Ageing human bone: factors affecting its biomechanical properties and the role of collagen

Bone collagen tensile properties of the aging human proximal femur

Despite the dominant role of bone mass in osteoporotic fractures, aging bone tissue properties must be thoroughly understood to improve osteoporosis management. In this context, collagen content and integrity are considered important factors, although limited research has been conducted on the tensile behavior of demineralized compact bone in relation to its porosity and elastic properties in the native mineralized state. Therefore, this study aims (i) at examining the age-dependency of mineralized bone and collagen micromechanical properties; (ii) to test whether, and if so to which extent, collagen properties contribute to mineralized bone mechanical properties. Two cylindrical cortical bone samples from fresh frozen human anatomic donor material were extracted from 80 proximal diaphyseal sections from a cohort of 24 female and 19 male donors (57 to 96 years at death). One sample per section was tested in uniaxial tension under hydrated conditions. First, the native sample was tested elastically (0.25 % strain), and after demineralization, up to failure. Morphology and composition of the second specimen was assessed using micro-computed tomography, Raman spectroscopy, and gravimetric methods. Simple and multiple linear regression were employed to relate morphological, compositional, and mechanical variables with age and sex. Macro-tensile properties revealed that only elastic modulus of native samples was age dependent whereas apparent elastic modulus was sex dependent (p < 0.01). Compositional and morphological analysis detected a weak but significant age and sex dependency of relative mineral weight (r = -0.24, p < 0.05) and collagen disorder ratio (I∼1670/I∼1640, r = 0.25, p < 0.05) and a strong sex dependency of bone volume fraction while generally showing consistent results in mineral content assessment. Young's modulus of demineralized bone was significantly related to tissue mineral density and Young's modulus of native bone. The results indicate that mechanical properties of the organic phase, that include collagen and non-collagenous proteins, are independent of donor age. The observed reduction in relative mineral weight and corresponding overall stiffer response of the collagen network may be caused by a reduced number of mineral-collagen connections and a lack of extrafibrillar and intrafibrillar mineralization that induces a loss of waviness and a collagen fiber pre-stretch.

Treatment affects load to failure and microdamage accumulation in healthy and osteolytic rat vertebrae

Bone turnover and microdamage are impacted by the presence of skeletal metastases which can contribute to increased fracture risk. Treatments for metastatic disease may further impact bone quality. This exploratory study aimed to establish an initial understanding of microdamage accumulation and load to failure in healthy and osteolytic rat vertebrae following focal and systemic cancer treatment (docetaxel (DTX), stereotactic body radiotherapy (SBRT), or zoledronic acid (ZA)). Osteolytic spine metastases were developed in 6-week-old athymic female rats via intracardiac injection of HeLa human cervical cancer cells (day 0). Additional rats served as healthy controls. Rats were either untreated, received SBRT to the T10-L6 vertebrae on day 14 (15 Gy, two fractions), DTX on day 7 or 14, or ZA on day 7. Rats were euthanized on day 21. Tumor burden was assessed with bioluminescence images acquired on day 14 and 21, histology of the excised T11 and L5 vertebrae, and ex-vivo μCT images of the T13-L4. Microstructural parameters (bone volume/total volume, trabecular number, spacing, thickness, and bone mineral density) were measured from L2 vertebrae. Load to failure was measured with axial compressive loading of the L1-L3 motion segments. Microdamage accumulation was labeled in T13 vertebrae with BaSO4 staining and was visualized with high resolution μCT imaging. Microdamage volume fraction was defined as the ratio of BaSO4 to bone volume. DTX administered on day 7 reduced tumor growth significantly (p < 0.05). Microdamage accumulation was found to be increased by the presence of metastases but was reduced by all treatments with ZA showing the largest improvement in HeLa cell injected rats. Load to failure was decreased in untreated and SBRT HeLa cell injected rats compared to healthy controls (p < 0.01). There was a moderate negative correlation between load to failure and microdamage volume fraction in vertebrae from rats injected with HeLa cells (R = -0.35, p = 0.031). Strong correlations were also found between microstructural parameters and load to failure and microdamage accumulation. Several factors, including the presence of osteolytic lesions and use of cancer therapies, influence microdamage accumulation and load to failure in rat vertebrae. Understanding the impact of these treatments on fracture risk of metastatic vertebrae is important to improve management of patients with spinal metastases.

