An Apparatus to Record the Hydrostatic Pressure Changes Occurring during the Cementation of Dowel-Retained Prostheses

A new apparatus to measure the hydrostatic pressures developed during the cementation of dowel-retained prostheses has been developed and is described. The initial results obtained using this apparatus have been shown to vary from zero if the dowel is coated with cement, to 1.1 times 104 kN/m2 when cement is placed into the prepared hole for an optimally fitting dowel. Avenues of further research using this apparatus are mentioned.

In situ thin film growth stresses during chemical ordering

The in situ growth stress and postgrowth stress relaxation during the L1(0) chemical ordering of Fe0.54Pt0.46 thin films have been characterized. The compressive stress is reduced with an increase in order parameter. The postgrowth stress relaxation rate increased with the order parameter and is rationalized in terms of an increase in the interfacial energy contribution at the grain boundaries because of chemical order. Density functional theory calculations were performed to quantify possible diffusion pathways and binding energies for Fe and Pt that may mitigate surface migration.

Skeletal phenotype of mice with a null mutation in Cav 1.3 L-type calcium channel

This study aimed to understand the role of Cav1.3, one of the four L-type voltage sensitive calcium channels (VSCC) alpha(1) subunits, in the skeletal response to mechanical loading and intermittent PTH treatment. The Cav1.3 mRNA is expressed in osteoblasts. The Cav1.3 mRNA level in male wild type mice is higher than those in female. Loss of Cav1.3 resulted in a smaller skeleton in male mice as indicated by significantly lower body weight, less bone mineral content and smaller cross-sectional area of femoral midshaft. However, the osteogenic response to mechanical loading of the ulna was normal in Cav1.3(-/-) compared to the normal control mice. Male mice Cav1.3(-/-) were then treated daily with PTH at a dose of 40 microg/kg. A 6-week course of intermittent PTH treatment enhanced bone mineral content and mechanical strength equally in wild type control and Cav1.3 null mice. We also found that Cav1.2 subunit significantly increases in the absence of Cav1.3 gene. In conclusion, Cav1.3 is involved in bone metabolism, especially in male mice. Cav1.3 does not mediate osteoblast response to mechanical loading and PTH. Our data suggest that Cav1.1 and Cav1.2 subunits may substitute for Cav1.3 to maintain bone response to mechanical loading.

Heritability of lumbar trabecular bone mechanical properties in baboons

Genetic effects on mechanical properties have been demonstrated in rodents, but not confirmed in primates. Our aim was to quantify the proportion of variation in vertebral trabecular bone mechanical properties that is due to the effects of genes. L3 vertebrae were collected from 110 females and 46 male baboons (6-32 years old) from a single extended pedigree. Cranio-caudally oriented trabecular bone cores were scanned with microCT then tested in monotonic compression to determine apparent ultimate stress, modulus, and toughness. Age and sex effects and heritability (h(2)) were assessed using maximum likelihood-based variance components methods. Additive effects of genes on residual trait variance were significant for ultimate stress (h(2)=0.58), toughness (h(2)=0.64), and BV/TV (h(2)=0.55). When BV/TV was accounted for, the residual variance in ultimate stress accounted for by the additive effects of genes was no longer significant. Toughness, however, showed evidence of a non-BV/TV-related genetic effect. Overall, maximum stress and modulus show strong genetic effects that are nearly entirely due to bone volume. Toughness shows strong genetic effects related to bone volume and shows additional genetic effects (accounting for 10% of the total trait variance) that are independent of bone volume. These results support continued use of bone volume as a focal trait to identify genes related to skeletal fragility, but also show that other focal traits related to toughness and variation in the organic component of bone matrix will enhance our ability to find additional genes that are particularly relevant to fatigue-related fractures.

Rapamycin impairs trabecular bone acquisition from high-dose but not low-dose intermittent parathyroid hormone treatment

The osteo-anabolic effects of intermittent parathyroid hormone (PTH) treatment require insulin-like growth factor (IGF) signaling through the IGF-I receptor. A major downstream target of the IGF-I receptor (via Akt) is the mammalian target of rapamycin (mTOR), a kinase involved in protein synthesis. We investigated whether the bone-building effects of intermittent PTH require functional mTOR signaling. Mice were treated with daily PTH 1-34 (0, 10, 30, or 90 microg/kg) for 6 weeks in the presence or absence of rapamycin, a selective inhibitor of mTOR. We found that all PTH doses were effective in enhancing bone mass, whether rapamycin was present or not. Rapamycin had little to no effect on the anabolic response at low (10 microg) PTH doses, small effects in a minority of anabolic measures at moderate doses (30 microg), but the anabolic effects of high-dose PTH (90 microg) were consistently and significantly suppressed by rapamycin ( approximately 4-36% reduction). Serum levels of Trap5b, a marker of resorption, were significantly enhanced by rapamycin, but these effects were observed whether PTH was absent or present. Our data suggest that intermittent PTH, particularly at lower doses, is effective in building bone mass in the presence of rapamycin. However, the full anabolic effects of higher doses of PTH are significantly suppressed by rapamycin, suggesting that PTH might normally activate additional pathways (including mTOR) for its enhanced high-dose anabolic effects. Clinical doses of intermittent PTH could be an effective treatment for maintaining or increasing bone mass among patients taking rapamycin analogs for unrelated health issues.

Skeletal health: primate model of postmenopausal osteoporosis

Currently, the nonhuman primate is the most widely used large animal model to evaluate the safety and efficacy of new drug entities to treat or prevent estrogen-deficiency-induced bone loss and osteoporosis. Surgical ovariectomy (OVX) induces a state of high bone turnover and rapid bone loss establishing a new steady-state bone mass within 8-9 months. Many systems in the monkey are similar to humans, including skeletal and reproductive physiology and the immune system, making this a plausible model suitable to evaluate the effects of new bone drugs. The long-term sequelae following OVX and withdrawal of monthly exposure to cyclic reproductive hormones in older female monkeys (cynomolgus and rhesus) mimics estrogen depletion and postmenopausal bone loss occurring in women. Characterization of the primate model revealed an apparent limitation to the extent of bone loss. Animals lose bone mass after OVX, but the extent of the bone loss cannot be described as osteoporotic. The small differences between OVX and sham-operated controls in many important bone measurements is overcome by including 15-20 animals per group to provide adequate statistical power. The long-term, at least 16 month, bone safety studies performed to satisfy regulatory guidelines provide an opportunity to study treatment effects for an extended period not covered in shorter-term safety studies. In vivo end-points such as densitometry and biochemical markers translate easily to clinical use, while biomechanical end-points that cannot be measured clinically can be used to predict fracture prevention. To date, the monkey OVX model has been used to support submissions for many new drugs including anabolics, bisphosphonates and selective estrogen receptor modulators. Despite its limitations, the OVX monkey model remains the best characterized of the large animal models of osteopenia and has become integral to osteoporosis drug development.

