Bone strength: current concepts

Fourier transform infrared imaging microspectroscopy and tissue-level mechanical testing reveal intraspecies variation in mouse bone mineral and matrix composition

Fracture susceptibility is heritable and dependent upon bone morphology and quality. However, studies of bone quality are typically overshadowed by emphasis on bone geometry and bone mineral density. Given that differences in mineral and matrix composition exist in a variety of species, we hypothesized that genetic variation in bone quality and tissue-level mechanical properties would also exist within species. Sixteen-week-old female A/J, C57BL/6J (B6), and C3H/HeJ (C3H) inbred mouse femora were analyzed using Fourier transform infrared imaging and tissue-level mechanical testing for variation in mineral composition, mineral maturity, collagen cross-link ratio, and tissue-level mechanical properties. A/J femora had an increased mineral-to-matrix ratio compared to B6. The C3H mineral-to-matrix ratio was intermediate of A/J and B6. C3H femora had reduced acid phosphate and carbonate levels and an increased collagen cross-link ratio compared to A/J and B6. Modulus values paralleled mineral-to-matrix values, with A/J femora being the most stiff, B6 being the least stiff, and C3H having intermediate stiffness. In addition, work-to-failure varied among the strains, with the highly mineralized and brittle A/J femora performing the least amount of work-to-failure. Inbred mice are therefore able to differentially modulate the composition of their bone mineral and the maturity of their bone matrix in conjunction with tissue-level mechanical properties. These results suggest that specific combinations of bone quality and morphological traits are genetically regulated such that mechanically functional bones can be constructed in different ways.

Structural basis of growth-related gain and age-related loss of bone strength

If bone strength was the only requirement of skeleton, it could be achieved with bulk, but bone must also be light. During growth, bone modelling and remodelling optimize strength, by depositing bone where it is needed, and minimize mass, by removing it from where it is not. The population variance in bone traits is established before puberty and the position of an individual's bone size and mass tracks in the percentile of origin. Larger cross-sections have a comparably larger marrow cavity, which results in a lower volumetric BMD (vBMD), thereby avoiding bulk. Excavation of a marrow cavity thus minimizes mass and shifts the cortex radially, increasing rigidity. Smaller cross-sections are assembled by excavating a smaller marrow cavity leaving a relatively thicker cortex producing a higher vBMD, avoiding the fragility of slenderness. Variation in cellular activity around the periosteal and endocortical envelopes fashions the diverse shapes of adjacent cross-sections. Advancing age is associated with a decline in periosteal bone formation, a decline in the volume of bone formed by each basic multicellular unit (BMU), continued resorption by each BMU, and high remodelling after menopause. Bone loss in young adulthood has modest structural and biomechanical consequences because the negative BMU balance is driven by reduced bone formation, remodelling is slow and periosteal apposition continues shifting the thinned cortex radially. But after the menopause, increased remodelling, worsening negative BMU balance and a decline in periosteal apposition accelerate cortical thinning and porosity, trabecular thinning and loss of connectivity. Interstitial bone, unexposed to surface remodelling becomes more densely mineralized, has few osteocytes and greater collagen cross-linking, and accumulates microdamage. These changes produce the material and structural abnormalities responsible for bone fragility.

Influence of age at menarche on forearm bone microstructure in healthy young women

BACKGROUND

Shorter estrogen exposure from puberty onset to peak bone mass attainment may explain how late menarche is a risk factor for osteoporosis. The influence of menarcheal age (MENA) on peak bone mass, cortical, and trabecular microstructure was studied in 124 healthy women aged 20.4 +/- 0.6 (sd) yr.

METHODS

At distal radius, areal bone mineral density (aBMD) was measured by dual-energy x-ray absorptiometry, and volumetric bone mineral density (BMD) and microstructure were measured by high-resolution peripheral computerized tomography, including: total, cortical, and trabecular volumetric BMD and fraction; trabecular number, thickness, and spacing; cortical thickness (CTh); and cross-sectional area (CSA).

