Femoral neck cortical bone in female and male hip fracture cases: Differential contrasts in cortical width and sub-periosteal porosity in 112 cases and controls

OBJECTIVES

To quantitate differences between cases of hip fracture and controls in cortical width around the mid-femoral neck in men and women.

METHODS

Over 5 years, 64 (14 male) participants over age 55 (mean 79) years, who had never taken bone-active drugs and suffered intra-capsular hip fracture treated by arthroplasty, donated their routinely discarded distal intra-capsular femoral neck bone for histomorphometry. After embedding, complete femoral neck cross sections from the cut surface near the narrowest part of the neck were stained with von Kossa and cortical width was measured radially every 5 degrees of arc. Control material (n = 48, 25 male) was available through consented post mortems prior to the year 2000. Cortical widths were averaged for circumferential octants, each representing 45 degrees of arc. Divergence of individual cortical widths from their means was also examined.

RESULTS

Because sections were required to have a complete cortex, sampling was biased towards cases with sub-capital versus trans-cervical fractures. Compared to sex- and age matched controls, male cases showed larger relative differences in cortical widths than female cases. Unexpectedly, cortical widths in female but not male cases also showed marked over-representation of extremely narrow (<0.1 mm) cortical widths, located mainly posteriorly. The numbers of these very narrow cortical widths observed per subject retrospectively predicted female fracture status in logistic regression independently of mean cortical width values. Together with mean cortical width differences, the numbers of measured cortical widths <0.1 mm (out of 72 measured) raised the sensitivity of predicting fracture status in women from 48 to 80% at 80% specificity. In almost all cases, very narrow cortical widths were identified in regions enclosing a cortical pore roofed on its endosteal surface by thin structural bone defined a priori as trabecular.

CONCLUSIONS

Cortical widths <0.1 mm probably reflect zones where endosteal cortex has been trabecularised through expansion of an un-refilled sub-endosteal canal close to the periosteum. Persistent cortical defects occurring near the periosteal surface, where mechanical loading exerts its greatest stresses, are likely to result in extremes of localized concentrations of stress during a fall, unknown in young normal fallers. Such defects have the potential to help explain the excess of hip fractures among elderly women. Prevention of sub-periosteal tunnelling by osteoclasts might explain in part the additional benefits, beyond an increase in bone density, of treatments that reduce excessive bone resorption or else stimulate new bone formation on previously resorbed surfaces.

Early Experience with a Highly Sensitive Bioassay for A.C.T.H

A new bioassay for A.C.T.H. has been devised utilising non-proliferative organ culture of segments of guinea pig adrenal. The cultured adrenal is exposed to standard or unknown A.C.T.H. and sections stained for the presence of reducing groups. The intensity of staining in the zona reticularis is measured by scanning and integrating microdensitometry and is inversely proportional to the logarithm of the concentration of A.C.T.H. in the culture medium over the range 0.0027–27 pg/ml.

Preliminary results using the assay, which greatly exceeds others in sensitivity, on human plasma are similar to those obtained with standard techniques.

The fragile elderly hip: mechanisms associated with age-related loss of strength and toughness

Every hip fracture begins with a microscopic crack, which enlarges explosively over microseconds. Most hip fractures in the elderly occur on falling from standing height, usually sideways or backwards. The typically moderate level of trauma very rarely causes fracture in younger people. Here, this paradox is traced to the decline of multiple protective mechanisms at many length scales from nanometres to that of the whole femur. With normal ageing, the femoral neck asymmetrically and progressively loses bone tissue precisely where the cortex is already thinnest and is also compressed in a sideways fall. At the microscopic scale of the basic remodelling unit (BMU) that renews bone tissue, increased numbers of actively remodelling BMUs associated with the reduced mechanical loading in a typically inactive old age augments the numbers of mechanical flaws in the structure potentially capable of initiating cracking. Menopause and over-deep osteoclastic resorption are associated with incomplete BMU refilling leading to excessive porosity, cortical thinning and disconnection of trabeculae. In the femoral cortex, replacement of damaged bone or bone containing dead osteocytes is inefficient, impeding the homeostatic mechanisms that match strength to habitual mechanical usage. In consequence the participation of healthy osteocytes in crack-impeding mechanisms is impaired. Observational studies demonstrate that protective crack deflection in the elderly is reduced. At the most microscopic levels attention now centres on the role of tissue ageing, which may alter the relationship between mineral and matrix that optimises the inhibition of crack progression and on the role of osteocyte ageing and death that impedes tissue maintenance and repair. This review examines recent developments in the understanding of why the elderly hip becomes fragile. This growing understanding is suggesting novel testable approaches for reducing risk of hip fracture that might translate into control of the growing worldwide impact of hip fractures on our ageing populations.

Fourier transform infrared imaging of femoral neck bone: reduced heterogeneity of mineral-to-matrix and carbonate-to-phosphate and more variable crystallinity in treatment-naive fracture cases compared with fracture-free controls

After the age of 60 years, hip fracture risk strongly increases, but only a fifth of this increase is attributable to reduced bone mineral density (BMD, measured clinically). Changes in bone quality, specifically bone composition as measured by Fourier transform infrared spectroscopic imaging (FTIRI), also contribute to fracture risk. Here, FTIRI was applied to study the femoral neck and provide spatially derived information on its mineral and matrix properties in age-matched fractured and nonfractured bones. Whole femoral neck cross sections, divided into quadrants along the neck's axis, from 10 women with hip fracture and 10 cadaveric controls were studied using FTIRI and micro-computed tomography. Although 3-dimensional micro-CT bone mineral densities were similar, the mineral-to-matrix ratio was reduced in the cases of hip fracture, confirming previous reports. New findings were that the FTIRI microscopic variation (heterogeneity) of the mineral-to-matrix ratio was substantially reduced in the fracture group as was the heterogeneity of the carbonate-to-phosphate ratio. Conversely, the heterogeneity of crystallinity was increased. Increased variation of crystallinity was statistically associated with reduced variation of the carbonate-to-phosphate ratio. Anatomical variation in these properties between the different femoral neck quadrants was reduced in the fracture group compared with controls. Although our treatment-naive patients had reduced rather than increased bending resistance, these changes in heterogeneity associated with hip fracture are in another way comparable to the effects of experimental bisphosphonate therapy, which decreases heterogeneity and other indicators of bone's toughness as a material.

Extracellular matrix mineralization promotes E11/gp38 glycoprotein expression and drives osteocytic differentiation

Osteocytes are terminally differentiated osteoblasts which reside in a mineralized extracellular matrix (ECM). The factors that regulate this differentiation process are unknown. We have investigated whether ECM mineralization could promote osteocyte formation. To do this we have utilised MLO-A5 pre-osteocyte-like cells and western blotting and comparative RT-PCR to examine whether the expression of osteocyte-selective markers is elevated concurrently with the onset of ECM mineralization. Secondly, if mineralization of the ECM is indeed a driver of osteocyte formation, we reasoned that impairment of ECM mineralization would result in a reversible inhibition of osteocyte formation. Supplementation of MLO-A5 cell cultures with ascorbic acid and phosphate promoted progressive ECM mineralization as well as temporally associated increases in expression of the osteocyte-selective markers, E11/gp38 glycoprotein and sclerostin. Consistent with a primary role for ECM mineralization in osteocyte formation, we also found that inhibition of ECM mineralization, by omitting phosphate or adding sodium pyrophosphate, a recognized inhibitor of hydroxyapatite formation, resulted in a 15-fold decrease in mineral deposition that was closely accompanied by lower expression of E11 and other osteocyte markers such as Dmp1, Cd44 and Sost whilst expression of osteoblast markers Ocn and Col1a increased. To rule out the possibility that such restriction of ECM mineralization may produce an irreversible modification in osteoblast behaviour to limit E11 expression and osteocytogenesis, we also measured the capacity of MLO-A5 cells to re-enter the osteocyte differentiation programme. We found that the mineralisation process was re-initiated and closely allied to increased expression of E11 protein after re-administration of phosphate or omission of sodium pyrophosphate, indicating an ECM mineralization-induced restoration in osteocyte formation. These results emphasise the importance of cell-ECM interactions in regulating osteoblast behaviour and, more importantly, suggest that ECM mineralization exerts pivotal control during terminal osteoblast differentiation and acquisition of the osteocyte phenotype.