Bone parameters are improved with intermittent dosing of vitamin D3 and calcitonin

Intermittent combination of an anabolic agent to promote bone formation and an antiresorptive agent that would prevent further bone loss is a theoretically attractive approach for restoring bone mass. We tested the potential of intermittently dosed calcitriol and calcitonin (CT) to restore bone properties in ovariectomized (Ovx) rats. Rats had Ovx or sham surgery at 8 weeks old and 4 weeks later were assigned to experimental groups: (1) sham vehicle, (2) Ovx vehicle, (3) Ovx + parathyroid hormone (PTH, 40 microg/kg), and (4) Ovx + calcitriol (2 microg/kg) + CT (2 microg/kg). Group 3 received PTH every week throughout the study, and group 4 received calcitriol at weeks 1, 3, 5, and 7 and CT at weeks 2, 4, 6, and 8. Dosing was carried out for 8 weeks with serum, and micro-computed tomographic analysis was done at 0, 4, and 8 weeks. Femurs and tibias were used for radiological analyses and for mechanical testing. Dosing with PTH improved bone mass and structure of cancellous bone at metaphyses of tibias and femurs as well as properties of cortical bone including geometry and strength. Intermittent dosing with calcitriol and CT was less potent in correcting loss of cancellous bone relative to treatment with PTH and had no effect on cortical bone parameters. However, intermittent dosing with calcitriol and CT was robust enough to improve cancellous bone mass and structure through bone formation without causing deleterious side effects. Our data provide additional evidence that therapies can be devised to ameliorate the skeletal defects associated with established osteoporosis.

High impact exercise is more beneficial than dietary calcium for building bone strength in the growing rat skeleton

The benefits of impact exercise and dietary calcium on bone development are controversial. We used inbred rats under highly controlled conditions to test the independent and combined effects of impact exercise and physiological levels of calcium intakes on the growing skeleton. Forty growing F-344 female rats were fed diets containing either 100% (Ca+; 0.5% Ca) or 40% (Ca(-); 0.2% Ca) of their calcium requirements. Half of each dietary group was subjected to either 10 impacts per day from 45 cm freefall drops (Impact+), or no impact (Impact(-)). All rats received a free choice of physical activity period daily. After 8 weeks, the mechanical strength, volumetric density, geometry, and microarchitecture of their ulnae were measured. Body weight and bone length did not differ among groups. On both diets, freefall impact resulted in greater bone strength, cross-sectional moments of inertia, and endosteal and periosteal circumferences in the shaft. Only Ca+ resulted in greater shaft volumetric bone mineral density (vBMD) but that did not affect shaft breaking strength. In the bone ends, both Impact+ and Ca+ positively affected density and structure of both cortical and trabecular bone but the effects of Impact+ were more pervasive. In the proximal end, Impact+ resulted in greater bone volume fraction (BV/TV) in the trabecular bone due to greater trabecular thickness, and cortical thickness was greater due to a smaller endosteal circumference. Impact+ exerted a compensatory effect on vBMD and BV/TV in Ca(-) rats at the proximal site. In Impact(-) rats only, Ca+ resulted in greater total and cortical vBMD and BV/TV in the proximal ulna. Impact+ and Ca+ exerted additive effects on cortical bone area (BA) in the proximal ulna and on total BA, periosteal circumference, and trabecular vBMD in the distal ulna. In conclusion, impact exercise was more beneficial than adequate dietary calcium to growing bones, although sufficient dietary calcium was beneficial in rats not subjected to impact exercise.

Effects of treatment with parathyroid hormone 1-84 on quantity and biomechanical properties of thoracic vertebral trabecular bone in ovariectomized rhesus monkeys

Osteoporosis is characterized by impaired bone quality leading to increased susceptibility to fracture, particularly of the thoracic spine. However, the lumbar spine is studied most commonly. We investigated the effects of 16 months of treatment with full-length parathyroid hormone (PTH) 1-84 (5, 10, or 25 microg/kg) on bone mineral density (BMD) and on architecture and biomechanical properties of trabecular bone at the thoracic spine of ovariectomized (OVX) adult rhesus monkeys and compared the results with those from the lumbar spine. At baseline, 9 months after surgery, dual-energy X-ray absorptiometric BMD at T9-T12 was 7% lower in OVX than in sham animals. All PTH(1-84) doses increased BMD to sham levels within 7 months. Micro-computed tomography of T10 vertebrae showed that trabecular bone volume and connectivity were higher in PTH(1-84)-treated animals than in sham controls, primarily through a significantly greater trabecular number. Peripheral quantitative computed tomography of trabecular bone cores from T11 and T12 confirmed that PTH(1-84) increased BMD. Compression testing of the cores showed that PTH(1-84) treatment increased stiffness, modulus, yield load, and yield stress to levels significantly greater than in sham animals, with the largest effect in the 10 microg/kg group (35-54% greater than in OVX controls). Thus, PTH(1-84) treatment increased BMD and the biomechanical properties of trabecular bone at the thoracic spine of OVX rhesus monkeys. The 10 microg/kg dose produced the greatest effect on trabecular strength, possibly because the highest dose stimulated bone remodeling excessively. Importantly, the changes observed were similar to those in lumbar vertebrae, thereby validating extrapolation of results from the lumbar to the thoracic spine.

Bone quality and bone strength in BXH recombinant inbred mice

The relationship between bone quality and strength was studied in 11 BXH recombinant inbred (RI) strains of mice. The bone quality parameters studied were bone mineralization, microhardness, architecture, and connectivity. Previous studies have demonstrated considerable variability in bone density, biomechanical properties, and microstructure among inbred strains of mice. In particular, C3H/HeJ (C3H) mice exhibit thicker femoral and vertebral cortices and fewer trabeculae in the vertebral body compared with C57BL/6J (B6) mice, despite having similar vertebral bone strength. A set of RI mouse strains has been generated from B6 and C3H (denoted BXH) in an attempt to isolate genetic regulation of numerous traits, including bone. The objective of this study was to investigate relationships among bone quality and bone strength in femurs and vertebrae among BXH RI mice. The study involved 11 BXH RI strains of female mice (n = 5-7) as well as the B6 and C3H progenitor strains. Parameters contributing to bone quality were evaluated, including BMD, bone mineralization, microhardness, architecture, and connectivity. There was a strong correlation between femoral and vertebral BMD in all strains (P < 0.001) except in BXH-9 and -10 (P < 0.001). Within the vertebrae, cortical bone was more mineralized than trabecular bone, and a strong correlation existed between the two (P < 0.001). However, cortical microhardness did not differ from trabecular microhardness. Cortical bone was more mineralized in the femur than in the vertebrae and significantly harder, by 30%. There was a wide range in trabecular connectivity, architecture, and femur geometry among BXH RI strains. BMD explained 43% of vertebral bone strength but only 11% of femoral bone strength. Trabecular connectivity explained an additional 8% of vertebral strength, while mineralization and femur geometry explained 7% and 50% of femoral strength, respectively. Different bone quality parameters had varying influences on bone mechanical properties, depending on bone site. BMD may play a larger role in explaining bone strength in the vertebrae than in the femur. Moreover, cortical bone in the femur is harder than in vertebrae. The control of cortical bone material properties may be site-dependent.

