Spatial distribution of the human enamel fracture toughness with aging

Effectiveness of changing the color of darker teeth is potentiated by association with violet LED light

BACKGROUND

The effectiveness of in-office bleaching protocols performed with violet LED light either combined with a bleaching agent containing 37% carbamide peroxide, or not, was determined by comparing teeth with different degrees of darkening.

METHODOLOGY

Eighty bovine incisors were separated into groups of "light" teeth (luminosity greater than or equal to B3) and "dark" teeth (less than or equal to A3.5) to receive the protocols: HP - 35% hydrogen peroxide (Whiteness HP), CP - 37% carbamide peroxide (Whiteness SuperEndo), LED - violet LED light (Bright Max Whitening), CPLED - CP associated with the LED. For color analysis the CIEL*a*b* e WID, ΔEab, ΔE00 e ΔWID parameters were used. Data were analyzed using Mann-Whitney, Kruskal-Wallis, Dunn, Friedman or Nemenyi tests (α = 5%).

RESULTS

HP and CP resulted in similar color change values (ΔEab, ΔE00 e ΔWID) for light and dark teeth (p > 0.05). Dark teeth showed better bleaching effectiveness (ΔEab, ΔE00 e ΔWID) than light teeth when CPLED was used (p < 0.05). LED showed color change that were below the limits of acceptability and perceptibility for ΔWID.

CONCLUSION

light teeth are effectively bleached with the use of HP or CP, whereas dark teeth respond better to treatment with the CPLED protocol. Violet LED used alone did not show a satisfactory result.

The influence of age and orthodontic debonding on the prevalence and severity of enamel craze lines

BACKGROUND

Craze lines may cause esthetic concerns, especially when noted on the incisors. Various light sources with additional recording apparatus have been proposed to visualize craze lines, but a standardized clinical protocol is yet to be determined. This study aimed to validate the application of near-infrared imaging (NIRI) from intraoral scans to evaluate craze lines and to determine the influence of age and orthodontic debonding on their prevalence and severity.

METHODS

The NIRI of maxillary central incisors from a full-mouth intraoral scan and photographs from an orthodontic clinic (N = 284) were collected. The prevalence of craze lines and influence of age and orthodontic debonding history on severity were evaluated.

RESULTS

Craze lines were detected reliably as white lines distinguishable from dark enamel using the NIRI from intraoral scans. The craze line prevalence was 50.7%, which was significantly higher in patients 20 years or older than in patients younger than 20 years (P < .001), with more frequent severe craze lines for those 40 years or older than in patients younger than 30 years (P < .05). Prevalence or severity was similar between patients with and without an orthodontic debonding history regardless of the type of appliance.

CONCLUSION

The prevalence of craze lines in the maxillary central incisor was 50.7%, with a higher prevalence in adults than in adolescents. Orthodontic debonding did not affect the severity of craze lines.

PRACTICAL IMPLICATIONS

Craze lines were reliably detected and documented by means of applying NIRI from intraoral scans. Intraoral scanning can provide new clinical information on enamel surface characteristics.

Biomechanical Modelling for Tooth Survival Studies: Mechanical Properties, Loads and Boundary Conditions-A Narrative Review

Recent biomechanical studies have focused on studying the response of teeth before and after different treatments under functional and parafunctional loads. These studies often involve experimental and/or finite element analysis (FEA). Current loading and boundary conditions may not entirely represent the real condition of the tooth in clinical situations. The importance of homogenizing both sample characterization and boundary conditions definition for future dental biomechanical studies is highlighted. The mechanical properties of dental structural tissues are presented, along with the effect of functional and parafunctional loads and other environmental and biological parameters that may influence tooth survival. A range of values for Young's modulus, Poisson ratio, compressive strength, threshold stress intensity factor and fracture toughness are provided for enamel and dentin; as well as Young's modulus and Poisson ratio for the PDL, trabecular and cortical bone. Angles, loading magnitude and frequency are provided for functional and parafunctional loads. The environmental and physiological conditions (age, gender, tooth, humidity, etc.), that may influence tooth survival are also discussed. Oversimplifications of biomechanical models could end up in results that divert from the natural behavior of teeth. Experimental validation models with close-to-reality boundary conditions should be developed to compare the validity of simplified models.

