Effects of 3- and 5-year treatment with risedronate on bone mineralization density distribution in triple biopsies of the iliac crest in postmenopausal women

Effects of 1 year of daily teriparatide treatment on iliacal bone mineralization density distribution (BMDD) in postmenopausal osteoporotic women previously treated with alendronate or risedronate

Anabolic treatment with teriparatide of postmenopausal osteoporotic patients previously treated with bisphosphonates is a new therapeutic approach. However, its effects on the bone mineralization density distribution (BMDD) are unknown. We studied paired transiliac bone biopsy samples taken before and after 1 year of treatment with recombinant human parathyroid hormone peptide 1-34 (teriparatide) from 16 osteoporotic women treated with either alendronate (priorALN) or risedronate (priorRIS) for at least 2 years and subsequently treated for 12 months with teriparatide. Cancellous (Cn.) and cortical (Ct.) BMDD values were measured using quantitative backscattered electron imaging. At baseline, BMDD values of priorALN and priorRIS women were similar and within the normal range. One year of teriparatide treatment caused significant effects on the BMDD. Analyzing changes from baseline for each bisphosphonate group separately, priorALN patients revealed increases in the portion of low mineralized bone areas (Cn.Ca(Low) +25.9%, Ct.Ca(Low) +62.0%, both p < .05) and Ct. heterogeneity of mineralization (Ct.Ca(Width) +22.8%, p < .001). PriorRIS patients showed increased mineralization heterogeneity (Cn.Ca(Width) +14.8%, p < .05, and Ct.Ca(Width) +15.8%, p < .001). Analysis of the influence of the prior bisphosphonate treatment showed that the BMDD response to 1 year of teriparatide treatment did not depend on the type of prior bisphosphonate. In consequence, priorALN and priorRIS groups were combined. The pooled groups revealed increased Cn.Ca(Width) and Ct.Ca(Width) (+10.7%, p < .01, and +19.6%, p < .001, respectively) as well as increased Cn.Ca(Low) and Ct.Ca(Low) (+18.2%, p < .05, and +36.6%, p < .01, respectively). In summary, our findings indicate a significant effect of teriparatide on BMDD when administered subsequent to a bisphosphonate in agreement with teriparatide's anabolic action.

Atypical fractures of the femur and bisphosphonate therapy: A systematic review of case/case series studies

Atypical fractures of the femur below the lesser trochanter have been reported in patients treated with bisphosphonates. We performed a systematic literature search of case/case series studies to better define the clinical presentation and to identify characteristics that may predispose patients to such fractures. We considered only women treated with a bisphosphonate at a dosing regimen used for the prevention or treatment of osteoporosis and we included also eight own unpublished cases. We identified 141 women with atypical fractures of the femur, mean age of 67.8+/-11.0 years, who were treated with bisphosphonate for 71.5+/-40.0 months (range=3-192 months). The results of this analysis allow identification of patients on bisphosphonate treatment at risk of developing atypical fractures, define fractures better as predominantly insufficiency fractures, illustrate that long-term bisphosphonate treatment is not a prerequisite for their development, recognize the use of glucocorticoids and proton pump inhibitors as important risk factors, but do not provide insights in the pathogenesis of these fractures and raise questions that need to be addressed in properly designed studies.

Bisphosphonate treatment modifies canine bone mineral and matrix properties and their heterogeneity

Bone loss and alterations in bone quality are major causes leading to bone fragility in postmenopausal women. Although bisphosphonates are well known to reduce bone turnover and prevent bone loss in postmenopausal osteoporosis, their effects on other bone properties are not fully characterized. Changes in bone mineral and matrix properties may contribute to the anti-fracture efficacy observed with bisphosphonate treatments. The aim of this work was to analyze the effect of a 1-year treatment with either alendronate or risedronate, at low and high doses, on spatially resolved bone material and compositional properties that could contribute to the fracture efficacy of these agents. Distal tibias from 30 normal beagles that had been treated daily for 1 year with oral doses of vehicle (Veh), alendronate (Aln) at 0.2 or 1 mg/kg, and risedronate (Ris) at 0.1 or 0.5 mg/kg were analyzed by Fourier Transform Infrared imaging (FTIRI) to assess the changes in both mineral and matrix properties in discrete bone areas. The widths at half maximum of the pixel histograms for each FTIRI parameter were used to assess the heterogeneity of the bone tissue. Aln and Ris increased the mineral content and the collagen maturity mainly in cancellous bone and at the endocortical surface. Significant differences were observed in the mineral content and in the hydroxyapatite crystallinity distribution in bone tissue, which can contribute to reduced ductility and micro-crack accumulation. No significant differences were observed between low and high dose nor between Aln and Ris treatments. These results show that pharmacologic suppression of bone turnover increases the mineral and matrix bone tissue maturity in normal cancellous and endocortical bone areas where bone turnover is higher. These positive effects for decreased fracture risk are also associated with a loss of bone heterogeneity that could be one factor contributing to increased bone tissue brittleness and micro-crack accumulation.

Evidence of reduced bone turnover and disturbed mineralization process in a boy with Stickler syndrome

We describe a tall-statured 14-year-old boy who illustrated the full phenotypic and radiographic features of Stickler syndrome type I. A bone biopsy showed evidence of reduced bone mass and bone turnover, such as reduced BV/TV (-43%), TbTh (-29%), and OS/BS (-48%), Ob.S/BS (-27%), and Oc/BS (-47%) compared to "age-matched" controls. Moreover, there was evidence that the mineralization process was severely disturbed. Quantitative backscattered electron imaging revealed that the bone mineralization density distribution (BMDD) of cancellous (Cn) as well as cortical (Ct) bone was shifted toward lower mineralization compared to a young control reference cohort. BMDD parameters of mean degree of mineralization, Cn Ca (-9.8%) and Ct Ca (-18.0%), were dramatically decreased. To the best of our knowledge this is the first clinical report describing bone biopsy findings in a boy with Stickler syndrome. Such a severe undermineralization of bone matrix might essentially contribute to the compromised mechanical competence of the skeleton found in this patient.

