Fragility fractures in men with idiopathic osteoporosis are associated with undermineralization of the bone matrix without evidence of increased bone turnover

The relative contribution of bone microarchitecture and matrix composition to implant fixation strength in rats

Bone microarchitectural parameters significantly contribute to implant fixation strength but the role of bone matrix composition is not well understood. To determine the relative contribution of microarchitecture and bone matrix composition to implant fixation strength, we placed titanium implants in 12-week-old intact Sprague-Dawley rats, ovariectomized-Sprague-Dawley rats, and Zucker diabetic fatty rats. We assessed bone microarchitecture by microcomputed tomography, bone matrix composition by Raman spectroscopy, and implant fixation strength at 2, 6, and 10 weeks postimplantation. A stepwise linear regression model accounted for 83.3% of the variance in implant fixation strength with osteointegration volume/total volume (50.4%), peri-implant trabecular bone volume fraction (14.2%), cortical thickness (9.3%), peri-implant trabecular crystallinity (6.7%), and cortical area (2.8%) as the independent variables. Group comparisons indicated that osseointegration volume/total volume was significantly reduced in the ovariectomy group at Week 2 (~28%) and Week 10 (~21%) as well as in the diabetic group at Week 10 (~34%) as compared with the age matched Sprague-Dawley group. The crystallinity of the trabecular bone was significantly elevated in the ovariectomy group at Week 2 (~4%) but decreased in the diabetic group at Week 10 (~3%) with respect to the Sprague-Dawley group. Our study is the first to show that bone microarchitecture explains most of the variance in implant fixation strength, but that matrix composition is also a contributing factor. Therefore, treatment strategies aimed at improving bone-implant contact and peri-implant bone volume without compromising matrix quality should be prioritized.

How to manage osteoporosis before the age of 50

This narrative review discusses several aspects of the management of osteoporosis in patients under 50 years of age. Peak bone mass is genetically determined but can also be affected by lifestyle factors. Puberty constitutes a vulnerable period. Idiopathic osteoporosis is a rare, heterogeneous condition in young adults due in part to decreased osteoblast function and deficient bone acquisition. There are no evidence-based treatment recommendations. Drugs use can be proposed to elderly patients at very high risk. Diagnosis and management of osteoporosis in the young can be challenging, in particular in the absence of a manifest secondary cause. Young adults with low bone mineral density (BMD) do not necessarily have osteoporosis and it is important to avoid unnecessary treatment. A determination of BMD is recommended for premenopausal women who have had a fragility fracture or who have secondary causes of osteoporosis: secondary causes of excessive bone loss need to be excluded and treatment should be targeted. Adequate calcium, vitamin D, and a healthy lifestyle should be recommended. In the absence of fractures, conservative management is generally sufficient, but in rare cases, such as chemotherapy-induced osteoporosis, antiresorptive medication can be used. Osteoporosis in young men is most often of secondary origin and hypogonadism is a major cause; testosterone replacement therapy will improve BMD in these patients. Diabetes is characterized by major alterations in bone quality, implying that medical therapy should be started sooner than for other causes of osteoporosis. Primary hyperparathyroidism, hyperthyroidism, Cushing's syndrome and growth hormone deficiency or excess affect cortical bone more often than trabecular bone.

Recovery of bone mineralization and quality during asfotase alfa treatment in an adult patient with infantile-onset hypophosphatasia

