Canine cancellous bone microarchitecture after one year of high-dose bisphosphonates

Clodronate disodium does not produce measurable effects on bone metabolism in an exercising, juvenile, large animal model

Bisphosphonates are commonly used to treat and prevent bone loss, but their effects in active, juvenile populations are unknown. This study examined the effects of intramuscular clodronate disodium (CLO) on bone turnover, serum bone biomarkers (SBB), bone mineral density (BMD), bone microstructure, biomechanical testing (BT), and cartilage glycosaminoglycan content (GAG) over 165 days. Forty juvenile sheep (253 ± 6 days of age) were divided into four groups: Control (saline), T0 (0.6 mg/kg CLO on day 0), T84 (0.6 mg/kg CLO on day 84), and T0+84 (0.6 mg/kg CLO on days 0 and 84). Sheep were exercised 4 days/week and underwent physical and lameness examinations every 14 days. Blood samples were collected for SBB every 28 days. Microstructure and BMD were calculated from tuber coxae (TC) biopsies (days 84 and 165) and bone healing was assessed by examining the prior biopsy site. BT and GAG were evaluated postmortem. Data, except lameness data, were analyzed using a mixed-effects model; lameness data were analyzed as ordinal data using a cumulative logistic model. CLO did not have any measurable effects on the skeleton of sheep. SBB showed changes over time (p ≤ 0.03), with increases in bone formation and decreases in some bone resorption markers. TC biopsies showed increasing bone volume fraction, trabecular spacing and thickness, and reduced trabecular number on day 165 versus day 84 (p ≤ 0.04). These changes may be attributed to exercise or growth. The absence of a treatment effect may be explained by the lower CLO dose used in large animals compared to humans. Further research is needed to examine whether low doses of bisphosphonates may be used in active juvenile populations for analgesia without evidence of bone changes.

Denosumab treatment is associated with decreased cortical porosity and increased bone density and strength at the proximal humerus of ovariectomized cynomolgus monkeys

Upper extremity fractures, including those at the humerus, are common among women with postmenopausal osteoporosis. Denosumab was shown to reduce humeral fractures in this population; however, no clinical or preclinical studies have quantified the effects of denosumab on humerus bone mineral density or bone microarchitecture changes. This study used micro-computed tomography (μCT) and computed tomography (CT), alongside image-based finite element (FE) models derived from both modalities, to quantify the effects of denosomab (DMAb) and alendronate (ALN) on humeral bone from acutely ovariectomized (OVX) cynomolgus monkeys. Animals were treated with 12 monthly injections of s.c. vehicle (VEH; n = 10), s.c. denosumab (DMAb; 25 mg/kg, n = 9), or i.v. alendronate (ALN; 50 μg/kg, n = 10). Two more groups received 6 months of VEH followed by 6 months of DMAb (VEH-DMAb; n = 7) or 6 months of ALN followed by 6 months of DMAb (ALN-DMAb; n = 9). After treatment, humeri were harvested and μCT was used to quantify tissue mineral density, trabecular morphology, and cortical porosity at the humeral head. Clinical CT imaging was also used to quantify trabecular and cortical bone mineral density (BMD) at the ultra-proximal, proximal, 1/5 proximal and midshaft of the bone. Finally, μCT-based FE models in compression, and CT-based FE models in compression, torsion, and bending, were developed to estimate differences in strength. Compared to VEH, groups that received DMAb at any time demonstrated lower cortical porosity and/or higher tissue mineral density via μCT; no effects on trabecular morphology were observed. FE estimated strength based on μCT was higher after 12-months DMAb (p = 0.020) and ALN-DMAb (p = 0.024) vs. VEH; respectively, FE predicted mean (SD) strength was 4649.88 (710.58) N, and 4621.10 (1050.16) N vs. 3309.4 (876.09) N. All antiresorptive treatments were associated with higher cortical BMD via CT at the 1/5 proximal and midshaft of the humerus; however, no differences in CT-based FE predicted strength were observed. Overall, these results help to explain the observed reductions in humeral fracture rate following DMAb treatment in women with postmenopausal osteoporosis.

Effects of anti-resorptive treatment on the material properties of individual canine trabeculae in cyclic tensile tests

Osteoporosis is defined as a decrease of bone mass and strength, as well as an increase in fracture risk. It is conventionally treated with antiresorptive drugs, such as bisphosphonates (BPs) and selective estrogen receptor modulators (SERMs). Although both drug types successfully decrease the risk of bone fractures, their effect on bone mass and strength is different. For instance, BP treatment causes an increase of bone mass, stiffness and strength of whole bones, whereas SERM treatment causes only small (4%) increases of bone mass, but increased bone toughness. Such improved mechanical behavior of whole bones can be potentially related to the bone mass, bone structure or material changes. While bone mass and architecture have already been investigated previously, little is known about the mechanical behavior at the tissue/material level, especially of trabecular bone. As such, the goal of the work presented here was to fill this gap by performing cyclic tensile tests in a wet, close to physiologic environment of individual trabeculae retrieved from the vertebrae of beagle dogs treated with alendronate (a BP), raloxifene (a SERM) or without treatments. Identification of material properties was performed with a previously developed rheological model and of mechanical properties via fitting of envelope curves. Additionally, tissue mineral density (TMD) and microdamage formation were analyzed. Alendronate treatment resulted in a higher trabecular tissue stiffness and strength, associated with higher levels of TMD. In contrast, raloxifene treatment caused a higher trabecular toughness, pre-dominantly in the post-yield region. Microdamage formation during testing was not affected by either anti-resorptive treatment regimens. These findings highlight that the improved mechanical behavior of whole bones after anti-resorptive treatment is at least partly caused by improved material properties, with different mechanisms for alendronate and raloxifene. This study further shows the power of performing a mechanical characterization of trabecular bone at the level of individual trabeculae for better understanding of clinically relevant mechanical behavior of bone.

