Babul Borah

Three-dimensional microimaging (MRμI and μCT), finite element modeling, and rapid prototyping provide unique insights into bone architecture in osteoporosis

With the proportion of elderly people increasing in many countries, osteoporosis has become a growing public health problem, with rising medical, social, and economic consequences. It is well recognized that a combination of low bone mass and the deterioration of the trabecular architecture underlies osteoporotic fractures. A comprehensive understanding of the relationships between bone mass, the three‐dimensional (3D) architecture of bone and bone function is fundamental to the study of new and existing therapies for osteoporosis. Detailed analysis of 3D trabecular architecture, using high‐resolution digital imaging techniques such as magnetic resonance microimaging (MRμI), micro‐computed tomography (μCT), and direct image analysis, has become feasible only recently. Rapid prototyping technology is used to replicate the complex trabecular architecture on a macroscopic scale for visual or biomechanical analysis. Further, a complete set of 3D image data provides a basis for finite element modeling (FEM) to predict mechanical properties. The goal of this paper is to describe how we can integrate three‐dimensional microimaging and image analysis techniques for quantitation of trabecular bone architecture, FEM for virtual biomechanics, and rapid prototyping for enhanced visualization. The integration of these techniques provide us with an unique ability to investigate the role of bone architecture in osteoporotic fractures and to support the development of new therapies. Anat Rec (New Anat) 265:101–110, 2001.

Risedronate Preserves Trabecular Architecture and Increases Bone Strength in Vertebra of Ovariectomized Minipigs as Measured by Three‐Dimensional Microcomputed Tomography

Risedronate reduces the risk of new vertebral fractures up to 70% within 1 year of treatment in patients with osteoporosis. Both increases in bone mass and preservation of bone architecture are thought to contribute to antifracture effects. Our objectives were to determine the effects of risedronate on trabecular bone mass and architecture and to determine the relative contributions of mass and architecture to strength in the vertebra of ovariectomized (OVX) minipigs. The minipigs were OVX at 18 months of age and were treated daily for 18 months with either vehicle or risedronate at doses of 0.5 mg/kg per day or 2.5 mg/kg per day. The three‐dimensional (3D) bone architecture of the L4 vertebral cores of Sinclair S1 minipigs was evaluated by 3D microcomputed tomography (μCT). Compared with the OVX control, the vertebral bone volume (bone volume/tissue volume [BV/TV]) was higher in both treated groups (p < 0.05). The architectural changes were more significant at the 2.5‐mg/kg dose and were more prevalent at the cranial‐caudal ends compared with the midsection. At the higher dose, the trabecular thickness (Tb.Th), trabecular number (Tb.N), and connectivity were higher, and marrow star volume (Ma.St.V) and trabecular separation (Tb.Sp) were lower (p < 0.05). The trabecular separation variation index(TSVI), a new measure to approximate structural variations, was smaller in the 2.5‐mg/kg‐treated group (p < 0.05). In this group, a significant preservation of trabeculae orthogonal to the cranial‐caudal axis was confirmed by a decrease in the degree of anisotropy (DA) and an increase in the percent Cross‐strut (%Cross‐strut; p < 0.05). Both normalized maximum load (strength) and normalized stiffness of the same vertebral cores were higher in the 2.5‐mg/kg risedronate group compared with the OVX group (p < 0.05). BV/TV alone could explain 76% of the variability of the bone strength. The combination of bone volume and architectural variables explained >90% of the strength. The study showed that risedronate preserved trabecular architecture in the vertebra of OVX minipigs, and that bone strength is tightly coupled to bone mass and architecture.

Risedronate Preserves Bone Architecture in Early Postmenopausal Women In 1 Year as Measured by Three-Dimensional Microcomputed Tomography