Hypophosphatasia (HPP) is a hereditary musculoskeletal disorder characterized by low serum alkaline phosphatase (ALP) activity leading to poor bone mineralization. On a micro-morphological level, this may not only be reflected by an enrichment of osteoid but also a degradation of bone quality. Asfotase alfa is an enzyme replacement therapy that was recently demonstrated to improve bone mineralization as well as clinical status (e.g. growth, muscle strength and quality of life). However, the underlying changes of bone quality parameters on asfotase alfa treatment are currently not known. In the present study, we report a 24-year-old woman with genetically confirmed infantile-onset HPP and recurrent fractures. While the initiated asfotase alfa treatment was followed by rapid clinical improvements (i.e., disappearance of bone marrow edema, increase of muscle strength), the BMD assessed by DXA at the hip and spine increased moderately at two years follow-up. A detailed skeletal assessment using high-resolution peripheral quantitative computed tomography (HR-pQCT) and a high-resolution analysis of two consecutive iliac crest bone biopsies revealed only minor improvements of bone microarchitecture but a remarkable reduction of osteoid parameters. Furthermore, the high mineralization heterogeneity at baseline assessed by quantitative backscattered electron imaging (qBEI) decreased after 2 year of asfotase alfa treatment. Finally, we found an increase in mineral maturation reflected by higher mineral-to-matrix and carbonate-to-phosphate ratios using Fourier transform infrared spectroscopy (FTIR) imaging as well as increased local mechanical properties using reference point indentation (RPI). Taken together, our findings provide evidence for an improvement of bone quality indices beyond the mere reduction of osteoid indices and thereby contribute to the understanding of fracture risk reduction in HPP patients on asfotase alfa treatment.

Selective Serotonin Reuptake Inhibitors (SSRIs) and Markers of Bone Turnover in Men

Selective serotonin reuptake inhibitors (SSRIs) have been shown to have a clinically significant impact on bone metabolism. To explore this further, we aimed to determine whether these agents are associated with serum markers of bone turnover utilising a population-based sample of men (n = 1138; 20-96 year) participating in the Geelong Osteoporosis Study. Blood samples were obtained and the bone resorption marker, C-telopeptide (CTx) and formation marker, type 1 procollagen amino-terminal-propeptide (PINP) were measured. Anthropometry and socio-economic status (SES) were determined and information on medication use and lifestyle was obtained via questionnaire. Lifetime mood disorders were assessed using semi-structured clinical interviews. Thirty-seven (3.3%) men reported using SSRIs. Age was an effect modifier in the association between SSRIs and markers of bone turnover. Among younger men (20-60 year; n = 557), adjusted mean CTx and PINP values were 12.4% [16.7 (95% CI 14.6-18.8) vs 19.1 (95% CI 18.7-19.4) pg/ml, p = 0.03] and 13.6% [5.6 (95% CI 4.9-6.3) vs 6.4 (95% CI 6.3-6.6) pg/ml, p = 0.02] lower among SSRI users compared to non-users, respectively. No differences in SSRI use and markers of bone turnover were detected among older men (61-94 year; all p > 0.05). These patterns persisted after further adjustment for activity, alcohol, smoking, SES, depression, bone active medications and other antidepressants. Our data suggest that SSRI use is associated with alterations in bone turnover markers among younger men. The observed decreases in both CTx and PINP are likely to contribute to a low bone turnover state and increased skeletal fragility with this potential imbalance between formation and resorption resulting in subsequent bone loss.

Bone quality changes associated with aging and disease: a review

Bone quality encompasses all the characteristics of bone that, in addition to density, contribute to its resistance to fracture. In this review, we consider changes in architecture, porosity, and composition, including collagen structure, mineral composition, and crystal size. These factors all are known to vary with tissue and animal ages, and health status. Bone morphology and presence of microcracks, which also contribute to bone quality, will not be discussed in this review. Correlations with mechanical performance for collagen cross-linking, crystallinity, and carbonate content are contrasted with mineral content. Age-dependent changes in humans and rodents are discussed in relation to rodent models of disease. Examples are osteoporosis, osteomalacia, osteogenesis imperfecta (OI), and osteopetrosis in both humans and animal models. Each of these conditions, along with aging, is associated with increased fracture risk for distinct reasons.