No Effect of Long-Term Risedronate Use on Cartilage and Subchondral Bone in an Experimental Rabbit Model of Osteoarthritis

Osteoarthritis (OA) is the most prevalent degenerative joint disease in animals and humans. It is characterized by pain, articular cartilage damage and joint stiffness. It has been suggested that the status of the subchondral bone compartment plays an important role in the initiation and progression of OA. Bisphosphonates have been proposed as a potential disease-modifying treatment for OA, however their effectiveness is not yet clear. Twenty-four male adult New Zealand rabbits were used to evaluate the effects of risedronate on the subchondral bone quality and cartilage degradation in a long-term model of experimentally induced OA. Animals underwent an anterior cruciate ligament transection and partial medial meniscectomy or sham operation in only one knee, which was randomly chosen, using the contralateral as healthy control. Animals were divided into three groups (n = 8): untreated control group and sham surgery control group; both groups received only vehicle; and risedronate group, treated with 2.5 mg orally weekly for 24 weeks. Stifle joints were harvested and scanned using a high-resolution micro-CT to evaluate the subchondral plate and trabecular bone changes. The macroscopic evaluation and histological analysis were determined using an adapted Osteoarthritis Research Society International scoring scheme to assess the cartilage degeneration. The lateral and medial femoral condyle and tibial plateau were evaluated. Additionally, the histological synovial membrane assessment was carried out. Sample analysis showed that the experimental model induced osteoarthritic changes in the operated joints, whereas in sham-operated rabbits, almost no histological changes were observed on articular cartilage surfaces. In terms of macroscopic and histological analyses, risedronate-treated animals did not show improved cartilage health compared with untreated operated rabbits, but a slightly anti-inflammatory activity was observed in the synovial membrane. Risedronate administration showed a slight tendency to increase subchondral bone plate thickness in lateral compartments but, it did not show conservation of periarticular bone and was not be able to suppress the osteophyte formation. In conclusion, long-term risedronate use did not demonstrate a positive effect on reducing the cartilage damage, and failed to prevent the subchondral bone changes and osteophytogenesis in an experimental rabbit model of OA.

Fifty years of bisphosphonates: What are their mechanical effects on bone?

After fifty years of experience with several generations of bisphosphonates (BPs), and 25 years after these drugs were approved for use in humans, their mechanical effects on bone are still not fully understood. Certainly, these drugs have transformed the treatment of osteoporosis in both men and women. There is no question that they do prevent fractures related to low bone mass, and there is widespread agreement that they increase strength and stiffness of the vertebrae. There is less consensus, however, about their effects on cortical bone, or on bone tissue properties in either trabecular or cortical bone, or their effects with longer periods of treatment. The consensus of most studies, both those based on ovariectomized and intact animal models and on testing of human bone, is that long-term treatment and/or high doses with certain BPs make the bone tissue more brittle and less tough. This translates into reduced energy to fracture and potentially a shorter bone fatigue life. Many studies have been done, but Interpretation of the results of these studies is complicated by variations in which BP is used, the animal model used, dose, duration, and methods of testing. Duration effects and effects on impact properties of bone are gaps that should be filled with additional testing.

Intrinsic material property differences in bone tissue from patients suffering low-trauma osteoporotic fractures, compared to matched non-fracturing women

Osteoporotic (low-trauma) fractures are a significant public health problem. Over 50% of women over 50yrs. of age will suffer an osteoporotic fracture in their remaining lifetimes. While current therapies reduce skeletal fracture risk by maintaining or increasing bone density, additional information is needed that includes the intrinsic material strength properties of bone tissue to help develop better treatments, since measurements of bone density account for no more than ~50% of fracture risk. The hypothesis tested here is that postmenopausal women who have sustained osteoporotic fractures have reduced bone quality, as indicated with measures of intrinsic material properties compared to those who have not fractured. Transiliac biopsies (N=120) were collected from fracturing (N=60, Cases) and non-fracturing postmenopausal women (N=60, age- and BMD-matched Controls) to measure intrinsic material properties using the nano-indentation technique. Each biopsy specimen was embedded in epoxy resin and then ground, polished and used for the nano-indentation testing. After calibration, multiple indentations were made using quasi-static (hardness, modulus) and dynamic (storage and loss moduli) testing protocols. Multiple indentations allowed the median and variance to be computed for each type of measurement for each specimen. Cases were found to have significantly lower median values for cortical hardness and indentation modulus. In addition, cases showed significantly less within-specimen variability in cortical modulus, cortical hardness, cortical storage modulus and trabecular hardness, and more within-specimen variability in trabecular loss modulus. Multivariate modeling indicated the presence of significant independent mechanical effects of cortical loss modulus, along with variability of cortical storage modulus, cortical loss modulus, and trabecular hardness. These results suggest mechanical heterogeneity of bone tissue may contribute to fracture resistance. Although the magnitudes of differences in the intrinsic properties were not overwhelming, this is the first comprehensive study to investigate, and compare the intrinsic properties of bone tissue in fracturing and non-fracturing postmenopausal women.

Bone Micro-CT Assessments in an Orchidectomised Rat Model Supplemented with Eurycoma longifolia

Recent studies suggested that Eurycoma longifolia, a herbal plant, may have the potential to treat osteoporosis in elderly male. This study aimed to determine the effects of Eurycoma longifolia supplementation on the trabecular bone microarchitecture of orchidectomised rats (androgen-deficient osteoporosis model). Forty-eight-aged (10–12 months old) Sprague Dawley rats were divided into six groups of sham-operated (SHAM), orchidectomised control (ORX), orchidectomised + 7 mg/rat testosterone enanthate (TEN) and orchidectomised + Eurycoma longifolia 30 mg/kg (EL30), orchidectomised + Eurycoma longifolia 60 mg/kg (EL60), orchidectomised + Eurycoma longifolia 90 mg/kg (EL90). Rats were euthanized following six weeks of treatment. The left femora were used to measure the trabecular bone microarchitecture using micro-CT. Orchidectomy significantly decreased connectivity density, trabecular bone volume, and trabecular number compared to the SHAM group. Testosterone replacement reversed all the orchidectomy-induced changes in the micro-CT parameters. EL at 30 and 60 mg/kg rat worsened the trabecular bone connectivity density and trabecular separation parameters of orchidectomised rats. EL at 90 mg/kg rat preserved the bone volume. High dose of EL (90 mg/kg) may have potential in preserving the bone microarchitecture of orchidectomised rats, but lower doses may further worsen the osteoporotic changes.

