Metabolic bone disease and bone quality

On the progress of study concerned with pathology of metabolic bone disease such as osteoporosis, it has been known that most of bone strength can be explained by bone volume. As bone volume can be determine by bone mineral density (BMD) with dual-energy X-ray absorptiometry (DXA), it has been widely used for diagnosis of osteoporosis or efficacy of treatment. However, with the advance of bone morphometry, decrease of bone strength or existence of insufficiency fracture is influenced by not only loss of BMD but also deterioration of bone quality especially bone microstructure. In this chapter, we will give an outline of change of bone quality in metabolic bone disease.     

Metabolic bone markers and bone cell biology

Various metabolic bone markers have been developed in order to analyze each process of bone resorption and bone formation. By evaluating the two processes using bone markers, an imbalance between bone resorption and formation can be estimated. Metabolic bone markers have already been in clinical use for the early diagnosis and the assessment of treatment efficacy in osteoporotic patients and for the diagnosis of cancer-induced bone diseases. Further elucidation of the mechanisms of formation and secretion, metabolic clearance, diurnal rhythm as well as their changes in various disorders should enable us to evaluate bone turnover at a real-time scale and to utilize for the diagnosis of a variety of metabolic bone diseases.     

Measurements of bone mass and bone density

X-ray-based procedures are available to measure bone mineral density in vitro at almost any skeletal site. These bone density measurements are not useful in the diagnosis of the cause of bone loss but at present are the only tests available for assessing bone mass prior to the occurrence of irreversible changes such as fractures or vertebral compression, which are easily recognizable on x-rays. When fractures are present, the severity of the bone loss and the risk for future fractures can be assessed. Repeated measurements permit estimation of the rate of bone loss, which gives useful information for monitoring treatment effect or course of the disease. Measurement of total body calcium is of less clinical importance because of the predominantly trabecular bone loss that generally occurs in metabolic bone disease. Dual-energy x-ray absorptiometry (DEXA) and quantitative computed tomography (QCT) of the spine are of about equal clinical value in the first approach to the patient with metabolic bone disease, although DEXA allows greater variety in sampling sites. For repeated measurements, DEXA provides better precision at significantly lower radiation burden. For bone mineral measurements, the lumbar spine appears to be the most sensitive skeletal site.     

Alcohol-induced bone loss and deficient bone repair

BACKGROUND: Chronic consumption of excessive alcohol eventually results in an osteopenic skeleton and increased risk for osteoporosis. Alcoholics experience not only increased incidence of fractures from falls, but also delays in fracture healing compared with non-alcoholics. In this review the term "alcohol-induced bone disease" is used to refer to these skeletal abnormalities. Alcohol-induced osteopenia is distinct from osteoporoses such as postmenopausal osteoporosis and disuse osteoporosis. Gonadal insufficiency increases the rate of bone remodeling, whereas alcohol decreases this rate. Thus, histomorphometric studies show different characteristics for the bone loss that occurs in these two disease states. In particular, alcohol-induced osteopenia results mainly from decreased bone formation rather than increased bone resorption. Human, animal and cell culture studies of the effects of alcohol on bone strongly suggest alcohol has a dose-dependent toxic effect on osteoblast activity. The capacity of bone marrow stromal cells to differentiate into osteoblasts has a critical role in the cellular processes involved in the maintenance of the adult human skeleton by bone remodeling. Chronic alcohol consumption suppresses osteoblastic differentiation of bone marrow cells and promotes adipogenesis. In fracture healing, the effect of alcohol is to suppress synthesis of an ossifiable matrix, possibly due to inhibition of cell proliferation and maldifferentiation of mesenchymal cells in the repair tissue. This results in the deficient bone repair observed in animal studies, characterized by repair tissue of lower stiffness, strength and mineral content. Current knowledge of cellular effects and molecular mechanisms involved in alcohol-induced bone disease is insufficient to develop interventional strategies for its prevention and treatment. OBJECTIVES: The objectives of this review are 1) to identify the characteristics of alcohol-induced bone loss and deficient bone repair as revealed in human and animal studies, 2) to determine the current understanding of the cellular effects underlying both skeletal abnormalities, and 3) to suggest directions for future studies to resolve current ambiguities regarding the cellular basis of alcohol-induced bone disease.     

Histomorphometric analysis of bone in metabolic bone disease

Most of the tools necessary for a detailed study of bone remodeling are now available. These methods will enable us to substitute simple surface-based histomorphometry with pertinent derived variables that reveal more information about the processes going on. Because of the inherent variability of the methods, bone histomorphometry is less well suited for investigations in single individuals. However, in terms of basic investigations of cell and tissue activity in grouped materials of patients with metabolic bone disease and the evaluation of treatment regimens in these materials, bone histomorphometric investigations still yield the most detailed analyses.     