Ibandronate increases sclerostin levels and bone strength in male patients with idiopathic osteoporosis

The pathomechanism of male idiopathic osteoporosis (MIO) differs from postmenopausal osteoporosis with regard to alterations in osteoblast activity. We evaluated intravenous ibandronate (IBN) in 25 MIO patients with fragility fractures in a prospective, monocentric, single-arm, and open-label study for 24 months. The impact and changes of sclerostin (Scl), Dickkopf-1 (DKK-1), CTX, and PINP were examined. Additionally, volumetric cortical, trabecular and areal bone mineral density (BMD), trabecular bone score (TBS), and finite element analyses (FEA) were evaluated. Compared to baseline, median Scl levels were increased after 1 month (Δ 121%, p < 0.0001) and remained elevated for 12 months. DKK-1 decreased (p < 0.001) to a lesser extent until month 9 with values comparable to baseline at study endpoint. Early changes (baseline-month 1) of Scl negatively correlated with early changes of DKK-1 (-0.72), CTX (-0.82), and PINP (-0.55; p < 0.005 for all). The overall changes over the 24 months study period of Scl negatively correlated with decreased CTX (-0.32) and DKK-1 levels (-0.57, p < 0.0001 for both); CTX and PINP changes positively correlated at each time point (p < 0.001). Volumetric hip BMD increased by 12 and 18%, respectively (p < 0.0001 for both). Cross-sectional moment of inertia and section modulus for total hip significantly improved (p < 0.05 for all). Areal BMD at total hip, spine, and TBS increased. FEA displayed an increase in bone strength both in the hip (17%) and vertebrae (13%, all p < 0.0001) at anatomical sites susceptible for fragility fracture. IBN increases Scl and improves cortical and trabecular bone strength with early and ongoing vigorous suppression of bone resorption.

Relationship of bone mineralization density distribution (BMDD) in cortical and cancellous bone within the iliac crest of healthy premenopausal women

Bone mineralization density distribution (BMDD) is an important determinant of bone mechanical properties. The most available skeletal site for access to the BMDD is the iliac crest. Compared to cancellous bone much less information on BMDD is available for cortical bone. Hence, we analyzed complete transiliac crest bone biopsy samples from premenopausal women (n = 73) aged 25-48 years, clinically classified as healthy, by quantitative backscattered electron imaging for cortical (Ct.) and cancellous (Cn.) BMDD. The Ct.BMDD was characterized by the arithmetic mean of the BMDD of the cortical plates. We found correlations between Ct. and Cn. BMDD variables with correlation coefficients r between 0.42 and 0.73 (all p < 0.001). Additionally to this synchronous behavior of cortical and cancellous compartments, we found that the heterogeneity of mineralization densities (Ct.Ca(Width)), as well as the cortical porosity (Ct.Po) was larger for a lower average degree of mineralization (Ct.Ca(Mean)). Moreover, Ct.Po correlated negatively with the percentage of highly mineralized bone areas (Ct.Ca(High)) and positively with the percentage of lowly mineralized bone areas (Ct.Ca(Low)). In conclusion, the correlation of cortical with cancellous BMDD in the iliac crest of the study cohort suggests coordinated regulation of bone turnover between both bone compartments. Only in a few cases, there was a difference in the degree of mineralization of >1wt % between both cortices suggesting a possible modeling situation. This normative dataset of healthy premenopausal women will provide a reference standard by which disease- and treatment-specific effects can be assessed at the level of cortical bone BMDD.

Changes in the degree of mineralization with osteoporosis and its treatment

The diagnosis of osteoporosis is based on low bone mineral density (BMD) and/or the occurrence of fragility fractures. The majority of patients, however, have also abnormally low bone matrix mineralization. The latter is indicative of alterations in bone turnover rates and/or in kinetics of mineral accumulation within the newly formed bone matrix. Osteoporosis therapies can alter the bone matrix mineralization according to their action on bone turnover and/or mineralization kinetics. Antiresorptives, including the most widely used bisphosphonates, reduce the bone turnover rate resulting in a decrease in heterogeneity and an increase in the degree of mineralization toward to or even beyond normal values. Anabolic agents increase the bone volume and the amount of newly formed bone resulting in a likely transient decrease in mean degree and homogeneity of mineralization. Hence, the measurement of bone matrix mineralization is a sensitive tool to evaluate the response to therapy.

