Gut microbial-derived short-chain fatty acids and bone: a potential role in fracture healing.

Bone healing complications such as delayed healing or non-union affect 5-10 % of patients with a long-bone fracture and lead to reduced quality of life and increased health-care costs. The gut microbiota and the metabolites they produce, mainly short-chain fatty acids (SCFAs), have been shown to impact nearly all organs of the human body including bone. SCFAs show broad activity in positively influencing bone healing outcomes either by acting directly on cell types involved in fracture healing, such as osteoblasts, osteoclasts, chondrocytes and fibroblasts, or indirectly, by shaping an appropriate anti-inflammatory and immune regulatory response. Due to the ability of SCFAs to influence osteoblast and osteoclast differentiation, SCFAs may also affect the integration of orthopaedic implants in bone. In addition, SCFA-derivatives have already been used in a variety of tissue engineering constructs to reduce inflammation and induce bone tissue production. The present review summarises the current knowledge on the role of the gut microbiota, in particular through the action of SCFAs, in the individual stages of bone healing and provides insights into how SCFAs may be utilised in a manner beneficial for fracture healing and surgical reconstruction.

The effects of metabolic syndrome, obesity, and the gut microbiome on load-induced osteoarthritis.

OBJECTIVE: Metabolic syndrome is characterized by obesity, hyperglycemia, hypertension, insulin resistance, and dyslipidemia. Metabolic syndrome is associated with osteoarthritis (OA), but it is unclear if the association is attributable to increased mechanical loading on joints caused by obesity or other aspects of metabolic syndrome. Here we examined the effects of altered metabolism, obesity, and the gut microbiome on load-induced OA. DESIGN: Cartilage damage was induced through cyclic compressive loading in four groups of adult male mice: Toll-like receptor-5 deficient (TLR5KO) mice that develop metabolic syndrome due to alterations in the gut microbiome, TLR5KO mice submitted to chronic antibiotics to prevent metabolic syndrome (TLR5KOΔMicrobiota), C57BL/6J mice fed a high fat diet to cause obesity (HFD), and untreated C57BL/6J mice (WT). Loading was applied for 2 weeks (n = 10-11/group) or 6 weeks (n = 10-11/group). RESULTS: After 2 weeks of loading, cartilage damage (OARSI score) was not different among groups. After 6 weeks of loading, HFD mice had increased load-induced cartilage damage, while TLR5KO mice had cartilage damage comparable to WT mice. TLR5KOΔMicrobiota mice had less cartilage damage than other groups. HFD mice had elevated serum inflammatory markers. Each group had a distinct gut microbiome composition. CONCLUSIONS: Severe obesity increased load-induced cartilage damage, while milder changes in adiposity/metabolic syndrome seen in TLR5KO mice did not. Furthermore, the effects of systemic inflammation/obesity on cartilage damage depend on the duration of mechanical loading. Lastly, reduced cartilage damage in the TLR5KOΔMicrobiota mice suggests that the gut microbiome may influence cartilage pathology.

Bone Mechanical Function and the Gut Microbiota.

The primary function of bone in the body is to resist mechanical forces. Impairment of the mechanical performance of bone is therefore the primary clinical challenge presented by bone disease. Failure to resist forces associated with activities of daily living leads to fragility fracture. In this chapter we review the characteristics of bone that influence mechanical performance and fracture risk, how bone remodeling and modeling alter mechanically relevant characteristics of bone, and the potential for the gut microbiome to alter bone mechanical performance and risk of fragility fracture. The ability of bone to resist fragility fracture is determined by characteristics of bone tissue at scales ranging from nanometers to centimeters. Bone remodeling and modeling can influence whole bone shape and density, but the effects of bone remodeling on bone tissue mechanical properties are not as well understood. The gut microbiome may influence bone by altering nutrient absorption, stimulating the immune system or through translocation of microbes and microbial products across the gut endothelium. Although there is evidence that the microbiome can alter bone density and bone remodeling, the ability of the microbiome to cause changes in tissue material properties, whole bone strength, and fragility fracture remains to be determined.

