Effects of ibandronate on bone quality: preclinical studies

Ibandronate Suppresses Changes in Apatite Orientation and Young's Modulus Caused by Estrogen Deficiency in Rat Vertebrae

Bone material quality is important for evaluating the mechanical integrity of diseased and/or medically treated bones. However, compared to the knowledge accumulated regarding changes in bone mass, our understanding of the quality of bone material is lacking. In this study, we clarified the changes in bone material quality mainly characterized by the preferential orientation of the apatite c-axis associated with estrogen deficiency-induced osteoporosis, and their prevention using ibandronate (IBN), a nitrogen-containing bisphosphonate. IBN effectively prevented bone loss and degradation of whole bone strength in a dose-dependent manner. The estrogen-deficient condition abnormally increased the degree of apatite orientation along the craniocaudal axis in which principal stress is applied; IBN at higher doses played a role in maintaining the normal orientation of apatite but not at lower doses. The bone size-independent Young's modulus along the craniocaudal axis of the anterior cortical shell of the vertebra showed a significant and positive correlation with apatite orientation; therefore, the craniocaudal Young's modulus abnormally increased under estrogen-deficient conditions, despite a significant decrease in volumetric bone mineral density. However, the abnormal increase in craniocaudal Young's modulus did not compensate for the degradation of whole bone mechanical properties due to the bone loss. In conclusion, it was clarified that changes in the material quality, which are hidden in bone mass evaluation, occur with estrogen deficiency-induced osteoporosis and IBN treatment. Here, IBN was shown to be a beneficial drug that suppresses abnormal changes in bone mechanical integrity caused by estrogen deficiency at both the whole bone and material levels.

Treatment with the combination of ibandronate plus eldecalcitol has a synergistic effect on inhibition of bone resorption without suppressing bone formation in ovariectomized rats

Bisphosphonates are widely used in the treatment of osteoporosis and contribute to the reduction of bone fractures. Ibandronate (IBN) is a highly potent, nitrogen-containing bisphosphonate, which is administered orally or intravenously at extended dosing intervals. Vitamin D or active vitamin D3 derivatives are also used in the treatment of osteoporosis, and are often used in combination with other drugs. In this study, we investigated the effect of treatment with the combination of once-monthly s.c. dosing of IBN plus once-daily oral eldecalcitol (ELD), an active vitamin D3 derivative, using aged ovariectomized (OVX) rats. Treatment was started the day after OVX, and analyses were performed 4, 8, and 12 weeks thereafter by determination of bone markers, bone mineral density, biomechanical properties, and histomorphometry. The combination treatment showed a synergistic effect in increasing both lumbar and femoral BMD, and resulted in a significant increase in bone ultimate load. The combination of IBN plus ELD acted synergistically to reduce bone resorption, whereas bone formation did not decrease any more than with monotherapy with either IBN or ELD. Bone formation independent of bone resorption (a process known as 'minimodeling') was not changed in vehicle treated OVX rats despite the increase in bone turnover. ELD upregulated minimodeling, which was however not diminished in the combination treatment. In conclusion, treatment with the combination of IBN plus ELD was beneficial in the treatment of osteoporosis in aged OVX rats. It exhibited a synergistic inhibitory effect on bone resorption and keeps bone formation at the level of sham controls. This uncoupling of bone resorption/bone formation was affected, to some extent, by minimodeling-based bone formation which is independent of bone resorption. This combination regimen which showed synergistic effect on BMD and bone ultimate load without inhibition of bone formation may be beneficial in long-term osteoporosis treatment to prevent bone fractures.

Bone Mineral Density Changes after Orchiectomy using a Scrotal Approach in Rats

Objective

To investigate a suitable animal model for studies of male osteoporosis. Osteoporosis has a particularly high incidence in postmenopausal women, resulting in a substantial amount of research with respect to this disease in women. However, research on osteoporosis in men is still lacking.

Methods

Twenty 10-week-old male Sprague Dawley rats were used in this study, including 4 rats used to establish a baseline bone mineral density (BMD). The other 16 rats were divided into two groups: a sham surgery group (n=8), which underwent a sham operation, and an orchiectomized rat group (OCX) (n=8), which underwent bilateral OCX at 10 weeks of age. Bone mineral density was measured in 4 rats from both the sham surgery group and the OCX group 8 weeks after the surgery, while BMD in the remainder of the rats was measured 10 weeks post-surgery.