Molecular evidence of osteoblast dysfunction in elderly men with osteoporotic hip fractures

Osteoporosis is extremely frequent in post-menopausal women; nevertheless, osteoporosis in men is also a severe and frequently occurring but often underestimated disease. Increasing evidence links bone loss in male idiopathic osteoporosis and age related osteoporosis to osteoblast dysfunction rather than increased osteoclast activity as seen in postmenopausal osteoporosis. The aim of this study was to investigate gene expression of osteoblast related genes and of bone architecture in bone samples derived from elderly osteoporotic men with hip fractures (OP) in comparison to bone samples from age matched men with osteoarthritis of the hip (OA). Femoral heads and adjacent neck tissue were collected from 12 men with low-trauma hip fractures and consecutive surgical hip replacement. Bone samples of age matched patients undergoing hip replacement due to osteoarthritis served as controls. One half of the bone samples was subjected to RNA extraction, reverse transcription, and real-time polymerase chain reactions. The second half of the bone samples was analyzed by static histomorphometry. From each half samples from four different regions, the central and subcortical region of the femoral head and neck, were analyzed. OP patients displayed a significantly decreased RUNX2, Osterix and SOST expression compared to OA patients. Major microstructural changes in OP bone were seen in the subcortical region of the neck and were characterized by a significant decrease of bone volume, and a significant increase of trabecular separation. In conclusion, decreased local gene expression of RUNX2 and Osterix in men with hip fractures strongly supports the concept of osteoblast dysfunction in male osteoporosis. Major microstructural changes in the trabecular structure associated with osteoporotic hip fractures in men are localized in the subcortical region of the femoral neck.

Intravenous treatment with ibandronate normalizes bone matrix mineralization and reduces cortical porosity after two years in male osteoporosis: a paired biopsy study

The spectrum of therapeutic options and the amount of clinical trials for male osteoporosis (mOP) is lower than those for postmenopausal osteoporosis. Therefore, we examined the effects of 24 months of ibandronate (IBN) treatment (3 mg/3 mL intravenously every 3 months) on bone material quality in 19 subjects with mOP within an open-label, single-center, prospective phase III study (Eudract number 2006-006692-20). Patients (median age [25th, 75th percentiles] 53.0 [44.5; 57.0] years) were included if they had low bone mineral density (BMD) and/or at least one low trauma fracture and no secondary cause of osteoporosis. The primary endpoint was to evaluate IBN effects on cancellous (Cn.) and cortical (Ct.) bone mineralization density distribution (BMDD) based on quantitative backscattered electron imaging (qBEI) of paired transiliacal bone biopsies (baseline, 24 months). Secondary endpoints included changes in areal bone mineral density (BMD by dual-energy X-ray absorptiometry [DXA]) and serum markers of bone turnover including type I collagen peptides CrossLaps (CTX), procollagen type 1 amino-terminal propeptide (P1NP), and osteocalcin (OC). At baseline, cancellous bone matrix mineralization from mOP was lower than published reference data (mean degree of mineralization Cn.CaMean -1.8%, p < 0.01). IBN treatment increased calcium concentrations versus baseline (Cn.CaMean +2.4%, Ct.CaMean, +3.0% both p < 0.01), and reduced heterogeneity of mineralization (Cn.CaWidth -14%, p = 0.044; Ct.CaWidth, -16%, p = 0.001), leading to cancellous BMDD within normal range. IBN treatment was associated with a decrease in porosity of mineralized cortical tissue (-25%, p = 0.01); increases in BMD at the lumbar spine, the femoral neck, and the total hip (+3.3%, +1.9%, and +5.6%, respectively, p ≤ 0.01); and reductions in CTX (-37.5%), P1NP (-44.4%), and OC (-36.3%, all p < 0.01). Our BMDD findings are in line with the reduction of bone turnover markers and the increase in BMD by IBN in our patients and suggest that the latter mainly reflects the increase in matrix mineralization and the reduction of cortical porosity in this cohort with mOP.

