F.S.L. Wong

Best Clinical Practice Guidance for clinicians dealing with children presenting with Molar-Incisor-Hypomineralisation (MIH)

BACKGROUND: The European Academy of Paediatric Dentistry (EAPD) has long recognised the necessity of promoting further research and knowledge regarding the dental defect described as molar-incisor-hypomineralisation (MIH). Following the establishment by EAPD of the defect diagnostic criteria in 2003, the publication of various papers and a whole issue assigned to the defect in the European Archives of Paediatric Dentistry (2008), an Interim Seminar and Workshop on MIH was organized in Helsinki in 2009. RESULT: The outcome of this event is the present consensus paper on the prevalence, diagnosis, aetiology and treatment for children and adolescents presenting with MIH. A clear diagnostic proposal and a treatment decision-making guide are presented together with suggestions on aetiology and guidance forfuture research. CONCLUSION: MIH is an important clinical problem that often concerns both the general dental and specialist paediatric dentists; the present ‘best clinical practice guidance’ aims to further help clinicians dealing with the condition.

Preparation and characterization of a novel bioactive restorative composite based on covalently coupled polyurethane-nanohydroxyapatite fibres

Nanohydroxyapatite (n-HAp) was prepared using a sol-gel method. n-HAp powder was obtained from the gel form by heat treatment followed by grinding using ball milling. A novel polyurethane composite material was prepared by chemically binding the hydroxyapatite to the diisocyanate component in the polyurethane backbone through solvent polymerization. The procedure involved the stepwise addition of monomeric units of the polyurethane and optimizing the reagent concentrations. The resultant composite material was electrospun to form fibre mats. The fibres were less than 1mum in thickness and contained no beads or irregularities. Chemical structural characterization of both the ceramics and the novel polymers were carried out by Fourier transform infrared and Raman spectroscopy. X-ray diffraction, scanning electron microscopy (SEM), transmission electron microscopy and Brunauer-Emmett-Teller surface area analysis were also employed to observe the crystal lattice and size and surface area of the n-HAp. Further characterization (by energy-dispersive X-ray analysis and SEM) of the spun fibres revealed the presence of elements associated with hydroxyapatite and polyurethane without the presence of any loose particles of hydroxyapatite, indicating the formation of the covalent bond between the ceramics and the polymer backbone.

Detection of a Novel Mutation in X-linked Amelogenesis Imperfecta

Amelogenesis imperfecta (AI) is a heterogeneous group of inherited disorders of defective enamel formation. The major protein involved in enamel formation, amelogenin, is encoded by a gene located at Xp22.1-Xp22.3. This study investigated the molecular defect producing a combined phenotype of hypoplasia and hypomineralization in a family with the clinical features and inheritance pattern of X-linked amelogenesis imperfecta (XAI). Genomic DNA was prepared from buccal cells sampled from family members. The DNA was subjected to the polymerase chain-reaction (PCR) in the presence of a series of oligonucleotide primers designed to amplify all 7 exons of the amelogenin gene. Cloning and sequencing of the purified amplification products identified a cytosine deletion in exon VI at codon 119. The deletion resulted in a frameshift mutation, introducing a premature stop signal at codon 126, producing a truncated protein lacking the terminal 18 amino acids. Identifying mutations assists our understanding of the important functional domains within the gene, and finding another novel mutation emphasizes the need for family-specific diagnosis of amelogenesis imperfecta.

DETERMINATION OF MINERAL CONCENTRATION IN DENTAL ENAMEL FROM X-RAY ATTENUATION MEASUREMENTS

The mineral content of dental enamel is commonly measured by X-ray attenuation experiments. Most studies have used contact microradiography in which intensities are measured with photographic film which is convenient and gives high spatial resolution. However photon counting intensity measurements are to be preferred in many experiments (longitudinal and scanning microradiography, and microtomography), as illustrated here, because they have a larger dynamic range and greater sensitivity to small intensity changes. Additionally, the detector and specimen are well separated which allows the pseudo-continuous study of de- and remineralization. The mineral content is often quoted as 95 wt% or 87 vol% hydroxyapatite for permanent human enamel. This determination from attenuation experiments requires accurate values of elemental mass attenuation coefficients and a number of assumptions. The effects of possible choices of these are considered and it is shown that the most important is the density of enamel mineral used in conversion of wt% to vol%. If the density is taken as 2.99 g cm(-3), as recently suggested (J.C. Elliott, Dental Enamel, Ciba Foundation Symposium 205, Wiley, Chichester, pp. 54-72, 1997), instead of 3.15 g cm(-3) as for hydroxyapatite, the calculated vol% is approximately 93 instead of approximately 87.