Results

Femoral BMD at 8 weeks post-surgery was found to be significantly lower in the OCX group compared to the sham group; a finding that was also similar 10 weeks post-surgery.

Conclusion

8 weeks after undergoing orchiectomy performed via a scrotal, white rats are a suitable model for studies of male osteoporosis.

Bisphosphonates and bone quality

Bisphosphonates (BPs) are bone-avid compounds used as first-line medications for the prevention and treatment of osteoporosis. They are also used in other skeletal pathologies such as Paget's and metastatic bone disease. They effectively reduce osteoclast viability and also activity in the resorptive phase of bone remodelling and help preserve bone micro-architecture, both major determinants of bone strength and ultimately of the susceptibility to fractures. The chemically distinctive structure of each BP used in the clinic determines their unique affinity, distribution/penetration throughout the bone and their individual effects on bone geometry, micro-architecture and composition or what we call 'bone quality'. BPs have no clinically significant anabolic effects. This review will touch upon some of the components of bone quality that could be affected by the administration of BPs.

Prolonged alendronate treatment prevents the decline in serum TGF-β1 levels and reduces cortical bone strength in long-term estrogen deficiency rat model

INTRODUCTION

While the anti-resorptive effects of the bisphosphonates (BPs) are well documented, many questions remain about their mechanisms of action, particularly following long-term use. This study evaluated the effects of alendronate (Ale) treatment on TGF-β1 signaling in mesenchymal stem cells (MSCs) and osteocytes, and the relationship between prolonged alendronate treatment on systemic TGF-β1 levels and bone strength.

METHODS

TGF-β1 expression and signaling were evaluated in MSCs and osteocytic MLO-Y4 cells following Ale treatment. Serum total TGF-β1 levels, a bone resorption marker (DPD/Cr), three-dimensional microCT scans and biomechanical tests from both the trabecular and cortical bone were measured in ovariectomized rats that either received continuous Ale treatment for 360 days or Ale treatment for 120 days followed by 240 days of vehicle. Linear regression tests were performed to determine the association of serum total TGF-β1 levels and both the trabecular (vertebrae) and cortical (tibiae) bone strength.

RESULTS

Ale increased TGF-β1 signaling in the MSCs but not in the MLO-Y4 cells. Ale treatment increased serum TGF-β1 levels and the numbers of TGF-β1-positive osteocytes and periosteal cells in cortical bone. Serum TGF-β1 levels were not associated with vertebral maximum load and strength but was negatively associated with cortical bone maximum load and ultimate strength.

CONCLUSIONS

The increase of serum TGF-β1 levels during acute phase of estrogen deficiency is likely due to increased osteoclast-mediated release of matrix-derived latent TGF-β1. Long-term estrogen-deficiency generally results in a decline in serum TGF-β1 levels that are maintained by Ale treatment. Measuring serum total TGF-β1 levels may help to determine cortical bone quality following alendronate treatment.

Positive alterations of viscoelastic and geometric properties in ovariectomized rat femurs with concurrent administration of ibandronate and PTH

Besides bone mineral density (BMD), structural and nano-level viscoelastic properties of bone are also crucial determinants of bone strength. However, treatment induced viscosity changes in osteoporotic bone have seldom been characterized. In this study, the effects of anabolic, antiresorptive and concurrent treatments on ovariectomized rat bones were thoroughly analyzed using multiple bone strength parameters. A total of 52 female Sprague-Dawley rats of 3 months age were divided into 5 groups and subjected to sham (SHM group) or ovariectomy surgery (OVX, PTH, IBN and COM groups). Weekly low-dose parathyroid hormone (PTH) and/or ibandronate or its vehicle was administered subcutaneously to the respective groups starting from 4th week post-surgery. Four rats per group were euthanized every 4 weeks and their femurs were harvested. The BMD, micro-architectural parameters, cortical bone geometry and viscoelastic parameters were measured at the distal femoral metaphysis. Our results showed that PTH, ibandronate or its concurrent treatment can effectively reverse ovariectomy induced deteriorations in both trabecular and cortical bone. Different drugs had selective effects especially in preserving geometric and viscoelastic properties of the bone. The concurrent administration of PTH and ibandronate was shown to offer an added advantage in preserving mean BMD and had a positive effect on cortical bone geometry, resulting from an increased periosteal formation and a decreased endocortical resorption. Viscosity (η) was prominently restored in combined treatment group. It is in accordance with an observed denser alignment of collagen fibers and hydroxyapatite crystal matrix with fewer pores, which may play an important role in hindering fracture propagation.

