Klaus Klaushofer

Atypical Subtrochanteric and Diaphyseal Femoral Fractures: Report of a Task Force of the American Society for Bone and Mineral Research

Bisphosphonates (BPs) and denosumab reduce the risk of spine and nonspine fractures. Atypical femur fractures (AFFs) located in the subtrochanteric region and diaphysis of the femur have been reported in patients taking BPs and in patients on denosumab, but they also occur in patients with no exposure to these drugs. In this report, we review studies on the epidemiology, pathogenesis, and medical management of AFFs, published since 2010. This newer evidence suggests that AFFs are stress or insufficiency fractures. The original case definition was revised to highlight radiographic features that distinguish AFFs from ordinary osteoporotic femoral diaphyseal fractures and to provide guidance on the importance of their transverse orientation. The requirement that fractures be noncomminuted was relaxed to include minimal comminution. The periosteal stress reaction at the fracture site was changed from a minor to a major feature. The association with specific diseases and drug exposures was removed from the minor features, because it was considered that these associations should be sought rather than be included in the case definition. Studies with radiographic review consistently report significant associations between AFFs and BP use, although the strength of associations and magnitude of effect vary. Although the relative risk of patients with AFFs taking BPs is high, the absolute risk of AFFs in patients on BPs is low, ranging from 3.2 to 50 cases per 100,000 person‐years. However, long‐term use may be associated with higher risk (∼100 per 100,000 person‐years). BPs localize in areas that are developing stress fractures; suppression of targeted intracortical remodeling at the site of an AFF could impair the processes by which stress fractures normally heal. When BPs are stopped, risk of an AFF may decline. Lower limb geometry and Asian ethnicity may contribute to the risk of AFFs. There is inconsistent evidence that teriparatide may advance healing of AFFs.

Alendronate increases degree and uniformity of mineralization in cancellous bone and decreases the porosity in cortical bone of osteoporotic women

The strength of bone is correlated with bone mass but is also influenced significantly by other factors such as structural properties of the matrix (e.g., collagen mutations) and the mineral. Changes at all levels of this organization could contribute to fracture risk. We investigated the effects of alendronate (Aln) treatment on the density of mineralization and the ultrastructure of the mineral/collagen composite, size and habitus of mineral particles in iliac cancellous bone, as well as on the porosity of iliac cortical bone from postmenopausal osteoporotic women. Twenty-four transiliac bone biopsies from Phase III Aln (10 mg/day) trials (placebo and Aln after 2 and 3 years of treatment, n = 6 per group) were studied. The mineral structure was investigated by quantitative backscattered electron imaging (qBEI) and by scanning small-angle X-ray scattering (scanning-SAXS). qBEI histograms reflect the bone mineralization density distribution (BMDD), whereas SAXS patterns characterize the size and arrangement of the mineral particles in bone. We found that: (i) the relative calcium content of osteoporotic bone was significantly lower than that of data-base controls; (ii) mineralization was significantly higher and more uniform after Aln treatment; (iii) size and habitus of the mineral particles was not different between placebo and Aln-treated groups; and (iv) the porosity of cortical bone was reduced significantly by Aln treatment. We conclude that Aln treatment increases the degree and uniformity of bone matrix mineralization without affecting the size and habitus of the mineral crystals. It also decreases the porosity of the corticalis. Together these effects may contribute to the observed reduction in fractures.

Nucleation and Growth of Mineral Crystals in Bone Studied by Small-Angle X-Ray Scattering

SummaryThe mechanism of calcification in bone and related tissues is a matter of current interest. The mean size and the arrangement of the mineral crystals are important parameters difficult to obtain by electron microscopy. Furthermore, most studies have been carried out on poorly calcified model systems or chemically treated samples. In the work presented here, native bone was studied as a function of age by a quantitative small-angle X-ray scattering method (SAXS). Bone samples (calvariae and ulnae) from rats and mice were investigated. Measurements were performed on native bone immediately after dissection for samples up to 1 mm thick. The size, shape, and predominant orientation of the mineral crystals in bone were obtained for embryonal, young, and adult animals. The results indicate that the mineral nucleates as thin layers of calcium phosphate within the hole zone of the collagen fibrils. The mineral nuclei subsequently grow in thickness to about 3 nm, which corresponds to maximum space available in these holes.

