Risedronate preserves bone architecture in postmenopausal women with osteoporosis as measured by three-dimensional microcomputed tomography

Human cancellous bone from T12-L1 vertebrae has unique microstructural and trabecular shear stress properties

Increase of trabecular stress variability with loss of bone mass has been implicated as a mechanism for increased cancellous bone fragility with age and disease. In the current study, a previous observation that trabecular shear stress estimates vary along the human spine such that the cancellous tissue from the thoracic 12 (T12)-lumbar 1 (L1) junction experiences the highest trabecular stresses for a given load was tested as a formal hypothesis using multiple human spines. Thoracic 4, T5, T7, T9, T10, T12, L1, L2, L4 and L5 vertebrae from 10 human cadaver spines were examined. One specimen in the central anterior region was cored in the supero-inferior (SI) direction and another in the postero-lateral region was cored in the transverse (TR) direction from each vertebra. Micro-CT-based large-scale finite element models were constructed for each specimen and compression in the long axis of the cylindrical specimens was simulated. Cancellous bone modulus and the mean, the standard deviation, variability and amplification of trabecular von Mises stresses were computed. Bone volume fraction, trabecular number, trabecular thickness, trabecular separation, connectivity density and degree of anisotropy were calculated using 3D stereology. The results were analyzed using a mixed model in which spine level was modeled using a quadratic polynomial. The maximum of trabecular shear stress amplification and minimum of bone volume fraction were found in the cancellous tissue from the T12-L1 location when results from the samples of the same vertebra were averaged. When groups were separated, microstructure and trabecular stresses varied with spine level, extrema being at the T12-L1 levels, for the TR specimens only. SI/TR ratio of measured parameters also had quadratic relationships with spine level, the extrema being located at T12-L1 levels for most parameters. For microstructural parameters, these ratios approached to a value of one at the T12-L1 level, suggesting that T12-L1 vertebrae have more uniform cancellous tissue properties than other levels. The mean intercept length in the secondary principal direction of trabecular orientation could account for the variation of all mechanical parameters with spine level. Our results support that cancellous tissue from T12-L1 levels is unique and may explain, in part, the higher incidence of vertebral fractures at these levels.

Comparison of quantitative cancellous bone connectivity analyses at two- and three-dimensional levels in dialysis patients

Assessment of cancellous bone connectivity has the potential to aid in predicting fracture risk. Today, cancellous bone connectivity is generally assessed using bone sections obtained from biopsy. However, how reliably such two-dimensional (2-D) analyses visualize the 3-D properties has not been evaluated. Biopsied iliac bone samples were obtained from 47 chronic hemodialysis patients. Bone samples were observed using a microfocus X-ray computed tomography (microCT) system en bloc, and the cancellous bone microstructure was quantitatively assessed at both the 2- and 3-D levels. Cancellous bone microarchitecture was successfully reconstructed from the data obtained by the microCT system. Most of the results from node-strut analysis (NSA) revealed no statistically significant correlations between the 2- and 3-D analyses, with the exception that the number of nodes (N.Nd/TV) showed a mild but significant correlation. In contrast, the marrow space star volumes (V*m) of the 2- and 3-D analyses were highly correlated. NSA parameters including N.Nd/TV showed significant correlations with V*m at the 3-D level. In conclusion, V*m values were similar in the 2- and 3-D analyses, while most of the 2-D NSA parameters did not reflect the 3-D ones. Since V*m and most of the NSA parameters were correlated in the 3-D analyses, 2-D NSA would seem to have serious limitations for the assessment of cancellous bone microstructural properties. Further studies will thus be needed to establish appropriate methods for assessing cancellous bone connectivity in clinical practice.

Biochemical markers of bone turnover: potential use in the investigation and management of postmenopausal osteoporosis

INTRODUCTION

The aim was to analyse data on the use of biochemical bone turnover markers (BTM) in postmenopausal osteoporosis.

METHODS

We carried out a comparative analysis of the most important papers concerning BTM in postmenopausal osteoporosis that have been published recently.

RESULTS

The BTM levels are influenced by several factors. They are moderately correlated with BMD and subsequent bone loss. Increased levels of bone resorption markers are associated with a higher risk of fracture. Changes in the BTM during the anti-osteoporotic treatment (including combination therapy) reflect the mechanisms of action of the drugs and help to establish their effective doses. Changes in the BTM during the anti-resorptive treatment are correlated with their anti-fracture efficacy.

CONCLUSION

Biological samples should be obtained in a standardised way. BTM cannot be used for prediction of the accelerated bone loss at the level of the individual. BTM help to detect postmenopausal women who are at high risk of fracture; however, adequate practical guidelines are lacking. BTM measurements taken during the anti-resorptive therapy help to identify non-compliers. They may improve adherence to the anti-resorptive therapy and the fall in the BTM levels that exceeds the predefined threshold improves patients' persistence with the treatment. There are no guidelines concerning the use of BTM in monitoring anti-osteoporotic therapy in postmenopausal women.

Effect of temporal changes in bone turnover on the bone mineralization density distribution: a computer simulation study

The heterogeneous distribution of mineral content in trabecular bone reflects the continuous renewal of bone material in bone remodeling and the subsequent increase in mineral content in the newly formed bone packets. The bone mineralization density distribution (BMDD) is typically used to describe this nonuniform mineral content of the bone matrix. Our mathematical model describes changes of the BMDD of trabecular bone as a function of bone resorption and deposition rates and the mineralization kinetics in a newly formed bone packet. Input parameters used in the simulations were taken from experimental studies. The simulations of the time evolution of the BMDD after increase in bone turnover (perimenopausal period) resulted in a shift of the BMDD toward lower values of the mineral content. Transiently, there was a broadening of the BMDD configuration partly showing two peaks, which points to a strongly heterogeneous distribution of the mineral. Conversely, when the remodeling rate was reduced (antiresorptive therapy), the BMDD shifted toward higher values of the mineral content. There was a transient narrowing of the distribution before broadening again to reach the new steady state. Results from this latter simulation are in good agreement with measurements of the BMDD of patients after 3 and 5 yr of treatment with risedronate. Based on available experimental data on bone remodeling, this model gives reliable predictions of changes in BMDD, an important factor of bone material quality. With the availability of medications with a known effect on bone turnover, this knowledge opens the possibility for therapeutic manipulation of the BMDD.

Trabecular bone structure analysis in the limited spatial resolution regime of in vivo MRI

RATIONALE AND OBJECTIVES

To develop a method for processing and visualization of trabecular bone networks on the basis of magnetic resonance (MR) images acquired in the limited spatial resolution regime of in vivo imaging at which trabecular thickness is comparable to voxel size.

MATERIALS AND METHODS

A sequence of processing steps for analyzing the topologic structure of trabecular bone networks is presented and evaluated using three types of datasets: images of synthetic structures with various levels of superimposed Gaussian noise, micro-computed tomographic images of human trabecular bone downsampled to in vivo resolution, and in vivo micro-MR images from a prior longitudinal study investigating the structural implications of testosterone treatment of hypogonadal men. The simulated images were analyzed at a voxel size of 150 microm(3), the clinical MR image data had been acquired with 137 x 137 x 410 microm(3) voxel size. The technique is a modification to the virtual bone biopsy processing chain that involves a sinc convolution step immediately preceding binarization, and employs the Manzanera-Bernard thinning algorithm for obtaining the three-dimensional skeleton before topologic classification. The detectability of plate and rod bone elements was also analyzed theoretically.

RESULTS

As compared with previously published techniques, the approach produced a more accurate bone skeleton in the micro-computed tomographic and simulation experiments, with clear improvement in preservation of rod and plate elements. Simulations suggest that rods are detectable down to a diameter of approximately 50% of the MR image voxel length, whereas plates can be detected at thicknesses of 20% or more of voxel length. For in vivo studies, it was shown that the method could recover the treatment response in terms of the ensuing topologic changes in patients undergoing antiresorptive treatment.

CONCLUSIONS

The algorithm for processing of in vivo micro-MR images of trabecular bone is superior to prior approaches in preserving the topology of the network in the presence of noise.