Emissivity evaluation of human enamel and dentin

Background: Human enamel and dentin temperatures have been assessed with non-contact infrared imaging devices for safety and diagnostic capacity and require an emissivity parameter to enable absolute temperature measurements. Emissivity is a ratio of thermal energy emitted from an object of interest, compared to a perfect emitter at a given temperature and wavelength, being dependent on tissue composition, structure, and surface texture. Evaluating the emissivity of human enamel and dentin is varied in the literature and warrants review. The primary aim of this study was to evaluate the emissivity of the external and internal surface of human enamel and dentin, free from acquired or developmental defects, against a known reference point. The secondary aim was to assess the emissivity value of natural caries in enamel and dentin.

Method: Fourteen whole human molar teeth were paired within a thermally stable chamber at 30°C. Two additional teeth (one sound and one with natural occlusal caries–ICDAS caries score 4 and radiographic score RB4) were sliced and prepared as 1-mm-thick slices and placed on a hot plate at 30°C within the chamber. A 3M Scotch Super 33 + Black Vinyl Electrical Tape was used for the known emissivity reference-point of 0.96. All samples were allowed to reach thermal equilibrium, and a FLIR SC305 infrared camera recorded the warming sequence. Emissivity values were calculated using the Tape reference point and thermal camera software.

Results: The external enamel surface mean emissivity value was 0.96 (SD 0.01, 95% CI 0.96–0.97), whereas the internal enamel surface value was 0.97 (SD 0.01, 95% CI 0.96–0.98). The internal crown-dentin mean emissivity value was 0.94 (SD 0.02, 95% CI 0.92–0.95), whereas the internal root-dentin value was 0.93 (SD 0.02, 95% CI 0.91–0.94) and the surface root-dentin had a value of 0.84 (SD 0.04, 95% CI 0.77–0.91). The mean emissivity value of the internal enamel surface with caries was 0.82 (SD 0.05, 95% CI 0.38–1.25), and the value of the internal crown-dentin with caries was 0.73 (SD 0.08, 95% CI 0.54–0.92).

Conclusion: The emissivity values of sound enamel, both internal and external, were similar and higher than those of all sound dentin types in this study. Sound dentin emissivity values diminished from the crown to the root and root surface. The lowest emissivity values were recorded in caries lesions of both tissues. This methodology can improve emissivity acquisition for comparison of absolute temperatures between studies which evaluate thermal safety concerns during dental procedures and may offer a caries diagnostic aid.

An Assessment of Mineral Concentration of Dental Enamel Neighbouring Hypothetical Orthodontic Brackets Using X-ray Microtomography

OBJECTIVES

Differences in the mineral concentration (MC) level of dental enamel may represent a precursor of white spot lesions adjacent to fixed orthodontic brackets. The aim of the current in vitro study was to compare the MC level central, occlusal and cervical to orthodontic attachments.

METHODS AND MATERIALS

A total of 16 enamel blocks were obtained from sound human premolar samples extracted for orthodontic reasons. The buccal portion of the dental enamel blocks was divided into central, occlusal, and cervical regions and then imaged and measured to calculate the level of MC using quantitative X-ray microtomography methods (XMT) at each site.

RESULTS

There was a substantial variation in the mineral concentration with the lowest level being detected in the cervical region when compared with other regions. The MC of the gingival zone was significantly lower than that of the middle zone (P<0.05) and was insignificantly lower than that of the occlusal zone.

CONCLUSION

Within the limitations of the current study, it can be concluded that the cervical region of the permanent enamel had the lowest mineral concentration using XMT. The cervical region may therefore be more vulnerable to the development of white spot lesions (WSLs) adjacent to a fixed orthodontic appliance during orthodontic treatment.

CLINICAL SIGNIFICANCE

Using X-ray microtomography lower mineral concentration in the cervical region of the enamel was observed. This may make these areas particularly susceptible to demineralisation during fixed appliance-based orthodontic treatment and may influence the bond strength of fixed orthodontic attachments.

Human tooth enamel tuft drapes revealed by microtomography

OBJECTIVE

The tufts of human dental enamel are structures located at the enamel-dentin junction and whose origin has not been clearly established. Although studies have highlighted their protein content and hypomineralization, none has been able to shed light on their 3D structure. The aim of this study was to reveal the whole structure using high-resolution conventional microtomography.