Theoretical consideration of the effect of drug holidays on BMD and tissue age.

UNLABELLED: It has been suggested that some patients undergoing prolonged treatment for osteoporosis with anti-resorptive agents may benefit from discontinuing treatment. Here we use a computer simulation of bone cell activity to estimate changes in bone mineral density (BMD) and tissue age when treatment is discontinued. INTRODUCTION: Although anti-resorptive agents are effective at reducing fracture risk, questions remain regarding how long patients should continue treatment and how long treatment should be discontinued. Suspending treatment as part of a drug holiday may reduce the risk of adverse effects, but may also lead to reduced BMD. METHODS: We use a computer simulation of the bone remodeling process to estimate how BMD and mean tissue age are changed after treatment is suspended. Mean tissue age is studied because increased tissue age has been associated with impaired bone quality and has been linked to the risk of adverse effects. RESULTS: Our simulations suggest that BMD gains from anti-resorptive therapy can be lost over time, especially with anti-resorptive agents that have little residual effects. With regard to mean tissue age, the simulations suggest that increases in tissue age from anti-resorptive treatment are long lasting; increases in mean tissue age caused by treatment may remain for as long as 15 years after treatment is suspended. After stopping treatment, reductions in BMD are expected to occur long before mean tissue age returns to normal. CONCLUSIONS: Our simulations suggest that, when using a long-lasting anti-resorptive agent, 1- to 5-year drug holidays may have little effect on BMD in most patients but that drug holiday intervals that maintain BMD are unlikely to reverse alterations in tissue age caused by treatment. Our analysis echoes recent reviews suggesting patient selection and monitoring when anti-resorptive treatment is discontinued.

Microbiome-induced Increases and Decreases in Bone Tissue Strength can be Initiated After Skeletal Maturity.

Recent studies in mice have indicated that the gut microbiome can regulate bone tissue strength. However, prior work involved modifications to the gut microbiome in growing animals and it is unclear if the same changes in the microbiome, applied later in life, would change matrix strength. Here we changed the composition of the gut microbiome before and/or after skeletal maturity (16 weeks of age) using oral antibiotics (ampicillin + neomycin). Male and female mice (n=143 total, n=12-17/group/sex) were allocated into five study groups:1) Unaltered, 2) Continuous (dosing 4-24 weeks of age), 3) Delayed (dosing only 16-24 weeks of age), 4) Initial (dosing 4-16 weeks of age, suspended at 16 weeks), and 5) Reconstituted (dosing from 4-16 weeks following by fecal microbiota transplant from Unaltered donors). Animals were euthanized at 24 weeks of age. In males, bone matrix strength in the femur was 25-35% less than expected from geometry in mice from the Continuous (p= 0.001), Delayed (p= 0.005), and Initial (p=0.040) groups as compared to Unaltered. Reconstitution of the gut microbiota, however, led to a bone matrix strength similar to Unaltered animals (p=0.929). In females, microbiome-induced changes in bone matrix strength followed the same trend as males but were not significantly different, demonstrating sex-related differences in the response of bone matrix to the gut microbiota. Minor differences in chemical composition of bone matrix were observed (Raman spectroscopy). Our findings indicate that microbiome-induced impairment of bone matrix in males can be initiated and/or reversed after skeletal maturity. The portion of the femoral cortical bone formed after skeletal maturity (16 weeks) is small; however, this suggests that microbiome-induced changes in bone matrix occur without osteoblast/osteoclast turnover using an, as of yet unidentified mechanism. These findings add to evidence that the mechanical properties of bone matrix can be altered in the adult skeleton.

TET enzymes and 5hmC epigenetic mark: new key players in carcinogenesis and progression in gynecological cancers.