Mineralization density distribution of postmenopausal osteoporotic bone is restored to normal after long-term alendronate treatment: qBEI and sSAXS data from the fracture intervention trial long-term extension (FLEX)

Long-term treatment studies showed that the therapeutic effects of alendronate (ALN) were sustained over a 10-year treatment period. However, data on the effects on intrinsic bone material properties by long-term reduction of bone turnover are still sparse. We analyzed transiliacal bone biopsies of a subgroup of 30 Fracture Intervention Trial Long-Term Extension (FLEX) participants (n = 6 were treated for 10 years with ALN at dose of 10 mg/day, n = 10 were treated for 10 years with ALN at dose of 5 mg/day, and n = 14 were treated for 5 years with ALN plus a further 5 years with placebo) by quantitative backscattered electron imaging (qBEI) and scanning small-angle X-ray scattering (sSAXS) to determine the bone mineralization density distribution (BMDD) and the mineral particle thickness parameter T. BMDD data from these FLEX participants were compared with those from a previously published healthy population (n = 52). Compared with 5 years of ALN plus 5 years of placebo 10 years of ALN treatment (independent of the dose given) did not produce any difference in any of the BMDD parameters: The weighted mean (Ca(mean)), the typical calcium concentration (Ca(peak)), the heterogeneity of mineralization (Ca(width)), the percentage of low-mineralized bone areas (Ca(low)), and the portion of highly mineralized areas (Ca(high)) were not different for the patients who continued ALN from those who stopped ALN after 5 years. Moreover, no significant differences for any of the BMDD parameters between the FLEX participants and the healthy population could be observed. In none of the investigated cases were abnormally high mineralization or changes in mineral particle thickness observed (Ca(high) and T were both in the normal range). The findings of this study support the recommendation that antiresorptive treatment with ALN should be maintained for 5 years. Even with longer treatment durations of up to 10 years, though, no negative effects on bone matrix mineralization were observed.

Trabecular packet-level lamellar density patterns differ by fracture status and bone formation rate in white females

Spatial patterns of mineralization for human iliac crest cancellous bone were measured from images obtained by quantitative backscattered electron microscopy. Biopsies collected from vertebral fracture patients and healthy individuals with high or low bone formation rate (BFR(s)) were examined (fracture/low BFR(s): N=12, fracture/high BFR(s): N=10, normal/low BFR(s): N=12, normal/high BFR(s): N=15). 20 by 20 pixel square areas or smaller were sampled from superficial and deep remodeling packets. Mean (Z(mean)) and standard deviation (SD) of mineralization were measured, and coefficients of variation (CV=SD/Z(mean)) were calculated. Fast Fourier transform analysis was used to quantify the distribution of the mineral in the packets. "FFT_ratio" was defined as the ratio magnitude of the principal spatial frequency to the average atomic number density. A higher FFT_ratio occurred in specimens with more pronounced alternating layers of light and dark as visible in the backscattered electron image, which was defined as lamellar patterning. Two-way ANOVA revealed that the coefficients of variation of mineralization for both superficial and deep packets were significantly lower in fracture patients than in normal individuals. However, the interaction between turnover rate and group (fracture/non-fracture) indicated that the difference in packet CV occurred among the low turnover individuals and not among those with high turnover. Mean mineralization levels and CV between deep and superficial packets were highly correlated. Regressions of packet CV of mineralization and FFT_ratio were highly significant (p<0.001) for all packets pooled and for packets divided by group (fracture/normal). However, analyses of packet CV and FFT_ratio by individual were variable (R(2) from 0.00338 to 0.700). Packet-level mineralization variability may be associated with fracture toughness, and fracture patients had less variable packet-level mineralization. The result that the packet CV varied significantly between fracture and non-fracture individuals with low turnover suggests that for low turnover subjects without fracture, high variability in mineralization may have a protective effect. In high turnover patients, the accelerated turnover may prevent the lamellar variability from developing over time. Strong correlations between CV and Z(mean) for both superficial and deep packets imply that newly formed bone is created similarly to older bone within an individual. Fourier transform results show that the mineralization variability found within packets is associated with lamellar patterning. Lamellar structure has been hypothesized to guide microcrack propagation in order to optimize bone strength and toughness. Osteoporotics with fracture had less pronounced lamellation than healthy normals and may be more prone to fracture.

Combination of nanoindentation and quantitative backscattered electron imaging revealed altered bone material properties associated with femoral neck fragility

Osteoporotic fragility fractures were hypothesized to be related to changes in bone material properties and not solely to reduction in bone mass. We studied cortical bone from the superior and inferior sectors of whole femoral neck sections from five female osteoporotic hip fracture cases (74-92 years) and five nonfractured controls (75-88 years). The typical calcium content (Ca(Peak)) and the mineral particle thickness parameter (T) were mapped in large areas of the superior and inferior regions using quantitative backscattered electron imaging (qBEI) and scanning small-angle X-ray scattering, respectively. Additionally, indentation modulus (E) and hardness (H) (determined by nanoindentation) were compared at the local level to the mineral content (Ca(Ind)) at the indent positions (obtained from qBEI). Ca(Peak) (-2.2%, P = 0.002), Ca(Ind) (-1.8%, P = 0.048), E (-5.6%, P = 0.040), and H (-6.0%, P = 0.016) were significantly lower for the superior compared to the inferior region. Interestingly, Ca(Peak) as well as Ca(Ind) were also lower (-2.6%, P = 0.006, and -3.7%, P = 0.002, respectively) in fracture cases compared to controls, while E and H did not show any significant reduction. T values were in the normal range, independent of region (P = 0.181) or fracture status (P = 0.551). In conclusion, it appears that the observed femoral neck fragility is associated with a reduced mineral content, which was not accompanied by a reduction in stiffness and hardness of the bone material. This pilot study suggests that a stiffening process in the organic matrix component contributes to bone fragility independently of mineral content.

The degree and distribution of cortical bone mineralization in the human femoral shaft change with age and sex in a microradiographic study

BACKGROUND

The incidence of osteoporotic hip fractures increases with age, more sharply in women than in men, as a result of qualitative and quantitative bone alterations. Mineralization (a qualitative parameter) showed no differences with age or sex in cancellous bone in earlier studies. Few studies assessed such differences in cortical bone, a major contributor to femoral bone strength. The aim of this in vitro cross-sectional study of a large group of human femoral midshafts was to look for age- and sex-related differences in the degree and distribution of cortical mineralization that might be implicated in bone fragility.

METHODS

Cortical bone specimens from 193 femurs were studied using quantitative microradiography, with an aluminum step-wedge reference. The femurs were from 99 females and 94 males in a Caucasian anthropological collection covering a broad age spectrum. We determined the mean degree of mineralization of osteons (On.DMB-Al), interstitial tissue (Int.DMB-Al), and total bone (Tt.DMB-Al), and representative parameters of density histograms. Results were expressed as relative values. Age- and sex-related differences in DMB-Al values were evaluated using non-parametric tests.

RESULTS

Degree of tissue mineralization (Tt.DMB-Al) decreased significantly with age in females (r=-0.257; P=0.010) but did not change in males. Tt.DMB-Al was higher in females than males until 50 years of age (P=0.001) but was lower in elderly females than elderly males (P=0.016). DMB-Al distribution varied significantly with sex and age. The first DMB-Al quartiles in osteons and interstitial tissue were not different between males and females, but the third quartile and interquartile range differed significantly (P=0.032 and P=0.000, respectively). The mineralization difference between the two tissues indicated greater bone heterogeneity in females than males (P=0.000).