Risedronate reduces the risk of vertebral fractures by up to 70% within the first year of treatment. Increases in bone mineral density or decreases in bone turnover markers explain only a portion of the anti-fracture effect, suggesting that other factors, such as changes in trabecular bone architecture, also play a role. Our objective was to determine the effects of risedronate on bone architecture by analyzing iliac crest bone biopsy specimens using three-dimensional microcomputed tomography (3-D µCT). Biopsy specimens were obtained at baseline and after 1 year of treatment from women enrolled in a double-blind, placebo-controlled study of risedronate 5 mg daily for the prevention of early postmenopausal bone loss. Trabecular architecture deteriorated in the placebo group (n = 12), as indicated by a 20.3% decrease in bone volume (25.1% vs. 20.0%, P = 0.034), a 13.5% decrease in trabecular number (1.649 vs. 1.426 mm−1, P = 0.052), a 13.1% increase in trabecular separation (605 vs. 684 µm, P = 0.056), and an 86.2% increase in marrow star volume (3.251 vs. 6.053 mm3, P = 0.040) compared with baseline values. These changes in architectural parameters occurred in the presence of a concomitant decrease from baseline in lumbar spine bone mineral density (−3.3%, P = 0.002), as measured by dual energy x-ray absorptiometry. There was no statistically significant (P < 0.05) deterioration in the risedronate-treated group (n = 14) over the 1-year treatment period. Comparing the actual changes between the two groups, the placebo group experienced decreases in bone volume (placebo, −5.1%; risedronate, +3.5%; P = 0.011), trabecular thickness (placebo, −20 µm; risedronate, +23 µm; P = 0.032), and trabecular number (placebo, −0.223 mm−1; risedronate, +0.099 mm−1; P = 0.010), and increases in percent plate (placebo, +2.79%; risedronate, −3.23%; P = 0.018), trabecular separation (placebo, +79 µm; risedronate, −46 µm; P = 0.010) and marrow star volume (placebo, +2.80 mm3 ; risedronate, −2.08mm3; P = 0.036), compared with the risedronate group. These data demonstrate that trabecular architecture deteriorated significantly in this cohort of early postmenopausal women, and that this deterioration was prevented by risedronate. Although there is no direct link in this study between fracture and preservation of architecture, it is reasonable to infer that the preservation of bone architecture may play a role in risedronate’s anti-fracture efficacy.

Evaluation of changes in trabecular bone architecture and mechanical properties of minipig vertebrae by three-dimensional magnetic resonance microimaging and finite element modeling

The study objective was to analyze the three‐dimensional (3D) trabecular architecture and mechanical properties in vertebral specimens of young and mature Sinclair minipigs to assess the relative contribution of architecture to bone strength. We used 3D magnetic resonance microimaging (MRμI) and direct image analysis to evaluate a set of standard structural measurements and new architectural descriptors of trabecular bone in biopsy specimens from L2, L3, and L4 vertebrae (n = 16 in each group) from young (mean age, 1.2 years) and mature (mean age, 4.8 years) minipigs. The measurements included bone volume/tissue volume (BV/TV), marrow star volume (Ma.St.V), connectivity density (ConnD), and two new parameters, percent platelike trabeculae (% plate) and percent bone in the load direction (% boneLD). The % plate, calculated from surface curvature, allowed the delineation of plates from rods. The % boneLD quantified the percentage of bone oriented along the long axis of the vertebral body. We showed that 3D MRμI can detect the subtle changes in trabecular architecture between the two age groups. ConnD, star volume, % plate, % boneLD, and BV/TV were found to be more effective than the model‐based, derived indices (trabecular thickness [Tb.Th], trabecular separation [Tb.Sp], and trabecular number [Tb.N]) in differentiating the structural changes. BV/TV, % plate, and % boneLD significantly increased (p < 0.05) in all three vertebral sites of the mature minipigs. The significant decrease in ConnD and star volume in the mature vertebra was consistent with the concurrent increase of platelike trabecular bone (p < 0.05). Overall, ConnD, star volume, % plate, and % boneLD provided a coherent picture of the architectural changes between the two age groups. Apparent modulus and maximum stress were determined experimentally on biopsy specimens from L2 vertebrae (n = 16). When apparent modulus was predicted using 3D MRμI data sets as input for finite element modeling (FEM), the results were similar to the experimentally determined apparent modulus (p = 0.12). Both methods were then used to compare the young and the mature animals; the experimental and predicted apparent modulus were significantly higher for the mature group (p = 0.003 and 0.012, respectively). The experimental maximum stress in the vertebra of the mature animals was twice as high as that for the young animals (p = 0.006). Bone quantity (BV/TV or bone mineral content [BMC]) alone could explain approximately 74–85% of the total variability in stress and modulus. The inclusion of either ConnD or % boneLD with BV/TV in a multiple regression analysis significantly improved the predictability of maximum stress, indicating that architecture makes additional contributions to compressive strength in normal minipig vertebra.