Effects of daily treatment with parathyroid hormone 1-84 for 16 months on density, architecture and biomechanical properties of cortical bone in adult ovariectomized rhesus monkeys

Treatment with parathyroid hormone 1-84 (PTH) or teriparatide increases osteonal remodeling and decreases bone mineral density (BMD) at cortical (Ct) bone sites but may also increase bone size. Decreases in BMD and increases in size exert opposing effects on bone strength. In adult ovariectomized (OVX) rhesus monkeys, we assessed the effects of daily PTH treatment (5, 10 or 25 microg/kg) for 16 months on BMD at the radial, tibial and femoral diaphyses, and on biomechanical properties (3-point bending) of radial cortical bone and the femoral diaphysis. PTH treatment did not affect areal BMD measured by dual-energy X-ray absorptiometry at the tibial diaphysis but caused a rapid, dose-related decrease at the distal radial diaphysis. Peripheral quantitative computed tomography at the radial and femoral diaphyses confirmed a significant PTH dose-related decrease in volumetric Ct.BMD caused primarily by increased cortical area. Significant increases in cortical thickness were the result of nonsignificant increases in periosteal length and decreases in endocortical length. Histomorphometry revealed increased endocortical bone formation at the tibial diaphysis and rib, higher Haversian remodeling at the rib and increased cortical porosity at the rib and tibia. Biomechanical testing at the femoral diaphysis showed that PTH treatment had no effect on peak load, but significantly decreased stiffness and increased work-to-failure (the energy required to break the bone). Similar changes occurred in radial cortical beams but only stiffness was changed significantly. Thus, PTH treatment of OVX rhesus monkeys decreased BMD and stiffness of cortical bone but did not affect peak load, likely because of increased bone size. However, PTH treatment increased the energy required to break the femur making it more resistant to fracture.

The skeletal responsiveness to mechanical loading is enhanced in mice with a null mutation in estrogen receptor-beta

Mechanical loading caused by physical activity can stimulate bone formation and strengthen the skeleton. Estrogen receptors (ERs) play some role in the signaling cascade that is initiated in bone cells after a mechanical load is applied. We hypothesized that one of the ERs, ER-beta, influences the responsiveness of bone to mechanical loads. To test our hypothesis, 16-wk-old male and female mice with null mutations in ER-beta (ER-beta(-/-)) had their right forelimbs subjected to short daily loading bouts. The loading technique used has been shown to increase bone formation in the ulna. Each loading bout consisted of 60 compressive loads within 30 s applied daily for 3 consecutive days. Bone formation was measured by first giving standard fluorochrome bone labels 1 and 6 days after loading and using quantitative histomorphometry to assess bone sections from the midshaft of the ulna. The left nonloaded ulna served as an internal control for the effects of loading. Mechanical loading increased bone formation rate at the periosteal bone surface of the mid-ulna in both ER-beta(-/-) and wild-type (WT) mice. The ulnar responsiveness to loading was similar in male ER-beta(-/-) vs. WT mice, but for female mice bone formation was stimulated more effectively in ER-beta(-/-) mice (P < 0.001). We conclude that estrogen signaling through ER-beta suppresses the mechanical loading response on the periosteal surface of long bones.

Effects of treatment of ovariectomized adult rhesus monkeys with parathyroid hormone 1-84 for 16 months on trabecular and cortical bone structure and biomechanical properties of the proximal femur

Treatment of monkeys and humans with parathyroid hormone (PTH) 1-84 stimulates skeletal remodeling, which increases trabecular (Tb) bone mineral density (BMD) but decreases cortical (Ct) BMD at locations where these bone types predominate. We report the effects of daily PTH treatment (5, 10, or 25 microg/kg) of ovariectomized (OVX) rhesus monkeys for 16 months on bone structure and biomechanical properties at the proximal femur, a mixed trabecular and cortical bone site. PTH reversed the OVX-induced decrease in BMD measured by dual-energy X-ray absorptiometry at the proximal femur, femoral neck, and distal femur. Peripheral quantitative computed tomography confirmed a significant decrease in Ct.BMD and an increase in Tb.BMD at the total proximal femur and at the proximal and distal femoral metaphyses. The decrease in Ct.BMD resulted primarily from increased area because cortical bone mineral content was unaffected by PTH. Histomorphometry revealed that PTH significantly increased the trabecular bone formation rate (BFR) as well as trabecular bone volume and number. PTH did not affect periosteal or haversian BFR at the femoral neck, but cortical porosity was increased slightly. PTH had no effects on stiffness or peak load measured using a shear test, whereas work-to-failure, the energy required to fracture, was increased significantly. Thus, PTH treatment induced changes in trabecular and cortical bone at the proximal femur that were similar to those occurring at sites where these bone types predominate. Together, the changes had no effect on stiffness or peak load but increased the energy required to break the proximal femur, thereby making it more resistant to fracture.

Daily treatment of aged ovariectomized rats with human parathyroid hormone (1-84) for 12 months reverses bone loss and enhances trabecular and cortical bone strength

Most studies that have investigated the anabolic effects of parathyroid hormone (1-84) (PTH) or PTH fragments on the skeleton of ovariectomized (OVX) rats have evaluated the short-term effects of high-dose PTH(1-34) in young animals. This study used densitometry, histomorphometry, and biomechanical testing to evaluate the effects of 12-month daily treatment with low-dose PTH (15 or 30 microg/kg) in rats that were 10 months old at baseline, 4 months after OVX. Bone mineral density (BMD) and bone strength were reduced substantially in control OVX rats. The 15 microg/kg dose of PTH restored BMD to levels similar to those in sham animals within 6 months at the lumbar spine, distal and central femur, and whole body and maintained the BMD gain from 6 to 12 months. The 30 microg/kg dose produced greater effects. Both PTH doses normalized the trabecular bone volume-to-total volume ratio (BV/TV) at lumbar vertebra 3 but not at the proximal tibia (where baseline BV/TV was very low), solely by increasing trabecular thickness. PTH dose-dependently increased bone formation by increasing the mineralizing surface, but only the 30 microg/kg dose increased resorption. PTH increased cortical BMD, area, and thickness, primarily by increasing endocortical bone formation, and restored all measures of bone strength to levels similar to those in sham animals at all skeletal sites. PTH increased bone mass safely; there was no osteoid accumulation, mineralization defect, or marrow fibrosis and there were no abnormal cells. Thus, long-term PTH therapy normalized bone strength in the aged OVX rat, a model of postmenopausal osteoporosis, through increased bone turnover and enhanced formation of both trabecular and cortical bone.

Identification of a quantitative trait locus on rat chromosome 4 that is strongly linked to femoral neck structure and strength

Risk factors for osteoporotic hip fracture include reduced bone mineral density and poor structure of the femoral neck, both of which are heritable traits. Previously, we showed that despite similar body size, Fischer 344 (F344) rats have significantly different skeletal traits compared with Lewis (LEW) rats. To identify a gene or genes regulating fracture risk at the femoral neck, we mapped quantitative trait loci (QTL) for femoral neck density and structure phenotypes using a 595 F2 progeny derived from the inbred F344 and LEW strains of rats. Femoral neck phenotypes included volumetric bone mineral density (vBMD), neck width, femoral neck cross-sectional area and polar moment of inertia (Ip). A 20-cM genome-wide scan was performed using 118 microsatellite markers and linkage analysis was conducted to identify chromosomal regions harbor QTL for femoral neck phenotypes. Strong evidence of linkage (P<0.01) to femoral neck vBMD was observed on chromosomes (Chrs) 1, 2, 4, 5, 7, 10 and 15. QTL affecting femoral neck structure and biomechanical properties were detected only on Chr 4 where the F344 alleles were shown to improve femoral neck structure, whereas these alleles had no effect on bone measurements at the lumbar spine and only modest effects at the femoral midshaft. In contrast, QTL on Chrs 1, 2 and 10 affected multiple skeletal sites. Several QTL regions in this study are homologous to human chromosomal regions, where linkage to femoral neck and related phenotypes has been reported previously. These findings represent an important first step in localizing and identifying genes that influence hip fragility.