Bisphosphonate effects on bone turnover, microdamage, and mechanical properties: what we think we know and what we know that we don't know

The bisphosphonates (BPs) have been useful tools in our understanding of the role that bone remodeling plays in skeletal health. The purpose of this paper is to outline what we know, and what is still unknown, about the role that BPs play in modulating bone turnover, how this affects microdamage accumulation, and ultimately what the effects of these changes elicited by BPs are to the structural and the material biomechanical properties of the skeleton. We know that BPs suppress remodeling site-specifically, probably do not have a direct effect on formation, and that the individual BPs vary with respect to speed of onset, duration of effect and magnitude of suppression. However, we do not know if these differences are meaningful in a clinical sense, how much remodeling is sufficient, the optimal duration of treatment, or how long it takes to restore remodeling to pre-treatment levels following withdrawal. We also know that suppression is intimately tied to microdamage accumulation, which is also site-specific, that BPs impair targeted repair of damage, and that they can reduce the energy absorption capacity of bone at the tissue level. However, the BPs are clearly effective at preventing fracture, and generally increase bone mineral density and whole bone strength, so we do not know whether these changes in damage accumulation and repair, or the mechanical effects at the tissue level, are clinically meaningful. The mechanical effects of BPs on the fatigue life of bone, or BP effects on bone subject to an impact, are entirely unknown. This paper reviews the literature on these topics, and identifies gaps in knowledge that can be addressed with further research.

The effects of high-dose, long-term alendronate treatment on microarchitecture and bone mineral density of compact and trabecular bone in the proximal femur of adult male rabbits

INTRODUCTION

Despite the widespread use of bisphosphonates, its effects on normal bone microarchitecture of the proximal femur are still poorly studied. The purpose of this study was to determine the effects of long-term high-dose treatment of alendronate on microstructure and bone mineral density of cancellous, cortical compact and subchondral compact bone of the femoral head and neck region in normal adult male rabbits.

MATERIALS AND METHODS

Thirty-two adult, male rabbits were randomized into and were treated with either alendronate or placebo for 6 and 12 months. Micro-QCT measurements were taken in the (1) trabecular region, (2) cortical region of the femoral neck and (3) the subchondral region of the femoral head.

RESULTS

In the trabecular region of the femoral head, alendronate treatment significantly increased vBMD at 6 and 12 months (+21.0%, p < 0.05 and +26.8%, p < 0.05, respectively) and BVF (29.6%, p < 0.05 and 35.6%, p < 0.05, respectively) with significantly altered bone microarchitecture when compared with their placebo group; 6- and 12-month alendronate treatment significantly increased the vBMD and thickness and decreased the porosity of the subchondral bone in the femoral head.

CONCLUSION

High-dose alendronate treatment led to significant and differential changes in bone microarchitecture in trabecular, cortical and subchondral bone of the proximal femur of adult male rabbits.

Heritability of lumbar trabecular bone mechanical properties in baboons

Genetic effects on mechanical properties have been demonstrated in rodents, but not confirmed in primates. Our aim was to quantify the proportion of variation in vertebral trabecular bone mechanical properties that is due to the effects of genes. L3 vertebrae were collected from 110 females and 46 male baboons (6-32 years old) from a single extended pedigree. Cranio-caudally oriented trabecular bone cores were scanned with microCT then tested in monotonic compression to determine apparent ultimate stress, modulus, and toughness. Age and sex effects and heritability (h(2)) were assessed using maximum likelihood-based variance components methods. Additive effects of genes on residual trait variance were significant for ultimate stress (h(2)=0.58), toughness (h(2)=0.64), and BV/TV (h(2)=0.55). When BV/TV was accounted for, the residual variance in ultimate stress accounted for by the additive effects of genes was no longer significant. Toughness, however, showed evidence of a non-BV/TV-related genetic effect. Overall, maximum stress and modulus show strong genetic effects that are nearly entirely due to bone volume. Toughness shows strong genetic effects related to bone volume and shows additional genetic effects (accounting for 10% of the total trait variance) that are independent of bone volume. These results support continued use of bone volume as a focal trait to identify genes related to skeletal fragility, but also show that other focal traits related to toughness and variation in the organic component of bone matrix will enhance our ability to find additional genes that are particularly relevant to fatigue-related fractures.

Glucocorticoid induced osteopenia in cancellous bone of sheep: validation of large animal model for spine fusion and biomaterial research

STUDY DESIGN

Glucocorticoid with low calcium and phosphorus intake induces osteopenia in cancellous bone of sheep.

OBJECTIVE

To validate a large animal model for spine fusion and biomaterial research.

SUMMARY OF BACKGROUND DATA

A variety of ovariectomized animals has been used to study osteoporosis. Most experimental spine fusions were based on normal animals, and there is a great need for suitable large animal models with adequate bone size that closely resemble osteoporosis in humans.

METHODS

Eighteen female skeletal mature sheep were randomly allocated into 3 groups, 6 each. Group 1 (GC-1) received prednisolone (GC) treatment (0.60 mg/kg/day, 5 times weekly) for 7 months. Group 2 (GC-2) received the same treatment as GC-1 for 7 months followed by 3 months without treatment. Group 3 was left untreated and served as the controls. All sheep received restricted diet with low calcium and phosphorus during experiment. After killing the animals, cancellous bone specimens from the vertebra, femurs, and tibias were micro-CT scanned and tested mechanically. Serum biomarkers were determined.