CONCLUSIONS

In this in vitro cross-sectional study of anterior midfemoral cortical specimens, the degree and distribution of mineralization varied with age and sex. In females, mineralization started at a higher level than in males but was lower in the sixth decade, falling below the level in males. Mineralization was far more stable throughout life in males. In elderly females, the lower degree and greater heterogeneity of mineralization may have consequences on bone strength and the risk of fracture.

Cortical bone mineralization differences between hip-fractured females and controls. A microradiographic study

The strength of bone depends on both bone quantity and bone quality. One determinant of bone quality is the degree of mineralization of bone tissue (DMB). To assess the role for DMB in osteoporotic hip fractures, we compared the degree of mineralization in femoral neck cortex from 23 women with hip fractures (age, 65-96 years) and 14 female controls (age, 75-103 years) using quantitative microradiography calibrated with an aluminum step wedge. Variables were DMB in osteons (oDMB(Al)mean) and interstitial tissue (exDMB(Al)mean). Wilcoxon signed-rank tests were used to compare oDMB(Al)mean to exDMB(Al)mean in each group, and Mann-Whitney tests to compare oDMB(Al)mean and exDMB(Al)mean between hip-fracture patients and controls. DMB was significantly lower in the osteons than in the interstitial tissue in both groups (hip-fracture group, P=0.000; control group, P=0.001). DMB values in osteons and interstitial tissue were significantly greater in the hip-fracture patients than in the controls (P=0.007 and P=0.005, respectively). These cross-sectional data suggest that bone fragility may be related to a higher degree of tissue mineralization.

The combination therapy with alfacalcidol and risedronate improves the mechanical property in lumbar spine by affecting the material properties in an ovariectomized rat model of osteoporosis

Background

We conducted the present study to investigate the therapeutic effects of a combination treatment of alfacalcidol (ALF) and risedronate (RIS) on the bone mechanical properties of bone and calcium (Ca) metabolism using an ovariectomized (OVX) rat model of osteoporosis.

Methods

Female Wistar rats were OVX- or sham-operated at 40 weeks of age. Twelve weeks post-surgery, rats were randomized into seven groups: 1) sham + vehicle, 2) OVX + vehicle, 3) OVX + ALF 0.025 μg/kg/day, 4) OVX + ALF 0.05 μg, 5) OVX + RIS 0.3 mg, 6) OVX + RIS 3.0 mg, 7) OVX + ALF 0.025 μg + RIS 0.3 mg. Each drug was administered orally five times a week for 12 weeks. After treatment, we evaluated the mechanical properties of the lumbar vertebra and femoral midshaft. In the lumbar vertebra, structural and material analyses were performed using micro-computed tomography (micro-CT) and microbeam X-ray diffraction (micro-XRD), respectively. Biochemical markers in serum and urine were also determined.

Results

(1) With respect to improvement in the mechanical strength of the lumbar spine and the femoral midshaft, the combination treatment of ALF and RIS at their sub-therapeutic doses was more effective than each administered as a monotherapy; (2) In the suppression of bone resorption and the amelioration of microstructural parameters, the effects of ALF and RIS were considered to be independent and additive; (3) The improvement of material properties, such as microstructural parameters and the biological apatite (Bap) c-axis orientation, contributed to the reinforcement of spinal strength; and (4) The combination treatment of ALF and RIS normalized urinary Ca excretion, suggesting that this treatment ameliorated the changes in Ca metabolism.

Conclusion

These results demonstrate that the combination treatment of ALF and RIS at their sub-therapeutic doses can improve the mechanical properties of the spine as well as the femur and ameliorate changes in Ca metabolism in an animal model of osteoporosis, suggesting that the combination treatment of ALF and RIS has a therapeutic advantage over each monotherapy for the treatment of osteoporosis.

Normative data on mineralization density distribution in iliac bone biopsies of children, adolescents and young adults

Bone mineralization density distribution (BMDD) as assessed by quantitative backscattered electron imaging (qBEI) in iliac crest bone biopsies has become in the last years a powerful diagnostic tool to evaluate the effect of metabolic bone diseases and/or therapeutic interventions on the mineralization status of the bone material. However until now, normative reference data are only available for adults. The aim of the present study is to close this gap and establish normative data from children and compare them with reference BMDD data of adults. qBEI analyses were performed on bone samples from 54 individuals between 1.5 and 23 years without metabolic bone diseases, which were previously used as study population to establish normative histomorphometric standards. In the trabecular compartment, none of the BMDD parameters showed a significant correlation with age. The BMDD was shifted towards lower mineralization density (CaMean -5.6%, p<0.0001; CaPeak -5.6%, p<0.0001; CaLow +39.0% p<0.001; CaHigh -80.7%, p<0.001) and the inter-individual variation was higher compared to the adult population. The cortices appeared to be markedly less mineralized (CaMean -3.1%, p<0.0001) than cancellous bone due to higher amounts of low mineralized secondary bone. However, the cortical BMDD parameters showed a strong correlation (r=0.38 to 0.85, with p<0.001 to<0.0001) with cancellous BMDD parameters. In conclusion, this study provides evidence that BMDD parameters in growing healthy subjects are relatively constant and that these data can be used as normative references in pediatrics osteology. The larger inter-individual variability compared to adults is most likely related to alterations of the bone turnover rate during growth.

Theoretical analysis of alendronate and risedronate effects on canine vertebral remodeling and microdamage

Bisphosphonates suppress bone remodeling activity, increase bone volume, and significantly reduce fracture risk in individuals with osteoporosis and other metabolic bone diseases. The objectives of the current study were to develop a mathematical model that simulates control and 1 year experimental results following bisphosphonate treatment (alendronate or risedronate) in the canine fourth lumbar vertebral body, validate the model by comparing simulation predictions to 3 year experimental results, and then use the model to predict potential long term effects of bisphosphonates on remodeling and microdamage accumulation. To investigate the effects of bisphosphonates on bone volume and microdamage, a mechanistic biological model was modified from previous versions to simulate remodeling in a representative volume of vertebral trabecular bone in dogs treated with various doses of alendronate or risedronate, including doses equivalent to those used for treatment of post-menopausal osteoporosis in humans. Bisphosphonates were assumed to affect remodeling by suppressing basic multicellular unit activation and reducing resorption area. Model simulation results for trabecular bone volume fraction, microdamage, and activation frequency following 1 year of bisphosphonate treatment are consistent with experimental measurements. The model predicts that trabecular bone volume initially increases rapidly with 1 year of bisphosphonate treatment, and continues to slowly rise between 1 and 3 years of treatment. The model also predicts that microdamage initially increases rapidly, 0.5-1.5-fold for alendronate or risedronate during the first year of treatment, and reaches its maximum value by 2.5 years before trending downward for all dosages. The model developed in this study suggests that increasing bone volume fraction with long term bisphosphonate treatment may sufficiently reduce strain and damage formation rate so that microdamage does not accumulate above that which is initiated in the first two years of treatment.