Bone remodeling at the iliac crest can predict the changes in remodeling dynamics, microdamage accumulation, and mechanical properties in the lumbar vertebrae of dogs

We previously demonstrated that suppression of bone remodeling allows microdamage to accumulate, thereby leading to reduced bone toughness in dog bone. In this study we evaluated the relationships between bone remodeling at the iliac crest and skeletal activation frequency, microdamage accumulation, or biomechanical properties of lumbar vertebrae using the same dogs to determine whether bone remodeling at the iliac crest can predict damage accumulation and mechanical parameters of the lumbar spine following treatment with antiresorptive agents. Thirty-six female beagles, 1 to 2 years old, were divided into three groups. The control group was treated daily for 12 months with saline vehicle. The remaining two groups were treated daily with oral risedronate at a dose of 0.5 mg/kg/day, or alendronate at 1.0 mg/kg/day orally. The doses of these bisphosphonates were 5 to 6 times the clinical doses approved for treatment of osteoporosis in humans. After sacrifice, the right ilium and L2 vertebra were assigned to histomorphometry. The left ilium and L3 vertebra were used for microdamage analysis. The L4 vertebra was mechanically tested to failure in compression, and bone toughness calculated from the stress-strain curve. There was a strong positive relationship for activation frequency (Ac.f) between ilium and lumbar vertebrae (r2 = 0.82; P < 0.0001). Iliac crest Ac.f underestimates Ac.f in L2, but L2 Ac.f reaches a minimum threshold and does not decline further when iliac crest Ac.f is below 0.10/yr. Microdamage (Cr.S.Dn) accumulation at the ilium was significantly associated with increased microdamage accumulation in the L3 lumbar vertebra (r2 = 0.43, P < 0.0001). The data also show that bisphosphonate treatment increased Cr.S.Dn at a faster rate in L3 than in the iliac crest. Although bisphosphonate treatment decreased bone toughness in L4, this decrease demonstrated no relationship to decreased Ac.f in the ilium. These results clearly indicate that bone remodeling data obtained from iliac crest biopsy could be used to estimate the activation frequency and microdamage burden in the vertebral column.

Mechanosensitivity of the rat skeleton decreases after a long period of loading, but is improved with time off

After the initial adaptation to large mechanical loads, it appears as though the skeleton's responsiveness to exercise begins to wane. To counteract the waning effects of long-term mechanical loading, "time off" may be needed to improve the responsiveness of bone cells to future mechanical signals and reinitiate bone formation. The aim of this study was to determine whether bone becomes less sensitive to long-term mechanical loading and whether time off is needed to improve mechanosensitivity. Fifty-seven female Sprague-Dawley rats (7-8 months of age) were randomized to one of following groups: Group 1 loading was applied for 5 weeks followed by 10 weeks of time off (1 x 5); Group 2 loading was applied for 5 weeks, followed by time off for 5 weeks and loading again for 5 weeks (2 x 5); Group 3 loading was applied continuously for 15 weeks (3 x 5); Group 4 age-matched control group; and Group 5 baseline control group. An axial load was applied to the right ulna for 360 cycles/day, at 2 Hz, 3 days/week at 15 N. At the end of the intervention, all three loaded groups showed similar increases in bone mass, cortical area, and I(MIN) in response to mechanical loading(.) Bone formation rate of the loaded ulna was increased in the first 5 weeks of loading for all three loaded groups; however, during the last 5 weeks, it was only significantly increased in the group that had time off (2 x 5) (P < 0.05). The group that had time off (2 x 5) also showed greater improvements in work to failure compared to the group loaded for 5 weeks (1 x 5) and the entire 15 weeks (3 x 5). A second experiment showed that the waning effect of long-term loading on the skeleton is not a result of aging. In conclusion, mechanical loading of the rat ulna results in large improvements in bone formation during the first 5 weeks of loading, but continual loading decreases the osteogenic response. Having time off increases bone formation and improves the resistance to fracture.

Estrogen receptor beta: the antimechanostat?

We have known for sometime that sex hormones influence the growth, preservation, and loss of bone tissue in the skeleton. However, we are only beginning to recognize how estrogen influences the responsiveness of the skeleton to exercise. Frost's mechanostat theory proposes that estrogen reduces the mechanical strain required to initiate an osteogenic response, but this may only occur at the endocortical and trabecular bone surfaces. The discovery of estrogen receptors alpha and beta may help us to understand the bone surface-specific effects of exercise. Findings from estrogen receptor knockout mice suggest that the activity of ERalpha may explain the positive interaction between estrogen and exercise on bone formation near marrow, that is, endocortical and trabecular bone surfaces. Estrogen inhibits the anabolic exercise response at the periosteal surface, and this we propose is due to the activation of ERbeta. Signaling through this receptor retards periosteal bone formation and suppresses gains in bone size and bone strength, and for these reasons it behaves as an antimechanostat.

Adaptations to free-fall impact are different in the shafts and bone ends of rat forelimbs

Impact exercise can have beneficial effects on the growing skeleton. To understand what changes it promotes in the shafts and ends of weight-bearing bones, we measured the effects of impact from repetitive free falls in growing rats. Fischer 344 female rats, 6.5 wk old, were assigned to one of three groups ( n = 10 each). Controls were not dropped, whereas those subjected to impact were dropped from 30 or 60 cm. Rats in both free-fall groups were dropped 10 times per day for 8 wk. Leg bones were mechanically tested, and their cross-sectional area (CSA), cross-sectional moments of inertia, and volumetric bone mineral density (BMD) were measured by peripheral quantitative computed tomography. In the shafts of the forelimbs, but not the hindlimbs, free-fall impact resulted in greater ultimate breaking force, minimum and maximum second moments of area, and CSA but not BMD. In the bone ends of the forelimb and tibial bones, trabecular BMD increased but CSA did not. Landing from 30 and 60 cm produced peak impact forces of 12.0 and 16.7 times the standing forefoot weight for each front leg and of 4.5 and 7.7 times the standing hind foot weight for each hind foot. Overall, free-fall impact affected the forelimbs by increasing trabecular bone density in the bone ends and improving the strength at the shaft as a result of geometric improvements. These results indicate that adaptation to impact may occur by different mechanisms in bone end and shaft regions.