Effect of in vitro ribosylation on the dynamic fracture behavior of mature bovine cortical bone

In this study, the fracture behavior of ribosylated bovine cortical bone is investigated under loading conditions simulating a fall event. Single edge notched specimens, separated into a control group (n = 11) and a ribosylated group (n = 8), were extracted from the mid-diaphysis of a single bovine femur harvested from a mature cow. A seven-day ribosylation process results in the accumulation of Advanced-Glycation End Products (AGEs) cross-links and AGE adducts. Specimens were subjected to symmetric three point bending (opening mode) and an impact velocity of 1.6 m/s using a drop tower. Near-crack displacement fields up to fracture initiation are determined from high-speed images post-processed using digital image correlation. A constrained over-deterministic least squares regression and orthotropic material linear elastic fracture mechanics theory are used to extract the in-plane critical stress intensity factors at fracture initiation (i.e., fracture initiation toughness values). Statistically significant differences were not observed when comparing the in-plane fracture initiation toughness values (p≥0.96) or energy release rate (p=0.90) between the control and seven-day ribosylated groups. The intrinsic variability of bone may require high sample numbers in order to achieve an adequately powered experiment when assessing dynamic fracture behavior. While there are no detectable differences due to the ribosylation treatment investigated, this is likely due to the limited sample sizes utilized.

Multiscale theoretical model shows that aging-related mechanical degradation of cortical bone is driven by microstructural changes in addition to porosity

This study aims to gain mechanistic understanding of how aging-related changes in the microstructure of cortical bone drive mechanical consequences at the macroscale. To that end, cortical bone was modeled as a bundle of elastic-plastic, parallel fibers, which represented osteons and interstitial tissue, loaded in uniaxial tension. Distinct material properties were assigned to each fiber in either the osteon or interstitial fiber "families." Models representative of mature (20-60 yrs.) bone, and elderly (60+) bone were created by modeling aging via the following changes to the input parameters: (i) increasing porosity from 5% to 15%, (ii) increasing the ratio of the number of osteon fibers relative to interstitial fibers from 40% to 50%, and (iii) changing the fiber material properties from representing mature bone samples to representing elderly bone samples (i.e., increased strength and decreased toughness of interstitial fibers together with decreased toughness of osteon fibers). To understand the respective contributions of these changes, additional models isolating one or two of each of these were also created. From the computed stress-strain curve for the fiber bundle, the yield point (ϵy, σy), ultimate point (ϵu, σu), and toughness (UT) for the bundle as a whole were measured. We found that changes to all three input parameters were required for the model to capture the aging-related decline in cortical bone mechanical properties consistent with those previously reported in the literature. In both mature and elderly bundles, rupture of the interstitial fibers drove the initial loss of strength following the ultimate point. Plasticity and more gradual rupture of the osteons drove the remainder of the response. Both the onset and completion of interstitial fiber rupture occurred at lower strains in the elderly vs. mature case. These findings point to the importance of studying microstructural changes beyond porosity, such as the area fraction of osteons and the material properties of osteon and interstitial tissue, in order to further understanding of aging-related changes in bone.

Quantifying microcracks on fractured bone surfaces – Potential use in forensic anthropology

Bone fracture surface morphology (FSM) can provide valuable information on the cause of failure in forensic and archaeological applications and it depends primarily on three factors, the loading conditions (like strain rate), the ambient conditions (wet or dry bone material) and the quality of bone material itself. The quality of bone material evidently changes in taphonomy as a result of the decomposition process and that in turn is expected to affect FSM. Porcine bones were fractured by a standardised impact during the course of soft tissue decomposition, at 28-day intervals, over 140 days (equivalent to 638 cooling degree days). Measurements of the associated microcracks on the fractured cortical bone surfaces indicated a progressive increase in mean length during decomposition from around 180 μm-375 μm. The morphology of these microcracks also altered, from multiple intersecting microcracks emanating from a central point at 0-28 cumulative cooling degree days, to longer linear cracks appearing to track lamellae as soft tissue decomposition progressed. The implications of these findings are that taphonomic changes of bone may offer the real possibility of distinguishing perimortem and taphonomic damage and also provide a new surrogate parameter for estimation of post-mortem interval (PMI) in forensics.