Spectroscopic markers of bone quality in alendronate-treated postmenopausal women

Comparison of infrared spectroscopic images of sections from biopsies of placebo-treated post-menopausal women and women treated for 3 years with 10 mg/day alendronate demonstrated significant increases in cortical bone mineral content, no alterations in other spectroscopic markers of "bone quality," but a decrease in tissue heterogeneity.

METHODS

The material properties of thick sections from iliac crest biopsies of seven alendronate-treated women were compared to those from ten comparably aged post-menopausal women without bone disease, using infrared spectroscopic imaging at approximately 7 microm spatial resolution. Parameters evaluated were mineral/matrix ratio, crystallinity, carbonate/amide I ratio, and collagen maturity. The line widths at half maximum of the pixel histograms for each parameter were used as measures of heterogeneity.

RESULTS

The mineral content (mineral/matrix ratio) in the cortical bone of the treated women's biopsies was higher than that in the untreated control women. Crystallinity, carbonate/protein, and collagen maturity indices were not significantly altered; however, the pixel distribution was significantly narrowed for all cortical and trabecular parameters with the exception of collagen maturity in the alendronate treatment group.

CONCLUSIONS

The increases in mineral density and decreased fracture risk associated with bisphosphonate treatment may be counterbalanced by a decrease in tissue heterogeneity, which could impair tissue mechanical properties. These consistent data suggest that alendronate treatment, while increasing the bone mass, decreases the tissue heterogeneity.

Effects of tumor-induced osteomalacia on the bone mineralization process

Fibroblast growth factor 23 (FGF23) overexpression has been identified as a causative factor for tumor-induced osteomalacia (TIO) characterized by hypophosphatemia due to increased renal phosphate wasting, low 1,25(OH)(2)D(3) serum levels, and low bone density. The effects of long-lasting disturbed phosphate homeostasis on bone mineralization are still not well understood. We report on a patient with a 12-year history of TIO, treated with 1,25(OH)(2)D(3) and phosphate, who finally developed hyperparathyroidism with gland hyperplasia before the tumor could be localized in the scapula and removed. During surgery a transiliac bone biopsy was obtained. FGF23 expression in the tumor cells was confirmed by in situ hybridization. Serum FGF23 levels as measured by ELISA were found to be extremely elevated before and decreased after removal of the tumor. Bone histology/histomorphometry and measurement of bone mineralization density distribution using quantitative backscattered electron imaging were performed on the bone biopsy. The data showed important surface osteoidosis and a slightly increased osteoblast but markedly decreased osteoclast number. The mineralized bone volume (-11%) and mineralized trabecular thickness (-18%) were low. The mean degree of mineralization of the bone matrix (-7%), the most frequent calcium concentration (-4.1%), and the amounts of fully mineralized bone (-40.3%) were distinctly decreased, while the heterogeneity of mineralization (+44.5%) and the areas of primary mineralization (+131.6%) were dramatically increased. We suggest that the elevated levels of FGF23 and/or low phosphate concentrations disturb the mineralization kinetics in vivo without affecting matrix mineralization of pre-existing bone packets.

The influence of boundary conditions and loading mode on high-resolution finite element-computed trabecular tissue properties

A widely used technique for determining the material properties of trabecular tissue is to perform combined experimental and computational testing of trabecular structures in order to calibrate effective tissue properties. To better understand the nature of such properties, we tested n=25 cores of human vertebral trabecular bone under two different boundary conditions (endcap and PMMA embedding) and loading modes (compression and torsion). High-resolution (20 microm) finite element models that explicitly modeled the different experimental conditions were constructed and sensitivity studies were performed to quantify errors arising from uncertainties between model and experiment. Mean (+/-S.D.) effective tissue modulus for the four groups ranged from 9.6+/-1.9 to 11.5+/-3.5 GPa, and the overall mean was 10.3+/-2.4 GPa. For the endcap tests, mean values were the same regardless of loading mode, suggesting that the effective tissue modulus is representative of true material behavior. However, on a specimen-specific basis, the various repeated measures of effective tissue modulus were only moderately correlated with each other (R2=27% to 81%), indicating that the individual measures can be subject to appreciable random errors. The sensitivity studies on the endcap tests indicated that models using lower resolution (40 microm element size) and roller-type platens boundary conditions overestimated effective tissue modulus by 42% on average, although preliminary tests with higher-density femoral neck bone indicated less sensitivity to modeling issues. We conclude that effective tissue properties derived from micro-finite element models do have biomechanical significance if measured correctly, although individual measures of tissue properties may have poor precision.

Prolonged treatments with antiresorptive agents and PTH have different effects on bone strength and the degree of mineralization in old estrogen-deficient osteoporotic rats

Current approved medical treatments for osteoporosis reduce fracture risk to a greater degree than predicted from change in BMD in women with postmenopausal osteoporosis. We hypothesize that bone active agents improve bone strength in osteoporotic bone by altering different material properties of the bone. Eighteen-month-old female Fischer rats were ovariectomized (OVX) or sham-operated and left untreated for 60 days to induce osteopenia before they were treated with single doses of either risedronate (500 microg/kg, IV), zoledronic acid (100 microg/kg, IV), raloxifene (2 mg/kg, PO, three times per week), hPTH(1-34) (25 microg/kg, SC, three times per week), or vehicle (NS; 1 ml/kg, three times per week). Groups of animals were killed after days 60 and 180 of treatment, and either the proximal tibial metaphysis or lumbar vertebral body were studied. Bone volume and architecture were assessed by muCT and histomorphometry. Measurements of bone quality included the degree of bone mineralization (DBM), localized elastic modulus, bone turnover by histomorphometry, compression testing of the LVB, and three-point bending testing of the femur. The trabecular bone volume, DBM, elastic modulus, and compressive bone strength were all significantly lower at day 60 post-OVX (pretreatment, day 0 study) than at baseline. After 60 days of all of the bone active treatments, bone mass and material measurements agent were restored. However, after 180 days of treatment, the OVX + PTH group further increased BV/TV (+30% from day 60, p < 0.05 within group and between groups). In addition, after 180 days of treatment, there was more highly mineralized cortical and trabecular bone and increased cortical bone size and whole bone strength in OVX + PTH compared with other OVX + antiresorptives. Treatment of estrogen-deficient aged rats with either antiresorptive agents or PTH rapidly improved many aspects of bone quality including microarchitecture, bone mineralization, turnover, and bone strength. However, prolonged treatment for 180 days with PTH resulted in additional gains in bone quality and bone strength, suggesting that the maximal gains in bone strength in cortical and trabecular bone sites may require a longer treatment period with PTH.