Idiopathic osteoporosis in men

Over the last decade, the increasingly significant problem of osteoporosis in men has begun to receive much more attention than in the past. In particular, recent observations from large scale population studies in males led to an advance in the understanding of morphologic basis of growth, maintenance and loss of bone in men, as well as new insights about the pathophysiology and treatment of this disorder. While fracture risk consistently increases after age 65 in men (with up to 50 % of cases due to secondary etiologies), osteoporosis and fractures may also occur in young or middle aged males in the absence of an identifiable etiology. For this category (so called idiopathic osteoporosis), there are still major gaps in knowledge, particularly concerning the etiology and the clinical management. This article provides a summary of recent developments in the acquisition and maintenance of bone strength in men, as well as new insights about the pathogenesis, diagnosis, and treatment of idiopathic osteoporosis.

TRPV4 deficiency causes sexual dimorphism in bone metabolism and osteoporotic fracture risk

We explored the role of transient receptor potential vanilloid 4 (TRPV4) in murine bone metabolism and association of TRPV4 gene variants with fractures in humans. Urinary and histomorphometrical analyses demonstrated reduced osteoclast activity and numbers in male Trpv4(-/-) mice, which was confirmed in bone marrow-derived osteoclast cultures. Osteoblasts and bone formation as shown by serum procollagen type 1 amino-terminal propeptide and histomorphometry, including osteoid surface, osteoblast and osteocyte numbers were not affected in vivo. Nevertheless, osteoblast differentiation was enhanced in Trpv4(-/-) bone marrow cultures. Cortical and trabecular bone mass was 20% increased in male Trpv4(-/-) mice, compared to sex-matched wild type (Trpv4(+/+)) mice. However, at the same time intracortical porosity was increased and bone matrix mineralization was reduced. Together, these lead to a maximum load, stiffness and work to failure of the femoral bone, which were not different compared to Trpv4(+/+) mice, while the bone material was less resistant to stress and less elastic. The differential impacts on these determinants of bone strength were likely responsible for the lack of any changes in whole bone strength in the Trpv4(-/-) mice. None of these skeletal parameters were affected in female Trpv4(-/-) mice. The T-allele of rs1861809 SNP in the TRPV4 locus was associated with a 30% increased risk (95% CI: 1.1-1.6; p=0.013) for non-vertebral fracture risk in men, but not in women, in the Rotterdam Study. Meta-analyses with the population-based LASA study confirmed the association with non-vertebral fractures in men. This was lost when the non-population-based studies Mr. OS and UFO were included. In conclusion, TRPV4 is a male-specific regulator of bone metabolism, a determinant of bone strength, and a potential risk predictor for fractures through regulation of bone matrix mineralization and intra-cortical porosity. This identifies TRPV4 as a unique sexually dimorphic therapeutic and/or diagnostic candidate for osteoporosis.

Bone mineralization: from tissue to crystal in normal and pathological contexts

Bone is a complex and structured material; its mechanical behavior results from an interaction between the properties of each level of its structural hierarchy. The degree of mineralization of bone (bone density measured at tissue level) and the characteristics of the mineral deposited (apatite crystals) are major determinants of bone strength. Bone remodeling activity acts as a regulator of the degree of mineralization and of the distribution of mineral at the tissue level, directly impacting bone mechanical properties. Recent findings have highlighted the need to understand the underlying process occurring at the nanostructure level that may be independent of bone remodeling itself. A more global comprehension of bone qualities will need further works designed to characterize what are the consequences on whole bone strength of changes at nano- or microstructure levels relative to each other.

