Showing papers in "Journal of Musculoskeletal & Neuronal Interactions in 2006"

Biology of tendon injury: healing, modeling and remodeling

Abstract: Tendon disorders are frequent, and are responsible for much morbidity both in sport and the workplace. Although the presence of degenerative changes does not always lead to symptoms, pre-existing degeneration has been implicated as a risk factor for acute tendon rupture. The term tendinopathy is a generic descriptor of the clinical conditions in and around tendons arising from overuse. The terms "tendinosis" and "tendinitis/tendonitis" should only be used after histopathological examination. Disordered healing is seen in tendinopathy, and inflammation is not typically seen. In acute injuries, the process of tendon healing is an indivisible process that can be categorized into three overlapping phases for descriptive purposes. Tendon healing can occur intrinsically, via proliferation of epitenon and endotenon tenocytes, or extrinsically, by invasion of cells from the surrounding sheath and synovium. Despite remodeling, the biochemical and mechanical properties of healed tendon tissue never match those of intact tendon. Tendon injuries account for considerable morbidity, and often prove disabling for several months, despite what is considered appropriate management. Chronic problems caused by overuse of tendons probably account for 30% of all running-related injuries, and the prevalence of elbow tendinopathy in tennis players can be as high as 40%. The basic cell biology of tendons is still not fully understood, and the management of tendon injury poses a considerable challenge for clinicians. This article describes the structure of tendons, and reviews the pathophysiology of tendon injury and healing.

Clinical aspects, pathology and pathophysiology of osteoarthritis

Abstract: Osteoarthritis (OA) is the most common joint disorder, and there is evidence that a majority of individuals over the age of 65 have radiographic and/or clinical evidence of OA. The most frequently affected sites are the hands, knees, hips and spine. Importantly, the symptoms are often associated with significant functional impairment, as well as signs and symptoms of inflammation, including pain, stiffness and loss of mobility 1. Anatomic analysis and application of histopatho-logical and imaging techniques have helped to define the natural history of OA with respect to the structural alterations in the articular cartilage 1-6. They also have demonstrated that OA is not exclusively a disorder of articular cartilage. Multiple components of the joint are adversely affected by OA, including the peri-articular bone, synovial joint lining and adjacent supporting connective tissue elements 1-6. The characteristic structural changes in OA include the progressive loss of articular cartilage, increased subchondral plate thickness, formation of new bone at the joint margins (osteophytes) and the development of subchondral bone cysts 7-9. In addition, at the junction of the articular hyaline cartilage and adjacent subchondral bone, in the region of the so-called tidemark, there is a remnant of calcified cartilage. As OA progresses, there is evidence of vascular invasion and advancement of this zone of calcified cartilage into the artic-ular cartilage that further contributes to a decrease in artic-ular cartilage thickness 2,10-14. These structural alterations in the articular cartilage and peri-articular bone may lead to modification of the contours of the adjacent articulating surfaces 7-9,15,16. These changes, as well as the accompanying alterations in subchondral bone remodeling and modulus, may further contribute to the development of an adverse biomechanical environment and enhance the progression of the articular cartilage deterioration 15-19. Multiple factors have been shown to affect the progression of OA, including the presence of polyarticular disease, increasing age, associated intra-articular crystal deposition, obesity, joint instability and/or malalignment, muscle weakness and peripheral neuropathy 1,20-22. These factors can be segregated into categories that include hereditary contributions , mechanical factors and the effects of ageing. There are several lines of evidence indicating that genetic factors contribute to the risk of OA 23,24. These include the results of epidemiological studies, analysis of patterns of familial clustering, twin studies and the characterization of rare genetic disorders. For example, twin studies have shown that the influence of genetic factors may approach 70% in certain skeletal sites. Linkage analyses and …

Mechanical stimulation in vivo reduces osteocyte expression of sclerostin.

Abstract: Osteocytes are by far the most numerous cell type in bone. Their population density, distribution, extensive communication networks, and fluid-filled lacuno-canalicular environment make these cells ideal mechanosensors in bone’s adaptive process. Despite these attributes, very little data have been generated that implicate the osteocyte network as the primary mechanosensory cell type, to the exclusion of the other cell types (e.g., osteoblasts, bone lining cells). The discovery of a mechanically modulated osteocyte-specific factor, particularly a secreted factor that had the propensity to reach effector cell populations, would provide a molecular basis for osteocytic reception and initiation of mechanotransduction. Sclerostin, the protein product of the SOST gene, is an osteocyte-specific cysteine-knot secreted glycoprotein that is a potent inhibitor of bone formation. Sclerostin can bind and inhibit Lrp5, a Wnt co-receptor that we have shown to be required for mechanotransduction. We investigated the regulation of sclerostin expression in mechanically loaded and control (non-loaded) mouse ulnar diaphyses to determine whether this osteocyte-specific factor was under mechanoregulation. To this end, we subjected the right forelimb of four male 18-week-old C57BL/6 mice to two brief (60 sec) bouts of in vivo mechanical loading (60 cycles, 2 Hz, ~1,800 IÂ) using a non-invasive rodent ulna-loading model. Twenty-four hours after the last session, the mice were sacrificed, and the right and left forelimb bones were dissected free, fixed in 4% paraformaldehyde, decalcified in 10% EDTA, embedded in paraffin, sectioned at 8 Im, and immunolabeled for sclerostin. We measured the number of sclerostin-positive osteocyte cell bodies in the medial, lateral and central cortex of each ulnar section, corresponding to different strain environments. The sections revealed strong reactivity in osteocytes but not in any other cell type (bone lining cells, osteoblasts, marrow cells). Qualitatively, the loaded ulnar sections revealed a clear difference from control ulna sections in the degree of sclerostin staining, particularly in canaliculi. Quantitatively, loading reduced the number of sclerostin-positive osteocyte cell bodies by 76% in the medial cortex but by only 10% in the central cortex. Sclerostin-positive osteocytes were reduced by 30% in the lateral cortex. The degree of reduction in sclerostin expression corresponded to strain distribution in the ulnar midshaft during exogenous loading, suggesting that sclerostin reduction is dose responsive to strain magnitude. In conclusion, mechanical loading resulted in a clear and significant reduction in sclerostin protein levels in the ulnar diaphysis after 24 hours. Our data have led us to hypothesize that mechanical loading is detected by osteocytes and causes a reduction in sclerostin secretion, which in turn reduces the inhibition of Lrp5 signaling in nearby osteoblasts, ultimately leading to enhanced osteogenesis. J Musculoskelet Neuronal Interact 2006; 6(4):354