Biomechanical effects of teriparatide in women with osteoporosis treated previously with alendronate and risedronate: Results from quantitative computed tomography-based finite element analysis of the vertebral body

Previous antiresorptive treatment may influence the anabolic response to teriparatide. The OPTAMISE (Open-label Study to Determine How Prior Therapy with Alendronate or Risedronate in Postmenopausal Women with Osteoporosis Influences the Clinical Effectiveness of Teriparatide) study reported greater increases in biochemical markers of bone turnover and volumetric bone mineral density (BMD) when 12 months of teriparatide treatment was preceded by 2 years or more of risedronate versus alendronate treatment. The objective of this study was to use quantitative computed tomography (CT)-based nonlinear finite element modeling to evaluate how prior therapy with alendronate or risedronate in postmenopausal women with osteoporosis influences the biomechanical effectiveness of teriparatide. Finite element models of the L1 vertebra were created from quantitative CT scans, acquired before and after 12 months of therapy with teriparatide, from 171 patients from the OPTAMISE study. These models were subjected to uniaxial compression. Total BMD-derived bone volume fraction (BV/TV(d), i.e., bone volume [BV]/total volume [TV]), estimated from quantitative CT-based volumetric BMD, vertebral stiffness, and failure load (strength) were calculated for each time measurement point. The results of this study demonstrated that 12 months of treatment with teriparatide following prior treatment with either risedronate or alendronate increased BMD-derived BV/TV(d), the predicted vertebral stiffness, and failure load. However, the effects of teriparatide were more pronounced in patients treated previously with risedronate, which is consistent with the findings of the OPTAMISE study. The mean (+/-standard error) increase in stiffness was greater in the prior risedronate group than the prior alendronate group (24.6+/-3.2% versus 14.4+/-2.8%, respectively; p=0.0073). Similarly, vertebral failure load increased by 27.2+/-3.5% in the prior risedronate group versus 15.3+/-3.1% in the prior alendronate group (p=0.0042). The mechanical variables increased in greater proportion than BV/TV(d), which increased by 6.9+/-0.9% versus 4.6+/-0.8% in the prior-risedronate and prior-alendronate groups, respectively (p=0.0290). Our finding indicated that while teriparatide can be used with success on patients who have previously undergone treatment with risedronate and alendronate, it demonstrated greater anabolic effect on biomechanical properties in prior-risedronate patients in the first year of teriparatide treatment.

Application of LC-NMR and LC-MS to the identification of degradation products of a protease inhibitor in dosage formulations

LC-NMR and LC-MS were applied to the characterization of six degradation products of a protease inhibitor, N-hydroxy-1,3-di-[4-ethoxybenzenesulphonyl]-5,5-dimethyl-[1,3]c yclohexyldiazine-2-carboxamide, in a dosage formulation. A reversed-phase HPLC method was developed for the separation of the parent compound and its six degradation products. LC-MS was then utilized to obtain the molecular weight and fragmentation information using an electrospray ionization (ESI) interface in the positive ion mode. LC-NMR was employed to acquire detailed structural information using a selective solvent suppression pulse sequence in the stop flow mode. This work demonstrated the usefulness of this integrated approach for the rapid and unambiguous identification of drug compounds and their degradation products in dosage formulations.

¹⁸F-fluoride PET as a non-invasive imaging biomarker for determining treatment efficacy of bone active agents at the hip

The functional imaging technique of 18F‐fluoride positron emission tomography (18F‐PET) allows the noninvasive quantitative assessment of regional bone formation at any skeletal site, including the spine and hip. The aim of this study was to determine if 18F‐PET can be used as an early biomarker of treatment efficacy at the hip. Twenty‐seven treatment‐naive postmenopausal women with osteopenia were randomized to receive teriparatide and calcium and vitamin D (TPT group, n = 13) or calcium and vitamin D only (control group, n = 14). Subjects in the TPT group were treated with 20 µg/day teriparatide for 12 weeks. 18F‐PET scans of the proximal femur, pelvis, and lumbar spine were performed at baseline and 12 weeks. The plasma clearance of 18F‐fluoride to bone, Ki, a validated measurement of bone formation, was measured at four regions of the hip, lumbar spine, and pelvis. A significant increase in Ki was observed at all regions of interest (ROIs), including the total hip (+27%, p = 0.002), femoral neck (+25%, p = 0.040), hip trabecular ROI (+21%, p = 0.017), and hip cortical ROI (+51%, p = 0.001) in the TPT group. Significant increases in Ki in response to TPT were also observed at the lumbar spine (+18%, p = 0.001) and pelvis (+42%, p = 0.001). No significant changes in Ki were observed for the control group. Changes in BMD and bone turnover markers were consistent with previous trials of teriparatide. In conclusion, this is the first study to our knowledge to demonstrate that 18F‐PET can be used as an imaging biomarker for determining treatment efficacy at the hip as early as 12 weeks after initiation of therapy.