Applicability of ToF-SIMS for monitoring compositional changes in bone in a long-term animal model

Time-of-flight secondary ion mass spectrometry (ToF-SIMS) is a well-established technique in material sciences but has not yet been widely explored for implementation in life sciences. Here, we demonstrate the applicability and advantages of ToF-SIMS analysis for the study of minerals and biomolecules in osseous tissue. The locally resolved analysis of fragment ions deriving from the sample surface enables imaging and differentiation of bone tissue and facilitates histology on non-stained cross sections. In a rat model, bilateral ovariectomy combined with either a multi-deficiency diet or steroid treatment was carried out to create osteoporotic conditions. We focused our study on the Ca content of the mineralized tissue and monitored its decline. Calcium mass images of cross sections show the progressive degenerative changes in the bone. We observed a decreased Ca concentration in the edge region of the trabeculae and a decline in the Ca/P ratio. Additionally, we focused on the non-mineralized matrix and identified fragment ions that are characteristic for the collagen matrix. We observed trabeculae with wide ranges of non-mineralized collagen for the diet group owing to an impaired mineralization process. Here, the advantage of coeval monitoring of collagen and minerals indicated an osteomalacic model rather than an osteoporotic one.

Selective serotonin reuptake inhibitors inhibit human osteoclast and osteoblast formation and function

BACKGROUND

Selective serotonin reuptake inhibitors (SSRIs) are widely used antidepressants and one of the most commonly used medications. There is growing concern that SSRIs, which sequester in bone marrow at higher concentrations than brain or blood, increase bone fragility and fracture risk. However, their mechanism of action on human osteoclasts (OC) and osteoblasts (OB) differentiation remains unclear.

METHODS

Expression of serotonin receptors (5-HTR), transporter (5-HTT), and tryptophan hydroxylase 1 (TPH1) was assessed in human OC (precursors and mature) and OB (nonmineralizing and mineralizing) by polymerase chain reaction. OC formation and resorption was measured in the presence of 5 SSRIs. OBs cultured with SSRIs for 28 days were assessed for alkaline phosphatase (ALP) and bone mineralization. Cell viability and apoptosis were determined by annexin V flow cytometry.

RESULTS

OCs and OB expressed TPH1, 5-HTT, and 5-HTR1B. The 5-HTR2A was expressed only in OB, whereas 5-HTR2B expression increased from precursor to mature OC. All SSRIs (except citalopram) dose-dependently inhibited OC formation and resorption between 1 μmol/L and 10 μmol/L; order of potency: sertraline > fluoxetine > paroxetine > fluvoxamine > citalopram. Similarly, SSRIs (except citalopram) inhibited ALP and bone mineralization by OB but only at 30 μmol/L. Apoptosis was induced by SSRIs in OC and OB in an identical pattern to inhibitory effects. Serotonin treatment had no effect on either OC or OB parameters.

CONCLUSIONS

These data demonstrate that SSRIs differentially inhibit bone cell function via apoptosis. This may explain the mechanisms of bone loss with chronic use and aid clinical choices.

A case-control study of fractures in men with idiopathic osteoporosis: fractures are associated with older age and low cortical bone density

OBJECTIVES

To determine biochemical, radiological and micro-architectural bone factors related to fragility fractures in idiopathic male osteoporosis (IMO) patients. IMO is a rare disorder characterized by low areal bone mineral density (aBMD) (Z-score<-2) occurring in men after excluding secondary causes of low BMD.

METHODS

We conducted a case-control study in 31 patients with fragility fracture (IMO F+) that had occurred after the age of 40 years and 37 without fracture (IMO F-). We first compared IMO group to 40 age-matched disease-free men. We measured aBMD and bone micro-architectural indices at distal radius and tibia sites with a HR-pQCT scan (XtremeCT) using standard and extended cortical analysis. Urine and blood samples were collected in order to determine the levels of bone-turnover markers and the potential determinant of bone fragility. Models of analysis of covariance, including age, height and weight as adjustment factors, were used to compare the groups.

