How do bisphosphonates prevent fractures?

Time to Benefit of Bisphosphonate Therapy for the Prevention of Fractures Among Postmenopausal Women With Osteoporosis: A Meta-analysis of Randomized Clinical Trials

Importance

The clinical decision to initiate bisphosphonate therapy for the treatment of osteoporosis requires balancing shorter-term harms and burdens (eg, gastroesophageal irritation or severe musculoskeletal pain) with longer-term benefits in reducing potential fractures.

Objective

To assess the time to benefit (TTB) of bisphosphonate therapy for the prevention of nonvertebral and other fractures among postmenopausal women with osteoporosis.

Data Sources

Randomized clinical trials (RCTs) were identified from systematic reviews commissioned by the US Preventive Services Task Force (1 review), the Agency for Healthcare Research and Quality (1 review), the Cochrane Library (2 reviews), and the Endocrine Society (1 review).

Study Selection

Studies selected were RCTs involving postmenopausal women with a diagnosis of osteoporosis based on existing vertebral fractures or bone mineral density T scores of -2.5 or lower. The selection process was focused on studies of alendronate, risedronate, and zoledronic acid because they are guideline-recommended first-line agents for reducing nonvertebral fractures. Studies were excluded if they did not focus on women with a primary diagnosis of osteoporosis, had no placebo arm, or had a lack of data on time to fracture.

Data Extraction and Synthesis

Random-effects Weibull survival curves were fitted and Markov chain Monte Carlo methods were used to estimate the absolute risk reduction (ARR) and TTB for each study. These estimates were pooled using a random-effects meta-analysis model.

Main Outcomes and Measures

The primary outcome was the time to 3 different ARR thresholds (0.002, 0.005, and 0.010) for the first nonvertebral fracture. Secondary outcomes included the time to 4 ARR thresholds (0.001, 0.002, 0.005, and 0.010) for hip fracture, any clinical fracture, and clinical vertebral fracture.

Results

Of 67 full-text articles identified, 10 RCTs comprising 23 384 postmenopausal women with osteoporosis were included either as the original RCT or part of subsequently published pooled analyses. Among the studies, the number of participants ranged from 994 to 7765, with mean (SD) age ranging from 63 (7) years to 74 (3) years and follow-up duration ranging from 12 to 48 months. The pooled meta-analysis found that 12.4 months (95% CI, 6.3-18.4 months) were needed to avoid 1 nonvertebral fracture per 100 postmenopausal women receiving bisphosphonate therapy at an ARR of 0.010. To prevent 1 hip fracture, 200 postmenopausal women with osteoporosis would need to receive bisphosphonate therapy for 20.3 months (95% CI, 11.0-29.7 months) at an ARR of 0.005. In addition, 200 postmenopausal women with osteoporosis would need to receive bisphosphonate therapy for 12.1 months (95% CI, 6.4-17.8 months) to avoid 1 clinical vertebral fracture at an ARR of 0.005.

Conclusions and Relevance

This meta-analysis found that the TTB of bisphosphonate therapy was 12.4 months to prevent 1 nonvertebral fracture per 100 postmenopausal women with osteoporosis. These results suggest that bisphosphonate therapy is most likely to benefit postmenopausal women with osteoporosis who have a life expectancy greater than 12.4 months.

Improvements in hip trabecular, subcortical, and cortical density and mass in postmenopausal women with osteoporosis treated with denosumab

In the FREEDOM study, denosumab treatment (60 mg every 6 months) decreased bone resorption, increased bone mineral density (BMD), and reduced new vertebral, nonvertebral, and hip fractures over 36 months in postmenopausal women with osteoporosis. In a subset of these women, hip quantitative computed tomography (QCT) was performed at baseline and months 12, 24, and 36. These scans were analyzed using Medical Image Analysis Framework (MIAF) software, which allowed assessment of total hip integral, trabecular, subcortical, and cortical compartments; the cortical compartment was further divided into 2 areas of interest (outer and inner cortex). This substudy reports changes in BMD and bone mineral content (BMC) from baseline and compared placebo with denosumab over 36 months of treatment (placebo N=26; denosumab N=36). Denosumab treatment resulted in significant improvements in total hip integral volumetric BMD (vBMD) and BMC from baseline at each time point. At month 36, the mean percentage increase from baseline in total hip integral vBMD and BMC was 6.4% and 4.8%, respectively (both p<0.0001). These gains were accounted for by significant increases in vBMD and BMC in the trabecular, subcortical, and cortical compartments. In the placebo group, total hip integral vBMD and BMC decreased at month 36 from baseline by -1.5% and -2.6%, respectively (both p<0.05). The differences between denosumab and placebo were also significant at months 12, 24, and 36 for integral, trabecular, subcortical, and cortical vBMD and BMC (all p<0.05 to <0.0001). While the largest percentage differences occurred in trabecular vBMD and BMC, the largest absolute differences occurred in cortical vBMD and BMC. In summary, denosumab significantly improved both vBMD and BMC from baseline and placebo, assessed by QCT MIAF, in the integral, trabecular, subcortical, and cortical hip compartments, all of which are relevant to bone strength.