Bone density after teriparatide in patients with or without prior antiresorptive treatment: one-year results from the EUROFORS study

OBJECTIVE

Recombinant teriparatide, a bone anabolic agent, is given to treatment-naïve and pre-treated patients with severe osteoporosis, but few data exist comparing the response to teriparatide in these groups. EUROFORS (the EUROpean study of FORSteo‡) enrolled postmenopausal women with established osteoporosis who were either treatment-naïve or had prior antiresorptive (AR) treatment with or without documented inadequate clinical response. The objective of the secondary analysis described here was to evaluate the interim bone mineral density (BMD) response in these groups after one year of open-label teriparatide therapy.

RESEARCH DESIGN AND METHODS

Postmenopausal women with established osteoporosis who enrolled in a prospective, randomized, controlled trial received open-label teriparatide 20 µg/day for the first year. With respect to their prior osteoporosis treatment history, they were retrospectively allocated to one of three groups: treatment-naïve (n = 204), prior treatment with an antiresorptive drug (AR-pretreated) (n = 240), or prior antiresorptive treatment with inadequate response (inadequate AR-responders) (n = 421). BMD was measured by dual energy x-ray absorptiometry.

RESULTS

Lumbar spine BMD increased from baseline (p < 0.001) in the three groups (mean, 95% CI); treatment-naïve: 8.4% (7.4%, 9.3%); AR-pretreated: 7.1% (6.3%, 7.9%); inadequate AR-responders: 6.2% (5.6%, 6.9%). Total hip BMD increased from baseline in the treatment-naïve (p < 0.001): 1.8% (1.1%, 2.5%) but did not change in the AR-pretreated: 0.4% (-0.2%, 1.1%) or inadequate AR-responders: -0.3% (-0.9%, 0.2%). Treatment-emergent adverse events were similar in the three groups.

CONCLUSION

One year of teriparatide significantly (p < 0.001) increased spine BMD in all groups, and total hip BMD in the treatment-naïve group. Because of the limitations of this interim analysis (most importantly, the short duration of treatment and lack of a control group), further study is needed to determine the optimal treatment duration to reach the potential BMD gains at the proximal femur in patients with prior antiresorptive drug use (mostly bisphosphonates).

CLINICAL TRIAL REGISTRATION

clinicaltrials.gov, nct00191425.

Influence of early and late zoledronic acid administration on vertebral structure and strength in ovariectomized rats

An annual infusion of zoledronic acid (ZOL) reduces fracture risk in osteoporotic patients. Previously, we showed that a single ZOL injection inhibited changes in bone microstructure and strength in rat tibiae after ovariectomy. Here, we determined the effects of a single ZOL injection as preventive and restorative treatment on the bone microstructure and strength in lumbar and caudal vertebrae of ovariectomized (OVX) rats. Twenty-nine female 35-week-old Wistar rats were divided into four groups: SHAM-OVX (n = 9), OVX (n = 5), OVX and early ZOL (n = 8), and OVX and late ZOL (n = 7). ZOL was given once (20 microg/kg body weight s.c.) at OVX in the early ZOL group and 8 weeks later in the late ZOL group; rats were killed 16 weeks after OVX. Trabecular and cortical bone microarchitecture were measured in lumbar (L3) and caudal (Cd6) vertebrae using micro-computed tomography, and compressive mechanical properties were determined in L3 vertebrae. Compared to SHAM-OVX, OVX rats had significantly lower BV/TV; SMI, Tb.N, Tb.Sp, and Conn.D tended to be deteriorated in lumbar vertebrae, while both ZOL groups did not differ from the SHAM-OVX group. Both ZOL groups had significantly higher BV/TV than OVX; the early ZOL group also had significantly lower SMI and higher Tb.Th. OVX tended to decrease mechanical properties, while early and late ZOL treatment inhibited OVX-induced degeneration. Neither OVX nor ZOL induced changes in the trabecular microarchitecture of caudal vertebrae. In summary, in adult rats a single ZOL injection inhibited OVX-induced changes in lumbar vertebral bone microarchitecture and strength.

Treatment of osteoporosis: all the questions we still cannot answer

Many questions remain with regard to the treatment of osteoporosis. The increase in bone mineral density due to bisphosphonates accounts for only a quarter of their antifracture action. The studies of bone mineral density equivalences that have validated certain modes of administration are therefore not relevant. No comparative study on the same patient cohort has been carried out, and the inclusion criteria of the pilot studies on the drugs currently available are all different. This makes any comparison impossible, and thus, the choice of treatment is subjective. Certain combinations (antiresorptive drugs and anabolic agents) are logical, but no major study is available to validate them. Drugs with a long latency period could interfere with later treatments or allow therapeutic windows, but the data in the literature in support of these hypotheses are scarce. The osteoporotic patient will require treatment for 10 to 30 years, depending on the age at diagnosis, but the available data on the efficacy and nontoxicity of the drugs are based on studies of a duration of only 3 or 5 years.

Side-to-side and within-side variability of 3D bone microarchitecture by conventional micro-computed tomography of paired iliac crest biopsies

Bone microarchitecture in osteoporosis can be characterized by examining iliac bone biopsies and treatment effects assessed by comparing a baseline biopsy from one side to a posttreatment biopsy from the other side, a method that assumes limited side-to-side variability. New techniques based on micro-computed tomography (microCT) provide information on the three-dimensional (3D) microarchitecture of bone. We used microCT to measure side-to-side and within-side variability of 3D microarchitectural parameters of trabecular and cortical bone in paired iliac-crest biopsies, one from each side. A Bordier needle trephine was used to collect biopsies from 30 postmenopausal female cadavers (mean age, 73.7+/-10.7 years; range, 55-96 years). Biopsies were chemically defatted then imaged using a desktop microCT scanner (voxel size, 10.77 microm). Parameters measured in trabecular bone consisted of bone volume/tissue volume (BV/TV, %), direct trabecular thickness and trabecular spacing (Tb.Th and Tb.Sp, microm) using the sphere method, bone surface/bone volume (BS/BV, mm(-1)), trabecular number (Tb.N, mm(-1)), structure model index (SMI), trabecular pattern factor (Tb.Pf), and degree of anisotropy (DA). In cortical bone, we measured cortical thickness (Cort.Th), porosity (Cort.Porosity), and pore diameter (Po.Dm). For trabecular bone parameters, reproducibility as assessed from two microCT acquisitions ranged from 4.1% to 6.9%. To assess side-to-side variability, we matched the volumes of interest selected in the right and left iliac crests. The mean difference in absolute individual percent variation (mAbsDelta(ind)) between the two sides ranged from 10.8% to 14.8% for all trabecular parameters except Tb.Pf (74%) and SMI (84%). In cortical bone, mAbsDelta(ind) were 11.6% for Po.Dm, 15.1% for Cort.Porosity, and 27.6% for Cort.Th. To assess within-side variability, we divided the trabecular iliac crest volume into three equal parts, one adjacent to each cortex and one in the middle. Values of mAbsDelta(ind) versus the middle part were ranging from 7.6% for Tb.Sp to 26.2% for BV/TV. Thus, within-side variability was similar in magnitude to side-to-side variability. The considerable differences in robustness across trabecular parameters indicate a need for selecting the most stable parameters, most notably for longitudinal studies of small numbers of patients. Acquisition by microCT and image analysis must comply with stringent quality criteria, especially the distance from the cortices must be standardized.

Differential effects of bone structural and material properties on bone competence in C57BL/6 and C3H/He inbred strains of mice

The femoral neck is a relevant and sensitive site for studying the degree of osteopenia. Engineering principles predict that bone structural parameters, like cross-sectional geometry, are important determinants of bone mechanical parameters. Mechanical parameters are also directly affected by the material properties of the bone tissue. However, the relative importance of structural and material properties is still unknown. The aim of this study was to compare bone competence and structural parameters between a murine strain showing a low bone mass phenotype, C57BL/6 (B6), and another one showing a high bone mass phenotype, C3H/He (C3H), in order to better determine the role of bone structure and geometry in bone failure behavior. Murine femora of 12- and 16-week-old B6 and 12- and 16-week-old C3H inbred strains were mechanically tested under axial loading of the femoral head. In order to assess the structural properties, we performed three-dimensional morphometric analyses in five different compartments of the mouse femur using micro-computed tomography. The mechanical tests revealed that B6 femora became stiffer, stronger, and tougher at 12-16 weeks, while bone brittleness stayed constant. C3H bone stiffness increased, but strength remained constant, work to failure decreased, and bone became more brittle. These age effects indicated that B6 did not reach peak bone properties at 16 weeks of age and C3H did reach maximal skeletal biomechanical properties before 16 weeks of age. Our investigations showed that 83% of the strength of the femoral neck in the B6 strain was explained by cortical thickness at this location; in contrast, in C3H none of the mechanical properties of the femoral neck was explained by bone structural parameters. The relative contributions of bone structural and material properties on bone strength are different in B6 and C3H. We hypothesize that these different contributions are related to differences at the ultrastructural level of bone that affect bone failure.