Histomorphometric analysis of the spine and femur in ovariectomized rats using micro-computed tomographic scan

OBJECTIVE

The purpose of this study was to evaluate the different patterns of bone loss between the lumbar spine and the femur after ovariectomy in rats.

METHODS

Twenty-four female Sprague-Dawley rats underwent a sham operation (the sham group) or bilateral ovariectomy (the ovariectomized group). Four and eight weeks after operation, six rats from each of the two groups were euthanized. Serum biochemical markers of bone turnover including osteocalcin and alkaline phosphatase (ALP), which are sensitive biochemical markers of bone formation, and the telopeptide fragment of type I collagen C-terminus (CTX), which is a sensitive biochemical marker of bone resorption, were analyzed. Bone histomorphometric parameters of the 4th lumbar vertebrae and femur were determined by micro-computed tomography.

RESULTS

Ovariectomized rats were found to have higher osteocalcin, ALP and CTX levels than sham controls. Additionally, 8 weeks after ovariectomy in the OVX group, serum levels of osteocalcin, ALP and CTX were significantly higher than those of 4 weeks after ovariectomy. Bone loss after ovariectomy was more extensive in the 4th lumbar spine compared to the femur. Bone loss in the 4th lumbar spine was mainly caused by trabecular thinning, but in the femur, it was mainly caused by trabecular elimination.

CONCLUSION

The present study demonstrates different patterns of bone loss between the 4th lumbar spine and the femur in ovariectomized rats. Therefore, when considering animal models of osteoporosis, it is important that bone sites should be taken into account.

Daily or monthly ibandronate prevents or restores deteriorations of bone mass, architecture, biomechanical properties and markers of bone turnover in androgen-deficient aged rats

AIM

The aim of this study was to investigate the effects of the bisphosphonate ibandronate (IBN) in a male osteoporosis animal model.

METHODS

Two studies were performed in 9-month-old orchidectomised (ORX) or sham-operated rats. In prevention study, subcutaneous IBN was administered daily (1 μg/kg) or monthly (28 μg/kg every 28 days) starting on day of surgery for 5 months. In treatment study, the same treatment started 6 months after ORX. After sacrifice, bone analyses by dual-energy X-ray absorptiometry, 3-dimensional micro-computed tomography, and 3-point bending were performed in femora or vertebrae. Serum tartrate-resistant acid phosphatase 5b (TRAP-5b) and aminoterminal propeptide of collagen I (PINP) were analysed for resorption and osteocalcin (BGP) for bone formation.

RESULTS

In both studies, ORX resulted in significant femoral and vertebral bone loss and microarchitectural deterioration after 5 months of ORX, and became more pronounced after 11 months. Biomechanical strength was also decreased. Serum levels for TRAP-5b and BGP increased while PINP levels were reduced or unchanged. Both daily and monthly IBN prevented or even restored ORX-induced changes in both studies, with the intermittent regimen showing a improvement in efficacy with respect to many of the biomechanical parameters.

Ibandronate does not reduce the anabolic effects of PTH in ovariectomized rat tibiae: a microarchitectural and mechanical study

Osteoporosis remains a challenging problem. Understanding the regulation on osteoclast and osteoblast by drugs has been of great interest. Both anabolic and anti-resorptive drugs yield positive results in the treatment of osteoporosis. However, whether the concurrent administration of parathyroid hormone (1-34) and ibandronate may offer an advantage over monotherapy is still unknown. This study, therefore, attempts to compare the efficacy of two therapeutical approaches and to investigate the beneficial effects in concurrent therapy in a rat model using three-point bending, pQCT and μCT analysis. A total of 60 female Sprague-Dawley rats of age 10 to 12 weeks were divided into 5 groups (SHAM, OVX+VEH, OVX+PTH, OVX+IBAN, OVX+PTH+IBAN) and subjected to ovariectomy or sham surgery accordingly. Low-dose parathyroid hormone (PTH) and/or ibandronate or its vehicle were administered subcutaneously to the respective groups starting from 4th week post-surgery at weekly intervals. Three rats from each group were euthanized every 2 weeks and their tibiae were harvested. The tibiae were subjected to metaphyseal three-point bending, pQCT and μCT analysis. Serum biomarkers for both bone formation (P1NP) and resorption (CTX) were studied. A total of 11 indices showed a significant difference between SHAM and OVX+VEH groups, suggesting the successful establishment of osteoporosis in the rat model. Compared to the previous studies which showed impedance from bisphosphonates in combination therapy with PTH, our study revealed that ibandronate does not block the anabolic effects of PTH in ovariectomized rat tibiae. Maximum load, strength-strain indices and serum bone formation markers of OVX+PTH+IBAN group are significantly higher than both monotherapy groups. With the proper ratio of anabolic and anti-resorptive drugs, the effect could be more pronounced.