DESIGN

Ten adult mandibular first and second molars and two primary mandibular first molars were sectioned and scanned with microcomputed tomography with a resolution between 4.7 and 5 micrometers. By determining the threshold discriminating dentin and tufts, we were able to reconstruct 3D meshes.

RESULTS

We revealed the exact pattern of the tufts in adult molars and discovered their distribution, their dynamics, and the existence of a regular undulation, forming a particular angle of approximately 30 degrees with the dentin surface. A spatial frequency of approximately 160 micrometers would be compatible with the variation in the orientation of groups of dental enamel rods. In contrast, the present setting is not sufficient to extract similar information for primary teeth.

CONCLUSIONS

Enamel tufts have a specific pattern, with an oriented draped form and are regularly spaced. The possible connection between these undulations and the Hunter-Schreger bands (diazonias and parazonias) needs to be studied.

Mechanical failure of posterior teeth due to caries and occlusal wear- A modelling study

OBJECTIVES

The purpose of the present work is to explore the effect of occlusal wear and different types and degrees of caries on the mechanical performance and structural integrity of posterior teeth.

METHODS

Three-dimensional (3D) computational models with different combinations of caries parameters (caries location, caries size and caries induced pulp shrinkage) and occlusal wear factors (enamel thickness, marginal ridge height and cuspal slope) were developed and analyzed using the extended finite element method (XFEM) to identify the stress distribution, crack initiation load and ultimate fracture load values. The effect of a non-drilling conservative treatment using resin infiltration on the recovery of mechanical properties of carious molar teeth was also investigated.

RESULTS

Presence of fissural caries, worn proximal marginal ridge and decreased enamel thickness due to occlusal wear, imparted a significant negative effect on the crack initiation load value of the lower molar models. Accordingly, models with intact and strong proximal marginal ridge, generally exhibited higher crack initiation loading, regardless of caries size and location. Presence of fissure caries drastically decreased (55%-70%) the crack initiation load compared to sound teeth. The depth of the fissural lesion and the presence of proximal caries did not have a major effect on crack initiation load values. However, increasing the caries size resulted in lower final fracture load values in most of the cases. Accordingly, the groups with combined and connected large fissural and proximal lesions experienced the largest drop in the fracture load values compared to sound tooth models. The worst condition consisted of two connected large proximal and fissural caries with no proximal marginal ridge, in which the fracture load dramatically decreased to only 25% of that for sound teeth with intact marginal ridge. On the other hand, decreased cuspal slope due to occlusal wear and shrinkage of the pulp due to caries appeared to have a protective role and a direct relation with the fracture resistance of the tooth. Following the application of resin infiltration on the carious models, the crack initiation load and the fracture load could recover up to 75% and 90% of the values for the corresponding sound tooth models, respectively.

SIGNIFICANCE

Presence of fissural caries, if not treated (either with remineralization, resin infiltration or restoration), can be a major risk factor in the initiation of tooth fracture. When combined with decreased enamel thickness and loss of proximal marginal ridge due to mechanical or chemical wear, the weakening effect of the caries will be amplified specially in teeth with steep cuspal slopes. The application of a conservative treatment with resin infiltration can be an effective approach in prevention of further mechanical failure of demineralized enamel. The findings of this study emphasize the importance of early interventions in the management of caries for the prevention of future cuspal or tooth fracture especially in subjects with higher risk factors for tooth fracture such as caries, wear and bruxism.

A machine learning approach to investigate the materials science of enamel aging

Understanding aging of tooth tissues is critical to the development of patient-centric oral healthcare. Yet, the traditional methods for analyzing the composition-structure-property relationships of hard tissues have limitations when considering aging and other factors.

OBJECTIVE

To apply unsupervised machine learning tools to pursue an understanding of relationships between the composition and mechanical behavior of aging enamel.

METHODS

Molar teeth were collected from primary (age ≤ 8), young adult (24 ≤ age ≤ 46) and old adult (55 ≤ age) donors. The hardness and elastic modulus were quantified using nanoindentation as a function of distance from the Dentin Enamel Junction (DEJ) within the cervical, cuspal and inter-cuspal regions of the enamel crown. Similarly, a co-located analysis of the chemical composition and structure was performed using Raman spectroscopy. A Self-Organizing Maps (SOMs) algorithm was implemented to identify multi-dimensional composition-property relationships.