Effects of risedronate on fracture risk in postmenopausal women with osteopenia

This posthoc analysis of four trials studied the efficacy of risedronate to reduce fragility fractures in postmenopausal women with osteopenia (i.e., T-scores between -1 and -2.5). Risedronate reduced the fracture risk by 73% (p = 0.023) in this population of women with low femoral neck bone mass and no prevalent vertebral fractures.

INTRODUCTION

Low bone mass represents an increasing health risk and burden. Half of fragility fractures occur in osteopenic women underscoring the need for treatments reducing fracture risk. This analysis reports the effect of risedronate to reduce fragility fracture risk in osteopenic women without prevalent vertebral fractures.

METHODS

Postmenopausal women with osteopenia, defined as femoral neck T-score between -1 and -2.5 by DXA and no prevalent vertebral fractures, were identified from four controlled randomized trials (BMD Multinational, BMD North America, VERT Multinational and VERT North America). The risk reduction for fragility fractures in patients receiving 5 mg risedronate daily for 1.5 to 3 years compared to placebo was assessed. An additional sensitivity analysis excluded patients who were osteopenic at the femoral neck but had a BMD lower than -2.5 SD at the lumbar spine.

RESULTS

Six hundred and twenty postmenopausal women with osteopenia were included, receiving either placebo (n = 309) or risedronate 5 mg (n = 311). Risedronate reduced the risk of fragility fractures by 73% over 3 years versus placebo (p = 0.023); cumulative fragility fracture incidence was 6.9% in placebo-treated versus 2.2% in risedronate-treated patients. The magnitude of the effect was similar in the sensitivity analysis subset.

CONCLUSION

Risedronate significantly reduced the risk of fragility fractures in postmenopausal women with osteopenia (femoral neck T-score between -1 and -2.5 SD) and no prevalent vertebral fractures.

In vivo determination of bone structure in postmenopausal women: a comparison of HR-pQCT and high-field MR imaging

Bone structural measures obtained by two noninvasive imaging tools-3T MRI and HR-pQCT-were compared. Significant but moderate correlations and 2- to 4-fold discrepancies in parameter values were detected, suggesting that differences in acquisition and analysis must be considered when interpreting data from these imaging modalities.

INTRODUCTION

High-field MRI and high resolution (HR)-pQCT are currently being used in longitudinal bone structure studies. Substantial differences in acquisition and analysis between these modalities may influence the quantitative data produced and could potentially influence clinical decisions based on their results. Our goal was to compare trabecular and cortical bone structural measures obtained in vivo by 3T MRI and HR-pQCT.

MATERIALS AND METHODS

Postmenopausal osteopenic women (n = 52) were recruited for this study. HR-pQCT imaging of the radius and tibia was performed using the XtremeCT scanner, with a voxel size of 82 x 82 x 82 microm(3). MR imaging was performed on a 3T Signa scanner using SSFP imaging sequences, with a pixel size of 156 x 156 microm(2) and slice thickness of 500 microm. Structure parameters were calculated using standard HR-pQCT and MRI analysis techniques. Relationships between measures derived from HR-pQCT, MRI, and DXA were studied.

RESULTS

Significant correlations between HR-pQCT and MRI parameters were found (p < 0.0001) and were strongest for Tb.N (r(2) = 0.52), Ct.Th (r(2) = 0.59), and site-specific Tb.Sp (r(2) = 0.54-0.60). MRI and HR-pQCT provided statistically different values of structure parameters (p < 0.0001), with BV/TV and Tb.Th exhibiting the largest discrepancies (MR/HR-pQCT = 3-4). Although differences in the Tb.N values were statistically significant, the mean differences were on the order of our reproducibility measurements. Systematic differences between MRI and HR-pQCT analysis procedures leading to discrepancies in cortical thickness values were observed, with MRI values consistently higher. Minimal correlations were found between MRI or HR-pQCT parameters and DXA BMD or T-score, except between HR-pQCT measures at the radius and the ultradistal radius T-scores, where moderate correlations were found (r(2) = 0.19-0.58).

CONCLUSIONS

This study provides unique insight into two emerging noninvasive tools for bone structure evaluation. Our findings highlight the significant influence of analysis technique on results of in vivo assessment and underscore the importance of accounting for these differences when interpreting results from these modalities.

Salmon calcitonin: a review of current and future therapeutic indications

Salmon calcitonin, available as a therapeutic agent for more than 30 years, demonstrates clinical utility in the treatment of such metabolic bone diseases as osteoporosis and Paget's disease, and potentially in the treatment of osteoarthritis. This review considers the physiology and pharmacology of salmon calcitonin, the evidence based research demonstrating efficacy and safety of this medication in postmenopausal osteoporosis with potentially an effect on bone quality to explain its abilities to reduce the risk of spine fracture, the development of an oral salmon calcitonin preparation, and the therapeutic rationale for this preparation's chondroprotective effect in osteoarthritis.

Bone degeneration and recovery after early and late bisphosphonate treatment of ovariectomized wistar rats assessed by in vivo micro-computed tomography

Bisphosphonates are antiresorptive drugs commonly used to treat osteoporosis. It is not clear, however, what the influence of the time point of treatment is. Recently developed in vivo micro-computed tomographic (CT) scanners offer the possibility to study such effects on bone microstructure in rats. The aim of this study was to determine the influence of early and late zoledronic acid treatment on bone in ovariectomized rats, using in vivo micro-CT. Twenty-nine female Wistar rats were divided into the following groups: ovariectomy (OVX, n = 5), OVX and zoledronic acid (ZOL) at week 0 (n = 8), OVX and ZOL at week 8 (n = 7), and sham (n = 9). CT scans were made of the proximal tibia at weeks 0, 2, 4, 8, 12, and 16; and bone structural parameters were determined in the metaphysis. Two fluorescent labels were administered to calculate dynamic histomorphometric parameters. At week 16, all groups were significantly different from each other in bone volume fraction (BV/TV), connectivity density, and trabecular number (Tb.N), except for the early ZOL and control groups which were not significantly different for any structural parameter. After ZOL treatment at week 8, BV/TV, structure model index, Tb.N, and trabecular thickness significantly improved in the late ZOL group. The OVX and ZOL groups showed, respectively, higher and lower bone formation rates than the control group. Early ZOL treatment inhibited all bone microstructural changes seen after OVX. Late ZOL treatment significantly improved bone microstructure, although the structure did not recover to original levels. Early ZOL treatment resulted in a significantly better microstructure than late treatment. However, late treatment was still significantly better than no treatment.

Fracture risk remains reduced one year after discontinuation of risedronate

One year after discontinuation of three year's treatment with risedronate, BMD decreased at the lumbar spine and femoral neck and bone turnover markers returned to control group levels. Despite these changes, the risk of new morphometric vertebral fractures remained lower in previous risedronate patients compared with previous control patients.

INTRODUCTION

Differences in bisphosphonate pharmacology and pharmacokinetics could influence persistence or resolution of the effects once treatment is stopped. We investigated changes in intermediate markers--bone mineral density (BMD) and bone turnover markers (BTM)--and fracture risk after discontinuation of treatment with risedronate.

METHODS

Patients who received risedronate 5 mg daily (N = 398) or placebo (N = 361) during the VERT-NA study stopped therapy per protocol after 3 years but continued taking vitamin D (if levels at study entry were low) and calcium and were reassessed one year later.