One mechanostat or many? Modifications of the site-specific response of bone to mechanical loading by nature and nurture.

Abstract: The concept of the mechanostat was not new in 1983, when Harold Frost coined the term to describe a mechanism by which bone responded to habitual exercise and changes in loading with structurally appropriate alterations in bone architecture. However, the word "mechanostat" has a meaning that is immediately apparent, and its adoption has led to a much wider appreciation of the process of functional adaptation by other scientists than those whose primary research focus is in the biology of adaptation. One problem exists though: it is widely thought that in a single individual, there is a setting for the mechanostat, just as a single thermostat might set the temperature for a whole house, and this is reflected in the idea that bones throughout the skeleton require a specific strain magnitude for maintenance. Increases in loading above that threshold are expected to induce bone formation and a stiffer structure that then experiences again the habitual strain magnitude. Reductions in strain magnitude supposedly induce resorption to reduce tissue mass and architectural properties so that the lower loading restores habitual strain magnitude. That widely held belief of a single unifying number of strain is fundamentally flawed. The purpose of this article is to explain the real basis of the mechanostat; that the skeleton responds to a complex strain stimulus, made up of numerous different parameters, of which peak magnitude is only one, and that the strain stimulus is different in different parts of the skeleton, so there is no universal number to describe a tissue strain magnitude that underlies the mechanostat's setting. Furthermore, males and females have different responses to loading, and those responses change in response to many factors including genetic constitution, age, concomitant disease, nutrient availability, and exposure to drugs or biochemicals. In summary then, there is not a single mechanostat controlling the skeleton of each of us. At a fundamental tissue level, small functional units of bone each have their own multifactorial threshold target strain stimuli for a given set of dynamic modifying influences. Understanding the biology behind the way that each of these mechanostats functions independently is likely to have pervasive consequences on our ability to control bone mass by manipulation of loading, either directly through different exercise regimens, or in a targeted manner using tailored site and individual specific pharmaceuticals.

Adaptation of the tendon to mechanical usage.

Abstract: Tendons primarily function as contractile force transmitters, but their mechanical properties may change dependent upon their level of mechanical usage. Using an ultrasound-based technique we have assessed tendon mechanical properties in vivo in a number of conditions representing different levels of mechanical usage. Ageing alters tendon mechanical properties; stiffness and modulus were lower in older adults by 10 and 14%, respectively, compared to young adults. Increased levels of exercise loading in old age can however partly reverse this process, as tendon stiffness and modulus were found to increase by 65 and 69%, respectively. Complete unloading due to bed rest or spinal cord injury both reduce tendon stiffness and modulus, however, only chronic unloading due to spinal cord injury seems to cause tendon atrophy. Alterations in tendon mechanical properties due to changes in the levels of loading have implications for the speed of force transmission, the muscle’s operating range and the likelihood of tendon strain injury.

Adaptive processes in skeletal muscle: molecular regulators and genetic influences.

Abstract: Skeletal muscle is a highly adaptable tissue. It responds to environmental and physiological challenges by changes in size, fibre type and metabolism. All of these responses are underpinned by our genes and it is therefore generally assumed that genetic variation between individuals may account for the differences in musculature and athletic capabilities between people. Research into the genetic influences of our muscle is at an embryonic stage, but some early insight into potential regulators has recently emerged, which is reflected in this review. Broad heritability, which appears to affect muscle size and strength more than metabolism has been assessed in twin and sibling studies. It appears to account for more inter-individual variation in the young as opposed to older people. However, the studies reported to date do demonstrate a large degree of diversity, which is probably predominantly due to different methodological approaches being adopted as well as distinct populations being studied. At a molecular level, there has been enormous progress in identifying regulators of atrophy and hypertrophy though the study of knock-out and transgenic animals and also through the utilisation of cell culture models. Among others, the insulin-like growth factors, calcineurin, desmin, myf5, mrf4, MyoD and myogenin have been identified as positive regulators of muscle size, while TNF-alpha, myostatin and components of the ubiquitin pathway have been recognized as regulators of muscle wasting. However, given the ethical and mechanistic constraints of performing similar studies in humans, difficulties have arisen when attempting to translate the animal and cell culture findings to humans. However, the current search for target "exercise genes" in humans has yielded the first successful results. Variations in the genes encoding for: the angiotensin converting enzyme, alpha-actinin 3, bradykinin, ciliary neurotrophic factor, interleukin-15, insulin-like growth factor II, myostatin and the vitamin D-receptor have all been found to account for some of the inter-subject variability in muscle strength or size. However, the influences of these genetic variations are somewhat weak, and not always reproducible and furthermore they are predominantly based in young healthy people. Hence, a key topic, namely the molecular mechanisms of muscle frailty in the elderly still remains to be elucidated.

Determinants of musculoskeletal frailty and the risk of falls in old age.