RESULTS

The hardness and elastic modulus are positively correlated to crystallinity and negatively correlated with carbonate substitution. Furthermore, the effects from fluoridation on the age-dependent properties of enamel is non-linear and depends on its location. The contributions of fluoridation to the enamel properties are different in the cervical and non-cervical regions and appear to be unique within primary and senior adult teeth.

SIGNIFICANCE

Based on the findings, unsupervised learning methods can reveal complicated non-linear structure-property relationships in tooth tissues and help to understand the materials science of aging and its consequences.

In vivo examination of enamel microcracks after orthodontic debonding: Is there a need for detailed analysis?

INTRODUCTION

Our aim was to assess changes in the number of enamel microcracks (EMCs) after removing metal brackets in teeth with and without visible EMCs before the bonding procedure.

METHODS

Before bonding, 13 patients having teeth with visible EMCs and 13 subjects whose teeth were free of EMCs were included in the study. All patients were asked to complete a questionnaire with a detailed medical history at the beginning of treatment and after removing metal brackets. The number of teeth with visible EMCs and the number of premolars without EMCs were recorded for each subject twice, that is, before bonding and after debonding, together with the tooth sensitivity assessments elicited by compressed air and cold testing.

RESULTS

The number of visible EMCs in premolars increased after removing metal brackets. EMCs were recorded in at least 25.0% of all evaluated teeth for the patients having teeth with and without visible EMCs at the beginning of treatment. However, the changes in the number of visible EMCs were not significantly different (P = 0.619) between the groups. For the subjects with visible EMCs, tooth sensitivity caused by cold was registered nearly 3 times more often after removing brackets compared with the patients without EMCs prior bonding.

CONCLUSIONS

Formation of EMCs was noticed after debonding. Changes in the number appeared to be similar for the subjects with and without visible EMCs before bonding. Higher incidence of EMCs was associated with more frequent tooth sensitivity perceptions after removing brackets.

Aging and Senescence of Dental Pulp and Hard Tissues of the Tooth

The ability to consume a meal using one's own teeth influences an individual's quality of life. In today's global aging society, studying the biological changes in aging teeth is important to address this issue. A tooth includes three hard tissues (enamel, dentin, and cementum) and a soft tissue (dental pulp). With advancing age, these tissues become senescent; each tissue exhibits a unique senescent pattern. This review discusses the structural alterations of hard tissues, as well as the molecular and physiological changes in dental pulp cells and dental pulp stem cells during human aging. The significance of senescence in these cells remains unclear. Thus, there is a need to define the regulatory mechanisms of aging and senescence in these cells to aid in preservation of dental health.

Investigation on the Gradient Nanomechanical Behavior of Dental Fluorosis Enamel

This study aims to investigate the gradient nanomechanical behavior of dental fluorosis enamel and provide appropriate selection criteria for restorative materials. The nanomechanical properties of the outer, middle, and inner layers of normal tooth enamel, mild dental fluorosis enamel, and severe dental fluorosis enamel were tested by nanoindentation under an applied load of 2000 μN and holding time of 30 s. The nanotribological properties were then evaluated through nanoscratch tests under an applied load of 1000 μN. In addition, the nanotribological property of the outer layer of dental fluorosis enamel was compared with that of four restorative materials, namely, lithium disilicate glass-ceramic (IPS e.max CAD), polymer-infiltrated-ceramic network (PICN), composite resin block (Lava™ ultimate), and conventional composite resin (Fltek™ Z350XT). The nanohardness and elastic modulus of mild dental fluorosis enamel increased from the outer to the middle layers and then decreased from the middle to the inner layers. By contrast, the changed displacement, friction coefficient, and nanoscratch depth and width decreased from the outer to the middle layers and then increased from the middle to the inner layers. In severe dental fluorosis enamel, nanohardness and elastic modulus increased from the outer to the inner layers, but the changed displacement, friction coefficient, and nanoscratch depth and width decreased from the outer to the inner layers. The nanoscratch depth and width of Lava™ ultimate were similar to those of the outer layer of the mild dental fluorosis enamel. The gradient nanomechanical behavior of dental fluorosis enamel significantly differed from that of normal tooth enamel. Dental materials with a wear resistance similar to that of the opposing enamel are a good choice for restoring dental fluorosis (trial registration: WCHSIRB-D-2014-126, registered 25 December 2014).