Association between nanoscale strains and tissue level nanoindentation properties in age-related hip-fractures

Measurement of the properties of bone as a material can happen in various length scales in its hierarchical and composite structure. The aim of this study was to test the tissue level properties of clinically-relevant human bone samples which were collected from donors belonging to three groups: ageing donors who suffered no fractures (Control); untreated fracture patients (Fx-Untreated) and patient who experienced hip fracture despite being treated with bisphosphonates (Fx-BisTreated). Tissue level properties were assessed by (a) nanoindentation and (b) synchrotron tensile tests (STT) where strains were measured at the 'tissue', 'fibril' and 'mineral' levels by using simultaneous Wide-angle - (WAXD) and Small angle- X-ray diffraction (SAXD). The composition was analysed by thermogravimetric analysis and material level endo- and exo-thermic reactions by differential scanning calorimetry (TGA/DSC3+). Irrespective of treatment fracture donors exhibited significantly lower tissue, fibril and mineral strain at the micro and nanoscale respectively and had a higher mineral content than controls. In nanoindentation only nanohardness was significantly greater for Controls and Fx-BisTreated versus Fx-Untreated. The other nanoindentation parameters did not vary significantly across the three groups. There was a highly significant positive correlation (p < 0.001) between organic content and tissue level strain behaviour. Overall hip-fractures were associated with lower STT nanostrains and it was behaviour measured by STT which proved to be a more effective approach for predicting fracture risk because evidently it was able to demonstrate the mechanical deficit for the bone tissue of the donors who had experienced fractures.

Tooth hop variability in human and nonhuman bone: Effect on the estimation of saw blade TPI

Forensic research has demonstrated that tooth hop (TH) is a valuable measurement from saw-cut bones as it can be used to estimate teeth-per-inch (TPI) of a saw used in postmortem dismemberment cases. However, error rates for TPI estimation are still under development and knowledge of how bone tissue affects TH measurements remains unclear. The purpose of this research was to investigate the effects of tissue variability through the use of different taxa on the accuracy and precision of TH measurements in the bone to estimate TPI of the blade. A total of 1766 TH measurements were analyzed from human, pig, and deer long bones cut by two 7 TPI saw blades of different tooth type. Fifty distance-between-teeth measurements before and after sawing were collected directly from each blade for comparison to bone-measured TH to assess potential effects of tooth wear on TH variability. ANOVA and F tests were used to compare mean TH and variance, respectively, by saw-species (i.e., crosscut-deer, rip-deer) and species groups (i.e., all deer, all pig), with significance determined at the p < 0.05 level. TH measurements were converted to usable TPI ranges, which would typically be presented in a forensic report. It is concluded that significant differences in TH (mm) do not necessarily reflect significant differences in associated TPI ranges of suspect blades. Forensic reports should report mean TPI ± 1.5-2.5 TPI while providing a sample size indicating number of TH measured rather than just number of cuts or cut surfaces examined.

Increased tissue-level storage modulus and hardness with age in male cortical bone and its association with decreased fracture toughness

The incidence of bone fracture increases with age, due to both declining bone quantity and quality. Toward the goal of an improved understanding of the causes of the age-related decline in the fracture toughness of male cortical bone, nanoindentation experiments were performed on femoral diaphysis specimens from men aged 21-98 years. Because aged bone has less matrix-bound water and dry bone is less viscoelastic, we used a nanoindentation method that is sensitive to changes in viscoelasticity. Given the anisotropy of bone stiffness, longitudinal (n = 26) and transverse (n = 25) specimens relative to the long axis of the femur diaphysis were tested both dry in air and immersed in phosphate buffered saline solution. Indentation stiffness (storage modulus) and hardness increased with age, while viscoelasticity (loss modulus) was independent of donor age. The increases in indentation stiffness and hardness with age were best explained by increased mineralization with age. Indentation stiffness and hardness were negatively correlated with previously acquired fracture toughness parameters, which is consistent with a tradeoff between material strength and toughness. In keeping with the complex structure of bone, a combination of tissue-level storage modulus or hardness, bound water, and osteonal area in regression models best explained the variance in the fracture toughness of male human cortical bone. On the other hand, viscoelasticity was unchanged with age and was not associated with fracture toughness. In conclusion, the age-related increase in stiffness and hardness of male cortical bone may be one of the multiple tissue-level characteristics that contributes to decreased fracture toughness.

Age related changes of rib cortical bone matrix and the application to forensic age-at-death estimation