DNA methylation is an epigenetic mechanism involving the transfer of a methyl group onto the C5 position of the cytosine to form 5-methylcytosine (5mC). In general, DNA methylation in cancer is associated with the repression of the expression of tumor suppressor genes (TSG) and the demethylation with the overexpression of oncogenes. DNA methylation was considered a stable modification for a long time, but in 2009, it was reported that DNA methylation is a dynamic modification. The Ten-Eleven-Translocations (TET) enzymes include TET1, TET2, and TET3 and participate in DNA demethylation through the oxidation of 5mC to 5-hydroxymethylcytosine (5hmC). The 5hmC oxidates to 5-formylcytosine (5fC) and 5-carboxylcitosine (5caC), which are replaced by unmodified cytosines via Thymine-DNA Glycosylase (TDG). Several studies have shown that the expression of TET proteins and 5hmC levels are deregulated in gynecological cancers, such as cervical (CC), endometrial (EC), and ovarian (OC) cancers. In addition, the molecular mechanisms involved in this deregulation have been reported, as well as their potential role as biomarkers in these types of cancers. This review shows the state-of-art TET enzymes and the 5hmC epigenetic mark in CC, EC, and OC.

From Beyond the Pale to the Pale Riders: The Emerging Association of Bacteria with Oral Cancer.

Oral cancer, predominantly oral squamous cell carcinoma (OSCC), is the eighth-most common cancer worldwide, with a 5-y survival rate <50%. There are numerous risk factors for oral cancer, among which periodontal disease is gaining increasing recognition. The creation of a sustained dysbiotic proinflammatory environment by periodontal bacteria may serve to functionally link periodontal disease and oral cancer. Moreover, traditional periodontal pathogens, such as Porphyromonas gingivalis, Fusobacterium nucleatum, and Treponema denticola, are among the species most frequently identified as being enriched in OSCC, and they possess a number of oncogenic properties. These organisms share the ability to attach and invade oral epithelial cells, and from there each undergoes its own unique molecular dialogue with the host epithelium, which ultimately converges on acquired phenotypes associated with cancer, including inhibition of apoptosis, increased proliferation, and activation of epithelial-to-mesenchymal transition leading to increased migration of epithelial cells. Additionally, emerging properties of structured bacterial communities may increase oncogenic potential, and consortia of P. gingivalis and F. nucleatum are synergistically pathogenic within in vivo oral cancer models. Interestingly, however, some species of oral streptococci can antagonize the phenotypes induced by P. gingivalis, indicating functionally specialized roles for bacteria in oncogenic communities. Transcriptomic data support the concept that functional, rather than compositional, properties of oral bacterial communities have more relevance to cancer development. Collectively, the evidence is consistent with a modified polymicrobial synergy and dysbiosis model for bacterial involvement in OSCC, with driver mutations generating a conducive microenvironment on the epithelial boundary, which becomes further dysbiotic by the synergistic action of bacterial communities.

Trabecular microfracture precedes cortical shell failure in the rat caudal vertebra under cyclic overloading.

Microscopic tissue damage has been observed in otherwise healthy cancellous bone in humans and is believed to contribute to bone fragility and increased fracture risk. Animal models to study microscopic tissue damage and repair in cancellous bone would be useful, but it is currently not clear how loads applied to a whole animal bone are related to the amount and type of resulting microdamage in cancellous bone. In the current study we determine the relationship between applied cyclic compressive overloading and the resulting amount of microdamage in isolated rat tail vertebrae, a bone that has been used previously for in vivo loading experiments. Rat caudal vertebrae (C7-C9, n = 22) were potted in bone cement and subjected to cyclic compressive loading from 0 to 260 N. Loading was terminated in the secondary and tertiary phases of the creep-fatigue curve using custom data-monitoring software. In cancellous bone, trabecular microfracture was the primary form of microdamage observed with few microcracks. Trabecular microfracture prevalence increased with the amount of cyclic loading and occurred in nine out of 10 specimens loaded into the tertiary phase. Only small amounts of microdamage were observed in the cortical shell of the vertebrae, demonstrating that, under axial cyclic loading, damage occurs primarily in regions of cancellous bone before overt fracture of the bone (macroscopic cracks in the cortical shell). These experiments in isolated rat tail vertebrae suggest that it may be possible to use an animal model to study the generation and repair of microscopic tissue damage in cancellous bone.

Biomechanical allometry in hominoid thoracic vertebrae.