Steps for targeting exercise towards the skeleton to increase bone strength

Osteoporosis is a disease of bone fragility resulting mostly from low bone mass and a concomitant increase in the risk for fracture. Exercise is a commonly prescribed intervention for osteoporosis because bone tissue is mechanosensitive. The ability of mechanical stimuli to influence bone biology has been known for over a century, but it has been only in the past several decades that great gains have been made in terms of understanding factors that influence this response. By understanding these factors, steps can be developed to maximize the osteogenic effects of exercise on the skeleton and potentially reduce the incidence of bone fractures. This paper outlines these steps. They include: 1) starting exercise when young while the skeleton is most responsive to mechanical stimuli; 2) selecting exercises that are dynamic and high-impact to maximize osteogenic responses, such as jumping for the lower extremity and racquet sports for the upper extremity; 3) exercising the specific skeletal regions you want to strengthen as the bone response to mechanical loading is highly site-specific; 4) exercising briefly, yet often to offset the desensitization of skeletal mechanotransduction pathways; and 5) continuing to exercise as you age to prevent bone loss and reduce the risk of falls. Following these steps will help to promote skeletal health at all ages and may reduce an individuals risk for fracture by augmenting bone mass and size during youth, while reducing age-related bone loss and the risk for falls in adulthood.

Mechanotransduction in the cortical bone is most efficient at loading frequencies of 5-10 Hz

A dose-response relationship has been shown between loading frequency and cortical bone adaptation for frequencies of up to 10 Hz, and is presumed to persist with further increases in frequency. Studies herein aimed to investigate cortical bone adaptation to loading frequencies of 1, 5, 10, 20 and 30 Hz. Two studies were performed in adult C57BL/6 mice using the ulna axial compression-loading model. In the first study, the histomorphometric response of the ulna was studied when loaded for 120 cycles day(-1) for 3 days at one of the five frequencies and one of two load magnitudes (1.5 or 2.0 N). In the second study, the changes in ulna geometry and mechanical properties were studied following loading for 5 min day(-1), 3 days week(-1) for 4 weeks at one of the five frequencies and one of two load magnitudes (1.0 or 1.6 N). Preliminary strain gauge measurements showed that frequency had no effect on mechanical strain per unit load. In study 1, loading frequency significantly influenced bone adaptation when loading at 2.0 N, with loading at 10 Hz resulting in significantly greater adaptation than with loading at other frequencies. In study 2, loading frequency significantly influenced the change in geometry when loading at 1.6 N, with loading at 5, 10 or 30 Hz resulting in significantly greater change than with loading at 1 Hz. Loading at 5 Hz also resulted in significantly greater change than with loading at 20 Hz. No frequency effect was found on any of the mechanical properties at either load. Overall, we found cortical bone adaptation to mechanical loading to increase with increasing loading frequency up to 5-10 Hz and to plateau with frequencies beyond 10 Hz. The mechanism for this nonlinear frequency response is not known; however, based on strain gauge measurements, we do not believe it resulted from dampening associated with high frequency loading through the flexed carpal joint. The obtained findings may relate to the mechanism of mechanotransduction within the bone. This requires further investigation.

The relationship between muscle size and bone geometry during growth and in response to exercise

As muscles become larger and stronger during growth and in response to increased loading, bones should adapt by adding mass, size, and strength. In this unilateral model, we tested the hypothesis that (1) the relationship between muscle size and bone mass and geometry (nonplaying arm) would not change during different stages of puberty and (2) exercise would not alter the relationship between muscle and bone, that is, additional loading would result in a similar unit increment in both muscle and bone mass, bone size, and bending strength during growth. We studied 47 competitive female tennis players aged 8-17 years. Total, cortical, and medullary cross-sectional areas, muscle area, and the polar second moment of area (I(p)) were calculated in the playing and nonplaying arms using magnetic resonance imaging (MRI); BMC was assessed by DXA. Growth effects: In the nonplaying arm in pre-, peri- and post-pubertal players, muscle area was linearly associated BMC, total and cortical area, and I(p) (r = 0.56-0.81, P < 0.09 to < 0.001), independent of age. No detectable differences were found between pubertal groups for the slope of the relationship between muscle and bone traits. Post-pubertal players, however, had a higher BMC and cortical area relative to muscle area (i.e., higher intercept) than pre- and peri-pubertal players (P < 0.05 to < 0.01), independent of age; pre- and peri-pubertal players had a greater medullary area relative to muscle area than post-pubertal players (P < 0.05 to < 0.01). Exercise effects: Comparison of the side-to-side differences revealed that muscle and bone traits were 6-13% greater in the playing arm in pre-pubertal players, and did not increase with advancing maturation. In all players, the percent (and absolute) side-to-side differences in muscle area were positively correlated with the percent (and absolute) differences in BMC, total and cortical area, and I(p) (r = 0.36-0.40, P < 0.05 to < 0.001). However, the side-to-side differences in muscle area only accounted for 11.8-15.9% of the variance of the differences in bone mass, bone size, and bending strength. This suggests that other factors associated with loading distinct from muscle size itself contributed to the bones adaptive response during growth. Therefore, the unifying hypothesis that larger muscles induced by exercise led to a proportional increase in bone mass, bone size, and bending strength appears to be simplistic and denies the influence of other factors in the development of bone mass and bone shape.

Congenic mice reveal sex-specific genetic regulation of femoral structure and strength

Genetic linkage studies in C3H/HeJ (C3H) and C57BL/6J (B6) mice identified several chromosomal locations or quantitative trait loci (QTL) linked to femoral volumetric bone mineral density (vBMD). From QTL identified on chromosomes (chr) 1, 4, 6, 13, and 18, five congenic mouse strains were developed. In each of these mice, genomic DNA from the QTL region of the donor C3H strain was transferred into the recipient B6 strain. Here we report the effects of donated C3H QTL on femoral structure, cortical vBMD and bending strength. Femoral structure was quantified by the polar moment of inertia (Ip) at the mid-diaphysis, which reflects the bending or torsional rigidity of the femur. Although the C3H progenitor mice have a smaller Ip than B6 progenitor mice, the congenic mice carrying the C3H segment at Chr 4 had significantly increased Ip in both males and females, giving these mice stronger femora. In female mice from the congenic Chr 1 strain, Ip was increased whereas male mice from the Chr 1 strain had smaller femoral cross-sections and significantly reduced Ip. This sex-specific effect on femoral structure was seen to a lesser extent in Chr 18 congenic mice. In addition, cortical vBMD was measured using peripheral quantitative computed tomography. Cortical vBMD was similar among most congenic strains except in Chr 6 congenic mice, where cortical vBMD was significantly less in females, but not in males. We conclude that (1) chromosomal QTL from C3H mice, which are genetically linked to total femoral vBMD, also regulate femoral structure; (2) the QTL on Chr 4 improves femoral structure and strength; (3) QTL on Chr 1 and 18 impart sex-specific effects on femoral structure; and (4) the QTL on Chr 6 imparts a sex-specific effect on cortical vBMD and femoral strength.