Osteocyte recruitment declines as the osteon fills in: interacting effects of osteocytic sclerostin and previous hip fracture on the size of cortical canals in the femoral neck

There is little information on the distribution of osteocytes within the individual cortical osteon, but using direct 3-D imaging in a single subject, Hannah et al. found a gradient with a two-fold higher density of cells adjacent to the cement line compared to near the canal. Since a limiting factor for bone formation might be the availability of osteoblasts due to their recruitment as osteocytes, we studied distributions of osteonal osteocytes in frozen sections of the femoral neck cortex. Osteocytes were stained with an anti-sclerostin antibody and counter-stained with toluidine blue. Adjacent sections were stained for alkaline phosphatase (ALP). Each osteonal osteocyte was categorised as being sclerostin-positive (scl+) or negative (scl-). ImageJ was used to measure the perimeter and area of each osteon and canal, while special purpose routines were used to measure the minimum distances of each osteocyte from the cement line and the canal. Canal area was strongly correlated with osteon area. Osteocytes were most dense close to the cement line; and their areal density within the matrix declined up to three-fold between the cement line and the canal, depending on osteon diameter. Large and small osteons had similar densities of osteocytes close to the cement line, but fractured neck of femur cases had significantly lower densities of osteocytes close to the canal. Higher osteocyte density close to the canal was associated with ALP expression. It is concluded that entombment of osteocytes newly drawn from the osteoblast pool into the mineralising matrix is independent of preceding bone resorption depth. As osteonal infilling proceeds, osteocyte formation declines more rapidly than matrix formation, leading to a progressive reduction in osteocyte density. A shrinking supply of precursor osteoblasts due to previous osteocyte recruitment, apoptosis, or both could produce this effect. In a statistically significant contrast, sclerostin negative osteocytes adjacent to the canal had the expected effect of reducing canal size in controls but this was not seen in hip fracture. This demonstrated the failure of osteonal osteoblasts to sustain bone formation through a complete remodelling cycle in osteoporosis, perhaps due to insufficient osteoblasts remaining capable of mineralized matrix formation. The failure of osteocytic sclerostin suppression to associate with bone formation in these osteons might alternatively be explained by downstream interference with sclerostin's effect on wnt signalling.

Patients with sclerosteosis and disease carriers: human models of the effect of sclerostin on bone turnover

Sclerosteosis is a rare bone sclerosing dysplasia, caused by loss-of-function mutations in the SOST gene, encoding sclerostin, a negative regulator of bone formation. The purpose of this study was to determine how the lack of sclerostin affects bone turnover in patients with sclerosteosis and to assess whether sclerostin synthesis is decreased in carriers of the SOST mutation and, if so, to what extent this would affect their phenotype and bone formation. We measured sclerostin, procollagen type 1 amino-terminal propeptide (P1NP), and cross-linked C-telopeptide (CTX) in serum of 19 patients with sclerosteosis, 26 heterozygous carriers of the C69T SOST mutation, and 77 healthy controls. Chips of compact bone discarded during routine surgery were also examined from 6 patients and 4 controls. Sclerostin was undetectable in serum of patients but was measurable in all carriers (mean 15.5 pg/mL; 95% confidence interval [CI] 13.7 to 17.2 pg/mL), in whom it was significantly lower than in healthy controls (mean 40.0 pg/mL; 95% CI 36.9 to 42.7 pg/mL; p < 0.001). P1NP levels were highest in patients (mean 153.7 ng/mL; 95% CI 100.5 to 206.9 ng/mL; p = 0.01 versus carriers, p = 0.002 versus controls), but carriers also had significantly higher P1NP levels (mean 58.3 ng/mL; 95% CI 47.0 to 69.6 ng/mL) than controls (mean 37.8 ng/mL; 95% CI 34.9 to 42.0 ng/mL; p = 0.006). In patients and carriers, P1NP levels declined with age, reaching a plateau after the age of 20 years. Serum sclerostin and P1NP were negatively correlated in carriers and age- and gender-matched controls (r = 0.40, p = 0.008). Mean CTX levels were well within the normal range and did not differ between patients and disease carriers after adjusting for age (p = 0.22). Our results provide in vivo evidence of increased bone formation caused by the absence or decreased synthesis of sclerostin in humans. They also suggest that inhibition of sclerostin can be titrated because the decreased sclerostin levels in disease carriers did not lead to any of the symptoms or complications of the disease but had a positive effect on bone mass. Further studies are needed to clarify the role of sclerostin on bone resorption.

Sclerostin and the regulation of bone formation: Effects in hip osteoarthritis and femoral neck fracture

Remodeling imbalance in the elderly femoral neck can result in thin cortices and porosity predisposing to hip fracture. Hip osteoarthritis protects against intracapsular hip fracture. By secreting sclerostin, osteocytes may inhibit Wnt signaling and reduce bone formation by osteoblasts. We hypothesised that differences in osteocytic sclerostin expression might account for differences in osteonal bone-formation activity between controls and subjects with hip fracture or hip osteoarthritis. Using specific antibody staining, we determined the osteocytic expression of sclerostin within osteons of the femoral neck cortex in bone removed from subjects undergoing surgery for hip osteoarthritis (hOA: 5 males, 5 females, 49 to 92 years of age) or hip fracture fixation (FNF: 5 males, 5 females, 73 to 87 years of age) and controls (C: 5 males, 6 females, 61 to 90 years of age). Sclerostin expression and distances of each osteocyte to the canal surface and cement line were assessed for all osteonal osteocytes in 636 unremodeled osteons chosen from fields ( approximately 0.5 mm in diameter) with at least one canal staining for alkaline phosphatase (ALP), a marker of bone formation. In adjacent sections, ALP staining was used to classify basic multicellular unit (BMUs) as quiescent or actively forming bone (ALP(+)). The areal densities of scl(-) and scl(+) osteocytes (number of cells per unit area) in the BMU were inversely correlated and were strong determinants of ALP status in the BMU. In controls and hip fracture patients only, sclerostin-negative osteocytes were closer to osteonal surfaces than positively stained cells. Osteon maturity (progress to closure) was strongly associated with the proportion of osteonal osteocytes expressing sclerostin, and sclerostin expression was the chief determinant of ALP status. hOA patients had 18% fewer osteocytes per unit bone area than controls, fewer osteocytes expressed sclerostin on average than in controls, but wide variation was seen between subjects. Thus, in most hOA patients, there was increased osteonal ALP staining and reduced sclerostin staining of osteocytes. In FNF patients, newly forming osteons were similar in this respect to hOA osteons, but with closure, there was a much sharper reduction in ALP staining that was only partly accounted for by the increased proportions of osteonal osteocytes staining positive for sclerostin. There was no evidence for a greater effect on ALP expression by osteocytes near the osteonal canal. In line with data from blocking antibody experiments, osteonal sclerostin appears to be a strong determinant of whether osteoblasts actively produce bone. In hOA, reduced sclerostin expression likely mediates increased osteoblastic activity in the intracapsular cortex. In FNF, full osteonal closure is postponed, with increased porosity, in part because the proportion of osteocytes expressing sclerostin increases sharply with osteonal maturation.

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.

Genetic selection for fast growth generates bone architecture characterised by enhanced periosteal expansion and limited consolidation of the cortices but a diminution in the early responses to mechanical loading

Bone strength is, in part, dependent on a mechanical input that regulates the (re)modelling of skeletal elements to an appropriate size and architecture to resist fracture during habitual use. The rate of longitudinal bone growth in juveniles can also affect fracture incidence in adulthood, suggesting an influence of growth rate on later bone quality. We have compared the effects of fast and slow growth on bone strength and architecture in the tibiotarsi of embryonic and juvenile birds. The loading-related biochemical responses (intracellular G6PD activity and NO release) to mechanical load were also determined. Further, we have analysed the proliferation and differentiation characteristics of primary tibiotarsal osteoblasts from fast and slow-growing strains. We found that bones from chicks with divergent growth rates display equal resistance to applied loads, but weight-correction revealed that the bones from juvenile fast growth birds are weaker, with reduced stiffness and lower resistance to fracture. Primary osteoblasts from slow-growing juvenile birds proliferated more rapidly and had lower alkaline phosphatase activity. Bones from fast-growing embryonic chicks display rapid radial expansion and incomplete osteonal infilling but, importantly, lack mechanical responsiveness. These findings are further evidence that the ability to respond to mechanical inputs is crucial to adapt skeletal architecture to generate a functionally appropriate bone structure and that fast embryonic and juvenile growth rates may predispose bone to particular architectures with increased fragility in the adult.

Bone structure and remodelling in stroke patients: early effects of zoledronate

INTRODUCTION

We have reported that after an acute stroke, intravenous zoledronate prevented bone loss in the hemiplegic hip. Participants from the trial also volunteered for trans-iliac bone biopsy, to assess the early effects of stroke and zoledronate on iliac bone remodelling.

METHODS

Patients with acute stroke were randomly assigned to a single intravenous dose of zoledronate 4 mg or placebo within 5 weeks of stroke. Biopsies from 14 patients (3 female, 11 male, mean age 71+/-11) were suitable for analysis. These were taken at mean 10 weeks (+/-2) post-stroke, and included 5 patients who had received zoledronate. Histomorphometry was performed on undecalcified sections using light and fluorescence microscopy. Static and dynamic indices of remodelling were compared to a local reference range from healthy controls. Osteoclasts and their precursors were identified on frozen sections using tartrate resistant acid phosphatase (TRAP) staining. Dual-energy x-ray absorptiometry (DXA) of the proximal femora was performed at baseline and 6 months later.

RESULTS

The eroded surface in cancellous bone (ES/BS) was significantly higher in stroke patients than controls (5.7% vs. ref 1.6%, p<0.0001). Although ES/BS did not differ between zoledronate and placebo-treated groups, there were significantly fewer osteoclasts and their precursors in zoledronate-treated individuals (p=0.023). Bone formation indices (osteoid surface, OS/BS and mineralising surface, MS/BS) were significantly lower in stroke patients than controls and although OS/BS was higher in the zoledronate group than the placebo group (p=0.033), MS/BS was not different (p=0.924). There were no differences between hemiplegic and unaffected sides for any histomorphometric parameter despite asymmetric reductions in hip bone mineral density (p=0.013).

