Hind limb muscles influence the architectural properties of long bones in frogs

The Mechanostat Theory states that osteocytes sense both the intensity and directionality of the strains induced by mechanical usage and modulate the bone design accordingly. In long bones, this process may adapt anterior-posterior and lateral-medial strength to their mechanical environment showing regional specificity. Anuran species are ideal for analyzing the muscle-bone relationships related to the different mechanical stresses induced by their many locomotor modes and habitat uses. This work aimed to explore the relationships between indicators of the force of the most relevant muscles to locomotion and the mechanical properties of femur and tibia fibula in preserved samples of three anuran species with different habitat use (aquatic, arboreal) and locomotion modes (swimmer, jumper, walker/climber). For that purpose, we measured the anatomical cross-sectional area of each dissected muscle and correlated it with the moments of inertia and bone strength indices. Significant, species-specific covariations between muscle and bone parameters were observed. Pseudis platensis, the aquatic swimmer, showed the largest muscles, followed by Boana faber, the jumper and Phyllomedusa sauvagii, the walker/climber. As we expected, bigger muscles correlate with bone parameters in all the species. Nevertheless, smaller muscles also play an important role in bone design. In aquatic species, muscle interaction enhances mostly lateral bending strength throughout the femur and lateral and antero-posterior bending strength in the tibia fibula. In the jumper species, muscles affected the femur and tibia fibula mostly in anterior-posterior bending. In the walker/climber species, responses involving both antero-posterior and lateral bending strengths were observed in the femur and tibia fibula. These results show that bones will be more or less resistant to lateral and antero-posterior bending according to the different mechanical challenges of locomotion in aquatic vs. arboreal habitats. This study provides new evidence of the muscle-bone relationships in three frog species associated with their different locomotion and habitat uses, highlighting the crucial role of muscle in determining the architectural properties of bones.

Biomechanical properties of anuran long bones: correlations with locomotor modes and habitat use

Long bones are subjected to mechanical loads during locomotion that will influence their biomechanical properties through a feedback mechanism (the bone mechanostat). This mechanism adapts the spatial distribution of the mineralized tissue to resist compression, bending and torsion. Among vertebrates, anurans represent an excellent group to study long bone properties because they vary widely in locomotor modes and habitat use, which enforce different skeletal loadings. In this study, we hypothesized that (a) the cortical bone mass, density and design of anuran femur and tibiofibula would reflect the mechanical influences of the different locomotor modes and habitat use, and (b) the relationships between the architectural efficiency of cortical design (cross-sectional moments of inertia) and the intrinsic stiffness of cortical tissue [cortical mineral density; the 'distribution/quality' (d/q) relationship] would describe some inter-specific differences in the efficiency of the bone mechanostat to improve bone design under different mechanical loads. To test this hypothesis, we determined tomographic (peripheral quantitative computed tomography) indicators of bone mass, mineralization, and design along the femur and tibiofibula of four anuran species with different modes of locomotion and use of habitat. We found inter-specific differences in all measures between the distal and proximal ends and mid-diaphysis of the bones. In general, terrestrial-hopper species had the highest values. Arboreal-walker species had the lowest values for all variables except for cortical bone mineral density, which was lowest in aquatic-swimmer species. The d/q relationships showed similar responses of bone modeling as a function of cortical stiffness for aquatic and arboreal species, whereas terrestrial-hoppers had higher values for moments of inertia regardless of the tissue compliance to be deformed. These results provide new evidence regarding the significant role of movement and habitat use in addition to the biomechanical properties of long bones within a morpho-functional and comparative context in anuran species.

Multi-Scale Approach for the Evaluation of Bone Mineralization in Strontium Ranelate-Treated Diabetic Rats

Long-term diabetes mellitus can induce osteopenia and osteoporosis, an increase in the incidence of low-stress fractures, and/or delayed fracture healing. Strontium ranelate (SrR) is a dual-action anti-osteoporotic agent whose use in individuals with diabetic osteopathy has not been adequately evaluated. In this study, we studied the effects of an oral treatment with SrR and/or experimental diabetes on bone composition and biomechanics. Young male Wistar rats (half non-diabetic, half with streptozotocin/nicotinamide-induced diabetes) were either untreated or orally administered 625 mg/kg/day of SrR for 6 weeks. After sacrifice, femora from all animals were evaluated by a multi-scale approach (X-ray diffraction, Fourier transform infrared spectroscopy, inductively coupled plasma optical-emission spectrometry, static histomorphometry, pQCT, and mechanical testing) to determine chemical, crystalline, and biomechanical properties. Untreated diabetic animals (versus untreated non-diabetic) showed a decrease in femoral mineral carbonate content, in cortical thickness and BMC, in trabecular osteocyte density, in maximum load supported at rupture and at yield point, and in overall toughness at mid-shaft. Treatment of diabetic animals with SrR further affected several parameters of bone (some already impaired by diabetes): crystallinity index (indicating less mature apatite crystals); trabecular area, BMC, and vBMD; maximum load at yield point; and structural elastic rigidity. However, SrR was also able to prevent the diabetes-induced decreases in trabecular osteocyte density (completely) and in bone ultimate strength at rupture (partially). Our results indicate that SrR treatment can partially but significantly prevent some bone structural mechanical properties as previously affected by diabetes, but not others (which may even be worsened).

Fibula: The Forgotten Bone-May It Provide Some Insight On a Wider Scope for Bone Mechanostat Control?

The human fibula responds to its mechanical environment differently from the tibia accordingly with foot usage. Fibula structure is unaffected by disuse, and is stronger concerning lateral bending in soccer players (who evert and rotate the foot) and weaker in long-distance runners (who jump while running) with respect to untrained controls, along the insertion region of peroneus muscles. These features, strikingly associated to the abilities of the fibulae of predator and prey quadrupeds to manage uneven surfaces and to store elastic energy to jump, respectively, suggest that bone mechanostat would control bone properties with high selective connotations beyond structural strength.

Meagre effects of disuse on the human fibula are not explained by bone size or geometry

Fibula response to disuse is unknown; we assessed fibula bone in spinal cord injury (SCI) patients and able-bodied counterparts. Group differences were smaller than in the neighbouring tibia which could not be explained by bone geometry. Differential adaptation of the shank bones may indicate previously unknown mechanoadaptive behaviours of bone.

INTRODUCTION

The fibula supports only a small and highly variable proportion of shank compressive load (-8 to +19 %), and little is known about other kinds of stresses. Hence, whilst effects of habitual loading on tibia are well-known, fibula response to disuse is difficult to predict.

METHODS

Therefore, we assessed fibular bone strength using peripheral quantitative computed tomography (pQCT) at 5 % increments from 5 to 90 % distal-proximal tibia length in nine participants with long-term spinal cord injury (SCI; age 39.2 ± 6.2 years, time since injury 17.8 ± 7.4 years), representing a cross-sectional model of long-term disuse and in nine able-bodied counterparts of similar age (39.6 ± 7.8 years), height and mass.

RESULTS

There was no group difference in diaphyseal fibula total bone mineral content (BMC) (P = 0.22, 95 % CIs -7.4 % to -13.4 % and +10.9 % to +19.2 %). Site by group interactions (P < 0.001) revealed 27 and 22 % lower BMC in SCI at 5 and 90 % (epiphyseal) sites only. Cortical bone geometry differed at mid and distal diaphysis, with lower endocortical circumference and greater cortical thickness in SCI than able-bodied participants in this region only (interactions both P < 0.01). Tibia bone strength was also assessed; bone by group interactions showed smaller group differences in fibula than tibia for all bone parameters, with opposing effects on distal diaphysis geometry in the two bones (all Ps < 0.001).

CONCLUSIONS

These results suggest that the structure of the fibula diaphysis is not heavily influenced by compressive loading, and only mid and distal diaphysis are influenced by bending and/or torsional loads. The fibula is less influenced by disuse than the tibia, which cannot satisfactorily be explained by differences in bone geometry or relative changes in habitual loading in disuse. Biomechanical study of the shank loading environment may give new information pertaining to factors influencing bone mechanoadaptation.

Imaging of the muscle-bone relationship

Muscle can be assessed by imaging techniques according to its size (as thickness, area, volume, or alternatively, as a mass) and architecture (fiber length and pennation angle), with values used as an anthropometric measure or a surrogate for force production. Similarly, the size of the bone (as area or volume) can be imaged using MRI or pQCT, although typically bone mineral mass is reported. Bone imaging measures of mineral density, size, and geometry can also be combined to calculate bone's structural strength-measures being highly predictive of bone's failure load ex vivo. Imaging of muscle-bone relationships can, hence, be accomplished through a number of approaches by adoption and comparison of these different muscle and bone parameters, dependent on the research question under investigation. These approaches have revealed evidence of direct, mechanical muscle-bone interactions independent of allometric associations. They have led to important information on bone mechanoadaptation and the influence of muscular action on bone, in addition to influences of age, gender, exercise, and disuse on muscle-bone relationships. Such analyses have also produced promising diagnostic tools for clinical use, such as identification of primary, disuse-induced, and secondary osteoporosis and estimation of bone safety factors. Standardization of muscle-bone imaging methods is required to permit more reliable comparisons between studies and differing imaging modes, and in particular to aid adoption of these methods into widespread clinical practice.