Two inbred rat strains that differ substantially in hip fragility

One approach to identifying the genetic influences on skeletal phenotypes involves the creation and genetic mapping of a population of the second filial (F2) offspring derived from a cross of two inbred strains of rodents. The two inbred strains should be chosen based upon a large difference in the phenotype of interest, e.g., bone fragility. We found previously that considerable variation exists in fragility phenotypes among inbred strains of rats, and the phenotypic variation was site specific. In particular, two inbred rat strains, Copenhagen 2331 (COP) and Dark Agouti (DA), were found to differ significantly in femoral neck geometry and strength. The aim of this study was to further characterize hip fragility in COP and DA rats at 6 months of age using peripheral quantitative computed tomography (pQCT), microcomputed tomography (mCT), and biomechanical tests. COP rats had a significantly wider femoral head (P = 0.04) and neck (P = 0.007), significantly larger bone area and cortical bone area in femoral neck (P = 0.03 and P = 0.02, respectively), significantly greater total bone mineral content (BMC) and cortical BMC in femoral neck (P = 0.01 and P = 0.001, respectively), and 65% greater femoral neck cross-sectional moment of inertia (P = 0.02), as compared with DA rats. As a result, COP rats had 22% higher ultimate force (Fu), 68% higher ultimate displacement (du), and 81% higher work to failure (U) than DA rats in the femoral neck biomechanical test (P = 0.04, P = 0.01, and P = 0.02, respectively). The biomechanical properties for the femoral midshaft and lumbar vertebrae were virtually the same in the two rat strains, suggesting a hip-specific genetic effect on bone strength. These data indicate that significant phenotypic variation at the femoral neck site exists between these two inbred strains, and COP rats appear to have genes that specifically enhance the femoral neck structural properties and strength. Therefore these two inbred strains, COP with DA, may facilitate effective genetic studies of hip fragility.

The effect of mechanical loading on the size and shape of bone in pre-, peri-, and postpubertal girls: a study in tennis players

Exercise during growth results in biologically important increases in bone mineral content (BMC). The aim of this study was to determine whether the effects of loading were site specific and depended on the maturational stage of the region. BMC and humeral dimensions were determined using DXA and magnetic resonance imaging (MRI) of the loaded and nonloaded arms in 47 competitive female tennis players aged 8-17 years. Periosteal (external) cross-sectional area (CSA), cortical area, medullary area, and the polar second moments of area (I(P), mm4) were calculated at the mid and distal sites in the loaded and nonloaded arms. BMC and I(P) of the humerus were 11-14% greater in the loaded arm than in the nonloaded arm in prepubertal players and did not increase further in peri- or postpubertal players despite longer duration of loading (both, p < 0.01). The higher BMC was the result of a 7-11% greater cortical area in the prepubertal players due to greater periosteal than medullary expansion at the midhumerus and a greater periosteal expansion alone at the distal humerus. Loading late in puberty resulted in medullary contraction. Growth and the effects of loading are region and surface specific, with periosteal apposition before puberty accounting for the increase in the bone's resistance to torsion and endocortical contraction contributing late in puberty conferring little increase in resistance to torsion. Increasing the bone's resistance to torsion is achieved by modifying bone shape and mass, not necessarily bone density.

Mechanotransduction in bone: genetic effects on mechanosensitivity in mice

Bone formation is enhanced by mechanical loading, but human exercise intervention studies have shown that the response to mechanical loading is variable, with some individuals exhibiting robust osteogenic responses while others respond modestly. Thus, mechanosensitivity - the ability of bone tissue to detect mechanical loads - could be under genetic control. We applied controlled mechanical loading to the ulnae of 20-week-old (adult) female mice derived from three different inbred strains (C3H/He, C57BL/6, and DBA/2), and measured the bone formation response with fluorochrome labels. Mechanical properties, including mechanical strain, second moments of area, and cortical bone material properties, were measured in a group of calibration animals not subjected to in vivo loading. The C3H/He mice were significantly less responsive to mechanical loading than the other two biological strains. Material properties (flexural elastic modulus, ultimate stress) were greatest in the C3H/He cortical tissue. Geometric and areal properties at the midshaft ulna were also greatest in the C3H/He mice. Based on the presumed role of osteocytes in strain detection, we measured osteocyte lacuna population densities in decalcified midshaft ulna sections. Osteocyte lacuna density was not related to mechanosensitivity. Our data suggest that bone mechanosensitivity has a significant genetic component. Identification of the genes that exert their influence on mechanosensitivity could ultimately lead to therapies that enhance bone mass and reduce fracture susceptibility.

Skeletal efficacy with parathyroid hormone in rats was not entirely beneficial with long-term treatment

We report the consequences of prolonged treatment with recombinant human parathyroid hormone (1-34) (PTH) in male and ovariectomized female rats with mature skeletons. Intact male and osteopenic, ovariectomized, female F-344 rats were evaluated after 1 year of treatment with 0, 8, or 40 microg/kg/day s.c. PTH. Males and females were about 6 months of age at study initiation; females were ovariectomized (Ovx) for 5 weeks before initiation of PTH treatment. PTH did not affect the survival of either intact males or ovariectomized females. Qualitative histopathology showed expected changes associated with aging in kidneys and proximal tibiae, with no treatment-related anomalies after 1 year of PTH administration. PTH slightly increased the femoral length of ovariectomized females but not that of males. No significant differences in femoral length were observed between sham and Ovx controls. Proximal femora of the males and ovariectomized females given the high dose of 40 microg/kg showed 211 and 186% greater trabecular bone area, 118 and 94% greater cortical thickness, 170 and 189% greater trabecular number, and 321 and 404% greater connectivity (node-to-node struts) compared with respective vehicle controls. Increased trabecular and endocortical surface apposition coincided with a 78 and 70% loss of marrow space for males and females treated with PTH, respectively. Biomechanical strength (ultimate load) of the femoral neck increased by 73 and 76%, respectively, in males and ovariectomized females. Cortical bone analyses of the femoral midshaft showed 105 and 72% increases in bone mineral content, 67 and 55% increases in bone mineral density, and 22 and 10% increases in cross-sectional area for males and ovariectomized females, respectively, with altered shape of femora. Biomechanical analyses of the midshaft showed substantial increases in strength and stiffness but a reduction in ultimate strain, which was likely due to the altered geometry of the midshaft for PTH groups. Aging effects on strength of vertebra and femoral midshaft were reversed by PTH treatment. In summary, the 1-year treatment duration, which represents about 50% of lifetime, did not affect survival and was not associated with any treatment-related anomalies in the kidney or skeleton. PTH reversed the aging process in bones but not kidneys and substantially increased bone mass and strength to well beyond normally attained levels. However, compared with short-term studies reported previously, there seemed to be no advantages to extending PTH treatment to 12 months in rat bones.

Do bone cells behave like a neuronal network?

Bone cells are organized into an interconnected network, which extends from the osteocytes within bone to the osteoblasts and lining cells on the bone surfaces. There is experimental evidence suggesting that bone tissue exhibits basic properties of short- and long-term memory. An analogy might be made between the bone cell network and neuronal systems. For instance, recent studies suggest that the neurotransmitter glutamate may play a role in cell-to-cell communication among bone cells. Glutamate is a key neurotransmitter involved in learning and memory in reflex loops and the hippocampus. The simplest forms of memory include habituation (desensitization) and sensitization. It is argued that bone cells exhibit habituation to repeated mechanical stimuli and sensitization to mechanical loading by parathyroid hormone (PTH). Acquired long-term memory of a mechanical loading environment may influence the responsiveness of bone tissue to external stimuli. For instance, bone tissue from the skull shows markedly different responses to several stimuli, e.g., mechanical loading, disuse, and PTH, compared with long bones. We speculate that the history of weight bearing imparts long-term cellular memory to the bone cell network that modulates the cellular response to a wide variety of stimuli.