Abstract: Neuromuscular parameters that describe locomotion are indispensable variables for the diagnosis and treatment of frailty, fall risk and osteoporosis. A scientifically-based standardized locomotor assessment should be an essential part of medical examinations in research and clinical practice. There has been no previous consensus regarding which test procedures should be included in a locomotor assessment. The goal of this article is to provide a rationale for the selection of appropriate locomotor tests in a comprehensive locomotor assessment for elderly patients. We propose that a locomotor assessment should comprise the parameters that have been proven predictive for both falls and impending disability. The parameters should be measured in the standard units of physics. Therefore, we propose the following tests for a standardized locomotor assessment: (1) Self-selected gait velocity as the single best measure of general locomotor status and a good predictor of age-related adverse events; (2) Chair rise test (timed 5 chair rises) which measures power on vertical movement and the hip surrounding muscles as the most important neuromuscular risk factor for falls and fall-related fractures; (3) Tandem standing and tandem walking to measure postural capacity (balance) to the side; (4) Timed up and go test as a global screening procedure; (5) Clinical gait analysis with special focus on regularity; and (6) At least on a research level, movement must be measured referring to the terms of physics by mechanography. Mechanography (Leonardo force plate system, Novotec Pforzheim, Germany) records the time course of ground reaction forces, velocity of the vertical movements of the center of mass and power during unrestricted physiological movements. In the mechanogram the eccentric and concentric phases of movements can be differentiated and the storage of energy in the elastic elements of the body can be examined. The kinetics of human movement is explained by mechanograms of a two-legged jump. The ground reaction forces resulting from a jump down from a height of 0,46 m are demonstrated as a performance that is representative for human coordination. One goal of this text is to underline the insights that arise if the rules of physics are applied to human movement. A deeper understanding enables us to create more effective treatments for disorders of the muscle-bone unit. Bringing physics and cybernetics into the field of osteoporosis is a great heritage of Harold Frost.

The influence of mechanical stimulation on osteocyte apoptosis and bone viability in human trabecular bone.

Abstract: It has been shown previously using in vivo and ex vivo animal models, that cyclical mechanical stimulation is capable of maintaining osteocyte viability through the control of apoptotic cell death. Here we have studied the effect of mechanical stimulation on osteocyte viability in human trabecular bone maintained in a 3-D bioreactor system. Bone samples, maintained in the bioreactor system for periods of 3, 7 and 27 days, were subjected to either cyclical mechanical stimulation which engendered a maximum of 3,000 microstrain in a waveform corresponding to physiological jumping exercise for 5 minutes daily or control unloading. Unloading resulted in a decrease in osteocyte viability within 3 days that was accompanied by increased levels of cellular apoptosis. Mechanical stimulation significantly reduced apoptosis (p< or =0.032) and improved the maintenance of osteocyte viability in bone from all patient samples. The percentage Alkaline Phosphatase (ALP) labelled bone surface was significantly increased (p< or =0.05) in response to mechanical stimulation in all samples as was the Bone Formation Rate (BFR/BS) (p=0.005) as determined by calcein label incorporation in the 27-day experiment. These data indicate that in this model system, mechanical stimulation is capable of maintaining osteocyte viability in human bone.

Cellular, molecular, and matrix changes in cartilage during aging and osteoarthritis.

Abstract: Several anecdotal statements have commonly been used to describe the relationship between aging and degenerative cartilage disease or osteoarthritis (OA). For example, it is commonly stated that there are two fundamental mechanisms that lead to OA; either abnormal loading on normal cartilage or normal loading on abnormal cartilage. The first case refers to relatively young individuals who have damage to a joint that causes structural instability and almost inevitably leads to OA. The second case refers to older individuals who have likely experienced changes in cartilage associated with aging that render the cartilage unable to support relatively normal loading. Another relationship frequently observed is that "OA" does not equal "old age". This statement summarizes the finding that changes in cartilage with age are likely universal, however development of OA, while common, is not universal. While these are concepts that are generalizations, there is a basis in the research literature to support them 1 . This review will deal with a comparison of changes that take place in cartilage with normal aging and during the pathogenesis of osteoarthritis. The focus will be on alterations in chondrocyte homeostasis including changes in viability, proliferation and gene expression. The consequences of these changes on the composition of the matrix will be discussed. Finally, data will be presented on the importance of endoplasmic reticulum stress in chondrocyte biology and its possible occurrence in human OA.

Deletions of genes encoding calcitonin/alpha-CGRP, amylin and calcitonin receptor have given new and unexpected insights into the function of calcitonin receptors and calcitonin receptor-like receptors in bone.