RESULTS

Median MENA was 12.9 yr. Mean aBMD T score of the whole cohort was slightly positive. aBMD was inversely correlated to MENA for total radius (R = -0.21; P = 0.018), diaphysis (R = -0.18; P = 0.043), and metaphysis (R = -0.19; P = 0.031). Subjects with MENA more than the median [LATER: 14.0 +/- 0.7 (+/-sd) yr] had lower aBMD than those with MENA less than the median (EARLIER: 12.1 +/- 0.7 yr) in total radius (P = 0.026), diaphysis (P = 0.042), and metaphysis (P = 0.046). LATER vs. EARLIER displayed lower total volumetric BMD (315 +/- 54 vs. 341 +/- 56 mg HA/cm(3); P = 0.010), cortical volumetric BMD (874 +/- 49 vs. 901 +/- 44 mg HA/cm(3); P = 0.003), and CTh (774 +/- 170 vs. 849 +/- 191 microm; P = 0.023). CTh was inversely related to CSA (R = -0.46; P < 0.001). In LATER reduced CTh was associated with 5% increased CSA.

CONCLUSIONS

In healthy young adult women, a 1.9-yr difference in mean MENA was associated with lower radial aBMD T score, lower CTh without reduced CSA, a finding compatible with less endocortical accrual. It may explain how late menarche is a risk factor for forearm osteoporosis.

A new quality of bone ultrasound research

Quantitative ultrasound (QUS) methods have strong power to predict osteoporotic fractures, but they are also very relevant for the assessment of bone quality. A representative sample of recent studies addressing these topics can be found in this special issue. Further pursuit of these methods will establish micro-QUS imaging methods as tools for measuring specific aspects of bone quality. Once this is achieved, we will be able to link such data to the clinical QUS methods used in vivo to determine which aspects of bone quality cause QUS to be a predictor of fracture risk that is independent of bone mineral density (BMD). Potentially this could lead to the development of a new generation of QUS devices for improved and expanded clinical assessment. Good quality of basic science work will thus lead to good quality of clinical patient examinations on the basis of a more detailed assessment of bone quality.

The Asn19Lys substitution in the osteoclast inhibitory lectin (OCIL) gene is associated with a reduction of bone mineral density in postmenopausal women

Osteoclast inhibitory lectin (OCIL) is a newly recognized inhibitor of mouse and human osteoclast differentiation whose cellular expression is similar to that of receptor activator of nuclear factor kappaB (RANKL). The main objective of the present work was to elucidate whether naturally occurring single-nucleotide polymorphisms (SNPs) in this gene could be associated with bone mass in postmenopausal women. To that end, we studied the association of bone mineral density (BMD) measured by dual-energy X-ray absorptiometry with two nonsynonymous SNPs in the OCIL gene resulting in Asn19Lys and Leu23Val substitutions in a population of 500 postmenopausal Spanish women. A weak association was detected for Asn19Lys SNP with femoral neck (FN) BMD and lumbar spine (LS) BMD in the whole population. When the population was stratified by age, however, the association was strong in older women (> or =53 years). Thus, in this group of participants, women with CG/GG genotype displayed reductions of 5.6% and 6.7% in FN BMD and LS BMD adjusted by age and body mass index (BMI), respectively, compared to women with CC genotype. The Asn19Lys SNP alleles explained about 7% of BMD variance in older women but only 1.7-3.9% in the whole population in regression models including age and BMI. In conclusion, women with a lysine (GG genotype) at position 19 of the OCIL protein displayed lower BMD at femoral neck and at lumbar spine sites than women having an asparagine residue. Since the OCIL protein inhibits osteoclast differentiation, this amino acid substitution could have consequences for OCIL functionality.

Role of genetic background in determining phenotypic severity throughout postnatal development and at peak bone mass in Col1a2 deficient mice (oim)

