Mechanical properties and microstructure of hypomineralised enamel of permanent teeth

Longitudinal Evaluation of the Structural Integrity of Teeth Affected by Molar Incisor Hypomineralisation

The aim of this prospective cohort study was to evaluate the risk of posteruptive breakdown and the development of caries lesions in teeth with molar incisor hypomineralisation (MIH). A total of 367 permanent incisors and first molars, affected and not affected by MIH lesions, of 45 children with MIH from Araraquara, São Paulo, Brazil, were evaluated at intervals from 6 to 12 months by assessing the severity of MIH, the presence of tooth caries lesions and the treatment needed. During the study period, all patients received preventive care. The data were analysed using Fisher's exact test and actuarial method survival analysis. Significant associations were also found in teeth between the presence of MIH and a DMFT index >0 in all periods and also between the need for treatment and the presence of MIH. The teeth affected by MIH opacities were healthy in 99% of incisors and 93% of molars at the end of the 12-month period. Due to the high likelihood of maintaining the tooth structure in opacities, the complete or premature removal of the affected area is not justified.

XRMA and ToF-SIMS Analysis of Normal and Hypomineralized Enamel

Molar incisor hypomineralization (MIH) is a developmental disturbance of the enamel. This study presents analyses of hypomineralized and normal enamel in first molar teeth diagnosed with MIH, utilizing time-of-flight secondary ion mass spectrometry area analyses and X-ray microanalysis of area and spot profiles in uncoated samples between gold lines which provide electrical conductivity. Statistical analysis of mean values allows discrimination of normal from MIH enamel, which has higher Mg and lower Na and P. Inductive analysis using complete data sets for profiles from the enamel surface to the enamel-dentin junction found that Mg, Cl and position in the profile provide useful discrimination criteria. Element profiles provide a visual complement to the inductive analysis and several elements also provide insight into the development of both normal and MIH enamel. The higher Mg content and different Cl profiles of hypomineralized enamel compared with normal enamel are probably related to a relatively short period during the development of ameloblasts between birth and the 1st year of life.

Aesthetic and functional rehabilitation of the primary dentition affected by amelogenesis imperfecta

The objective of this case report was to describe the oral rehabilitation of a five-year-old boy patient diagnosed with amelogenesis imperfecta (AI) in the primary dentition. AI is a group of hereditary disorders that affects the enamel structure. The patient was brought to the dental clinic complaining of tooth hypersensitivity during meals. The medical history and clinical examination were used to arrive at the diagnosis of AI. The treatment was oral rehabilitation of the primary molars with stainless steel crowns and resin-filled celluloid forms. The main objectives of the selected treatment were to enhance the esthetics, restore masticatory function, and eliminate the teeth sensitivity. The child was monitored in the pediatric dentistry clinic at four-month intervals until the mixed dentition stage. Treatment not only restored function and esthetic, but also showed a positive psychological impact and thereby improved perceived quality of life. The preventive, psychological, and curative measures of a young child with AI were successful. This result can encourage the clinicians to seek a cost-effective technique such as stainless steel crowns, and resin-filled celluloid forms to reestablish the oral functions and improve the child's psychosocial development.

The effect of resin infiltration and oxidative pre-treatment on microshear bond strength of resin composite to hypomineralised enamel

BACKGROUND

Reduced bond strengths of resin composites to hypomineralised enamel increase restorative failure.

AIM

To investigate if the adhesion of resin composite to hypomineralised enamel can be improved by pre-treatments: resin infiltration, oxidative pre-treatment followed by a resin infiltration, or oxidative pre-treatment.

DESIGN

Twenty-one enamel specimens in each of five Groups: 1) Normal enamel; 2) Hypomineralised enamel; 3) Hypomineralised enamel pre-treated with a resin infiltrant, (Icon(®)); 4) Hypomineralised enamel pre-treated with 5.25% sodium hypochlorite then treatment with resin infiltrant; 5) Hypomineralised enamel pre-treated with 5.25% sodium hypochlorite. A resin composite rod was bonded to each specimen using Clearfil™ SE bond as the adhesive (hereafter termed 'routine bonding'), then subjected to microshear bond strength (MSBS) testing.

RESULTS

Overall, the mean MSBS between the five groups differed significantly (P = 0.001). Pre-treatment of hypomineralised enamel with 5.25% sodium hypochlorite with or without subsequent resin infiltration in Groups 4 and 5 prior to routine bonding resulted in increased mean MSBS compared to Groups 2 and 3, with mean MSBS values not differing significantly when compared to routine bonding to normal enamel.

CONCLUSION

Increased bond strength of resin composite to hypomineralised enamel was obtained by pre-treatment of hypomineralised enamel specimens with 5.25% sodium hypochlorite with or without subsequent resin infiltration.