Abstract: It has been suggested that skeletal nerves fibers may play important roles in neuro-osteogenic interactions. This view is partly based upon information obtained from immunohistochemical studies, chemical and surgical denervation experiments and clinical observations in patients with stroke and spinal cord injury, indicating the presence of a network of nerve fibers in the skeleton and that defective signalling in skeletal nerve fibers affects remodelling of bone. This view is also supported by data showing that functional receptors for signalling molecules in skeletal nerve fibers are expressed in bone cells and that activation of these receptors leads to profound effects on bone forming osteoblasts and bone resorbing osteoclasts. Convincing evidence for a role of neuronal signalling in bone metabolism has been provided by gene deletion approaches in which it has been shown that leptin-sensitive and neuropeptide Y-sensitive receptors in hypothalamus are important for bone remodelling in mice. Recently, gene deletion experiments have shown that calcitonin gene-related peptide (CGRP), one of the neuropeptides present in skeletal nerve fibers, is an important physiological regulator of bone formation at the level of osteoblast activity. CGRP belongs to the calcitonin (CT) family of peptides also including CT, amylin and adrenomedullin, as well as the recently described intermedin and calcitonin receptor-stimulating peptide. These peptides utilize two seven transmembrane G protein-coupled receptors - the calcitonin receptor (CTR) and the calcitonin receptor- like receptor (CRLR) - which can dimerize with three different single transmembrane proteins, making up the RAMP family. Associations between RAMPs and either CTR or CRLR give rise to seven distinct, molecularly characterized, receptors for CT, CGRP, amylin and adrenomedullin. Deletions of the genes for ligands in the CT family of peptides and for one of the receptors have revealed unexpected findings that have changed our view on the role of these peptides in bone remodelling. It was anticipated that deletions of the CT/alpha-CGRP and CTR genes would lead to bone loss, since CT has been shown to inhibit bone resorption in vitro and in vivo and has been used to treat patients with excessive bone resorption. Surprisingly, it was found that CT/alpha-CGRP-/- and CTR+/- mice have increased bone mass due to increased bone formation. Mice with deletion of the amylin gene, however, exhibited bone loss due to enhanced bone resorption. Selective deletion of the alpha-CGRP gene also leads to bone loss, but due to decreased bone formation. Thus, our understanding of the role of the CT family of peptides has been changed dramatically and much more data have to be gained before we fully understand the roles these peptides have in bone biology.

Ethnic difference in osteoporosis-related phenotypes and its potential underlying genetic determination.

Abstract: Osteoporosis is a serious health problem in both Caucasians and Asians. Caucasians and Asians are two distinct major ethnic groups, which may have differential genetic determination underlying complex genetic diseases such as osteoporosis. However, to date, there has been no systematic review focusing on the aspect of ethnic difference in risk to osteoporosis and its potential underlying genetic determination between Asians and Caucasians. Here, we firstly review diverse aspects of osteoporosis-related differences, including the differences of epidemiology of osteoporotic fractures, peak bone mass, bone loss, bone area, bone geometry and drug treatment response between Asians and Caucasians. Then, we provide some potential genetic evidence on the different heritability and inheritance mode of bone phenotypes, the different osteoporosis candidate genes and the differential results in related molecular studies between them, to explain the above osteoporosis-related phenotypic differences. The results suggest that the osteoporosis-related phenotypic differences between Asians and Caucasians may be partially the result of the different ethnic genetic background. The present review may increase our understanding of potential different mechanisms related to ethnicity in pathogenesis of osteoporosis for effective and potentially customized treatments in different major ethnic groups.

Downregulation of SOST/sclerostin by PTH: a novel mechanism of hormonal control of bone formation mediated by osteocytes.

Abstract: Both chronic excess of PTH, as in hyperparathyroidism, and intermittent elevation of PTH (by daily injections) increase the number of osteoblasts. However, whereas the former condition can lead to bone catabolism, intermittent administration of PTH causes bone anabolism. The striking difference between bone loss and bone gain in the two conditions might result from a negative vs. positive balance between formation and resorption within each bone remodeling unit, or from de novo bone formation not coupled to previous resorption in the case of intermittent PTH administration. In any event, increased osteoblast number can be achieved by increased osteoblast production from progenitors, or decreased osteoblast apoptosis, or a combination of the two events. Studies in mice indicate that chronic and intermittent PTH increase osteoblast number by distinct mechanisms (Figure 1). Thus, whereas the increase in osteoblast number and the anabolic effect of intermittent PTH in cancellous bone can be accounted for by attenuation of osteoblast apoptosis, chronic elevation of endogenous PTH had no effect on osteoblast survival. The osteoblast specific transcription factor Runx2 is required for the antiapoptotic effect of PTH; however, PTH also stimulates proteasomal proteolysis of Runx2. Based on this, we have reasoned that repeated injections of the hormone are needed to inhibit osteoblast apoptosis because the duration of the PTH-induced survival signaling is self-limited by downregulation of Runx2; and that the inability of chronic elevation of PTH to attenuate osteoblast apoptosis may be due to a decrease in Runx2 levels below the threshold needed for survival signaling. Nevertheless, the increase in osteoblasts seen with chronic PTH elevation cannot be accounted for by inhibition of osteoblast apoptosis and therefore must result from increased osteoblast production. Recent studies indicate that the osteoblastogenic action of chronic elevation of PTH may result from actions of the hormone on osteocytes via changes in Sost expression. Consistent with evidence that Sost is upregulated by Runx2 and that PTH induces proteasomal degradation of Runx2 protein, continuous elevation of PTH dramatically reduces the expression of Sost mRNA and sclerostin in osteocytes in vivo and in vitro. These findings demonstrate a direct effect of PTH on osteocytes and were independently confirmed by other investigators. PTH also acts on stromal/osteoblastic cells to stimulate the production of growth factors and J Musculoskelet Neuronal Interact 2006; 6(4):358-359

Effect of smoking and smoking cessation on bone mass, bone remodeling, vitamin D, PTH and sex hormones

Abstract: Objective To assess the effect of smoking and smoking cessation on bone density, bone remodeling markers, sex hormones, and vitamin D-PTH axis in healthy young subjects. Materials and methods We studied 74 healthy people (31 men, 43 women; mean age 32.2 (7) years) divided into 52 never smokers and 22 smokers, 15 of which stopped smoking for one month. Results Male smokers compared with never smokers showed lower BMD (0.971 (0.11) g/cm(2) vs. 1.069 (0.09) g/cm(2), P=0.042); higher plasma estrone levels (32.37 (10.13) pg/mL vs. 20.91 (5.46) pg/mL, P=0.001); and lower serum iPTH levels (16.2 (3.5) pg/mL vs. 28.8 (2.0) pg/mL, P=0.008). In women, BMD values were similar in smokers than in never smokers, but 25-hydroxyvitamin D levels were lower in smokers (31.9 (15.1) ng/mL vs. 16.8 (9.9) ng/mL, P=0.002). After adjusting by age and coffee consumption, female smokers had higher urinary-NTX levels than never smokers. After smoking cessation, statistically significant decreases of 25-hydroxyvitamin D and SHBG plasma levels were observed in men and women, respectively. Conclusions Tobacco increases bone resorption and affects bone mass by some alterations in sex hormone metabolism, but also importantly by alterations on the vitamin D-PTH axis.

