Presence of pyrophosphate in bone from an atypical femoral fracture site: A case report

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.

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.

Magnesium whitlockite - omnipresent in pathological mineralisation of soft tissues but not a significant inorganic constituent of bone

Whitlockite is a calcium phosphate that was first identified in minerals collected from the Palermo Quarry, New Hampshire. The terms magnesium whitlockite [Mg-whitlockite; Ca₁₈Mg₂(HPO₄)₂(PO₄)₁₂] and beta-tricalcium phosphate [β-TCP; β-Ca₃(PO₄)₂] are often used interchangeably since Mg-whitlockite is not easily distinguished from β-Ca₃(PO₄)₂ by powder X-ray diffraction although their crystalline structures differ significantly. Being both osteoconductive and bioresorbable, Mg-whitlockite is pursued as a synthetic bone graft substitute. In recent years, advances in development of synthetic Mg-whitlockite have been accompanied by claims that Mg-whitlockite is the second most abundant inorganic constituent of bone, occupying as much as 20-35 wt% of the inorganic fraction. To find evidence in support of this notion, this review presents an exhaustive summary of Mg-whitlockite identification in biological tissues. Mg-whitlockite is mainly found in association with pathological mineralisation of various soft tissues and dental calculus, and occasionally with enamel and dentine. With the exception of high-temperature treated tumoural calcified deposits around interphalangeal and metacarpal joints and rhomboidal Mg-whitlockite crystals in post-apoptotic osteocyte lacunae in human alveolar bone, this unusual mineral has never been detected in the extracellular matrix of mammalian bone. Characterisation techniques capable of unequivocally distinguishing between different calcium phosphate phases, such as high-resolution imaging, crystallography, and/or spectroscopy have exclusively identified bone mineral as poorly crystalline, ion-substituted, carbonated apatite. The idea that Mg-whitlockite is a significant constituent of bone mineral remains unsubstantiated. Contrary to claims that such biomaterials represent a bioinspired/biomimetic approach to bone repair, Mg-whitlockite remains, exclusively, a pathological biomineral. STATEMENT OF SIGNIFICANCE: Magnesium whitlockite (Mg-whitlockite) is a unique calcium phosphate that typically features in pathological calcification of soft tissues; however, an alarming trend emerging in the synthetic bioceramics community claims that Mg-whitlockite occupies 20-35 wt% of bone mineral and therefore synthetic Mg-whitlockite represents a biomimetic approach towards bone regeneration. By providing an overview of Mg-whitlockite detection in biological tissues and scrutinising a diverse cross-section of literature relevant to bone composition analysis, this review concludes that Mg-whitlockite is exclusively a pathological biomineral, and having never been reported in bone extracellular matrix, Mg-whitlockite does not constitute a biomimetic strategy for bone repair.

Medical Management of Patients After Atypical Femur Fractures: a Systematic Review and Recommendations From the European Calcified Tissue Society

CONTEXT

Atypical femur fractures (AFFs) are serious adverse events associated with bisphosphonates and often show poor healing.

EVIDENCE ACQUISITION

We performed a systematic review to evaluate effects of teriparatide, raloxifene, and denosumab on healing and occurrence of AFF.

EVIDENCE SYNTHESIS

We retrieved 910 references and reviewed 67 papers, including 31 case reports, 9 retrospective and 3 prospective studies on teriparatide. There were no RCTs. We pooled data on fracture union (n = 98 AFFs on teriparatide) and found that radiological healing occurred within 6 months of teriparatide in 13 of 30 (43%) conservatively managed incomplete AFFs, 9 of 10 (90%) incomplete AFFs with surgical intervention, and 44 of 58 (75%) complete AFFs. In 9 of 30 (30%) nonoperated incomplete AFFs, no union was achieved after 12 months and 4 (13%) fractures became complete on teriparatide. Eight patients had new AFFs during or after teriparatide. AFF on denosumab was reported in 22 patients, including 11 patients treated for bone metastases and 8 without bisphosphonate exposure. Denosumab after AFF was associated with recurrent incomplete AFFs in 1 patient and 2 patients of contralateral complete AFF. Eight patients had used raloxifene before AFF occurred, including 1 bisphosphonate-naïve patient.