A soluble bone morphogenetic protein type IA receptor increases bone mass and bone strength

Diseases such as osteoporosis are associated with reduced bone mass. Therapies to prevent bone loss exist, but there are few that stimulate bone formation and restore bone mass. Bone morphogenetic proteins (BMPs) are members of the TGFβ superfamily, which act as pleiotropic regulators of skeletal organogenesis and bone homeostasis. Ablation of the BMPR1A receptor in osteoblasts increases bone mass, suggesting that inhibition of BMPR1A signaling may have therapeutic benefit. The aim of this study was to determine the skeletal effects of systemic administration of a soluble BMPR1A fusion protein (mBMPR1A-mFc) in vivo. mBMPR1A-mFc was shown to bind BMP2/4 specifically and with high affinity and prevent downstream signaling. mBMPR1A-mFc treatment of immature and mature mice increased bone mineral density, cortical thickness, trabecular bone volume, thickness and number, and decreased trabecular separation. The increase in bone mass was due to an early increase in osteoblast number and bone formation rate, mediated by a suppression of Dickkopf-1 expression. This was followed by a decrease in osteoclast number and eroded surface, which was associated with a decrease in receptor activator of NF-κB ligand (RANKL) production, an increase in osteoprotegerin expression, and a decrease in serum tartrate-resistant acid phosphatase (TRAP5b) concentration. mBMPR1A treatment also increased bone mass and strength in mice with bone loss due to estrogen deficiency. In conclusion, mBMPR1A-mFc stimulates osteoblastic bone formation and decreases bone resorption, which leads to an increase in bone mass, and offers a promising unique alternative for the treatment of bone-related disorders.     

Aseptic bone/bone marrow necrosis in leukaemia

Aseptic (avascular) bone/bone marrow necrosis (ABN) was found in 21 leukaemic patients during a period of 14 yr, with a frequency of 6.5% in autopsied leukaemic patients, and with the highest frequency in ALL. 8 patients (38%) complained of bone pain. X-ray examination was carried out in 7 of these with a positive result in 3 (43%). Only 1 patient had elevated S-Alkaline phosphatase. Leucocytosis was found in 12 patients. Only 1 patient (ALL) received steroid in a rather high dose during 6 weeks. At autopsy ABN was found localized to the femur in all patients and 2 patients also had ABN in other bones. Post-mortem X-ray examination demonstrated changes in 8 of 15 cases (53%), with osteolysis in 6 and sclerosis in 2. 19 patients had had recent ABN with some fibroblast proliferation. In 4 of these, appositional bone formation had started. 2 patients had sclerosis as the only change. The pathogenesis of ABN is not known; an important factor, however, may be ischaemia due to vascular obstruction.     

Injectable bone

Tissue engineering applies the principles of biology and engineering to the development of functional substitutes,for example 1) cells, 2) scaffolds, 3) some cytokines, for lost tissues. The meaning of this bone regeneration is that it decreases the needed tissues and burden of patients. In this time, we applied to mesenchymal stem cells (MSCs) from the own bone marrow as cell sources. MSCs are thought to be multipotent cells that can replicate. And we also used a beta-tricalcium phosphate (beta-TCP) as a scaffold and fibrin glue as materials to regenerate a injectable bone and we injected into the subcutaneous space on the dorsum of the rat. After 8 weeks of implantation, it could be confirmed newly formed bone and fibrin glue functioned as a injectable materials without loosing the cell activity and the proliferation of MSCs. Next we applied Platelet Rich Plasma (PRP) to improve the ability of osteogenesis. PRP contains some cytokines and are expected to promote the increase of osteogenesis. The merit is not immunity rejection from autologous blood collected in the immediate preoperative period. The admixtures of PRP or fibrin glue have fluidity and gel-like consistency as the thrombin mixing. They can be injected with a syringe in tissues. We named this "Injectable Bone". According to the histological observations, the MSCs with PRP were well formed mature bone and neovascularization compared with control (defect only) after 8 weeks implantation. These results demonstrated that the mixture of MSCs and PRP were useful as injectable bone substitute and its ability of bone regeneration is almost equal to autogenous particulate cancellous bone and marrow (PCBM).     

Recombinant human bone morphogenetic protein induces bone formation.