CONCLUSION

Stroke patients had higher resorption indices and lower bone forming surfaces than controls, consistent with uncoupling of bone remodelling. These findings are preliminary and a larger study is required to evaluate the contributions of gender, age and hemiplegic status to the remodelling imbalance. Zoledronate therapy was associated with a reduction in osteoclastic cell numbers consistent with its known mode of action in bone.

Increased longitudinal growth in rats on a silicon-depleted diet

Silicon-deficiency studies in growing animals in the early 1970s reported stunted growth and profound defects in bone and other connective tissues. However, more recent attempts to replicate these findings have found mild alterations in bone metabolism without any adverse health effects. Thus the biological role of silicon remains unknown. Using a specifically formulated silicon-depleted diet and modern methods for silicon analysis and assessment of skeletal development, we undertook, through international collaboration between silicon researchers, an extensive study of long-term silicon depletion on skeletal development in an animal. 21-day old female Sprague-Dawley rats (n=20) were fed a silicon-depleted diet (3.2 microg Si/g feed) for 26 weeks and their growth and skeletal development were compared with identical rats (n=10) on the same diet but with silicon added as Si(OH)(4) to their drinking water (53.2 microg Si/g water); total silicon intakes were 24 times different. A third group of rats, receiving a standard rodent stock feed (322 microg Si/g feed) and tap water (5 microg Si/g water), served as a reference group for optimal growth. A series of anthropometric and bone quality measures were undertaken during and following the study. Fasting serum silicon concentrations and especially urinary silicon excretion were significantly lower in the silicon-deprived group compared to the supplemented group (P=0.03 and 0.004, respectively). Tibia and soft-tissue silicon contents did not differ between the two groups, but tibia silicon levels were significantly lower compared to the reference group (P<0.0001). Outward adverse health effects were not observed in the silicon-deprived group. However, body lengths from week 18 onwards (P<0.05) and bone lengths at necropsy (P Collapse

Key Words

• bone growth
• sprague–dawley rats
• silicon deficiency
• orthosilicic acid supplementation
• bone and soft-tissue silicon concentrations
• tibia calcium and phosphorus

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MESH Headings

• Animal Feed
• Animals
• Body Weight
• Bone Density/drug effects
• Bone Development
• Bone Remodeling
• Bone and Bones/drug effects
• Bone and Bones/metabolism
• Bone and Bones/pathology
• Chondrocytes/drug effects
• Female
• Rats
• Rats, Sprague-Dawley
• Silicon/deficiency
• Silicon/metabolism
• Tetracycline/pharmacology
• Tibia/pathology
• Tomography, X-Ray Computed/methods

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Grants

• Wellcome Trust
• MC_U105960399 Medical Research Council

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A Single Infusion of Zoledronate Prevents Bone Loss After Stroke

BACKGROUND AND PURPOSE

Stroke is a major risk factor for hip fracture. Patients with intermediate rather than severe or mild stroke deficits at the time of hospital discharge have the most fractures. This proof-of-concept study evaluated the efficacy of a single infusion of zoledronate, an intravenous bisphosphonate, in preserving hip bone density after stroke.

METHODS

In a 1-year randomized, double-blind, placebo-controlled, clinical trial, 27 newly hemiplegic patients (6 females, 21 males) with acute stroke were assigned to receive 4 mg of the intravenous zoledronate (n=14) or placebo (n=13) within 35 days. Strict inclusion criteria were followed-up to ensure recruited patients were likely to have residual functional impairment. Both groups received calcium and vitamin D supplementation. The primary outcome measure was the change in bone mineral density (BMD; Lunar Prodigy) at the hemiplegic hip during the year of investigation.

RESULTS

The treatment was generally well tolerated. Mean total hip BMD was unchanged in the hemiplegic hip of the zoledronate group (mean 0.0% change), whereas in the placebo group the total hip BMD changed by -5.5%, with the greatest bone loss observed in the trochanteric subregion (mean, -8.1%). On the unaffected side the mean change in total hip BMD was +1.0% with zoledronate versus a mean change of -2.7% without. Repeated measures ANOVA confirmed the significance of the differences between groups at both hips (hemiplegic, P<0.001; unaffected, P=0.002).

CONCLUSIONS

Stroke patients were protected from the deleterious effects of hemiplegia on hip bone density for at least 1 year after a single infusion of zoledronate.

Effects of physical activity on evolution of proximal femur structure in a younger elderly population

INTRODUCTION

For a fixed weight, a wider bone of standardised length is stiffer. But moving the cortices away from the centre of mass risks creating structural (elastic) instability, and hip fractures have been postulated to occur as a consequence of buckling of the thinned supero-lateral femoral neck cortex during a fall. We hypothesised that stereotyped physical activity (e.g., walking) may help conserve bending resistance (section modulus, Z) through redistribution of bone tissue, but it might be at the expense of supero-lateral cortical stability.

METHODS

Hip structural analysis (HSA) software applied to DXA scans was used to derive measurements of section modulus and distances of a cross-section's centre of mass from the supero-lateral cortical margin (lateral distance, in cm). DXA scans were obtained on 1361 men and women in the EPIC-Norfolk population-based prospective cohort study. Up to 4 repeat DXA scans were done in 8 years of follow-up. Weight, height and activities of daily living were assessed on each occasion. A detailed physical activity and lifestyle questionnaire was administered at baseline. The lateral distance was measured on three narrow cross-sections with good precision: narrow neck (NN, coefficient of variation 2.6%), intertrochanter (IT) and shaft (S). A linear mixed model was used to assess associations with predictors.

RESULTS

Ageing was associated with medial shifting of the centre of mass, so that lateral distance increased. Both greater weight and height were associated with greater lateral distance (P<0.0001). Among physical activity-related variables, walking/cycling for >1 h/day (P=0.025), weekly time spent on moderate impact activity (P=0.003), forced expiratory volume in 1 s (NN and IT, P<0.026) and lifetime physical activity (IT, P<0.0001) were associated with higher lateral distance. However, after adjusting for these variables, activities of daily living scores (NN, P<0.0001) and weekly time spent on low impact hip flexing activities were associated with shorter lateral distance (P=0.001). Greater baseline lateral distance was significantly associated with increased risk of subsequent hip fracture (n=26) in females (P<0.05, all regions) independently of age, height and bone mineral content.

CONCLUSION

The age-related shift medially of the centre of mass of the femoral neck and trochanter may have adverse effects on fracture resistance in the event of a fall, so compromising the beneficial effects of walking on fitness, strength and risk of falling. The role of more diverse physical activity patterns in old age that impose loading on the supero-lateral cortex of the femur, involving for example hip flexion and stretching, needs investigation for their ability to correct this medial shifting of the centre of mass.

Sex hormone status may modulate rate of expansion of proximal femur diameter in older women alongside other skeletal regulators

CONTEXT

Little is known of associations between hip geometry and skeletal regulators. This is important because geometry is a determinant of both hip function and resistance to fracture.

OBJECTIVE

We aimed to determine the effects of sex hormone status and other candidate regulators on hip geometry and strength.

SUBJECTS AND METHODS

A random sample of 351 women aged 67-79 had two to four hip dual-energy x-ray absorptiometry scans performed over 8 yr of follow-up. Hip structural analysis software was used to measure subperiosteal diameter (PD) and the distance from the center of mass to the lateral cortical margin (d-lat) on three 5-mm-thick cross-sectional regions: narrow neck, intertrochanter, and shaft. Section modulus (Z), bone mineral density (grams per centimeter squared), and an index of bone mineral content (cross-sectional area) were calculated as estimators of bone strength. Serum analytes measured at baseline included SHBG, estradiol, PTH, creatinine, albumin, vitamin D metabolites, and glutamate- and gamma-carboxyglutamate-osteocalcin (OC). A linear mixed model was used to model associations with predictor variables, including testing whether the predictors significantly modified the effect of aging.

RESULTS

Aging was associated with increasing PD and d-lat, and higher baseline SHBG significantly modified this effect, in the case of PD, increasing the rates of change at the narrow neck region by 19% for SHBG level 2 sd higher than population mean (P = 0.026). Higher baseline creatinine was independently associated with faster increases in PD and d-lat with aging (P < 0.041). Z declined faster with aging if baseline PTH was higher, and higher albumin had a contrary effect. Z was positively associated with free estradiol and inversely associated with SHBG and glutamate-OC.

CONCLUSION

These results show large effects of SHBG on the regulation of proximal femur expansion and bending resistance, probably acting as a surrogate for low bioavailable estrogen. Potentially important effects for fracture resistance in old age were also revealed for PTH, markers related to renal function and the nutritional markers albumin and undercarboxylated OC.

Infant growth influences proximal femoral geometry in adulthood

The relationship between early growth and adult femoral geometry has not been studied previously. In 333 adults, we were able to show that infant weight predicts femoral width and cross-sectional moment of inertia but not femoral neck length. These results support the hypothesis that growth in early life leads to persisting differences in proximal femoral geometry.

INTRODUCTION

Both femoral geometry and bone mass have been shown independently to predict both hip strength and fracture risk. Whereas growth during intrauterine and early postnatal life has been shown to influence adult bone mass, the relationship between growth in early life and adult femoral geometry has not been described previously.

MATERIALS AND METHODS

We studied the relationship between growth during early life, adult hip geometry, and proximal femur bone mass in a sample of 333 men and women (60-75 years of age), for whom birth weight and weight at 1 year of age were recorded. Hip geometry was derived using Hip Structure Analysis software from proximal femur DXA scans (Hologic QDR 1000).