Efficacy of anti-sclerostin monoclonal antibody BPS804 in adult patients with hypophosphatasia

BACKGROUND. Hypophosphatasia (HPP) is a rare genetic disorder resulting in variable alterations of bone formation and mineralization that are caused by mutations in the ALPL gene, encoding the tissue-nonspecific alkaline phosphatase (ALP) enzyme. METHODS. In this phase IIA open-label, single-center, intra-patient, dose-escalating study, adult patients with HPP received 3 ascending intravenous doses of 5, 10, and 20 mg/kg BPS804, a fully human anti-sclerostin monoclonal antibody, on days 1, 15, and 29, respectively. Patients were followed for 16 weeks after the last dose. We assessed the pharmacodynamics, pharmacokinetics, preliminary efficacy, and safety of BPS804 administrations at specified intervals during treatment and follow-up. RESULTS. Eight patients (mean age 47.8 years) were enrolled in the study (6 females, 2 males). BPS804 treatment increased mean ALP and bone-specific ALP enzymatic activity between days 2 and 29. Transient increases in the bone formation markers procollagen type-I N-terminal propeptide (PINP), osteocalcin, and parathyroid hormone as well as a transient decrease in the bone resorption marker C-telopeptide of type I collagen (CTX-1) were observed. Lumbar spine bone mineral density showed a mean increase by day 85 and at end of study. Treatment-associated adverse events were mild and transient. CONCLUSION. BPS804 treatment was well tolerated and resulted in increases in bone formation biomarkers and bone mineral density, suggesting that sclerostin inhibition could be applied to enhance bone mineral density, stability, and regeneration in non-life-threatening clinical situations in adults with HPP. TRIAL REGISTRATION. Clinicaltrials.gov NCT01406977. FUNDING. Novartis Institutes for BioMedical Research, Basel, Switzerland.

Architecture Is One of the Determinants of Bone Strength

In our recent publication in JBMR, we assessed the effects of risedronate on trabecular architecture and compressive strength of lumbar vertebrae from ovariectomized (OVX) minipigs. Regression analysis showed that trabecular bone volume (BV/TV) explained 76% of the variability of compressive strength. Adding architectural parameters to BV/TV in a three-parameter multiple linear regression model increased the R value to 91%. We concluded that risedronate improves the three-dimensional trabecular architecture in the vertebra of OVX minipigs in a way that contributes to an increase in bone strength. In their letter to the editor, Drs Kiebzak and Miller commented that our data lacked critical information, namely, ash weight or mineral content. Our decision not to include bone mineral content (BMC) or ash weight was based on data from a previous study on minipig vertebral cores in which there was a strong correlation between BV/TV and BMC (R 0.82). For completeness, however, we now report dual-energy X-ray absorptiometry (DXA) data that we collected from the minipig vertebral cores before microcomputed tomography measurements. After 18 months of treatment, risedronate significantly increased bone mineral density (BMD) compared with OVX controls (0.303 0.040 vs. 0.248 0.037 g/cm). Trabecular BV/TV (measured by microcomputed tomography) and BMD (measured by DXA) correlated strongly (R 0.96). In a simple linear regression model, BMD correlated strongly (R 0.80) with strength. Adding the architectural parameters TSVI and Tb.N to BMD in a three-parameter multiple linear regression model increased R to 0.91. The conclusion, therefore, is that although BV/TV and BMD correlate strongly with compressive strength, neither explains all the variance in bone strength. Although the contribution of trabecular architecture to bone strength, as indicated by change in R, was less than 15% in these minipigs, trabecular bone volume was high ( 25–33%). Architecture is likely to contribute more to bone strength and fracture risk in low bone mass conditions, and this is supported by data from several studies. Key architectural parameters have an exponential relationship with BV/TV. Furthermore, there were larger changes in architectural parameters when BV/TV was less than 11%, a value that has been suggested as threshold for spontaneous vertebral fracture. For trabecular bone in human biopsy specimens from different skeletal sites, BV/TV explained only 37–67% of the variance in elastic constants, especially at low bone volume. The prediction of the mechanical properties was significantly improved when architectural indices were included with bone volume. These data show that the relative contribution of architecture to strength increases as the bone mass decreases. It was not our intention to downplay the importance of BMD. Low BMD is clearly associated with a high risk of fragility fracture. However, there is now strong clinical evidence that an increase in BMD does not fully explain the fracture benefits of antiresorptive agents. It is appropriate, therefore, that we revisit the paradigm that BMD alone is an adequate surrogate for bone strength. We agree with Drs Kiebzak and Miller that the effectiveness of antiresorptive therapies for osteoporosis may be related to increase in BMD as well as to changes in one or several other factors that contribute to better bone quality. Our focus on the role of architecture does not preclude the importance of other components such as decreased bone turnover and osteocyte viability. Continued investigation into the effects of treatments on bone quality will improve our understanding of the pathophysiology and treatment of bone diseases.