Effects of intracortical porosity on fracture toughness in aging human bone: a microCT-based cohesive finite element study

The extent to which increased intracortical porosity affects the fracture properties of aging and osteoporotic bone is unknown. Here, we report the development and application of a microcomputed tomography based finite element approach that allows determining the effects of intracortical porosity on bone fracture by blocking all other age-related changes in bone. Previously tested compact tension specimens from human tibiae were scanned using microcomputed tomography and converted to finite element meshes containing three-dimensional cohesive finite elements in the direction of the crack growth. Simulations were run incorporating age-related increase in intracortical porosity but keeping cohesive parameters representing other age-related effects constant. Additional simulations were performed with reduced cohesive parameters. The results showed a 6% decrease in initiation toughness and a 62% decrease in propagation toughness with a 4% increase in porosity. The reduction in toughnesses became even more pronounced when other age-related effects in addition to porosity were introduced. The initiation and propagation toughness decreased by 51% and 83%, respectively, with the combined effect of 4% increase in porosity and decrease in the cohesive properties reflecting other age-related changes in bone. These results show that intracortical porosity is a significant contributor to the fracture toughness of the cortical bone and that the combination of computational modeling with advanced imaging improves the prediction of the fracture properties of the aged and the osteoporotic cortical bone.

Pamidronate treatment of pediatric fracture patients on chronic steroid therapy

Pediatric nephrology and rheumatology patients with steroid-induced osteopenia are at risk of skeletal fracture. Bisphosphonate therapy has not been routinely advocated as a primary or secondary intervention for steroid-associated fractures in this population. This case control study evaluates the role of pamidronate therapy as a secondary fracture intervention. Children with symptomatic pathological fractures of the axial spine or ribs were treated with pamidronate 1 mg/kg/dose (n=17) IV at 60-day intervals for 1 yr (n=15) or 2 yr (n=2). Bone mineral density of L1-L4 (BMD) was assessed prior to treatment and at six-month intervals, and compared to 17 disease-age-gender-steroid dose-matched control patients. Alkaline phosphatase, calcium, phosphate, PTH, renal biochemistry, and 24-hr urine collections for CrCl, N-telopeptide/creatinine ratio, phosphate excretion, and calcium excretion were obtained every two months in the pamidronate population. Pamidronate caused a first exposure transient flu-like illness lasting <24 h in three patients and one patient had a new pathological fracture. No adverse events of hypocalcemia, allergic reaction or thrombophlebitis were noted. All patients reported improvement of skeletal pain. Despite ongoing steroid treatment, pamidronate significantly increased L1-L4 BMD Z-scores (mean+/-SE) relative to baseline (pamidronate vs control: 0-6 months: 0.27+/-0.14 vs -0.82+/-0.31; 0-12 months: 0.63+/-0.17 vs -0.46+/-0.27; 0-18 months: 0.55+/-0.32 vs 0.17+/-0.27; 0-24 months: 0.15+/-0.21 vs -0.23+/-0.22; 0-30 or 36 months: 0.77+/-0.71 vs -0.68+/-0.25) with repeated measures ANOVA assessment (F=11.27, p=0.0057). This study supports the safety and efficacy of pamidronate in steroid-induced fractures in pediatric nephrology and rheumatology patients.

A theoretical analysis of long-term bisphosphonate effects on trabecular bone volume and microdamage

Bisphosphonates increase bone mass and reduce fracture risk, but their anti-resorptive action may lead to increases in fatigue microdamage. To investigate how bisphosphonate effects influence changes in bone volume and microdamage in the long term, a strain-adaptive model of bone remodeling and microdamage balance was developed for a continuum-level volume of postmenopausal trabecular bone by invoking Frost's mechanostat hypothesis. Both disuse and fatigue microdamage were assumed to stimulate the activation frequency of basic multicellular units (BMUs) such that bone remodeling served to remove excess bone mass and microdamage. Bisphosphonate effects were simulated as follows: low, intermediate, high, or complete suppression of BMU activation frequency either without a change in resorption by the BMU or with an independent decrease in resorption while the bone formation process was unaffected (i.e., formation initially exceeded resorption). Of the bisphosphonate effects, a reduction in resorption relative to formation dictated the long-term gain in bone volume while the potency of activation frequency suppression controlled the rate of gain. A plateau in the bone mass gain that typically occurs in clinical studies of bisphosphonate treatment was predicted by the model because the resultant reduction in strain forced bone formation by the BMU to decrease over time until it matched the reduction in BMU resorption. A greater suppression of activation frequency proportionally increased microdamage, but the accumulation was limited over the long term as long as remodeling was incompletely suppressed. The results of the model suggest creating bisphosphonates that provide minimal suppression of remodeling and a large decrease in BMU resorption because this would minimize damage accumulation and increase bone mass, respectively.