Georges Boivin

Bilateral fractures of the femur diaphysis in a patient with rheumatoid arthritis on long-term treatment with alendronate: clues to the mechanism of increased bone fragility.

Unusual fractures of the femur diaphysis have been reported in patients treated with alendronate and, although no causal relationship has been established, excessive suppression of bone turnover and length of treatment with alendronate have been implicated in their pathogenesis. We report here clinical, biochemical, and radiological findings of a patient with rheumatoid arthritis and multiple risk factors for fractures who was treated with alendronate for 8 yr and developed spontaneous bilateral subtrochanteric/diaphyseal fractures. Bone biopsies obtained form the iliac crest and the femur showed decreased bone formation with histomorphometric evidence of markedly increased bone resorption at the femur. These results show for the first time that an imbalance between bone resorption and bone formation at the affected bone is associated with the occurrence of these atypical femur fractures. The cause of this imbalance is currently unknown, and further studies of the epidemiology and pathogenesis of diaphyseal femur fractures are warranted.

The role of mineralization and organic matrix in the microhardness of bone tissue from controls and osteoporotic patients.

Degree of mineralization of bone (DMB) is a major intrinsic determinant of bone strength at the tissue level but its contribution to the microhardness (Vickers indentation) at the intermediary level of organization of bone tissue, i.e., Bone Structural Units (BSUs), has never been assessed. The purpose of this study was to analyze the relationship between the microhardness, the DMB and the organic matrix, measured in BSUs from human iliac bone biopsies. Iliac bone samples from controls and osteoporotic patients (men and women), embedded in methyl methacrylate, were used. Using a Vickers indenter, microhardness (kg/mm2) was measured, either globally on surfaced blocks or focally on 100 microm-thick sections from bone samples (load of 25 g applied during 10 sec; CV=5%). The Vickers indenter was more suited than the Knoop indenter for a tissue like bone in which components are diversely oriented. Quantitative microradiography performed on 100 microm-thick sections, allowed measurement of parameters reflecting the DMB (g/cm3). Assessed on the whole bone sample, both microhardness and DMB were significantly lower (-10% and -7%, respectively) in osteoporotic patients versus controls (p<0.001). When measured separately at the BSU level, there were significant positive correlations between microhardness and DMB in controls (r2=0.36, p<0.0001) and osteoporotic patients (r2=0.43, p<0.0001). Mineralization is an important determinant of the microhardness, but did not explain all of its variance. To highlight the role of the organic matrix in bone quality, microhardness of both osteoid and adjacent calcified matrix were measured in iliac samples from subjects with osteomalacia. Microhardness of organic matrix is 3-fold lower than the microhardness of calcified tissue. In human calcanei, microhardness was significantly correlated with DMB (r2=0.33, p=0.02) and apparent Youngs modulus (r2=0.26, p=0.03). In conclusion, bone microhardness measured by Vickers indentation is an interesting methodology for the evaluation of bone strength and its determinants at the BSU level. Bone microhardness is linked to Youngs modulus of bone and is strongly correlated to mineralization, but the organic matrix accounts for about one third of its variance.

Mineral maturity and crystallinity index are distinct characteristics of bone mineral

The purpose of this study was to test the hypothesis that mineral maturity and crystallinity index are two different characteristics of bone mineral. To this end, Fourier transform infrared microspectroscopy (FTIRM) was used. To test our hypothesis, synthetic apatites and human bone samples were used for the validation of the two parameters using FTIRM. Iliac crest samples from seven human controls and two with skeletal fluorosis were analyzed at the bone structural unit (BSU) level by FTIRM on sectionsxa02–4xa0μm thick. Mineral maturity and crystallinity index were highly correlated in synthetic apatites but poorly correlated in normal human bone. In skeletal fluorosis, crystallinity index was increased and maturity decreased, supporting the fact of separate measurement of these two parameters. Moreover, results obtained in fluorosis suggested that mineral characteristics can be modified independently of bone remodeling. In conclusion, mineral maturity and crystallinity index are two different parameters measured separately by FTIRM and offering new perspectives to assess bone mineral traits in osteoporosis.

In osteoporotic women treated with strontium ranelate, strontium is located in bone formed during treatment with a maintained degree of mineralization.