Forensic anthropology includes, amongst other applications, the positive identification of unknown human skeletal remains. The first step in this process is an assessment of the biological profile, that is: sex, age, stature and ancestry. In forensic contexts, age estimation is one of the main challenges in the process of identification. Recently established admissibility criteria are driving researchers towards standardisation of methodological procedures. Despite these changes, experience still plays a central role in anthropological examinations. In order to avoid this issue, age estimation procedures (i) must be presented to the scientific community and published in peer reviewed journals, (ii) accurately explained in terms of procedure and (iii) present clear information about the accuracy of the estimation and possible error rates. In order to fulfil all these requirements, a number of methods based on physiological processes which result in biochemical changes in various tissue structures at the molecular level, such as modifications in DNA-methylation and telomere shortening, racemization of proteins and stable isotopes analysis, have been developed. The current work proposes a new systematic approach in age estimation based on tracing physicochemical and mechanical degeneration of the rib cortical bone matrix. This study used autopsy material from 113 rib specimens. A set of 33 parameters were measured by standard bio-mechanical (nanoindentation and microindentation), physical (TGA/DSC, XRD and FTIR) and histomorphometry (porosity-ImageJ) methods. Stepwise regressions were used to create equations that would produce the best 'estimates of age at death' vs real age of the cadavers. Five equations were produced; in the best of cases an equation counting 7 parameters had an R² = 0.863 and mean absolute error of 4.64 years. The present method meets all the admissibility criteria previously described. Furthermore, the method is experience-independent and as such can be performed without previous expert knowledge of forensic anthropology and human anatomy.

Establishing a minimum PMI for bone sun bleaching in a UK environment with a controlled desert-simulated comparison

Microenvironments play a significant part in understanding the post-mortem interval in forensic taphonomy. Recently, the value of weathering factors in relation to obtaining a PMI has been investigated further. In this study, observations were made to calculate the length of time it takes for three different bone elements (femur, rib, and scapula) to bleach in a UK summer and winter. This research also investigated whether there were any physicochemical modifications to the bone caused by bleaching. Porcine femora, scapulae, and ribs were placed into open and shaded areas of an outdoor research facility located in Oxfordshire, UK, during summer (July-Sep) and winter months (Dec-Mar). The specimens were monitored at 3-week intervals using photography, and an observational scoring method was developed to quantify the extent of bleaching. As temperatures are typically much lower in the UK compared with warmer climates, a controlled indoor-simulated desert experiment was also undertaken to be used as a control. This allowed sun bleaching and changes to the bone chemistry to be monitored in a controlled, high-UV environment for comparison with the UK outdoor experiments. Fourier transform infrared spectroscopy (FTIR) was employed to analyze physicochemical modifications to both the mineral and organic components of the bone. The FTIR was used to calculate crystallinity index (CI), mineral to organic ratio, and the relative amount of carbonate concentrations. Weather data was collected and a positive correlation was found between both ultraviolet (UV) levels and accumulated degree days (ADD) when compared with observational bleaching scores. Bleaching (whitening) of the bone samples occurred in both seasons but at different rates, with the bleaching process occurring at a slower rate in winter. During summer, the initial bleaching process was evident at 6 weeks, and by 9 weeks, the bones were an off-white colour. During the winter period, whitening of the bone started at 9 weeks; however, only the scapula and rib samples displayed a similar off-white colour. This colouration was observed at 13 weeks rather than at 9 weeks. The desert simulation samples started bleaching in a similar pattern to the outdoor samples after 1 week but the bones did not fully bleach. The bone chemistry, based on physicochemical properties obtained from the FTIR, showed a significant statistical difference between the simulated desert and winter season when compared against a control sample. For the winter samples, the mineral to organic ratio was significantly higher than that in the control, suggesting a reduction in the proportion of organic. For the samples in the simulated desert environment, the crystallinity index was significantly higher than that in the control samples, suggesting an increase in crystallinity. The results of this experiment support the fact that it is possible to achieve bleaching in a UK environment and that the minimal time frame for this to occur differs in seasons.

Validity of the use of porcine bone in forensic cut mark studies

Porcine bone is often used as a substitute for human bone in forensic trauma studies, but little has been published on its comparative mechanical behavior. The factors affecting mechanical properties and therefore selection of bone models are complex and include the age of the animal at death, and physiological loading conditions, the latter being of particular relevance when using a quadrupedal animal as a human substitute. The regional variation in hardness of adult and infant porcine bones was investigated using Vickers' indentation tests and compared to published data for human limb bones to relate differences to inherent genetic effects and loading influences, and to examine the validity of the porcine-human model. Significant differences in hardness were observed both along and around the adult porcine humerus and femur, but no significant differences were found along the length of the infant bones. Significant differences were found between the forelimb and hindlimb, but only in the infant specimens. The hardness values for porcine adult cortical bone from the femur (52.23 ± 1.00 kg mm^-2 ) were comparable to those reported in the literature for adult human cortical bone from the fibula, ilium, and calcaneus. These data will help inform subject selection in terms of both species and bone type for use in future trauma studies.