Effect of temporal changes in bone turnover on the bone mineralization density distribution: a computer simulation study

The heterogeneous distribution of mineral content in trabecular bone reflects the continuous renewal of bone material in bone remodeling and the subsequent increase in mineral content in the newly formed bone packets. The bone mineralization density distribution (BMDD) is typically used to describe this nonuniform mineral content of the bone matrix. Our mathematical model describes changes of the BMDD of trabecular bone as a function of bone resorption and deposition rates and the mineralization kinetics in a newly formed bone packet. Input parameters used in the simulations were taken from experimental studies. The simulations of the time evolution of the BMDD after increase in bone turnover (perimenopausal period) resulted in a shift of the BMDD toward lower values of the mineral content. Transiently, there was a broadening of the BMDD configuration partly showing two peaks, which points to a strongly heterogeneous distribution of the mineral. Conversely, when the remodeling rate was reduced (antiresorptive therapy), the BMDD shifted toward higher values of the mineral content. There was a transient narrowing of the distribution before broadening again to reach the new steady state. Results from this latter simulation are in good agreement with measurements of the BMDD of patients after 3 and 5 yr of treatment with risedronate. Based on available experimental data on bone remodeling, this model gives reliable predictions of changes in BMDD, an important factor of bone material quality. With the availability of medications with a known effect on bone turnover, this knowledge opens the possibility for therapeutic manipulation of the BMDD.

Glucocorticoid-induced bone loss in mice can be reversed by the actions of parathyroid hormone and risedronate on different pathways for bone formation and mineralization

OBJECTIVE

Glucocorticoid excess decreases bone mineralization and microarchitecture and leads to reduced bone strength. Both anabolic (parathyroid hormone [PTH]) and antiresorptive agents are used to prevent and treat glucocorticoid-induced bone loss, yet these bone-active agents alter bone turnover by very different mechanisms. This study was undertaken to determine how PTH and risedronate alter bone quality following glucocorticoid excess.

METHODS

Five-month-old male Swiss-Webster mice were treated with the glucocorticoid prednisolone (5 mg/kg in a 60-day slow-release pellet) or placebo. From day 28 to day 56, 2 groups of glucocorticoid-treated animals received either PTH (5 microg/kg) or risedronate (5 microg/kg) 5 times per week. Bone quality and quantity were measured using x-ray tomography for the degree of bone mineralization, microfocal computed tomography for bone microarchitecture, compression testing for trabecular bone strength, and biochemistry and histomorphometry for bone turnover. In addition, real-time polymerase chain reaction (PCR) and immunohistochemistry were performed to monitor the expression of several key genes regulating Wnt signaling (bone formation) and mineralization.

RESULTS

Compared with placebo, glucocorticoid treatment decreased trabecular bone volume (bone volume/total volume [BV/TV]) and serum osteocalcin, but increased serum CTX and osteoclast surface, with a peak at day 28. Glucocorticoids plus PTH increased BV/TV, and glucocorticoids plus risedronate restored BV/TV to placebo levels after 28 days. The average degree of bone mineralization was decreased after glucocorticoid treatment (-27%), but was restored to placebo levels after treatment with glucocorticoids plus risedronate or glucocorticoids plus PTH. On day 56, RT-PCR revealed that expression of genes that inhibit bone mineralization (Dmp1 and Phex) was increased by continuous exposure to glucocorticoids and glucocorticoids plus PTH and decreased by glucocorticoids plus risedronate, compared with placebo. Wnt signaling antagonists Dkk-1, Sost, and Wif1 were up-regulated by glucocorticoid treatment but down-regulated after glucocorticoid plus PTH treatment. Immunohistochemistry of bone sections showed that glucocorticoids increased N-terminal Dmp-1 staining while PTH treatment increased both N- and C-terminal Dmp-1 staining around osteocytes.

CONCLUSION

Our findings indicate that both PTH and risedronate improve bone mass, degree of bone mineralization, and bone strength in glucocorticoid-treated mice, and that PTH increases bone formation while risedronate reverses the deterioration of bone mineralization.

Quantitative imaging of musculoskeletal tissue

Quantitative imaging of musculoskeletal tissue, including radiography, computed tomography (CT), and magnetic resonance imaging (MRI), has become the essential methodology in clinical practice for diagnosis and monitoring of various musculoskeletal conditions. Furthermore, quantitative imaging technologies have become indispensable for research and development in diseases of the human skeleton. Standardized methods of image analysis have been developed through the years to quantify measurements on bone and cartilage with high precision and accuracy. Key areas of musculoskeletal disease where quantitative imaging is currently employed are osteoporosis and arthritis.

Identification of material parameters based on Mohr-Coulomb failure criterion for bisphosphonate treated canine vertebral cancellous bone

Nanoindentation has been widely used to study bone tissue mechanical properties. The common method and equations for analyzing nanoindentation, developed by Oliver and Pharr, are based on the assumption that the material is linearly elastic. In the present study, we adjusted the constraint of linearly elastic behavior and use nonlinear finite element analysis to determine the change in cancellous bone material properties caused by bisphosphonate treatment, based on an isotropic form of the Mohr-Coulomb failure model. Thirty-three canine lumbar vertebrae were used in this study. The dogs were treated daily for 1 year with oral doses of alendronate, risedronate, or saline vehicle at doses consistent, on a mg/kg basis, to those used clinically for the treatment of post-menopausal osteoporosis. Two sets of elastic modulus and hardness values were calculated for each specimen using the Continuous Stiffness Measurement (CSM) method (E(CSM) and H(CSM)) from the loading segment and the Oliver-Pharr method (E(O-P) and H(O-P)) from the unloading segment, respectively. Young's modulus (E(FE)), cohesion (c), and friction angle (phi) were identified using a finite element model for each nanoindentation. The bone material properties were compared among groups and between methods for property identification. Bisphosphonate treatment had a significant effect on several of the material parameters. In particular, Oliver-Pharr hardness was larger for both the risedronate- and alendronate-treated groups compared to vehicle and the Mohr-Coulomb cohesion was larger for the risedronate-treated compared to vehicle. This result suggests that bisphosphonate treatment increases the hardness and shear strength of bone tissue. Shear strength was linearly predicted by modulus and hardness measured by the Oliver-Pharr method (r(2)=0.99). These results show that bisphosphonate-induced changes in Mohr-Coulomb material properties, including tissue shear cohesive strength, can be accurately calculated from Oliver-Pharr measurements of Young's modulus and hardness.