Application of scanning microradiography and x-ray microtomography to studies of bones and teeth.

In scanning microradiography (SMR), a thin section is stepped across a 15-μm diameter X-ray beam and the transmitted intensity measured at each point. This technique has permitted more accurate measurements of the spatial variation of the mineral concentration in sections of dentin and enamel than conventional photographic microradiography. Moreover, because the section is not in close contact with an emulsion, SMR allows continuous study while the specimen is bathed in a reaction solution. The present studies have been particularly directed to gaining an understanding of the formation and repair of carious lesions in teeth: one particular puzzle is subsurface demineralization, in which the initial loss of mineral appears to take place some 20 to 50 μm below the tooth surface. SMR studies are reported here on the demineralization in dilute acids and the subsequent partial remineralization in supersaturated calcium phosphate solutions in model systems for dental caries. In order to develop a theoretical model for de- and remineralization of carious lesions, it is necessary to quantify transport processes within the tooth. To this end, we are developing a method of measuring effective diffusion coefficients of strongly X-ray-absorbing ions in water within permeable solids in which the diffusion coefficient varies with position. The method uses sequential concentration/distance profiles determined by SMR. As a test, diffusion coefficients of potassium iodide in water within a permeable glass frit have been measured. X-ray microtomography (XMT) can be carried out by adding an axis of rotation to the SMR apparatus. Using this method, linear absorption coefficients, and hence mineral concentrations, can be measured in 15 X 15 X 15-μm3 voxels. This has advantages over SMR in that superposition within the depth of the section and errors in determining its thickness are avoided. XMT studies of de- and remineralization similar to those described above for SMR, and also XMT studies of the variation in mineral concentration in the cortical bone of a rat femur along its length, are reported.

Deciduous enamel defects in low-birth-weight children: correlated X-ray microtomographic and backscattered electron imaging study of hypoplasia and hypomineralization

Enamel does not remodel, and disturbances occurring during development may remain in the tooth as a permanent record of the upset. Mineralization in prenatal and postnatal deciduous enamel was studied in the shed deciduous incisors of low-birth-weight (LBW: < 2kg) children. The specific objective was to gain further insight into the mechanism of formation of developmental defects of enamel. Sections at a resolution of 22–40 μm were reconstructed using X-ray microtomography (μCT) giving absolute measurements of linear absorption coefficient for AgKα radiation. Detail to ca. 1 μm resolution was obtained using automated, digital backscattered electron (BSE) imaging of PMMA-embedded material. Matching the histograms of BSE and μCT images made possible the calibration of the mean atomic number-dependent signal in the BSE images. The comparison of abnormal, affected enamel regions and post-recovery, normal, unaffected regions could be made in the same teeth, since these zones were easily recognized from the distribution of hypoplasia and hypomineralization. The μCT values, converted to calculated mineral densities, ranged from 2.3 g cm-3 to 2.6 g cm-3 in LBW hypoplastic, and between 2.65 and 2.78 g cm-3 in control primary enamel and post-defect, post-natal LBW enamel. Hypoplasia with or without minimal hypomineralization indicated recovery of the ameloblasts in the maturation phase. Disturbance during late matrix formation and early maturation resulted in hypoplasia and hypomineralization.