Ageing bone fractures: The case of a ductile to brittle transition that shifts with age

Human bone becomes increasingly brittle with ageing. Bones also fracture differently under slow and fast loadings, being ductile and brittle, respectively. The effects of a combination of these two factors have never been examined before. Here we show that cortical bone is most fracture-resistant at the physiologically prevalent intermediate strain rates of 10^-3 s^-1 to 10^-2 s^-1 such as they occur in walking or running, slightly weaker at slower quasistatic and much weaker at fast impact loading rates. In young cortical bone (15 years of age) the ductile-to-brittle transition (DBT) occurs at strain rates of 10^-2 s^-1, in old cortical bone (85 yrs) at speeds lower by a factor of 10 to 40. Other research has shown that the energy required to break bone (per unit of fracture surface) drops as much as 60% between these two ages. Therefore, DBT seems to compound the well-known phenomenon of 'brittle old bones'. Old bones can only cope with slow movement, young ones with both slow and fast movement. These observed material characteristics of (i) a shift of the DBT and (ii) a reduced energy absorption capacity appear to contribute at least as much to the loss of bone quality as the various quantity based (lowered bone density and mineral content) explanations of the past. They also provide a new powerful paradigm, which allows us to demonstrate mechanically, and uniquely, how human bone becomes increasingly brittle with age.

Evaluating the evidentiary value of the analysis of skeletal trauma in forensic research: A review of research and practice

Understanding bone trauma characteristics is a fundamental component of forensic investigations that can assist in understanding the nature of blunt trauma related deaths. The variation of each blunt force trauma (BFT) injury is dependent on a magnitude of factors including, age, sex, health, angle of impact, impact mechanism, impact force and clothing, making BFT one of the more difficult area of trauma to interpret solely based on skeletal fractures. A detailed literature review was performed to assess the value and scientific rigour of the current research in forensic anthropology, forensic pathology and biomechanics that attempts to provide an objective framework in which forensic practitioners can assess and interpret BFT injuries. Four areas of research which investigate the analysis of BFT are examined. These included research involving experiments on animal models; experiments on human models (and synthetic models); computer modelling, and research/publications including 'mild', 'moderate' and 'severe' as descriptions of impact force, resulting from trauma. Also briefly discussed is how BFT research is framed within medicolegal contexts. While many published works have contributed to the understanding of the biomechanics of BFT, more research that can provide an objective means to accurately assess and interpret BFT injures is required.

Protein synthesis rates of muscle, tendon, ligament, cartilage, and bone tissue in vivo in humans

Skeletal muscle plasticity is reflected by a dynamic balance between protein synthesis and breakdown, with basal muscle tissue protein synthesis rates ranging between 0.02 and 0.09%/h. Though it is evident that other musculoskeletal tissues should also express some level of plasticity, data on protein synthesis rates of most of these tissues in vivo in humans is limited. Six otherwise healthy patients (62±3 y), scheduled to undergo unilateral total knee arthroplasty, were subjected to primed continuous intravenous infusions with L-[ring-¹³C₆]-Phenylalanine throughout the surgical procedure. Tissue samples obtained during surgery included muscle, tendon, cruciate ligaments, cartilage, bone, menisci, fat, and synovium. Tissue-specific fractional protein synthesis rates (%/h) were assessed by measuring the incorporation of L-[ring-¹³C₆]-Phenylalanine in tissue protein and were compared with muscle tissue protein synthesis rates using a paired t test. Tendon, bone, cartilage, Hoffa’s fat pad, anterior and posterior cruciate ligament, and menisci tissue protein synthesis rates averaged 0.06±0.01, 0.03±0.01, 0.04±0.01, 0.11±0.03, 0.07±0.02, 0.04±0.01, and 0.04±0.01%/h, respectively, and did not significantly differ from skeletal muscle protein synthesis rates (0.04±0.01%/h; P>0.05). Synovium derived protein (0.13±0.03%/h) and intercondylar notch bone tissue protein synthesis rates (0.03±0.01%/h) were respectively higher and lower compared to skeletal muscle protein synthesis rates (P<0.05 and P<0.01, respectively). Basal protein synthesis rates in various musculoskeletal tissues are within the same range of skeletal muscle protein synthesis rates, with fractional muscle, tendon, bone, cartilage, ligament, menisci, fat, and synovium protein synthesis rates ranging between 0.02 and 0.13% per hour in vivo in humans.

Clinical trial registration: NTR5147

Raman Biomarkers Are Associated with Cyclic Fatigue Life of Human Allograft Cortical Bone

BACKGROUND

Structural bone allografts are an established treatment method for long-bone structural defects resulting from such conditions as traumatic injury and sarcoma. The functional lifetime of structural allografts depends on resistance to cyclic loading (cyclic fatigue life), which can lead to fracture at stress levels well below the yield strength. Raman spectroscopy biomarkers can be used to non-destructively assess the 3 primary components of bone (collagen, mineral, and water), and may aid in optimizing allograft selection to decrease fatigue fracture risk. We studied the association of Raman biomarkers with the cyclic fatigue life of human allograft cortical bone.