Evaluation of X-ray microanalysis for characterization of dental enamel

Elemental analysis of dental hard tissues is of importance. The aim of this study is to evaluate X-ray microanalysis (XRMA) of bovine enamel in a scanning electron microscope (SEM) with different coatings. The buccal surface of bovine incisors was polished flat, one-third was coated with carbon, one-third with gold, leaving one-third uncoated for XRMA in an SEM equipped with an energy-dispersive microanalysis system. The elements oxygen, sodium, magnesium, phosphorous, chlorine, potassium, and calcium were analyzed using their respective characteristic K X-ray series. Comparisons were made with analyses of glass produced by fusion of the bovine enamel, showing that oxygen analyses using the K X-ray series are reliable and preferable to calculating oxygen by stoichiometry for natural enamel. For the gold-coated and uncoated analyses, carbon was also measured using the K X-ray series. Small area Analyses in small areas (80 × 80 μm) in variable pressure-SEM mode with low vacuum (20 Pa), without any coating, midway between 40 μm wide gold lines 140 μm apart to avoid build-up of electrostatic charge is the preferred method, especially if carbon is included in the analysis. The analyses of bovine enamel are sufficiently reproducible to be regarded as quantitative for all elements except carbon.

Molar incisor hypomineralisation: prevalence and clinical presentation in school children of the northern region of India

There is rarity of prevalence data on molar incisor hypomineralisation (MIH) for the Indian population and the majority of data originated from European countries.

AIM

To report on prevalence and defect characteristics of MIH for school children of the northern Indian region.

METHODS

A cross-sectional survey including 1,792, 6-9-year-old school children of Chandigarh, India was carried out using European Academy of Paediatric Dentistry (EAPD) 2003 criteria for diagnosis of MIH. In addition to descriptive analysis for distribution of various defects, comparative data analysis was carried out for inter-comparison of distribution and type of defect amongst two phenotypes, MH [first permanent molar (FPMs) involvement] and M + IH (simultaneous involvement of molars and incisors). Similar comparative analysis was performed for four subgroups on the basis of number of affected surfaces/subjects.

RESULTS

A prevalence of 6.31% was reported. FPMs (2.83 ± 0.874/subject) were more commonly affected than permanent incisors (1.19 ± 1.614/subjects). White/creamy opacity without post-eruptive breakdown (PEB) was the most common lesion, seen in 85% of subjects. MH phenotype was seen in 44% of subjects and 56% exhibited M + IH phenotype. A trend toward greater severity was seen in M + IH phenotype when compared to MH phenotype. A greater number of surfaces presented with white/creamy opacities without PEB (p < 0.05). With an increase in the number of surfaces involved the severity of MIH also increased with more frequent presence of brown defects with PEB.

CONCLUSION

With concomitant involvement of incisors, more severe presentation of MIH was seen. Also, with increase in number of affected surfaces a parallel increase in severity as well as extent of lesions was observed.

Characterisation of developmentally hypomineralised human enamel

OBJECTIVES

To investigate and clarify physical and chemical properties of enamel affected by molar incisor hypomineralisation (MIH).

METHODS

A series of in vitro studies were performed on extracted molars affected by MIH and sound teeth for controls. Tooth sections underwent Vickers microhardness testing before lapping and subsequent transverse microradiographic analysis and examination under polarised light microscopy. Carbonate content was determined by CO2 release from acid digestion. Unprepared and fractured surfaces were examined under scanning electron microscopy.

RESULTS

MIH-affected molars demonstrated a severe degree of hypomineralisation with an average mineral content of only 58.8%vol% mineral. Vickers microhardness was significantly reduced in MIH compared with controls (1.8±1.1 v 4.4±1.0 GPa, p<0.05) and polarised light microscopy revealed the bulk of MIH lesions had a porosity of ≤5% but also substantial areas of ≥10% and smaller areas exceeding 25% porosity. A surface layer was frequently observed on both intact and broken-down lesions and cervical regions of MIH teeth were typically spared. Carbonate content of MIH enamel was higher than control samples (6.6±2.1 v 4.4±1.1 wt%, p<0.05). Scanning electron microscopy showed that both the enamel rod and surface ultrastructure were defective. Clinical characteristics did not consistently correlate with all properties.

CONCLUSIONS

The properties of MIH-affected enamel significantly differ from those of normal enamel and were highly variable, however some common characteristics were observed. Implications for aetiology and clinical management are discussed.

Mechanical Properties of Enamel Nanocomposite

For adult Indian premolar teeth, we report for the first time ever the simultaneous evaluations of nanohardness, Young's modulus, and fracture toughness of the enamel nanocomposite. The nanohardness and Young's moduli were evaluated from near the beginning of the middle enamel region to within ~10 μm of the dentino-enamel junction (DEJ) and in the dentin region using the nanoindentation technique. The fracture toughness from near the middle of the enamel region to near the DEJ zone was measured using the microindentation technique. The deformation was studied using scanning electron microscopy (SEM) and field emission scanning electron microscopy (FESEM). The relative differences in the extents of biomineralization in the enamel and dentin regions were studied by the energy dispersive X-ray (EDS) technique. The variations of the toughness of the enamel as a function of the toughness of the protein matrix phase have been analyzed which showed that the predicted value of the toughness of the protein present in the nanocomposite was comparable to that of other bioproteins reported in the literature. Further, the work of fracture estimated from the measured value of toughness of the enamel nanocomposite agreed well with the experimental data reported in the literature.

An in vivo investigation of salivary properties, enamel hypomineralisation, and carious lesion severity in a group of Iraqi schoolchildren

BACKGROUND

Hypomineralised enamel is a prevalent, congenital defect vulnerable to deteriorate post-eruptively particularly in the presence of an unfavourable oral environment.

