Duration-Dependent Increase of Human Bone Matrix Mineralization in Long-Term Bisphosphonate Users with Atypical Femur Fracture

Differences in tissue-level properties as assessed by nano-scratching in patients with and without atypical femur fractures on long-term bisphosphonate therapy: a proof-of-concept pilot study

Atypical femur fractures (AFFs) are a well-established complication of long-term bisphosphonate (BP) therapy, but their pathogenesis is not fully understood. Although many patients on long-term BP therapy have severe suppression of bone turnover (SSBT), not all such patients experience AFF, even though SSBT is a major contributor to AFF. Accordingly, we evaluated tissue level properties using nano-scratch testing of trans-iliac bone biopsy specimens in 12 women (6 with and 6 without AFF matched for age and race). Nano-scratch data were analyzed using a mixed-model ANOVA with volume-normalized scratch energy as a function of AFF (Yes or No), region (periosteal or endosteal), and a first-order interaction between region and AFF. Tukey post hoc analyses of the differences of least squared means of scratch energy were performed and reported as significant if p<.05. The volume-normalized scratch energy was 10.6% higher in AFF than in non-AFF patients (p=.003) and 17.9 % higher in the periosteal than in the endosteal region (p=.004). The differences in normalized scratch energy are suggestive of a higher hardness of the bone tissue after long-term BP therapy. The results of this study are consistent with other studies in the literature and demonstrate the efficacy of using Nano-Scratch technique to evaluate bone tissue that exhibits SSBT and AFF. Further studies using nano-scratch may help quantify and elucidate underlying mechanisms for the pathogenesis of AFF.

Is there genetic susceptibility for atypical femoral fractures?

The mechanisms underlying AFF remain unclear, with hypotheses including bone turnover suppression and morphological variation. Recent studies have suggested a potential genetic susceptibility to AFF. A scoping review was conducted using PubMed to identify studies published since 2016. Twenty-one studies were identified, focusing on histological and genetic analysis of AFF patients and Bisphosphonates users. Biopsies and imaging modalities were used to assess histological and morphometric parameters, while genetic sequencing was performed to identify variants in target genes. Genetic studies identified variants in geranylgeranyl diphosphate synthase 1 (GGPS1) and CYP1A1 genes, which play roles in osteoclast function and drug metabolism, respectively. Functional analysis revealed reduced enzymatic activity in mutant variants of these genes, which could be further inhibited by BP use. Other genes, such as ATRAID, ALPL, and COL1A2, were also associated with AFF. Histomorphometric studies supported the hypothesis of bone turnover suppression in AFF, with alterations in tissue mechanical properties and microarchitecture observed, particularly in cortical bone. The findings suggest a potential genetic susceptibility to AFF, with variants in GGPS1 and CYP1A1 genes affecting osteoblast and osteoclast function. Bone turnover suppression and altered tissue properties contribute to the pathogenesis of AFF.

An integrated experimental-computational framework to assess the influence of microstructure and material properties on fracture toughness in clinical specimens of human femoral cortical bone

Microstructural and compositional changes that occur due to aging, pathological conditions, or pharmacological treatments alter cortical bone fracture resistance. However, the relative importance of these changes to the fracture resistance of cortical bone has not been quantified in detail. In this technical note, we developed an integrated experimental-computational framework utilizing human femoral cortical bone biopsies to advance the understanding of how fracture resistance of cortical bone is modulated due to modifications in its microstructure and material properties. Four human biopsy samples from individuals with varying fragility fracture history and osteoporosis treatment status were converted to finite element models incorporating specimen-specific material properties and were analyzed using fracture mechanics-based modeling. The results showed that cement line density and osteonal volume had a significant effect on crack volume. The removal of cement lines substantially increased the crack volume in the osteons and interstitial bone, representing straight crack growth, compared to models with cement lines due to the lack of crack deflection in the models without cement lines. Crack volume in the osteons and interstitial bone increased when mean elastic modulus and ultimate strength increased and mean fracture toughness decreased. Crack volume in the osteons and interstitial bone was reduced when material property heterogeneity was incorporated in the models. Although both the microstructure and the heterogeneity of the material properties of the cortical bone independently increased the fracture toughness, the relative contribution of the microstructure was more significant. The integrated experimental-computational framework developed here can identify the most critical microscale features of cortical bone modulated by pathological processes or pharmacological treatments that drive changes in fracture resistance and improve our understanding of the relative influence of microstructure and material properties on fracture resistance of cortical bone.