RESULTS

In the year off treatment, spine BMD decreased significantly, but remained higher than baseline (p < or = 0.001) and placebo (p < 0.001), with similar findings at the femoral neck and trochanter. Urinary NTX and bone-specific alkaline phosphatase, which decreased significantly with treatment, were not significantly different from placebo after 1 year off treatment. Despite the changes in intermediate markers, the incidence of new morphometric vertebral fractures was 46% lower in the former risedronate group compared with the former placebo group (RR 0.54 [95% CI, 0.34, 0.86, p = 0.009]).

CONCLUSIONS

Despite the apparent resolution of effect on BMD and BTM, the risk reduction of new vertebral fractures remained in the year after treatment with risedronate was stopped.

Interindividual and intraspecimen variability of 3-D bone microarchitectural parameters in iliac crest biopsies imaged by conventional micro-computed tomography

Bone microarchitecture of the iliac bone is used to characterize the properties of bone tissue in osteoporosis, particularly in pharmacological studies. Trabecular bone is known to be heterogeneous media. For a few years, the analysis of three-dimensional (3-D) bone microarchitecture has been based on micro-computed tomography (micro-CT). To assess the interindividual variability (inter-indVar) and the intrasample variability (intra-sampVar) of iliac crest biopsies, we used a Bordier needle trephine in 35 postmenopausal female cadavers (mean age, 74.4 +/- 10.4 years). Finally, we had at our disposal 32 individual iliac crests to assess the inter-indVar and 21 oriented specimens to assess the intra-sampVar. All the samples were chemically defatted, and the images were performed with a desktop micro-CT with a voxel size of 10.77 microm. We measured trabecular bone parameters: bone volume/tissue volume (BV/TV %), trabecular thickness and spacing (Tb. Th*, Tb.Sp* microm), bone surface/bone volume (BS/BV, 1/mm), the trabecular number (Tb.N, 1/mm), structure model index (SMI), trabecular pattern factor (Tb.Pf), and degree of anisotropy (DA). We also measured cortical bone parameters: cortical thickness (Cort.Th), porosity (PoV/TV), and pore diameter (Po.Dm). For the inter-indVar, we analyzed a fixed volume of interest corresponding to 119.8 mm(3) centered on each iliac crest. To assess the intra-sampVar, we divided the whole trabecular volume into three equal height parts (external, middle, internal). BV/TV, Tb.N, and PoV/TV were negatively correlated with age and Tb.Sp* and SMI were positively correlated. The mean difference of absolute individual variations in percentage with the middle area used as a reference, comparatively to external and internal areas, ranged from 6.6% (Tb.Sp*) to 27.8% (BV/TV), except Tb.Pf, which showed large variability. There was no difference between external and internal areas, with a tendency for lower values of BV/TV, Tb.Th*, and Tb.N in the middle of the iliac crest and higher values of Tb.Sp* and BS/BV. The evaluation of bone microarchitecture of iliac crest samples on micro-CT images is reliable. The heterogeneity of bone inside the iliac crest is noticeable as leading to analyzing the largest possible quantity of bone, with standardized location, according to cortex but without any assumption of orientation.

Over-suppression of bone turnover: does it exist?

Bone-turnover suppression is central to the therapeutic benefit of many interventions used to prevent osteoporotic fractures. There are theoretical concerns that long-term suppression may have adverse effects on bone strength, although at present no direct evidence exists that this happens. Nevertheless, further research is required to establish the optimal duration of treatment with antiresorptive agents.

Risedronate prevents the loss of microarchitecture in glucocorticoid-induced osteoporosis in rats

Osteoporosis is a severe complication of glucocorticoid treatment. Bisphosphonates are a powerful therapeutic option to prevent osteoporotic fractures. The aims of this study were: a) to determine bone alterations induced by therapy with glucocorticoids (GC); b) to establish the efficacy of risedronate (Ris) in the prevention of these effects. We studied 40 female Sprague-Dawley rats randomly divided into 4 groups of treatment, administered 3 times a week sc: 1.

CONTROL

vehicle of methylprednisolone (GC) + vehicle of Ris; 2. Ris: Ris 5 mug/kg body weight vehicle of GC; 3. GC: GC 7 mg/kg + vehicle of Ris; 4. GC+Ris: GC 7 mg/kg, Ris 5 microg/kg. Animals were treated for 30 days and then were sacrificed. Densitometry was performed at baseline and at the end of the treatment. Right tibiae were removed for histomorphometric analyses. The GC group showed a 7% decrease in bone density vs controls (p<0.05), while the GC+Ris group was associated with a 3.5% increase in bone density vs controls (p<0.05). In the GC group, histomorphometric evaluations showed reduced bone volume (BV/TV) and thinning of trabeculae (Tb.Th) vs controls (BV/TV: 31+/-1 vs 35+/-1%, p<0.05; Tb.Th: 43+/-2 vs 50+/-3 microm, p<0.01; Ac.f: 1.8+/-0.2 vs 1.6+/-0.3 N/yr). The GC+Ris group had increased BV/TV and Tb.Th, and reduced Ac.f vs the GC group. Ris also maintained trabecular microarchitecture. At the histological level, glucocorticoid-induced osteoporosis was characterized by decreased bone volume, reduced osteoblastic activity, and deterioration of microarchitecture. Ris counteracted these effects both by prolonging osteoblast activity, and by maintaining bone microarchitecture.

Bone quality: what is it and how is it measured?

Bone quality describes aspects of bone composition and structure that contribute to bone strength independently of bone mineral density. These include bone turnover, microarchitecture, mineralisation, microdamage and the composition of bone matrix and mineral. New techniques to assess these components of bone quality are being developed and should produce important insights into determinants of fracture risk in untreated and treated disease.

Increases in BMD correlate with improvements in bone microarchitecture with teriparatide treatment in postmenopausal women with osteoporosis

Increases in BMD are correlated with improvements in 2D and 3D trabecular microarchitecture indices with teriparatide treatment. Therefore, improvements in trabecular bone microarchitecture may be one of the mechanisms to explain how BMD increases improve bone strength during teriparatide treatment.

INTRODUCTION

Bone strength is determined by BMD and other elements of bone quality, including bone microarchitecture. Teriparatide treatment increases BMD and improves both cortical and trabecular bone microarchitecture. Increases in lumbar spine (LS) BMD account for approximately 30-41% of the vertebral fracture risk reduction with teriparatide treatment. The relationship between increases in BMD and improvements in cortical and trabecular microarchitecture has not yet been studied.

MATERIALS AND METHODS

The relationship between increases in BMD and improvements in cortical and trabecular microarchitecture after teriparatide treatment was assessed using data from a subset of patients who had areal BMD measurements and structural parameters from transiliac bone biopsies in the Fracture Prevention Trial. 2D histomorphometric and 3D microCT parameters were measured at baseline and 12 (n = 21) or 22 (n = 36) mo. LS BMD was assessed at baseline and 12 and 18 mo, and femoral neck (FN) BMD was measured at baseline and 12 mo. Pearson correlation was performed to assess the relationship between actual changes in BMD and actual changes in microarchitectural parameters.

RESULTS

Changes in LS BMD at 12 mo were significantly correlated with improvements in trabecular bone structure at 22 mo: 2D bone volume (r = 0.45, p = 0.02), 2D mean wall thickness (r = 0.41, p = 0.03), 3D bone volume (r = 0.48, p = 0.006), 3D trabecular thickness (r = 0.44, p = 0.01), 3D trabecular separation (r = -0.37, p = 0.04), 3D structural model index (r = -0.54, p = 0.001), and 3D connectivity density (r = 0.41, p = 0.02). Changes in LS BMD at 18 mo had similar correlations with improvements in bone structure at 22 mo. Changes in FN BMD at 12 mo were significantly correlated with changes in 2D mean wall thickness (r = 0.56, p = 0.002), 3D bone volume (r = 0.51, p = 0.004), 3D trabecular thickness (r = 0.44, p = 0.01), 3D trabecular separation (r = -0.46, p = 0.01), and 3D structural model index (r = -0.55, p = 0.001).

CONCLUSIONS

Increases in BMD are correlated with improvements in trabecular microarchitecture in iliac crest of patients with teriparatide treatment. Therefore, improvements in trabecular bone microarchitecture may be one of the mechanisms to explain how BMD increases improve bone strength during teriparatide treatment.

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.

