This study examines the variability in the anatomy of mandibles of differing ages and different stages of tooth loss. Mandibles from individuals between 19 and 96 y were sectioned into 2 mm thick vertical planeparallel slices and cleaned of marrow and periosteum. The apparent density (mass per unit volume in g\ml) from midline (MID) and mental foramen region (MF) sites was determined by weighing the slices and dividing by a volume calculated as the product of section thickness and the mean area of the 2 sides of the section. The cortical thickness of the inferior border and the basal and alveolar bone heights were measured in radiographs of the slices. Mandibular apparent density was negatively correlated with the cross sectional area (midline r l k0.48, mental foramen r l k0.45), and at the midline was significantly greater in edentulous than in dentate individuals (means (p...) edentulous n l 13 : 1.43 (p0.07) g\ml ; dentate n l 17 : 1.27 (p0.04) g\ml, P 0.05). Where a large enough age range was available, mandibular apparent bone density showed a significant increase with age (midline males : r l 0.53, n l 18) especially for dentate individuals (r l 0.91, n l 8). There was a correlation between the apparent densities at the two sites in the same mandible (r l 0.64), with the values obtained for the midline being significantly greater than for the mental foramen region (midline 1.34 (p0.04) g\ml ; mental foramen 1.19 (p0.04) g\ml, P 0.001, paired t test). The mandible shows great interindividual variability, but there may be a considerable reduction in cross sectional girth of the mandible following tooth loss, and, unlike postcranial sites, an increase in apparent density with age.

Following tooth loss, the mandible shows an extensive loss of bone in some individuals. This may pose a significant problem in the prosthodontic restoration of function and esthetics. The many factors which have been proposed as being responsible for the inter-individual variation in post-extraction remodeling mean that a perfunctory analysis of the literature, in which well-controlled, relevant studies are scarce, may not provide the whole story. This article reviews the local and systemic factors which may play a role in the post-extraction remodeling of the mandible. Since severe residual ridge resorption may occur even when the bone status in the rest of the skeleton is good and vice versa, it is concluded that local functional factors are of paramount significance. It is now essential to determine how they can be modified and applied to help maintain ridge height and quality in our aging, edentulous population.

The incidence of limb bone fracture and subsequent morbidity and mortality due to excessive bone loss is increasing in the progressively ageing populations of both men and women. In contrast to bone loss in the weight-bearing limb, bone mass in the protective skull vault is maintained. One explanation for this could be anatomically diverse bone matrix characteristics generated by heterogeneous osteoblast populations. We have tested the hypothesis that adult bones demonstrate site-specific characteristics, and report differences at the organ, cell and transcriptome levels. Limb bones contain greater amounts of polysulphated glycosaminoglycan stained with Alcian Blue and have significantly higher osteocyte densities than skull bone. Site-specific patterns persist in cultured adult bone-derived cells both phenotypically (proliferation rate, response to estrogen and cell volumes), and at the level of specific gene expression (collagen triple helix repeat containing 1, reelin and ras-like and estrogen-regulated growth inhibitor). Based on genome-wide mRNA expression and cluster analysis, we demonstrate that bones and cultured adult bone-derived cells segregate according to site of derivation. We also find the differential expression of genes associated with embryological development (Skull: Zic, Dlx, Irx, Twist1 and Cart1; Limb: Hox, Shox2, and Tbx genes) in both adult bones and isolated adult bone-derived cells. Together, these site-specific differences support the view that, analogous to different muscle types (cardiac, smooth and skeletal), skull and limb bones represent separate classes of bone. We assign these differences, not to mode of primary ossification, but to the embryological cell lineage; the basis and implications of this division are discussed.

This study examined the tissue level mineralisation density distribution in mandibles from 88 adult humans. were sectioned vertically in midline (MID), mental foramen (MF), and third molar (M3) regions. Surgical fragments from M3 were obtained from individuals aged 16-38 y. All specimens were cleaned, embedded in PMMA, micromilled and examined by digital 20 kV backscattered electron (BSE) stereology. Quantitation was based on rescaling image histograms to the signal range between a monobrominated (0) and a monoiodinated (255) dimethacrylate resin standard. Mineralisation density increased with age (r l 0.70 ; P 0.0001) : the mean for 39 individuals aged between 16 and 50 y was significantly lower (P 0.0001) than for 35 individuals over 51 y (mean (p...) : 158.20 (1.63) and 174.71 (1.27) normalised grey level units respectively). There was good correlation in mean mineralisation density between different sites in the same mandible, but MID was significantly less highly mineralised than the other sites : MID 173.90, MF 177.34, M3 177.11 (P 0.002 and 0.01 for MF and M3 respectively ; paired t test), as was the alveolar bone density when compared with the bone of the inferior cortex (e.g. MID : 171.13 (1.53) and 174.46 (1.14) P 0.0001). No sex difference was found. Partially dentate mandibles generally had regions of higher mineralisation than fully dentate and edentulous mandibles. The lowest density bone occurred at the alveolar crest anteriorly and superolingually at M3, matching sites of net resorption following tooth loss. Highest densities were found inferolingually at MID, inferiorly at MF and buccally at M3, matching the sites thought to experience the highest functional strains. This stresses the importance that local factors may have in the remodelling of the edentulous mandible. Morphology showed that there is a preponderance of highly mineralised cement lines, and of packets containing dead, mineralised, osteocytes.Key words : Osteoporosis ; bone tissue density fractionation ; ageing ; osteocyte cell death. A greater understanding of the factors affecting the turnover of mandibular bone is of relevance to most areas of dentistry. Of major current interest is the relative importance that local and systemic factors have in influencing mandibular residual ridge reduction. However, before it is possible to study any association between bone turnover of the mandible with the turnover at other sites in the skeleton it is necessary to establish first the changes that occur within the bone with ageing and with tooth loss.The main constituents of bone tissue are protein

Changes in bone structure as a function of age have been studied by simple inspection, x-ray imaging, stereo-photography, deep field optical microscopy, circularly polarised light microscopy, and scanning electron microscopy (SEM), including both topographic and compositional backscattered electron (BSE) imaging modes. The study of bone as a three-dimensional object, rather than in thin sections, enables us to envisage modelling and remodelling processes in context. The study of ultra-flat block surfaces permits the acquisition of data from an effectively very thin layer in the block face, and to examine bone as a spectrum of tissue types varying in the degree of mineralisation. Particular attention has been paid in our earlier studies to the iliac crest, lumbar vertebral bodies, femoral mid-shaft, neck and head and parietal and frontal skull bones. Recently, we have compared findings from these sites with observations on the mandible. We conclude, from our new imaging data, that common generalisations about the changes in bone in ageing and osteoporosis are too simplified, and that the mandible differs sufficiently from post-cranial skeletal sites that it would be unwise to extrapolate from findings in the jaw to the circumstances elsewhere.