Osteogenesis imperfecta (OI) is a genetically and clinically heterogeneous disease characterized by extreme bone fragility. Although fracture numbers tend to decrease post-puberty, OI patients can exhibit significant variation in clinical outcome, even among related individuals harboring the same mutation. OI most frequently results from mutations in type I collagen genes, yet how genetic background impacts phenotypic outcome remains unclear. Therefore, we analyzed the phenotypic severity of a known proalpha2(I) collagen gene defect (oim) on two genetic backgrounds (congenic C57BL/6J and outbred B6C3Fe) throughout postnatal development to discern the phenotypic contributions of the Col1a2 locus relative to the contribution of the genetic background. To this end, femora and tibiae were isolated from wildtype (Wt) and homozygous (oim/oim) mice of each strain at 1, 2 and 4 months of age. Femoral geometry was determined via muCT prior to torsional loading to failure to assess bone structural and material biomechanical properties. Changes in mineral composition, collagen content and bone turnover were determined using neutron activation analyses, hydroxyproline content and serum pyridinoline crosslinks. muCT analysis demonstrated genotype-, strain- and age-associated changes in femoral geometry as well as a marked decrease in the amount of bone in oim/oim mice of both strains. Oim/oim mice of both strains, as well as C57BL/6J (B6) mice of all genotypes, had reduced femoral biomechanical strength properties compared to Wt at all ages, although they improved with age. Mineral levels of fluoride, magnesium and sodium were associated with biomechanical strength properties in both strains and all genotypes at all ages. Oim/oim animals also had reduced collagen content as compared to Wt at all ages. Serum pyridinoline crosslinks were highest at two months of age, regardless of strain or genotype. Strain differences in bone parameters exist throughout development, implicating a role for genetic background in determining biomechanical strength. Age-associated improvements indicate that oim/oim animals partially compensate for their weaker bone material, but never attain Wt levels. These studies indicate the importance of genetic background in determining phenotypic severity, but the presence of the proalpha2(I) collagen gene defect and age of the animal are the primary determinants of phenotypic severity.

Age trends in proximal femur geometry in men: variation by race and ethnicity

Data on bone architecture in diverse male populations are limited. We examined proximal femur geometry in 1,190 black, Hispanic, and white men. Cross-sectional analyses indicate greatest bone strength among black men, and greater age-related differences in bone strength among Hispanic men than other subjects at the narrow neck and intertrochanter regions of the proximal femur.

INTRODUCTION

Although race/ethnic differences in bone mass are well-documented, less is known about differences in bone architecture. We examined proximal femur geometry in a diverse, randomly-sampled population of 1,190 community-dwelling men (age 30-79 y).

METHODS

Dual X-ray absorptiometry scans were obtained for 355 black, 394 Hispanic, and 441 white subjects. Measures were obtained for the narrow neck (NN), intertrochanter (IT) and shaft regions of the proximal femur via hip structural analysis. Analyses considered bone mineral density (BMD, g/cm2), outer diameter (cm), cross-sectional area (CSA, cm2), section modulus (Z, cm3), and buckling ratio (BR). Results were adjusted for height, weight and physical activity level.

RESULTS

Black subjects exhibited greater age-specific BMD, CSA and Z, than their white counterparts. For instance, at age 50 y, NN BMD was approximately 11% higher among black men (p < 0.001). Hispanic men exhibited sharper age-related differences in NN and IT BMD than did others. IT BMD, for instance, decreased by 2.4% with 10 y age among Hispanic subjects, but had virtually no age trend in others (p < 0.001).

CONCLUSIONS

These results imply greater bone strength among black American men than among their white counterparts, and may indicate elevated fracture risk among older Hispanic American subpopulations.

Topography of acoustical properties of long bones: from biomechanical studies to bone health assessment

The article presents a retrospective view on the assessment of long bones condition using topographical patterns of the acoustic properties. The application of ultrasonic point-contact transducers with exponential waveguides on a short acoustic base for detailed measurements in human long bones by the surface transmission was initiated during the 1980s in Latvia. The guided wave velocity was mapped on the surface of the long bones and the topographical patterns reflected the biomechanical peculiarities. Axial velocity profiles obtained in vivo by measurements along the medial surface of tibia varied due to aging, hypokinesia, and physical training. The method has been advanced at Artann Laboratories (West Trenton, NJ) by the introduction of multifrequency data acquisition and axial scanning. The model studies carried out on synthetic phantoms and in bone specimens confirmed the potential to evaluate separately changes of the bone material properties and of the cortical thickness by multifrequency acoustic measurements at the 0.1 to 1 MHz band. The bone ultrasonic scanner (BUSS) is an axial mode ultrasonometer developed to depict the acoustic profile of bone that will detect the onset of bone atrophy as a spatial process. Clinical trials demonstrated a high sensitivity of BUSS to osteoporosis and the capability to assess early stage of osteopenia.