AIMS

To assess the influence of salivary characteristics on the clinical presentation of hypomineralisation lesions diagnosed in first permanent and second primary molars and to evaluate caries severity in relation to the defect's clinical presentation.

DESIGN

Recruitment consisted of 445 seven- to nine-year-old participants, of whom 152 were diagnosed as having molar hypomineralisation (MH); the remaining unaffected subjects (N = 293) were considered their controls for saliva analysis. Dental caries status was assessed in 300 subjects of saliva sub-sample, equally divided as MH-affected and non-affected children. The International Caries Detection and Assessment System was used for caries detection. Salivary flow rates, viscosity, pH, and buffering capacity were determined.

RESULTS

Molar hypomineralisation-affected children have significantly higher mean caries scores compared to the non-affected group. Dentinal carious lesions were ten times more frequent in teeth with post-eruptive breakdown (PEB) than with teeth with opacities only. Low salivary flow rates (LSFR), moderately viscous saliva, and low pH were significantly more common in the affected group. LSFR and moderate and highly acidic saliva were more likely associated with PEB.

CONCLUSION

Demarcated hypomineralised enamel is a dynamic defect highly influenced by individual characteristics of the oral environment.

Role of crystal arrangement on the mechanical performance of enamel

The superior mechanical properties of enamel, such as excellent penetration and crack resistance, are believed to be related to the unique microscopic structure. In this study, the effects of hydroxyapatite (HAP) crystallite orientation on the mechanical behavior of enamel have been investigated through a series of multiscale numerical simulations. A micromechanical model, which considers the HAP crystal arrangement in enamel prisms, the hierarchical structure of HAP crystals and the inelastic mechanical behavior of protein, has been developed. Numerical simulations revealed that, under compressive loading, plastic deformation progression took place in enamel prisms, which is responsible for the experimentally observed post-yield strain hardening. By comparing the mechanical responses for the uniform and non-uniform arrangement of HAP crystals within enamel prisms, it was found that the stiffness for the two cases was identical, while much greater energy dissipation was observed in the enamel with the non-uniform arrangement. Based on these results, we propose an important mechanism whereby the non-uniform arrangement of crystals in enamel rods enhances energy dissipation while maintaining sufficient stiffness to promote fracture toughness, mitigation of fracture and resistance to penetration deformation. Further simulations indicated that the non-uniform arrangement of the HAP crystals is a key factor responsible for the unique mechanical behavior of enamel, while the change in the nanostructure of nanocomposites could dictate the Young's modulus and yield strength of the biocomposite.

Demineralization of enamel in primary second molars related to properties of the enamel

Enamel structure is of importance in demineralization. Differences in porosity in enamel effect the rate of demineralization, seen between permanent and deciduous teeth. Individual differences have been shown in the mean mineral concentration values in enamel, the role of this in demineralization is not thoroughly investigated. The aim of this study was to study variations of depths of artificial lesions of demineralization and to analyze the depth in relation to variations in the chemical and mineral composition of the enamel. A demineralized lesion was created in second primary molars from 18 individuals. Depths of lesions were then related to individual chemical content of the enamel. Enamel responded to demineralization with different lesion depths and this was correlated to the chemical composition. The carbon content in sound enamel was shown to be higher where lesions developed deeper. The lesion was deeper when the degree of porosity of the enamel was higher.

In vitro evaluation of the effects of a fluoride-releasing composite on enamel demineralization around brackets

OBJECTIVES

The aim of this in vitro study was to evaluate the effectiveness of a fluoride-releasing bonding agent in inhibiting enamel demineralization around orthodontic brackets after the exposure to a demineralizing solution.

MATERIALS AND METHODS

Twenty-six extracted upper molars were bonded with two different composites: Transbond XT (TXT) and Transbond Plus (TPlus), fluoride-releasing (both 3M Unitek, Monrovia, CA, USA). The samples were exposed to an acid lactic solution for three days and then subjected to Metallographic Optical Microscope (MOM) and Scanning Electron Microscope/Energy Dispersive X-Ray (SEM/EDX) analyses. Enamel surface was examined in different areas: un-treated, etched and primer-painted, un-treated area with no acid exposure, central area with bracket bonded. The maximum demineralization depths and the fluoride content at 100, 200 and 300 μm depth were evaluated.

RESULTS

MOM analysis showed statistically significant (p<0.001) differences in demineralization depth for TPlus group compared to TXT group with lower values for the first one. EDX analysis confirmed the presence of fluoride in TPlus group.

CONCLUSIONS

The fluoride content of TPlus appeared able to weakly reduce the enamel demineralization.

Electron microscopy

Correlative light (LM) and transmission electron microscopic (TEM) analysis is useful, if ultrastructural details of cells need to be related to functional aspects which can only be examined at the LM level. The first protocol presented here introduces a relatively simple way of obtaining TEM images which, on the one hand, reveal ultrastructural details of individual cells and, on the other hand, are large enough to allow a correlation with light micrographs. The second protocol describes a technique for estimating mineral densities of hard tissues using backscattered electron images obtained with a scanning electron microscope. This technique can be used to analyze the mineralization processes which occur throughout tooth formation.