Changes in bone remodelling and antifracture efficacy of intermittent bisphosphonate therapy: implications from clinical studies with ibandronate

Bisphosphonates reduce the rate of bone resorption and bone remodelling. Given daily, they decrease the risk of fractures in postmenopausal osteoporosis. When bisphosphonates were given at extended drug-free intervals this antifracture efficacy was generally not seen. This may be due to the different pattern of bone remodelling changes. Data from randomised clinical studies of ibandronate, given orally or intravenously, at different doses and for variable time intervals to women with osteoporosis were examined to explore the relationship between intermittent bisphosphonate therapy, changes in bone resorption and fracture risk. The magnitude of the reduction of the rate of bone resorption at the end of the drug-free interval rather than its fluctuation pattern after bisphosphonate administration determines antifracture efficacy, provided that these fluctuations occur within the premenopausal range. Prolongation of the drug-free interval beyond 2 weeks should be compensated by a dose higher than the cumulative daily dose.

Transmenopausal changes in the trabecular bone structure

INTRODUCTION

Post-menopausal osteoporosis is a disorder of excess skeletal fragility, due partly to changes in bone microstructure. Menopause is known to result in bone loss and reduction in bone mechanical strength. However, the mechanism and nature of microstructural changes at menopause need more detailed description and analyses. The overall hypothesis for this analysis is that the variables describing trabecular bone micro-architecture will be affected by changes in the hormonal status of women just prior to, and early after, last menses, and that volumetric bone density, and trabecular structure will decline significantly. The study was designed to capture true longitudinal transmenopausal changes in three-dimensional (3-D) trabecular bone architecture. Currently, minimal data exist regarding these features.

MATERIALS AND METHODS

Transilial biopsies specimens were obtained from healthy pre-menopausal women (age >46), and repeated at 12 months after the last menstrual period. Bone architecture was quantified in 38 paired specimens using micro-computed tomography (micro-CT-40, Scanco) techniques. Bone biopsies were embedded for histomorphomteric analyses and parts of the analyses have been published elsewhere. Embedded bone biopsies were scanned at 30-mum resolution such that the region of interest was similar to that in the two-dimensional (2-D) histomorphometric analyses. Paired t-tests were used to compare the pre- and post-menopausal bone structural data from each technique.

RESULTS

There was good correlation between standard histomorphometric (2-D) and micro-CT (3-D) measurements. Most of the variables characterizing bone structure in post-menopausal women (from micro-CT) significantly decreased (BV/TV, trabecular number, apparent and tissue density). In addition, both trabecular spacing (Tb.S) and the structure model index (SMI) increased in the post-menopausal women suggesting transformation of trabecular bone from plate- to rod-like structure. The 3-D trabecular connectivity density (Conn.D) was negatively correlated with activation frequency (Ac.f).

CONCLUSIONS

These data suggest that 3-D micro-CT measurements (longitudinal) are comparable to those of standard histomorphometry, and that most of the bone structural measurements are sensitive to changes in women's hormonal status across menopause.

Skeletal actions of intermittent parathyroid hormone: effects on bone remodelling and structure

Intermittent administration of parathyroid hormone peptides has anabolic skeletal effects and reduces fracture risk in postmenopausal women with osteoporosis but the cellular and structural mechanisms by which these effects are mediated have not been fully established. In cancellous and endocortical bone, there is evidence that both modelling and remodelling-based formation contribute to the increase in bone mass although the contribution of these at different time points in the response to PTH has not been established. Despite the large increase in spine bone mineral density, however, significant increases in iliac crest cancellous bone volume and trabecular thickness have not been consistently demonstrated, possibly reflecting site-specific differences in PTH-induced skeletal effects and/or the large sampling and measurement variance associated with assessment of iliac crest cancellous bone volume and structure. In iliac crest cortical bone, increased cortical thickness has been demonstrated, due at least in part to increased endosteal bone formation; there is also some evidence for increased formation on periosteal surfaces. At some sites an increase in cortical porosity may also occur and the overall effects on cortical bone strength, particularly at the hip, remain to be established. Studies in iliac crest bone indicate a trend towards a lower mineralisation density of bone matrix and increased heterogeneity of mineralisation, consistent with new bone formation. In addition, there is a reduction in mineral crystallinity and a shift towards more divalent collagen cross-links, indicating a change towards a younger bone profile. The potential clinical implications of these effects on bone are currently unknown. The stimulatory effect of PTH peptides on bone formation may favour their use in low turnover bone disease and in states of advanced bone loss. Furthermore, if beneficial effects on cortical bone strength are confirmed, efficacy at non-vertebral sites might be superior to those observed with antiresorptive drugs. Better definition of the effects of intermittent PTH administration on cancellous and cortical bone remodelling and structure at different skeletal sites may inform these speculations and is an important area for future research.

Interrelationships between bone microarchitecture and strength in ovariectomized monkeys treated with teriparatide

Bone microarchitecture measured at the iliac crest at 6 mo was confirmed to be a reasonable surrogate for, and a predictor of, architecture and strength of the femoral neck and lumbar vertebra after 18 mo of teriparatide treatment. However, the data taken together showed the importance of cortical bone volume for vertebra to assess pharmacological effects on bone quality.

INTRODUCTION

Improvements in bone architecture with teriparatide treatment are suggested to contribute to fracture risk reduction in osteoporotic patients. Teriparatide significantly improves microarchitecture in the iliac crest of humans by stimulating bone modeling and remodeling processes that differ dramatically from those induced by antiresorptives. The relationship between improvements of bone microarchitecture and improvements of bone strength with teriparatide treatment has not yet been fully studied.

MATERIALS AND METHODS

Ovariectomized monkeys were administered vehicle (n = 20); teriparatide 1.0 microg/kg/d (n = 19); or teriparatide 5.0 microg/kg/d (n = 21) for 18 mo. Iliac crest biopsies were obtained at 6 and 15 mo after initiation of treatment. Animals were killed after 18 mo of treatment, and adjacent vertebrae or contralateral proximal femora were processed for biomechanical or histomorphometric analyses. Pearson correlation analyses were performed to assess the relationship between biomechanical and static histomorphometric parameters of lumbar vertebra, femoral neck, and iliac crest biopsies.

RESULTS

Static histomorphometric parameters of the 6- and 15-mo biopsies were significantly correlated with the vertebral and femoral neck parameters obtained at 18 mo of teriparatide treatment. Iliac crest biopsy parameters at 6 and 15 mo also correlated with vertebral and femoral neck strength at 18 mo. Static histomorphometry of the lumbar vertebra and femoral neck at 18 mo also significantly correlated with strength at these sites. However, cortical bone volume of the lumbar vertebrae had the strongest correlation with vertebral and femoral neck strength (r = 0.74 and 0.71, respectively).

CONCLUSIONS

Teriparatide dose dependently improved cortical and trabecular microarchitecture of vertebra and femoral neck, as well as trabecular microarchitecture of the iliac crest. Bone microarchitecture at all sites was significantly correlated with lumbar vertebra and femoral neck strength. Cortical bone volume of vertebra had the strongest correlation with vertebral and femoral neck strength. Therefore, structural improvement seemed to be part of the mechanism for improved strength observed with teriparatide treatment. Trabecular bone architecture of the iliac crest at 6 mo also correlated with vertebral and femoral neck strength, as did femoral neck (cortical and trabecular) histomorphometry and trabecular histomorphometry of vertebra after 18 mo of treatment. Because clinical assessment of cortical bone volume is not readily possible for vertebra noninvasively, these findings confirm the importance of iliac crest biopsies to monitor skeletal health and show that biopsies are a reasonable surrogate to assess spine and femoral neck structure and function.