Osteocytes as multifunctional cells

Abstract: Bone is often thought of as being a passive, inactive tissue like a skeleton hanging in the anatomy lab. Often, bone tissue is envisioned statically in terms of two dimensions similar to a ‘shapshot’ of a histology slide. However, quite to the contrary, bone undergoes considerable turnover as compared to other organs in the body. Modeling during growth is dramatic and even in adult bone, 2–5% turnover per year occurs in the long bone and 30% in alveolar bone. Any experimental approach can bias or have an effect on human interpretations of biological processes and events. Scientists should always be constantly asking how our perceptions are being modified by our experimental approaches. Bone biologists can easily visualize in vitro and in vivo the dynamic nature of osteoclasts with their resorption lacunae and rapid removal of bone, which occurs relatively rapidly in days. Osteoblasts are less dynamic, with new bone formation occurring in weeks. Many individuals still view the osteocyte as being a passive, inactive cell that merely acts as a ‘place holder’ in bone. Again, this perspective has most likely been perpetuated by histological approaches to the study of bone. Decades ago there were pioneers in the bone field who proposed that the osteocyte is not a passive cell, but a cell with the potential to have several functions. Credit is given to several of these pioneers below, while contrasting with most recent advances due to the availability of state of the art technology.

Increased osteogenic response to exercise in metaphyseal versus diaphyseal cortical bone

Abstract: Recent experimental data suggest that the anabolic response of bone to changes in physical activity and mechanical loading may vary among different skeletal elements, and even within different regions of the same bone. In order to better understand site-specific variation in bone modeling we used an experimental protocol in which locomotor activity was increased in laboratory mice with regular treadmill exercise for only 30 min/day. We predicted that the regular muscle contractions that occur during exercise would significantly increase cortical bone formation in these animals, and that the increase in cortical bone mass would vary between metaphyseal and diaphyseal regions. Cortical bone mass, density, and bone geometry were compared between these two regions using pQCT technology. Results indicate that exercise increases bone mineral content (BMC) in the mid-diaphysis by approximately 20%, whereas bone mass in the metaphyseal region is increased by approximately 35%. Endosteal and periosteal circumference at the midshaft are increased with exercise, whereas increased periosteal circumference is accompanied by marked endosteal contraction at the metaphysis, resulting in an increase in cortical area of more than 50%. These findings suggest that the osteogenic response of cortical bone to exercise varies significantly along the length of a bone, and more distal regions appear most likely to exhibit morphologic changes when loading conditions are altered.

Subchondral bone failure in overload arthrosis: a scanning electron microscopic study in horses.

Abstract: Mechanical overload leads to a common arthrosis in the metacarpal condyle of the fetlock joint of racehorses. This is usually asymptomatic but severe forms can cause lameness. Subchondral bone failure is often present and the predictability of the site provided an opportunity to study of the progression of bone failure from microcracks to actual collapse of subchondral bone. Twenty-five fetlock condyles from racehorses with various stages of disease were selected. Stages ranged from mild through severe subchondral bone sclerosis, to the collapse of bone and indentation or loss of cartilage known as 'traumatic osteochondrosis'. Parasagittal slices were radiographed and examined with scanning electron microscopy. Fine matrix cracks were seen in the subchondral bone layer above the calcified cartilage and suggested loss of water or other non-collagenous components. The earliest microcracks appeared to develop in the sclerotic bone within 1-3 mm of the calcified cartilage layer and extend parallel to it in irregular branching lines. Longer cracks or microfractures appeared to develop gaps as fragmentation occurred along the margins. Occasional osteoclastic resorption sites along the fracture lines indicated activated remodeling may have caused previous weakening. In one sample, smoothly ground fragments were found in a fracture gap. Bone collapse occurred when there was compaction of the fragmented matrix along the microfracture. Bone collapse and fracture lines through the calcified cartilage were associated with indentation of articular cartilage at the site.

Bone adaptation to altered loading after spinal cord injury: a study of bone and muscle strength.

Abstract: Bone loss from the paralysed limbs after spinal cord injury (SCI) is well documented. Under physiological conditions, bones are adapted to forces which mainly emerge from muscle pull. After spinal cord injury (SCI), muscles can no longer contract voluntarily and are merely activated during spasms. Based on the Ashworth scale, previous research has suggested that these spasms may mitigate bone losses. We therefore wished to assess muscle forces after SCI with a more direct measure and compare it to measures of bone strength. We hypothesized that the bones in SCI patients would be in relation to the loss of muscle forces. Six male patients with SCI 6.4 (SD 4.3) years earlier and 6 age-matched, able-bodied control subjects were investigated. Bone scans from the right knee were obtained by pQCT. The knee extensor muscles were electrically stimulated via the femoral nerve, isometric knee extension torque was measured and patellar tendon force was estimated. Tendon force upon electrical stimulation in the SCI group was 75% lower than in the control subjects (p<0.01). Volumetric bone mineral density of the patella and of the proximal tibia epiphysis were 50% lower in the SCI group than in the control subjects (p<0.01). Cortical area was lower by 43% in the SCI patients at the proximal tibia metaphysis, and by 33% at the distal femur metaphysis. No group differences were found in volumetric cortical density. Close curvilinear relationships were found between stress and volumetric density for the tibia epiphysis (r(2)=0.90) and for the patella (r(2)=0.91). A weaker correlation with the tendon force was found for the cortical area of the proximal tibia metaphysis (r(2)=0.63), and none for the distal femur metaphysis. These data suggest that, under steady state conditions after SCI, epiphyseal bones are well adapted to the muscular forces. For the metaphysis of the long bones, such an adaptation appears to be less evident. The reason for this remains unclear.