Amelogenesis imperfecta: Report of a case and review of literature

Amelogenesis imperfecta (AI) is a diverse collection of inherited diseases that exhibit quantitative or qualitative tooth enamel defects in the absence of systemic manifestations. Also known by varied names such as Hereditary enamel dysplasia, Hereditary brown enamel, Hereditary brown opalescent teeth, this defect is entirely ectodermal, since mesodermal components of the teeth are basically normal. The AI trait can be transmitted by either autosomal dominant, autosomal recessive, or X-linked modes of inheritance. Genes implicated in autosomal forms are genes encoding enamel matrix proteins, namely: enamelin and ameloblastin, tuftelin, MMP-20 and kallikrein - 4. This article presents a case reported to Dr. D. Y. Patil, Dental College and Hospital, Pune, India, along with a review of this often seen clinical entity.

Molar-incisor hypomineralisation: prevalence and defect characteristics in Iraqi children

BACKGROUND.  Little prevalence data relating to molar incisor hypomineralisation (MIH) exist for Middle East populations. AIM.  To evaluate the prevalence and the clinical features of MIH in school-aged children residing in Mosul City, Iraq. DESIGN.  A cluster sample of 823 7- to 9-year-old children had their first permanent molars and incisors (index teeth) evaluated using the European Academy of Paediatric Dentistry (EAPD) criteria for MIH. The examinations were conducted at schools by a calibrated examiner. RESULTS.  Of the children examined, 177 (21.5%) had hypomineralisation defects in at least one index tooth, 153 (18.6%) had at least one affected first molar or first molars and incisors and were considered as having MIH. The most commonly affected teeth were maxillary molars. Demarcated creamy white opacities were the most frequent lesion type. Dental restorations and tooth extraction because of MIH were uncommon. Children with three or more affected teeth were 3.7 times more likely to have enamel breakdown when compared with those children having only one or two affected teeth. CONCLUSIONS.  Molar incisor hypomineralisation was common amongst Iraqi children. Demarcated opacities were more prevalent than breakdown. The severity of the lesions increased with the number of affected teeth. The more severe the defect, the greater the involved tooth surface area.

Increase in severity of molar-incisor hypomineralization and its relationship with the colour of enamel opacity: a prospective cohort study

BACKGROUND

Predicting risk of posteruptive enamel breakdown (PEB) of molar-incisor hypomineralization (MIH) opacity is a difficult but important clinical task. Therefore, there is a need to evaluate these aspects through longitudinal studies.

OBJECTIVE

The aim of this longitudinal study was to analyse the relationship between colours of MIH opacity of children aged 6-12 (baseline) and other clinical and demographic variables involved in the increase in severity of MIH.

MATERIALS AND METHODS

A blinded prospective 18-month follow-up was conducted with 147 individuals presenting mild MIH. Tooth-based incidence of increase in severity of MIH (PEB or atypical restorations) was used as dependent measurement. Enamel opacities were recorded according to colour shades of white, yellow and brown, allowing assessment of susceptibility to structural loss over time, according to colour of MIH opacity. Poisson regression models were used to adjust the results for demographic and clinical variables.

RESULTS

Brown and yellow MIH opacities were at higher risk for PEB and atypical restorations than those of white ones, even after adjustment for clinical and demographic variables.

CONCLUSION

Teeth presenting mild MIH severity associated with yellow and brown enamel opacities were at high risk for increase in severity of MIH than lighter ones. This result could help clinicians determine a risk-based treatment for children with MIH.

Fluoride reduces the expression of enamel proteins and cytokines in an ameloblast-derived cell line

OBJECTIVE

To investigate the effects of two different fluoride concentrations on the expression of enamel proteins, alkaline phosphatase (ALP), cytokines and interleukins by an ameloblast-derived cell line.

METHODS

Murine ameloblast-derived cells (LS-8), mouse odontogenic epithelia, were exposed to 1 or 5ppm sodium fluoride (NaF) (0.46 and 2.25ppm F, respectively) for 1, 3 and 7 days. The effect of NaF on the mRNA expression of enamel proteins was quantified; the secretion of cytokines, and interleukins, and the alkaline phosphatase (ALP) activity, into the cell culture medium was measured and compared to untreated controls. The effect on cell growth after 1- and 3-days in culture was measured using BrdU incorporation.

RESULTS

Fluoride at 2.25ppm reduced mRNA expression of the structural enamel matrix proteins amelogenin (amel), ameloblastin (ambn), enamelin (enam), and the enamel protease matrix metallopeptidase-20 (MMP-20). Similarly several vascularisation factors (vascular endothelial growth factor (VEGF), monocyte chemoattractant proteins (MCP-1) and interferon inducible protein 10 (IP-10), was also reduced by 2.25ppm fluoride. ALP activity and proliferation were stimulated by 0.46ppm fluoride but inhibited by 2.25ppm fluoride.

CONCLUSIONS

These results indicate that fluoride may impact on the expression of structural enamel proteins and the protease responsible for processing these proteins during the secretory stage of amelogenesis and go some way to explaining the mineralization defect that characterises fluorotic enamel.

Molar incisor hypomineralization: prevalence, severity and clinical consequences in Brazilian children

BACKGROUND

The prevalence of molar incisor hypomineralization (MIH) varies considerably around the world; however, few studies have examined MIH in South American countries.

OBJECTIVE

To evaluate the prevalence, severity, and clinical consequences of MIH in Brazilian children residing in rural and urban areas of the municipality of Botelhos, Minas Gerais, Brazil.