Bisphosphonates: mode of action and pharmacology

The profound effects of the bisphosphonates on calcium metabolism were discovered over 30 years ago, and they are now well established as the major drugs used for the treatment of bone diseases associated with excessive resorption. Their principal uses are for Paget disease of bone, myeloma, bone metastases, and osteoporosis in adults, but there has been increasing and successful application in pediatric bone diseases, notably osteogenesis imperfecta. Bisphosphonates are structural analogues of inorganic pyrophosphate but are resistant to enzymatic and chemical breakdown. Bisphosphonates inhibit bone resorption by selective adsorption to mineral surfaces and subsequent internalization by bone-resorbing osteoclasts where they interfere with various biochemical processes. The simpler, non-nitrogen-containing bisphosphonates (eg, clodronate and etidronate) can be metabolically incorporated into nonhydrolysable analogues of adenosine triphosphate (ATP) that may inhibit ATP-dependent intracellular enzymes. In contrast, the more potent, nitrogen-containing bisphosphonates (eg, pamidronate, alendronate, risedronate, ibandronate, and zoledronate) inhibit a key enzyme, farnesyl pyrophosphate synthase, in the mevalonate pathway, thereby preventing the biosynthesis of isoprenoid compounds that are essential for the posttranslational modification of small guanosine triphosphate (GTP)-binding proteins (which are also GTPases) such as Rab, Rho, and Rac. The inhibition of protein prenylation and the disruption of the function of these key regulatory proteins explains the loss of osteoclast activity. The recently elucidated crystal structure of farnesyl diphosphate reveals how bisphosphonates bind to and inhibit at the active site via their critical nitrogen atoms. Although bisphosphonates are now established as an important class of drugs for the treatment of many bone diseases, there is new knowledge about how they work and the subtle but potentially important differences that exist between individual bisphosphonates. Understanding these may help to explain differences in potency, onset and duration of action, and clinical effectiveness.

Bone texture analysis on direct digital radiographic images: precision study and relationship with bone mineral density at the os calcis

Assessment of bone microarchitecture in complement to bone mineral density (BMD) exam could improve prediction of osteoporotic fractures. A high-resolution X-ray prototype was developed to assess microarchitecture quality. Images were obtained on os calcis; then, three texture parameters were calculated on the same region of interest (ROI): a fractal parameter, a run-length parameter, and a co-occurrence parameter. This work describes the reproducibility of this method. We also examine the relationship between texture parameters and BMD at a site-matched ROI. Measurements on the left heel were performed on 30 healthy women, on the same day, with repositioning for short-term precision error. An additional measurement was done at 1 week to evaluate mid-term precision error on 14 subjects. Os calcis images from 10 healthy women were used to evaluate both intra- and interobserver reproducibility. Thirty other healthy patients were measured successively on two similar devices for interprototype comparison. BMD and texture analyses of the left heel were obtained from 57 women. Short-term precision errors ranged 1.16-1.24% according to the texture parameter. Mid-term precision error was slightly higher than short-term precision for the mean Hurst exponent parameter. Comparisons of texture parameters and BMD at a site-matched ROI on the os calcis showed no significant relationships. The results also show that the use of this high-resolution digital X-ray device improves the reproducibility of parameter measurement compared to the indirect digitization of radiologic films previously used.

Effects of teriparatide and alendronate on vertebral strength as assessed by finite element modeling of QCT scans in women with osteoporosis

FE modeling was used to estimate the biomechanical effects of teriparatide and alendronate on lumbar vertebrae. Both treatments enhanced predicted vertebral strength by increasing average density. This effect was more pronounced for teriparatide, which further increased predicted vertebral strength by altering the distribution of density within the vertebra, preferentially increasing the strength of the trabecular compartment.

INTRODUCTION

Teriparatide 20 microg/day (TPTD) and alendronate 10 mg/day (ALN) increase areal, measured by DXA, and volumetric, measured by QCT, lumbar spine BMD through opposite effects on bone remodeling. Using finite element (FE) modeling of QCT scans, we sought to compare the vertebral strength characteristics in TPTD- and ALN-treated patients.

MATERIALS AND METHODS

A subset of patients (N = 28 TPTD; N = 25 ALN) from the Forteo Alendronate Comparator Trial who had QCT scans of the spine at baseline and postbaseline were analyzed. The QCT scans were analyzed for compressive strength of the L(3) vertebra using FE modeling. In addition, using controlled parameter studies of the FE models, the effects of changes in density, density distribution, and geometry on strength were calculated, a strength:density ratio was determined, and a response to bending was also quantified.

RESULTS

Both treatments had positive effects on predicted vertebral strength characteristics. At least 75% of the patients in each treatment group had increased strength of the vertebra at 6 months compared with baseline. Patients in both treatment groups had increased average volumetric density and increased strength in the trabecular bone, but the median percentage increases for these parameters were 5- to 12-fold greater for TPTD. Larger increases in the strength:density ratio were also observed for TPTD, and these were primarily attributed to preferential increases in trabecular strength.

CONCLUSIONS

These results provide new insight into the effects of these treatments on estimated biomechanical properties of the vertebra. Both treatments positively affected predicted vertebral strength through their effects on average BMD, but the magnitudes of the effects were quite different. Teriparatide also affected vertebral strength by altering the distribution of density within the vertebra, so that overall, teriparatide had a 5-fold greater percentage increase in the strength:density ratio.

Preventative ibandronate treatment has the most beneficial effect on the microstructure of bone in experimental tumor osteolysis

We investigated the effect of ibandronate on three-dimensional (3-D) microstructure and bone mass in experimentally induced tumor osteolysis. Walker carcinosarcoma cells were implanted into the left femur of female rats that received 26-day ibandronate pretreatment followed by continued therapy or ibandronate posttreatment only. A tumor-only group received isotonic saline. At endpoint, excised femurs were scanned using microcomputed tomography (microCT) to assess bone volume density, bone mineral content, trabecular number/thickness, and separation for cortical plus trabecular bone or trabecular bone alone. Compared with the nonimplanted right femur, bone volume and surface density and trabecular number and thickness were reduced in the distal left femur following tumor cell implantation. microCT analysis revealed greater cortical and trabecular bone mineral content in the preventative and interventional (pre-post tumor) ibandronate group, and the interventional (post-tumor) ibandronate group, versus the tumor-only group. Bone volume density was significantly higher in pre-post and post-tumor groups compared to the tumor-only group. After preventative and interventional ibandronate, bone volume density and trabecular thickness were 13% and 60% greater, respectively, than in the post-tumor treatment group. 3-D microCT images confirmed microstructural changes. We conclude that combined interventional and preventative ibandronate preserves bone strength and integrity more than intervention alone.

Risedronate decreases biochemical markers of cartilage degradation but does not decrease symptoms or slow radiographic progression in patients with medial compartment osteoarthritis of the knee: results of the two-year multinational knee osteoarthritis structural arthritis study

OBJECTIVE

Bisphosphonates have slowed the progression of osteoarthritis (OA) in animal models and have decreased pain in states of high bone turnover. The Knee OA Structural Arthritis (KOSTAR) study, which is the largest study to date investigating a potential structure-modifying OA drug, tested the efficacy of risedronate in providing symptom relief and slowing disease progression in patients with knee OA.

METHODS

The study group comprised 2,483 patients with medial compartment knee OA and 2-4 mm of joint space width (JSW), as determined using fluoroscopically positioned, semiflexed-view radiography. Patients were enrolled in 2 parallel 2-year studies in North America and the European Union. These studies evaluated the efficacy of risedronate at dosages of 5 mg/day, 15 mg/day, 35 mg/week (in Europe), and 50 mg/week (in North America) compared with placebo in reducing signs and symptoms, as measured by the Western Ontario and McMaster Universities Osteoarthritis (WOMAC) index and patient global assessment (PGA) scores, and in slowing radiographic progression.

RESULTS

A reduction of approximately 20% in signs and symptoms, as measured by WOMAC subscales and PGA scores, was observed in all groups, with no treatment effect of risedronate demonstrated. Risedronate did not significantly reduce radiographic progression as measured by decreased JSW or using a dichotomous definition of progression (joint space loss of >or=0.6 mm). Thirteen percent of patients receiving placebo demonstrated significant disease progression over 2 years. A dose-dependent reduction in the level of C-terminal crosslinking telopeptide of type II collagen, a cartilage degradation marker associated with progressive OA, was seen in patients who received risedronate. No increase in the number of adverse events was demonstrated for risedronate compared with placebo.

CONCLUSION

Although risedronate (compared with placebo) did not improve signs or symptoms of OA, nor did it alter progression of OA, a reduction in the level of a marker of cartilage degradation was observed. A sustained clinically relevant improvement in signs and symptoms was observed in all treatment and placebo groups.

