Composite 3D printing of biomimetic human teeth.

Human teeth are mechanically robust through a complex structural composite organisation of materials and morphology. Efforts to replicate mechanical function in artificial teeth (typodont teeth), such as in dental training applications, attempt to replicate the structure and morphology of real teeth but lack tactile similarities during mechanical cutting of the teeth. In this study, biomimetic typodont teeth, with morphology derived from X-ray microtomography scans of extracted teeth, were 3D printed using an approach to develop novel composites. These composites with a range of glass, hydroxyapatite and porcelain reinforcements within a methacrylate-based photopolymer resin were compared to six commercial artificial typodont teeth. Mechanical performance of the extracted human teeth and 3D printed typodont teeth were evaluated using a haptic approach of measuring applied cutting forces. Results indicate 3D printed typodont teeth replicating enamel and dentine can be mechanically comparable to extracted human teeth despite the material compositions differing from the materials found in human teeth. A multiple parameter variable of material elastic modulus and hardness is shown to describe the haptic response when cutting through both human and biomimetic, highlighting a critical dependence between the ratio of material mechanical properties and not absolute material properties in determining tooth mechanical performance under the action of cutting forces.

Reduction of fibrillar strain-rate sensitivity in steroid-induced osteoporosis linked to changes in mineralized fibrillar nanostructure.

As bone is used in a dynamic mechanical environment, understanding the structural origins of its time-dependent mechanical behaviour - and the alterations in metabolic bone disease - is of interest. However, at the scale of the mineralized fibrillar matrix (nanometre-level), the nature of the strain-rate dependent mechanics is incompletely understood. Here, we investigate the fibrillar- and mineral-deformation behaviour in a murine model of Cushing's syndrome, used to understand steroid induced osteoporosis, using synchrotron small- and wide-angle scattering/diffraction combined with in situ tensile testing at three strain rates ranging from 10-4 to 10-1 s-1. We find that the effective fibril- and mineral-modulus and fibrillar-reorientation show no significant increase with strain-rate in osteoporotic bone, but increase significantly in normal (wild-type) bone. By applying a fibril-lamellar two-level structural model of bone matrix deformation to fit the results, we obtain indications that altered collagen-mineral interactions at the nanoscale - along with altered fibrillar orientation distributions - may be the underlying reason for this altered strain-rate sensitivity. Our results suggest that an altered strain-rate sensitivity of the bone matrix in osteoporosis may be one of the contributing factors to reduced mechanical competence in such metabolic bone disorders, and that increasing this sensitivity may improve biomechanical performance.

Comparison of the Efficacy of Different Fluoride Varnishes on Dentin Remineralization During a Critical pH Exposure Using Quantitative X-Ray Microtomography.

OBJECTIVES:: The objective of this in vitro study was to quantify the amount of mineral change in demineralized dentin at pH 5.5 after the application of dental varnishes containing fluoride with casein phosphopeptide-amorphous calcium phosphate, fluoride and bioglass, or fluoride alone. METHODS AND MATERIALS:: A total of 12 extracted human sound mandibular premolar root samples were coated with an acid-resistant varnish, leaving a 2 × 3 mm window at the outer root surface. These root specimens were then randomly divided into four groups and separately subjected to the demineralizing cycle at a pH of 4.8 for five days to create artificial caries-like lesions in dentin. Subsequently, each sample was imaged using quantitative x-ray microtomography (XMT) at a 15-µm voxel size. Each test group then received one of the following treatments: dental varnish containing casein phosphopeptide-amorphous calcium phosphate and fluoride (CPP-ACP, MI varnish, GC Europe), bioglass and fluoride (BGA, Experimental, Dentsply Sirona), or fluoride alone (NUPRO, Dentsply Sirona), as well as a control group, which received no treatment. These samples were kept in deionized water for 12 hours. The thin layer of varnish was then removed. All samples including the nonvarnish group were subjected to the second demineralizing cycle at pH 5.5 for five days. The final XMT imaging was then carried out following the second demineralizing cycle. XMT scan was also carried out to varnish samples at 25 µm voxel size. The change in mineral concentration in the demineralized teeth was assessed using both qualitative and quantitative image analysis. RESULTS:: There was an increase in radiopacity in the subtracted images of all varnish groups; a significant increase in mineral content, 12% for the CPP-ACP and fluoride ( p≤0.05 and p≤0.001), 25% BGA ( p≤0.001), and 104% fluoride alone varnish ( p≤0.001). There was an increase in the size of radiolucency in the lesion area with a significant decrease in mineral content in the nonvarnish group, 10% ( p≤0.05 and p≤0.001). CONCLUSIONS:: There was encouraging evidence of a remineralization effect following the application of dental varnish on dentin and also an observed resistance to demineralization during the acidic challenge in all cases. However, a dental varnish containing fluoride alone appeared to have a much greater effect on dentin remineralization when compared with CPP-ACP with fluoride and bioglass with fluoride.