Differential bone and muscle recovery following hindlimb unloading in skeletally mature male rats.

Abstract: This study was designed to track the recovery of bone and muscle properties after 28 days of hindlimb unloading (HU) in skeletally mature male rats in order to quantify the degree and timing of the expected mismatch between bone and muscle properties. Outcome variables were in vivo plantarflexor peak isometric torque and proximal tibial volumetric bone mineral density (vBMD). Proximal tibia vBMD was significantly lower than age-matched controls (-7.8%) after 28 days of HU, continued to decrease through day 28 of recovery (-10%) and did not recover until day 84 of recovery. Plantarflexor peak isometric torque was significantly reduced after 28 days of HU (-13.9%). Further reductions of isometric torque occurred after 7 days of recovery (-15%), but returned to age-matched control levels by day 14. The functional relationship between bone and muscle (vBMD/isometric torque) tended to increase after 28 days of HU (+7.8%), remained elevated after 7 days of reloading (+9.1%) and was significantly lower than age-matched controls on day 28 (-13.6%). This relatively rapid return of muscle strength, coupled with continued depression of bone density at the proximal tibia metaphysis, may increase the risk for skeletal injury during recovery from prolonged periods of reduced mechanical loading.

Recent advances in the genetics of osteoporosis

Abstract: It has been known for over 20 years that osteoporosis is highly influenced by genetic factors. Bone mineral density (BMD) has also been shown to be highly heritable. Other known risk factors for osteoporotic fractures such as reduced bone quality, femoral neck geometry and bone turnover are now also known to be heritable. Susceptibility to osteoporosis is mediated, in all likelihood, by multiple genes each having small effect. Different approaches are being used currently to identify the many genes responsible. These include linkage studies in man and experimental animals as well as candidate gene studies and alterations in gene expression. Linkage studies have identified multiple quantitative trait loci (QTL) for regulation of BMD and, with twin studies, have indicated that the effects of these loci are partly site-dependent and sex-specific. On the whole, the genes responsible for BMD regulation at these QTL have not yet been isolated. Most studies have used the candidate gene approach. The vitamin D receptor gene (VDR), the collagen type I alpha 1 gene (COLIA1) and estrogen receptor gene (ER) alpha have been most widely investigated and found to play a role in regulating BMD, but the effects are modest and together probably account for less than 5% of the heritable contribution to BMD. Genes may vary in their influence of particular intermediate phenotypes, and we now know that not all genes influencing BMD will be important in fracture. In addition, the study of other diseases such as osteoarthritis and metabolic bone syndromes may prove fruitful in highlighting genes which overlap to osteoporosis as well. As large scale genetic testing becomes more cost-effective, recent findings have illustrated the potential of novel approaches. These include combining large multi-national populations for candidate gene analysis, metaanalyses, DNA pooling studies and gene expression studies.

Can exercise prevent osteoporosis

Abstract: Commonly used definitions of osteoporosis rely upon the measurement of bone mass or bone mineral density and regard the difference between osteopenia and osteoporosis as gradual. An alternative definition has been proposed by Harold Frost, suggesting that osteopenia is the bone’s physiological response to disuse. On the contrary, true osteoporoses imply the bone’s inability to adapt to the loads imposed on them by their habitual mechanical usage. As a consequence, fractures occur with no or very little trauma in osteoporotic, but not in osteopenic bones. There is now ample evidence that mechanical stimuli can increase strength. Accordingly, exercise, in particular some new forms of it that involve high strain rates, seems to be preventing bone loss and possibly also induces increases in bone mass even at older ages. Hence, exercise may ameliorate osteopenia in the sense of Frost’s definition. However, exercise must be feared to facilitate rather than to ameliorate the occurrence of true osteoporoses, e.g., due to microdamage accumulation. This is in sharp contrast to the general ‘understanding’.

Perspective: cerebral palsy as a model of bone development in the absence of postnatal mechanical factors.

Abstract: To ensure optimal skeletal development, mechanical loading is imperative. The consequences of the removal of, or complete absence of, mechanical loading are illustrated by the clinical condition of cerebral palsy (CP). Clinical and radiological evaluation of children with CP provides an insight into how the growing skeleton develops when mechanical loading is reduced due to non-physiological muscle function. The poor bone status or "physiologic osteopenia" that these children suffer is multifactorial compromised of both mechanical and non-mechanical effects; primarily it is the lack of normal loading from the musculature which causes the development of a bone incapable of withstanding daily activities. Fractures occur during daily activities such as dressing and handling. Increased bone resorption during periods of immobilisation after fracture or surgery, also increases bone fragility. Trials of physical, nutritional and pharmacological treatments in CP children result in increased bone mineral density. Trials that include fracture prevention as the primary end point are required in this vulnerable group of children.

Bone geometric response to chronic disuse following stroke: a pQCT study.