Vertebral osteoporosis and trabecular bone quality

Vertebral fractures due to osteoporosis commonly occur under non-traumatic loading conditions. This problem affects more than 1 in 3 women and 1 in 10 men over a lifetime. Measurement of bone mineral density (BMD) has traditionally been used as a method for diagnosis of vertebral osteoporosis. However, this method does not fully account for the influence of changes in the trabecular bone quality, such as micro-architecture, tissue properties and levels of microdamage, on the strength of the vertebra. Studies have shown that deterioration of the vertebral trabecular architecture results in a more anisotropic structure which has a greater susceptibility to fracture. Transverse trabeculae are preferentially thinned and perforated while the remaining vertical trabeculae maintain their thickness. Such a structure is likely to be more susceptible to buckling under normal compression loads and has a decreased ability to withstand unusual or off-axis loads. Changes in tissue material mechanical properties and levels of microdamage due to osteoporosis may also compromise the fracture resistance of vertebral trabecular bone. New diagnostic techniques are required which will account for the influence of these changes in bone quality. This paper reviews the influence of the trabecular architecture, tissue properties and microdamage on fracture risk for vertebral osteoporosis. The morphological characteristics of normal and osteoporotic architectures are compared and their potential influence on the strength of the vertebra is examined. The limitations of current diagnostic methods for osteoporosis are identified and areas for future research are outlined.

New mechanisms and targets in the treatment of bone fragility

Bone modelling and remodelling are cell-mediated processes responsible for the construction and reconstruction of the skeleton throughout life. These processes are chiefly mediated by locally generated cytokines and growth factors that regulate the differentiation, activation, work and life span of osteoblasts and osteoclasts, the cells that co-ordinate the volumes of bone resorbed and formed. In this way, the material composition and structural design of bone is regulated in accordance with its loading requirements. Abnormalities in this regulatory system compromise the material and structural determinants of bone strength producing bone fragility. Understanding the intercellular control processes that regulate bone modelling and remodelling is essential in planning therapeutic approaches to prevention and treatment of bone fragility. A great deal has been learnt in the last decade. Clinical trials carried out exclusively with drugs that inhibit bone resorption have identified the importance of reducing the rate of bone remodelling and so the progression of bone fragility to achieved fracture reductions of approx. 50%. These trials have also identified limitations that should be placed upon interpretation of bone mineral density changes in relation to treatment. New resorption inhibitors are being developed, based on mechanisms of action that are different from existing drugs. Some of these might offer resorption inhibition without reducing bone formation. More recent research has provided the first effective anabolic therapy for bone reconstruction. Daily injections of PTH (parathyroid hormone)-(1–34) have been shown in preclinical studies and in a large clinical trial to increase bone tissue mass and reduce the risk of fractures. The action of PTH differs from that of the resorption inhibitors, but whether it is more effective in fracture reduction is not known. Understanding the cellular and molecular mechanisms of PTH action, particularly its interactions with other pathways in determining bone formation, is likely to lead to new therapeutic developments. The recent discovery through mouse genetics that PTHrP (PTH-related protein) is a crucial bone-derived paracrine regulator of remodelling offers new and interesting therapeutic targets.

Osteoporosis prevention and therapy: preserving and building strength through bone quality

While bone mineral density measurements play a central role in osteoporosis management, the degree to which increases in this parameter contribute to bone fragility, fracture risk, and the therapeutic efficacy of osteoporosis agents is controversial. Indeed, bone strength is also significantly dependent upon bone turnover and bone quality, including microarchitecture, mineralization, and geometry. Given the critical role of these factors, it is essential to understand how they are affected by therapeutic agents. Even though a number of technological advances, such as microcomputed tomography, magnetic resonance imaging, and computerized analysis of radiographic patterns, help to provide critical information toward a more comprehensive assessment of bone turnover and bone quality, clinical trials addressing these factors are scarce. This article provides a review of studies relating to how osteoporosis therapies impact parameters of bone strength and quality.

Effects of osteoporosis medications on bone quality

In clinical practice, the quantitative evaluation of bone tissue relies on dual-energy X-ray absorptiometry (DXA) measurements of bone mineral density (BMD) values, which are closely associated with the risk of osteoporotic fracture. However, only a small fraction of the antifracture effect of bone resorption inhibitors is ascribable to BMD gains (4% with raloxifene and 16-28% with alendronate and risedronate). Bone quality encompasses a number of bone tissue properties that govern mechanical resistance, such as bone geometry, cortical properties, trabecular microarchitecture, bone tissue mineralization, quality of collagen and bone apatite crystal, and presence of microcracks. All these properties are dependent on bone turnover and its variations. In populations, the decreases in bone resorption markers achieved with resorption inhibitors may predict in part the decrease in fracture risk. At the spine, however, this correlation exists down to a 40% fall in bone resorption markers; larger drops did not provide further protection against fractures in patients taking risedronate in one evaluation of this relationship. Osteoporosis medications can exert favorable effects on bone size and cortical thickness. Such effects have been documented with teriparatide (PTH 1-34), which is the unique purely anabolic treatment for osteoporosis available to date. More surprising are the favorable effects on bone size seen with some of the bone resorption inhibitors such as neridronate in adults with osteogenesis imperfecta. Similarly, estrogens and alendronate can increase femoral neck size in postmenopausal women. Preservation of the trabecular microarchitecture was demonstrated first with risedronate and subsequently with alendronate. In placebo-controlled studies, a deterioration in trabecular microarchitecture occurred within 1 to 3 years in the placebo groups but not in the bisphosphonate groups. Teriparatide, in contrast, improves trabecular microarchitecture, in particular by increasing connectivity and improving the plate-rod distribution. The minerals within trabecular or cortical bone can be evaluated using microradiography or synchrotron micro-computed tomography. Marked or prolonged secondary mineralization may result in poor bone quality. Increased bone mineralization is among the key effects of bone resorption inhibitors, most notably bisphosphonates. Prolonged use of the most potent bisphosphonates may lead to unwanted effects related to excessive mineralization. Microcracks may play a physiological role; however, a large number of microcracks may be deleterious via an effect on osteocytes. Excessive mineralization may promote the development of multiple microcracks. Studies of bone crystal and collagen properties with several bone resorption inhibitors, including risedronate and raloxifene, showed no harmful effects. An increasing number (several hundreds) of mandibular osteonecrosis associated with bisphosphonate therapy has been reported. The typical patient was receiving injectable bisphosphonate therapy for bone cancer and had undergone dental work shortly before bisphosphonate administration. The mechanism of this adverse effect is poorly understood.

Bone material properties in trabecular bone from human iliac crest biopsies after 3- and 5-year treatment with risedronate

Long-term effects of risedronate on bone mineral maturity/crystallinity and collagen cross-link ratio in triple iliac crest biopsies of osteoporotic women were evaluated. In this double-blinded study, 3- and 5-year treatment with risedronate arrested the tissue aging encountered in untreated osteoporosis and in osteoporosis treated with other antiresorptives. This effect may be contributing to risedronate's antifracture efficacy.

INTRODUCTION

Risedronate is widely used in the treatment of osteoporosis. It reduces bone turnover, increases BMD, and decreases fracture risk. To date, there are no data available on the long-term effects of risedronate on bone material properties in humans.

MATERIALS AND METHODS

Osteoporotic women enrolled in the VERT-NA trial received either risedronate (5 mg/day, orally) or placebo for up to 5 years. All subjects received calcium. They also received vitamin D supplementation if deficient at baseline. Triple iliac crest biopsies were collected from a subset of these subjects at baseline, 3 years, and 5 years. Mineral maturity/crystallinity and collagen cross-link ratio was measured in these biopsies using Fourier transform infrared imaging.

RESULTS

Patients that received placebo exhibited increased mineral maturity/crystallinity and collagen cross-link ratio after 3 and 5 years compared with baseline values. On the contrary, patients that received risedronate retained baseline values in both bone material indices throughout. A more spatially detailed analysis revealed that this was achieved mainly through beneficial effects on active bone-forming areas. Surprisingly, patients that received risedronate achieved premenopausal values at bone-forming areas in both indices after 5 years of treatment.

CONCLUSION

Long-term treatment with risedronate affects bone material properties (mineral maturity/crystallinity and collagen cross-link ratio) and arrests the tissue aging apparent in untreated osteoporosis. These changes at the material level of the bone matrix may contribute to risedronate's rapid and sustained antifracture efficacy in osteoporotic patients.