RESULTS

Compared to their controls, IMO patients showed marked disturbance of their micro-architectural parameters at tibia and radius affecting both trabecular and cortical parameters. IMO F+ subjects were significantly older than IMO F- subjects (58 ± 8 vs. 53 ± 9 yrs, p=0.01). BMD Z-score at the total-hip was significantly lower in IMO F+ (-1.3 ± 0.5 vs. -0.9 ± 0.8 g/cm(2), p=0.01). After adjustment, trabecular micro-architectural parameters, biochemical markers and hormonal parameters were not different in the 2 groups. At distal tibia, cortical v-BMD was significantly lower in IMO F+ patients (799 ± 73 vs. 858 ± 60 mg/cm(3), p=0.03), while cortical thickness was not different.

CONCLUSION

Our results show that patients with IMO display a marked disturbance of trabecular and cortical bone micro-architecture, and that age and low cortical density are determinants of the fracture occurrence.

Bone material properties in premenopausal women with idiopathic osteoporosis

Idiopathic osteoporosis (IOP) in premenopausal women is characterized by fragility fractures at low or normal bone mineral density (BMD) in otherwise healthy women with normal gonadal function. Histomorphometric analysis of transiliac bone biopsy samples has revealed microarchitectural deterioration of cancellous bone and thinner cortices. To examine bone material quality, we measured the bone mineralization density distribution (BMDD) in biopsy samples by quantitative backscattered electron imaging (qBEI), and mineral/matrix ratio, mineral crystallinity/maturity, relative proteoglycan content, and collagen cross-link ratio at actively bone forming trabecular surfaces by Raman microspectroscopy and Fourier transform infrared microspectroscopy (FTIRM) techniques. The study groups included: premenopausal women with idiopathic fractures (IOP, n = 45), or idiopathic low BMD (Z-score ≤ -2.0 at spine and/or hip) but no fractures (ILBMD, n = 19), and healthy controls (CONTROL, n = 38). BMDD of cancellous bone showed slightly lower mineral content in IOP (both the average degree of mineralization of cancellous bone [Cn.Ca(Mean) ] and mode calcium concentration [Cn.Ca(Peak) ] are 1.4% lower) and in ILBMD (both are 1.6% lower, p < 0.05) versus CONTROL, but no difference between IOP and ILBMD. Similar differences were found when affected groups were combined versus CONTROL. The differences remained significant after adjustment for cancellous mineralizing surface (MS/BS), suggesting that the reduced mineralization of bone matrix cannot be completely accounted for by differences in bone turnover. Raman microspectroscopy and FTIRM analysis at forming bone surfaces showed no differences between combined IOP/ILBMD groups versus CONTROL, with the exceptions of increased proteoglycan content per mineral content and increased collagen cross-link ratio. When the two affected subgroups were considered individually, mineral/matrix ratio and collagen cross-link ratio were higher in IOP than ILBMD. In conclusion, our findings suggest that bone material properties differ between premenopausal women with IOP/ILBMD and normal controls. In particular, the altered collagen properties at sites of active bone formation support the hypothesis that affected women have osteoblast dysfunction that may play a role in bone fragility.

Na+/H+ exchanger regulatory factor 1 (NHERF1) directly regulates osteogenesis

Bone formation requires synthesis, secretion, and mineralization of matrix. Deficiencies in these processes produce bone defects. The absence of the PDZ domain protein Na(+)/H(+) exchange regulatory factor 1 (NHERF1) in mice, or its mutation in humans, causes osteomalacia believed to reflect renal phosphate wasting. We show that NHERF1 is expressed by mineralizing osteoblasts and organizes Na(+)/H(+) exchangers (NHEs) and the PTH receptor. NHERF1-null mice display reduced bone formation and wide mineralizing fronts despite elimination of phosphate wasting by dietary supplementation. Bone mass was normal, reflecting coordinated reduction of bone resorption and formation. NHERF1-null bone had decreased strength, consistent with compromised matrix quality. Mesenchymal stem cells from NHERF1-null mice showed limited osteoblast differentiation but enhanced adipocyte differentiation. PTH signaling and Na(+)/H(+) exchange were dysregulated in these cells. Osteoclast differentiation from monocytes was unaffected. Thus, NHERF1 is required for normal osteoblast differentiation and matrix synthesis. In its absence, compensatory mechanisms maintain bone mass, but bone strength is reduced.