We have purified and characterized active recombinant human bone morphogenetic protein (BMP) 2A. Implantation of the recombinant protein in rats showed that a single BMP can induce bone formation in vivo. A dose-response and time-course study using the rat ectopic bone formation assay revealed that implantation of 0.5-115 micrograms of partially purified recombinant human BMP-2A resulted in cartilage by day 7 and bone formation by day 14. The time at which bone formation occurred was dependent on the amount of BMP-2A implanted; at high doses bone formation could be observed at 5 days. The cartilage- and bone-inductive activity of the recombinant BMP-2A is histologically indistinguishable from that of bone extracts. Thus, recombinant BMP-2A has therapeutic potential to promote de novo bone formation in humans.     

Age-related bone loss: old bone, new facts

The human skeleton serves several functions for the body: support, locomotion, protection of vital organs, and housing of bone marrow. Bone remodeling is the result of the interactions of multiple elements, including osteoblasts, osteoclasts, hormones, growth factors, and cytokines, the end result being the maintenance of the bone architecture and to maintain systemic calcium homeostasis. In early life, a careful balance exists between bone formation by osteoblasts and bone resorption by osteoclasts. With aging, the process of coupled bone formation is affected by the reduction of osteoblast differentiation, activity, and life span which is further potentiated in the perimenopausal years with hormone deprivation and increased osteoclast activity. Age-related bone loss is thus not only a consequence of hormone deprivation, but also the result of changes in bone formation and cell-cell interactions with a unique pathophysiology. In this review, we describe the cellular and metabolic changes associated with aging bone and present recent evidence regarding cell differentiation within the bone marrow. We also consider the mechanism of programmed cell death, apoptosis, as being an important determinant of aging in bone as well as describe possible future interventions to prolong the life span of osteoblasts.     

Histologic diagnosis of metabolic bone diseases: bone histomorphometry

Histomorphometry or quantitative histology is the analysis on histologic sections of bone resorption parameters, formation and structure. It is the only technique that allows a dynamic evaluation of the activity of bone modelling after labelling with tetracycline. Moreover, the new measurement procedures through the use of the computer allow an assessment of bone microarchitecture too. Histomorphometric bone biopsy is a reliable and well-tolerated procedure. Complications are reported only in 1% of the subjects (hematoma, pain, transient neuralgia). Histomorphometry is used to exclude or confirm the diagnosis of osteomalacia. It is employed in the evaluation of bone damage associated with particular treatments (for example, anticonvulsants) or in case of rare bone diseases (osteogenesis imperfecta, systemic mastocytosis). It is also an essential approach when clinical, biochemical and other diagnostic data are not consistent. Finally, it is a useful method to understand the pathophysiologic mechanisms of drugs. The bone sample is taken at the level of iliac crest under local anesthesia. It is then put into methyl-metacrilate resin where the sections are prepared for the microscopic analysis of the various histomorphometric parameters.     

Sex differences in cancellous and cortical bone strength, bone mineral content and bone density.

The purpose of this study was to examine sex differences in cancellous and cortical bone strength, bone mineral content (BMC) and bone density of excised cadaver vertebral and phalangeal bones. The samples were age-matched. Bone strength was measured as the mechanical force required to crush or break the bones. Two parameters of bone strength were used on the vertebrae; the force at the first deviation from linearity and the mean force during the consolidation before final failure. The force at first deviation from linearity was not significantly different between the sexes, but there was a significant difference in the consolidation force. The mean men's phalangeal strength was twice that of the women's. BMC and BMC/BW of both types of bone were statistically different between the sexes. Radiographic photodensity measures on the vertebrae showed no sex differences. Cortical diameters of the finger bones were significantly greater in males.     

Recovery of bone mass and normalization of bone turnover in long-term survivors of allogeneic bone marrow transplantation