The pQCT 'Bone Strength Indices' (BSIs, SSI). Relative mechanical impact and diagnostic value of the indicators of bone tissue and design quality employed in their calculation in healthy men and pre- and post-menopausal women

The pQCT-assessed Bone Strength Indices (BSI's, SSI) depend on the product of a 'quality' indicator, the cortical vBMD (vCtD), and a 'design' indicator, one of the cross-sectional moments of inertia or related variables (MIs) in long bones. As the MIs vary naturally much more than the vCtD and represent different properties, it could be that the variation of the indices might not reflect the relative mechanical impact of the variation of their determinant factors in different individuals or circumstances. To understand this problem, we determined the vCtD and MI's in tibia scans of 232 healthy men and pre- and post-MP women, expressed in SD of the means calculated for each group, and analyzed the independent influence of 1 SD unit of variation of each factor on that of the indices by multiple correlations. Results showed: 1. that the independent influence of the MIs on the indices was generally larger than that of the vCtD, and 2. that in post-MP women the influence of the vCtD was larger than it was in the other groups. This confirms the view that inter-individual variation of vCtD is comparatively small, and that mechanical competence of human bone is mostly determined by 'design' factors.

Mineralization- and remodeling-unrelated improvement of the post-yield properties of rat cortical bone by high doses of olpadronate

Some pharmacologic effects on bone modeling may not be evident in studies of remodeling skeletons. This study analyzes some effects of olpadronate on cortical bone modeling and post-yield properties in femurs diaphyses (virtually only-modeling bones) of young rats by mid-diaphyseal pQCT scans and bending tests. We studied 20/22 male/female animals traetad orally with olpadronate (45-90 mg/kg/d, 3 months) and 8/9 untreated controls. Both OPD doses enhanced diaphyseal cross-sectional moments of inertia (CSMI) with no change in cortical vBMD and elastic modulus. Yield stiffness and strength were mildly increased. Post-yield strength, deflection and energy absorption were strikingly enhanced. Ultimate strength was enhanced mainly because of effects on bone mass/geometry and post-yield properties. The large improvement of post-yield properties could be explained by improvements in bone geometry. Improvements in bone mass/geometry over weight-bearing needs suggest an enhanced modeling-related response to mechanical stimuli. Effects on tissue microstructural factors (not measured) could not be excluded. Results reveal novel olpadronate effects on bone strength and toughness unrelated to tissue mineralization and stiffness, even at high doses. Further studies could establish whether this could also occur in modeling-remodeling skeletons. If so, they could counteract the negative impact of anti-remodeling effects of bisphosphonates on bone strength.

pQCT-assessed relationships between diaphyseal design and cortical bone mass and density in the tibiae of healthy sedentary and trained men and women

In a pQCT study of running-trained and untrained men and women we had shown that bone mass distribution along the tibia was adapted to the usage-derived stress pattern. To study the possible association between the efficiency of diaphyseal design and bone material stiffness, we extend the analysis of the same sample to correlate pQCT indicators of the distribution (CSMIs), mass (BMC), and density (vBMD) of cortical bone tissue as descriptors of "distribution/mass" (d/m) or "distribution/quality" (d/q) relationships. The d/m and d/c curves followed positive (exponential) and negative (hyperbolic-like) equations, respectively. Distribution curves of r coefficients throughout the bone were all bell-shaped, reaching a maximum towards the mid-diaphysis. The CSMIs and BMC were higher, and vBMD was lower in men than women and in runners than non-runners. The d/m relationships were described by unique curves for all groups while d/q relationships were better adjusted to separate curves for men and women. Results support that: 1. diaphyseal design reflects the relative influence of bending/torsion stress along the bones, tending to minimize bone mass; 2. there is a trade-off between cortical bone "quality" and distribution; 3. d/m and d/q relationships are related to bone mechanical environment, and 4. d/q relationships are affected by sex.

Structural analysis of the human tibia in men with spinal cord injury by tomographic (pQCT) serial scans

Spinal cord injury (SCI), as a primarily neurological disorder that causes muscular atrophy, is well known to be associated with sub-lesional bone losses. These losses are more pronounced from epiphyseal than from diaphyseal regions. We hypothesized that this discrepancy may be explained by anatomical variation in endocortical circumference. Nine men who had attracted SCI 9 to 32 (mean 21.4) years prior to study inclusion were matched to able bodied control (Ctrl) people by age, height and weight. Serial scans by peripheral quantitative computed tomography were obtained from the tibia at steps corresponding to 5%-steps of the tibias length (s05 to s95, from distal to the proximal end of the tibia). As expected, SCI people had lower total bone mineral content (vBMC.tot) than able bodied control people (P<0.001 at all sites). This group difference (DeltavBMC.tot) was more pronounced at the distal and proximal tibia than in the shaft (P<0.001), and it amounted to 51% at s05, to 22% at s40, and to 47% at s95. Both endocortical and periosteal circumference were better predictors of DeltavBMC.tot (R(2)=0.98 and R(2)=0.97, respectively; P<0.001 in both cases) than vBMC.tot (R(2)=0.58, P<0.001), suggesting that anatomical variation in geometry, rather than in bone mass can explain differential rates of bone loss after SCI. Moreover, the s04:s38 ratio in vBMC.tot was found to be 1.00 (95% confidence interval: 0.95-1.05) in the Ctrl group, and 0.63 in the SCI group (P<0.001, 95% confidence interval: 0.54-0.68). These findings offer a rationale to account for the discrepancy between epiphyseal and diaphyseal bone losses following SCI. The suggestion is that the bone adaptive responses involved are limited in time, and that the reduced surface:volume ratio constitutes a limit within the available time window, in particular in the diaphysis. Finally, the drastically reduced s04:s38 vBMC.tot ratio observed in the SCI group in this study provides a rationale to scrutinize this Capozza index also in other studies as a general indicator of immobilisation-induced bone loss.

Structural analysis of the human tibia by tomographic (pQCT) serial scans

This study analyses the evaluation of tomographic indicators of tibia structure, assuming that the usual loading pattern shifts from uniaxial compression close to the heel to a combined compression, torsion and bending scheme towards the knee. To this end, pQCT scans were obtained at 5% intervals of the tibia length (S5-S95 sites from heel to knee) in healthy men and women (10/10) aged 20-40 years. Indicators of bone mass [cortical area, cortical/total bone mineral content (BMC)], diaphyseal design (peri/endosteal perimeters, cortical thickness, circularity, bending/torsion moments of inertia - CSMIs), and material quality [(cortical vBMD (bone mineral density)] were determined. The longitudinal patterns of variation of these measures were similar between genders, but male values were always higher except for cortical vBMD. Expression of BMC data as percentages of the minimal values obtained along the bone eliminated those differences. The correlative variations in cortical area, BMC and thickness, periosteal perimeter and CSMIs along the bone showed that cortical bone mass was predominantly associated with cortical thickness toward the mid-diaphysis, and with bone diameter and CSMIs moving more proximally. Positive relationships between CSMIs (y) and total BMC (x) showed men's values shifting to the upper-right region of the graph and women's values shifting to the lower-left region. Total BMC decayed about 33% from S5 to S15 (where minimum total BMC and CSMI values and variances and maximum circularity were observed) and increased until S45, reaching the original S5 value at S40. The observed gender-related differences reflected the natural allometric relationships. However, the data also suggested that men distribute their available cortical mass more efficiently than women. The minimum amount and variance of mass indicators and CSMIs, and the largest circularity observed at S15 reflected the assumed adaptation to compression pattern at that level. The increase in CSMIs (successively for torsion, A-P bending, and lateral bending), the decrease in circularity values and the changes in cortical thickness and periosteal perimeter toward the knee described the progressive adaptation to increasing torsion and bending stresses. In agreement with the biomechanical background, the described relationships: (i) identify the sites at which some changes in tibial stresses and diaphyseal structure take place, possibly associated with fracture incidence; (ii) allow prediction of mass indicators at any site from single determinations; (iii) establish the proportionality between the total bone mass at regions with highly predominant trabecular and cortical bone of the same individual, suitable for a specific evaluation of changes in trabecular mass; and (iv) evaluate the ability of bone tissue to self-distribute the available cortical bone according to specific stress patterns, avoiding many anthropometric and gender-derived influences.