Short-term effects of high-dose zoledronic acid treatment on bone mineralization density distribution after orthotopic liver transplantation

Patients with "hepatic" bone disease exhibit increased fracture incidence. The effects on bone material properties, their changes due to orthotopic liver transplantation (OLT), as well as zolendronate (ZOL) treatment have not yet been investigated. We studied bone mineralization density distribution (BMDD) in paired transiliacal biopsies (at and 6 months after OLT) from patients (control CON n = 18, treatment group ZOL n = 21, the latter treated with i.v. ZOL at doses of 4 mg/month) for how bone at the material level was affected by the "hepatic" disease in general, as well as by OLT and ZOL in particular. (1) BMDD parameters at baseline reflected disturbed bone matrix mineralization in "hepatic" bone disease combined with low turnover. Trabecular bone displayed a decrease in mean and most frequent calcium concentration (Ca(MEAN) -2.9% and Ca(PEAK) -2.8%, respectively; both P < 0.001), increased heterogeneity of mineralization (Ca(WIDTH) +12.2%, P = 0.01), and increased percentage of bone areas with low mineralization (Ca(LOW) +32.4%, P = 0.02) compared to normal; however, there were no differences compared to cortical bone. (2) Six months after OLT, ZOL-treated trabecular bone displayed reduced Ca(LOW) (-32.0%, P = 0.047), cortical bone increased Ca(MEAN) (+4.2%, P = 0.009), increased Ca(PEAK) (+3.3%, P = 0.040), and decreased Ca(LOW) (-55.7, P = 0.038) compared to CON and increased Ca(MEAN) compared to baseline (+1.9, P = 0.032) without any signs of hyper- or defective mineralization. These changes as consequence of the antiresorptive action of ZOL visible already after 6 months result in beneficial effects on bone matrix mineralization, likely contributing to the significant decrease in fracture incidence observed in these patients 2 years post transplantation.

Effectiveness of risedronate in osteoporotic postmenopausal women with inflammatory bowel disease: a prospective, parallel, open-label, two-year extension study

OBJECTIVE

To evaluate long-term efficacy of risedronate in osteoporotic postmenopausal patients with inflammatory bowel disease (IBD).

DESIGN

A prospective, parallel, open-label, 2-year extension study of a randomized, double-blind, 1-year clinical trial. Eighty-one osteoporotic postmenopausal women with IBD were treated with risedronate (n = 40) or placebo (n = 41). Bone mineral density (BMD), biochemical bone turnover markers, and vertebral and nonvertebral fractures were assessed throughout the study. Data were analyzed using the intent-to-treat principle.

RESULTS

Significant (P < 0.05) differences were observed between risedronate and placebo groups at 1-, 2-, and 3-year follow-up visits in bone turnover markers and in lumbar spine, trochanter, and femoral neck BMD. In participants treated with risedronate, the percentage of changes from baseline in bone turnover markers and in lumbar spine, trochanter, and femoral neck BMD were significantly (P < 0.05) higher at 2- and 3-year follow-up in comparison with baseline and 1-year follow-up, with a significant (P < 0.05) difference between the 2- and 3-year follow-up visits. At the end of the study, the cumulative risk of vertebral and nonvertebral fractures was significantly (P < 0.05) lower in the risedronate group than in the placebo group. The relative risk for new vertebral fractures was 0.456 (95% CI: 0.134-1.559, P = 0.211) and 0.296 (95% CI: 0.121-0.721, P = 0.007) and was 0.209 (95% CI: 0.023-1.867, P = 0.161) and 0.137 (95% CI: 0.030-0.620, P = 0.010), respectively, for new nonvertebral fractures after 2 and 3 years of risedronate treatment.

CONCLUSIONS

In postmenopausal osteoporotic women with IBD, long-term treatment with risedronate is effective in increasing BMD and reducing vertebral and nonvertebral fracture risk.

Long-term protective effects of zoledronic acid on cancellous and cortical bone in the ovariectomized rat

Current bisphosphonate therapies effectively prevent bone loss in postmenopausal women. We studied the effect of a single intravenous dose of ZOL in ovariectomized rats. Protection from bone loss was dose dependent, lasting for up to 32 weeks, supporting the rationale for an annual intravenous dosing regimen of ZOL for treatment of postmenopausal osteoporosis.

INTRODUCTION

Once-yearly dosing with zoledronic acid (ZOL) 5 mg can increase BMD and reduce fracture rate in postmenopausal women with low BMD. The primary objective of this study was to determine the duration of bone protective effects of a single dose of ZOL in ovariectomized rats, an animal model of postmenopausal osteopenia. Secondary objectives were to determine the effects on bone turnover and mechanical properties.

MATERIALS AND METHODS

Female Wistar rats (10 per group) received single intravenous doses of ZOL 0.8, 4, 20, 100, or 500 microg/kg, alendronate 200 microg/kg, or isotonic saline 4 days before bilateral ovariectomy. Sham-operated controls were pretreated with saline. Mass and density of cancellous and cortical bone (pQCT) were measured at 4-wk intervals for 32 wk. Bone architecture (microCT), bone formation dynamics (fluorochrome label-based histomorphometry), and biomechanical strength in compression testing were also assessed at 32 wk.

RESULTS

Ovariectomy-associated BMD loss was significantly attenuated for 32 wk by ZOL >or=4 microg/kg for total BMD, ZOL >or=20 microg/kg for cortical BMD, and ZOL >or=4 microg/kg for cancellous BMD (p < 0.01 versus ovariectomized controls). Alendronate 200 microg/kg was of equivalent potency to ZOL 20 microg/kg. Ovariectomy-associated decreases in trabecular architectural parameters were dose-dependently attenuated by ZOL. Alendronate 200 microg/kg was equivalent to ZOL 20 microg/kg. The bone resorption marker TRACP5b indicated transient suppression of elevated osteoclast activity by ZOL relative to OVX-rats even at the lowest dose of 0.8 microg/kg, whereas at 100-500 microg/kg, the effect was significant relative to the OVX control for the entire duration of the study of 32 wk. Bone formation parameters were not significantly affected by ZOL 20 microg/kg but were significantly reduced by ZOL 100-500 microg/kg. Alendronate 200 microg/kg was equivalent to ZOL 100 microg/kg. ZOL produced dose-related improvements in bone strength parameters after ovariectomy. Alendronate 200 microg/kg was of similar potency to ZOL 20 microg/kg.

CONCLUSIONS

The duration and magnitude of the bone-protecting effect of a single intravenous dose of ZOL in ovariectomized rats is dose dependent and lasts for up to 32 wk. Compared with alendronate, ZOL shows 10-fold higher potency in preventing bone loss. These data support the use of an annual intravenous ZOL dosing regimen for the treatment of osteoporosis.

Evidence that abnormal high bone mineralization in growing children with osteogenesis imperfecta is not associated with specific collagen mutations

Osteogenesis imperfecta type I (OI-I) represents the mildest form of OI. The collagen I mutations underlying the disorder can be classified as quantitative mutations that lead to formation of a decreased amount of normal collagen or qualitative mutations where structurally aberrant collagen chains are generated. However, the phenotypic consequences of a particular mutation are not well understood. Transiliac bone biopsies from 19 young OI-I patients (age range 2.0-14.1 years) and 19 age-matched controls were used to assess bone histomorphometric parameters and bone mineralization density distribution, measured by quantitative backscattered electron imaging. Thirteen of the OI-I patients were affected by quantitative and six patients by qualitative mutations. Compared to age-matched controls, iliac bone samples in the OI group were smaller and had thinner cortices and less trabecular bone. Resorption parameters were similar between groups, whereas surface-based parameters of bone formation were considerably higher in OI patients than in controls with the exception of bone formation rate per osteoblast surface, which was reduced in OI. Backscattered electron imaging revealed a higher mean mineralization density (+7%, P < 0.001) in OI-I patients than in age-matched controls, which was accompanied by a reduced heterogeneity of mineralization (-13%, P < 0.001). However, the increase of mean degree of mineralization in OI did not exceed the average level of normal adult bone. No differences were found between the two mutation types. In summary, the tissue- and material-level abnormalities found in OI-I (low bone mass and increased mineral content of the matrix) seem to be independent of the collagen mutations.