The Tooth: Its Structure and Properties

This article provides a brief review of recent investigations concerning the structure and properties of the tooth. The last decade has brought a greater emphasis on the durability of the tooth, an improved understanding of the fatigue and fracture behavior of the principal tissues, and their importance to tooth failures. The primary contributions to tooth durability are discussed, including the process of placing a restoration, the impact of aging, and challenges posed by the oral environment. The significance of these findings to the dental community and their importance to the pursuit of lifelong oral health are highlighted.

Trend-analysis of dental hard-tissue conditions as function of tooth age

OBJECTIVE

This retrospective in-vitro study investigated tooth age effect on dental hard-tissue conditions.

METHODS

Unidentified extracted premolars (n = 1500) were collected and their individual age was estimated (10-100 (±10) years old (yo)) using established dental forensic methods Dental caries, fluorosis and tooth wear (TW) were assessed using the International Caries Detection and Assessment System (ICDAS; 0-5 for crown and 0-2 for root), Thylstrup-Fejerskov (TFI; 0-9) and Basic Erosive Wear Examination (BEWE; 0-3) indices, respectively. Staining and color were assessed using the modified-Lobene (MLI) (0-3) and VITA shade (B1-C4) indices, respectively. Relationships between indices and age were tested using regression models.

RESULTS

Starting at age ∼10yo, presence of caries increased from 35% to 90% at ∼50yo (coronal), and from 0% to 35% at ∼80yo (root). Caries severity increased from ICDAS 0.5 to 2 at ∼40yo and from ICDAS 0 to 0.5 at ∼60yo for coronal and root caries, respectively. Presence of TW increased from 25% (occlusal) and 15% (smooth-surfaces) to 100% at ∼80yo. TW severity increased from BEWE 0.5 to 2 at ∼50yo (occlusal) and ∼0.3 to 1.5 at ∼50yo (smooth-surfaces). Percentage and severity of fluorosis decreased from 70% to 10% at ∼80yo, and from TFI 1 to 0 at ∼90yo, respectively. Percentage of extrinsic staining increased from 0% to 85% at ∼80yo and its severity increased from MLI 0 to 2 at ∼70yo. Color changed from A3 to B3 at ∼50yo (crown), and from C2 to A4 at ∼85yo (root).

CONCLUSIONS

Aging is proportionally related to the severity of caries, TW, staining, and inversely to dental fluorosis. Teeth become darker with age.

Deformation behaviour of aged coronal dentin

OBJECTIVE

This study aimed to identify the changes in the time-dependent deformation response of coronal dentin with ageing and its relationship with changes in chemical composition.

BACKGROUND

The structural behaviour of dentin with ageing is affected by changes in the density and diameter of its dentinal tubules (ie porosity), as well as changes in chemical composition throughout the tooth. However, little is known about the time-dependent deformation behaviour of aged dentin and the importance of its hierarchical structure and variations in chemical composition.

MATERIALS AND METHODS

The spherical indentation response of aged coronal dentin was analysed in the outer, middle and inner regions, and its time-dependent deformation response was modelled in terms of its microstructure and chemical composition using a model recently proposed for young dentin.

RESULTS

The viscous deformation behaviour of aged dentin followed a power-law response with a decrease in the stress exponent when compared to young dentin. These results can be explained by cross-linking of the collagen present in the tissue.

CONCLUSION

A decrease in the deformation ability of aged dentin was found. This behaviour could be a result of a dissolution process and reprecipitation of the minerals present in intertubular dentin into the dentinal tubules.

Correlation between micro-hardness and mineral content in healthy human enamel

BACKGROUND

Enamel is the hardest and the stiffest tissue in the human body. The enamel undergoes multidirectional stresses, withstands multimillion chewing cycles, all while protecting the internal dentin and pulp from damage due to mechanical overload and exposure to the harsh chemical environment of the mouth. Raman spectroscopy allows to study enamel mineral content in a non-destructive and site-specific way. While Raman spectroscopy has been applied in other studies to assess tooth mineralization, there are no studies that examine the relationship between micro-hardness and mineral content of the untreated enamel. An understanding of this relationship is extremely important in a clinical context. The effect of various agents on enamel hardness was investigated, though the relationship between healthy enamel mineral content and micro-hardness remains obscure.