RESULTS

There were significant (p < 0.002) relationships between weight at age 1 year and measures of femoral width as well as intertrochanteric (IT) cross-sectional moment of inertia (CSMI), but not with femoral neck length. The relationships with measures of femoral width but not CSMI remained after adjusting for adult body weight and were independent of proximal femoral BMC.

CONCLUSIONS

These results support the hypothesis that different patterns of growth in utero and during the first year of life lead to persisting differences in proximal femoral geometry, thereby mediating in part the effects of early growth on risk of hip fracture in adulthood.

Reduced vitamin D in acute stroke

BACKGROUND AND PURPOSE

Stroke leads to a reduction in bone mineral density, altered calcium homeostasis, and an increase in hip fractures. Vitamin D deficiency is well documented in long-term stroke survivors and is associated with post-stroke hip fractures. Less is known regarding levels in acute stroke.

METHODS

We compared the serum 25-dihydroxyvitamin D levels of 44 patients admitted to an acute stroke unit with first-ever stroke with results obtained by measuring 96 healthy ambulant elderly subjects every 2 months for 1 year. Statistical Z scores of serum vitamin D were then calculated after seasonal adjustment for the month of sampling.

RESULTS

The mean Z score of vitamin D in acute stroke was -1.4 SD units (95% CI, -1.7, -1.1), with 77% of patients falling in the insufficient range.

CONCLUSIONS

Reduced vitamin D was identified in the majority of patients with acute stroke throughout the year and may have preceded stroke. Vitamin D is a potential risk marker for stroke, and the role of vitamin D repletion in enhancing musculoskeletal health after stroke needs to be explored.

Osteoclastic cortical erosion as a determinant of subperiosteal osteoblastic bone formation in the femoral neck's response to BMU imbalance. Effects of stance-related loading and hip fracture

Femoral neck fractures have previously been shown to be associated with increased cortical and endocortical remodeling, reduced wall thickness of endocortical packets and cortical porosity. Femoral neck width is associated positively with history of lifetime physical activity; so we hypothesized that exposure to mechanical loading may influence the subperiosteal osteoblastic response to the weakening effect of intracortical bone resorption. In 21 femoral neck biopsies from female subjects (13 with hip fracture), there was a positive association between osteoblastic periosteal alkaline phosphatase expression shown in frozen sections and the percentage of cortical canals internal to the subperiosteal surface showing evidence of osteoclastic erosion (Goldner's stain; p =0.03). This was stronger in the plane of locomotor loading and particularly strong in the inferior (compression) cortex ( p =0.002). In 35 cases and 23 age/gender-matched postmortem controls, osteoid-bearing cortical canals (%) were significantly elevated in the fracture cases compared with the controls within the anterior region. There was also a significant correlation between cortical and endocortical %OS/BS (percentage osteoid surface to bone surface) (fracture, n =12; control, n =12) over the whole biopsy ( p =0.041). Generally, these associations of intracortical with endocortical remodeling were consistent with both envelopes being regulated by common processes. These results support the concept that the slow growth of femoral neck width by subperiosteal apposition of bone occurs directly or, otherwise, in response to the weakening of the cortex as it is "trabecularized" by imbalance of bone multicellular units (BMU). This process, in turn, depends on cortical thinning and enlargement of canals with the formation of giant, composite osteons, the whole being more marked in cases of future hip fracture.

Sclerostin is a delayed secreted product of osteocytes that inhibits bone formation

Osteocytes are the most abundant cells in bone and are ideally located to influence bone turnover through their syncytial relationship with surface bone cells. Osteocyte-derived signals have remained largely enigmatic, but it was recently reported that human osteocytes secrete sclerostin, an inhibitor of bone formation. Absent sclerostin protein results in the high bone mass clinical disorder sclerosteosis. Here we report that within adult iliac bone, newly embedded osteocytes were negative for sclerostin staining but became positive at or after primary mineralization. The majority of mature osteocytes in mineralized cortical and cancellous bone was positive for sclerostin with diffuse staining along dendrites in the osteocyte canaliculi. These findings provide for the first time in vivo evidence to support the concept that osteocytes secrete sclerostin after they become embedded in a mineralized matrix to limit further bone formation by osteoblasts. Sclerostin did not appear to influence the formation of osteocytes. We propose that sclerostin production by osteocytes may regulate the linear extent of formation and the induction or maintenance of a lining cell phenotype on bone surfaces. In doing so, sclerostin may act as a key inhibitory signal governing skeletal microarchitecture.

Relation between age, femoral neck cortical stability, and hip fracture risk

BACKGROUND

Hip fracture risk rises 100 to 1000-fold over 60 years of ageing. Loss of resistance to bending is not a major feature of normal ageing of the femoral neck. Another cause of fragility is local buckling or elastic instability. Bones adapt to their local experience of mechanical loading. The suggestion that bipedalism allows thinning of the underloaded superolateral femoral neck cortex arises from the failure of walking to transmit much mechanical load to this region. We aimed to measure whether elastic instability increases greatly with age since it might trigger hip fracture in a sideways fall.

METHODS

We measured with computed tomography the distribution of bone in the mid-femoral neck of 77 proximal femurs from people who died suddenly aged 20-95 years. We then calculated the critical stress, from the geometric properties and density of the cortical zone most highly loaded in a sideways fall, as a threshold for elastic instability.

FINDINGS

With normal ageing, this thin cortical zone in the upper femoral neck became substantially thinner. Relative to mean values at age 60 years, female cortical thickness declined by 6.4% (SD 1.1) per decade (p<0.0001), and critical stress by 13.2% (4.3) per decade (p=0.004) in the superoposterior octant compressed most in a sideways fall. Similar, but significantly smaller, effects were evident in men (p=0.004). This thinning compromised the capacity of the femur to absorb energy independently of osteoporosis. Patients with hip fracture had further reduced stability.

INTERPRETATION

As women age, hip fragility increases because underloading of the superolateral cortex leads to atrophic thinning. Because walking does not sufficiently load the upper femoral neck, the fragile zones in healthy bones may need strengthening, for example with more well targeted exercise.

Osteocytic expression of constitutive NO synthase isoforms in the femoral neck cortex: a case-control study of intracapsular hip fracture

NO is an osteocytic signaling molecule that can inhibit osteoclasts. The NO synthases eNOS and nNOS were expressed by >50% of osteonal osteocytes in controls. Hip fracture cases showed +NOS osteocytes only in deep osteonal bone, and 25-35% reduced expression overall. These data are consistent with increased osteonal vulnerability to deep osteoclastic attack.

INTRODUCTION

Osteocytes may regulate the response to mechanical stimuli in bone through the production of local signaling molecules such as NO derived from the NO synthase eNOS. Because NO is inhibitory to osteoclastic resorption, it has been suggested that osteocytes expressing eNOS act as sentinels, confining resorption within single osteons. Recently, nNOS has been shown to be present in osteocytes of adult human bone.

MATERIALS AND METHODS

Cross-sections of the femoral neck (eight female cases of intracapsular hip fracture and seven postmortem controls; age, 68-91 years) were analyzed by immunohistochemistry. The percentages of osteocytes expressing each of these two isoforms were calculated, and their distances to the nearest canal surface were measured.

RESULTS

The percentage of +nNOS osteocytes was lower in the fracture cases than in the controls (cases: 43.12 +/- 1.49, controls: 56.68 +/- 1.45; p < 0.0001). Compared with nNOS, eNOS expression was further reduced (p = 0.009) in the cases but was not different in the controls (cases: 36.41 +/- 1.53, controls: 56.47 +/- 2.41; p < 0.0001). The minimum distance of +eNOS or +nNOS osteocytes to a canal surface was higher in the cases compared with controls (eNOS: controls; 44.4 +/- 2.2 microm, cases: 61.7 +/- 2.0 microm; p < 0.0001; nNOS: controls: 52.4 +/- 1.7 microm, cases: 60.2 +/- 2.1 microm; p = 0.0039). +eNOS osteocytes were closer to the canal surfaces than +nNOS osteocytes in the controls by 8.00 +/- 4.0 microm (p = 0.0012).

CONCLUSION

The proportions of osteocytes expressing nNOS and eNOS were both reduced in the fracture cases, suggesting that the capacity to generate NO might be reduced. Furthermore, the reduction in NOS expression occurs in those osteocytes closest to the canal surface, suggesting that the ability of NO to minimize resorption depth might be impaired. Further studies are needed on the regulation of the expression and activity of these distinct NOS isoforms.