Constant mineralization density distribution in cancellous human bone.

The degree of mineralization of bone matrix is an important factor in determining the mechanical competence of bone. The remodeling and modeling activities of bone cells together with the time course of mineralization of newly formed bone matrix generate a characteristic bone mineralization density distribution (BMDD). In this study we investigated the biological variance of the BMDD at the micrometer level, applying a quantitative backscattered electron imaging (qBEI) method. We used the mean calcium concentration (Ca(Mean)), the most frequent calcium concentration (Ca(Peak)), and full width at half maximum (Ca(Width)) to characterize the BMDD. In none of the BMDD parameters were statistically significant differences found due to ethnicity (15 African-American vs. 27 Caucasian premenopausal women), skeletal site variance (20 ilium, 24 vertebral body, 13 patella, 13 femoral neck, and 13 femoral head), age (25 to 95 years), or gender. Additionally, the interindividual variance of Ca(Mean) and Ca(Peak), irrespective of biological factors, was found to be remarkably small (SD < 2.1% of means). However, there are significant changes in the BMDD in the case of bone diseases (e.g., osteomalacia) or following clinical treatment (e.g., alendronate). From the lack of intraindividual changes among different skeletal sites we conclude that diagnostic transiliac biopsies can be used to determine the BMDD variables of cancellous bone for the entire skeleton of the patient. In order to quantify deviations from normal mineralization, a reference BMDD for adult humans was calculated using bone samples from 52 individuals. Because we find the BMDD to be essentially constant in healthy adult humans, qBEI provides a sensitive means to detect even small changes in mineralization due to bone disease or therapeutic intervention.

Scanning small angle X-ray scattering analysis of human bone sections

Abstract. Scanning small-angle X-ray scattering (scanning SAXS) was applied for the first time on bone to compare results from SAXS directly with those from other position-sensitive methods, such as light and polarized light microscopy, back-scattered electron imaging, and radiographic imaging. Since scanning SAXS is a nondestructive method of investigation, images from all these techniques could be obtained from the same bone sections. Thus, it could be shown that both the collagen and the mineral crystals were predominantly aligned parallel to the trabeculae and, therefore, to principle stress directions. Moreover, the mean crystal thickness as determined by scanning SAXS was found to be different at various positions inside the trabecular and cortical structure. Finally, it could be shown that scanning SAXS is suitable for detecting local changes in bone material, e.g., due to fluoride treatment.

Characteristics of mineral particles in the human bone/cartilage interface.

Bone and cartilage consist of different organic matrices, which can both be mineralized by the deposition of nano-sized calcium phosphate particles. We have studied these mineral particles in the mineralized cartilage layer between bone and different types of cartilage (bone/articular cartilage, bone/intervertebral disk, and bone/growth cartilage) of individuals aged 54 years, 12 years, and 6 months. Quantitative backscattered electron imaging and scanning small-angle X-ray scattering at a synchrotron radiation source were combined with light microscopy to determine calcium content, mineral particle size and alignment, and collagen orientation, respectively. Mineralized cartilage revealed a higher calcium content than the adjacent bone (p<0.05 for all samples), whereas the highest values were found in growth cartilage. Surprisingly, we found the mineral platelet width similar for bone and mineralized cartilage, with the exception of the growth cartilage sample. The most striking result, however, was the abrupt change of mineral particle orientation at the interface between the two tissues. While the particles were aligned perpendicular to the interface in cartilage, they were oriented parallel to it in bone, reflecting the morphology of the underlying organic matrices. The tight bonding of mineralized cartilage to bone suggests a mechanical role for the interface of the two elastically different tissues, bone and cartilage.