Considerable differences in spinal morphology have been noted between humans and other hominoids. Although comparative analyses of the external morphology of vertebrae have been performed, much less is known regarding variations in internal morphology (density) and biomechanical performance among humans and closely related non-human primates. In the current study we utilize density calibrated computed tomography images of thoracic vertebral bodies from hominoids (n=8-15 per species, human specimens 20-40 years of age) to obtain estimates of vertebral bone strength in axial compression and anteroposterior bending and to determine how estimates of strength scale with animal body mass. Our biomechanical analysis suggests that the strength of thoracic vertebral bodies is related to body mass (M) through power law relationships (y proportional, variant M(b)) in which the exponent b is 0.89 (reduced major axis) for prediction of axial compressive strength and is equal to 1.89 (reduced major axis) for prediction of bending strength. No differences in the relationship between body mass and strength were observed among hominoids. However, thoracic vertebrae from humans were found to be disproportionately larger in terms of vertebral length (distance between cranial and caudal endplates) and overall vertebral body volume (p<0.05). Additionally, vertebral bodies from humans were significantly less dense than in other hominoids (p<0.05). We suggest that reduced density in human vertebral bodies is a result of a systemic increase in porosity of cancellous bone in humans, while increased vertebral body volume and length are a result of functional adaptation during growth resulting in a vertebral bone structure that is just as strong, relative to body mass, as in other hominoids.

Three-dimensional surface texture visualization of bone tissue through epifluorescence-based serial block face imaging.

Serial block face imaging is a microscopy technique in which the top of a specimen is cut or ground away and a mosaic of images is collected of the newly revealed cross-section. Images collected from each slice are then digitally stacked to achieve 3D images. The development of fully automated image acquisition devices has made serial block face imaging more attractive by greatly reducing labour requirements. The technique is particularly attractive for studies of biological activity within cancellous bone as it has the capability of achieving direct, automated measures of biological and morphological traits and their associations with one another. When used with fluorescence microscopy, serial block face imaging has the potential to achieve 3D images of tissue as well as fluorescent markers of biological activity. Epifluorescence-based serial block face imaging presents a number of unique challenges for visualizing bone specimens due to noise generated by sub-surface signal and local variations in tissue autofluorescence. Here we present techniques for processing serial block face images of trabecular bone using a combination of non-uniform illumination correction, precise tiling of the mosaic in each cross-section, cross-section alignment for vertical stacking, removal of sub-surface signal and segmentation. The resulting techniques allow examination of bone surface texture that will enable 3D quantitative measures of biological processes in cancellous bone biopsies.

PINK1 Silencing Modifies Dendritic Spine Dynamics of Mouse Hippocampal Neurons.

PTEN-induced kinase 1 (PINK1) mutations can cause early-onset Parkinson's disease and patients are likely to develop cognitive decline, depression, and dementia. Several neurophysiological studies have demonstrated PINK1 deficiency impairs striatal and hippocampal presynaptic plasticity. Dendritic spine postsynaptic abnormalities are common in neurological diseases; however, whether PINK1 silencing modifies dendritic spine dynamics of hippocampal neurons is unclear. To address this question, confocal images of mouse cultured hippocampal neurons transfected with plasmids to silence PINK1 were analyzed. These studies revealed that PINK1 silencing increased density of thin spines and reduced head size of stubby spines. Immunoblotting analysis uncovered that PINK1 silencing decreased expression of postsynaptic density proteins (PSD95 and Shank) and glutamate receptors (NR2B and mGluR5). We also found PINK1 silencing regulated dendritic spine morphology by actin regulatory proteins (RhoGAP29 and ROCK2) and regulated neuronal survival by decreased Akt activation. These results suggest PINK1 may regulate postsynaptic plasticity in hippocampal neurons generating presymptomatic alterations in dendritic spines that eventually could lead to the neurodegeneration and cognitive decline often seen in Parkinson's disease.

Can quantitative computed tomography detect bone morphological changes associated with catastrophic proximal sesamoid bone fracture in Thoroughbred racehorses?

