel Hardness and Elastic modulus values were higher in premolars (6.9 GPa and 269 GPa), followed by canines (6.5 GPa and 230 GPa) and incisors (4.9 GPa and 187 GPa). Dentine Hardness and Elastic modulus values were higher in canines. CaO and P2O5 were the components with higher oxide weight percentage in both enamel and dentine. CONCLUSIONS: Understanding the microstructure, elemental composition and mechanical properties of polar bear teeth can help elucidate the biology and functional morphology of this globally threatened species and could be used as a proxy for studies with fossil ursids.

Quantification of enamel decussation in gracile and robust capuchins (Cebus, Sapajus, Cebidae, Platyrrhini).

Multiple behavioral and biomechanical analyses have demonstrated that capuchin monkeys (Cebus and Sapajus) are specialized for breaking down hard-object foods as compared to other cebid monkeys. In addition to a complex suite of craniodental adaptations, it has specifically been demonstrated that capuchins possess highly complex dental enamel, with extensive Hunter-Schreger banding and other decussation, that likely serve as an adaptation to resist crack propagation during hard-object feeding. Furthermore, it has been demonstrated that robust capuchins (Sapajus spp., formerly Cebus apella) demonstrate further adaptation for hard-object feeding than other capuchins, routinely breaking down extremely mechanically challenging foods. However, there has been no comparison of dental enamel complexity in robust versus gracile capuchins, to assess whether the dental enamel in Sapajus follows this same pattern of further specialization. Therefore, this study compares dental enamel complexity in images of dental thin sections from a sample of robust versus gracile capuchins using image compression ratio (ICR) analysis. ICR is a variable that correlates with enamel complexity, such that higher ICR values are indicative of increased complexity in the form of enamel decussation. We found no significant difference between robust and gracile capuchins when assessing all teeth in our sample together, however, we did find that robust capuchins have significantly higher ICR values than gracile capuchins for canine teeth, specifically. Our results support prior studies suggesting that robust capuchins are specialized to generate increased masticatory loads with their anterior dentition, specifically, as compared to gracile species.

Bandas de Hunter-Schreger: propuesta terminológica / Hunter-Schreger bands: terminological propose

Las Bandas de Hunter-Schreger (BHS), son bandas claras y oscuras, que se aprecian en el esmalte debido a la distínta curvatura de los prismas del esmalte, resultan ser ampliamente mencionadas en estudios, tanto odontológicos como antropológicos, mas no se ven reflejadas en Terminologia Histologica. El objetivo del presente trabajo fue realizar un análisis del término BHS, con el fin de proponerlo como nuevo término histológico, y así poder ser incluido en Terminologia Histologica por la Federative International Programme for Anatomical Terminology (FIPAT). Luego del análisis en textos de estudio y publicaciones científicas, proponemos en reemplazo del término BHS, por dos términos: diazone prismatica (diá del griego δι á 'a través de' y del griego zon(e) ζωνη 'faja', 'zona de la tierra') y parazone prismatica (pará gr. 'a lo largo de' y del griego zon(e) ζωνη 'faja', 'zona de la tierra'), definiendo así términos más descriptivos y que no utilizan epónimos, tal como lo establece la Terminología Internacional. Proponer nuevos términos que estén más acorde con los señalado por la International Federation of Associations of Anatomists (IFAA) y la propia terminología, presenta grandes desafíos; un término no sólo es una palabra que hace referencia a una estructura morfológica, sino que también es una unidad del lenguaje, que une a la comunidad morfológica en un solo lenguaje. Por lo cual proponemos que el término sea incluido por la FIPAT en próximas discusiones.

Hunter-Schreger Bands (HSB), are light and dark bands, which can be seen in the enamel due to the different curvature of the enamel prisms, they are widely mentioned in studies, both dental and anthropological, but are not reflected in Terminologia Histologica. The aim of the present work was to carry out an analysis of the HSB term, in order to propose it as a new histological term, and thus be able to be included in Terminologia Histologica by the Federative International Program for Anatomical Terminology (FIPAT). After the analysis in study texts and scientific publications, we propose in replacement of the term HSB, for two terms: diazone prismatica (diá of Greek δι á 'through' and of Greek zon(e) ζωνη 'strip', 'zone of the earth') and parazone prismatica (pará gr. 'along' and from the Greek zon(e) ζωνη 'strip', 'zone of the earth'), thus defining the most descriptive and noneponymous terms, as the International Terminology says. Propose new terms that are more in agreement with those indicated by the International Federation of Associations of Anatomists (IFAA) and the own terminology, presents great answers; A term is not only a word that refers to a morphological structure, but also a unit of language, which unites the morphological community in a single language. So we propose that the term sea included by the FIPAT in the next discussions.