Bone mineralization density distribution in health and disease

Human cortical and trabecular bones are formed by individual osteons and bone packets, respectively, which are produced at different time points during the (re)modeling cycle by the coupled activity of bone cells. This leads to a heterogeneously mineralized bone material with a characteristic bone mineralization density distribution (BMDD) reflecting bone turnover, mineralization kinetics and average bone matrix age. In contrast to BMD, which is an estimate of the total amount of mineral in a scanned area of whole bone, BMDD describes the local mineral content of the bone matrix throughout the sample. Moreover, the mineral content of the bone matrix is playing a pivotal role in tuning its stiffness, strength and toughness. BMDD of healthy individuals shows a remarkably small biological variance suggesting the existence of an evolutionary optimum with respect to its biomechanical performance. Hence, any deviations from normal BMDD due to either disease and/or treatment might be of significant biological and clinical relevance. The development of appropriate methods to sensitively measure the BMDD in bone biopsies led to numerous applications of BMDD in the evaluation of diagnosis and treatment of bone diseases, while advancing the understanding of the bone material, concomitantly. For example, transiliacal bone biopsies of postmenopausal osteoporotic women were found to have mostly lower mineralization densities than normal, which were partly associated by an increase of bone turnover, but also caused by calcium and Vit-D deficiency. Antiresorptive therapy causes an increase of degree and homogeneity of mineralization within three years of treatment, while normal mineralization levels are not exceeded. In contrast, anabolic therapy like PTH decreases the degree and homogeneity of matrix mineralization, at least transiently. Osteogenesis imperfecta is generally associated with increased matrix mineralization contributing to the brittleness of bone in this disease, though bone turnover is usually increased suggesting an alteration in the mineralization kinetics. Furthermore, BMDD measurements combined with other scanning techniques like nanoindentation, Fourier transform infrared spectroscopy and small angle X-ray scattering can provide important insights into the structure-function relation of the bone matrix, and ultimately a better prediction of fracture risk in diseases, and after treatment.

Changes in vertebral strength-density and energy absorption-density relationships following bisphosphonate treatment in beagle dogs

We aimed to determine the effects of bisphosphonates on mechanical properties independent of changes in bone density. Our results show that at equivalent bone densities, vertebrae from beagles treated with bisphosphonate have equivalent bone strength and reduced bone energy absorption compared to those from untreated animals.

INTRODUCTION

Assessing the relationship between mechanical properties and bone density allows a biomechanical evaluation of bone quality, with differences at a given density indicative of altered quality. The purpose of this study was to evaluate the strength-density and energy absorption-density relationships in vertebral bone following a one-year treatment with clinical doses of two different bisphosphonates in beagle dogs.

METHODS

Areal bone mineral density (aBMD) and compressive mechanical properties (ultimate load and energy absorption) were assessed on lumbar vertebrae from skeletally mature beagle dogs treated with vehicle (VEH), alendronate (ALN), or risedronate (RIS). Relationships among properties were assessed using analyses of covariance.

RESULTS

Neither treatment altered the strength-density relationship compared to VEH, suggesting increases in vertebral strength with bisphosphonate-treatment are explained by increased density. The energy absorption-density relationship was altered by ALN, resulting in significantly lower energy absorption capacity at a given aBMD compared to both VEH (-22%) and RIS (-14%).

CONCLUSIONS

These data document that after adjusting for increased aBMD, vertebrae from animals treated with bisphosphonates have similar strength as those from untreated animals. Conversely, when adjusted for increased aBMD, alendronate treatment, but not risedronate treatment, significantly reduces the energy required for vertebral fracture, indicative of an alteration in bone quality.

The degree of bone mineralization is maintained with single intravenous bisphosphonates in aged estrogen-deficient rats and is a strong predictor of bone strength

The treatment of osteoporotic women with bisphosphonates significantly reduces the incidence of bone fractures to a degree greater than can be explained by an increase in bone mineral density. In this study, 18-month Fischer 344 rats were ovariectomized and treated with a single dose of risedronate (intravenous, iv, 500 microg), zoledronic acid (iv, 100 microg) or continuous raloxifene (2 mg/kg, po, 3x/week). High resolution microCT was used to measure lumbar vertebral bone microarchitecture, the degree of bone mineralization (DBM) and the distribution of mineral. Small angle X-ray scattering was used to investigate mineral crystallinity. We found prolonged estrogen deficiency, reduced trabecular bone volume, and increased micro architecture bone compression strength lowered the degree of mineralization. Treatment with resorptive agents (bisphosphonates>raloxifene) prevented the loss of mineralization, trabecular bone volume and bone compression strength. Crystal size was not changed with OVX or with anti-resorptive treatments. In conclusion, in the aged estrogen-deficient rat model, single intravenous doses of two bisphosphonates were effective in maintaining the compressive bone strength for 180 days by reducing bone turnover, and maintaining the DBM to a greater degree than with raloxifene.

The effects of geometric and threshold definitions on cortical bone metrics assessed by in vivo high-resolution peripheral quantitative computed tomography

This study evaluates in vivo methods for calculating cortical thickness (Ct.Th) with respect to sensitivity to tissue-level changes in mineralization and the ability to predict whole-bone mechanical properties. Distal radial and tibial images obtained from normal volunteers using high-resolution peripheral quantitative computed tomography (HR-pQCT) were segmented using three thresholds including the manufacturer default and +/-5% in terms of equivalent mineral density. Ct.Th was determined in two ways: using a direct three-dimensional (3D) method and using an annular method, where cortical bone volume is divided by periosteal surface area. D(comp) (mg HA/cm(3)) was calculated based on the mean density-calibrated linear attenuation values of the cortical compartment. Finite element analysis was performed to evaluate the predictive ability of the annular and direct Ct.Th methods. Using the direct 3D method, a +/-5% change in threshold resulted in a 2% mean difference in Ct.Th for both the radius and tibia. An average difference of 5% was found using the annular method. The change in threshold produced changes in D(comp) ranging 0.50-1.56% for both the tibia and radius. Annular Ct.Th correlated more strongly with whole-bone apparent modulus (R(2)=0.64 vs. R(2)=0.41). Both thickness calculation methods and threshold selection have a direct impact on cortical parameters derived from HR-pQCT images. Indirectly, these results suggest that moderate changes in tissue-level mineralization can affect cortical measures. Furthermore, while the direct 3D Ct.Th method is less sensitive to threshold effects, both methods are moderate predictors of mechanical strength, with the annular method being the stronger correlate.