SummaryIn postmenopausal osteoporotic women and up to 3xa0years of treatment with strontium ranelate, strontium was present only in recently deposited bone tissue resulting from formation activity during the period of treatment. Strontium was shown to be dose-dependently deposited into this newly formed bone with preservation of the mineralization.IntroductionInteractions between strontium (Sr) and bone mineral and its effects on mineralization were investigated in women treated with strontium ranelate.MethodsBone biopsies from osteoporotic women were obtained over 5-year strontium ranelate treatment from phases II and III studies. Bone samples obtained over 3-year treatment were investigated by X-ray microanalysis for bone Sr uptake and focal distribution, and by quantitative microradiography for degree of mineralization. On some samples, Sr distribution (X-ray cartography) was analyzed on whole sample surfaces and the percentage of bone surface containing Sr was calculated. Bone Sr content was chemically measured on whole samples.ResultsIn treated women, Sr was exclusively present in bone formed during treatment; Sr deposition depended on the dose with higher focal content in new bone structural units than in old ones constantly devoid of Sr, even after 3-year treatment. A plateau in global bone Sr content was reached after 3xa0years of treatment. Cartography illustrated the extent of surfaces containing Sr, and formation activity during strontium ranelate treatment was higher in cancellous than in cortical bone. Mineralization was maintained during treatment.ConclusionThe quality of bone mineral was preserved after treatment with strontium ranelate, supporting the safety of this agent at the bone tissue level.

Mechanisms of the Anabolic Effects of Teriparatide on Bone: Insight From the Treatment of a Patient With Pycnodysostosis

Pycnodysostosis is an extremely rare genetic osteosclerosis caused by cathepsin K deficiency. We hypothesized that teriparatide, a potent anabolic agent used in the treatment of osteoporosis, might reduce skeletal fragility by activating bone turnover. We studied a typical case of pycnodysostosis in a 37‐yr‐old woman who exhibited short stature, skull and thorax deformities, and a history of severe fragility fractures. Cathepsin K gene sequencing was performed. Before and after 6 mo of 20 μg/d teriparatide, biochemical markers of bone turnover were measured, and 3D bone structure and microarchitecture was assessed in vivo by HR‐pQCT. Qualitative and quantitative analysis of transiliac bone biopsies were performed, and the degree of mineralization was evaluated by quantitative microradiography. In vitro assessment of bone resorption was performed after separation and differentiation of CD14+ monocytes from peripheral blood. Bone structure assessed by HR‐pQCT on the radius and tibia showed augmentation of cortical and trabecular density. Transiliac bone biopsy showed highly increased bone mass (+63% versus age‐ and sex‐matched controls), a decrease in bone remodeling without evidence of active osteoblasts, and a severe decrease in the dynamic parameters of bone formation (mineralizing surfaces, −90% and bone formation rate, −93% versus age‐ and sex‐matched controls). This depressed bone turnover probably explained the increased degree of mineralization. The presence of a novel missense mutation leading to an A141V amino acid substitution confirmed a genetic defect of cathepsin K as the cause of the disease. The deficiency of active osteoclasts was confirmed by an in vitro study that showed a decreased concentration of CD14+ monocytes (the precursor of osteoclasts) in blood. These osteoclasts had low resorptive activity when incubated on bone slices. After 6 mo of teriparatide, the structure, microarchitecture, and turnover of bone—assessed by HR‐pQCT, histology, and bone turnover markers—remained unchanged. Our data strongly suggest that some features of the osteoclastic phenotype—that are absent in pycnodysostosis—are a prerequisite for the anabolic effect of PTH on osteoblasts.

Effects of preexisting microdamage, collagen cross-links, degree of mineralization, age, and architecture on compressive mechanical properties of elderly human vertebral trabecular bone.

Previous studies have shown that the mechanical properties of trabecular bone are determined by bone volume fraction (BV/TV) and microarchitecture. The purpose of this study was to explore other possible determinants of the mechanical properties of vertebral trabecular bone, namely collagen cross‐link content, microdamage, and mineralization. Trabecular bone cores were collected from human L2 vertebrae (nu2009=u200949) from recently deceased donors 54–95 years of age (21 men and 27 women). Two trabecular cores were obtained from each vertebra, one for preexisting microdamage and mineralization measurements, and one for BV/TV and quasi‐static compression tests. Collagen cross‐link content (PYD, DPD, and PEN) was measured on surrounding trabecular bone. Advancing age was associated with impaired mechanical properties, and with increased microdamage, even after adjustment by BV/TV. BV/TV was the strongest determinant of elastic modulus and ultimate strength (r2u2009=u20090.44 and 0.55, respectively). Microdamage, mineralization parameters, and collagen cross‐link content were not associated with mechanical properties. These data indicate that the compressive strength of human vertebral trabecular bone is primarily determined by the amount of trabecular bone, and notably unaffected by normal variation in other factors, such as cross‐link profile, microdamage and mineralization.