Effects of biomechanical stress on bones in animals

The signals that allow bone to adapt to its mechanical environment most likely involve strain-mediated fluid flow through the canalicular channels. Fluid can only be moved through bone by cyclic loading, and the shear stresses generated on bone cells are proportional to the rate of loading. The proportional relation between fluid shear stresses on cells and loading rate predicts that the magnitude of bone's adaptive response to loading should be proportional to strain rate. For lower loading frequencies within the physiologic range, experimental evidence shows this is true. It is also true that the mechanical sensitivity of bone cells saturates quickly, and that a period of recovery either between loading cycles or between periods of exercise can optimize adaptive response. Together, these concepts suggest that short periods of exercise, with a 4-8 h rest period between them, are a more effective osteogenic stimulus than a single sustained session of exercise. The data also suggest that activities involving higher loading rates are more effective for increasing bone formation, even if the duration of the activity is short.

Suppression of prostaglandin synthesis with NS-398 has different effects on endocortical and periosteal bone formation induced by mechanical loading

Prostaglandins mediate adaptive bone formation induced by mechanical loading. Inhibition of cyclooxygenase-2 (COX-2) with NS-398 effectively blocks loading-induced osteogenesis on the endocortical bone surface of the tibia. In this study, we compared the effects of selective inhibition of COX-2 with NS-398 on mechanically induced osteogenesis at the endocortical surface (tibia) with that on the periosteal surface (ulna). We further tested the effect of NS-398 administered at different times before (3 hrs or 30 min) or after (30 min) mechanical loading. Mechanical loading induced lamellar bone formation on the endocortical surface of the tibia and the periosteal surface of the ulna. Oral administration of either indomethacin or NS-398 3 hrs before loading significantly decreased loading-induced bone formation rate (BFR) and mineralizing surface (MS/BS), but not mineral apposition rate (MAR), at the endocortical surface of the tibia and the periosteal surface of the ulna. NS-398 reduced loading-induced MS/BS by 96% on the endocortical surface of the tibia, but only by 37% on the periosteal surface of the ulna (significantly different from endocortical, P <0.05). Indomethacin reduced MS/BS and BFR to a lesser extent than NS-398 and did not have different effects on the periosteal and endocortical surfaces. These data suggest that the endocortical bone adaptive response to mechanical loading is more dependent upon COX-2 activity than is the periosteal bone response. Intraperitoneal injection of NS-398 3 hrs before loading suppressed load-induced bone formation rate at the endocortical surface of the tibia significantly more (27%) than when administered 30 min before loading. When NS-398 was given 30 min after loading, bone formation was not significantly suppressed. These data suggest that a primary cellular mechanism of bone formation following brief bouts of mechanical loading involves release of prostaglandins from cells at the time mechanical loading is applied, rather than new prostaglandin synthesis associated with a mechanically induced COX-2 expression.

Is skeletal mechanotransduction under genetic control?

Studies of twins have established that peak bone mass is about 70% heritable. The skeletal response to exercise contributes to peak bone mass, as mechanical loading increases skeletal mass during growth and development. It is possible that the skeletal responsiveness to mechanical loading is under genetic control, so that some individuals will build stronger bones with exercise. This appears to be the case in mice. Long bones in mice of the C3H/He strain are largely unresponsive to mechanical loading. Ironically, this strain of mice has very high bone density. Perhaps the genes that regulate BMD are not the same as those that regulate mechanical loading response. Studies of recombinant inbred and congenic strains derived from C3H mice will help to identify genes influencing bone size, density and responsiveness to mechanical loading.

Biomechanics of bone: determinants of skeletal fragility and bone quality

Bone fragility can be defined by biomechanical parameters, including ultimate force (a measure of strength), ultimate displacement (reciprocal of brittleness) and work to failure (energy absorption). Bone fragility is influenced by bone size, shape, architecture and tissue 'quality'. Many osteoporosis treatments build bone mass but also change tissue quality. Antiresorptive therapies, such as bisphosphonates, substantially reduce bone turnover, impairing microdamage repair and causing increased bone mineralization, which can increase the brittleness of bone. Anabolic therapies, such as parathyroid hormone (PTH-(1-84)) or teriparatide (PTH-( 1-34)), increase bone turnover and porosity, which offset some of the positive effects on bone strength. Osteoporosis therapies may also affect bone architecture by causing the redistribution of bone structure. Restructuring of bone during treatment may change bone fragility, even in the absence of drug effects on bone mineral density (BMD). This effect may explain why some drugs can affect fracture incidence disproportionately to changes in BMD. For instance, in a recent clinical trial, PTH-(1-34) therapy caused a dose-related increase in spinal BMD without any dose-dependent effect on the observed decrease in spinal fracture incidence. This apparent disassociation between spinal BMD and bone fragility is probably due to effects of PTH-(1-34) on bone architecture within vertebral bodies. While it has been shown that BMD is highly heritable, bone mineral distribution and architecture are also under strong genetic influence. Recent findings suggest that different genes regulate trabecular and cortical structures within lumbar vertebrae, producing a wide range of bone architectural designs. These findings suggest that there is no single optimal bone architecture; instead many different architectural solutions produce adequate bone strength.

Sexual dimorphism in vertebral fragility is more the result of gender differences in age-related bone gain than bone loss

Spine fractures usually occur less commonly in men than in women. To identify the structural basis for this gender difference in vertebral fragility, we studied 1013 healthy subjects (327 men and 686 women) and 76 patients with spine fractures (26 men and 50 women). Bone mineral content (BMC), cross-sectional area (CSA), and volumetric bone mineral density (vBMD) of the third lumbar vertebral body (L3) were measured by posteroanterior (PA) and lateral scanning using dual-energy X-ray absorptiometry (DXA). In this cross-sectional study, the diminution in peak vertebral body BMC from young adulthood to old age was less in men than in women (6% vs. 27%). This diminution was the net result of two opposing changes occurring concurrently throughout adult life: the removal of bone adjacent to marrow on the inner (endosteal) surface by bone resorption and the deposition of bone on the outer (periosteal) surface by bone formation. For L3, we estimated that men resorbed 3.7 g and deposited 3.1 g, producing a net loss of 0.6 g from young adulthood to old age and women resorbed 3.1 g and deposited only 1.2 g, producing a net loss of 1.9 g. Thus, based on our indirect estimates of periosteal gain and endosteal loss across life, the observed net diminution in BMC during aging was less in men than women because absolute periosteal bone formation was greater in men than women (3.1 g vs. 1.2 g) not because absolute bone resorption was less in men. On the contrary, the absolute amount of bone resorbed was greater in men than women (3.7 g vs. 3.1 g). Periosteal bone formation also increased vertebral body CSA 3-fold more in men than in women, distributing loads onto a larger CSA, so that the load imposed per unit CSA decreased twice as much in men than in women (13% vs. 5%). In men and women with spine fractures, CSA and vBMD were reduced relative to age-matched controls. However, vBMD was no different to the adjusted vBMD in age-matched controls derived assuming controls had no periosteal bone formation during aging. Thus, large amounts of bone are resorbed in men as well as in women, accounting for the age-related increase in spine fractures in both genders. Periosteal bone formation increases CSA and offsets bone loss in both genders but more greatly in men, accounting for the lower incidence of spine fractures in men than in women. We speculate that reduced periosteal bone formation, during growth or aging, may be in part responsible for both reduced vertebral size and reduced vBMD in men and women with spine fractures. Sexual dimorphism in vertebral fragility is more the result of gender differences in age-related bone gain than age-related bone loss.