Age-dependency in bone mass and geometry: a pQCT study on male and female master sprinters, middle and long distance runners, race-walkers and sedentary people

OBJECTIVE

To investigate whether athletic participation allows master athletes to preserve their good bone health into old age.

METHODS

Bone strength indicators of the tibia and the radius were obtained of master runners and race-walkers (n=300) competing at World and European Master Championships and of 75 sedentary controls, all aged 33-94 yrs.

RESULTS

In the tibia, diaphyseal cortical area (Ar.Ct), polar moment of resistance (RPol) and trabecular bone mineral density (vBMD) were generally greater in athletes than controls at all ages. In the athletes, but not the controls, Ar.Ct, RPol (females) and trabecular vBMD were negatively correlated with age (p<0.01). Radius measures were comparable between athlete and control groups at all ages. The amalgamated data revealed negative correlations of age with Ar.Ct, RPol (females), cortical vBMD and trabecular vBMD (males; p<0.005) and positive correlations with endocortical circumference (p<0.001).

CONCLUSION

This cross-sectional study found age-related differences in tibial bone strength indicators of master athletes, but not sedentary controls, thus, groups becoming more similar with advancing age. Age-related differences were noticeable in the radius too, without any obvious group difference. Results are compatible with the notion that bones adapt to exercise-specific forces throughout the human lifespan.

Vertical jump performance after 90 days bed rest with and without flywheel resistive exercise, including a 180 days follow-up

Muscle atrophy and neuromuscular de-conditioning occur in response to space flight and bed-rest. In this study, we investigated the efficacy of flywheel training to conserve jumping power and height during 90 days bed rest. Twenty-four young healthy men underwent strict bed-rest (-6 degrees head down tilt) for 90 days. Eight participants were assigned to a flywheel group (FW) and 16 to a control group (Ctrl). The ground reaction force was measured during vertical jump tests twice during baseline data collection, and on day 4, 7, 14, 90 and 180 of recovery. In half of the participants, jump tests were also performed within minutes after re-ambulation and on four more occasions during the first 2 days of recovery. Jump height was reduced from 40.6 cm (SD 6.1 cm) during the first baseline measurement to 27.6 cm (SD 5.6 cm) on day 4 of recovery in Ctrl, but only from 38.6 cm (SD 3.9 cm) to 34.4 cm (SD 6.5 cm) in FW (P < 0.001). At the same time, peak power was reduced from 47.4 W/kg (SD 8.0 W/kg) to 34.5 W/kg in Ctrl, but only from 46.2 W/kg (6.0 W/kg) to 42.2 W/kg SD 4.6 W/kg) in FW (P < 0.001). Jump height and peak power were completely recovered after 163 and 140 days in Ctrl, respectively, and after 72 and 18 days in FW (regression analysis). In conclusion, flywheel exercise could effectively offset neuromuscular de-conditioning during bed-rest, and led to full recovery at an earlier stage. These findings nourish the hope that adequate training paradigms can fully sustain neuromuscular function under microgravity conditions.

Olpadronate: a new amino-bisphosphonate for the treatment of medical osteopathies

Olpadronate is a nitrogenated bisphosphonate. Although it shares the therapeutic and pharmacological properties of pamidronate and alendronate, it has a greater dosage amplitude, more predictable effects and a greater digestive tolerability than other bisphosphates. Therefore, it may be more appropriate in the treatment of medical osteopathies, by both oral and parenteral routes of administration. According to various experimental and human models, the pharmacological potency of olpadronate is 5- to 10-times higher than that of pamidronate and close to that of alendronate. The two methyl groups bound to the nitrogen atom give the compound a high water solubility, which is about 8-times higher than that of the two other bisphosphonates. The lack of a terminal amino group in the side-chain of the molecule and the absence of crystallised forms of the compound in the digestive tract (due to its high water solubility) may avoid the potential for inducing oesophageal and gastrointestinal side-effects. These features may explain the high tolerability reported after the administration of doses of olpadronate (by the oral route) up to 5- to 10-times higher than the maximum tolerated dose of alendronate in Paget's bone disease and bone metastases, thus widening the possibilities for its clinical usage. In addition, initial pharmacokinetic studies suggest that olpadronate's oral bioavailability would fit into a confidence range of 2-4%, which contrasts with the erratic absorption shown by other highly potent bisphosphonates. The clinical efficacy demonstrated in preliminary studies in Paget's bone disease (including ultra-short treatments), and also in single-dose iv. therapy of hypercalcaemia of malignancies, renders olpadronate among the most promising bisphosphonate compounds, with potential use in the treatment of a variety of bone-involving diseases, such as osteoporosis, malignancies and rheumatoid arthritis.

Biomechanical impact of aluminum accumulation on the pre- and post-yield behavior of rat cortical bone

In order to analyze the effects on whole-bone behavior of aluminum accumulation, 14 rats, aged 90 days, received i.p. doses of 27 mg/day of elemental Al, as Al(OH)3, during 26 weeks, while 14 rats remained as controls. Their femur diaphyses were studied tomographically by peripheral quantitative computed tomography (pQCT) and mechanically tested in bending. The load/deformation curves obtained allowed distinction between effects observed during the linearly elastic (Hookean) and nonlinear (non-Hookean, or plastic) behaviors of bones before and after the yield point, respectively. Treatment reduced the cortical bone mineralization (volumetric bone mineral density [BMD], -2%; P < 0.01), with a negative impact on the bending stiffness (Young's elastic modulus) and the yield stress of cortical bone (-18% and -13%; P < 0.05). Despite the absence of any cortical mass increase (cross-sectional area), improved spatial distribution of cortical tissue concerning anterior-posterior bending stress (cross-sectional moment of inertia, 10%; P < 0.05) occurred through a modulation of modeling drifts. Up to the yield point, neither the structural strength (load supported), the strain, nor the structural stiffness (load/deformation ratio) of the diaphyses were affected. This suggests an adaptive response of bone geometry to the impairment of bone material stiffness. However, Al intoxication significantly reduced the ultimate load and the post-yield fraction of that load (-6% and -27%; P < 0.05). This suggests that the proposed, adaptive response could have improved bone design so as to make it adequate for maintaining a normal pre-yield diaphyseal stiffness and strength according to the bone mechanostat theory, but not so as to provide complete protection of the diaphyseal post-yield (and ultimate) strength. Although a relative inhibition of bone formation could not be discarded, an Al-induced impairment of the bone ability to resist loads beyond the yield point could have caused the unusual disparity observed between effects on bone (elastic) stiffness and (ultimate) strength. In addition, to explain the unusual finding, these results suggest that little-studied microstructural factors (spatial arrangement of elements within the mineralized tissue) affecting the post-yield behavior of cortical bone, regardless of bone mineralization in these and other conditions, ought to be further investigated in specifically designed studies as a novel, promising resource in skeletal research.

Impact of Parathyroid Status and Ca and Vitamin-D Supplementation on Bone Mass and Muscle-Bone Relationships in 208 Belarussian Children after Thyroidectomy because of Thyroid Carcinoma

This observational study analyzes Ca-P metabolism and its impact on bone mass accrual and density and the muscle-bone mass/mass relationships in male and female children and adolescents who were parathyroidectomized because of thyroid carcinoma. Two hundred and eight children and adolescents (119 girls and 89 boys) from Gomel city (Belarus) and its rural surroundings were referred to our institution after having undergone total thyroidectomy for the treatment of advanced papillary thyroid cancer. A subgroup of children with demonstrated primary hypoparathyroidism received dihydrotachysterol (AT-10) and/or Ca supplementation. Among routine procedures over a maximum follow-up period of 5 years (average 3.7 years, maximum 8 visits), whole-body scans were taken using dual energy X-ray absorptiometry (DXA) at each visit in order to determine whole-body bone mineral content (TBMC), projected "areal" bone mineral density (TBMD), total lean mass (TLM) and total fat mass (TFM). The average serum Ca, P and AP concentrations over the whole observation period were significantly different between the groups; however, TBMC z-scores for all studied children were statistically similar in all visits. In girls, no between-group differences in height- and weight-controlled TBMC and TBMD or the TBMC/TLM ratio were observed (ANCOVA) and supplementation exerted no effect on these data, suggesting that the total bone mass accrual was not impaired by PTH deficiency in the studied conditions. However, non-supplemented boys showed lower values of the TBMC/TLM ratio than girls, and supplementation normalized these values in direct correlation with the induced improvement in serum P availability to bone. Results indicate that the primary impairment in parathyroid function and bone metabolism indicators in the thyroidectomized children was unrelated to any measurable change in crude bone mass values. However, in boys this condition impaired the TBMC/TLM ratio in such a way that the administered supplementation could normalize it as a function of improved P availability. Girls' skeleton seemed to have been naturally protected against the negative metabolic effect of the studied condition. An estrogen-induced enhancement of the biomechanical impact of muscle contractions on bone mass and structure could not be excluded in this group.