Approaches to 3D printing teeth from X-ray microtomography.

Artificial teeth have several advantages in preclinical training. The aim of this study is to three-dimensionally (3D) print accurate artificial teeth using scans from X-ray microtomography (XMT). Extracted and artificial teeth were imaged at 90 kV and 40 kV, respectively, to create detailed high contrast scans. The dataset was visualised to produce internal and external meshes subsequently exported to 3D modelling software for modification before finally sending to a slicing program for printing. After appropriate parameter setting, the printer deposited material in specific locations layer by layer, to create a 3D physical model. Scans were manipulated to ensure a clean model was imported into the slicing software, where layer height replicated the high spatial resolution that was observed in the XMT scans. The model was then printed in two different materials (polylactic acid and thermoplastic elastomer). A multimaterial print was created to show the different physical characteristics between enamel and dentine. LAY DESCRIPTION: Objectives Trainee dentists practice procedures using artificial teeth that are far from real teeth. Using x-rays and 3D printing technology the project will recreate a real tooth, artificially. Methods X-rays produce a 3D image that can be printed out as a physical replica, after several conversions of files. Different settings can be used to allow the printed model, to be as accurate as possible. Data were collected on the forces from a dental drill on a tooth's surface, to measure hardness and resistance. Results Multiple teeth replicas were printed with a high accuracy. The materials printed did not mimic actual tooth properties, but using the data from real teeth, materials can be tested in future.

Bone matrix development in steroid-induced osteoporosis is associated with a consistently reduced fibrillar stiffness linked to altered bone mineral quality.

Glucocorticoid-induced osteoporosis (GIOP) is a major secondary form of osteoporosis, with the fracture risk significantly elevated - at similar levels of bone mineral density - in patients taking glucocorticoids compared with non-users. The adverse bone structural changes at multiple hierarchical levels in GIOP, and their mechanistic consequences leading to reduced load-bearing capacity, are not clearly understood. Here we combine experimental X-ray nanoscale mechanical imaging with analytical modelling of the bone matrix mechanics to determine mechanisms causing bone material quality deterioration during development of GIOP. In situ synchrotron small-angle X-ray diffraction combined with tensile testing was used to measure nanoscale deformation mechanisms in a murine model of GIOP, due to a corticotrophin-releasing hormone promoter mutation, at multiple ages (8-, 12-, 24- and 36 weeks), complemented by quantitative micro-computed tomography and backscattered electron imaging to determine mineral concentrations. We develop a two-level hierarchical model of the bone matrix (mineralized fibril and lamella) to predict fibrillar mechanical response as a function of architectural parameters of the mineralized matrix. The fibrillar elastic modulus of GIOP-bone is lower than healthy bone throughout development, and nearly constant in time, in contrast to the progressively increasing stiffness in healthy bone. The lower mineral platelet aspect ratio value for GIOP compared to healthy bone in the multiscale model can explain the fibrillar deformation. Consistent with this result, independent measurement of mineral platelet lengths from wide-angle X-ray diffraction finds a shorter mineral platelet length in GIOP. Our results show how lowered mineralization combined with altered mineral nanostructure in GIOP leads to lowered mechanical competence. SIGNIFICANCE STATEMENT: Increased fragility in musculoskeletal disorders like osteoporosis are believed to arise due to alterations in bone structure at multiple length-scales from the organ down to the supramolecular-level, where collagen molecules and elongated mineral nanoparticles form stiff fibrils. However, the nature of these molecular-level alterations are not known. Here we used X-ray scattering to determine both how bone fibrils deform in secondary osteoporosis, as well as how the fibril orientation and mineral nanoparticle structure changes. We found that osteoporotic fibrils become less stiff both because the mineral nanoparticles became shorter and less efficient at transferring load from collagen, and because the fibrils are more randomly oriented. These results will help in the design of new composite musculoskeletal implants for bone repair.