Bone mineral density changes after ovariectomy in rats as an osteopenic model : stepwise description of double dorso-lateral approach

OBJECTIVE

This study describes a method for inducing osteopenia using bilateral ovariectomy (OVX), which causes significant changes in bone mineral density (BMD) in rats.

METHODS

Twenty-five 10-week-old female Sprague Dawley rats were used. Five rats were euthanized after two weeks, and BMD was measured in their femora. The other 20 rats were assigned to one of two groups : a sham group (n = 10), which underwent a sham operation, and an OVX group (n = 10), which underwent bilateral OVX at 12 weeks of age. After six weeks, five rats from each group were euthanized, and BMD was measured in their femora. The same procedures were performed in the remaining rats form each group eight weeks later.

RESULTS

The femur BMD was significantly lower in the six-week OVX group than in the six-week sham group, and in the eight-week OVX group than in the eight-week sham group.

CONCLUSION

Bilateral OVX is a safe method for creating an osteopenic rat model. The significant decrease in BMD appears six weeks after bilateral OVX.

Therapeutic potential of genetically modified adult stem cells for osteopenia

Adult stem cells have therapeutic potential because of their intrinsic capacity for self-renewal, especially for bone regeneration. The present study shows the utility of ex vivo modified mesenchymal stem cells (MSC) to enhance bone density in an immunocompetent mouse model of osteopenia. MSC were transduced ex vivo with a recombinant adeno-associated virus 2 (rAAV2) expressing bone morphogenetic protein 2 (BMP2) under the transcriptional control of collagen type-1alpha promoter. To enrich bone homing in vivo, we further modified the cells to transiently express the mouse alpha4 integrin. The modified MSC were systemically administered to ovariectomized, female C57BL/6 mice. Effects of the therapy were determined by dual-energy X-ray absorptiometry, 3D micro-CT, histology and immunohistochemistry for up to 6 months. Results indicated that mice transplanted with MSC expressing BMP2 showed significant increase in bone mineral density and bone mineral content (P < 0.001) with relatively better proliferative capabilities of bone marrow stromal cells and higher osteocompetent pool of cells compared to control animals. Micro-CT analysis of femora and other bone histomorphometric analyses indicated more trabecular bone following MSC-BMP2 therapy. Results obtained by transplanting genetically modified MSC from green fluorescent protein transgenic mouse suggested that production of BMP2 from transplanted MSC also influenced the mobilization of endogenous progenitors for new bone formation.

Determination of rat vertebral bone compressive fatigue properties in untreated intact rats and zoledronic-acid-treated, ovariectomized rats

SUMMARY

Compressive fatigue properties of whole vertebrae, which may be clinically relevant for osteoporotic vertebral fractures, were determined in untreated, intact rats and zoledronic-acid-treated, ovariectomized rats. Typical fatigue behavior was found and was similar to that seen in other species. Fatigue properties were comparable between both groups.

INTRODUCTION

Osteoporosis is often treated with bisphosphonates, which reduce fracture risk. Effects of bisphosphonates on fatigue strength, which may be clinically relevant for vertebral fractures, are unknown. We determined vertebral, compressive fatigue properties in normal and zoledronic acid (ZOL)-treated, OVX rats.

METHODS

Thirty-five-week old Wistar rats were divided into SHAM-OVX (n = 7) and OVX with ZOL treatment (n = 5; single injection, 20 microg/kg b.w. s.c.). After 16 weeks, vertebral trabecular microarchitecture and cortical thickness were determined using micro-CT. Vertebrae were cyclically compressed in load-control at 2 Hz starting at 0.75% apparent strain. A line parallel to the apparent strain curve was drawn at 0.5% higher offset, after which the intersection was defined as the time to failure and the apparent strain at failure. Data were compared using Student's t test.

RESULTS

Morphology and fatigue properties were the same in both groups. Samples failed between 10 min and 15 h. Force-displacement curves displayed typical fatigue behavior. Displacement increased over time due to mostly creep and to decreasing secant stiffness.