Bone mineralization density and femoral neck fragility

The traditional view of osteoporotic fractures is that they result from a reduction in bone mass combined with alterations in the micro-architecture. Apart from the effects of bone remodeling, the material properties of the remaining bone are thought to be unaffected. To test this, we compared the degree of matrix mineralization in femoral neck biopsies taken from cases of intracapsular hip fracture with age- and sex-matched postmortem controls. Whole femoral neck biopsies from seven female hip fracture cases (72-90 years) and nine controls (68-94 years) were embedded in methylmethacrylate, and sections stained with Solochrome Cyanin R for analysis of osteoid. The blocks were then diamond micro-milled, carbon coated, and analyzed for the degree of matrix mineralization using halogenated dimethacrylate standards for quantitative backscattered electron (qBSE) imaging (20 kV, entire block face, sampling interval 5 microm). The BSE gray scale was adjusted such that 0 corresponds to an electron backscattering coefficient of 0.1159 (approximately 1.70 g/ml) and 255-0.1519 (approximately 2.18 g/ml). Remodeling and mineralization data were analyzed for both the whole biopsy face and on a regional (anterior; inferior, posterior, or superior) basis. Over the whole biopsy, the level of mineralization was lower in the cases than the postmortem controls (-2.8%, P < 0.05). In both cases and controls, cortical mineralization was higher in the inferior (compressive) region compared with superior (tensile) region (P < 0.05). Mineralization was lower in all regions of the cases (inferior: -3.3%; posterior: -3.1%; anterior: -2.7%; superior: -1.6%) compared to the controls. Mineralization density in cancellous bone was not regionally dependent but was lower in the fracture cases (-3.5%; P = 0.001). Although there were weak relationships between osteoid formation (%O.Ar/B.Ar) and the mean level of mineralization in both cortical (P = 0.068) and cancellous (P < 0.01) bone, adjustment for this did not markedly affect the case-control differences. In conclusion, this study has shown that in cases of intracapsular hip fracture, matrix mineralization is reduced in the femoral neck. Unexpectedly, in view of the likely role of mild to moderate vitamin D deficiency osteopathy in hip fracture, this decreased mineralization was independent of osteoid indices and therefore potentially independent of bone age. This raises the possibility that alterations in the bone matrix such as excessive glycation or changes in the composition of the collagen fibrils affect its mineralization in hip fracture cases.

NOS isoforms in adult human osteocytes: multiple pathways of NO regulation?

Until now, eNOS has been considered to be the predominant osteocytic nitric oxide synthase (NOS) isoform in bone. We previously studied the distribution of eNOS protein expression in the human femoral neck because of its possible involvement in the response to load. Studies in rat and human fracture callus have shown that nNOS mRNA is expressed sometime after fracture, but no study has yet immunolocalized NOS isoforms in mature adult human bone. In this study, we have examined the distribution of NOS isoforms in iliac osteocytes. Frozen sections (10 microm) were cut from transiliac biopsies from 8 female osteoporotic patients (range, 56-80 years) and from 7 female postmortem femoral neck biopsies (range, 65-90 years). Sections were incubated overnight in antiserum for eNOS, nNOS, or iNOS followed by peroxidase/VIP substrate detection. We used eNOS and iNOS antisera directed against the C-terminus. For nNOS, three different antisera were used, two binding to different C-terminal epitopes and one binding to N-terminal epitope. Sections were then incubated in propidium iodide or methyl green to detect all osteocytes. eNOS antibody was able to detect eNOS epitopes in osteocytes. All three nNOS antibodies detected nNOS epitopes in osteocytes, but those directed against the C-terminus had higher detection rates. iNOS was rarely seen. In the iliac crest, the percentage of osteocytes positive for nNOS was higher than that for eNOS (cortical: nNOS 84.04%, eNOS 61.78%, P < 0.05; cancellous: nNOS 82.33%, eNOS 65.21%, P < 0.05). In the femoral neck, the percentage of osteocytes positive for nNOS (60.98%) was also higher than that for eNOS (40.41%), although this difference was not statistically significant. In conclusion, both eNOS and nNOS isoforms are present in osteocytes in the iliac crest and femoral neck.

Femoral neck fragility: genes or environment?

As with other diseases that show exponentially increasing rates, hip fracture probably requires multiple prior events to become manifest. It is common ground that there are genetic, environmental, lifestyle and perhaps dietary determinants of risk of osteoporotic fracture as well as interactions between them. The key to secondary prevention is the understanding of how these components can be integrated into an effective assessment of the major risks of hip fracture after a previous fracture has occurred. The key to primary prevention is to understand both the pathological and physiological basis of hip fragility. It is not unreasonable to suppose that in a western lifestyle our limited and stereotypic patterns of locomotion from middle age onwards may offer considerably less protection than, for example, the more physically demanding activity of subsistence farming.

Discrimination between cases of hip fracture and controls is improved by hip structural analysis compared to areal bone mineral density. An ex vivo study of the femoral neck

In vivo bone densitometry is affected by measurement inaccuracies arising from the assumptions made about soft tissue and marrow composition. This study tested the hypothesis that section modulus (SM, a measure of bending resistance) when measured ex vivo, would discriminate cases of hip fracture from controls better than areal bone mineral density (aBMD). The biopsies were from (n = 22, female) subjects that had suffered an intracapsular hip fracture. The control material (n = 24, female) was from post-mortem subjects. Serial peripheral quantitative computed tomography (pQCT) 1-mm thick cross-sectional images of femoral neck previously embedded in methacrylate were obtained with the Densiscan 1000 pQCT densitometer and matched for lateral location. The image voxels were converted to units of bone mass, which were then used to derive the section modulus. The data were used to derive means from which receiver operating characteristic (ROC) curves could be generated. The area under the curves (AUC) showed that discrimination between the fracture cases and controls was better for SM than aBMD [SM: AUC = 0.83 (95% confidence interval: 0.71, 0.96), aBMD: AUC = 0.70 (0.54, 0.85); P = 0.034]. To simulate the forces experienced during a sideways fall, the model's neutral axis was rotated by 210 degrees. The results for section modulus were predictable from those at 0 degrees (r(2) = 0.97). We conclude that biomechanical analysis of the distribution of bone within the femoral neck may offer a marked improvement in the ability to discriminate patients with an increased risk of intracapsular fracture. Progress towards implementing this form of analysis in clinical densitometry should improve its diagnostic value, but may depend in part on better image resolution and more accurate corrections for the variability between subjects in regional soft tissue composition.

Ghrelin and des-octanoyl ghrelin promote adipogenesis directly in vivo by a mechanism independent of the type 1a growth hormone secretagogue receptor

Ghrelin promotes fat accumulation, despite potent stimulation of the lipolytic hormone, GH. The function of the major circulating isoform of ghrelin, des-octanoyl ghrelin, is unclear, because it does not activate the GH secretagogue receptor (GHS-R1a) and lacks the endocrine activities of ghrelin. We have now addressed these issues by infusing ghrelin, des-octanoyl ghrelin, or synthetic GHS-R1a agonists into three rat models with moderate, severe, or total GH deficiency. We show that in the context of significant GH secretion, the adipogenic effect of systemic ghrelin infusion is pattern dependent. However, this adipogenic action is not mediated by the pituitary hormones. Using a novel unilateral local infusion strategy, we demonstrate that ghrelin promotes bone marrow adipogenesis in vivo by a direct peripheral action. Surprisingly, this effect was also observed with des-octanoyl ghrelin, whereas a potent synthetic GHS-R1a agonist was ineffective. Thus, these adipogenic effects are mediated by a receptor other than GHS-R1a. This is the first in vivo demonstration of a direct adipogenic effect of des-octanoyl ghrelin, a major circulating form of ghrelin that lacks GH-releasing activity. We suggest that the ratio of ghrelin and des-octanoyl ghrelin production could help regulate the balance between adipogenesis and lipolysis in response to nutritional status.

Hip section modulus, a measure of bending resistance, is more strongly related to reported physical activity than BMD

We hypothesized that measures of physical activity would have a closer relationship with section modulus (SM), an indicator of bending resistance, than with bone mineral density (BMD) because physical activity might expand the bony envelope, which tends to reduce BMD for a constant bone mineral content. Four hundred twenty-three men and 436 women (mean age 72 years, SD =3) were recruited from a prospective population-based cohort study to a study of hip bone loss. Hip BMD was measured on two occasions 2-5 years apart (mean 2.7, DXA-Hologic 1,000 W). Hip structural analysis (HSA) software was used to calculate SM and BMD from the DXA scans on three narrow regions: the narrow neck (NN), intertrochanter (IT) and shaft (S). A physical activity and lifestyle questionnaire was administered at baseline. Multivariate repeated measures analysis of variance was used to model the associations between personal attributes (weight, height, age), physical activity and lifestyle variables with SM, cross-sectional area (CSA), sub-periosteal diameter (PD) and BMD. Men and women were analysed together after tests for interactions with gender, which were found not to be significant. In all regions female gender was associated with having lower values of all outcomes, and body weight was positively associated with all outcomes, i.e., SM, CSA, PD and BMD ( P<0.0001). Sub-periosteal diameter was positively associated with reported lifetime physical activity (IT and S, P<0.0001). There was a significant decline of BMD with age at the NN and S regions ( P<0.026), and the PD increased with age (NN and S, P<0.019). Previous fracture history was associated with having lower values of BMD, SM and CSA (except for S; P<0.022). Both section modulus and CSA were positively associated with heavy physical activity after age 50 years in all regions ( P<0.019), whereas NN BMD was the only BMD associate of heavy physical activity after 50 ( P=0.036). Time spent per week on recreational activities classified as no impact activity was positively associated with BMD, CSA and SM (multivariate P<0.016). In conclusion, proximal femur diameter is associated positively with reported life-long physical activity. If this is mediated through a loading related effect on sub-periosteal expansion, BMD would be an unsatisfactory outcome measure in physical activity studies since it is inversely related to projected bone area. SM in contrast was associated with several measures of recent physical activity and relates more directly to the bending experienced by the proximal femur in response to a given load. These data are consistent with an effect of mechanical loading to regulate bone strength through an anabolic effect maximal in the subperiosteal cortex, where the highest loading-related strains are experienced.