On fragmenting, densely mineralised acellular protrusions into articular cartilage and their possible role in osteoarthritis.

High density mineralised protrusions (HDMP) from the tidemark mineralising front into hyaline articular cartilage (HAC) were first described in Thoroughbred racehorse fetlock joints and later in Icelandic horse hock joints. We now report them in human material. Whole femoral heads removed at operation for joint replacement or from dissection room cadavers were imaged using magnetic resonance imaging (MRI) dual echo steady state at 0.23 mm resolution, then 26-µm resolution high contrast X-ray microtomography, sectioned and embedded in polymethylmethacrylate, blocks cut and polished and re-imaged with 6-µm resolution X-ray microtomography. Tissue mineralisation density was imaged using backscattered electron SEM (BSE SEM) at 20 kV with uncoated samples. HAC histology was studied by BSE SEM after staining block faces with ammonium triiodide solution. HDMP arise via the extrusion of an unknown mineralisable matrix into clefts in HAC, a process of acellular dystrophic calcification. Their formation may be an extension of a crack self-healing mechanism found in bone and articular calcified cartilage. Mineral concentration exceeds that of articular calcified cartilage and is not uniform. It is probable that they have not been reported previously because they are removed by decalcification with standard protocols. Mineral phase morphology frequently shows the agglomeration of many fine particles into larger concretions. HDMP are surrounded by HAC, are brittle, and show fault lines within them. Dense fragments found within damaged HAC could make a significant contribution to joint destruction. At least larger HDMP can be detected with the best MRI imaging ex vivo.

X-ray microtomography study to validate the efficacies of caries removal in primary molars by hand excavation and chemo-mechanical technique.

BACKGROUND/AIMS: Mechanical removal of carious dentine based on perceived hardness is subjective and tends to be excessively destructive; chemo-mechanical techniques have been proposed as being more objective and conservative. The aims of the present study are to use X-ray microtomography (XMT/micro-CT) to determine the three-dimensional mineral concentration distribution in sound, carious and excavated dentine using hand excavation (HE) and a chemo-mechanical, Carisolv (CS), removal technique for primary molars, and to compare the volume of sound dentine removed in order to validate the efficacies of these two techniques. METHODS: Twenty-one primary molars with open carious cavities were hemisected. The carious tissue in one half was then removed by HE and the other by CS. XMT scans were taken before and after caries removal. After alignment, subtracted XMT images from the two scans revealing the tissues removed were generated, from which mineral distributions were determined, and volumes of sound dentine removed by each technique were calculated. RESULTS: It was found that the sound dentine removed by HE and CS techniques accounted for 4.0 and 2.1% of total tissues removed, respectively. The mean cut-off linear attenuation coefficients at 40 keV to which HE and CS excavated to were 1.27 and 1.09 cm(-1), respectively. The corresponding Knoop hardness number for the cut-off for CS was 25 kg · mm(-2). CONCLUSION: It is concluded that using XMT, CS is validated to be more conservative than HE and preserves a layer of partially demineralised dentine with a mineral concentration > 0.97 g · cm(-3).

Hypophosphatemic rickets is associated with disruption of mineral orientation at the nanoscale in the flat scapula bones of rachitic mice with development.