RESULTS

In lumbar vertebra, the GC treatment resulted in significant decrease of cancellous bone volume fraction and trabecular thickness, and bone strength. However, the microarchitecture and bone strength of GC-2 recovered to a similar level of the controls. A similar trend of microarchitectural changes was also observed in the distal femur and proximal tibia of both GC treated sheep. The bone formation marker serum-osteocalcin was largely reduced in GC-1 compared to the controls, but recovered with a rebound increase at month 10 in GC-2.

CONCLUSION

The current investigation demonstrates that the changes in microarchitecture and mechanical properties were comparable with those observed in humans after long-term GC treatment. A prolonged GC treatment is needed for a long-term observation to keep osteopenic bone. This model resembles long-term glucocorticoid treated osteoporotic model, and is useful in preclinical studies.

Microarchitectural adaptations in aging and osteoarthrotic subchondral bone issues

The human skeleton optimizes its microarchitecture by elaborate adaptations to mechanical loading during development and growth. The mechanisms for adaptation involve a multistep process of cellular mechanotransduction stimulating bone modelling, and remodeling resulting in either bone formation or resorption. This process causes appropriate microarchitectural changes tending to adjust and improve the bone structure to its prevailing mechanical environment. Normal individual reaches peak bone mass at age between 25 and 30 years, and thereafter bone mass declines with age in both genders. The bone loss is accompanied by microarchitectural deterioration resulting in reduced mechanical strength likely leading to fragility fractures. With aging, inevitable bone loss occurs, which is frequently the cause of osteoporosis; and inevitable bone and joint degeneration happens, which often results in osteoarthrosis. These diseases are among the major health care problems in terms of socio-economic costs. The overall goals of the current series of studies were to investigate the age-related and osteoarthrosis (OA) related changes in the 3-D microarchitectural properties, mechanical properties, collagen and mineral quality of subchondral cancellous and cortical bone tissues. The studies included mainly two parts. For human subjects: aging- (I–IV) and early OArelated (V–VI) changes in cancellous bone properties were assessed. For OA guinea pig models (VII–IX), three topics were studied: firstly, the spontaneous, age-related development of guinea pig OA; secondly, the potential effects of hyaluronan on OA subchondral bone tissues; and thirdly, the effects on OA progression of an increase in subchondral bone density by inhibition of bone remodeling with a bisphosphonate. These investigations aimed to obtain more insight into the age-related and OA-related subchondral bone adaptations.

Anticipating bipedalism: trabecular organization in the newborn ilium

Trabecular bone structural organization is considered to be predominantly influenced by localized temporal forces which act to maintain and remodel the trabecular architecture into a biomechanically optimal configuration. In the adult pelvis, the most significant remodelling forces are believed to be those generated during bipedal locomotion. However, during the fetal and neonatal period the pelvic complex is non-weight bearing and, as such, structural organization of iliac trabecular bone cannot reflect direct stance-related forces. In this study, micro-computed tomography scans from 28 neonatal ilia were analysed, using a whole bone approach, to investigate the trabecular characteristics present within specific volumes of interest relevant to density gradients highlighted in a previous radiographic study. Analysis of the structural indices bone volume fraction, trabecular thickness, trabecular spacing and trabecular number was carried out to quantitatively investigate structural composition. Quantification of the neonatal trabecular structure reinforced radiographic observations by highlighting regions of significant architectural form which grossly parallel architectural differences in the adult pattern but which have previously been attributed to stance-related forces. It is suggested that the seemingly organized rudimentary scaffold observed in the neonatal ilium may be attributable to other non-weight bearing anatomical interactions or even to a predetermined genetic blueprint. It must also be postulated that whilst the observed patterning may be indicative of a predetermined inherent template, early non-weight bearing and late stance-related locomotive influences may subsequently be superimposed upon this scaffolding and perhaps reinforced and likely remodelled at a later age. Ultimately, the analysis of this fundamental primary pattern has core implications for understanding the earliest changes in pelvic trabecular architecture and provides a baseline insight into future ontogenetic development and bipedal capabilities.

Theoretical analysis of alendronate and risedronate effects on canine vertebral remodeling and microdamage

Bisphosphonates suppress bone remodeling activity, increase bone volume, and significantly reduce fracture risk in individuals with osteoporosis and other metabolic bone diseases. The objectives of the current study were to develop a mathematical model that simulates control and 1 year experimental results following bisphosphonate treatment (alendronate or risedronate) in the canine fourth lumbar vertebral body, validate the model by comparing simulation predictions to 3 year experimental results, and then use the model to predict potential long term effects of bisphosphonates on remodeling and microdamage accumulation. To investigate the effects of bisphosphonates on bone volume and microdamage, a mechanistic biological model was modified from previous versions to simulate remodeling in a representative volume of vertebral trabecular bone in dogs treated with various doses of alendronate or risedronate, including doses equivalent to those used for treatment of post-menopausal osteoporosis in humans. Bisphosphonates were assumed to affect remodeling by suppressing basic multicellular unit activation and reducing resorption area. Model simulation results for trabecular bone volume fraction, microdamage, and activation frequency following 1 year of bisphosphonate treatment are consistent with experimental measurements. The model predicts that trabecular bone volume initially increases rapidly with 1 year of bisphosphonate treatment, and continues to slowly rise between 1 and 3 years of treatment. The model also predicts that microdamage initially increases rapidly, 0.5-1.5-fold for alendronate or risedronate during the first year of treatment, and reaches its maximum value by 2.5 years before trending downward for all dosages. The model developed in this study suggests that increasing bone volume fraction with long term bisphosphonate treatment may sufficiently reduce strain and damage formation rate so that microdamage does not accumulate above that which is initiated in the first two years of treatment.