Bone remodeling at the endocortical surface of the human femoral neck: a mechanism for regional cortical thinning in cases of hip fracture

Endocortical remodeling and wall thickness (W.Th.) were measured in femoral neck bone from 12 female fracture cases (81.3 +/- 1.5 years) and 12 sex-matched controls (81.9 +/- 1.9 years). Regionally, osteoid and eroded surface were increased, whereas W.Th. was reduced. These processes likely contribute to cortical bone loss seen in hip fracture.

INTRODUCTION

Because periosteal expression of alkaline phosphatase was similar between cases and controls, we hypothesized that the mechanism causing the marked femoral neck cortical thinning associated with hip fracture may be net endocortical bone loss.

METHODS

Twelve female cases of femoral neck fracture (mean age = 81.3 +/- 1.5 years) and 12 age- and sex-matched postmortem controls (mean age = 81.9 +/- 1.9 years) were included in the study. Samples of their femoral neck bone were embedded in methyl methacrylate, sectioned at 10 microm, and stained with Solochrome cyanine R and Goldner's trichrome for the detection of osteoid (%OS/BS) and resorption surfaces (%ES/BS) respectively. In addition, wall thickness (W.Th.) and lamellar thickness (Lm.Th.) data were also collected from identifiable endocortical bone packets as a measure of formative potential.

RESULTS AND CONCLUSIONS

%OS/BS was significantly elevated in the anterior (control = 3.4 +/- 0.7: fracture = 11.0 +/- 2.3; p = 0.0001), inferior (3.4 +/- 1.0: 9.9 +/- 3.0; p = 0.0009), and posterior quadrants (3.2 +/- 0.8: 9.1 +/- 2.3; p = 0.0021). Only for anterior region was increased %ES/BS demonstrated in the fracture group (2.8 +/- 0.6: 5.3 +/- 0.7; p = 0.055). W.Th. (mm) was reduced only in the inferior region of the fracture cases (control = 33.7 +/- 1.2: fracture = 30.6 +/- 0.9; p = 0.013), whereas Lm.Th. was also reduced inferiorly (control = 2.7 +/- 0.08: fracture = 2.5 +/- 0.08; p = 0.042). These data suggest that an endocortical remodeling imbalance involving reduced bone formation within inferior region coupled with elevated anterior resorption may make an important contribution to the cortical thinning observed in cases of femoral neck fracture.

The effects of hormone replacement therapy on cortical bone in postmenopausal women. A histomorphometric study

Investigations of the actions of estrogen on the skeleton have mainly focused on cancellous bone and there are no reported histomorphometric studies of the effects of oestrogen on cortical bone in humans. The aim of this study was to investigate the effects of both conventional hormone replacement therapy (HRT) and high-dose oestradiol on cortical bone in postmenopausal women. Transiliac biopsies were obtained from nine postmenopausal women aged 54-71 yr before and after 2 yr (mean, 23.5 months) of conventional HRT and in seven postmenopausal women aged 52-67 yr after long-term, high-dose oestradiol implant therapy (at least 14 yr). Indices of bone turnover, remodeling, and cortical structure were assessed by image analysis. Cortical width was highest in the women treated with high-dose oestrogen therapy (2.29 +/- 0.78 mm; mean +/- SD) and lowest in untreated women (1.36 +/- 0.60 mm; P=0.014). The proportion of canals with an eroded surface was significantly lower in the high-dose oestrogen group than in women before or after conventional HRT (3.03 +/- 3.7% vs. 11.1 +/- 7.1% and 10.5 +/- 8.6%; P=0.017 and 0.05, respectively). Bone formation rate (microm2/microm/day) in untreated women was significantly higher than in the high-dose oestrogen group (0.121 +/- 0.072 vs. 0.066 +/- 0.045, respectively; P=0.05), values in women treated with conventional HRT being intermediate. Our results provide the first histomorphometric evidence in postmenopausal women of dose-dependent oestrogen-induced suppression of bone turnover in iliac crest cortical bone. There was also a trend toward higher wall width with increasing dose of oestrogen, consistent with the previously reported anabolic effect in cancellous bone.

Effects of gender, anthropometric variables, and aging on the evolution of hip strength in men and women aged over 65

Although gender differences in fall rates may partly explain the higher prevalence of fractures in elderly women than men, male bones may also be intrinsically stronger or suffer less structural degradation with age than those of women. We used hip structural analysis (HSA) to study gender differences in hip geometry and bone mineral density (BMD) as they evolved over time in elderly white men and women with the aim of identifying candidate biological pathways leading to heightened risk of hip fracture. We recruited 443 women and 439 men aged 67-79 years from a diet and cancer prospective population-based cohort study to a study of hip bone loss. Hip BMD was measured on two occasions 2-5 years apart by dual-energy X-ray absorptiometry and HSA software used to derive BMD and structural parameters at the narrow neck (NN), the intertrochanter (IT), and the shaft (S) regions. Structural indices calculated in each region were cross-sectional area (CSA)-amount of bone surface area in the cross section after excluding soft tissue space; section modulus (Z)-an index of bending resistance, subperiosteal width, endocortical width, cortical thickness; and cortical buckling ratio (CBR)-a measure of cortical instability. Compared to men, women had lower values of BMD, CSA, Z, subperiosteal width, endocortical width, and cortical thickness in all regions, except S endocortical width, after adjusting for weight, height, and age (P < 0.0001). CBR was higher in women than in men (P < 0.0001) in all regions. Longitudinal analysis of rates of change revealed faster rates of BMD decline in women than in men at the Hologic total hip, Hologic femoral neck, and IT regions (P < 0.029). Women had faster rates of subperiosteal and endosteal expansion than men at the NN (P < 0.011) and IT (P < 0.049) and faster increase in Z at the NN (P = 0.029). At the IT region, cortical thinning was faster in women than in men (P = 0.037) and CBR increased at a faster rate in women (P = 0.011). In conclusion, Z is lower in women than in men and expansion of the proximal femur occurs in both sexes, being faster in women than in men. Z does not decline at the same rate as BMD, implying that part of the effect of aging on BMD is due to expansion of the bony envelope without loss of bone mineral content. Faster expansion in the female femoral neck may in turn lead to greater fragility if wider diameter and thinner cortices become locally unstable.

Increased cancellous bone in the femoral neck of patients with coxarthrosis (hip osteoarthritis): a positive remodeling imbalance favoring bone formation

Osteoporosis is caused by an imbalance between bone resorption and formation which results in an absolute reduction in bone mass. In a previous study we highlighted a condition, osteoarthritis of the hip (coxarthrosis, cOA), where an imbalance between resorption and formation provided beneficial effects in the form of an absolute increase in bone mass. We demonstrated that the femoral neck in patients with cOA had increased cancellous bone area, connectivity and trabecular thickness which might contribute to the protection against fracture associated with the condition. The aim of the present study was to analyze forming and resorbing surfaces in coxarthritic cancellous bone to assess whether increased formation or reduced resorption could be responsible for these structural changes. Whole cross-sectional femoral neck biopsies were obtained from 11 patients with cOA and histomorphometric parameters compared with 14 age- and sex-matched cadaveric controls. The ratio of osteoid surface to bone surface was 121% ( p<0.001) higher in the cases but there was no significant difference in resorptive surface. The percentage osteoid volume to bone volume (%OV/BV; +270%, p<0.001) and osteoid width (O.Wi; +127%, p<0.001) were also higher in the cases. This study suggests that the increased cancellous bone mass seen in cases of cOA is due to increased bone formation rather than decreased bone resorption. Investigation of the cellular and biochemical basis for these changes might provide new insights into the pathogenesis of osteoarthritis and highlight novel biological mechanisms regulating bone multicellular unit (BMU) balance that could be relevant to developing new interventions against hip and other osteoporotic fractures.

Evidence for bone formation on the external "periosteal" surface of the femoral neck: a comparison of intracapsular hip fracture cases and controls

Age-related expansion of the external surface of the femoral neck in order to offset generalized bone loss is potentially an important mechanism whereby hip strength and hence resistance to hip fracture is maintained. However, it has been widely assumed that bone formation is precluded from this external interface due to the presence of a synovial membrane associated with the hip joint. In this study we have demonstrated histologically that bone formation does indeed occur on the outer "periosteal" surface of the proximal femoral neck. It was therefore hypothesized that an impairment or reduction in periosteal bone formation might be seen in cases of femoral neck fracture compared with age-matched controls. Qualitative analysis of whole femoral neck samples from female subjects and age- and sex-matched post-mortem controls demonstrated that these groups expressed similar distributions of the bone formation marker, alkaline phosphatase (AP), at the periosteal surface [whole biopsy mean % periosteal AP-positive surface: control=16.0 (range=0.5-43.0), fracture=13.4 (range=1.0-34.6), p=0.44]. In conclusion, despite a wide intersubject variation, bone formation at the femoral neck periosteum is a feature of elderly women even if they have had a hip fracture.