Health of herring gulls (Larus argentatus) in relation to breeding location in the early 1990s. III. Effects on the bone tissue

Health effects associated with the Great Lakes environment were assessed in adult herring gulls (Larus argentatus) in the early 1990s, including the size and quality of their bones. Femurs were excised from 140 individuals from 10 colonies distributed throughout the Great Lakes and 2 reference colonies in Lake Winnipeg (freshwater) and the Bay of Fundy (marine). Femurs of gulls from the Great Lakes differed from the freshwater or marine reference for 9 of 12 variables of size, composition, and strength assessed using peripheral quantitative computed tomography (pQCT) and biomechanical testing. Femurs of Great Lakes gulls were significantly smaller in length (-2.9%), periosteal circumference (-2.4%), and cross-sectional area (-5.4%) than freshwater reference birds. Femurs of the Great Lakes gulls had a lower significant cortical bone mineral content (-8.1%) and density (-2%) than the marine reference. A significant increase in the amount the bone could bend before it broke (+34%) and the energy required to break it (+44%) and a significant decrease (-16.3%) in stiffness during three-point biomechanical bending test were also detected in Great Lakes versus the freshwater gulls. These differences are indicative of impaired mineralization. When divided into high and low 2,3,7,8-tetrachlorodibenzo-p-dioxin toxicity equivalent (TCDD-TEQ) colonies, the amount the bone could bend before it broke and the energy required to break it were significantly higher in the high TEQ colonies, but not high polychlorinated biphenyl (PCB) colonies. Breeding location and dietary choices of Great Lakes herring gulls in the early 1990s resulted in modulations of physiological processes that affected the size, mineralization, and biomechanical properties of bone.

Novel loci regulating bone anabolic response to loading: expression QTL analysis in C57BL/6JXC3H/HeJ mice cross

Variations in the expression levels of bone marker genes among the inbred strains of mice in response to mechanical loading (ML) are largely determined by genetic factors. To explore this, we performed four-point bending on tibiae of 10-week female F2 mice of B6XC3H cross using 9N at 2 Hz, 36 cycles, once per day for 12 days. We collected tibiae from these mice for RNA extraction. We then measured the expression changes of bone marker genes, bone sialoprotein (BSP), alkaline phosphatase (ALP) and housekeeping genes, beta-actin and peptidylprolyl isomerase A (PPIA), by using real-time PCR in both the loaded and the non-loaded tibiae of F2 mice (n=241). A genome-wide scan was performed using 111 micro satellite markers in DNA sample collected from these mice. Mean increase in gene expression, expressed as fold change, ranges from 2.8 to 3.0 for BSP and 2.7 to 2.8 for ALP. Both showed a skewed distribution with a heritability response of 87 to 91%. Absence of significant correlation between the increased gene expression vs. body weight (BW) and bone size (BS) suggests that bone response to loading is independent of BS or BW. Non-parametric mapping (MapQTL program 5) revealed that BSP and ALP expression in response to bending was regulated by several significant and suggestive QTL: Loci regulating both BSP and ALP were located on Chr 8 (60.1 cM), 16 (45.9 cM), 17 (14.2 cM), 18 (38.0 cM) and Chr 19 (3.3 cM); Loci specific to BSP were found on Chrs 1 (LOD score 10.4 at 91.8 cM), 5 (5.2 at 73.2 cM) and 9 (7.0 at 13.1 cM); Loci regulating only ALP were found on Chrs 1 (7.6 at 46 and 75.4 cM), 3 (8.3 at 47 cM) and 4 (5.6 at 54.6 cM). QTLs on Chrs 1, 3, 8, 9, 17 and 18 correspond to QTLs we previously reported by pQCT measurements, thus validating these findings. In addition, we found that the QTL associated with non-loaded tibiae for BSP and ALP on Chrs 4, 16 and 18 was identical to the QTLs associated with ML. This finding suggests that regions on these chromosomes are responsible for natural variation in expression of BSP and ALP as well as for ML. This is the first expression study to provide evidence for the presence of multiple genetic loci regulating bone anabolic response to loading in the B6XC3H intercross and will lead to a better understanding of how exercise improves the skeletal mass.

Runx2: of bone and stretch

Runx2 is a key transcriptional modulator of osteoblast differentiation that plays a fundamental role in osteoblast maturation and homeostasis. Runx2-null mice despite normal skeletal patterning have no osteoblasts and consequently bone tissue. Mutations of the runx2 gene in humans cause cleidocranial dysplasia. As a member of the Runx family of transcription factors, Runx2 operates by binding to the osteoblast-specific cis-acting element 2 (OSE2), which is found in the regulatory region of all main osteoblast-related genes controlling their expression. Its expression and/or activity are dictated by a number of different external cues while multiple signalling pathways that affect osteoblast function merge to and are integrated by Runx2. Among the various stimuli that modulate Runx2 activity, mechanical loading (strain/stretching) has been revealed to be one of the most critical signals that connect Runx2 with osteoblast function and bone remodelling through mechanotransduction.