BACKGROUND: Fracture of the proximal sesamoid bones continues to be the most common fatal musculoskeletal injury in US racehorses. Identifying factors that influence fracture risk could lead to screening techniques to reduce catastrophic injury rates and improve animal welfare. OBJECTIVES: To identify morphological differences between proximal sesamoid bones of the contralateral limb of fracture and control horses and assess the feasibility of computed tomography (CT) to detect traits associated with proximal sesamoid bone fracture. We hypothesised that horses with proximal sesamoid bone fracture would have greater bone density. STUDY DESIGN: Cross-sectional cadaver morphological study. METHODS: Proximal sesamoid bone morphology was measured using high-resolution micro-CT images from 16 Thoroughbred racehorses (eight fracture, eight control) euthanised on New York racetracks. Nominal logistic regression models and receiver operating characteristic curves were created to assess the ability of CT-derived morphological traits to accurately classify fracture horses vs. controls. RESULTS: Bone volume fraction was greater in the fracture group (90.39 ± 1.76%) as compared to controls (87.20 ± 2.79%, P<0.0001). Bone volume fraction, bone width, trabecular thickness and degree of anisotropy were significantly different between fracture and control horses. Receiver operating characteristic curves showed that a combined model that incorporates bone volume fraction and width can identify fracture from control horses with an area under the curve of 0.938, indicating high accuracy at classifying fracture horses from controls. MAIN LIMITATIONS: The number of horses per group is small, although the total number of sesamoids imaged is reasonable (n = 62). In vivo CT at the resolution performed in this study is currently unattainable; however, density and width could be measured with quantitative CT. CONCLUSIONS: Differences in proximal sesamoid bone morphology were identified between fracture and control horses. As improved technology becomes accessible, quantitative CT could potentially be used as a clinical imaging technique to estimate proximal sesamoid bone fracture risk in Thoroughbred racehorses.

PRL -1149T allele (rs1341239) is associated with decreased risk of rheumatoid arthritis in population from southern Mexico: analysis of mRNA expression and PRL serum levels.

INTRODUCTION: Prolactin (PRL) is a sex hormone with immunomodulatory properties, and it is associated with the clinical activity of rheumatoid arthritis (RA). The -1149G>T polymorphism at the prolactin (PRL) gene has been associated with autoimmune diseases, but its functional effect is unclear. OBJECTIVE: To analyze the association of the PRL -1149G>T polymorphism with disease susceptibility, mRNA, and protein expression of PRL in RA patients from Southern Mexico. METHODS: We included 300 RA patients and 300 control subjects (CS). Genotypes were identified by the polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) technique, the PRL mRNA expression was determined by real-time PCR, and PRL serum levels were measured by enzyme-linked immunosorbent assay. RESULTS: Applying genetic models of inheritance (dominant, recessive, and additive), we found an association between the T allele and decreased RA susceptibility (OR = 0.55, 95% CI 0.35-0.87, p = 0.009; OR = 0.09, 95% CI 0.012-0.76, p = 0.011; OR = 0.49, 95% CI 0.32-0.76, p = 0.001, respectively). RA patients had higher mRNA expression and soluble levels of PRL than CS (p < 0.05). The PRL serum levels were similar in RA and CS according to genotypes. However, in CS, carriers of GT and TT genotypes showed lower PRL mRNA expression than GG genotype carriers (7.1-fold and 20-fold respectively, p = 0.006). CONCLUSIONS: This study demonstrated that the PRL -1149T allele is a genetic marker of decrease risk to RA in population from Southern Mexico, and it is associated with low PRL mRNA. KEY POINTS: • PRL -1149T allele is a marker of decreased RA susceptibility in population from southern Mexico. • PRL -1149TT genotype is associated with low PRL mRNA expression. • RA patients have higher mRNA expression and soluble levels of PRL than healthy subjects. • PRL serum levels are higher in those RA patients with < 2 years of disease evolution.

How can bone turnover modify bone strength independent of bone mass?