A DXA study of muscle-bone relationships in the whole body and limbs of 2512 normal men and pre- and post-menopausal women

A whole-body DXA study of 1450 healthy Caucasian individuals [Bone 22 (1998) 683] found that mineral mass, either crude (BMC) or statistically adjusted to fat mass (FM-adjusted BMC), correlated linearly with lean mass (LM, proportional to muscle mass). The results showed similar slopes but decreasing intercepts (ordinate values) in the order: pre-MP women > men > post-MP women > children. This supports the hypothesis that sex hormones influence the control of bone status by muscle strength in all species. Now we further study those relationships in 2512 healthy Hispanic adults (307 men, 753 pre-MP women, 1452 post-MP women), including separate determinations in their upper and lower limbs. The slopes of the BMC or FM-adjusted BMC vs. LM relationships were parallel in all the studied regions. However, region-related differences were found between the ordinates of the curves. In the whole body, the crude-BMC/LM relationships showed the same ordinate differences as previously observed. In the lower limbs, those differences were smaller in magnitude but highly significant, showing the order: pre-MP women > men = post-MP women. In the upper limbs, the decreasing ordinate order was: men > pre-MP women > post-MP women. After fat adjustment of the BMC, order in both limbs was: men > pre-MP women > post-MP women. Parallelism of the curves was maintained in all cases. LM had a larger independent influence on these results than FM, body weight, or age. The parallelism of the curves supports the idea that a common biomechanical control of bones by muscles occurs in humans. Results suggest that sex-hormone-associated differences in DXA-assessed muscle-bone proportionality in humans could vary according to the region studied. This could be related to the different weight-bearing nature of the musculoskeletal structures studied. Besides the obvious anthropometric associations, FM would exert a mechanical effect as a component of body weight, evident in the lower limbs, while muscle contractions would induce a more significant, dynamical effect in both lower and upper limbs. Muscles seem to exert a larger influence than FM, body weight, and age on BMC in the whole body and lower limbs, regardless of the gender and reproductive status of the individual. The muscle-bone relationships studied may provide a rationale for a future differential diagnosis between disuse-related and other types of osteopenia.

Biomechanical background for a noninvasive assessment of bone strength and muscle-bone interactions

New concepts and methods of study in bone biomechanics defy the prevailing idea that bone strength is determined by a systemically-controlled "mineralized mass" which grows until reaching a peak and then is lost at individually-specific rates. In case of bones, "mass" represents actually the substratum of a structure, the stiffness of which does not depend on the mass, but on the intrinsic stiffness and the spatial distribution of the mineralized material. A feed-back system called "bone mechanostat" seems to orient the osteoblastic and osteoclastic processes of bone, modeling and remodeling, according to the sensing by osteocytes of strains caused in the structure by mechanical usage of the skeleton, in specific directions as determined principally by the customary contractions of regional muscles and impact forces. The endocrine-metabolic systems, crucial for the normal skeletal development, modulate the work of osteocytes, blasts and clasts in a systemic way (i.e., not related to a specific direction of the stimuli). Therefore, they tend actually to interact with, rather than contribute to, the biomechanical control of bone structure. Furthermore, no feed-back loop enabling a cybernetic relationship of those systems with bone is known. Instead of passively letting hormones regulate their "mass" in order to optimize their strength, bones would actively self-regulate their architecture following an anisotropic pattern in order to optimize their stiffness (the only known variable to be ever controlled in the skeleton) and strength "despite of" the endocrine systems. Three practical questions derive from those ideas: 1. Osteoporoses are not "intense osteopenias" but "osteopenic fragilities". 2. The diagnosis of osteopenia could be solved densitometrically; but that of bone fragility is a biomechanical problem which requires auxiliary resources for evaluating the stiffness and the spatial distribution of the mineralized material. 3. Osteopenias and osteoporoses should be on time evaluated as related to the mass or strength of the regional muscles, respectively, in order to differentiate between the "primary" (intrinsic lesion of the mechanostat) or "secondary" (systemic) etiologies and the biomechanical origin (disuse) in each case, with important therapeutic implications.

Effects of teriparatide [recombinant human parathyroid hormone (1-34)] on cortical bone in postmenopausal women with osteoporosis

Treatment with teriparatide (rDNA origin) injection [teriparatide, recombinant human parathyroid hormone (1-34) [rhPTH(1-34)]] reduces the risk of vertebral and nonvertebral fragility fractures and increases cancellous bone mineral density in postmenopausal women with osteoporosis, but its effects on cortical bone are less well established. This cross-sectional study assessed parameters of cortical bone quality by peripheral quantitative computed tomography (pQCT) in the nondominant distal radius of 101 postmenopausal women with osteoporosis who were randomly allocated to once-daily, self-administered subcutaneous injections of placebo (n = 35) or teriparatide 20 microg (n = 38) or 40 microg (n = 28). We obtained measurements of moments of inertia, bone circumferences, bone mineral content, and bone area after a median of 18 months of treatment. The results were adjusted for age, height, and weight. Compared with placebo, patients treated with teriparatide 40 microg had significantly higher total bone mineral content, total and cortical bone areas, periosteal and endocortical circumferences, and axial and polar cross-sectional moments of inertia. Total bone mineral content, total and cortical bone areas, periosteal circumference, and polar cross-sectional moment of inertia were also significantly higher in the patients treated with teriparatide 20 microg compared with placebo. There were no differences in total bone mineral density, cortical thickness, cortical bone mineral density, or cortical bone mineral content among groups. In summary, once-daily administration of teriparatide induced beneficial changes in the structural architecture of the distal radial diaphysis consistent with increased mechanical strength without adverse effects on total bone mineral density or cortical bone mineral content.

Bone mass, bone strength, muscle-bone interactions, osteopenias and osteoporoses

Densitometrically, the skeleton is currently conceived as 'a systemically regulated mass of mineralized material that is born, grows, reaches a more or less high peak, and then declines faster or slower as to develop a correspondingly high or low fracture risk'. Alternatively, from a biomechanical point of view, the skeleton can be conceived as 'a biomechanically-regulated structure that can be systemically disturbed (in the cybernetic sense), the strength of which depends on the intrinsic stiffness (material properties) and the spatial distribution (architectural properties) of the mineralized tissue'. The biomechanical feedback system involved (bone 'mechanostat') would not control bone mass to optimize bone strength; it would rather control bone material quality and architecture (through a modulation of bone modeling and remodeling) in order to optimize bone stiffness. The natural stimuli for the bone mechanostat would be the customary strains of bone tissue (sensed by osteocytes) that are induced by gravitational forces and, more importantly, the contractions of regional muscles. According to this view, the development of any bone-weakening disease should be related to either (1) an intrinsic illness of the system (primary disturbances of bone cells), (2) a lack of mechanical stimulation (disuse-induced bone losses), or (3) a systemically-induced shift of the system's setpoint (systemic or secondary bone diseases). This short review aims to conciliate those views: (1) taking profit of the diagnostic possibilities provided by densitometric bone 'mass' determinations; (2) proposing other resources to assess bone mechanical properties; and (3) analyzing the muscle-bone interactions. These are crucial for achieving a differential diagnosis between disuse and primary or secondary bone disturbances, based either (1) on the densitometric determination of bone and muscle masses that would provide an anthropometric diagnosis of osteopenia (not osteoporosis because no extrapolations to bone strength can be made this way) or (2) on the cross-sectional analyses of bone structure or strength and muscle strength provided by bone tomography, magnetic resonance or other techniques that could afford a diagnosis of osteoporosis according to biomechanical criteria.