MATERIAL AND METHODS

Twenty human incisor teeth were obtained in compliance with the NIH guidelines and imaged site-specifically with a Raman microscope and evaluated with a Brinell hardness measurement device. The front portion of each tooth was divided into apical, medium and cervical regions and subsequently imaged with a Raman microscope in these three locations.

RESULTS AND CONCLUSIONS

The results demonstrated that enamel mineral content varies significantly between individuals and is correlated with the hardness of the enamel. Non-invasive, sample preparation free Raman spectroscopy was successfully employed to measure the mineral content of healthy enamel and it correlated the mineralization score to the hardness measurements of the selected cervical location. The overall level of enamel mineral content may serve as a robust predictor of patients' susceptibility to developing caries, and overall enamels wear resistance, thus allowing for the prevention of caries via clinically available methods of remineralization, fluoride treatment and frequent cleaning. Key words:Enamel, raman spectroscopy, micro-hardness, extracted teeth.

A portable fiber-optic raman spectrometer concept for evaluation of mineral content within enamel tissue

Background

Measurement of tooth enamel mineralization using a clinically viable method is essential since variation of mineralization may be used to monitor caries risk or in assessing the effectiveness of remineralization therapy. Fiber optic Raman systems are becoming more affordable and popular in context of biomedical applications. However, the applicability of fiber optic Raman systems for measurement of mineral content within enamel tissue has not been elucidated significantly in the prior literature.

Material and Methods

Human teeth with varying degrees of enamel mineralization were selected. In addition alligator, boar and buffalo teeth which have increasing amount of mineral content, respectively, were also included as another set of samples. Reference Raman measurements of mineralization were performed using a high-fidelity confocal Raman microscope.

Results

Analysis of human teeth by research grade Raman system indicated a 2-fold difference in the Raman intensities of v1 symmetric-stretch bands of mineral-related phosphate bonds and 7-fold increase in mineral related Raman intensities of animal teeth. However, fiber optic system failed to resolve the differences in the mineralization of human teeth.

Conclusions

These results indicate that the sampling volume of fiber optic systems extends to the underlying dentin and that confocal aperture modification is essential to limit the sampling volume to within the enamel. Further research efforts will focus on putting together portable Raman systems integrated with confocal fiber probe.

Key words:Enamel, mineral content, raman spectroscopy.

Evaluation of mineral content in healthy permanent human enamel by Raman spectroscopy

Background

An understanding of tooth enamel mineral content using a clinically viable method is essential since variations in mineralization may serve as an early precursor of a dental health issues, and may predict progression and architecture of decay in addition to assessing the success and effectiveness of the remineralization strategies.

Material and Methods

Twenty two human incisor teeth were obtained in compliance with the NIH guidelines and site specifically imaged with Raman microscope. The front portion of the teeth was divided into apical, medium and cervical regions and subsequently imaged with Raman microscope in these three locations.

Results

Measured mineralization levels have varied substantially depending on the regions. It was also observed that, the cervical enamel is the least mineralization as a populational average.

Conclusions

Enamel mineralization is affected by a many factors such as are poor oral hygiene, alcohol consumption and high intake of dietary carbohydrates, however the net effect manifests as overall mineral content of the enamel. Thus an early identification of the individual with overall low mineral content of the enamel may be a valuable screening tool in determining a group with much higher than average caries risk, allowing intervention before development of caries. Clinically applicable non-invasive techniques that can quantify mineral content, such as Raman analysis, would help answer whether or not mineralization is associated with caries risk.

Key words:Enamel, Raman spectroscopy, mineral content, dental caries.

On the importance of aging to the crack growth resistance of human enamel

With improvements in oral health and an overall increase in quality of life, the percentage of fully or largely dentate seniors is increasing. Understanding the effects of aging on the mechanical properties of teeth is essential to the maintenance of lifelong oral health. In this investigation the effects of aging on the fracture toughness of human enamel were evaluated from incremental crack growth experiments performed on tissue of donor teeth representing "young" (17 ⩽ age ⩽ 25) and "old" (age ⩾ 55) age groups. Results showed that the old enamel exhibited significantly lower resistance to fracture than that of the young tissue in two orthogonal directions of crack growth. For crack growth transverse to the enamel rods, the fracture toughness of the old enamel (0.37 ± 0.15 MPa m(0.5)) was nearly 70% lower than that of tissue from the young teeth (1.23 ± 0.20 MPa m(0.5)). Based on results from a mechanistic analysis of crack growth, the reduction in fracture resistance is attributed to a decrease in the degree of extrinsic toughening. The practice of restorative dentistry should account for these changes in tooth tissues in the treatment of senior patients.