The biomechanical basis of vertebral body fragility in men and women

The aim of this study was to quantify the biomechanical basis for vertebral fracture risk in elderly men and women. A bone is likely to fracture when the loads imposed are similar to or greater than its strength. To quantify this risk, we developed a fracture risk index (FRI) based on the ratio of the vertebral body compressive load and strength. Loads were determined by upper body weight, height, and the muscle moment arm, and strength was estimated from cross-sectional area (CSA) and volumetric bone mineral density (vBMD). With loads less than the strength of the bone, the FRI remains < 1. For any given load, once bone strength diminishes due to a falling vBMD, the FRI will increase. Should FRI approach or exceed unity, structural failure of the vertebra is likely. We measured vertebral body CSA vBMD of the middle zone of third lumbar vertebra by lateral and posteroanterior (PA) scanning using dual-energy X-ray absorptiometry (DXA) and calculated vertebral compressive stress (load per unit area) in 327 healthy men and 686 healthy women and 26 men and 55 postmenopausal women with vertebral fractures. Activities that require forward bending of the upper body caused approximately 10-fold more compressive stress on the vertebra compared with standing upright. Men and women had similar peak vBMD in young adulthood. Because men have greater stature than women, the loads imposed on the vertebral body are higher (3,754 +/- 65 N vs. 3,051 +/- 31 N; p < 0.001). However, because CSA also was higher in men than women, peak load per unit CSA (stress) did not differ by gender (317.4 +/- 4.7 N/cm2 vs. 321.9 +/- 3.3 N/cm2, NS). The FRI was similar in young men and women and well below unity (0.42 +/- 0.02 vs. 0.43 +/- 0.01; NS). Gender differences emerged during aging; CSA increased in both men and women but more so in men, so load per unit area (stress) diminished but more so in men than in women. vBMD decreased in both genders but less so in men. These changes were captured in the FRI, which increased by only 21% in men and by 102% in women so that only 9% of elderly men but 26% of elderly women had an FRI > or = 1. Men and women with vertebral fractures had an FRI that was greater than or equal to unity (1.03 +/- 0.13 vs. 1.35 +/- 0.13; p < 0.05) and was 2.04 SD and 2.26 SD higher than age-matched men and women, respectively. In summary and conclusion, young men and women have a similar vBMD, vertebral stress, and FRI. During aging, CSA increases more, and vBMD decreases less in men than in women. Thus, fewer men than women are at risk for fracture because fewer men than women have these structural determinants of bone strength below a level at which the loads exceed the bone's ability to tolerate them. Men and women with vertebral fractures have FRIs that are equal to or exceed unity. The results show that a fracture threshold for vertebrae can be defined using established biomechanical principles; whether this approach has greater sensitivity and specificity than the current BMD T score of -2.5 SD is unknown.

Shear strength and fatigue properties of human cortical bone determined from pure shear tests

Shear properties of bone have been inferred from torsion tests. However, torsion often causes spiral fracture planes that correspond to tensile rather than shear failure. We measured the shear properties of human cortical bone in both longitudinal and transverse directions using pure shear tests. Shearing applied transverse to the bone long axis caused fracture along a 45 degrees plane that coincided with maximum tension. This fracture pattern is similar to spiral fractures caused by torsion. Shear strength along the bone axis was 51.6 MPa or about 35% less than that determined using torsion tests. Fatigue tests of human cortical bone in pure shear were conducted. The results agreed well with previous measurements of cortical bone fatigue life in tension and compression, when normalized to strength. Using tibial shear strain magnitudes measured previously for human volunteers, we estimated the fatigue life of cortical bone for different activities, and speculate that shear fatigue failure is a probable cause of tibial stress fractures resulting from impact loading.

Mechanical loading of diaphyseal bone in vivo: the strain threshold for an osteogenic response varies with location

Bone tissue responds to elevated mechanical loading with increased bone formation, which is triggered either directly or indirectly by the mechanical strain engendered in the bone tissue. Previous studies have shown that mechanical strain magnitude must surpass a threshold before bone formation is initiated. The objective of this study was to estimate the strain thresholds at three different locations along the ulna of adult rats. We hypothesized that the strain threshold would be greater in regions of the ulna habitually subjected to larger mechanical strains. New bone formation was measured on the periosteal and endocortical surfaces of the ulnar diaphysis in adult female rats exposed to controlled dynamic loading. Axial, compressive loading was applied daily at five different magnitudes for a period of 2 weeks. Bone formation rate (BFR) was measured, using double-label histomorphometry at the ulnar middiaphysis and at locations 3 mm proximal and 3 mm distal to the middiaphysis. Loading induced lamellar bone formation on the periosteal surface that was greater at the distal ulnar location and lower at the proximal location when compared with the middiaphysis. Likewise, peak strains on the periosteal surface were greatest distally and less proximally. There was a significant dose-response relationship between peak strain magnitude and periosteal new bone formation when the mechanically induced strain surpassed a threshold. The strain threshold varied from 1343 microstrain (mu strain) proximally to 2284 mu strain at the midshaft to 3074 mu strain distally. Unlike the periosteal response to mechanical loading, there was not a clear dose-response relationship between applied load and bone formation on the endocortical surface. Endocortical strains were estimated to be < 20% of periosteal strains and may not have been sufficient to initiate a bone formation response. Our results show that the osteogenic response on the periosteal surface of the ulna depends on peak strain level once a strain threshold is surpassed. The threshold strain is largest distally, where locomotor bone strains are typically higher and smallest proximally where locomotor bone strains are lower.

Recovery periods restore mechanosensitivity to dynamically loaded bone

Bone cells are capable of sensing and responding to mechanical forces, but mechanosensitivity begins to decline soon after the stimulus is initiated. Under continued stimulation, bone is desensitized to mechanical stimuli. We sought to determine the amount of time required to restore mechanosensitivity to desensitized bone cells in vivo by manipulating the recovery time (0, 0.5, 1, 2, 4 or 8 h) allowed between four identical daily loading bouts. We also investigated the osteogenic effectiveness of shorter-term recovery periods, lasting several seconds (0.5, 3.5, 7 or 14 s), introduced between each of 36 identical daily loading cycles. Using the rat tibia four-point bending model, the right tibia of 144 adult female Sprague-Dawley rats was subjected to bending, sham bending or no loading. In the rats receiving recovery periods between loading bouts, histomorphometric measurements from the endocortical surface of the loaded and nonloaded control (left) tibiae revealed more than 100 % higher relative bone formation rates in the 8 h recovery group than in the 0 and 0.5 h recovery groups. Approximately 8 h of recovery was sufficient to restore full mechanosensitivity to the cells. In the rats allowed time to recover between load cycles, 14 s of recovery resulted in significantly higher (66–190 %) relative bone formation rates compared to any of the three shorter recovery periods. In both experiments, bone formation in the sham-bending animals was similar to that in the nonloaded control group. The results demonstrate the importance of recovery periods for (i) restoring mechanosensitivity to bone cells and (ii) maximizing the osteogenic effects of mechanical loading (exercise) regimens.