METHODS

Children aged 6 to 12 years (n = 918) with all four-first permanent molars erupted had these teeth evaluated according to the European Academy of Paediatric Dentistry (EAPD) criteria. The examinations were conducted by two previously trained examiners, and the dental impact caused by MIH was evaluated with the Decayed, Missing and Filled Teeth (DMFT) index (WHO).

RESULTS

Molar incisor hypomineralization was present in 19.8% of the 918 children, with a higher prevalence in rural areas. The majority of the defects presented were demarcated opacities without post-eruptive structural loss, which has been considered as mild defects. Children with MIH had higher DMFT values.

CONCLUSION

Despite the high prevalence of MIH, the severity of the defects was mild. The results indicate a positive association between MIH and the presence of dental caries.

Perturbed amelogenin secondary structure leads to uncontrolled aggregation in amelogenesis imperfecta mutant proteins

Mutations in amelogenin sequence result in defective enamel, and the diverse group of genetically altered conditions is collectively known as amelogenesis imperfecta (AI). Despite numerous studies, the detailed molecular mechanism of defective enamel formation is still unknown. In this study, we have examined the biophysical properties of a recombinant murine amelogenin (rM180) and two point mutations identified from human DNA sequences in two cases of AI (T21I and P41T). At pH 5.8 and 25 °C, wild type (WT) rM180 and mutant P41T existed as monomers, and mutant T21I formed lower order oligomers. CD, dynamic light scattering, and fluorescence studies indicated that rM180 and P41T can be classified as a premolten globule-like subclass protein at 25 °C. Thermal denaturation and refolding monitored by CD ellipticity at 224 nm indicated the presence of a strong hysteresis in mutants compared with WT. Variable temperature tryptophan fluorescence and dynamic light scattering studies showed that WT transformed to a partially folded conformation upon heating and remained stable. The partially folded conformation formed by P41T, however, readily converted into a heterogeneous population of aggregates. T21I existed in an oligomeric state at room temperature and, upon heating, rapidly formed large aggregates over a very narrow temperature range. Thermal denaturation and refolding studies indicated that the mutants are less stable and exhibit poor refolding ability compared with WT rM180. Our results suggest that alterations in self-assembly of amelogenin are a consequence of destabilization of the intrinsic disorder. Therefore, we propose that, like a number of other human diseases, AI appears to be due to the destabilization of the secondary structure as a result of amelogenin mutations.

Surface Integrity Governs the Proteome of Hypomineralized Enamel

Growing interest in the treatment and prevention of Molar/Incisor Hypomineralization (MIH) warrants investigation into the protein composition of hypomineralized enamel. Hypothesizing abnormality akin to amelogenesis imperfecta, we profiled proteins in hypomineralized enamel from human permanent first molars using a biochemical approach. Hypomineralized enamel was found to have from 3- to 15-fold higher protein content than normal, but a near-normal level of residual amelogenins. This distinguished MIH from hypomaturation defects with high residual amelogenins (amelogenesis imperfecta, fluorosis) and so typified it as a hypocalcification defect. Second, hypomineralized enamel was found to have accumulated various proteins from oral fluid and blood, with differential incorporation depending on integrity of the enamel surface. Pathogenically, these results point to a pre-eruptive disturbance of mineralization involving albumin and, in cases with post-eruptive breakdown, subsequent protein adsorption on the exposed hydroxyapatite matrix. These insights into the pathogenesis and properties of hypomineralized enamel hold significance for prevention and treatment of MIH.

Chemical, mechanical and morphological properties of hypomineralized enamel of permanent first molars

OBJECTIVE

The microstructure of hypomineralized enamel in permanent teeth has been described in several studies as less distinct prism sheaths and disorganized enamel with lack of organization of the enamel crystals. The mechanical properties, hardness and modulus of elasticity of the hypomineralized enamel have lower values compared with normal. The aim of this study was to examine normal and hypomineralized enamel using scanning electron microscopy (SEM), hardness measurements and X-ray microanalysis (XRMA).

MATERIAL AND METHODS

Four extracted hypomineralized permanent first molars, sectioned and cut in half, were analyzed with SEM, XRMA and hardness measurements.

RESULTS

An inverse relation was found between the micro hardness and the Ca:C ratio in hypomineralized and normal enamel. The acid-etched hypomineralized enamel appeared on SEM to be covered with a structureless layer and the prisms appeared disorganized, with thick prism sheaths and loosely packed crystallites. Furthermore, bacteria were found deep in porous hypomineralized enamel close to the enamel-dentin junction.

CONCLUSIONS

Teeth diagnosed with molar incisor hypomineralization have significantly lower hardness values in the hypomineralized enamel compared with normal enamel. The hardness values vary according to the morphological and chemical properties.

Mineral density of hypomineralised enamel

OBJECTIVES

To characterize molar-incisor hypomineralisation (MIH) defects of different severities quantitatively and qualitatively using X-ray microtomography (XMT) and to measure the range of reduction in mineral density (MD) of MIH enamel compared with the normal range.

METHODS

Ten sound teeth and ten MIH teeth were scanned using a commercial XMT system. Four hydroxyapatite phantoms of different densities were used as calibration standards with each scan. A calibration equation derived from the phantoms with each tooth was used for MD calibration. MD was traced from the cementum-enamel junction (CEJ) to the cusp tip and from the dentine-enamel junction (DEJ) to the outer enamel surface.