CONCLUSIONS

We established a technique to determine compressive fatigue properties in the rat vertebral body. Our initial results indicate that ZOL-treated OVX rats have similar vertebral fatigue properties as SHAM-OVX controls.

Ibandronate: a review of its use in the management of postmenopausal osteoporosis

Ibandronate (ibandronic acid; Bonviva, Boniva), a nitrogen-containing bisphosphonate available in once-monthly oral and quarterly intravenous formulations for intermittent administration, has been approved for the treatment of osteoporosis in postmenopausal women in the EU, the US and many other countries worldwide. The once-monthly oral formulation has also been approved for the prevention of postmenopausal osteoporosis in the US. Ibandronate is an effective and generally well tolerated bisphosphonate that offers an alternative to other bisphosphonates as a first-line treatment for postmenopausal osteoporosis. It occupies a similar position with respect to the prevention of osteoporosis in postmenopausal women at risk for the disease. The once-monthly oral and quarterly intravenous dosage regimens have the potential to improve treatment adherence and persistence, and hence clinical outcomes, compared with more frequently administered oral bisphosphonates. Intravenous ibandronate may be particularly useful for postmenopausal osteoporotic women who are noncompliant with, or are unable to tolerate or receive, oral bisphosphonates. Thus, intermittent ibandronate extends the range of pharmacological therapies for the treatment and prevention of postmenopausal osteoporosis.

Prolonged treatments with antiresorptive agents and PTH have different effects on bone strength and the degree of mineralization in old estrogen-deficient osteoporotic rats

Current approved medical treatments for osteoporosis reduce fracture risk to a greater degree than predicted from change in BMD in women with postmenopausal osteoporosis. We hypothesize that bone active agents improve bone strength in osteoporotic bone by altering different material properties of the bone. Eighteen-month-old female Fischer rats were ovariectomized (OVX) or sham-operated and left untreated for 60 days to induce osteopenia before they were treated with single doses of either risedronate (500 microg/kg, IV), zoledronic acid (100 microg/kg, IV), raloxifene (2 mg/kg, PO, three times per week), hPTH(1-34) (25 microg/kg, SC, three times per week), or vehicle (NS; 1 ml/kg, three times per week). Groups of animals were killed after days 60 and 180 of treatment, and either the proximal tibial metaphysis or lumbar vertebral body were studied. Bone volume and architecture were assessed by muCT and histomorphometry. Measurements of bone quality included the degree of bone mineralization (DBM), localized elastic modulus, bone turnover by histomorphometry, compression testing of the LVB, and three-point bending testing of the femur. The trabecular bone volume, DBM, elastic modulus, and compressive bone strength were all significantly lower at day 60 post-OVX (pretreatment, day 0 study) than at baseline. After 60 days of all of the bone active treatments, bone mass and material measurements agent were restored. However, after 180 days of treatment, the OVX + PTH group further increased BV/TV (+30% from day 60, p < 0.05 within group and between groups). In addition, after 180 days of treatment, there was more highly mineralized cortical and trabecular bone and increased cortical bone size and whole bone strength in OVX + PTH compared with other OVX + antiresorptives. Treatment of estrogen-deficient aged rats with either antiresorptive agents or PTH rapidly improved many aspects of bone quality including microarchitecture, bone mineralization, turnover, and bone strength. However, prolonged treatment for 180 days with PTH resulted in additional gains in bone quality and bone strength, suggesting that the maximal gains in bone strength in cortical and trabecular bone sites may require a longer treatment period with PTH.

Once-monthly oral ibandronate improves biomechanical determinants of bone strength in women with postmenopausal osteoporosis

CONTEXT

Bone strength and fracture resistance are determined by bone mineral density (BMD) and structural, mechanical, and geometric properties of bone. DESIGN, SETTING, AND OBJECTIVES: This randomized, double-blind, placebo-controlled outpatient study evaluated effects of once-monthly oral ibandronate on hip and lumbar spine BMD and calculated strength using quantitative computed tomography (QCT) with finite element analysis (FEA) and dual-energy x-ray absorptiometry (DXA) with hip structural analysis (HSA).

PARTICIPANTS

Participants were women aged 55-80 yr with BMD T-scores -2.0 or less to -5.0 or greater (n = 93).

INTERVENTION

Oral ibandronate 150 mg/month (n = 47) or placebo (n = 46) was administered for 12 months.