Effects of suppression of bone turnover on cortical and trabecular load sharing in the canine vertebral body

The relative biomechanical effects of antiresorptive treatment on cortical thickness vs. trabecular bone microarchitecture in the spine are not well understood. To address this, T-10 vertebral bodies were analyzed from skeletally mature female beagle dogs that had been treated with oral saline (n=8 control) or a high dose of oral risedronate (0.5mg/kg/day, n=9 RIS-suppressed) for 1 year. Two linearly elastic finite element models (36-mum voxel size) were generated for each vertebral body-a whole-vertebra model and a trabecular-compartment model-and subjected to uniform compressive loading. Tissue-level material properties were kept constant to isolate the effects of changes in microstructure alone. Suppression of bone turnover resulted in increased stiffness of the whole vertebra (20.9%, p=0.02) and the trabecular compartment (26.0%, p=0.01), while the computed stiffness of the cortical shell (difference between whole-vertebra and trabecular-compartment stiffnesses, 11.7%, p=0.15) was statistically unaltered. Regression analyses indicated subtle but significant changes in the relative structural roles of the cortical shell and the trabecular compartment. Despite higher average cortical shell thickness in RIS-suppressed vertebrae (23.1%, p=0.002), the maximum load taken by the shell for a given value of shell mass fraction was lower (p=0.005) for the RIS-suppressed group. Taken together, our results suggest that-in this canine model-the overall changes in the compressive stiffness of the vertebral body due to suppression of bone turnover were attributable more to the changes in the trabecular compartment than in the cortical shell. Such biomechanical studies provide an unique insight into higher-scale effects such as the biomechanical responses of the whole vertebra.

Effect of inappropriate and continuous therapy with alendronate for ten years on skeletal integrity - observations in two elderly patients

Alendronate is a potent aminobisphosphonate that has been used worldwide to decrease fracture risk in millions of post-menopausal women with and without osteoporosis, men with low bone mass, and in those with glucocorticoid- induced osteoporosis. A recent report of 9 patients with spontaneous atypical non-vertebral fractures during treatment with alendronate for up to 8 yr raised questions suggesting the possibility of severe suppression of bone turnover and resultant susceptibility to fracture. Our recent observations in 2 elderly women with inactive monostotic Paget's disease of bone who had been treated elsewhere continuously for this disease with alendronate for 10 yr at doses overall of 2 and 4 times the osteoporotic dose provided an opportunity to engage in the ongoing controversy over long-term safety of bisphosphonate therapy. Despite such therapy, skeletal integrity was maintained with normal bone densities and the absence of skeletal fractures. These observations do not support the suggestion of deleterious effects with longterm alendronate therapy.

Development of a low-dose anti-resorptive drug regimen reveals synergistic suppression of bone formation when coupled with disuse

Safe and effective countermeasures to spaceflight-induced osteoporosis are required to mitigate the potential for mission-critical fractures and ensure long-term bone health in astronauts. Two anti-resorptive drugs, the bisphosphonate zoledronic acid (ZOL) and the anti-receptor activator of NF-kappaB ligand protein osteoprotegerin (OPG), were investigated to find the minimum, comparable doses that yield a maximal increase in bone quality, while minimizing deleterious effects on turnover and mineralization. Through a series of five trials in normally loaded female mice (n = 56/trial), analysis of trabecular volume fraction and connectivity using microcomputed tomography, along with biomechanical testing, quantitative histomorphometry, and compositional analysis, was used to select 45 microg/kg ZOL and 500 microg/kg OPG as doses that satisfy these criteria. These doses were then examined for their ability to mitigate bone loss following short-term unloading through hindlimb suspension (HLS). Seventy-two mice were prophylactically administered ZOL, OPG, or PBS and assigned to loaded control or 2-wk HLS groups (n = 12 for each of 6 groups). Both anti-resorptives were able to preserve trabecular microarchitecture and femoral elastic and maximum force in HLS mice (+30-40% ZOL/OPG vs. PBS). In HLS mice, anti-resorptive dosing reduced resorption perimeter at the femoral endocortical surface by 30% vs. PBS. In loaded control mice, anti-resorptives produced no change in bone formation rate; however, reductions in bone formation rate brought about by HLS were exacerbated by anti-resorptive treatment, suggesting synergistic inhibition of osteoblasts during disuse. Refined anti-resorptive dosing will tend to target countermeasures to the period of disuse, resulting in faster recovery and less adverse effects for astronauts.

The effects of bone remodeling inhibition by alendronate on three-dimensional microarchitecture of subchondral bone tissues in guinea pig primary osteoarthrosis

We assessed whether increase of subchondral bone density enhances cartilage stress during impact loading, leading to progressive cartilage degeneration and accelerated osteoarthrosis (OA) progression. Sixty-six male guinea pigs were randomly divided into six groups. During a 9-week treatment period, four groups received twice-weekly subcutaneous injections of alendronate (ALN) in two doses: two groups received 10 microg/kg and two groups received 50 microg/kg. The two control groups received vehicle. After 9 weeks, one 10 microg/kg ALN group, one 50 microg/kg ALN group, and one control group were killed. The remaining three groups (17-week groups) were left for an additional 8 weeks, receiving the same treatment regimen before death. The left proximal tibiae were scanned by micro-computed tomography to quantify the microarchitecture of subchondral bone, followed by mechanical testing and determination of collagen and mineral. The control groups had typical OA-related cartilage degeneration at 9 and 17 weeks, whereas the 50 microg/kg ALN group had even worse degeneration in the medial condyle. It is unclear whether there is a direct or a secondary effect of ALN on the cartilage. The 9-week ALN group had significantly greater subchondral plate thickness. The 9- and 17-week groups had similar changes of cancellous bone microarchitecture, with greater volume fraction and connectivity and an extremely plate-like structure. The 9-week ALN group had greater bone mineral concentration, and the 17-week ALN group had reduced collagen concentration and greater mineral concentration. Treatment with ALN did not significantly change the mechanical properties of the cancellous bone.