Effectiveness of microabrasion technique for improvement of dental aesthetics

Objectives To investigate which types of enamel opacity are effectively treated by the microabrasion technique and whether this technique could be used as a diagnostic aid to determine the aetiology of these defects.Materials and method Thirty two patients who had enamel opacities affecting both upper central incisors were selected and the disfigurements were classified into four types: single line, multi-line, patched and diffused. The patients previous medical history, possible history of fluoride ingestion, presence of taurodontism and family history of similar enamel defects were recorded. Both incisors were treated with Prèma abrasive paste mixed with 18% hydrochloric acid. The aesthetic improvements were assessed by the patients and their parents and their satisfaction level after the treatment was recorded.Results Approximately two-thirds (65.6%) of the patients were satisfied with their appearance after microabrasion. Apart from four patients, the improved appearance was stable and acceptable to the remaining patients at the six month recall. Statistical analysis showed that acceptable improvement was found in patients with single line/patched types of defects but not in multi-line/diffused types (P=0.03). However, the aesthetic improvement was not related to the patients fluoride history, presence of taurodontism or the family history of enamel defects.Conclusion Microabrasion using Prèma abrasives with 18% HCl is effective in improving the appearance of enamel with single-line or patched opacities, indicating that these defects are a surface phenomenon. For the multi-line and diffused types, the defects appear to extend deeper into the enamel. The technique failed to assist in determining the aetiology of these defects.

An X-Ray Microtomography Study on the Mineral Concentration of Carious Dentine Removed during Cavity Preparation in Deciduous Molars

Dentists use a number of criteria in order to assess when a cavity is caries free, amongst which hardness is probably the most widely used. However, the judgement is subjective. X-ray microtomography (XMT) is a non-destructive microscopic technique that allows in vitro specimens to be scanned, manipulated and then rescanned. In this study, a high-definition XMT scanner was used to determine the mineral distribution of carious dentine in 10 deciduous molars, and the extent of dentine removed by an experienced clinician was investigated. For each tooth, after an initial XMT scan, caries was removed using a steel bur in a slow hand-piece. The tooth was then repositioned and rescanned. Mineral concentrations were calculated from the linear attenuation coefficients assuming the mineral phase to be hydroxyapatite and the organic phase to be collagen. The volume of dentine tissues removed was calculated by subtracting data of the second scan from the first. The results showed that the mean modal mineral concentration for the 10 teeth was 1.42 g · cm–3 for sound dentine. Because of uncertainty about collagen concentration in carious dentine, the mean modal mineral concentration for the carious dentine had a range of 0.37–0.5 g · cm–3. It was found that the subjective criteria used by the operator could lead to inconsistency of cavity preparation. The cavities could be overprepared by 8.5–44.3% in volume. However, the overpreparation was not uniform throughout the cavity: residual demineralised dentine could still be detected in the postoperative scan in isolated regions.

X‐ray microtomographic study of mineral distribution in enamel of mandibular rat incisors

X‐ray microtomography was used to study the mineral concentrations in sequential slices of enamel of 5 mandibular incisors which showed an increase from ∼ 1.0 to ∼ 2.7 g cm−3 from the apex towards the incisal end. For points at the same distance from the apex, there were differences up to 0.6 g cm−3 between the teeth. The change of mean concentrations in the slices with distance could be modelled as (different) saturating exponentials. Under the assumption of a uniform growth rate of a mandibular incisor of 0.6 mm per day and a common time origin for the start of maturation (taken as a mineral concentration of 1 g cm−3), the distances were transformed to a common time frame to give a pooled data set. A single saturating exponential could be fitted to this pooled transformed data; this was: Cm = 2.84−1.94exp (−0.18d) where Cm is the mean mineral concentration (g cm−3) and d the time (days) from the start of maturation. This gives an asymptotic concentration of 2.84 g cm−3 towards the incisal end, with a time constant of 7.7 days. The mineral concentration distribution functions were found to be more positively skewed closer to the apex, but more negatively skewed towards the incisal end. The difference between the higher mineral concentration in the outer enamel and the enamel near the amelodentinal junction (ADJ) was ∼ 3%. The direction of maximum increase in concentration from the outer enamel surface to the ADJ meets the boundary of the ADJ at ∼ 80°. Three dimensional surface rendering of isodensity contours showed that the previously described C‐shaped pattern of mineralisation is not solely a surface phenomenon, but extends through the depth of the enamel.

Computed tomography part II: The practical use of a single source and detector

Although a single-source, single-detector system has limitations, it can prove extremely useful for computed tomography studies. Such simple systems also have the advantages of relatively low set-up costs excellent results for many different types of measurements.