Metabolic bone disorders such as rickets are associated with altered in vivo muscular force distributions on the skeletal system. During development, these altered forces can potentially affect the spatial and temporal dynamics of mineralised tissue formation, but the exact mechanisms are not known. Here we have used a murine model of hypophosphatemic rickets (Hpr) to study the development of the mineralised nanostructure in the intramembranously ossifying scapulae (shoulder bone). Using position-resolved scanning small angle X-ray scattering (SAXS), we quantified the degree and direction of mineral nanocrystallite alignment over the width of the scapulae, from the load bearing lateral border (LB) regions to the intermediate infraspinous fossa (IF) tissue. These measurements revealed a significant (p<0.05) increase in mineral nanocrystallite alignment in the LB when compared to the IF region, with increased tissue maturation in wild-type mice; this was absent in mice with rickets. The crystallites were more closely aligned to the macroscopic bone boundary in the LB when compared to the IF region in both wild type and Hpr mice, but the degree of alignment was reduced in Hpr mice. These findings are consistent with a correlation between the nanocrystallites within fibrils and in vivo muscular forces. Thus our results indicate a relevant mechanism for the observed increased macroscopic deformability in rickets, via a significant alteration in the mineral particle alignment, which is mediated by an altered spatial distribution of muscle forces.

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/cm(3). 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.

Descending brain neurons in larval lamprey: spinal projection patterns and initiation of locomotion.

In larval lamprey, partial lesions were made in the rostral spinal cord to determine which spinal tracts are important for descending activation of locomotion and to identify descending brain neurons that project in these tracts. In whole animals and in vitro brain/spinal cord preparations, brain-initiated spinal locomotor activity was present when the lateral or intermediate spinal tracts were spared but usually was abolished when the medial tracts were spared. We previously showed that descending brain neurons are located in eleven cell groups, including reticulospinal (RS) neurons in the mesenecephalic reticular nucleus (MRN) as well as the anterior (ARRN), middle (MRRN), and posterior (PRRN) rhombencephalic reticular nuclei. Other descending brain neurons are located in the diencephalic (Di) as well as the anterolateral (ALV), dorsolateral (DLV), and posterolateral (PLV) vagal groups. In the present study, the Mauthner and auxillary Mauthner cells, most neurons in the Di, ALV, DLV, and PLV cell groups, and some neurons in the ARRN and PRRN had crossed descending axons. The majority of neurons projecting in medial spinal tracts included large identified Müller cells and neurons in the Di, MRN, ALV, and DLV. Axons of individual descending brain neurons usually did not switch spinal tracts, have branches in multiple tracts, or cross the midline within the rostral cord. Most neurons that projected in the lateral/intermediate spinal tracts were in the ARRN, MRRN, and PRRN. Thus, output neurons of the locomotor command system are distributed in several reticular nuclei, whose neurons project in relatively wide areas of the cord.

Changes in bone mineral and matrix in response to a soft diet.

Alterations in the magnitude of habitual mechanical loads upon the skeleton may not only affect bone architecture, but also influence the nature of the bone matrix. We tested the hypothesis that changing the mechanical consistency of the diet affects both the mineral and non-mineralized moieties of bone matrix. Female rats were fed a soft diet (powdered chow as a paste), while control animals were fed the standard chow. After 8 or 20 wks, animals were killed. Cranial (mandible, maxilla, parietal, and frontal) bones and ulnae were analyzed for mineralization density by quantitative backscattered electron microscopy, and sulphated glycosaminoglycan levels with alcian blue staining were measured by microdensitometry. The soft diet group showed a significant increase in mineralization density distribution at almost all cranial sites and a reduction in alcian blue staining in alveolar bone. Altering the consistency of the diet significantly affects mineral concentration and glycosaminoglycan content of alveolar bone.

Ovariectomy vs. hypofunction: their effects on rat mandibular bone.

Previous studies have suggested that the mandible may be more influenced by mechanical loading than by circulating hormone levels. We tested the hypothesis that hypofunction has a greater influence than ovariectomy on mandibular bone. Two-month-old rats were ovariectomized (OVX) or had maxillary molars removed from one side to induce unilateral mandibular hypofunction. Control animals remained untreated. After 5 months, animals were killed, and bones were assessed by micro-tomography (muCT), quantitative back-scattered electron analysis in an SEM (qBSE-SEM), and light microscopy. Mineralization density was reduced in calvarial, maxillary, and mandibular alveolar bone following OVX, yet was increased in lingual mandibular alveolar bone of the hypo-function animals compared with controls. OVX caused a reduction in osteocyte density in alveolar bone, while hypofunction showed an increase compared with controls. Hypofunction led to alveolar bone becoming more highly mineralized and more cellular, while ovariectomy caused a reduction in both mineralization density and osteocyte numbers.