High-dose bisphosphonate therapy in an urgent case of spontaneous multiple vertebral fractures in a 55 year old woman

An early postmenopausal Caucasian woman aged 55 sustained multiple vertebral fractures after a minor trauma. After exclusion of any kind of secondary osteoporosis, we administered due to clinical severity combined oral and cyclic intravenous bisphosphonate therapy (oral risedronate 35 mg/week, i.v. pamidronate 30 mg quarterly) with adequate calcium and vitamin D supplementation for 28 months. We performed a transiliac bone biopsy at baseline and at month 28. The paired samples were investigated by histomorphometry, by microCT-analysis for 3d structure and by qBEI representing bone mineral density distribution. Mineralisation of the bone matrix was not influenced by supplementation of calcium and vitamin D. Parameters of bone architecture and BMD improved; and a reduction of pain and increased mobility was observed. No further osteoporotic fractures occurred during the time of investigation.

Evidence that treatment with risedronate in women with postmenopausal osteoporosis affects bone mineralization and bone volume

Risedronate is used in osteoporosis treatment. Postmenopausal women enrolled in the Vertebral Efficacy with Risedronate Therapy trial received either risedronate (5 mg/day) or placebo for 3 years. Subjects received calcium and vitamin D supplementation if deficient at baseline. Lumbar spine bone mineral density (BMD) was measured at baseline and at 3 years. Quantitative back-scattered electron imaging (qBEI) was performed on paired iliac crest biopsies (risedronate, n = 18; placebo, n = 13) before and after treatment, and the mineral volume fraction in the trabecular bone was calculated. Combining dual-energy X-ray absorptiometric values with the mineral volume fraction for the same patients allowed us to calculate the relative change in trabecular bone volume with treatment. This showed that the effect on BMD was likely to be due partly to changes in matrix mineralization and partly due to changes in bone volume. After treatment, trabecular bone volume in the lumbar spine tended to increase in the risedronate group (+2.4%, nonsignificant) but there was a significant decrease (-3.7%, P < 0.05) in the placebo group. Calcium supplementation with adequate levels of vitamin D led to an approximately 3.3% increase in mineral content in the bone material independently of risedronate treatment. This increase was larger in patients with lower matrix mineralization at baseline and likely resulted from correction of calcium/vitamin D deficiency as well as from reduced bone remodeling. Combining BMD and bone mineralization density distribution data show that in postmenopausal osteoporosis 3-year treatment with risedronate preserves or may increase trabecular bone volume, unlike placebo. This analysis also allows, for the first time, separation of the contributions of bone volume and matrix mineralization to the increase in BMD.

New observations on bone quality in mild primary hyperparathyroidism as determined by quantitative backscattered electron imaging

Bone mineralization density distribution, an important aspect of bone material quality, was determined in mild primary hyperparathyroidism using quantitative backscattered electron imaging. A strong correlation between bone turnover status and degree and heterogeneity of mineralization was found. Further studies are needed before we can draw conclusions about fracture risk in this disorder.

INTRODUCTION

Mild primary hyperparathyroidism (PHPT) is best characterized by asymptomatic hypercalcemia, most commonly in the absence of classical signs and symptoms. Hence, there is need to characterize this disorder with particular attention to the skeleton.

MATERIALS AND METHODS

We analyzed bone mineralization density distribution (BMDD) in iliac crest bone biopsies from patients with PHPT in 51 subjects (16 men, 28-68 years of age; 35 women, 26-74 years of age) by quantitative backscattered electron imaging (qBEI). The BMDD variables quantified are as follows: Ca(MEAN), the weighted mean calcium concentration; Ca(PEAK), the most frequent Ca concentration; Ca(WIDTH), the width of the distribution, a measure of the mineralization homogeneity; Ca(LOW), the percentage of bone area that is mineralized below the fifth percentile in the reference range. The results were compared with a reference range that we have previously established.

RESULTS

The greatest differences were found in Ca(WIDTH) (+15.7%, p<0.0001) and Ca(LOW) (+44.7%, p<0001), both of which were significantly higher in PHPT than control. Ca(MEAN) was significantly lower (-2.5%, p<0.0001) in PHPT compared with controls. These differences were reversed in seven patients who underwent parathyroidectomy. Ca(MEAN) and Ca(PEAK) variables were negatively, whereas Ca(WIDTH) and Ca(LOW) were positively, correlated with dynamic variables of bone formation: mineralizing surface and bone formation rate as determined by histomorphometry. (r = +/-0.3-0.8; p=0.05-0.0001). These results, which represent the first BMDD measurements in mild PHPT using qBEI, show a reduction in the average mineralization density and an increase in the heterogeneity of the degree of mineralization. These changes correlate significantly with the bone turnover rate.

CONCLUSIONS

The results are consistent with our previous observations of increased bone turnover in this disease, and consequently, reduced mean age of bone tissue. Reduced mineralization density in patients with PHPT would be expected to reduce the stiffness of bone tissue. These observations are relevant to considerations of fracture risk in PHPT.

The bone mineralization density distribution as a fingerprint of the mineralization process

The inhomogeneous mineral content and its topographical distribution on a microscopic scale are major determinants of the mechanical quality of trabecular bone. The kinetics of bone tissue deposition and resorption together with the kinetics of the mineralization process determine the distribution of mineral in the tissue. The heterogeneity of the mineral content is described by the well-established bone mineralization density distribution (BMDD), which is experimentally accessible, e.g., using quantitative electron backscattering imaging (qBEI). In the present work, we demonstrate that the shape of the BMDD histogram of trabecular bone reflects directly the mineralization kinetics. Based on the experimental BMDD data of trabecular bone from healthy human adults and using a mathematical model for the remodeling and the mineralization process, the following main results were obtained. The peaked BMDD reflects necessarily a two-phase mineralization process with a fast primary phase and a slow secondary phase where the corresponding time constants differ three orders of magnitude. The obtained mineralization law, which describes the increase in the mineral content in a bone packet as a function of time, provides information not only about the initial mineralization surge, but also about the slow increase afterwards on the time scale of years. In addition to the mineralization kinetics the turnover rate of the remodeling process has a strong influence on the peak position and the shape of the BMDD. The described theoretical framework opens new possibilities for an analysis of experimentally measured BMDDs with respect to changes caused by diseases or treatments. It allows addressing whether changes in the BMDD have to be attributed to a variation in the turnover rate which consequently affects the density distribution or to a primary disorder in the mineralization process most likely reflecting alterations of the organic matrix. This is of important clinical interest because it helps to find therapeutic approaches directly targeting the primary etiological defects to correct the patients' BMDD towards normal BMDD.