Incidence of Atypical Femoral Fractures in Patients on Osteoporosis Therapy – a Registry‐based Cohort Study

Atypical femoral fractures (AFFs) have been reported in patients taking bisphosphonates (BPs) for osteoporosis therapy but also in patients with no exposure to these drugs. In contrast, less is known about the incidence of AFFs in patients taking denosumab. This registry‐based cohort study analyzed the incidence of AFFs in patients with suspected or confirmed osteoporosis who were included in the osteoporosis register of the Swiss Society of Rheumatology between January 2015 and September 2019. Statistical analyses included incidence rates, rate ratios, and hazard ratios for AFFs, and considered sequential therapies and drug holidays as time‐dependent covariates. Among the 9956 subjects in the cohort, 53 had subtrochanteric or femoral shaft fractures. Ten fractures occurred under BP or denosumab treatment and two under teriparatide therapy. Five fractures were classified as AFFs based on the revised American Society of Bone and Mineral Research case definition of AFFs from 2014. Three AFFs occurred in women being treated with denosumab at the time of diagnosis, all with prior BP use (10, 7, and 1 years, respectively). One AFF developed in a woman receiving ibandronate and one arose in a woman receiving glucocorticoids rather than antiresorptive therapy. The incidence of AFFs per 10,000 observed patient‐years was 7.1 in patients receiving denosumab and 0.9 in patients with BP‐associated AFFs, yielding a rate ratio of 7.9 (95% confidence interval [CI] 0.63–413), p = 0.073. The risk of AFFs was not significantly higher in patients receiving denosumab therapy compared with BP therapy (hazard ratio = 7.07, 95% CI 0.74–68.01, p = 0.090). We conclude that the risk of AFFs is low in patients taking BPs, denosumab, or both sequentially. All three patients with AFFs under denosumab therapy had undergone prior BP therapy. © 2022 The Authors. JBMR Plus published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research.

Bone Mineral and Organic Properties in Postmenopausal Women Treated With Denosumab for Up to 10 years

In postmenopausal women with osteoporosis, denosumab (DMAb) therapy through 10 years resulted in significantly higher degree of mineralization of bone, with a subsequent increase from years 2-3 to year 5 and no further difference between years 5 and 10. Our aim was to assess the variables reflecting the quality of bone mineral and organic matrix (Fourier transform infrared microspectroscopy), and the microhardness of bone (Vickers microindentation). Cross-sectional assessments were performed in blinded fashion on iliac bone biopsies from osteoporotic women (72 from FREEDOM trial, 49 from FREEDOM Extension trial), separately in cortical and cancellous compartments. After 2-3 years of DMAb, mineral/matrix ratio and microhardness of cortical bone were significantly higher compared with placebo, whereas mineral maturity, mineral crystallinity, mineral carbonation, and collagen maturity were not different in both bone compartments. Through 5 years of DMAb, mineral carbonation was significantly lower and mineral/matrix ratio, mineral maturity, and crystallinity were significantly higher versus 2-3 years and were not different between 5 and 10 years, with the exception of mineral maturity in cancellous bone. These data support a transition of mineral to more mature crystals (within physiological range) and the completeness of secondary mineralization within 5 years of DMAb treatment. Microhardness in cortical and cancellous compartments was significantly lower at 5 years of DMAb versus 2-3 years and was not different from years 5 to 10. The lower microhardness at years 5 and 10 is likely the result of maturation of the organic matrix in a persistently low state of bone remodeling over 5 and 10 years. © 2022 American Society for Bone and Mineral Research (ASBMR).

Skeletal outcomes of patients with osteogenesis imperfecta during drug holiday of bisphosphonates: a real-world study

PURPOSE

This study aimed to investigate the skeletal outcomes of patients with osteogenesis imperfecta (OI) who received bisphosphonate (BP) treatment and entered drug holiday after achieving an age- and sex-specific bone mineral density (BMD) reference.

METHODS

Patients with OI receiving BP treatment were enrolled when they entered drug holidays of BPs. The skeletal outcomes were evaluated in detail during the drug holiday, including BMD, X-ray of the bone, bone fracture incidence, and bone turnover biomarkers. The pathogenic mutations of OI were identified by next-generation sequencing and confirmed by Sanger sequencing.

RESULTS

A total of 149 OI patients (127 juveniles and 22 adults) who entered drug holidays after nearly 4 years of BP treatment were included. Areal BMD at the lumbar spine increased from 0.934 ± 0.151 to 0.990 ± 0.142 g/cm² and was stable in the second (1.029 ± 0.176 g/cm²) and third years (1.023 ± 0.174 g/cm²) of BP drug holidays, and BMD at the femoral neck, trochanter, and total hip had no significant change, but it was gradually inferior to that of the same-gender juveniles in the second and third years of the drug holiday. BMD at the lumbar spine and proximal hip did not change and was inferior to that of the same-gender adults. The average time of fractures fluctuated from 0.18 to 0.08 per year in juveniles, while only one adult suffered from a fracture during BP drug holidays. Bone turnover markers were in the normal range, except for a mildly high level of β-carboxy-terminal cross-linked telopeptide of type 1 collagen in the juvenile group. A total of 17 (11.4%) patients received BP retreatment because of bone loss during the drug holiday. OI type III and type IV and COL1A2 mutation were correlated to a longer duration of BP treatment to enter drug holidays (all p < 0.05). Old age at initial treatment (OR, 1.056) and OI type III (OR, 10.880) were correlated to a higher risk of BP retreatment.

CONCLUSIONS

OI patients will undergo nearly 4 years of BP treatment to achieve drug holidays. During the 3 years of the drug holiday, the patients' BMD is stable, and fracture incidence does not increase significantly. Patients are more inclined to need retreatment during drug holidays owing to the late start of BP treatment and more severe OI phenotypes.