Tooth size patterns in patients with hypodontia and supernumerary teeth.

AIMS: Anomalies of tooth number may not be isolated conditions but may have wider associations in the development of the dentition including tooth size. This study aimed to examine links between hypodontia, supernumerary teeth and crown size, considering the effect on the development of the whole dentition and so increase understanding of the aetiology of these conditions. METHODS AND RESULTS: The patients, who were all of European ancestry, were 60 young adults (30 males and 30 females) with hypodontia and 60 age and sex matched controls together with 60 young adults (39 males and 21 females) with supernumerary teeth and 60 age and sex matched controls. Hand measurements of mesiodistal and buccolingual dimensions were made of the teeth on dental study models using Mitutoyo electric callipers. The mean value of two measurements was used and intra-operator and inter-operator reliability determined. Patients with hypodontia had smaller teeth than the control group and this difference was statistically significant (p<0.05) for all teeth except the MD dimensions of 13, 23, 24 and 44. The difference in size was greatest for the BL dimensions in hypodontia patients. Further, the greater the number of missing teeth the smaller the tooth size. The hypodontia patients also showed higher variability in tooth dimensions than the control group. Patients with supernumerary teeth had larger teeth than the controls, with the greatest differences in the MD dimensions. In both hypodontia and supernumerary patients the differences in tooth size were generalised throughout the dentition. CONCLUSIONS: In anomalies of tooth number the size of teeth is also involved. In patients with hypodontia and supernumerary teeth the crown size of the whole dentition is affected. These findings are compatible with a multifactorial aetiology of these conditions.

Enamel mineral concentration in diabetic rodents.

AIM: This was to use x-ray microtomography (XMT), to assess the mineral composition and 3-D structure of enamel and bone in the teeth and skulls of diabetic rodents. METHODS: Three-dimensional images of the skull were reconstructed using computer generated false colour to highlight different levels of mineralization in bone and enamel. RESULTS: These showed that diabetic rodents exhibited more wear in their teeth. Deformities were observed in the alveolar process of the mandible and maxilla. Regions of extensive hypomineralization were found in the calvarial bone of skulls. The maximum mineral concentrations and the time constants for diabetic rodents were similar to the controls. The diabetic mice appeared to have random regions of hypomineralization and one diabetic rat had areas of hypoplasia in the mandibular incisors. CONCLUSIONS: Diabetes may have a detrimental influence on the function of ameloblasts in laying down enamel.

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 x 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 x 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.

Dentinal carious lesion in three dimensions.

AIM: The aim of this study was to show the morphology of the carious lesion in dentine in three dimensions (3D). DESIGN: A novel high-definition X-ray microtomography (XMT) scanner was used to scan 10 carious primary molars at a resolution of 15 x 15 x 15 microm3. A stack of approximately 640 XMT slices were recorded for each tooth. Using this data set and a volume rendering algorithm, each tooth was reconstructed in 3D. The VG Studio Max 1.0 visualization software package was used to make normal enamel and dentine transparent to show the carious lesions in 3D. A video film, comprised of the rendered images from 60 viewing angles rotating through 360 degrees , was produced to show the carious lesion and its relation to the pulp in a three-dimensional perspective (http://www.smd.qmul.ac.uk/dental/oralgrowdev/biophysics/xmt/images/carious.mpg). RESULTS: These images showed that carious lesions in dentine were bowl-shaped. The pulp adjacent to the carious lesion was also observed to mimic the base of the bowl-shaped lesion. CONCLUSIONS: It was concluded from the teeth studied that the shape at the base of the carious lesion in dentine is curved in 3D, rather than conical, as traditionally believed from two-dimensional image interpretation. Further 3D studies are needed to investigate whether the bowl-shaped carious lesions in dentine also apply to caries in other types of teeth.

Assessment of the fitness impacts on Escherichia coli of acquisition of antibiotic resistance genes encoded by different types of genetic element.