Age- and genotype-dependence of bone material properties in the osteogenesis imperfecta murine model (oim)

Cortical mineralization of long bones was studied in collagen alpha2(I)-deficient mice (oim) used as a model for human osteogenesis imperfecta. Aspects of the age development of the mice were characterized by combining nanometer- to micrometer-scale structural analysis with microhardness measurements. Bone structure was determined from homozygous (oim/oim) and heterozygous (oim/+) mice and their normal (+/+) littermates as a function of animal age by small-angle X-ray scattering (SAXS) and quantitative backscattered electron imaging (qBEI) measurements. SAXS studies found anomalies in the size and arrangement of bone mineral crystals in both homozygous and heterozygous mice aged 1-14 months. Generally, the crystals were smaller in thickness and less well aligned in these mice compared with control animals. An increase in the mean crystal thickness of the bone was found within all three genotypes up to an age of 3 months. Vickers hardness measurements were significantly enhanced for oim bone (homozygotes and heterozygotes) compared with controls. The microhardness values were correlated directly with increased mineral content of homozygous and heterozygous compared with control bone, as determined by qBEI analysis. There was also a significant increase of mineral content with age. Two possibilities for collagen-mineral association are discussed for explaining the increased hardness and mineral content of oim/oim bone, together with its decreased toughness and thinner mineral crystals. As a consequence of the present measurements, one model for oim bone could incorporate small and densely packed mineral crystals. A second model for possible collagen-mineral association in oim material would consist of two families of mineral crystals, one being smaller and the other being much larger than the crystals found in normal mouse long bones.

Effects of 3- and 5-Year Treatment With Risedronate on Bone Mineralization Density Distribution in Triple Biopsies of the Iliac Crest in Postmenopausal Women

Long‐term effects of risedronate on bone mineralization density distribution in triple transiliac crest biopsies of osteoporotic women were evaluated. In this double‐blinded study, 3‐ and 5‐year treatment with risedronate increased the degree and homogeneity of mineralization without producing hypermineralization. These changes at the material level of bone could contribute to risedronates antifracture efficacy.

Bone material properties in trabecular bone from human iliac crest biopsies after 3- and 5-year treatment with risedronate.

Long‐term effects of risedronate on bone mineral maturity/crystallinity and collagen cross‐link ratio in triple iliac crest biopsies of osteoporotic women were evaluated. In this double‐blinded study, 3‐ and 5‐year treatment with risedronate arrested the tissue aging encountered in untreated osteoporosis and in osteoporosis treated with other antiresorptives. This effect may be contributing to risedronates antifracture efficacy.

Mineralization of cancellous bone after alendronate and sodium fluoride treatment: A quantitative backscattered electron imaging study on minipig ribs

Fluoride stimulates bone formation, whereas bisphosphonates reduce bone resorption. In clinical trials, both treatments increase bone density, although sodium fluoride (NaF) increases and alendronate (bisphosphonate, ALN) decreases bone turnover. In a comparative study using minipigs an inverse correlation has been reported between bone turnover and elastic modulus. Small-angle X-ray scattering (SAXS) measurements of these bones revealed no structural deterioration of the collagen/mineral composite at the nanometer range for ALN-treated vertebra, whereas a slight increase of the average thickness of the mineral crystals as well as changes of the structure of the collagen/mineral composite were found in the bones of NaF-treated animals. In this study we used quantitative backscattered electron imaging (qBSE) to investigate the cancellous bones from ribs of minipigs treated with vehicle, NaF, or ALN. This method provides information on the local mineral concentration in the micrometer range. Mineralization spectra were obtained from each treatment group, and statistically significant differences between ALN and controls were found for the peak position, the peak height, the peak width, and the average calcium (Ca) concentration of the mineral distribution. The results reveal that the cancellous bone matrix was more uniformly mineralized after ALN treatment. The reduced bone turnover induced by ALN, documented histomorphometrically could be at the origin of this phenomenon. No significant differences were detected between NaF and control. Together with the earlier SAXS data these results may explain in part the increase in bone density and the improvement of biomechanical properties observed after ALN treatment in animals and in osteoporotic patients.