RESULTS

In sound teeth, MD increased from CEJ to cusp/incisal tip, while in MIH teeth MD dropped from the CEJ to the occlusal region, then increased again at the cusp tip. MD was highest midway between DEJ and outer enamel in sound teeth. In MIH, enamel showed normal thickness and MD was highest near the DEJ and then decreased towards the outer enamel. MD of MIH enamel was on average about 19% lower than sound enamel. The MIH defects seemed to follow the incremental lines of enamel formation.

CONCLUSIONS

MIH defects are hypomineralised defects of different severities that follow the natural incremental lines of enamel formation. Cuspal areas are usually only mildly affected and cervical enamel always appears to be sound.

Molar-incisor-hypomineralisation: a literature review

BACKGROUND

Molar-Incisor-Hypomineralisation (MIH) is a qualitative defect of 1-4 first permanent molars with or without the maxillary and mandibular permanent incisors. It seems to have been recognised first in the 1970s and prevalence varies between 2.8% and 25%, dependent upon the study.

METHODS

The dental literature was searched using a number of key terms entered into MEDLINE, the reference list of each paper as located was examined for further papers that had been missed in the initial search.

RESULTS

The review of the literature showed that teeth that are affected indicate a systemic cause at around the time of birth; investigators have put forward a number of possible causes; asthma, pneumonia, upper respiratory tract infections, otitis media, antibiotics, dioxins in mother's milk, tonsillitis and tonsillectomy and exanthamatous fevers of childhood. However, at the present time the aetiology remains unclear. Treatment of the affected permanent first molars can include restorations using adhesive intra-coronal restorations to extra-coronal restorations (e.g. preformed metal crowns). There is little evidence to support one option over another. In severe cases extraction at the optimum time may be the best option; allowing the permanent second molars to come forwards. There is little improvement in affected anterior teeth with microabrasion and direct or indirect composite resin restorations may be appropriate in some children. Ultrastructural and biochemical make up of MIH affected teeth seem to have been investigated less than other areas.

CONCLUSION

It is important that children with MIH are diagnosed as early as possible and managed appropriately; this will involve multidisciplinary input.

Effect of microstructure upon elastic behaviour of human tooth enamel

Tooth enamel is the stiffest tissue in the human body with a well-organized microstructure. Developmental diseases, such as enamel hypomineralisation, have been reported to cause marked reduction in the elastic modulus of enamel and consequently impair dental function. We produce evidence, using site-specific transmission electron microscopy (TEM), of difference in microstructure between sound and hypomineralised enamel. Built upon that, we develop a mechanical model to explore the relationship of the elastic modulus of the mineral-protein composite structure of enamel with the thickness of protein layers and the direction of mechanical loading. We conclude that when subject to complex mechanical loading conditions, sound enamel exhibits consistently high stiffness, which is essential for dental function. A marked decrease in stiffness of hypomineralised enamel is caused primarily by an increase in the thickness of protein layers between apatite crystals and to a lesser extent by an increase in the effective crystal orientation angle.

Structure and mechanical properties of Ank/Ank mutant mouse dental tissues--an animal model for studying periodontal regeneration

Enamel, dentine and cementum are dental tissues with distinct functional properties associated with their unique hierarchical structures. Some potential ways to repair or regenerate lost tooth structures have been revealed in our studies focused on examining teeth obtained from mice with mutations at the mouse progressive ankylosis (ank) locus. Previous studies have shown that mice with such mutations have decreased levels of extracellular inorganic pyrophosphate (PP(i)) at local sites resulting in ectopic calcification in joint areas and in formation of a significantly thicker cementum layer when compared with age-matched wild-type (WT) tissue [Ho AM, Johnson MD, Kingsley DM. Role of the mouse ank gene in control of tissue calcification and arthritis. Science 2000;289:265-70; Nociti Jr FH, Berry JE, Foster BL, Gurley KA, Kingsley DM, Takata T, et al. Cementum: a phosphate-sensitive tissue. J Dent Res 2002;81:817-21]. As a next step, to determine the quality of the cementum tissue formed in mice with a mutation in the ank gene (ank/ank), we compared the microstructure and mechanical properties of cementum and other dental tissues in mature ank/ank vs. age-matched WT mice. Backscattered scanning electron microscopy (SEM) imaging and transmission electron microscopy (TEM) analyses on mineralized tissues revealed no decrease in the extent of mineralization between ank/ank cementum vs. WT controls. Atomic-force-microscopy-based nanoindentation performed on enamel, dentine or cementum of ank/ank vs. age-matched WT molars revealed no significant difference in any of the tested tissues in terms of hardness and elastic modulus. These results indicate that the tissue quality was not compromised in ank/ank mice despite faster rate of formation and more abundant cementum when compared with age-matched WT mice. In conclusion, these data suggest that this animal model can be utilized for studies focused on defining mechanisms to promote cementum formation without loss of mechanical integrity.