Application of Image Compression Ratio Analysis as a Method for Quantifying Complexity of Dental Enamel Microstructure.

It is well recognized that enamel microanatomy in mammals reflects biomechanical demands placed upon teeth, as determined by mechanical properties of species' diets, use of teeth as weapons, and so forth. However, there are limited options for researchers wishing to perform large-scale comparisons of enamel microstructure with adaptive questions in mind. This is because to date there has been no efficient method for quantification and statistical analysis of enamel complexity. Our study proposes to apply a method previously developed for quantification of 3D tooth cusp morphology to the problem of quantifying microstructural enamel complexity. Here, we use image compression ratio (ICR) as a proxy variable for enamel complexity in 2D enamel photomicrographs taken using circularly polarized transmitted light microscopy. ICR describes the relationship between a digital image captured in an uncompressed file format and the identical image that has had its file size compressed using computer algorithms; more complex images receive less compression. In our analyses, ICR analysis is able to distinguish between images of teeth with simple, radial enamel and teeth with complex decussating enamel. Moreover, our results show a significant correlation between ICR and enamel complexity ranks assigned via visual assessment. Therefore, our results demonstrate that ICR analysis provides a viable methodology for efficient comparison of overall enamel complexity among dental samples. Anat Rec, 302:2279-2286, 2019. © 2019 American Association for Anatomy.

Digital enhancement of dental enamel microstructure images from intact teeth.

Dental enamel is formed by rod-like structures, the enamel prisms. Groups of prisms are packed together in successive horizontal layers of alternating directions, known as Hunter-Schreger bands (HSBs). HSBs are the major microstructural characteristic of mammalian enamel. The pattern of HSBs can vary among mammalian species and this variability may provide relevant information regarding the species life history and taxon identification. In human HSBs can be used as a biometric-based parameter for personal identification in automated systems. The analysis of HSBs has been hampered by technical difficulties. The low contrast between light and dark bands and variations in light intensity may hinder the observation of HSBs in digital images. This article describes a simple and efficient computational procedure that greatly enhances the contrast and minimizes the differences in the intensity of illumination in HSBs images. Its use can significantly increase the quality and the number of HSBs that can be recorded in intact teeth.

Optimizing the analysis of dental enamel microstructure in intact teeth.

In most mammalian species enamel prisms are regularly arranged in layers of alternating directions forming an angle of approximately 90°. These successive layers of prisms are known as Hunter-Schreger bands (HSBs). The analysis of HSBs may provide valuable information regarding the species life history, taxon and personal identification, with evident applicability in physical anthropology and forensics. Obtaining good quality digital images of HSBs in intact specimens is not always a feasible task. The major problems are the low contrast of images; the reflection of incident light, which may create areas of intense shine in digital images; and the abrupt decrease in the degree of illumination that occurs after light crosses the vertical cracks, frequently present in enamel. We show here that the area of intense shine can be minimized by a polarizing filter coupled to the camera objective, and the filling of enamel cracks with corn oil can reduce refraction of light in enamel cracks. These procedures can significantly increase the quality and the area of HSBs that can be recorded in intact teeth.

Comparative studies between mice molars and incisors are required to draw an overview of enamel structural complexity.

In the field of dentistry, the murine incisor has long been considered as an outstanding model to study amelogenesis. However, it clearly appears that enamel from wild type mouse incisors and molars presents several structural differences. In incisor, exclusively radial enamel is observed. In molars, enamel displays a high level of complexity since the inner part is lamellar whereas the outer enamel shows radial and tangential structures. Recently, the serotonin 2B receptor (5-HT2BR) was shown to be involved in ameloblast function and enamel mineralization. The incisors from 5HT2BR knockout (KO) mice exhibit mineralization defects mostly in the outer maturation zone and porous matrix network in the inner zone. In the molars, the mutation affects both secretory and maturation stages of amelogenesis since pronounced alterations concern overall enamel structures. Molars from 5HT2BR KO mice display reduction in enamel thickness, alterations of inner enamel architecture including defects in Hunter-Schreger Bands arrangements, and altered maturation of the outer radial enamel. Differences of enamel structure were also observed between incisor and molar from other KO mice depleted for genes encoding enamel extracellular matrix proteins. Thus, upon mutation, enamel analysis based exclusively on incisor defects would be biased. In view of the functional relationship between enamel structure and tooth morphogenesis, identification of molecular actors involved in amelogenesis requires comparative studies between mice molars and incisors.