Dynamics of recovery of morphometrical variables and pQCT-derived cortical bone properties after a short-term protein restriction in maturing rats

Severe protein restriction during the post-weaning period in the rat markedly reduces femoral bone mass and produces a number of alterations in the shaft biomechanical properties. Body weight and femur length show an immediate and complete catch-up during nutritional rehabilitation. The aim of the present investigation was to assess whether the accelerated bone growth that occurs during protein rehabilitation is accompanied by recovery of cortical bone properties. The dynamics of the recovery of both material and geometric properties were thus evaluated on the femoral diaphyses in 45-day old female rats after a 10-day period of dietary protein restriction by peripheral quantitative computed tomography (pQCT). Protein starvation led to marked reduction of both body weight and femoral length (37% and 14% at day 10, respectively) which showed a complete catch-up after 30 d of protein refeeding. Protein restriction was associated with the interruption of the natural increase in cortical area (CtCSA), volumetric cortical bone mineral content (vCtBMC) and volumetric cortical bone mineral density (vCtBMD) which were 19.7, 25.8, and 14%, respectively, in malnourished than in control rats at the end of the protein starvation period. These parameters recovered completely during protein refeeding. Treatment also reduced by 30% both rectangular (xCSMI) and polar (pCSMI) moments of inertia. Although an improvement of these architectural indicators occurred with time, an approximately 20% deficit was still present at the end of the observation period (70 d), as was the bone strength index (BSI). It is concluded that protein restriction affected the adaptation of diaphyseal design which should reduce the mechanical competence of the femoral diaphysis because of an inadequate architectural distribution of cortical bone, and that the alteration did not show complete catch-up during the studied period.

Postmenopausal changes in the distribution of the volumetric BMD of cortical bone. A pQCT study of the human leg

Three different regions of interest (ROIs) were defined in pQCT scans (XCT-3000 machine, Stratec, Germany) taken at the tibial mid-diaphyses of 12 pre-menopausal (pre-MP) and 12 post-menopausal (post-MP) women who were otherwise normal, according to the volumetric bone mineral density (vBMD) value of their corresponding pixels (voxels) as assessed by their respective attenuation values. They were classified as "low-vBMD" (LD-ROI, with a vBMD of 200-400 mg/cm(3)), corresponding chiefly to trabecular-subcortical bone; "medium-vBMD" (MD-ROI, vBMD = 400-800 mg/cm(3)), corresponding mainly to porous cortical bone or cortical-subcortical bone, and "high-vBMD" (HD-ROI, vBMD higher than 800 mg/cm(3)), corresponding to dense cortical bone. The fraction of the total cross-sectional bone area covered by the HD-ROI was 16% higher, and that covered by the MD-ROI 20% lower, in pre-MP than post-MP women. No differences concerning the LDROIs were found. A close, linearly negative relationship was found between the MD- and HD-ROI fractions in all the women together, with no inter-group differences in slope. The Stress-Strain Index (an indicator of the torsional stiffness and strength of the whole bones that involved both the vBMD and the spatial disposition of the HD bone in the cross-section - torsional moment of inertia -) correlated linearly and positively with the cross-sectional area of the HD-ROI, with a higher slope for pre-MP than post-MP women. Qualitatively, a. post-MP women showed a significantly more prevalent discontinuity of the voxels in the HD-ROI than pre-MP women, and b. the tendency of LD-ROIs to accumulate along the mechanically lesseffective (antero-posterior) axis of the image - a characteristic of pre-MP bones - was visually less evident in post-MP bones. These features describe non-invasively some changes induced by menopause in the human tibia that may be critical for defining the skeletal condition and to monitor the effects of treatments addressed either to protect or to improve mechanically the bone structure, beyond the possibilities of standard densitometry.

Analysis of biomechanical effects on bone and on the muscle-bone interactions in small animal models

Animal models are suitable to study many aspects of bone structure and strength. This article reviews some general principles of current bone biomechanics and describes the scope of the available methodology for biomechanical studies of the musculoskeletal system employing those models. The analysis comprises bone and muscle "mass" indicators provided by standard densitometry (DEXA); bone 'mass', 'apparent density', geometry or architectural design and strength and muscle strength indicators that can be determined by peripheral quantitative computed tomography (pQCT), and bone material and structural (whole-bone) properties than can be directly assessed by destructive mechanical tests. Some novel interrelationships that can be investigated that way are discussed, namely, 1. the pathogenetic analysis of the effects on whole-bone strength, 2. the discrimination between mineralization and microstructural factors as determinants of changes in the bone material or structural properties, 3. the evaluation of the interaction of a treatment with the ability of bone 'mechanostat' to optimize the bone architectural design by 'distribution / mass' and 'distribution / quality' curves, and 4. the analysis of effects on the musclebone interactions for a differential diagnosis between 'physiological' or 'disuse' and 'true' osteopenias and osteoporoses.

Bone quality parameters of the distal radius as assessed by pQCT in normal and fractured women

The aim of this study was to test the ability of some indicators of different aspects of bone quality (assessed by peripheral quantitative computed tomography in the distal radius) to discriminate between fractured and nonfractured individuals. The study compared 214 women aged 45-85 years, free of any bone-affecting treatment, of whom 107 had suffered a Colles' fracture in the previous 6 months and 107 did not. The determinations included bone tissue or mineral 'mass' indicators (trabecular, cortical and total volumetric mineral content, cortical bone area); bone 'density' estimates (trabecular, cortical and total volumetric mineral density), and the Cartesian (rectangular) and polar moments of inertia as influences of cross-sectional architecture on resistance to bending and torsional loads, respectively. The influences of body height, weight and age on the tomographic indicators were minimized by adjusting the data according to the partial coefficients of multiple stepwise regressions. The adjusted values of all the indicators were lower in fractured than in nonfractured groups. The prevalence of fractures was directly related to the actual values of the indicators, rather than the age or body habitus of the individuals. The significance of these differences between the assessed indicators decreased in the following order: trabecular 'mass' > trabecular 'density' > cortical or total 'mass' > cortical architecture > total or cortical 'density' indicators. Within the same type of bone, the tissue or mineral 'mass' indicators performed better than the 'density' indicators. The cortical bone density did not give useful information, probably because of technical difficulties. Odds-ratios and receiver-operating characteristic (ROC) analyses confirmed those features. The selected 'cut-off' values of the indicators as determined by the ROC curves (very close to those determined by the inflexion points of the logistic reression curves) may indicate reference limits to detect persons at risk of fracture according to the type of information provided by each variable. These results show that these tomographic indicators discriminate well between fractured and nonfractured individuals, and should be suitable to assess how total, cortical and trabecular bone strength in the distal radius could affect different kinds of strength regardless of the age or body habitus of the individual. Their ability to estimate fracture risk from different biomechanical points of view should be assessed by adequately designed, prospective studies.

Densitometric and tomographic analyses of musculoskeletal interactions in humans

Previous studies with standard densitometry (DXA) have suggested that the bone mass is strongly dependent on the muscle mass in the species, following a similar relationship at any age and sex hormones or related factors potentiate that relationship. Studies with pQCT indicated that the surplus bone mass per unit of muscle mass previously observed in premenopausal women would be stored in skeletal regions with relatively little mechanical relevance, thus avoiding remotion through mechanically oriented remodelling by the bone mechanostat. Scanning the distal radius with pQCT has also showed a highly significant, linear relationship between SSI of the distal radius and the dynamometric maximal bending moment of the forearm in normal men and women. In order to investigate similar relationships in regions that are inaccessible to pQCT, we used spinal radiographs and axial QCT. This study affords additional evidence to the previous references concerning the direct, significant impact of the regional muscle strength on the determination of the tomographic indicators of bone mechanical quality and their indirect repercussion of the skeletal condition (curvature of the spine).

Influence of age and sex in serum osteocalcin levels in thoroughbred horses

In this study, we assessed the potential value of free serum osteocalcin or bone gla protein (BGP), the most abundant non collagenous matrix protein found in bone and dentin, to reflect changes of bone turnover in thoroughbred horses. Levels of osteocalcin were analyzed in serum samples of 54 clinically normal animals divided into three groups (A, B, C) according to age: 8, 16-18 and 24-36 months, in order to determine the standard for young horses of different age and sex. Serum BGP was measured by an in-house developed double antibody radioimmunoassay using bovine antigen. The mean BGP levels (ng/ml) were 45.65 +/- 11.69; 33.65 +/- 16.65; 15.08 +/- 6.70 respectively for groups A, B and C; statistically significant differences were found between groups (A vs B and C; Bvs C). Difference between males and females was found significant in group C with higher values in the females: 18.75 +/- 5.00 against 14.43 +/- 10.47 i n the males. This can be considered a sex related effect on BGP serum levels after the onset of puberty. Correlation coefficient between age and serum BGP for females and males were r 5 20.598 ( P < 0.001) and r 5 200.807 (P < 0.001) respectively. A significant negative linear relationship could be established between these two parameters in males during the growth period. The regression equation between serum BGP and age for males was (month of age = 65.14-1.68. BGP). In the female group the gestation and lactation are variables that lower the correlation coefficient between age and serum BGP levels. These results suggest that serum BGP decreases in thoroughbred horses during the growth period, and significant differences between sexes were found only after the onset of puberty.