Bone Nanomechanical Properties and Relationship to Bone Turnover and Architecture in Patients With Atypical Femur Fractures: A Prospective Nested Case-Control Study

Atypical femur fractures (AFFs) are well‐established serious complication of long‐term bisphosphonate and denosumab therapy in patients with osteopenia or osteoporosis. To elucidate underlying mechanism(s) for the development of AFF, we performed a nested case‐control study to investigate bone tissue nanomechanical properties and prevailing bone microstructure and tissue‐level remodeling status as assessed by bone histomorphometry. We hypothesized that there would be differences in nanomechanical properties between patients with and without AFF and that bone microstructure and remodeling would be related to nanomechanical properties. Thirty‐two full‐thickness transiliac bone biopsies were obtained from age‐ and sex‐matched patients on long‐term bisphosphonate therapy with (n = 16) and without an AFF (n = 16). Standard histomorphometric measurements were made in each sample on three different bone envelopes (cancellous, intracortical, and endosteal). Iliac bone wall thickness was significantly lower on all three bone surfaces in patients with AFF than in those without AFF. Surface‐based bone formation rate was suppressed similarly in both groups in comparison to healthy premenopausal and postmenopausal women, with no significant difference between the two groups. Nanoindentation was used to assess material properties of cortical and cancellous bone separately. Elastic modulus was higher in cortical than in cancellous bone in patients with AFF as well as compared to the elastic modulus of cortical bone from non‐AFF patients. However, the elastic modulus of the cancellous bone was not different between AFF and non‐AFF groups or between cortical and cancellous bone of non‐AFF patients. Resistance to plastic deformation was decreased in cortical bone in both AFF and non‐AFF groups compared to cancellous bone, but to a greater extent in AFF patients. We conclude that long‐term bisphosphonate therapy is associated with prolonged suppression of bone turnover resulting in altered cortical remodeling and tissue nanomechanical properties leading to AFF. © 2021 The Authors. JBMR Plus published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research.

Biomechanical mechanisms of atypical femoral fracture

Antiresorptives such as bisphosphonates (BP) and denosumab are commonly used osteoporosis treatments that are effective in preventing osteoporotic fractures by suppressing bone turnover. Although these treatments reduce fracture risk, their long-term use has been associated with atypical femoral fracture (AFF), a rare potential side effect. Despite its rare occurrence, AFF has had a disproportionately significant adverse impact on society due to its severe outcomes such as loss of function and delayed healing. These severe outcomes have led to the decrease in the use and prescription of osteoporosis treatment drugs due to patient anxiety and clinician reluctance. This creates the risk for increasing osteoporotic fracture rates in the population. The existing information on the pathogenesis of AFF primarily relies on retrospective observational studies. However, these studies do not explain the underlying mechanisms that contribute to AFF, and therefore the mechanistic origins of AFF are still poorly understood. The purpose of this review is to outline the current state of knowledge of the mechanical mechanisms of AFF. The review focuses on three major potential mechanical mechanisms of AFF based on the current literature which are (1) macroscale femoral geometry which influences the stress/strain distribution in the femur under loading; (2) bone matrix composition, potentially altered by long-term remodeling suppression by BPs, which directly influences the material properties of bone and its mechanical behavior; and (3) microstructure, potentially altered by long-term remodeling suppression by BPs, which impacts fracture resistance through interaction with crack propagation. In addition, this review presents the critical knowledge gaps in understanding AFF and also discusses approaches to closing the knowledge gap in understanding the underlying mechanisms of AFF.

Cortical bone development, maintenance and porosity: genetic alterations in humans and mice influencing chondrocytes, osteoclasts, osteoblasts and osteocytes

Cortical bone structure is a crucial determinant of bone strength, yet for many years studies of novel genes and cell signalling pathways regulating bone strength have focused on the control of trabecular bone mass. Here we focus on mechanisms responsible for cortical bone development, growth, and degeneration, and describe some recently described genetic-driven modifications in humans and mice that reveal how these processes may be controlled. We start with embryonic osteogenesis of preliminary bone structures preceding the cortex and describe how this structure consolidates then matures to a dense, vascularised cortex containing an increasing proportion of lamellar bone. These processes include modelling-induced, and load-dependent, asymmetric cortical expansion, which enables the cortex's transition from a highly porous woven structure to a consolidated and thickened highly mineralised lamellar bone structure, infiltrated by vascular channels. Sex-specific differences emerge during this process. With aging, the process of consolidation reverses: cortical pores enlarge, leading to greater cortical porosity, trabecularisation and loss of bone strength. Each process requires co-ordination between bone formation, bone mineralisation, vascularisation, and bone resorption, with a need for locational-, spatial- and cell-specific signalling pathways to mediate this co-ordination. We will discuss these processes, and a number of cell-signalling pathways identified in both murine and human genetic studies to regulate cortical bone mass, including signalling through gp130, STAT3, PTHR1, WNT16, NOTCH, NOTUM and sFRP4.