Assessment of Microelastic Properties of Bone Using Scanning Acoustic Microscopy: A Face-to-Face Comparison with Nanoindentation

The current work aimed at comparing, on site-matched cortical bone tissue, the micron-level elastic modulus Ea derived from 200 MHz-scanning acoustic microscopy (SAM) acoustic impedance (Z) combined with bone mineral density (assessed by synchrotron radiation microcomputed tomography, SR-µCT) to nanoindentation modulus En. A good correlation was observed between En and Z (R2=0.67, p<0.0001, root mean square error RMSE=1.9 GPa). The acoustical elastic modulus Ea derived from Z showed higher values of E compared to nanoindentation moduli. We assumed that the discrepancy between Ea and En values may likely be due to the fixed assumed value of Poissons ratio while values comprised between 0.15 and 0.45 have been reported in the literature. Despite these differences, a highly significant correlation between Ea and En was found (R2=0.66, p<0.001, RMSE=1.8 GPa) suggesting that SAM can reliably be used as a modality to quantitatively map the local variations of tissue-level bone elasticity.

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.

Unexpected absence of effect of rhBMP-7 on distraction osteogenesis.

We asked whether locally applied rhBMP-7 with a Type I collagen carrier could enhance the consolidation phase in distraction osteogenesis. We performed unilateral transverse osteotomy of the tibia in 28 immature male rabbits. In half the animals (14 rabbits), 70 μg rhBMP-7 (28.5 mg rhBMP-7/CC, OP-1) was applied locally on the day of the osteotomy; the control group (14 rabbits) had no local application. After 7 days, 3 weeks of distraction was begun at a rate of 0.5 mm every 12 hours. Starting with Week 2 of distraction, we assessed radiographic, ultrasonographic, and densitometric parameters once per week. Animals were sacrificed after 3 weeks of distraction, and after 1, 2, and 3 weeks of additional consolidation. Radiographic evaluation revealed a tendency for increased bony union and bone differentiation in the control group compared with the rhBMP-7/CC group. The bone mineral content was higher in the control group at each time. Ultrasonographic evaluation revealed the development of a cyst in the regenerate in 92% of the animals in the rhBMP-7/CC group versus 0% in the control group. Ultrasonography and histomorphometry showed delayed regenerate development in the rhBMP-7/CC group. We suggest the poor results related to the relatively large bulk of the solid carrier of the OP-1 that might have acted as a mechanical obstacle to osteogenesis.

A new concept of gentamicin loaded HAP/TCP bone substitute for prophylactic action: in vivo pharmacokinetic study.

Despite systemic prophylaxis, infection rates after orthopedic surgery can reach more than 1%. A new HAP/TCP bone substitute loaded with 125xa0mg of gentamicin was designed for prophylactic use. Its aim was to enhance the efficacy of systemic prophylactic treatments by increasing the local antibiotic concentration. The release rate of gentamicin from the bone substitute was investigated after implantation in the femoral condyle of five sheep. In order to investigate the local and systemic gentamicin concentrations, synovial fluids and blood samples were assessed over a 5-day period. The mean gentamicin concentration peak in blood was 4.2xa0μg/ml and the mean local concentration in synovial fluids during the first 8xa0h was 305xa0μg/ml. After 48xa0h, the concentrations in blood and synovial fluids were less than 0.5xa0μg/ml. No remaining gentamicin was detected in bone substitutes explanted after 8xa0days of implantation. The gentamicin release rate from the bone substitutes assessed corresponds to the recommendations for the prophylactic use of antibiotics: high local concentration but limited in time (less than 48xa0h) not to select antibiotic-resistant bacterial strains. Our results indicated that this implant should be an effective prophylactic tool in orthopedic surgery in combination with systemic prophylaxis.