Dissociation of the pro-apoptotic effects of bisphosphonates on osteoclasts from their anti-apoptotic effects on osteoblasts/osteocytes with novel analogs

Bisphosphonates induce osteoclast apoptosis, thereby decreasing bone resorption and reducing the rate of bone remodeling. Earlier work from our group and others has demonstrated that, additionally, bisphosphonates prevent osteoblast and osteocyte apoptosis in vivo and in vitro, raising the possibility that perhaps part of their anti-fracture efficacy may result from preserving the integrity of the osteocyte network and prolonging the working time of bone forming cells. Whereas induction of osteoclast apoptosis results from inhibition of the mevalonate pathway or from conversion to toxic ATP analogs, prevention of osteoblastic cell apoptosis is mediated by connexin43 hemichannel opening and activation of the extracellular signal-regulated kinases (ERKs). We examined here the ability of several bisphosphonates, including novel analogs, to exert these two effects. All 16 bisphosphonates studied inhibited etoposide-induced apoptosis of MLO-Y4 osteocytic cells and osteoblastic cells derived from calvaria, with EC50 between 10(-12) and 10(-10) M. On the other hand, only 10 analogs induced apoptosis of RAW-264.7-cell-derived osteoclasts. Each of the 6 bisphosphonates that lack pro-apoptotic activity in osteoclasts but retain anti-apoptotic activity in osteoblasts and osteocytes has a structural-related analog that is active in both cell types. These findings indicate that the structural prerequisites for the anti-apoptotic effect of bisphosphonates on cells of the osteoblastic lineage are less stringent than the ones required to induce osteoclast apoptosis and confirm that bisphosphonates act on the two cell types by distinct mechanisms. Preservation of osteoblast and osteocyte viability without inducing osteoclast apoptosis by these bisphosphonates analogs opens new possibilities for the treatment of bone fragility in conditions in which a decrease in bone remodeling is not desirable.

Long-term risedronate treatment normalizes mineralization and continues to preserve trabecular architecture: sequential triple biopsy studies with micro-computed tomography

The objective of the study was to assess the time course of changes in bone mineralization and architecture using sequential triple biopsies from women with postmenopausal osteoporosis (PMO) who received long-term treatment with risedronate. Transiliac biopsies were obtained from the same subjects (n = 7) at baseline and after 3 and 5 years of treatment with 5 mg daily risedronate. Mineralization was measured using 3-dimensional (3D) micro-computed tomography (CT) with synchrotron radiation and was compared to levels in healthy premenopausal women (n = 12). Compared to the untreated PMO women at baseline, the premenopausal women had higher average mineralization (Avg-MIN) and peak mineralization (Peak-MIN) by 5.8% (P = 0.003) and 8.0% (P = 0.003), respectively, and lower ratio of low to high-mineralized bone volume (BMR-V) and surface area (BMR-S) by 73.3% (P = 0.005) and 61.7% (P = 0.003), respectively. Relative to baseline, 3 years of risedronate treatment significantly increased Avg-MIN (4.9 +/- 1.1%, P = 0.016) and Peak-MIN (6.2 +/- 1.5%, P = 0.016), and significantly decreased BMR-V (-68.4 +/- 7.3%, P = 0.016) and BMR-S (-50.2 +/- 5.7%, P = 0.016) in the PMO women. The changes were maintained at the same level when treatment was continued up to 5 years. These results are consistent with the significant reduction of turnover observed after 3 years of treatment and which was similarly maintained through 5 years of treatment. Risedronate restored the degree of mineralization and the ratios of low- to high-mineralized bone to premenopausal levels after 3 years of treatment, suggesting that treatment reduced bone turnover in PMO women to healthy premenopausal levels. Conventional micro-CT analysis further demonstrated that bone volume (BV/TV) and trabecular architecture did not change from baseline up to 5 years of treatment, suggesting that risedronate provided long-term preservation of trabecular architecture in the PMO women. Overall, risedronate provided sustained benefits on mineralization and architecture, two key determinants of bone strength, over 5 years lending support for its long-term efficacy in fracture risk reduction.

The effects of antifracture therapies on the components of bone strength: assessment of fracture risk today and in the future

OBJECTIVE

To summarize the current knowledge regarding the impact of the most common antifracture medications on the various determinants of bone strength.

METHODS

Relevant English-language articles acquired from Medline from 1966 to January 2005 were reviewed. Searches included the keywords bone AND 1 of the following: strength, remodeling, microcrack, structure, mineralization, collagen, organic, crystallinity, osteocyte, porosity, diameter, anisotropy, stress risers, or connectivity AND alendronate, estrogen, etidronate, hormone replacement therapy, parathyroid hormone, risedronate, OR teriparatide. Abstracts from relevant conference proceedings were also reviewed for pertinent information.

RESULTS

Antiresorptive therapies increase bone strength through decreasing bone turnover. This lower bone turnover results in a higher mean mineralization and decreases the number of active resorption pits within bone at any given time. These resorption pits are speculated to be areas of focal weakness and a higher number of them would, if all other things were equal, result in greater fragility. Parathyroid hormone therapy increases the rate of bone remodeling, which introduces many resorption pits, but this source of strength loss is thought to be compensated by rapid increases in bone mass.

CONCLUSIONS

Both the antiresorptives, particularly bisphosphonates, and the parathyroid hormone therapy increase bone strength; however, the changes that are elicited to achieve this differ significantly.

Important Determinants of Bone Strength

Osteoporosis is a systemic skeletal disorder characterized by compromised bone strength that predisposes individuals to increased fracture risk. Bone strength is determined by its material and structural properties. Bone mineral density (BMD) is a useful tool for diagnosis; however, this parameter provides information regarding only the quantity of mineral in bone, which is only one component of bone strength. Osteoporosis treatments have been shown to have beneficial effects on bone turnover, microarchitecture, and/or mineralization, all of which can help account for the reductions in fracture risk above and beyond changes in BMD. Newer noninvasive imaging methods are being developed that assess bone strength independent of BMD, and these methods should improve the assessment of fracture risk and response to treatment. These imaging methods are not currently available for routine clinical use, and therefore, clinicians need to continue for now to rely on surrogate markers of bone fragility, including BMD, prevalent fracture, and other important risk factors for fracture.

Bisphosphonates: From Bench to Bedside

The discovery and development of the bisphosphonates (BPs) as a major class of drugs for the treatment of bone diseases has been a fascinating journey that is still not over. In clinical medicine, several BPs are established as the treatments of choice for various diseases of excessive bone resorption, including Paget's disease of bone, myeloma and bone metastases, and osteoporosis. Bisphosphonates are chemically stable analogues of inorganic pyrophosphate, and are resistant to breakdown by enzymatic hydrolysis. Bisphosphonates inhibit bone resorption by being selectively taken up and adsorbed to mineral surfaces in bone, where they interfere with the action of the bone-resorbing osteoclasts. Bisphosphonates are internalized by osteoclasts and interfere with specific biochemical processes. Bisphosphonates can be classified into at least two groups with different molecular modes of action. The simpler non-nitrogen-containing bisphosphonates (such as clodronate and etidronate) can be metabolically incorporated into nonhydrolyzable analogues of adenosine triphosphate (ATP) that may inhibit ATP-dependent intracellular enzymes. The more potent, nitrogen-containing bisphosphonates (such as pamidronate, alendronate, risedronate, ibandronate, and zoledronate) are not metabolized in this way but can inhibit enzymes of the mevalonate pathway, thereby preventing the biosynthesis of isoprenoid compounds that are essential for the posttranslational modification of small GTP-binding proteins (which are also GTPases) such as rab, rho, and rac. The inhibition of protein prenylation and the disruption of the function of these key regulatory proteins explain the loss of osteoclast activity and induction of apoptosis. The key target for bisphosphonates is farnesyl pyrophosphate synthase (FPPS) within osteoclasts, and the recently elucidated crystal structure of this enzyme reveals how BPs bind to and inhibit at the active site via their critical N atoms. In conclusion, bisphosphonates are now established as an important class of drugs for the treatment of many bone diseases, and their mode of action is being unraveled. As a result their full therapeutic potential is gradually being realized.