The ratio of osteocytic incorporation to bone matrix formation in femoral neck cancellous bone: an enhanced osteoblast work rate in the vicinity of hip osteoarthritis

Recently it has been shown that an inactivating mutation in the TGFb-SMAD3 signaling pathway, which increases the conversion of osteoblasts to osteocytes, is accompanied by bone loss combined with increased osteocyte density. We hypothesized that increased matrix TGFb, known to occur in osteoarthritis, might cause the reverse of these effects in man. Because coxarthrosis (cOA) is associated with a reduced risk of femoral neck fracture, whole cross-section femoral neck biopsies were obtained from 11 patients with femoral neck fracture, 14 patients with cOA, and 22 age-and sex-matched controls. Lacunar density (Lc x mm2), osteocyte density (Ot x mm2), and cancellous wall width (Cn x W x Wi), were compared between cases of coxarthrosis, femoral neck fracture (FNF) and controls. In cOA, Lc.mm2 was reduced by 24% (P <0.001) while in FNF it was increased by 20% (P <0.001). Cn x W x Wi was increased in cOA by 22% (P <0.05) and in FNF was reduced by 27% (P <0.001). Lc x mm2 was inversely related to percentage cancellous bone area (adj. r2 = 0.373; P <0.01) and wall widths, r2 = 0.382, P <0.001. The reduction in osteocyte lacunar density coupled with increased wall width is consistent with a model of cOA effects on bone in which increased levels of matrix TGFb might prolong the effective lifespan or work rate of the osteoblast and delay its incorporation into the matrix as an osteocyte. One possible approach to strengthening bone in osteoporosis might be to enhance the effective lifespan of the osteoblast by modulating TGFb-related pathway activity in its local environment.

Increased femoral neck cancellous bone and connectivity in coxarthrosis (hip osteoarthritis)

Patients with coxarthrosis (cOA) have a reduced incidence of intracapsular femoral neck fracture, suggesting that cOA offers protection. The distribution of bone in the femoral neck was compared in cases of coxarthrosis and postmortem controls to assess the possibility that disease-associated changes might contribute to reduced fragility. Whole cross-section femoral neck biopsies were obtained from 17 patients with cOA and 22 age- and sex-matched cadaveric controls. Densitometry was performed using peripheral quantitated computed tomography (pQCT) and histomorphometry on 10-microm plastic-embedded sections. Cortical bone mass was not different between cases and controls (P > 0.23), but cancellous bone mass was increased by 75% in cOA (P = 0.014) and histomorphometric cancellous bone area by 71% (P < 0.0001). This was principally the result of an increase of apparent density (mass/vol) of cancellous bone (+45%, P = 0.001). Whereas cortical porosity was increased in the cases (P < 0.0001), trabecular width was also increased overall in the cases by 52% (P < 0.001), as was cancellous connectivity measured by strut analysis (P < 0.01). Where osteophytic bone was present (n = 9) there was a positive relationship between the amount of osteophyte and the percentage of cancellous area (P < 0.05). Since cancellous bone buttresses and stiffens the cortex so reducing the risk of buckling, the increased cancellous bone mass and connectivity seen in cases of cOA probably explain, at least in part, the ability of patients with cOA to resist intracapsular fracture of the femoral neck during a fall.

Bone marrow adipocytes: a neglected target tissue for growth hormone

Bone marrow (BM) contains numerous adipocytes. These share a common precursor with osteoblasts and chondrocytes, but their function is unknown. It is unclear what regulates the differentiation of these three different cell types, though their subsequent metabolic activity is under hormonal regulation. GH and estrogen stimulate bone growth and mineralization, by direct effects on chondrocytes and osteoblasts. GH also stimulates lipolysis in subcutaneous and visceral adipocytes. However, adipocytes in BM have largely been ignored as potential targets for GH or estrogen action. We have addressed this by measuring BM adipocyte number, perimeter and area as well as bone area and osteoblast activity in GH-deficient dwarf (dw/dw), normal, or ovariectomized (Ovx) rats, with or without GH, IGF-1, PTH, or estrogen treatment or high fat feeding. Marrow adipocyte numbers were increased 5-fold (P < 0.001) in dw/dw rats, and cell size was also increased by 20%. These values returned toward normal in dw/dw rats given GH but not when given IGF-1. Cancellous bone area and osteoblast number were significantly (P < 0.005) lower in dw/dw rats, though alkaline phosphatase (ALP) activity in individual osteoblasts was unchanged. GH treatment increased % osteoblast covered bone surface without affecting individual cell ALP activity. Ovariectomy in normal or dw/dw rats had no affect on marrow adipocyte number nor size, although estrogen treatment in ovariectomized (Ovx) normal rats did increase adipocyte number. Ovx decreased tibial cancellous bone area in normal rats (64%; P < 0.05) and decreased osteoblast ALP-activity (P < 0.01) but did not affect the percentage of osteoblast-covered bone surface. Estrogen replacement reversed these changes. While treatment with PTH by continuous sc infusion decreased cancellous bone (P < 0.05) and high fat feeding increased the size of BM adipocytes (P < 0.01), they did not affect BM adipocyte number. These results suggest that GH has a specific action on BM adipocytes that is not simply due to altered bone or fat metabolism. We conclude that the marrow adipocyte lineage is an important and specific target for GH action. The inverse relationship between adipocyte number and osteoblast covered bone surface, together with the well-known effects of GH on epiphysial chondrocytes leads us to propose that GH plays two important roles on cells of all three lineages. During differentiation, it regulates the numbers of each cell type that are maintained from the common precursor lineage. Subsequently it has cell-specific effects on the metabolic activities of the differentiated cells. In the case of marrow adipocytes, GH-dependent lipolysis could provide an important hormonally regulated local high energy source in bone.

Osteocyte density in aging subjects is enhanced in bone adjacent to remodeling haversian systems

The osteocyte is a candidate regulatory cell for bone remodeling. Previously, we demonstrated that there is a substantial (approximately 50%) loss of osteocytes from their lacunae in the cortex of the elderly femoral neck. Higher occupancy was evident in tissue exhibiting high remodeling and high porosity. The present study examines the distribution of osteocytes within individual osteonal systems at differing stages of the remodeling cycle. In 22 subjects, lacunar density, osteocyte density, and their quotient, the percent lacunar occupancy, was assessed up to a distance of 65 microm from the canal surface in six quiescent, resorbing, and forming osteons. In both forming (p = 0.024) and resorbing (p = 0.034) osteons, osteocyte densities were significantly higher in cases of hip fracture than controls. However, there were no significant between-group differences in lacunar occupancy. In both cases and controls, osteocyte density (p < 0.0001; mean difference +/-SEM: 157 +/- 34/mm2) and lacunar occupancy (p = 0.025; mean difference: 8.1 +/- 3.4%) were shown to be significantly higher in forming compared with quiescent osteons. Interestingly, resorbing systems also exhibited significantly elevated osteocyte density in both the fracture and the control group combined (mean difference 76 +/- 23/mm2; p = 0.003). Lacunar occupancy was also greater in resorbing compared with quiescent osteons (both groups combined: p = 0.022; mean difference: 5.7 +/- 2.3%). Elevated osteocyte density and lacunar occupancy in forming compared with quiescent systems was expected because of the likely effects of aging on quiescent osteons. However, the higher levels of these parameters in resorbing compared with quiescent systems was the opposite of what we expected and suggests that, in addition to their postulated mechanosensory role in the suppression of remodeling and bone loss, osteocytes might also contribute to processes initiating or maintaining bone resorption.

Patterns of osteocytic endothelial nitric oxide synthase expression in the femoral neck cortex: differences between cases of intracapsular hip fracture and controls

Evidence indicates that extensive amalgamation of adjacent resorbing osteons is responsible for destroying the microstructural integrity of the femoral neck's inferior cortex in osteoporotic hip fracture. Such osteonal amalgamation is likely to involve a failure to limit excessive resorption, but its mechanistic basis remains enigmatic. Nitric oxide (NO) inhibits osteoclastic bone destruction, and in normal bone cells its generation by endothelial nitric oxide synthase (eNOS, the predominant bone isoform) is enhanced by mechanical stimuli and estrogen, which both protect against fracture. To determine whether eNOS expression in osteocytes reflects their proposed role in regulating remodeling, we have examined patterns of osteocyte eNOS immunolabeling in the femoral neck cortex of seven cases of hip fracture and seven controls (females aged 68-96 years). The density of eNOS+ cells (mm(-2)) was 53% lower in the inferior cortex of the fracture cases (p < 0.0004), but was similar in the superior cortex. eNOS+ osteocytes were, on average, 22% further from their nearest blood supply, than osteocytes in general (p < 0.0001) and the nearest eNOS+ osteocyte was 57% further from its nearest canal surface (p < 0.0001). This differential distribution of eNOS+ osteocytes was significantly more pronounced in the cortices of fracture cases (p < 0.0001). We conclude that the normal regional and osteonal pattern of eNOS expression by osteocytes is disrupted in hip fracture, particularly at sites that are loaded most by physical activity. These results suggest that eNOS+ osteocytes may normally act as sentinels confining resorption within single osteons. A reduction in their number, coupled to an increase in their remoteness from canal surfaces, may thus permit the irreversible merging of resorbing osteons, and thus contribute to the marked increase in the fragility of osteoporotic bone.