Osteoporotic fractures are potential long-term complications of bone marrow transplantation (BMT). We previously reported that bone mineral density (BMD) of patients undergoing allogeneic BMT decreased by 6% to 9% during the first 6 months after BMT and that bone turnover rate was still increased 1 year after BMT. BMT patients do not need lifelong immunosuppressive treatment, which should offer favorable circumstances for the recovery of BMD. Thus, 27 (14 women, 13 men) of 29 long-term survivors of our previous study were invited to a follow-up study at a median of 75 months after BMT. From 12 months after BMT the BMD of the lumbar spine had increased by 2.4% (P = 0.002). The respective changes in femoral sites were +4.1% in the femoral neck (P = 0.087), 4.0% in the trochanter (P = 0.095), +4.7% in Ward's triangle (P = 0.072) and +1.4% in the total hip (P = 0.23). The markers of bone formation, serum osteocalcin and type I procollagen aminoterminal propeptide (PINP) had returned to control levels, but out of the markers of bone resorption the mean level of serum type I carboxyterminal telopeptide (ICTP) was 41% higher (P = 0.0001) and that of urinary type I collagen N-terminal telopeptide/creatinine (NTx) 41% lower (P = 0.0002) in patients than in controls. The mean serum 25-hydroxyvitamin D [25(OH)D] was 33% lower in patients (P = 0.0002), most of whom had hypovitaminosis D [serum 25(OH)D < or = 37 nmol/l]. Except for two, males had serum testosterone level lower than before BMT and four men had hypogonadism. In conclusion, in long-term survivors of allogeneic BMT BMD recovers and bone turnover state normalizes as compared to the situation 1 year after BMT. More attention should be paid to the vitamin D status of all recipients and to possible hypogonadism of male patients.     

Clinical application of bone metabolism markers to disuse bone atrophy

Mechanical stress on bone is one of the determinants of bone morphology, bone mineral density and bone strength. Therefore, disuse accelerates bone resorption, especially of cancellous bone, and consequently bone becomes atrophic and fragile. Osteocytes embedded in the bone matrix respond to mechanical load and changes of bone metabolism. The gapjunction of the long processes of osteocytes plays an important role in transmitting mechanical load through intracellular signal transmitters (cAMP, cGMP) and extracellular signal transmitters (PGE(2), IGF- I , IGF- II , TGF-beta) , to induce bone formation by osteoblasts and bone resorption by osteoclasts, or a combination of the two. Bone metabolism markers reflect activities of osteoblasts and osteoclasts, so they provide information on bone turnover and bone metabolism. Bone resorption markers characteristically increase and remain high during bed rest and disuse, whereas ambulation and exercise, as in kinesiotherapy, decrease bone resorption markers. As for treatments of disuse bone atrophy, from the bone metabolic and pharmacological viewpoint, anti-resorption agents are recommended to inhibit bone loss. Bisphosphonates which are strong anti-resorption agents, are expected to inhibit bone resorption. Calcitonin and vitamin K(2) which are also anti-resorption agents effectively, reduce bone loss due to disuse and bed rest.     

Notch信号通路与骨发生和骨重建

Notch信号通路对调节胚胎细胞和成体细胞增生、分化、凋亡以及前体细胞的自我更新具有重要意义。它可通过协调胚胎期生长板内软骨细胞分化和生长,控制骨组织正常发生。骨重建是对病损骨组织的再生和修复,主要由成骨细胞和破骨细胞参与,Notch信号除可对上述2种细胞增生、分化、成熟等进行调控外,也可介导依赖于成骨细胞的破骨细胞生成。本文试以Notch信号对软骨细胞、成骨细胞、破骨细胞等细胞学行为改变为主线,探讨Notch信号对上述2种骨生物学行为调节的作用机制。     

Mechanisms of bone resorption and new bone formation in spondyloarthropathies

Spondyloarthropathies (SpA) share clinical features such as sacroiliitis, axial immobility, and peripheral arthropathies. They also share a strong association with human leukocyte antigen-B27, implicating T cells and antigen-presenting cells in the disease process. Inflammation seems to underlie the pathogenesis of SpA, particularly in the axial skeleton and entheses. Pathologic bone loss and formation occur simultaneously in inflamed regions, suggesting an inflammation-induced dysregulation of osteoclast and osteoblast activity. Pro-inflammatory cytokines such as tumor necrosis factor-alpha (TNFα) appear to be central to the disease, because TNFα blockade has been shown to effectively improve clinical outcome. Other cytokines such as transforming growth factor-beta, interferon-gamma (IFNγ), and interleukin-18 are also likely to be important in SpA. Activated T cells have been shown to produce cytokines such as IFNγ and receptor activator of nuclear-factorkappaB ligand, with direct effects on osteoclastogenesis. The dual role of T cells in immunobiology and skeletal biology provides a possible link between human leukocyte antigen-B27, pro-inflammatory cytokines, and bone cells in SpA.     

Drug-induced bone pathology

Some drugs are potentially toxic, directly or indirectly, to bone tissue. However, the banality of the bone pathology renders a definitive association difficult. Osteomalacia can be triggered by anticonvulsants, aluminum, fluoride or diphosphonates. Osteoporosis is induced more commonly with corticosteroids, long-term heparin administration or hypothyroid hormonal replacement therapy. The link between the bone disease and the suspected drug is accepted only if there is no other risk factor and after histomorphometric confirmation of the bone pathology.