Side-artifact errors in yield strength and elastic modulus for human trabecular bone and their dependence on bone volume fraction and anatomic site

In the context of reconciling the mechanical properties of trabecular bone measured from in vitro mechanical testing with the true in situ behavior, recent attention has focused on the "side-artifact" which results from interruption of the trabecular network along the sides of machined specimens. The objective of this study was to compare the magnitude of the side-artifact error for measurements of elastic modulus vs. yield stress and to determine the dependence of these errors on anatomic site and trabecular micro-architecture. Using a series of parametric variations on micro-CT-based finite element models of trabecular bone from the human vertebral body (n=24) and femoral neck (n=10), side-artifact correction factors were quantified as the ratio of the side-artifact-free apparent mechanical property to the corresponding property measured in a typical experiment. The mean (+/-SD) correction factors for yield stress were 1.32+/-0.17 vs. 1.20+/-0.11 for the vertebral body and femoral neck (p<0.05), respectively, and the corresponding factors for modulus were 1.24+/-0.09 vs. 1.10+/-0.04 (p<0.0001). Correction factors were greater for yield stress than modulus (p<0.003), but no anatomic site effect was detected (p>0.29) after accounting for variations in bone volume fraction (BV/TV). Approximately 30-55% of the variation in the correction factors for modulus and yield stress could be accounted for by BV/TV or micro-architecture, representing an appreciable systematic component of the error. Although some scatter in the correction factor-BV/TV relationships may confound accurate correction of modulus and yield stress for individual specimens, side-artifact correction is nonetheless essential for obtaining accurate mean estimates of modulus and yield stress for a cohort of specimens. We conclude that appreciation and correction for the differential effects of the side-artifact in modulus vs. yield stress and their dependence on BV/TV may improve the interpretation of measured elastic and failure properties for trabecular bone.

A computational assessment of the independent contribution of changes in canine trabecular bone volume fraction and microarchitecture to increased bone strength with suppression of bone turnover

This study addressed the effects of changes in trabecular microarchitecture induced by suppressed bone turnover-including changes to the remodeling space-on the trabecular bone strength-volume fraction characteristics independent of changes in tissue material properties. Twenty female beagle dogs, aged 1-2 years, were treated daily with either oral saline (n=10 control) or high doses of oral risedronate (0.5mg/kg/day, n=10 suppressed) for a period of 1 year, the latter designed (and confirmed) to substantially suppress bone turnover. High-resolution micro-CT-based finite element models (18-mum voxel size) of canine trabecular bone cores (n=2 per vertebral body) extracted from the T-10 vertebrae were analyzed in both compressive and torsional loading cases. The same tissue-level material properties were used in all models, thus providing measures of tissue-normalized strength due only to changes in the microarchitecture. Suppressed bone turnover resulted in more plate-like architecture with a thicker and more dense trabecular structure, but the relationship between the microarchitectural parameters and volume fraction was unaltered (p>0.05). Though the suppressed group had a greater tissue-normalized strength as compared to the control group (p<0.001) for both compressive and torsional loading, the relationship between tissue-normalized strength and volume fraction was not significantly altered for compression (p>0.13) or torsion (p>0.09). In this high-density, non-osteoporotic animal model, the increases in tissue-normalized strength seen with suppression of bone turnover were entirely commensurate with increases in bone volume fraction and thus, no evidence of microarchitecture-related or "stress-riser" effects which may disproportionately affect strength were found.

Alendronate prevents bone loss and improves tendon-to-bone repair strength in a canine model

Previously we showed a loss of bone and a concomitant decrease in mechanical properties in the first 21 days after flexor tendon insertion site injury and repair in a canine model. The goal of this short-term study was to suppress bone loss after insertion site repair using alendronate in an attempt to prevent the reduction in biomechanical properties. Flexor tendons of the second and fifth digits of the right forelimbs of canines were injured and repaired. Dogs received a daily oral dose of alendronate (2 mg/kg). One digit in each dog also received a local dose of alendronate in the bone tunnel at the time of surgery. The repair was evaluated for bone mineral density (BMD) and biomechanical properties and compared to data from a previous study in which no alendronate was used. Alendronate was effective in protecting the distal phalanx from resorption during tendon-to-bone healing (BMD was 94 and 104% of control for systemic alendronate and for systemic plus local alendronate, respectively). Alendronate treatment prevented much of the decrease in ultimate load that occurs in the first 21 days. Without treatment, ultimate load was 42% of control. With systemic alendronate treatment and systemic plus local alendronate treatment, ultimate load was 78 and 69% of control, respectively. Failure mode was significantly different when comparing alendronate treatment to repair alone. A lower incidence of suture pull through was found in alendronate treated dogs, suggesting less tendon degeneration. Ultimate load can be improved in association with preventing the bone loss that normally occurs during the early period following tendon-to-bone repair. These initial short-term data demonstrate the potential for a clinical treatment that could enhance tendon-to-bone healing.

A method for patient-specific evaluation of vertebral cancellous bone strength: in vitro validation

BACKGROUND

In the context of osteoporosis, important determinants of the fracture risk are the apparent strength and stiffness of cancellous bone, as well as its brittleness and energy absorption capacity. Standard medical imaging, however, cannot measure these mechanical properties directly. Consequently, an estimation of the risk for fracture is made by correlating relative density or mineral density at a skeletal site with statistics of fracture occurrence, which provides limited and partial indications on fracture risks. A better method for evaluating the patient-specific mechanical properties of cancellous bone is therefore required.