CONCLUSIONS

There is no evidence-based indication in patients with AFF for teriparatide apart from reducing the risk of typical fragility fractures, although observational data suggest that teriparatide might result in faster healing of surgically treated AFFs. Awaiting further evidence, we formulate recommendations for treatment after an AFF based on expert opinion.

50 years of scanning electron microscopy of bone-a comprehensive overview of the important discoveries made and insights gained into bone material properties in health, disease, and taphonomy

Bone is an architecturally complex system that constantly undergoes structural and functional optimisation through renewal and repair. The scanning electron microscope (SEM) is among the most frequently used instruments for examining bone. It offers the key advantage of very high spatial resolution coupled with a large depth of field and wide field of view. Interactions between incident electrons and atoms on the sample surface generate backscattered electrons, secondary electrons, and various other signals including X-rays that relay compositional and topographical information. Through selective removal or preservation of specific tissue components (organic, inorganic, cellular, vascular), their individual contribution(s) to the overall functional competence can be elucidated. With few restrictions on sample geometry and a variety of applicable sample-processing routes, a given sample may be conveniently adapted for multiple analytical methods. While a conventional SEM operates at high vacuum conditions that demand clean, dry, and electrically conductive samples, non-conductive materials (e.g., bone) can be imaged without significant modification from the natural state using an environmental scanning electron microscope. This review highlights important insights gained into bone microstructure and pathophysiology, bone response to implanted biomaterials, elemental analysis, SEM in paleoarchaeology, 3D imaging using focused ion beam techniques, correlative microscopy and in situ experiments. The capacity to image seamlessly across multiple length scales within the meso-micro-nano-continuum, the SEM lends itself to many unique and diverse applications, which attest to the versatility and user-friendly nature of this instrument for studying bone. Significant technological developments are anticipated for analysing bone using the SEM.

Genetic Risk Factors for Atypical Femoral Fractures (AFFs): A Systematic Review

Atypical femoral fractures (AFFs) are uncommon and have been associated particularly with long‐term antiresorptive therapy, including bisphosphonates. Although the pathogenesis of AFFs is unknown, their identification in bisphosphonate‐naïve individuals and in monogenetic bone disorders has led to the hypothesis that genetic factors predispose to AFF. Our aim was to review and summarize the evidence for genetic factors in individuals with AFF. We conducted structured literature searches and hand‐searching of conference abstracts/reference lists for key words relating to AFF and identified 2566 citations. Two individuals independently reviewed citations for (i) cases of AFF in monogenetic bone diseases and (ii) genetic studies in individuals with AFF. AFFs were reported in 23 individuals with the following 7 monogenetic bone disorders (gene): osteogenesis imperfecta (COL1A1/COL1A2), pycnodysostosis (CTSK), hypophosphatasia (ALPL), X‐linked osteoporosis (PLS3), osteopetrosis, X‐linked hypophosphatemia (PHEX), and osteoporosis pseudoglioma syndrome (LRP5). In 8 cases (35%), the monogenetic bone disorder was uncovered after the AFF occurred. Cases of bisphosphonate‐naïve AFF were reported in pycnodysostosis, hypophosphatasia, osteopetrosis, X‐linked hypophosphatemia, and osteoporosis pseudoglioma syndrome. A pilot study in 13 AFF patients and 268 controls identified a greater number of rare variants in AFF cases using exon array analysis. A whole‐exome sequencing study in 3 sisters with AFFs showed, among 37 shared genetic variants, a p.Asp188Tyr mutation in the GGPS1 gene in the mevalonate pathway, critical to osteoclast function, which is also inhibited by bisphosphonates. Two studies completed targeted ALPL gene sequencing, an ALPL heterozygous mutation was found in 1 case of a cohort of 11 AFFs, whereas the second study comprising 10 AFF cases did not find mutations in ALPL. Targeted sequencing of ALPL, COL1A1, COL1A2, and SOX9 genes in 5 cases of AFF identified a variant in COL1A2 in 1 case. These findings suggest a genetic susceptibility for AFFs. A large multicenter collaborative study of well‐phenotyped AFF cases and controls is needed to understand the role of genetics in this uncommon condition. © 2017 The Authors JBMR Plus published by Wiley Periodicals, Inc. on behalf of American Society for Bone and Mineral Research.