METHODS

Twenty-one cortical bone specimens were machined from the femoral diaphyses of 4 human donors (a 63-year old man, a 61-year-old man, a 51-year-old woman, and a 48-year-old woman) obtained from the Musculoskeletal Transplant Foundation. Six Raman biomarkers were analyzed: collagen disorganization, mineral maturation, matrix mineralization, and 3 water compartments. The specimens underwent cyclic fatigue testing under fully reversed conditions (35 and 45 MPa), during which they were tested to fracture or to 30 million cycles ("runout"), simulating 15 years of moderate activity. A tobit censored linear regression model for cyclic fatigue life was created.

RESULTS

The multivariate model explained 60% of the variance in the cyclic fatigue life (R = 0.604, p < 0.001). Increases in Raman biomarkers for disordered collagen (coefficient: -2.74×10, p < 0.001) and for loosely collagen-bound water compartments (coefficient: -2.11×10, p < 0.001) were associated with a decreased cyclic fatigue life. Increases in Raman biomarkers for mineral maturation (coefficient: 3.50×10, p < 0.001), matrix mineralization (coefficient: 2.32×10, p < 0.001), tightly collagen-bound water (coefficient: 1.19×10, p < 0.001), and mineral-bound water (coefficient: 3.27×10, p < 0.001) were associated with an increased cyclic fatigue life. Collagen disorder accounted for 44% of the variance in the cyclic fatigue life, mineral maturation accounted for 6%, and all bound water compartments accounted for 3%.

CONCLUSIONS

Increasing baseline collagen disorder was associated with a decreased cyclic fatigue life and had the strongest correlation with the cyclic fatigue life of human cortical donor bone. This model should be prospectively validated.

CLINICAL RELEVANCE

Raman analysis is a promising tool for the non-destructive evaluation of structural bone allograft quality for load-bearing applications.

Microdamage as a Bone Quality Component: Practical Guidelines for the Two-Dimensional Analysis of Linear Microcracks in Human Cortical Bone

Microdamage is a component of bone quality believed to play an integral role in bone health. However, comparability between existing studies is fraught with issues due to highly variable methods of sample preparation and poorly defined quantification criteria. To address these issues, this article has two aims. First, detailed methods for preparation and analysis of linear microcracks in human ribs, specifically addressing troubleshooting issues cited in previous studies, are laid out. Second, new, partially validated criteria are proposed in an effort to reduce subjective differences in microcrack counts and measures, ensuring more comparable results between studies. Revised definitions based on current literature in conjunction with a digital atlas to reduce observer inaccuracy and bias are presented. The goal is to provide a practical methodology for bone biologists and biomechanists to collect and analyze linear microcracks for basic science research. © 2019 The Authors. JBMR Plus published by Wiley Periodicals, Inc. on behalf of American Society for Bone and Mineral Research.

Environmentally-Controlled Near Infrared Spectroscopic Imaging of Bone Water

We have designed an environmentally-controlled chamber for near infrared spectroscopic imaging (NIRSI) to monitor changes in cortical bone water content, an emerging biomarker related to bone quality assessment. The chamber is required to ensure repeatable spectroscopic measurements of tissues without the influence of atmospheric moisture. A calibration curve to predict gravimetric water content from human cadaveric cortical bone was created using NIRSI data obtained at six different lyophilization time points. Partial least squares (PLS) models successfully predicted bone water content that ranged from 0–10% (R = 0.96, p < 0.05, root mean square error of prediction (RMSEP) = 7.39%), as well as in the physiologic range of 4–10% of wet tissue weight (R = 0.87, p < 0.05, RMSEP = 14.5%). Similar results were obtained with univariate and bivariate regression models for prediction of water in the 0–10% range. Further, we identified two new NIR bone absorbances, at 6560 cm⁻¹ and 6688 cm⁻¹, associated with water and collagen respectively. Such data will be useful in pre-clinical studies that investigate changes in bone quality with disease, aging and with therapeutic use.