METHODS

In order to asses the mechanical properties of vertebral cancellous bone, we developed a finite element parametric model of lattice trabecular architecture that, in the future, will be suitable for use with bone imaging modalities. The model inputs are apparent morphological parameters (trabecular thickness and trabecular separation) and the bone mineral density. We conducted uniaxial compression tests on 36 canine vertebral cancellous bone specimens (C7 and L1) to validate model predictions of strength and stiffness in vitro.

FINDINGS

Predictions of strength and stiffness matched the experimental results within relative absolute errors of 17.7% and 12.8%, respectively (average of differences between model-predicted and measured values, divided by the average of measured values). We also employed the model for evaluation of strength and stiffness of human L1 and L5 vertebrae and found mean strength of 1.67 MPa (confidence interval 0.42 MPa) and mean elastic modulus of 190 MPa (confidence interval 50 MPa), which are well within the range of previously reported apparent strength and stiffness properties.

INTERPRETATION

The present model can be used to improve medical imaging-based evaluation of the spine in osteoporotic individuals by providing more specific information on the individual bone's susceptibility to fracture once clinical bone scans will be able to provide more reliable measures of trabecular thickness and separation.

A biomechanical perspective on bone quality

Observations that dual-energy X-ray absorptiometry (DXA) measures of areal bone mineral density cannot completely explain fracture incidence after anti-resorptive treatment have led to renewed interest in bone quality. Bone quality is a vague term but generally refers to the effects of skeletal factors that contribute to bone strength but are not accounted for by measures of bone mass. Because a clinical fracture is ultimately a mechanical event, it follows then that any clinically relevant modification of bone quality must change bone biomechanical performance relative to bone mass. In this perspective, we discuss a framework for assessing the clinically relevant effects of bone quality based on two general concepts: (1) the biomechanical effects of bone quality can be quantified from analysis of the relationship between bone mechanical performance and bone density; and (2) because of its hierarchical nature, biomechanical testing of bone at different physical scales (<1 mm, 1 mm, 1 cm, etc.) can be used to isolate the scale at which the most clinically relevant changes in bone quality occur. As an example, we review data regarding the relationship between the strength and density in excised specimens of trabecular bone and highlight the fact that it is not yet clear how this relationship changes during aging, osteoporosis development, and anti-resorptive treatment. Further study of new and existing data using this framework should provide insight into the role of bone quality in osteoporotic fracture risk.

The effects of side-artifacts on the elastic modulus of trabecular bone

Determining accurate density-mechanical property relationships for trabecular bone is critical for correct characterization of this important structure-function relation. When testing any excised specimen of trabecular bone, an unavoidable experimental artifact originates from the sides of the specimen where peripheral trabeculae lose their vertical load-bearing capacity due to interruption of connectivity, a phenomenon denoted here as the 'side-artifact'. We sought in this study to quantify the magnitude of such side-artifact errors in modulus measurement and to do so as a function of the trabecular architecture and specimen size. Using parametric computational analysis of high-resolution micro-CT-based finite-element models of cores of elderly human vertebral trabecular bone, a specimen-specific correction factor for the side-artifact was quantified as the ratio of the side-artifact-free apparent modulus (Etrue) to the apparent modulus that would be measured in a typical experiment (Emeasured). We found that the width over which the peripheral trabeculae were mostly unloaded was between 0.19 and 0.58 mm. The side-artifact led to an underestimation error in Etrue of over 50% in some specimens, having a mean (+/-SD) of 27+/-11%. There was a trend for the correction factor to linearly increase as volume fraction decreased (p=0.001) and as mean trabecular separation increased (p<0.001). Further analysis indicated that the error increased substantially as specimen size decreased. Two methods used for correcting for the side-artifact were both successful in bringing Emeasured into statistical agreement with Etrue. These findings have important implications for the interpretation of almost all literature data on trabecular bone mechanical properties since they indicate that such properties need to be adjusted to eliminate the substantial effects of side-artifacts in order to provide more accurate estimates of in situ behavior.

The roles of bone mineral density, bone turnover, and other properties in reducing fracture risk during antiresorptive therapy

Osteoporosis is a skeletal disorder characterized by compromised bone strength and increased risk of fracture. Properties related to bone strength include rate of bone turnover, bone mineral density, geometry, microarchitecture, and mean degree of mineralization. These properties (with or without bone density) are sometimes collectively referred to as bone quality. Antiresorptive agents may reduce fracture risk by several separate but interrelated effects on these individual properties. For example, antiresorptive agents have been reported to reduce bone turnover, stabilize or increase bone density, preserve or improve microarchitecture, reduce the number or size of resorption sites, and improve mineralization. Although changes in bone architecture and mineralization are not currently measurable in clinical practice, bone turnover is assessed easily in vivo and affects the other bone properties. Moreover, antiresorptive therapies that produce larger decreases in bone turnover markers together with larger increases in bone mineral density are associated with greater reductions in fracture risk, especially at sites primarily composed of cortical bone such as the hip. Reductions in fracture risk are the most convincing evidence of good bone quality. Data from well-designed randomized clinical trials with up to 10 years of continuous antiresorptive therapy have shown that certain antiresorptive agents effectively reduce fracture risk and (together with extensive preclinical data) suggest no deleterious effects on bone quality.