Abstract: The objectives of this study were to 1) assess volumetric bone geometry and density at the distal radius of individuals with chronic stroke and 2) assess whether bone strength is associated with measures of muscle strength and impairment Material and methods: Cross-sectional study of bone and muscle parameters in 15 community-dwelling people living with the residual effects of a stroke (between 1 and 9 years post-incident stroke) Results: The 4% site of the distal radius had significantly lower bone mineral content and density on the paretic side (p<0006) There was a significant difference in pQCT measures of bone cortical density (p<003), area (p<005) and bone strength [Stress-Strain Index; SSI] (p<001) (lower on the paretic side) at the 30% We found significant correlations between composite muscle strength score of the upper extremities and pQCT-generated bone strength Conclusions: This cross-sectional study highlights lower bone strength on the paretic limb and an adaptive response to disuse

Different changes of quantity due to aging in the psoas major and quadriceps femoris muscles in women.

Abstract: Bone fractures cause disabilities that leave the elderly bedridden and strengthening the muscles of the lower limbs, especially the quadriceps femoris, is the main kinematical method of preventing falls. Recently, however, it has become clear that the psoas major is critical for walking ability. We examined changes due to aging in the size of the psoas major compared with changes in the quadriceps femoris. Bone fractures are more frequent in women than in men; our participants (n=210) were therefore exclusively women ranging in age from 20 to 79 and divided into 6 age groups (n=35 each) in 10-year increments. Cross-sectional areas of the two muscles were measured by an MR scanner for a comparative estimation of muscle size. The psoas major showed the greatest quantity in subjects in their 20s, after which it declined steadily until the 60s and dramatically in the 70s, while the area of the quadriceps femoris was preserved until the 40s and showed no dramatic later decline. Exercise beyond regular daily activities is recommended to prevent the psoas major from decreasing in volume. We also recommend the development of a method of maintaining its muscle volume which would target women younger than 40 and older than 60.

What do we know about fracture risk in long-duration spaceflight?

Abstract: Summary The preponderance of evidence shows that, on average,crews of long-duration spaceflight appear to experiencesevere loss of bone mass, particularly in the lower skeleton.In the proximal femur, a recent study has shown that the lossof trabecular and cortical bone, taken together, may result inlosses of proximal femoral strength in the order of 15% fora six-month flight. These findings, taken together with dataon muscle loss and decreased postural stability in bed rest,indicate that the risk of fracture may increase due to bothdecreased bone strength and an increased risk of falling. Abetter understanding of whether the observed changes inbone strength and postural stability translate to increasedfracture risk will benefit from modeling of the loads associ-ated with physical activities and potential falls on the surfaceof the Moon and Mars. AcknowledgementsResearch was funded by NASA grants NNJ04HC7SA andNNJ04HF78G. References 1. Stupakov GP, Kazaykin VX, Kozlovsky AO, KorolevVV. Evaluation of changes in human axial skeletal bonestructures during long-term spaceflights.Kosmicheskaya Biologiya i AviakosmicheskayaMeditsina 1984; 18:33-37.2. Vogel JM, Whittle MW. Bone mineral changes; the sec-ond Skylab mission. Aviat Space Environ Med 1976;47:396-400.3. Whedon GD, Lutwak L, Reid J, Rambaut P, Whittle M,Smith M, Leach C. Mineral and nitrogen metabolic

Wnt signaling is involved in the inhibitory action of sclerostin on BMP-stimulated bone formation.

Abstract: 357 Sclerosteosis and van Buchem disease are two rare, closely related skeletal disorders characterized by a substantial increase in bone mass. Sclerosteosis is due to premature termination mutations in the SOST gene on chromosome 17q12-q21, whereas in van Buchem disease a 52 kb homozygous deletion downstream of the SOST gene was identified. The SOST gene encodes a protein, named sclerostin, of which the expression in the adult is highly restricted to osteocytes. In particular, sclerostin is localized in mature osteocytes in mineralized cortical and cancellous bone. Sclerostin expression is absent in the bones of both patients with sclerosteosis and van Buchem disease. Increase in bone mass is due to increased osteoblast activity as demonstrated by the predominance of cuboibal, activeappearing osteoblasts, increased double tetracycline label spacing, and increased osteoid levels that mineralize normally, in bone biopsies of affected individuals. Osteoclast numbers seem not to be affected. In vitro studies confirmed that sclerostin is a negative regulator of bone formation. Moreover, transgenic mice with overexpression of sclerostin are osteopenic. Sclerostin is a member of the Dan family of glycoproteins of which many members have been reported to antagonize the activity of several growth factors, including BMPs and Wnts. Sclerostin has been shown to inhibit BMP-stimulated bone formation in vitro. DNA-electroporation of the calf muscle of mice using expression plasmids for BMP and sclerostin showed that sclerostin also inhibited BMP-induced ectopic bone formation in vivo. The mechanism by which sclerostin inhibits BMP-stimulated bone formation is, however, unclear. Sclerostin has been reported to attenuate BMP-stimulated Smad phosphorylation, but were unable to find any antagonistic effect of sclerostin on direct BMP-induced responses like Smad phosphorylation and Smad-driven transcriptional reporter activation in several osteogenic cell lines, this in contrast to noggin and/or soluble BMPR. Transcriptional profiling analysis of osteoblastic cultures indicated that sclerostin specifically affects BMP and Wnt signaling out of many other growth signaling pathways.

Simvastatin did not prevent nor restore ovariectomy-induced bone loss in adult rats.

Abstract: Current published results on whether statins have beneficial effects on bone metabolism have been conflicting so far. In order to further investigate if statins were promising candidates for the treatment for osteoporosis, we conducted a study in which rats were ovariectomized (OVX) at 6 months of age, allowed to lose bone for 60 days and followed by oral administration of simvastatin at the dose levels of 0.3-10 mg/kg/d for 60 days. PGE2 (6 mg/kg) was used as a positive control. Study endpoints included bone histomorphometry on the proximal tibial metaphysis (PTM) and the tibial diaphysis (TX), dual-energy X-ray absorptiometry on the right femur and micro computed tomography (ICT) on the 5 th lumbar vertebra (LV). After 120 days of OVX, cancellous bone lost by 80% in the PTM and 18% in the LV accompanied by increased bone formation and resorption. Simvastatin at all dose levels did not affect bone volume, bone formation rate and bone erosion surface when compared to 120 day ovariectomized animals at all bone sites studied. By contrast, PGE2 restored cancellous and cortical bone area to sham control levels. In conclusion, this study demonstrated that unlike PGE2, oral administration of simvastatin did not have effects on cancellous or cortical bone formation and resorption; and consequently was not able to prevent further bone loss or restore bone mass in the osteopenic, OVX rats.