Transmission electron microscope characterisation of molar-incisor-hypomineralisation

Molar-incisor-hypomineralisation (MIH), one of the major developmental defects in dental enamel, is presenting challenge to clinicians due, in part, to the limited understanding of microstructural changes in affected teeth. Difficulties in the preparation of site-specific transmission electron microscope (TEM) specimens are partly responsible for this deficit. In this study, a dual-beam field emission scanning electron microscope (FESEM)/focused ion beam (FIB) milling instrument was used to prepare electron transparent specimens of sound and hypomineralised enamel. Microstructural analysis revealed that the hypomineralised areas in enamel were associated with marked changes in microstructure; loosely packed apatite crystals within prisms and wider sheath regions were identified. Microstructural changes appear to occur during enamel maturation and may be responsible for the dramatic reduction in mechanical properties of the affected regions. An enhanced knowledge of the degradation of structural integrity in hypomineralised enamel could shed light on more appropriate management strategies for these developmental defects.

Mechanical properties of human enamel as a function of age and location in the tooth

Aging and the related changes in mechanical behavior of hard tissues of the human body are becoming increasingly important. In this study the influence of aging on the mechanical behavior of human enamel was evaluated using 3rd molars from young (18 < or = age < or = 30 years) and old (55 < or = age) patients. The hardness and elastic modulus were quantified using nanoindentation as a function of distance from the Dentin-Enamel Junction (DEJ) and within three different regions of the crown (i.e. cervical, cuspal and inter-cuspal enamel). Results of the evaluation showed that the elastic modulus and hardness increased with distance from the DEJ in all three regions examined, regardless of patient age. The largest increases with distance from the DEJ occurred within the cervical region of the old enamel. Overall, the results showed that there were no age-dependent differences in properties of enamel near the DEJ. However, near the tooth's surface, both the hardness (p < 0.025) and elastic modulus (p < 0.0001) were significantly greater in the old enamel. At the surface of the tooth the average elastic modulus of "old" enamel was nearly 20% greater than that of enamel from the young patients.

Head posture and dental wear evaluation of bruxist children with primary teeth

The aim of the present study was to compare the head position and dental wear of bruxist and non-bruxist children with primary dentition.

METHODS

All the subjects had complete primary dentition, dental and skeletal class I occlusion and were classified as bruxist or non-bruxist according to their anxiety level, bruxism described by their parents and signs of temporomandibular disorders. The dental wear was drawn in dental casts and processed in digital format. Physiotherapeutic evaluation and a cephalometric radiograph with natural head position were also performed for each child to evaluate the cranio-cervical position for the bruxist group (n = 33) and the control group (n = 20). The variables of the two groups were compared, using the Student t-test and Mann-Whitney U-test.

RESULTS

A more anterior and downward head tilt was found in the bruxist group, with statistically significant differences compared with the controls. More significant dental wear was observed in the bruxist children.

CONCLUSIONS

Bruxism seems to be related to altered natural head posture and more intense dental wear. Further studies are necessary to explore bruxism mechanisms.

On the structure-property relationship of sound and hypomineralized enamel

Developmental defects in dental enamel pose significant clinical challenges which have highlighted our limited understanding of the structure and properties of this tissue. In this study, we first investigated the contact-size dependence of the physical properties of sound and hypomineralized enamel, and then examined the microstructure to establish a structural basis for their differing properties. Depth-sensing indentation tests were carried out over a wide range of peak loads in a direction perpendicular to the enamel prisms. Hypomineralized enamel demonstrated stronger penetration dependence for measured hardness and elastic modulus than sound enamel. The microstructure of sound and hypomineralized enamel was observed using field emission scanning electron microscopy and transmission electron microscopy with support of a focused ion beam milling system. Images of sound enamel showed barely distinguishable sheath regions with minimal organic presence. In contrast, hypomineralized enamel showed thicker sheath structures surrounding the prisms and higher levels of organic content within both the prisms and the sheath regions. It is argued that the higher organic content within prism structure was responsible for an initial lower hardness and elastic modulus of hypomineralized enamel under low-load indentation. As the indentation depth increased, the thicker organic-rich sheath regions played a more important role in reducing the mechanical properties of the hypomineralized enamel. On the basis of Spears finite element model [Spears IR. A three-dimensional finite element model of prismatic enamel: a re-appraisal of the data on the Young's modulus of enamel. J Dental Res 1997; 76(10):1690-97], elastic moduli of sound and hypomineralized enamel were predicted, which matched experimental results.

Combined FT-Raman and SEM Studies of the Effects of Er:YAG Laser Irradiation on Dentin

OBJECTIVE

This study evaluated the molecular and morphological changes on dentin elements after Er:YAG laser irradiation.

BACKGROUND DATA

Spectroscopy studies reporting the effects of Er:YAG laser irradiation as an alternative to acid etching are needed to better understand the laser's effects.

METHODS

The occlusal one-third of the crown of six human third molars was removed. The dentin surface was schematically divided into areas corresponding to four surface treatment groups: control (group C): 37% phosphoric acid etching; group I: Er:YAG laser 80 mJ; group II: Er:YAG laser 120 mJ; and group III: Er:YAG laser 180 mJ. The analysis was performed by scanning electron microscopy (SEM) and Fourier transform Raman spectroscopy (FT-Raman) before and after the treatments. Raman data were submitted to ANOVA and Bonferroni tests.