STATEMENT OF SIGNIFICANCE

The mechanical behavior of enamel has been studied for over 3 decades. Due to the limited volume of tissue available for evaluation, past work has been largely based on indentation methods. In this investigation we have evaluated the resistance to fracture of human enamel using a conventional fracture mechanics approach and incremental crack growth. We compared the fracture resistance of cuspal enamel obtained from the teeth of representative "young" and "old" donor groups. Our results show that there is a substantial reduction in the resistance to fracture with age, that it is anisotropic, and that the degradation is more severe than that which occurs to dentin. As such, we feel this work is a significant contribution to the field.

Longevity of resin-bonded fixed partial dental prostheses made with metal alloys

Objectives

The purpose of this study was to evaluate the clinical performance of resin-bonded fixed partial dental prostheses (RBFPDPs) made with metal alloys.

Materials and methods

The retention of 311 RBFPDPs from 226 patients fabricated from 1983 to 2013 using an adhesive resin was clinically evaluated. Partial or complete debonding of the RBFPDP or framework fracture was considered a treatment failure. All data were obtained from clinical examinations, and missing data were censored at the date of the last available information. The effect of the following factors on survival rate were investigated: patient gender, location (maxilla/mandible and anterior/posterior), number of missing teeth, number of abutment teeth, framework structure, type of metal alloy, patient age at the point of cementation, cement type, and distinction of the treating dentist. Data were analyzed with the Kaplan–Meier survival tests, log-rank tests, and Cox regression analyses (α = 0.05).

Results

The Kaplan–Meier survival rate was 41.2 % ± 6.5 % (standard error) at 28.8 years (last outcome event). Significant differences were found for patient age and treating dentist (p < 0.05). The risk of failure in younger patients was 1.7 times greater than that in older patients and that of inexperienced dentists was 2.0 times greater than that of dentist experienced and specialized in adhesive dentistry.

Conclusions

When fabricating RBFPDPs for younger patients, mechanical preparation for bonding may be necessary in consideration of the risk for debonding. Experienced dentists may achieve better results.

Clinical relevance

Mastery of skills is necessary to ensure excellent prognoses for RBFPDPs.

Radiotherapy effect on nano-mechanical properties and chemical composition of enamel and dentine

OBJECTIVE

To understand radiotherapy-induced dental lesions characterized by enamel loss or delamination near the dentine-enamel junction (DEJ), this study evaluated enamel and dentine nano-mechanical properties and chemical composition before and after simulated oral cancer radiotherapy.

DESIGN

Sections from seven non-carious third molars were exposed to 2 Gy fractions, 5 days/week for 7 weeks for a total of 70 Gy. Nanoindentation was used to evaluate Young's modulus, while Raman microspectroscopy was used to measure protein/mineral ratios, carbonate/phosphate ratios, and phosphate peak width. All measures were completed prior to and following radiation at the same four buccal and lingual sites 500 and 30 μm from the DEJ in enamel and dentine (E-500, E-30, D-30 and D-500).

RESULTS

The elastic modulus of enamel and dentine was significantly increased (P ≤ 0.05) following radiation. Based on Raman spectroscopic analysis, there was a significant decrease in the protein to mineral ratio (2931/430 cm(-1)) following radiation at all sites tested except at D-500, while the carbonate to phosphate ratio (1070/960 cm(-1)) increased at E-30 and decreased at D-500. Finally, phosphate peak width as measured by FWHM at 960 cm(-1) significantly decreased at both D-30 and D-500 following radiation.

CONCLUSIONS

Simulated radiotherapy produced an increase in the stiffness of enamel and dentine near the DEJ. Increased stiffness is speculated to be the result of the radiation-induced decrease in the protein content, with the percent reduction much greater in the enamel sites. Such changes in mechanical properties and chemical composition could potentially contribute to DEJ biomechanical failure leading to enamel delamination that occurs post-radiotherapy. However, other analyses are required for a better understanding of radiotherapy-induced effects on tooth structure to improve preventive and restorative treatments for oral cancer patients.