Gender-related differences in the relationship between densitometric values of whole-body bone mineral content and lean body mass in humans between 2 and 87 years of age

The mineral, lean, and fat contents of the human body may be not only allometrically but also functionally associated. This report evaluates the influence of muscle mass on bone mass and its age-related changes by investigating these and other variables in both genders in the different stages of reproductive life. We have analyzed the dual-energy X-ray absorptiometry (DEXA)-determined whole-body mineral content (TBMC), lean body mass (LBM), and fat body mass data (FBM) of 778 children and adolescents of both genders, aged 2-20 years [previously reported in Bone 16(Suppl.): 393S-399S; 1995], and of 672 age-matched men and women, aged 20-87 years. Bone mass (as assessed by TBMC) was found to be closely and linearly associated with muscle mass (as reflected by LBM) throughout life. This relationship was similar in slope and intercept in prepubertal boys and girls. However, while keeping the same slope of that relationship (50-54 g increase in TBMC per kilogram LBM): (1) both men and women stored more mineral per unit of LBM within the reproductive period than before puberty (13%-29% and 33%-58%, respectively); (2) women stored more mineral than age-matched men with comparable LBM (17%-29%) until menopause; and (3) postmenopausal women had lower values of bone mineral than premenopausal women, similar to those of men with comparable LBM. Men showed no age effect on the TBMC/LBM relationship after puberty. Multiple regression analyses showed that not only the LBM, but also the FBM and body height (but not body weight), influenced the TBMC, in that decreasing order of determining power. However, neither the FBM nor body height could explain the pre/postpubertal and the gender-related differences in the TBMC/LBM relationship. Accordingly: (1) calculated TBMC/LBM and FBM-adjusted TBMC/LBM ratios were lower in girls and boys from 2-4 years of age until puberty; (2) thereafter, females rapidly reached significantly higher ratios than age-matched men until menopause; and (3) then, ratios for women and age-matched men tended to equalize. A biomechanical explanation of those differences is suggested. Sex hormones or related factors could affect the threshold of the feedback system that controls bone remodeling to adapt bone structure to the strains derived from customary mechanical usage in each region of the skeleton (bone "mechanostat"). Questions concerning whether the mineral accumulation in women during the reproductive period is related or not to an eventual role in pregnancy or lactation, or whether the new bone is stored in mechanically optimal or less optimal regions of the skeleton, are open to discussion.

[Preclinical toxicology of bisphosphonates]

Bisphosphonates regulate bone turnover by inhibiting osteoclastic bone resorption. Due to their pharmacodynamic and pharmacokinetic characteristics, bisphosphonates have a special pharmacotoxicological profile related to their high degree of specificity: low or non-existent distribution in soft tissues and strong affinity for calcified tissues. Some general conclusions may be drawn from the pre-clinical toxicological studies, whose main aim is to identify the toxicity target organ/s and estimate the safety margins of a "prospective therapeutic agent" in laboratory animals. They are based on our own results and on data from the available literature as regards various bisphosphonates: Alendronate, Clodronate, Etidronate, Olpadronate and Pamidronate. Generally, very high doses of bisphosphonates are required to produce in different levels and incidence various extra-skeletical toxic side effects: local reaction, hypocalcemia (and its consequences on the cardiovascular system and the possibility of tetany), affection of the dental structures and renal dysfunction. Most of side effects may be related to the low solubility in biological fluids, the formation of calcium complexes, the potent inhibitory effect of endogenous or induced bone resorption as well as to its main excretion pathway. Some other side effects (on the eye, lungs and liver), may be related to repeated excessive high doses. A safety margin of 200 to 300 : 1 between the "toxic" and "pharmacological" doses may be estimated if the total quantity of Olpadronate given to various animal species in toxicological studies and in pharmacodynamic experimental models (osteopenias due to estrogen deprivation or immobilization and retinoid-induced hypercalcemia) is considered. If the toxic doses in animals are related to the highest doses suggested for human beings, then the ratio increases from 300 to 1000 : 1 depending on the pathology and the route of administration. As regards their effect on the bone, experimental data with the new bisphosphonates suggest a significant dissociation between pharmacologically active doses and those ones producing defective mineralization. The excessive inhibition of bone remodelling, due to the use of high doses in normal animals, is the natural consequence of the pharmacological effect of this family of compounds. A bisphosphonate's toxic potential effect on bone should not be evaluated in normal animals but in particular situations with a high bone turnover. Furthermore, the doses should be adjusted in order to regulate the magnitude of bone remodelling inhibition so as to take it to a normal level without totally suppressing it. Potency, safety margins, doses and proper administration schemes, should be considered as key elements for the optimum use of the therapeutic potentiality of these compounds.

[Effects of bisphosphonates on the mechanical efficiency of normal and osteopenic bones]

Bone mechanical competence (stiffness, strength) at organ level is determined by mechanical quality (intrinsic stiffness) and spatial distribution (macro-architecture) of bone material in cortical tissue (in every bone) and trabecular network (in vertebral bodies). These properties are inter-related and controlled according to mechanical usage by a feed-back mechanism known as mechanostat. Therefore, the effects on bone fragility of any treatment should be evaluated concerning the way they may have affected bone material or geometric properties as well as the mechanostatical interactions between them. Standard densitometry does not provide the necessary data, but some alternative methodologies (as peripheral quantitative computed tomography, pQCT) are being developed to complement or even substitute SPA, DPA or DXA determinations. Bisphosphonate (BP) effects on bone biomechanics have been studied only in animal models. Many sources of variation of results (type of compound, dose, mode of administration, species, race, sex, age, age since menopause, type of bone, remodeling ability of the skeleton, endocrine-metabolic status, interactions with other treatments, etc.) have been reported. In general terms, BPs are beneficial concerning cortical bone strength in purely modeling species (rodents) and trabecular strength in remodeling mammals (dogs, baboons). This positive action at organ level depends on independent improvements in bone macro-architecture (mainly affected by bone modeling) and material stiffness (chiefly affected by bone composition and remodeling). On one hand, bone macro-architecture has been positively affected by BPs in normal (not in ovariectomy (OX), steroid- or disuse-induced osteopenic) animals. On the other, bone material quality has been improved in the latter but not in the former. Mechanostatic interrelationships have been differently affected according to the compound employed. Results reported by ours and other laboratories concerning the three derivatives available nowadays in Argentina were reviewed and summarized. Pamidronate improved small rodents' cortical bone strength and geometric properties at low doses but impaired mineralization, material properties and strength at toxic doses. In normal, remodeling animals it improved mechanical properties in vertebral bodies but not in long bones. It also prevented the negative impact of OX-, steroid- or disuse-induced osteopenia in rats by improving bone material properties without affecting normal mechanostatic interrelationships. Olpadronate exerted positive effects on long-bone strength at any dose in normal rats and mice by improving cross-sectional properties and preserving both mineralization and material properties. These effects were highly dependent upon bone deformability, body weight, and mechanical usage of the limb as an evidence of an anabolic interaction induced on bone modeling and mechanostatic interrelationships. This compound also prevented the OX- or disuse-induced impairment in rat cortical long-bone strength and recovered rat cortical bone when given since 3 months after OX by improving only bone material quality. No interaction with bone mechanostat was detected in these studies. Alendronate effects on bone biomechanics in normal rats and dogs were positive only in long treatments. They were highly dependent on body weight of the animals, hence a positive interaction with bone mechanostat should be hypothesized. It also prevented the negative impact of OX in rat femurs by improving cortical material quality with no effect on cross-sectional properties, i.e., exerting an anti-catabolic interaction with bone mechanostat. The effects of all the three compounds were found positive for bone health, yet their mechanisms of action varied with type of bone and subject condition. A striking dissociation between (positive) effects on bone strength and (variable) effects on bone stiffness was repeatedly observed in these studies. Also an enla

On new opportunities for absorptiometry

Mechanical loads cause bone strains; and muscle forces, not body weight, cause the largest strains. The strains help to control the effects of bone modeling and remodeling on bone strength and "mass." When strains exceed a threshold range, modeling increases bone strength and "mass." When strains stay below a smaller threshold range, remodeling begins removing bone next to marrow. As a result, increasing muscle strength increases bone strength and "mass," and decreasing muscle strength decreases bone strength and "mass." Estrogen apparently lowers the remodeling threshold, which reduces bone losses. Loss of estrogen raises that threshold to cause losses of bone next to marrow. Such facts help to explain: 1. Bone loss in aging adults. 2. An increase in bone "mass" in girls at menarche. 3. The loss of bone during menopause. 4. The greater bone "mass" in obese than in slender subjects, and in weightlifters than in marathon runners. 5. And the pathogenesis of physiologic osteopenias and true osteoporoses. Thus new standards are needed for the relationships between bone and muscle strengths, and as functions of sex, age, race, disease, endocrine status, nutrition, vitamin and mineral intakes, medications, puberty, and menopause. Obtaining those standards and studying such relationships provide many new opportunities for studies that involve dual energy X-ray absorptiometry (DXA) and peripheral quantitative computer tomography (pQCT) and, perhaps some day, ultrasound and magnetic resonance imaging (MRI) techniques.