RESULTS

The SEM photomicrographs revealed open dentin tubules in the control group. The molars from groups I, II, and III showed partially open dentin tubules. SEM images showed that the laser-irradiated dentin surface was not favorable to the diffusion of monomers. A significant reduction of the spectra relative intensity was observed in group III specimens.

CONCLUSIONS

Er:YAG laser irradiation with 180 mJ could produce chemical changes in proteins, phosphate, and carbonate in dentin.

Nanoindentation derived stress–strain properties of dental materials

OBJECTIVES

The aim of the study is to investigate the stress-strain response of different dental materials, especially dental brittle materials, and compare them with enamel.

METHODS

A nano-based indentation system (Ultra Micro-Indentation System, UMIS-2000, CSIRO, Australia) was used to determine the indentation stress-strain response of two kinds of dental ceramics (Cerec 2 Mark II and Vita VM9), one kind of dental alloy (Wiron 99) and healthy enamel. A spherical indenter was used to test the materials with nanometer and micro-Newton displacement and force resolution. Assuming the elastic modulus remained constant, a plot of contact pressure versus contact strain, H-a/R, of each material was obtained.

RESULTS

By comparing the H-a/R curve of the different materials with enamel, it can be concluded that only the metallic alloy, has similar stress-strain response as enamel. Dental ceramics showed much higher yield stress response than enamel. VM9, a porcelain veneer component of crown/bridge structure, is slightly softer than its core, Mark II. The yield point for Mark II and VM9 are nearly 10 and 7GPa, respectively, and approximately 2GPa for Wiron alloy and enamel.

SIGNIFICANCE

H-a/R curves provide a new method to compare the mechanical properties of different dental materials. From the standpoint of structural reliability, strong and tough materials with primarily elastic response, such as toughened ceramics are required to enable dental crown/bridges to have long term reliability. On the other hand, materials with too high hardness or yield response may damage opposing teeth during occlusal contact. Future studies may establish a relationship between stress and strain property and abrasive wear of dental material.

Effects of Treatment for Manipulation of Teeth and Er:YAG Laser Irradiation on Dentin: A Raman Spectroscopy Analysis

OBJECTIVE

The main purpose of this study was to evaluate the utility of Raman spectroscopy analysis as a research tool to study the effects of Er:YAG laser etching on dentin mineral and organic components. A secondary aim was to study the effects of the decontamination process and the storage procedure on dentin components.

BACKGROUND DATA

There are no spectroscopy reports relating the effects of Er:YAG laser irradiation as an alternative to acid etching and the manipulation process on the dentin structure.

METHODS

Twelve non-carious human third molars were divided in two main groups: stored in thymol solution (group A, n = 6) or autoclaved (group B, n = 6). The specimens were either etched with 37% phosphoric acid (control subgroup) or irradiated with Er:YAG laser. Irradiated samples were divided into the following subgroups: I, II, and III (80 mJ, 3 Hz, 30 sec; 120 mJ, 3 Hz, 30 sec; and 180 mJ, 3 Hz, 30 sec, respectively). Samples were analyzed by Raman spectroscopy.

RESULTS

The mineral and organic dentin contents were more affected in autoclaved teeth than in the specimens stored in thymol. Peak area reduction in group A specimens treated with phosphoric acid and pulse energy of 80 mJ were the most conservative surface treatments regarding changes in the peak area of organic and inorganic dentin components.

CONCLUSION

The autoclaving process and pulse energies of 120 and 180 mJ produced greater reduction of organic and inorganic contents in dentin, associated with greater reduction in the areas of 968, 1077, 1460, and 1670 cm(1) Raman peaks.

Genes and related proteins involved in amelogenesis imperfecta

Dental enamel formation is a remarkable example of a biomineralization process. The exact mechanisms involved in this process remain partly obscure. Some of the genes encoding specific enamel proteins have been indicated as candidate genes for amelogenesis imperfecta. Mutational analyses within studied families have supported this hypothesis. Mutations in the amelogenin gene (AMELX) cause X-linked amelogenesis imperfecta, while mutations in the enamelin gene (ENAM) cause autosomal-inherited forms of amelogenesis imperfecta. Recent reports involve kallikrein-4 (KLK4), MMP-20, and DLX3 genes in the etiologies of some cases. This paper focuses mainly on the candidate genes involved in amelogenesis imperfecta and the proteins derived from them, and reviews current knowledge on their structure, localization within the tissue, and correlation with the various types of this disorder.

Decreased mineral content in MMP-20 null mouse enamel is prominent during the maturation stage

During enamel development, matrix metalloproteinase-20 (MMP-20, enamelysin) is expressed early during the secretory stage as the enamel thickens, and kallikrein-4 (KLK-4, EMSP1) is expressed later during the maturation stage as the enamel hardens. Thus, we investigated whether the physical properties of the secretory-/maturation-stage MMP-20 null enamel were significantly different from those of controls. We demonstrated that although, in relative terms, the weight percent of mature mineral in the MMP-20 null mouse enamel was only 7-16% less than that in controls, overall the enamel mineral was reduced by about 50%, and its hardness was decreased by 37%. Percent mineral content by weight was assessed at 3 different developmental stages. Remarkably, the biggest difference in mineral content between MMP-20 null and controls occurred in the nearly mature enamel, when MMP-20 is normally no longer expressed. This suggests that MMP-20 acts either directly or indirectly to facilitate the removal of maturation-stage enamel proteins.