The amount of bone turnover in the skeleton has been identified as a predictor of fracture risk independent of areal bone mineral density (aBMD) and is increasingly cited as an explanation for discrepancies between areal bone mineral density and fracture risk. A number of mechanisms have been proposed to explain how bone turnover influences bone biomechanics, including regulation of tissue degree of mineralization, the disconnection or fenestration of individual trabeculae by remodeling cavities, and the ability of cavities formed during the remodeling process to act as stress risers. While these mechanisms can influence bone biomechanics, they also modify bone mass. If bone turnover is to explain any of the observed discrepancies between fracture risk and areal bone mineral density, however, it must not only modify bone strength, but must also modify bone strength in excess of what would be expected from the associated change in bone mass. This article summarizes biomechanical studies of how tissue mineralization, trabecular disconnection, and the presence of remodeling cavities might have an effect on cancellous bone strength independent of bone mass. Existing data support the idea that all of these factors may have a disproportionate effect on bone stiffness and/or strength, with the exception of average tissue degree of mineralization, which may not affect bone strength independent of aBMD. Disproportionate effects of mineral content on bone biomechanics may instead come from variation in tissue degree of mineralization at the micro-structural level. The biomechanical explanation for the relationship between bone turnover and fracture incidence remains to be determined, but must be examined not in terms of bone biomechanics, but in terms of bone biomechanics relative to bone mass.

Validity of serial milling-based imaging system for microdamage quantification.

Understanding the three-dimensional distribution of microdamage within trabecular bone may help provide a better understanding of the mechanisms of bone failure. Toward that end, a novel serial milling-based fluorescent imaging system was developed for quantifying microscopic damage in three dimensions throughout cores of trabecular bone. The overall goal for this study was to compare two-dimensional (2D), surface-based measures of microdamage extracted from this new imaging system against those from more conventional histological section analyses. Human vertebral trabecular cores were isolated, stained en bloc with a series of chelating fluorochromes, monotonically loaded, and underwent microdamage quantification via the two methods. Bone area fraction measured by the new system was significantly correlated to that measured by histological point counting (p<0.001, R(2)=0.80). Additionally, the new system produced statistically equivalent (p=0.021) measures of damage fraction (mean+/-SD), Dx.AF=0.047+/-0.021, to that obtained from stereological point counting, Dx.AF=0.048+/-0.017, at a 10% difference level. These results demonstrate that this serial milling-based fluorescent imaging system provides a destructive yet practical alternative to more conventional histologic section analysis in addition to its ability to provide a better understanding of the three-dimensional nature of microdamage.

Expression of MIF and TNFA in psoriatic arthritis: relationship with Th1/Th2/Th17 cytokine profiles and clinical variables.

Psoriatic arthritis (PsA) is an autoimmune inflammatory disease associated with psoriasis. The cause of this pathology is still unknown, but research suggests the diseases are caused by a deregulated cytokine production. MIF is a cytokine associated with immunomodulation of Th1, Th2, and Th17 cytokine profiles in inflammatory diseases. Based on this knowledge, the aim of this study was to determine the association of MIF and TNFA expression with Th1, Th2, and Th17 cytokine profiles in serum levels of PsA patients. A cross-sectional study was performed in 50 PsA patients and 30 control subjects (CS). The cytokine profiles were quantified by BioPlex MagPix system and the mRNA expression levels by real-time PCR. TNFA mRNA expression was 138.81-folds higher in PsA patients than CS (p < 0.001). Regarding MIF mRNA expression, no significant differences were observed; however, a positive correlation was identified between MIF mRNA expression and PsA time of evolution (r = - 0.53, p = 0.009). An increase of Th1 (IFNγ: PsA = 37.1 pg/mL vs. CS = 17 pg/mL, p < 0.05; TNFα: PsA = 24.6 pg/mL vs. CS = 9.8 pg/mL, p < 0.0001) and Th17 cytokine profiles (IL-17: PsA = 6.4 pg/mL vs. CS = 2.7 pg/mL, p < 0.05; IL-22: PsA = 8.4 pg/mL vs. CS = 1.8 pg/mL, p < 0.001), were found in PsA patients. Th2 cytokines were not significantly different in both groups. In conclusion, a high expression of TNFA mRNA, as well as an increase of Th1 and Th17 cytokine profiles evaluated by IFNγ, TNFα, IL-17, and IL-22 cytokines, was observed in PsA patients.