Effects of surgical holes in mouse tibiae on bone formation induced by knee loading

Loads applied directly to the knee (knee loading) have recently been demonstrated to induce anabolic responses in femoral and tibial cortical bone. In order to examine the potential role of intramedullary pressure in generating those knee loading responses, we investigated the effects of drilling surgical holes that penetrated into the tibial medullary cavity and thereby modulated pressure alteration. Thirty-nine C57/BL/6 female mice in total were used with and without surgical holes, and the surgical holes were monitored with micro CT and histology. The left knee was loaded for 3 days, and the contralateral limb was treated as a sham-loaded control. Mice were sacrificed for bone histomorphometry 2 weeks after the last loading. Although the surgical hole induced bone formation in both loaded and non-loaded tibiae, due to regional and systemic acceleratory phenomenon the anabolic effect of knee loading was substantially diminished. Without the holes, knee loading significantly elevated cross-sectional cortical area, cortical thickness, mineralizing surface, mineral apposition rate, and bone formation rate on the periosteal surface. For example, the rate of bone formation was elevated 2.1 fold (p<0.001; middle diaphysis--50% site from the knee along the length of tibiae) and 2.7 fold (p<0.01; distal diaphysis--75% site). With the surgical holes, however, knee loading did not provide significant enhancement either at the 50% or 75% site in any of the histomorphometric measurements (p>0.05). The results support the idea that alteration of intramedullary pressure is necessary for knee loading to induce bone formation in the diaphysis.

Knee loading dynamically alters intramedullary pressure in mouse femora

Dynamic mechanical loads have been known to stimulate bone formation. Many biophysical factors such as number of daily loading cycles, bone strain, strain-induced interstitial fluid flow, molecular transport, and modulation of intramedullary pressure have been considered as potential mediators in mechanotransduction of bone. Using a knee loading modality that enhances anabolic responses in mouse hindlimb, we addressed a question: Do oscillatory loads applied to the knee induce dynamic alteration of intramedullary pressure in the femoral medullary cavity? To answer this question, mechanical loads were applied to the knee with a custom-made piezoelectric loader and intramedullary pressure in the femoral medullary cavity was measured with a fiber optic pressure sensor. We observed that in response to sinusoidal forces of 0.5 Hz and 10 Hz, pressure amplitude increased up to 4-N loads and reached a plateau at 130 Pa. This amplitude significantly decreased with a loading frequency above 20 Hz. To confirm alteration of intramedullary pressure, real-time motion of microparticles in a glass tube inserted to the femoral medullary cavity ex vivo was visualized. Taken together, these data reveal that knee loading dynamically alters intramedullary pressure as a function of loading intensities and frequencies.

Phenotypic plasticity and functional asymmetry in response to grip forces exerted by intercollegiate tennis players

This study examined phenotypic and functional responses to extreme asymmetry in hand use, and recorded the grip forces that occur during the tennis serve to assess the forces that contribute to asymmetry. Compressive grip performance was measured for Division I collegiate tennis players (24 female, 24 male) and nonathlete college students (18 females, 17 males) during three experiments: single-repetition maximum voluntary contraction, 30 consecutive repetitions, and a 30-s static hold. Tennis players had significant asymmetry in both forearm circumference and grip strength. The dominant hand of female tennis players produced 25% more force than the opposite hand, while the difference between hands of male tennis players was 18%. However, endurance over 30-repetitions and during the 30-s hold did not significantly differ between the limbs of tennis players. No significant asymmetry in forearm measurements, grip strength, or endurance was detected between the limbs of nonathletes. Grip forces generated during the tennis serve were recorded for 4 male tennis players, using a racket with a dynamometer incorporated into the handle. Peak handgrip forces generated during the serve (150-250 N) averaged 31-44% of the maximum grip force of the participant. Tennis players have increased muscle mass and strength of the dominant limb. Moderate forces, occurring hundreds of times each match and practice, are likely the primary forces contributing to this morphological and functional asymmetry.