P. Anderson

3D X-ray microscopic study of the extent of variations in enamel density in first permanent molars with idiopathic enamel hypomineralisation.

Objective To measure mineral concentration distributions within teeth with idiopathic enamel hypomineralisation, a condition in which developmental defects are seen in first permanent molars, and/or incisors.Design X-ray microtomographic and 3D x-ray microscopy.Setting UK University, 2001.Materials and methods X-ray microtomographic measurements of the extent of hypomineralisation in two affected molars and two contralateral controls extracted from the same patient.Results The control molars were visibly normal. The affected molars showed hypomineralised yellow opaque enamel with regions of breakdown. X-ray microtomographic images showed; a 20% reduction in mineral concentration of affected enamel (most cases involved full enamel thickness); hypomineralised enamel had a mineral concentration gradient opposite to that of normal enamel; regions of hypomineralisation distributed randomly throughout affected teeth, (apart from cervical region which was less severely affected).Conclusions The pattern of mineral concentration suggests a disturbance during the maturation process. Differences in susceptibility of the ameloblasts during different stages of dental development may explain the asymmetric distribution of the defects. Topical fluoride applications may help promote post eruption maturation of the surface layer in these teeth. The use of fissure sealants and adhesive materials appears to prevent further breakdown.

Preparation and characterisation of monoclinic hydroxyapatite and its precipitated carbonate apatite intermediate

Five 100 g batches of a carbonate apatite (the intermediate) were produced by heating an aqueous slurry of CaCO3 and CaHPO4 with an overall Ca/P mole ratio of 5/3 with vigorous stirring. Each intermediate produced by boiling off water was heated in vacuum at 1100 degrees C to remove carbonate, then steamed at 900 degrees C to ensure complete hydroxylation. Comparison of calculated and observed X-ray diffraction patterns showed final products containing 50-100 wt% monoclinic hydroxyapatite (remainder hexagonal). Rietveld refinements in P6(3)/m gave structures similar to several hydroxyapatite standards, including NIST SRM 2910, although there was no evidence from X-ray diffraction that the latter was in the monoclinic form. Refinements from standards and final products were slightly different from published single crystal data for Holly Springs hydroxyapatite. This is attributed to known impurities in mineral hydroxyapatite and indicates that parameters from the Rietveld refinements are closer to the true values for pure hydroxyapatite. Rietveld refinements for intermediates showed small, but significant differences from the final product, the largest being in O1x, O2x and O(H)z. All P-O bond lengths were shorter than in the final product, resulting in a 3.2% lower PO4 tetrahedron volume. The occupancies of P and Ca(2) were reduced. These differences are attributed to partial replacement of PO4(3) by CO3(2-) ions.

Effects of sodium hypochlorite solution on root dentine composition

Sodium hypochlorite (NaOCl) solution,≤5% w/v available chlorine (abbreviated subsequently to %), is widely used as an irrigant in root canal treatment of teeth, so its effects on dentine are of clinical importance. The effects of ∼0.5%, 3% and 5% NaOCl solution on the composition of root dentine were studied at ambient temperature. For dentine powder treated for 30 min, depletion of the organic phase was confirmed by infrared spectroscopy. Apatite lattice parameters showed no significant change, but NaCl was also detected by X-ray powder diffraction. The low solubility of apatite mineral in the NaOCl solutions was demonstrated by the constant weight of bulk enamel specimens immersed for seven days. The stability of the mineral phase was confirmed by scanning microradiography (SMR), an X-ray attenuation method employing photon counting. Repeated SMR measurements of the local mineral content of bulk samples of root dentine and a synthetic hydroxyapatite aggregate during exposure to pumped NaOCl solutions for 100 h showed no mineral loss. As predicted from apatite chemistry, reaction of NaOCl with the mineral phase can be excluded as a primary factor in changes in mechanical properties of treated dentine. Effects of retention of NaCl on endodontic sealants requires further investigation.

Rates of Mineral Loss in Human Enamel during in vitro Demineralization Perpendicular and Parallel to the Natural Surface

Human enamel is a structurally anisotropic material. The aim of this study was to investigate whether this structural anisotropy is reflected in the demineralization behaviour of enamel. Kinetics of demineralization of in vitro caries lesions with the direction of acid attack perpendicular to the natural surface of dental enamel from human premolar teeth were compared with kinetics when demineralized parallel to this surface. Pairs of enamel samples from the same tooth were demineralized under identical conditions. Loss of mineral with time was very nearly linear for both directions (consistent with the rate–controlling step being reaction at the advancing front rather than transport processes), but the perpendicular rate was, on average, about 14% higher than the parallel rate. The rate of demineralization parallel to the surface increased from the natural surface to the enamel–dentine junction by 10–25%, depending on sample. The origin of fine structure and slight departures from linearity in the loss of mineral with time plots are discussed. Mineral masses per unit area were determined from absorption of a 15–μm diameter X–ray beam using photon (AgKα) counting methods.