Super-osteons (remodeling clusters) in the cortex of the femoral shaft: influence of age and gender

Previous studies of cortical remodeling in the fractured femoral neck indicated that the merging of spatially clustered remodeling osteons could result in the formation of deleteriously large cavities associated with femoral neck fracture. This study aimed to identify whether remodeling osteons in the femoral shaft were also clustered and to assess the influence of age and gender. Microradiographic images of femoral mid-shaft cross-sections from 66 subjects over 21 years of age were analyzed to determine the number, size and location of all Haversian canals. Those most recently remodeled were identified using an edge-detection algorithm highlighting the most marked differential gradients in grey levels. Cluster analysis (JMP software) of these osteons identified the proportion of recently remodeled osteons that were within 0.75 mm clusters. As in the femoral neck, remodeling osteons were significantly more clustered than could occur by chance (real, 59.4%; random, 39.4%; P < 0.0001). The density of these clusters (number/mm(2)) was not significantly associated with subject age or gender but was greatest near the periosteum and decreased toward the marrow cavity (periosteal 0.043 +/- 0.004; mid-cortex 0.028 +/- 0.003; endosteal 0.017 +/- 0.002). Cortical porosity increased with age. The presence of giant canals (diameter >385 microm) was inversely related to the presence of clusters (R(2) = 0.237, P < 0.0001). This data suggest that remodeling osteons tend to be spatially colocalized in the shaft as they are in the neck of the femur and their presence is independent of age or gender. We propose that these remodeling clusters be termed super-osteons. The negative relationship between super-osteons and giant canals raises the intriguing possibility that loss of the control of remodeling depth results in the merging of osteonal systems to form deleteriously large cortical cavities with a marked reduction in bone strength.

Intracapsular hip fracture and the region-specific loss of cortical bone: analysis by peripheral quantitative computed tomography

Generalized bone loss within the femoral neck accounts for only 15% of the increase in intracapsular hip fracture risk between the ages of 60 and 80 years. Conventional histology has shown that there is no difference in cancellous bone area between cases of intracapsular fracture and age and sex-matched controls. Rather, a loss of cortical bone thickness and increased porosity is the key feature with the greatest change occurring in those regions maximally loaded during a fall (the inferoanterior [IA] to superoposterior [SP] axis). We have now reexamined this finding using peripheral quantitative computed tomography (pQCT) to analyze cortical and cancellous bone areas, density, and mass in a different set of ex vivo biopsy specimens from cases of intracapsular hip fracture (female, n = 16, aged 69-92 years) and postmortem specimens (female, n = 15, aged 58-95 years; male, n = 11, aged 56-86 years). Within-neck location was standardized by using locations at which the ratio of maximum to minimum external diameters was 1.4 and at more proximal locations. Cortical widths were analyzed using 72 radial profiles from the center of area of each of the gray level images using a full-width/half-maximum algorithm. In both male and female controls, cancellous bone mass increased toward the femoral head and the rate of change was gender independent. Cancellous bone mass was similar in cases and controls at all locations. Overall, cortical bone mass was significantly lower in the fracture cases (by 25%; p < 0.001) because of significant reductions in both estimated cortical area and density. These differences persisted at locations that are more proximal. The mean cortical width in the cases was significantly lower in the IA (22.2%;p = 0.002) and inferior regions (19%;p < 0.001). The SP region was the thinnest in both cases and controls. These data confirm that a key feature in the etiology of intracapsular hip fracture is the site-specific loss of cortical bone, which is concentrated in those regions maximally loaded during a fall on the greater trochanter. An important implication of this work is that the pathogenesis of bone loss leading to hip fracture must be by a mechanism that varies in its effect according to location within the femoral neck Key candidate mechanisms would include those involving locally reduced mechanical loading. This study also suggests that the development of noninvasive methodologies for analyzing the thickness and estimated densities of critical cortical regions of the femoral neck could improve detection of those at risk of hip fracture.

Osteocyte lacunar occupancy in the femoral neck cortex: an association with cortical remodeling in hip fracture cases and controls

In adult humans, osteocytes die and disappear from their lacunae in the cortex of bones which remodel slowly, such as the proximal femur, and osteocyte death is particularly prevalent in the elderly. We have investigated the statistical determinants of osteocyte density in microscopic fields (0.71 mm2) within thin, complete femoral neck cross-sections cut from biopsies embedded in methyl methacrylate and stained with solochrome cyanine R. Lacunae were counted under phase contrast and osteocytes within lacunae were counted in the same fields under epifluorescence. The percentage of lacunae containing an osteocyte varied between 12.4% and 99.2%, according to subject and quadrantic region of the cortex examined. The microscopic determinants of field-specific osteocyte density included the porosity measured in the field itself and the regional measurement of the proportion of cortical canals bearing osteoid. There was significant variation between subjects and, within subjects, between cortical regions. Also the inferior region showed a significantly higher density of lacunae than the superior region (+8.2%; P = 0.013). However, cases of fracture were not significantly different from controls with respect to osteocyte lacunar occupancy after adjusting for osteoid-bearing canals and porosity. It is concluded that in subjects in their 7th-9th decades of age, osteocyte lacunar occupancy is statistically associated with bone turnover, implying that high turnover (locally young bone age) might favor lacunar occupancy (ln% osteoid; P = 0.021). Alternative explanations of the association are that porosity reflects a better nutritional supply via the vasculature or that porosity of the cortex is associated with osteocyte density through an effect of osteocytes on bone remodeling.

A novel mechanism for induction of increased cortical porosity in cases of intracapsular hip fracture

It has been suggested that, in hip fracture, the cortex on the inferoanterior (IA) to superoposterior (SP) axis is thinned and shows increased porosity. This is dependent on the presence of giant canals (i.e., diameter >385 microm), which are related to clusters of remodeling osteons. To investigate further the relationship between remodeling and bone loss, osteonal diameter (On.Dm), wall thickness (W.Th), osteoid width (O.Wi), and extent (OS) were measured in femoral neck biopsies from 12 female intracapsular hip fracture cases and 11 age- and gender-matched controls. Over 83% of giant canals were "composite" osteonal systems in which a single canal was surrounded by multiple packets of osteonal bone. Among smaller canals, over 80% of systems had a canal encircled by a single cement line containing one packet of bone ("simple"). Composites were nearly twice as prevalent in fractures (fracture cases 9.8 +/- 0.7/25 mm(2), controls 5.3 +/- 0.4/25 mm(2), p < 0. 0001), and were dependent (R(2) = 0.52) on femoral neck region (p = 0.0008) and the regional distribution of clusters of remodeling osteons (p = 0.0045). Both the inferior (I) and anterior (A) regions had an elevated number of composites (I: 263% of control values, p = 0.0054; A: 202% of control values, p = 0.0092). On.Dm was similar in fracture cases and controls (simple: fracture cases 183 +/- 3 microm, controls 191 +/- 4 microm; composites: fracture cases 446 +/- 13 microm, controls 460 +/- 13 microm). W.Th in simples was similar in fracture cases and controls (fracture cases 51 +/- 0.8 microm, controls 49 +/- 0.7 microm), but composites had significantly (p < 0. 0001) thinner walls, with the reduction in fracture cases (31%) being twice that of controls (12%, p < 0.0001). There were no differences in O.Wi. It was unusual for osteoid to fully surround the composite canal surface; OS was 38% lower in composite than simple canals (p < 0.0001). This study indicates that, in the femoral neck cortex, the principal remodeling deficit in hip fracture is specific to composite osteons. Hip fracture cases had zonal increases in composite osteon density with reduced bone formation. The data suggest that generation of composite osteons is a plausible mechanism leading to increasing porosity and trabecularization of the cortex, thus weakening the cortex in regions maximally loaded on fall impact.

Spatial clustering of remodeling osteons in the femoral neck cortex: a cause of weakness in hip fracture?

Intracapsular femoral neck fractures are associated with decreased cortical width and increased proportions of Haversian canals with diameters greater than the normal mean plus 3 SD (i.e., >385 microm). Such canals might be formed if closely associated resorbing osteons merge; a cortical event analogous with the loss of cancellous connectivity. To test this, we investigated the pattern of osteon distribution in the aging femoral neck to determine if remodeling osteons were distributed in anatomical clusters. Femoral neck biopsies from female patients with intracapsular hip fractures (n = 13) were compared with age/gender-matched cadaveric controls (n = 13). Solochrome-stained sections were analyzed for Haversian canal location, canal diameter, and the presence of an osteoid surface. Clustering was investigated using statistical software with a cluster defined as two or more osteoid-bearing osteon centers within 0.75 mm of each other. Clusters occurred more frequently than would be expected by chance (p < 0.001). Fracture cases had more clusters per unit area (3.14 +/- 0.31 clusters/25 mm2 of cortical bone) than controls (1.89 +/- 0.22) (p = 0.002). In fracture cases, the antero-inferior, antero-superior, and infero-anterior regions had more clusters per 25 mm2 than comparable control regions (ant/inf: 4.12 +/- 0.79, 1.70 +/- 0.60,p = 0.025; ant/sup: 5.31 +/- 1.1, 1.80 +/- 0.59,p = 0.013; inf/ant: 3.15 +/- 0.49, 1.27 +/-0.29, p = 0.004). The mean number of clusters per 25 mm2 per region correlated with the mean porosity per region (adjusted r2 = 0.60;p = 0.014), and the total number of giant canals per region correlated with the total number of clusters per region (adjusted r2 = 0.58; p = 0.011). In conclusion, remodeling osteons are clustered or grouped anatomically, and fracture cases have more clusters than controls. Our data suggest that merging of adjacent, clustered osteons during resorption could lead to the rapid development of canals with excessive diameters and focal weakness. Clustering is greatest in those regions that we have previously shown to have the largest relative reductions in bone strength compared with controls and known to be maximally loaded during a sideways fall. This implicates the remodeling process underlying clustering of remodeling osteons in the aetiology of hip fracture.