The search for human osteoporosis genes.

Abstract: Osteoporosis is the most prevalent metabolic bone disease and a major clinical and public health problem. Heredity plays an important and well-established role in determining the lifetime risk of this disease. Major efforts are currently underway to identify the specific genes and their allelic variations that contribute to the heritable component to osteoporosis. A number of laboratories are using quantitative trait locus (QTL) methods of genome scanning in families and animal models to identify candidate genomic regions and, ultimately, the genes and genetic variations that lead to osteoporosis. Several chromosomal regions of the human genome have now been linked to osteoporosis-related phenotypes. Although the specific genes contributing to the majority of these linkage signals have not been identified, two positional candidate genes have now been identified: low density lipoprotein receptor-related protein 5 (LRP5) and bone morphogenetic protein 2 (BMP2). A number of QTL has also been identified by cross-breeding strains of mice with variable bone density and several of these QTL have been fine mapped, providing a rich new base for understanding osteoporosis. Genetic association analyses have also provided evidence for a modest relationship between allelic variants in several biological candidate genes and bone mass and the risk of fracture. These ongoing animal and human studies will provide a continuing source of new insight into the genetic regulation of bone and mineral metabolism and the molecular etiology of osteoporosis. The new insight that will emerge from this ongoing research should lead to new ways of diagnosing, preventing and treating the growing clinical and public health problem of osteoporosis.

The muscle-bone unit in adulthood: influence of sex, height, age and gynecological history on the bone mineral content and muscle cross-sectional area.

Abstract: Bone and muscle development are both strongly influenced by sex hormones. The purpose of this study was to examine the changes in bone and muscle parameters (bone mineral content - BMC, muscle cross-sectional area - MA) in 130 men aged 31 -60 years, and in 180 pre-menopausal women aged 30-53 years with respect to age, body height and, with the women, their gynecological history (age-at-menarche, number of pregnancies, duration of lactation and use of oral contraception). The study was performed using peripheral quantitative computed tomography (pQCT) at a 65% site of the forearm length. Both BMC and MA were dependent on body height (p<0.0001), but not on age. The BMC/MA ratio was dependent neither on age nor on body height in both genders. MA as well as BMC were found significantly higher in males than in females (p<0.0001 for both variables). We observed a significantly higher BMC/MA ratio in females than in males (p<0.0001). We found no effect either of the analyzed variables of gynecological history on bone/muscle characteristics. The findings highlight the necessity of involving height-adjusted parameters and BMC/MA ratio into bone analysis in adults.

Effects of joint unloading and reloading on human cartilage morphology and function, muscle cross-sectional areas, and bone density - a quantitative case report.

Abstract: Recent studies have shown that thinning of human cartilage occurs with unloading, but no data are available on the effect of remobilization (after immobilization) on knee joint cartilage status in humans. We examined a 36-year-old patient after 6 weeks of unilateral immobilization. Knee joint cartilage morphology (patella and tibia), patellar cartilage deformation, and thigh muscle cross-sectional areas were assessed with quantitative MR imaging and bone density with peripheral quantitative computed tomography (pQCT) during 24 months of remobilization. The immobilized limb displayed lower muscle cross-sectional areas (MCSA) of the knee extensors (-36%), lower bone density of the femur and tibia (-12/-6%), lower patellar cartilage thickness (-14%), but no side differences of tibial cartilage thickness. During remobilization, side differences decreased to -4% for knee extensor MCSAs, to -6%/-3% for femoral and tibial BMD, and to -8% for patellar cartilage thickness. No change was observed in tibial cartilage. Patellar deformation decreased from 9% to 4% after 15 months. In conclusion, we observed substantial changes of thigh MCSAs, but little (patella) to no (tibia) change in cartilage thickness during remobilization. These preliminary results indicate that human cartilage macro-morphology may be less adaptive to variations of the mechanical loading than muscle and bone.

The importance of the elastic and plastic components of strain in tensile and compressive fatigue of human cortical bone in relation to orthopaedic biomechanics

Abstract: The longevity, success, or failure of an orthopaedic implant is dependent on its osseointegration especially within the initial six months of the initial surgery. The development of strains plays a crucial role in both bone modelling and remodelling. For remodelling, in particular, strains of substantial values are required to activate the osteoblastic and osteoclastic activity for the osseointegration of the implant. Bone, however, is subject to ‘damage’ when strain levels exceed a certain threshold level. Damage is manifested in the form of microcracks; it is linked to increased elastic strain amplitudes and is accompanied by the development of ‘plastic’ (irrecoverable, residual) strains. Such strains increase the likelihood for the implant to subside or loosen. The present study examines the rates (per cycle) by which these two components of strain (elastic and ‘plastic’) develop during fatigue cycling in two loading modes, tension and compression. The results of this study show that these strain rates depend on the applied stress in both loading modes. It also shows that elastic and plastic strain rates can be linked to each other through simple power law relationships so that one can calculate or predict the latter from the former and vice versa. We anticipate that such basic bone biomechanics data would be of great benefit to both clinicians and bioengineers working in the field of FEA modelling applications and orthopaedic implant surgery.