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.

Scanning microradiographic studies of rates of in vitro demineralization in human and bovine dental enamel

The aim was to measure frequently and with precision the local integrated mineral loss through small areas of the natural surface of human and bovine enamel during in vitro demineralization using an X-ray photon-counting system (scanning microradiography). The method used was an adaptation of photographic longitudinal microradiography in which the attenuation of X-rays through the enamel is measured in the direction of acid attack, i.e., normal to the enamel surface. The mass of mineral (assumed to be hydroxyapatite) per unit exposed area was measured over 15 microm dia. circles at a series of positions as a function of time in blocks of human and bovine enamel immersed in 0.1 mol/l acetic acid buffered to pH 4.0 with NaOH. There was an initial period (approx. 45 h for human, approx. 75 h for bovine enamel) during which the mineral loss with time was sigmoidal, followed by a nearly linear loss for the remainder of the experiment, in some cases up to 500 h. The initial sigmoidal period may be due to properties of surface enamel or be associated with the development of a surface layer overlying subsurface demineralization. The essentially constant rate of mineral loss after the surface layer has formed confirms earlier observations and is consistent with a rate-limiting process occurring at the dissolving enamel surfaces of the advancing front, and not by transport of ions within the lesion. Small perturbations from a linear loss were seen, which were approximately periodic for human enamel. The slope of the linear period was rather constant within one human or bovine block, but variable between blocks without a clear distinction between human and bovine enamel.

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.

Damage in aligned-fibre SiC/Al quantified using a laboratory X-ray tomographic microscope

Abstract A first-generation laboratory X-ray tomographic microscope is used to non-destructively ‘section’ a continuous, aligned-fibre SiC/Al metal-matrix composite (MMC). Damage in the MMC associated with mechanical deformation is the principal focus of the study. Two types of deformation are examined: wedge loading (with and without load) and three-point bending. Quantification of crack opening and fibre fracture detection is found to be practical down to one-tenth of a pixel in the reconstructed sections

An X-ray Microtomographic Study of Natural White-spot Enamel Lesions

White-spot enamel lesions are an early presentation of dental caries and are ideally managed by non-invasive procedures. The aim of this study was to characterize white-spot enamel lesions by x-ray microtomography. In particular, mineral content across the lesion from the surface to the base of the lesion was measured and surface layers defined. Molars with long buccal white-spot enamel lesions were collected, photographed, and each sectioned to produce 3 500-µm-thick sections. The sections were mounted and imaged by quantitative x-ray microtomography at a 15-µm voxel size. We analyzed line profiles through the middle of each 3D image to determine mineral content and depth. The surface layer thickness of the lesions ranged from 35 to 130 µm, with the maximum mineral content in this layer being 74% to 100% of that of sound enamel. The average mineral content across the lesions ranged from 1.73 to 2.48 g/cm3. No significant differences could be found between lesions clinically categorized as active and those categorized as inactive. However, for depth-matched active and inactive lesions, the active lesions exhibited a more porous surface layer than the inactive lesions. White-spot enamel lesions are highly variable, with surface layers of considerable thickness.

Sequential 3D X-ray microtomographic measurement of enamel and dentine ablation by an Er:YAG laser

Objective To demonstrate the progression of crater growth during repeated sequential application of an Er:YAG laser to enamel and dentine, monitored using X-ray microtomography (XMT).Design A single centre study in which laser craters were created in blocks cut from human enamel and/or dentine under standardised and known conditions and then studied using XMT to obtain visualisation and quantification of the effects.Setting University setting, UK, 2001.Main outcome measures Success was judged by an ability to obtain useful 3D XMT reconstructions of the blocks during crater development, and to make measurements from these data. These measurements were compared with data obtained from similar studies using different measurement techniques.Results Time sequences of 2D and 3D images were obtained which demonstrated the progression of laser craters in enamel and dentine. Quantitative measurements from these data enabled values to be derived for the rate of progression of crater depth per unit energy, and the volume of hard tissue removed per unit energy. These values were compared with data derived from other studies and shown to be broadly comparable. However, the present study is unique in that these values were obtained from a series of measurements of the same craters over time.Conclusions 3D X-ray microtomography is shown to be a useful tool for quantitative measurements in dental research. For the Er:YAG laser, the relationship of laser crater depth and volume of mineral removed to applied energy was found to be linear.