Assessing matrix quality by Raman spectroscopy helps predict fracture toughness of human cortical bone

Developing clinical tools that assess bone matrix quality could improve the assessment of a person's fracture risk. To determine whether Raman spectroscopy (RS) has such potential, we acquired Raman spectra from human cortical bone using microscope- and fiber optic probe-based Raman systems and tested whether correlations between RS and fracture toughness properties were statistically significant. Calculated directly from intensities at wavenumbers identified by second derivative analysis, Amide I sub-peak ratio I₁₆₇₀/I₁₆₄₀, not I₁₆₇₀/I₁₆₉₀, was negatively correlated with K_init (N = 58; R² = 32.4%) and J-integral (R² = 47.4%) when assessed by Raman micro-spectroscopy. Area ratios (A₁₆₇₀/A₁₆₉₀) determined from sub-band fitting did not correlate with fracture toughness. There were fewer correlations between RS and fracture toughness when spectra were acquired by probe RS. Nonetheless, the I₁₆₇₀/I₁₆₄₀ sub-peak ratio again negatively correlated with K_init (N = 56; R² = 25.6%) and J-integral (R² = 39.0%). In best-fit general linear models, I₁₆₇₀/I_1640, age, and volumetric bone mineral density explained 50.2% (microscope) and 49.4% (probe) of the variance in K_init. I₁₆₇₀/I₁₆₄₀ and v₁PO₄/Amide I (microscope) or just I₁₆₇₀/I₁₆₄₀ (probe) were negative predictors of J-integral (adjusted-R² = 54.9% or 37.9%, respectively). While Raman-derived matrix properties appear useful to the assessment of fracture resistance of bone, the acquisition strategy to resolve the Amide I band needs to be identified.

Elevated Microdamage Spatially Correlates with Stress in Metastatic Vertebrae

Metastasis of cancer to the spine impacts bone quality. This study aims to characterize vertebral microdamage secondary to metastatic disease considering the pattern of damage and its relationship to stress and strain under load. Osteolytic and mixed osteolytic/osteoblastic vertebral metastases were produced in athymic rats via HeLa cervical or canine Ace-1 prostate cancer cell inoculation, respectively. After 21 days, excised motion segments (T12-L2) were µCT scanned, stained with BaSO₄ and re-imaged. T13-L2 motion segments were loaded in axial compression to induce microdamage, re-stained and re-imaged. L1 (loaded) and T12 (unloaded) vertebrae were fixed, sample blocks cut, polished and BSE imaged. µFE models were generated of all L1 vertebrae with displacement boundary conditions applied based on the loaded µCT images. µCT stereological analysis, BSE analysis and µFE derived von Mises stress and principal strains were quantitatively compared (ANOVA), spatial correlations determined and patterns of microdamage assessed qualitatively. BaSO₄ identified microdamage was found to be spatially correlated with regions of high stress in µFEA. Load-induced microdamage was shown to be elevated in the presence of osteolytic and mixed metastatic disease, with diffuse, crossed hatched areas of microdamage present in addition to linear microdamage and microfractures in metastatic tissue, suggesting diminished bone quality.

Limited field radiation therapy results in decreased bone fracture toughness in a murine model

Fragility fractures are a well-known complication following oncologic radiotherapy, and it is suspected that radiation-induced embrittlement of bone within the treatment field may contribute to fracture risk. To explore this phenomenon, a mouse model (BALB/cJ) of fractionated, limited field, bilateral hindlimb irradiation (4x5 Gy) was used. The effects of radiation on femoral (cortical) bone fracture toughness, morphology, and biochemistry-including advanced glycation end products (AGEs)-were quantified and compared to Sham group samples prior to irradiation and at 0, 4, 8, and 12 weeks post-irradiation. Additionally, alterations to bone fracture toughness mediated directly by radiation (independent of cellular mechanisms) were determined using devitalized mouse cadaver femurs. Finally, the contribution of AGEs to reduced fracture toughness was examined by artificially ribosylating mouse femurs ex vivo. These data demonstrate that in vivo irradiation results in an immediate (-42% at 0 weeks, p < 0.001) and sustained (-28% at 12 weeks, p < 0.001) decrease in fracture toughness with small changes in morphology (-5% in cortical area at 12 weeks), and minimal changes in bone composition (tissue mineral density, mineral:matrix ratio, and AGE content). Irradiation of devitalized femurs also reduced fracture toughness (-29%, p < 0.001), but to a lesser extent than was seen in vivo. While artificial ribosylation decreased fracture toughness with time, the extent of glycation needed to induce this effect exceeded the AGE accumulation that occurred in vivo. Overall, hindlimb irradiation induced a substantial and sustained decrease in bone fracture toughness. Approximately half of this decrease in fracture toughness is due to direct radiation damage, independent of cellular remodeling. Collagen glycation in vivo was not substantially altered, suggesting other matrix changes may contribute to post-radiotherapy bone embrittlement.