Long-bone biomechanics in mice selected for body conformation

Two lines of mice divergently selected from the control strain (CBi) against the positive phenotypic correlation between body weight (b.w.) and tail (skeletal) length were obtained (CBi/C: high weight, short tail; CBi/L: low weight, long tail). The selected animals showed a different relationship between body and skeletal masses. To compare the adequacy between biomass and load-bearing ability of the skeleton, and to describe the eventual role of bone mechanostat in the production of these changes, cross-sectional and bending properties of both femur diaphyses were determined in CBi, CBi/C, and CBi/L adult mice of both genders. Cortical bone material quality (elastic modulus) was reduced in the selected lines (p < 0.001), significantly less in CBi/C than in CBi/L. In contrast, cross-sectional design (b.w.-adjusted values of moment of inertia, CSMI) was largely improved (p < 0.001), significantly more in CBi/C than in CBi/L. These effects determined a greater stiffness and strength in CBi/C than in CBi/L or CBi weight-paired mice. The elevations of the negative regression lines between elastic modulus and CSMI ("distribution/quality" curves) decreased in the order CBi/C > CBi/L > CBi. Data show that selection improved diaphyseal stiffness and strength in CBi/C animals because of an architectural overcompensation for the reduced bone material quality. Therefore, an inadequate control of long-bone architectural design as a function of the mechanical quality of cortical bone and b.w. bearing could have been induced in that line. Assuming bone mechanostatic regulation to be genetically programmed, some of the corresponding biological determinants should be transmitted independently, because artificial selection separately affected material quality and architectural design. The possibility of transmission of an inadequate mechanostatic function (inability to adapt bone modeling to bone material quality as a function of the biomass to be supported) was also shown, as some genotypes could express architectural modifications that largely exceed bone material quality deterioration.

Mechanical validation of a tomographic (pQCT) index for noninvasive estimation of rat femur bending strength

Cross-sectional moment of inertia (CSMI) and volumetric cortical bone mineral density (vCtBMD) were assessed by peripheral quantitative computed tomography (pQCT) at femur midshafts from 103 Wistar female rats receiving 0 (n = 12) or 15-1000 mu g/kg/day sc of dexamethasone (n = 46) from 5 to 9 weeks of age, or 0 or 80 mg/kg 3/wk of AI(OH)(3) IP (n = 23,22) from 4 to 10 months of age. A bone strength index (BSI), calculated as the product CSMI x vCtBMD, was found to closely correlate (r = 0.94, R(2) = 0.89, p < 0.001) with the actual, mechanically tested bending breaking force of all bones. Correlation and determination coefficients obtained were higher than those usually reported employing different long-bone strength predictive formulae. The curve approached the origin and was linear throughout the wide range of CSMI, vCtBMD and BSI achieved because of age- and treatment-induced differences, showing a very low standard error of the estimate. Instead, different curve slopes and/or intercepts were found in separate analysis between data from each of the experiments when breaking force was correlated with CSMI or vCtBMD alone, or with the DEXA-assessed BMD of the mechanically assayed bone portion. Results suggest that noninvasive assessment of the BSI by means of pQCT technology provides an original tool for a precise and accurate estimation of long-bone bending strength that can be advantageously applied in crosssectional as well as longitudinal, in vivo studies employing animal models.

Intravenous olpadronate restores ovariectomy-affected bone strength. A mechanical, densitometric and tomographic (pQCT) study

Female Wistar rats aged 3 months were ovariectomized (OX, n = 27). Three months later they were given i.v. doses of 150 (6), 300 (7), or 600 (6) ug/kg 2/wk of olpadronate during 12 weeks or left as OX controls (OXc). Bending fracture load of femur diaphyses, reduced in OXc, was recovered by olpadronate. This effect was paralleled by changes in material quality indicators as DEXA-BMD, tomographic (volumetric) BMD, elastic modulus, and maximum elastic stress of cortical bone. No changes were induced by any of the treatments on cross-sectional area or moment of inertia. Diaphyseal stiffness, not reduced by OX, was enhanced to overnormal values by olpadronate at any dose. None of the treatments affected the normal mechanostatic interrelationships between cross-sectional architecture and bone material quality indicators. The positive effects described point out important differences in bisphosphonate action on bone biomechanics according to the experimental conditions assayed.

Tomographic (pQCT) and biomechanical effects of hPTH(1-38) on chronically immobilized or overloaded rat femurs

Six-month old rats chronically submitted to right hindlimb immobilization (IM) with mechanical overload (OL) of the left leg were treated 1 month later with 200 micrograms/kg/d of hPTH(1-38) for 15 or 75 days. Peripheral quantitative computed tomography (pQCT) scans and bending tests showed that hPTH increased cortical mass and volumetric BMD (vCtBMD) in both legs. However, elastic modulus of cortical bone and diaphyseal load-bearing capacity were improved only in OL bones. Improvement of diaphyseal strength was attributable to that of cortical bone quality, yet a stronger mechanostatic response of cortical modeling to bone material quality was also observed in treated OL bones. Data support hPTH(1-38) use for improving cortical bone mass and strength and point out a physical activity interaction with therapeutic results.

Perspectives of pQCT technology associated to biomechanical studies in skeletal research employing rat models

Assessment of bone material quality and architectural indicators by means of peripheral quantitative computed tomography (pQCT) offers a wide perspective for skeletal research employing noninvasive procedures. Some mechanically-validated examples of these pQCT applications in animal models are described. They concern (a) the analysis of bone mechanostatical interrelationships as shown by experimental "distribution/quality" curves, and (b) the noninvasive determination of bone strength. An attractive attempt to extrapolate the latter to human bone studies is also discussed.

Effects of on/off anabolic hPTH and remodeling inhibitors on metaphyseal bone of immobilized rat femurs. Tomographical (pQCT) description and correlation with histomorphometric changes in tibial cancellous bone

An anabolic effect of hPTH(1-38) (s.c. doses of 200 micrograms/kg/d during 75 days) on trabecular and cortical bone mass is tomographically described in the metaphyseal region of immobilized rat femurs using pQCT technology, in agreement with previous histomorphometrical studies of the proximal tibial metaphyses. Correlations between pQCT and histomorphometrical data showed that this effect derived from a stimulation of endosteal and trabecular bone modeling that induced a transference from trabecular to cortical bone mass. Loss of effects after withdrawal, resulting from a stimulation of bone remodeling, could be total or partially prevented by subsequent s.c. injections of risedronate (5 micrograms/kg/2/wk), 17-B-estradiol (10 micrograms/kg/d) or calcitonin (10 micrograms/kg/d) given during 60 days, in this order of effectiveness. The preventive potency was proportionally related to the reduction induced in histomorphometric indices of bone resorption.

Effects of large doses of olpadronate (dimethyl-pamidronate) on mineral density, cross-sectional architecture, and mechanical properties of rat femurs

As part of a safety-assessment study, doses of 8, 40, and 200 mg/kg per day, 6 days per week, of sodium olpadronate (dimethyl-APD, Me2-APD) were given by gavage to 10-week-old male and female rats during 27 weeks. Only the 200 mg/kg per day dose provoked toxic effects and a meaningful growth depression, regardless of the animal gender. In male animals, doses of 40 or 200 mg/kg per day improved strength, stiffness, and cross-sectional moment of inertia (CSMI) of femur diaphyses despite the toxic effects observed at the highest dose. Changes in bone mechanical properties were a consequence of those induced in CSMI. Regression analyses showed a treatment-induced improvement in bone modeling (as assessed by CSMI) for the same level of bone material stiffness (as expressed by calculated values of elastic modulus). The high dependency of results on body mass bearing suggested that these effects were exerted through an increase in the efficiency of bone mechanostat. Strikingly, they were not evident in female rats. If not related to a lower bone bioavailability of bisphosphonates in female rats as described by others, this phenomenon may have reflected: (1) their a smaller biomass; and/or (2) a less effective mechanostatic regulation of bone architecture derived from a higher bone material stiffness related to male animals. An increase of BMD with a predominance toward the distal region was observed in all femurs studied. This effect, unrelated to the observed changes in mechanical properties, seems to express a lack of remodeling of primary cartilage or bone tissue.