Dynamic finite element analysis and moving particle simulation of human enamel on a microscale

BACKGROUND

The study of biomechanics of deformation and fracture of hard biological tissues involving organic matrix remains a challenge as variations in mechanical properties and fracture mode may have time-dependency. Finite element analysis (FEA) has been widely used but the shortcomings of FEA such as the long computation time owing to re-meshing in simulating fracture mechanics have warranted the development of alternative computational methods with higher throughput. The aim of this study was to compare dynamic two-dimensional FEA and moving particle simulation (MPS) when assuming a plane strain condition in the modeling of human enamel on a reduced scale.

METHODS

Two-dimensional models with the same geometry were developed for MPS and FEA and tested in tension generated with a single step of displacement. The displacement, velocity, pressure, and stress levels were compared and Spearman׳s rank-correlation coefficients R were calculated (p<0.001).

RESULTS

The MPS and FEA were significantly correlated for displacement, velocity, pressure, and Y-stress.

CONCLUSIONS

The MPS may be further developed as an alternative approach without mesh generation to simulate deformation and fracture phenomena of dental and potentially other hard tissues with complex microstructure.

On the mechanical properties of tooth enamel under spherical indentation

The mechanical properties of tooth enamel generally exhibit large variations, which reflect its structural and material complexity. Some key properties were evaluated under localized contact, simulating actual functioning conditions. Prominent cusps of extracted human molar teeth were polished down ~0.7 mm below the cusp tip and indented by tungsten carbide balls. The internal damage was assessed after unloading from longitudinal or transverse sections. The ultimate tensile stress (UTS) was determined using a novel bilayer specimen. The damage is characterized by penny-like radial cracks driven by hoop stresses and cylindrical cracks driven along protein-rich interrod materials by shear stresses. Shallow cone cracks typical of homogeneous materials which may cause rapid tooth wear under repeat contact are thus avoided. The mean stress vs. indentation strain curve is highly nonlinear, attributable to plastic shearing of protein between and within enamel rods. This curve is also affected by damage, especially radial cracks, the onset of which depends on ball radius. Several material properties were extracted from the tests, including shear strain at the onset of ring cracks γ(F) (=0.14), UTS (=119 MPa), toughness K(C) (=0.94 MPa m(1/2)), a crack propagation law and a constitutive response determined by trial and error with the aid of a finite-element analysis. These quantities, which are only slightly sensitive to anatomical location within the enamel region tested, facilitate a quantitative assessment of crown failure. Causes for variations in published UTS and K(C) values are discussed.

Review of research on the mechanical properties of the human tooth

‘Bronze teeth' reflect the mechanical properties of natural teeth to a certain extent. Their mechanical properties resemble those of a tough metal, and the gradient of these properties lies in the direction from outside to inside. These attributes confer human teeth with effective mastication ability. Understanding the various mechanical properties of human teeth and dental materials is the basis for the development of restorative materials. In this study, the elastic properties, dynamic mechanical properties (visco-elasticity) and fracture mechanical properties of enamel and dentin were reviewed to provide a more thorough understanding of the mechanical properties of human teeth.

Enamel organic matrix: potential structural role in enamel and relationship to residual basement membrane constituents at the dentin enamel junction

Although mature enamel is predominantly composed of mineral, a previously uncharacterized organic matrix layer remains in the post-eruptive tissue that begins at the dentin enamel junction and extends 200-300 μm towards the outer tooth surface. Identification of the composition of this layer has been hampered by its insolubility; however, we have developed a single step method to isolate the organic enamel matrix relatively intact. After dissociative dissolution of the matrix with SDS and urea, initial characterization by Western blotting and gel zymography indicates the presence of type IV and type VII basement membrane collagens and active matrix metalloproteinase-20. When combined with data from transgenic knockout mice and from human mutations, these data suggest that the enamel organic matrix (EOM) and dentin enamel junction may have a structural and functional relationship with basement membranes, e.g. skin. To clarify this relationship, we hypothesize a "foundation" model which proposes that components of the EOM form a support structure that stabilizes the crystalline enamel layer, and bonds it to the underlying dentin along the dentin enamel junction. Since we have also co-localized an active matrix metalloproteinase to this layer, our hypothesis suggests that, under pathologic conditions, MMP-mediated degradation of the EOM could destabilize the enamel-dentin interface.