Substance P and acetylcholine are co-localized in the pathway mediating mucociliary activity in Rana pipiens.

Mucociliary activity is an important clearance mechanism in the respiratory system of air breathing vertebrates. Substance P (SP) and acetylcholine play a key role in the stimulation of the mucociliary transport in the frog palate. In this study, retrograde neuronal tracing was combined with immunocytochemistry for SP and choline acetyl transferase (ChAT) in the trigeminal ganglion and for neurokinin-1 receptor (NK1R) in the palate of Rana pipiens. The cells of origin of the palatine nerve were identified in the trigeminal ganglion using the retrograde tracer Fluorogold (FG). Optimal labeling of FG cells in the trigeminal ganglion was obtained at 96 h of exposure. Immunoflorescent shows that SP and acetylcholine are co-localized in 92% of the cells labeled with FG in the trigeminal ganglion. NK1 receptors were found in the membrane of epithelial and goblet cells of the palate. Ultrastructural study of the palate showed axonal-like endings with vesicles in connection with epithelial and goblet cells. These results further support the concerted action of both neurotransmitters in the regulation of mucociliary activity in the frog palate.

A biomechanical perspective on bone quality.

Observations that dual-energy X-ray absorptiometry (DXA) measures of areal bone mineral density cannot completely explain fracture incidence after anti-resorptive treatment have led to renewed interest in bone quality. Bone quality is a vague term but generally refers to the effects of skeletal factors that contribute to bone strength but are not accounted for by measures of bone mass. Because a clinical fracture is ultimately a mechanical event, it follows then that any clinically relevant modification of bone quality must change bone biomechanical performance relative to bone mass. In this perspective, we discuss a framework for assessing the clinically relevant effects of bone quality based on two general concepts: (1) the biomechanical effects of bone quality can be quantified from analysis of the relationship between bone mechanical performance and bone density; and (2) because of its hierarchical nature, biomechanical testing of bone at different physical scales (<1 mm, 1 mm, 1 cm, etc.) can be used to isolate the scale at which the most clinically relevant changes in bone quality occur. As an example, we review data regarding the relationship between the strength and density in excised specimens of trabecular bone and highlight the fact that it is not yet clear how this relationship changes during aging, osteoporosis development, and anti-resorptive treatment. Further study of new and existing data using this framework should provide insight into the role of bone quality in osteoporotic fracture risk.

Spatial relationships between bone formation and mechanical stress within cancellous bone.

Bone adapts to mechanical stimuli. While in vivo mechanical loading has been shown to increase the density of cancellous bone, theory suggests that the relationship between tissue stress/strain and subsequent bone formation occurs at the scale of individual trabeculae. Here we examine bone formation one week following mechanical stimulus. Three bouts of cyclic loading (300 cycles/day on 3 consecutive days) were applied to caudal vertebrae of female rats (n=7). Bone formation was determined using three-dimensional images of fluorescent markers of bone formation (0.7×0.7×5.0µm(3)) and local tissue stress/strain was determined using high-resolution finite element models. Three days of mechanical stimuli resulted in an increase in mineralizing surface (loaded: 17.68±2.17%; control: 9.05±3.20%; mean±SD) and an increase in the volume of bone formed (loaded: 7.09±1.97%; control: 1.44±0.50%). The number of bone formation sites was greater in loaded animals (650.71±118.54) than pinned not loaded controls (310.71±91.55), a difference that was explained by the number of formation sites at regions with large local tissue strain energy density (SED). In addition, the probability of observing bone formation was greater at locations of the microstructure experiencing greater SED, but did not exceed 32%, consistent with prior work. Our findings demonstrate that bone formation in the week following a short term mechanical stimulus occurs near regions of bone tissue experiencing high tissue SED, although the ability of finite element models to predict the locations of bone formation remains modest and further improvements may require accounting for additional factors such as osteocyte distribution or fluid flow.