Age-dependence of power spectral density and fractal dimension of bone mineralized matrix in atomic force microscope topography images: potential correlates of bone tissue age and bone fragility in female femoral neck trabeculae

There is an increasing interest in bone nano-structure, the ultimate goal being to reveal the basis of age-related bone fragility. In this study, power spectral density (PSD) data and fractal dimensions of the mineralized bone matrix were extracted from atomic force microscope topography images of the femoral neck trabeculae. The aim was to evaluate age-dependent differences in the mineralized matrix of human bone and to consider whether these advanced nano-descriptors might be linked to decreased bone remodeling observed by some authors and age-related decline in bone mechanical competence. The investigated bone specimens belonged to a group of young adult women (n = 5, age: 20-40 years) and a group of elderly women (n = 5, age: 70-95 years) without bone diseases. PSD graphs showed the roughness density distribution in relation to spatial frequency. In all cases, there was a fairly linear decrease in magnitude of the power spectra with increasing spatial frequencies. The PSD slope was steeper in elderly individuals (-2.374 vs. -2.066), suggesting the dominance of larger surface morphological features. Fractal dimension of the mineralized bone matrix showed a significant negative trend with advanced age, declining from 2.467 in young individuals to 2.313 in the elderly (r = 0.65, P = 0.04). Higher fractal dimension in young women reflects domination of smaller mineral grains, which is compatible with the more freshly remodeled structure. In contrast, the surface patterns in elderly individuals were indicative of older tissue age. Lower roughness and reduced structural complexity (decreased fractal dimension) of the interfibrillar bone matrix in the elderly suggest a decline in bone toughness, which explains why aged bone is more brittle and prone to fractures.

Human bone material characterization: integrated imaging surface investigation of male fragility fractures

The interrelation of calcium and phosphorus was evaluated as a function of bone material quality in femoral heads from male fragility fracture patients via surface analytical imaging as well as scanning microscopy techniques. A link between fragility fractures and increased calcium to phosphorus ratio was observed despite normal mineralization density distribution.

INTRODUCTION

Bone fragility in men has been recently recognized as a public health issue, but little attention has been devoted to bone material quality and the possible efficacy in fracture risk prevention. Clinical routine fracture risk estimations do not consider the quality of the mineralized matrix and the critical role played by the different chemical components that are present. This study uses a combination of different imaging and analytical techniques to gain insights into both the spatial distribution and the relationship of phosphorus and calcium in bone.

METHODS

X-ray photoelectron spectroscopy and time-of-flight secondary ion mass spectrometry imaging techniques were used to investigate the relationship between calcium and phosphorus in un-embedded human femoral head specimens from fragility fracture patients and non-fracture age-matched controls. The inclusion of the bone mineral density distribution via backscattered scanning electron microscopy provides information about the mineralization status between the groups.

RESULTS

A link between fragility fracture and increased calcium and decreased phosphorus in the femoral head was observed despite normal mineralization density distribution. Results exhibited significantly increased calcium to phosphorus ratio in the fragility fracture group, whereas the non-fracture control group ratio was in agreement with the literature value of 1.66 M ratio in mature bone.

CONCLUSIONS

Our results highlight the potential importance of the relationship between calcium and phosphorus, especially in areas of new bone formation, when estimating fracture risk of the femoral head. The determination of calcium and phosphorus fractions in bone mineral density measurements may hold the key to better fracture risk assessment as well as more targeted therapies.