Noninvasive monitoring of changes in structural cancellous bone parameters with a novel prototype micro-CT

Characterization of trabecular bone structures requires necropsy of animals followed by a labor-intense histomorphometric or ex vivo micro-CT analysis. We tested the novel vivaCT40 from Scanco Medical AG (Bassersdorf, Switzerland), which allows monitoring such changes repeatedly in anesthetized rats and mice. Postmenopausal osteoporosis: in 8-month-old ovariectomized (OVX) rats, the vivaCT40 was capable of picking up the decrease in trabecular bone volume and trabecular thinning as well as the decrease in the number of trabecular elements as a function of time. The bone anabolic effects of parathyroid hormone [hPTH(1-34)], which resulted in an increase in trabecular thickness but not their number, as well as the bone protective effect of the two antiresorptive agents zoledronic acid (ZA) and 17-alpha ethinylestradiol (aEE), were detected correctly with the vivaCT40. Adjuvans arthritis: the vivaCT40 allowed measuring trabecular bone loss caused by periarticular inflammation in a rat model of adjuvans arthritis and demonstrated the bone protective effect of dexamethasone (DM). In addition, it was possible to image the subtle erosive lesions in subchondral bone caused by the inflammatory processes. Tumor osteolysis: the vivaCT40 allowed monitoring of the progressive osteolytic response following the local administration of 4T1luc2000 tumor cells into the tibia metaphysis of nude mice. The potent protective effect of ZA on tumor osteolysis was demonstrated. In summary, the new vivaCT40 can monitor the effects of known agents and diseases such as osteoporosis, inflammatory arthritis, and tumor invasion on 3-D trabecular microarchitecture accurately, repeatedly, reliably, and quickly in anesthetized rats and mice. The scanner represents a breakthrough for noninvasive imaging and structural measurements in small rodents.

Bisphosphonate treatment affects trabecular bone apparent modulus through micro-architecture rather than matrix properties

Bisphosphonates are emerging as an important treatment for osteoporosis. But whether the reduced fracture risk associated with bisphosphonate treatment is due to increased bone mass, improved trabecular architecture and/or increased secondary mineralization of the calcified matrix remains unclear. We examined the effects of bisphosphonates on both the trabecular architecture and matrix properties of canine trabecular bone. Thirty-six beagles were divided into a control group and two treatment groups, one receiving risedronate and the other alendronate at 5-6 times the clinical dose for osteoporosis treatment. After one year, the dogs were killed, and samples from the first lumbar vertebrae were examined using a combination of micro-computed tomography, finite element modeling, and mechanical testing. By combining these methods, we examined the treatment effects on the calcified matrix and trabecular architecture independently. Conventional histomorphometry and microdamage data were obtained from the second and third lumbar vertebrae of the same dogs [Bone 28 (2001) 524]. Bisphosphonate treatment resulted in an increased apparent Young's modulus, decreased bone turnover, increased calcified matrix density, and increased microdamage. We could not detect any change in the effective Young's modulus of the calcified matrix in the bisphosphonate treated groups. The observed increase in apparent Young's modulus was due to increased bone mass and altered trabecular architecture rather than changes in the calcified matrix modulus. We hypothesize that the expected increase in the Young's modulus of the calcified matrix due to the increased calcified matrix density was counteracted by the accumulation of microdamage.

Efficacy and safety of long-term bisphosphonates in postmenopausal osteoporosis

Bisphosphonates have been in clinical use since the first approval of etidronate for myositis ossficans progressiva. They have now been used since the early 1990s for osteoporosis. As patients are continued on bisphosphonates and with consideration for bisphosphonate application in younger postmenopausal women, questions of 'how long to treat' are emerging in the clinical community. What is the evidence for continual efficacy and safety? The longest efficacy data where a placebo group was maintained is the 5-year risedronate vertebral fracture data, which demonstrated additional fracture reduction benefit with this bisphosphonate during years 4-5 of use. As bone mineral density (BMD) or biochemical markers of bone resorption (BCM) change very little for at least 1-2 years (and possibly longer) after discontinuing long-term (3-5 years) bisphosphonate use, it seems reasonable to suggest discontinuation for some indefinite period of time after 5 years of use in younger lower-risk postmenopausal women. Monitoring of both BMD and BCM may indicate when to consider reinitiation of bisphosphonate therapy. In higher-risk elderly postmenopausal women who are doing well on long-term bisphosphonate therapy as defined by a stable BMD, height and no incident fractures, the same discussion of a 'honeymoon' period of no treatment could be entertained. However, because there are no data on fracture events post-treatment even without changing BMD or BCM, this author is reluctant to stop bisphosphonates in elderly high-risk patients. My opinion on the continuation of the use in high-risk patients is also predicated on the observations that there does not appear to be any clinically important safety issues with long-term (10-year) use. Concerns about 'oversuppression' of bone turnover and accumulation of microdamage are theoretical and, so far, have no clinical basis for discontinuation in high-risk patients. It will be important, however, for the continual surveillance of bone safety issues by capturing postmarketing fracture data in patients on long-term therapy as well as long-term bone histomorphometry and measurements of bone quality to reassure clinicans about long-term bisphosphonate continuation.

Bone microarchitecture as an important determinant of bone strength

Structure and microarchitecture are determinant aspects of bone strength and essential elements for the assessment of bone mechanical properties. The main structural determinants of bone mechanical strength include width and porosity in the cortical bone; shape, width, connectivity, and anisotropy in the trabecular bone. There are several methods to assess bone architecture, particularly at the trabecular level. Two different approaches can be identified. The first is based on the use of optical microscopy and on the principles of quantitative histology, which evaluate microarchitecture two-dimensionally. The second applies the most modern diagnostic techniques, employing computed tomography and magnetic resonance to obtain and analyze three-dimensional images. From a clinical point of view, microarchitecture is an interesting aspect to study and define specific patterns, such as glucocorticoid-induced osteoporosis, or to evaluate bone alterations in transplanted patients. Microarchitecture seems to be a determinant of bone fragility independent of bone density. Moreover, bone microarchitecture seems to be important to understand the mechanisms of bone fragility as well as the action of the drugs used to prevent osteoporotic fractures. Several in vivo studies (on animals and humans) showed important findings on the effects of different treatments on microarchitecture. Bisphosphonates and parathyroid hormone seemed to preserve or even improve microarchitecture. These observations can provide an additional interpretation for the anti-fracture effect of drugs from a structural viewpoint. The challenge for the future will be to evaluate bone quality in vivo with the same or better resolution and accuracy than the invasive methods in use today.