Monophasic dose-response curves of betamethasone on geometric and mechanical properties of femur diaphyses in growing rats

The biomechanical repercussion of the corticoid-induced osteopenia (a severe consequence of long-term glucocorticoid therapy) was studied in cortical bone of small rodents. Growing rats receiving 12.5-3200 micrograms/kg/d of betamethasone (BMS) s.c. for 20 days suffered a log-dose related impairment in body weight gain and in mechanical (fracture load, bending stiffness) and cross-sectional properties (area, moment of inertia) of femur diaphyses. No changes in bone material properties (ability to stand stress, elastic modulus, energy absorption per unit volume) were observed. At variance with the biphasic dose-response curves (positive effects at low-medium doses, negative at high doses) previously obtained with cortisol in a similar model, only negative effects on every variable studied were observed in this experiment. Results suggest that BMS effects on cortical bone biomechanics derived mainly or completely from those induced on bone geometry (biomechanical correlate of corticoid-induced osteopenia) in the assayed conditions. Data are compatible with a BMS-induced change in the setpoint of bone mechanostat. Correlation of bone geometric and biomechanical data with body weight gain showed that the anti-anabolic effects of BMS on bone were proportionally less intense than those exerted on the whole biomass.

Dexamethasone effects on mechanical, geometric and densitometric properties of rat femur diaphyses as described by peripheral quantitative computerized tomography and bending tests

In previous studies with cortisol, betamethasone and oxazacort we attributed glucocorticoid effects on bone biomechanics to changes in bone mass and geometry rather than to an action on bone material properties. In this experiment, groups of 7 rats each received subcutaneous doses of 15.6, 31.2, 62.5, 125, 250, 500 or 1000 micrograms/kg per day of dexamethasone (DMS) and an additional 14 animals were controlled untreated for 4 weeks. Their fresh femurs were then scanned by peripheral quantitative computerized tomography (pQCT; XCT-960, Stratec, Germany) at the midshaft and submitted to three-point bending tests. In consonance with our earlier investigations, a significant, log-dose-related reduction in bone load-bearing capacity was observed, associated with an impairment in bone geometric properties (cross-sectional area and moment of inertia) and in body weight gain. However, the pQCT-assessed volumetric mineral density of cortical bone (vCtBMD; regarded as a material quality indicator in terms of mineralization) was significantly reduced by DMS following a dose-response relationship. Furthermore, a direct association was detected between vCtBMD and diaphyseal load-bearing capacity and stiffness. In contrast with our previous approach, data suggests that, apart from changes in bone geometric properties, glucocorticoid effects on bone material quality--as assessed by vCtBMD changes in this study--seem also to play a significant role in the determination of their biomechanical consequences.

Interrelationships between densitometric, geometric, and mechanical properties of rat femora: inferences concerning mechanical regulation of bone modeling

A compensation for differences in bone material quality by bone geometric properties in femora from two different strains of rats was previously shown by us. A feedback mechanism controlling the mechanical properties of the integrated bones was then proposed, in accordance with Frost's mechanostat theory. Evidence of such a system is now offered by the finding of a negative correlation between the modeling-dependent cross-sectional architecture (moment of inertia) and the mineral-dependent stiffness (elastic modulus) of bone material in the femoral diaphyses of 45 normal Wistar rats of different sexes, ages, and sizes. The strength and stiffness of the integrated diaphyses were found to depend on both cross-sectional inertia and body weight, not on bone mineral density. These findings are interpreted as supporting the hypothesis that the architectural efficiency of diaphyseal cross-sectional design resulting from the spatial orientation of bone modeling during growth is optimized as a function of the body weight-dependent bone strain history, within the constraints imposed by bone stiffness. Results suggest a modulating role of biomass, related to the system set point determination, and explain the usually observed lack of a direct correlation between mineral density and strength or stiffness of long bones in studies of geometrically inhomogeneous populations.

Determination of femur structural properties by geometric and material variables as a function of body weight in rats. Evidence of a sexual dimorphism

Femur diaphyses of male and female Wistar rats were densitometrically and biomechanically assayed. The BMD-dependent material properties were better in female than in male bones, but cross-section geometric properties were better in male femurs. As a result, mechanical properties of the integrated diaphyses were better in males, but differences disappeared after statistical adjustment of data to a common body weight. Results evidence a feed-back mechanism locally controlling the strain-dependent bone modelling and the corresponding cross-sectional design as related to bone stiffness, with a set-point adjusted to animal biomass. A sexual dimorphism of bone biomechanics is also described for the species.

Protective effects of disodium etidronate and pamidronate against the biomechanical repercussion of betamethasone-induced osteopenia in growing rat femurs

To assess the protective effect of bisphosphonates on the biomechanical repercussion of glucocorticoid-induced osteopenia, intraperitoneal doses of 1 or 10 mg/kg/d of disodium etidronate or 1 or 50 mg/kg/day of pamidronate were given to groups of 6 growing rats simultaneously receiving subcutaneous doses of 4.8 mg/kg/day of betamethasone for 20 days. Betamethasone impaired strength and stiffness of femur diaphyses through a reduction of geometric properties, abnormally enhancing bone ability to absorb energy. Both bisphosphonates partially prevented betamethasone effects on diaphyseal stiffness (but not strength) through positive, dose-related effects on material modulus of elasticity and slighter improvements in diaphyseal geometry, avoiding the enhancement of energy-absorbing ability and the subsequent tendency to production of comminute fractures. These results and others obtained treating normal rats with (pamidronate) APD suggest that the sign of bisphosphonate effects on bone biomechanics may depend not only on the type of compound but also on eventual interactions with concomitant treatments.

Interrelationships between geometric and mechanical properties of long bones from three rodent species with very different biomass: phylogenetic implications

In femora from rats or mice, the area and moment of inertia (but not the wall-lumen ratio) of a diaphyseal section correlated with biomass and were determinants of the strength and stiffness of the integrated bone. In otter metacarpals, however, the geometric variable typically associated with body weight and mechanical ability of the integrated bone was the wall-lumen ratio (not the sectional moment of inertia). These differences may be associated with the meaningfulness of body density for natural selection in diving species like otters. A negative relationship between wall-lumen ratio and material modulus of elasticity in bones from the three species pointed out the impossibility of increasing diaphyseal thickness and stiffness at the same time. The results are compatible with the hypothesis that ecologic habits are selectively more important than phylogenetic relationships between species for the determination of bone mechanical properties in the upper vertebrates.

Biphasic dose-response curves of cortisol effects on rat diaphyseal bone biomechanics

Doses of 8, 16 (low), 32, 48, 64 (medium), and 150 (high) mg/kg/day of cortisol were administered to groups of 8 growing rats each during 16 days, and their femurs were then submitted to 3-point bending tests at low strain rate. Low doses had no effect. Medium doses, previously shown to improve calcium (Ca) balance and weight gain in the species, augmented diaphyseal elastic and ultimate strength, stiffness, and plastic-to-elastic deformation ratio with respect to untreated controls. This effect was achieved either by enhancing bone mass (volume, sectional moment of inertia, wall/lumen ration) without changes in material quality parameters (32 mg/kg/day) or, conversely, by increasing bone tissue mechanical properties (stress, modulus of elasticity) not affecting bone geometry (48 and 64 mg/kg/day). The highest dose, known to depress Ca balance and weight gain, impaired diaphyseal mechanical performance in controls by substantially reducing bone mass without major variation in bone material properties, that is, developing a true osteopenic state in mechanical terms. The energy elastically absorbed per unit volume (proportional to the risk of comminute fractures) was greater with the highest dose because of enhanced deformability and diminished bone mass. The biphasic dose-response curves obtained, grossly parallel to those previously demonstrated for metabolic actions of cortisol in the same species, showed that biomechanical repercussion of this treatment on bone depends on different, dose-dependent effects which vary independently in temporal course, intensity, and sign.

Biomechanical effects of the full range of useful doses of (3-amino-1-hydroxypropylidene)-1,1-bisphosphonate (APD) on femur diaphyses and cortical bone tissue in rats

The effects of i.p. doses of 0.016, 0.16, 1.6, 5, 16, 50 and 160 microM/kg/day of APD, over a period of 23 days, on geometric and biomechanical properties of femoral diaphyses in bending were determined in groups of seven growing rats. Both elastic and ultimate strength increased with low doses and decreased with high doses. Geometric (mass) variables (diaphyseal volume, wall/lumen ratio) correlated positively, and material properties (limit elastic stress, modulus of elasticity) negatively, with log dose. Normal mass and improved quality at low doses, and improved mass and impaired quality data at high doses were obtained. No changes in sectional moment of inertia (Ix, an expression of bone architecture) were observed. Biphasic changes in diaphyseal strength expressed the effects of APD on material quality in spite of mass variation. The contrasting lack of changes in Ix may have reflected the blocking effect of APD on osteoclast-osteoblast communication, essential for directional modulation of remodelling.