OUTCOME MEASURES

The primary end point was total hip QCT BMD change from baseline; secondary end points included other QCT BMD sites, FEA, DXA, areal BMD, and HSA. All analyses were exploratory, with post hoc P values.

RESULTS

Ibandronate increased integral total hip QCT BMD and DXA areal BMD more than placebo at 12 months (treatment differences: 2.2%, P = 0.005; 2.0%, P = 0.003). FEA-derived hip strength to density ratio and femoral, peripheral, and trabecular strength increased with ibandronate vs. placebo (treatment differences: 4.1%, P < 0.001; 5.9%, P < 0.001; 2.5%, P = 0.011; 3.5%, P = 0.003, respectively). Ibandronate improved vertebral, peripheral, and trabecular strength and anteroposterior bending stiffness vs. placebo [7.1% (P < 0.001), 7.8% (P < 0.001), 5.6% (P = 0.023), and 6.3% (P < 0.001), respectively]. HSA-estimated femoral narrow neck cross-sectional area and moment of inertia and outer diameter increased with ibandronate vs. placebo (respectively 3.6%, P = 0.003; 4.0%, P = 0.052; 2.2%, P = 0.049).

CONCLUSIONS

Once-monthly oral Ibandronate for 12 months improved hip and spine BMD measured by QCT and DXA and strength estimated by FEA of QCT scans.

Anti-resorptives in the management of osteoporosis

Bone-active agents that decrease bone turnover (the anti-resorptive agents) have been, to date, the most thoroughly studied pharmacological agents for the management of osteoporosis in a variety of populations - postmenopausal, male, and glucocorticoid-induced osteoporosis - and have received both Food and Drug Administration (FDA) and Committee for Medicinal Products for Human Use (CHMP) as well as other worldwide registrations for the management of these conditions. While the mechanisms of action of 'anti-resorptives' as a class differ, their effect on increasing bone strength and reducing the risk of fragility fractures share common pathways: an increase in bone mineral content, and a reduction in bone turnover. Within the category of anti-resorptives: estrogen, selective estrogen receptor modulators, tibolone, calcitonin, bisphosphonates and denosumab all reduce vertebral fractures risk, but differ in their ability to reduce the risk of non-vertebral fractures in randomized clinical trials. This chapter will discuss the data on these effects for each class of anti-resorptive agent.

The role of mineralization and organic matrix in the microhardness of bone tissue from controls and osteoporotic patients

Degree of mineralization of bone (DMB) is a major intrinsic determinant of bone strength at the tissue level but its contribution to the microhardness (Vickers indentation) at the intermediary level of organization of bone tissue, i.e., Bone Structural Units (BSUs), has never been assessed. The purpose of this study was to analyze the relationship between the microhardness, the DMB and the organic matrix, measured in BSUs from human iliac bone biopsies. Iliac bone samples from controls and osteoporotic patients (men and women), embedded in methyl methacrylate, were used. Using a Vickers indenter, microhardness (kg/mm2) was measured, either globally on surfaced blocks or focally on 100 microm-thick sections from bone samples (load of 25 g applied during 10 sec; CV=5%). The Vickers indenter was more suited than the Knoop indenter for a tissue like bone in which components are diversely oriented. Quantitative microradiography performed on 100 microm-thick sections, allowed measurement of parameters reflecting the DMB (g/cm3). Assessed on the whole bone sample, both microhardness and DMB were significantly lower (-10% and -7%, respectively) in osteoporotic patients versus controls (p<0.001). When measured separately at the BSU level, there were significant positive correlations between microhardness and DMB in controls (r2=0.36, p<0.0001) and osteoporotic patients (r2=0.43, p<0.0001). Mineralization is an important determinant of the microhardness, but did not explain all of its variance. To highlight the role of the organic matrix in bone quality, microhardness of both osteoid and adjacent calcified matrix were measured in iliac samples from subjects with osteomalacia. Microhardness of organic matrix is 3-fold lower than the microhardness of calcified tissue. In human calcanei, microhardness was significantly correlated with DMB (r2=0.33, p=0.02) and apparent Young's modulus (r2=0.26, p=0.03). In conclusion, bone microhardness measured by Vickers indentation is an interesting methodology for the evaluation of bone strength and its determinants at the BSU level. Bone microhardness is linked to Young's modulus of bone and is strongly correlated to mineralization, but the organic matrix accounts for about one third of its variance.