OBJECTIVES: Little is known of the fitness cost that antibiotic resistance exerts on wild-type bacteria, especially in their natural environments. We therefore examined the fitness costs that several antibiotic resistance elements imposed on a wild-type Escherichia coli isolate, both in the laboratory and in a pig gut colonization model. METHODS: Plasmid R46, Tn1 and Tn7 and a K42R RpsL substitution were separately introduced into E. coli 345-2 RifC, a rifampicin-resistant derivative of a recent porcine isolate. The insertion site of Tn1 was determined by DNA sequencing. The fitness cost of each resistance element was assessed in vitro by pairwise growth competition and in vivo by regularly monitoring the recovery of strains from faeces for 21 days following oral inoculation of organic piglets. Each derivative of 345-2 RifC carrying a resistance element was grown in antibiotic-free broth for 200 generations and the experiments to assess fitness were repeated. RESULTS: RpsL K42R was found to impose a small fitness cost on E. coli 345-2 RifC in vitro but did not compromise survival in vivo. R46 imposed a cost both before and after laboratory passage in vitro, but only the pre-passage strain was at a disadvantage in vivo. The post-passage isolate had an advantage in pigs. Acquisition of Tn7 had no impact on the fitness of E. coli 345-2 RifC. Two derivatives containing Tn1 were isolated and, in both cases, the transposon inserted into the same cryptic chromosomal sequence. Acquisition of Tn1 improved fitness of E. coli 345-2 RifC in vitro and in vivo in the case of the first derivative, but in the case of a second, independent derivative, Tn1 had a neutral effect on fitness. CONCLUSIONS: The fitness impact imposed on E. coli 345-2 RifC by carriage of antibiotic resistance elements was generally low or non-existent, suggesting that once established, resistance may be difficult to eliminate through reduction in prescribing alone.

Synchrotron x-ray microtomographic investigation of mineral concentrations at micrometre scale in sound and carious enamel.

Synchrotron X-ray microtomography (XMT) was used to measure the linear attenuation coefficient (LAC) for 1.9-microm sidelength voxels within approximal brown spot lesions and sound human enamel. XMT demonstrated three-dimensional features, notably sheets with approximately 30 microm periodicity having low LAC, identified as regions of demineralization corresponding to Retzius lines. Quantitative three-dimensional measurements of mineral concentration, derived from LAC with assumption of a single model composition, were consistent with previous measurements of sound and carious enamel from microradiographic projections. The uncertainty in measurements of mineral concentration and mineral fraction volume was investigated by modelling enamel with a range of composition and component densities. This analysis showed that, although mineral concentration can be determined from LAC with an error of <0.2 g cm(-3), the variation in pore fraction volume within caries lesions cannot be reliably determined from X-ray attenuation measurements alone.

X-ray microtomographic study of mineral concentration distribution in deciduous enamel.

OBJECTIVE: To determine the mineral concentration distribution in deciduous enamel by quantitative X-ray microtomography (XMT). DESIGN: Tooth rods ( approximately 2 mm x 2 mm) were removed from the mid-buccal region of 11 deciduous molars. Three XMT slices were taken at 1.5, 2.0 and 2.5 mm from the amelocemental junction. The distribution and variation in mineral concentration of enamel were studied from the XMT images. RESULTS: The mean mineral concentration for all the teeth was 2.81 (S.D. = 0.065) g cm(-3). There was no notable difference in the mean mineral concentration values between the three XMT slices of each tooth. However, there was up to 8% variation between different teeth (2.69-2.92 g cm(-3)). Gradients of increasing mineral concentration from the amelodentinal junction (ADJ) to the external surface were found, ranging from 0.08 to 0.60 mg cm(-3) microm(-1) with a mean of 0.366 mg cm(-3) microm(-1). The mineral concentration gradients in the occlusal slices were steeper than those in the cervical slices. The difference in mineral concentration between the inner and outer enamel ranged from 1.5 to 8.7%. CONCLUSION: In view of the large variation in both the means and the gradients of mineral concentration in deciduous molars, the mineral distribution of each experimental tooth should be measured as baseline data in studies of caries progression.

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.