Injury risk functions based on population-based finite element model responses: Application to femurs under dynamic three-point bending

OBJECTIVE

The goal of this study was to explore a framework for developing injury risk functions (IRFs) in a bottom-up approach based on responses of parametrically variable finite element (FE) models representing exemplar populations.

METHODS

First, a parametric femur modeling tool was developed and validated using a subject-specific (SS)-FE modeling approach. Second, principal component analysis and regression were used to identify parametric geometric descriptors of the human femur and the distribution of those factors for 3 target occupant sizes (5th, 50th, and 95th percentile males). Third, distributions of material parameters of cortical bone were obtained from the literature for 3 target occupant ages (25, 50, and 75 years) using regression analysis. A Monte Carlo method was then implemented to generate populations of FE models of the femur for target occupants, using a parametric femur modeling tool. Simulations were conducted with each of these models under 3-point dynamic bending. Finally, model-based IRFs were developed using logistic regression analysis, based on the moment at fracture observed in the FE simulation. In total, 100 femur FE models incorporating the variation in the population of interest were generated, and 500,000 moments at fracture were observed (applying 5,000 ultimate strains for each synthesized 100 femur FE models) for each target occupant characteristics.

RESULTS

Using the proposed framework on this study, the model-based IRFs for 3 target male occupant sizes (5th, 50th, and 95th percentiles) and ages (25, 50, and 75 years) were developed. The model-based IRF was located in the 95% confidence interval of the test-based IRF for the range of 15 to 70% injury risks. The 95% confidence interval of the developed IRF was almost in line with the mean curve due to a large number of data points.

CONCLUSIONS

The framework proposed in this study would be beneficial for developing the IRFs in a bottom-up manner, whose range of variabilities is informed by the population-based FE model responses. Specifically, this method mitigates the uncertainties in applying empirical scaling and may improve IRF fidelity when a limited number of experimental specimens are available.

Non-destructive NIR spectral imaging assessment of bone water: Comparison to MRI measurements

Bone fracture risk increases with age, disease states, and with use of certain therapeutics, such as acid-suppressive drugs, steroids and high-dose bisphosphonates. Historically, investigations into factors that underlie bone fracture risk have focused on evaluation of bone mineral density (BMD). However, numerous studies have pointed to factors other than BMD that contribute to fragility, including changes in bone collagen and water. The goal of this study is to investigate the feasibility of using near infrared spectral imaging (NIRSI) to determine the spatial distribution and relative amount of water and organic components in whole cross-sections of bone, and to compare those results to those obtained using magnetic resonance imaging (MRI) methods. Cadaver human whole-section tibiae samples harvested from 18 donors of ages 27-97years underwent NIRSI and ultrashort echo time (UTE) MRI. As NIRSI data is comprised of broad absorbances, second derivative processing was evaluated as a means to narrow peaks and obtain compositional information. The (inverted) second derivative peak heights of the NIRSI absorbances correlated significantly with the mean peak integration of the water, collagen and fat NIR absorbances, respectively, indicating that either processing method could be used for compositional assessment. The 5797cm^-1 absorbance was validated as arising from the fat present in bone marrow, as it completely disappeared after ultrasonication. The MRI UTE-determined bound water content in tibial cortical bone samples ranged from 62 to 91%. The NIRSI water peaks at 5152cm^-1 and at 7008cm^-1 correlated significantly with the UTE data, with r=0.735, p=0.016, and r=0.71, p=0.0096, respectively. There was also a strong correlation between the intensity of the NIRSI water peak at 7008cm^-1 and the intensity of the collagen peak at 4608cm^-1 (r=0.69, p=0.004). Since NIRSI requires minimal to no sample preparation, this approach has great potential to become a gold standard modality for the investigation of changes in water content, distribution, and environment in pre-clinical studies of bone pathology and therapeutics.

Raman spectral markers of collagen denaturation and hydration in human cortical bone tissue are affected by radiation sterilization and high cycle fatigue damage

BACKGROUND

Thermal denaturation and monotonic mechanical damage alter the organic and water-related compartments of cortical bone. These changes can be detected using Raman spectroscopy. However, less is known regarding Raman sensitivity to detect the effects of cyclic fatigue damage and allograft sterilization doses of gamma radiation.

OBJECTIVE

To determine if Raman spectroscopic biomarkers of collagen denaturation and hydration are sensitive to the effects of (a) high cycle fatigue damage and (b) 25kGy irradiation.

METHODS

Unirradiated and gamma-radiation sterilized human cortical bone specimens previously tested in vitro under high-cycle (> 100,000 cycles) fatigue conditions at 15MPa, 25MPa, 35MPa, 45MPa, and 55MPa cyclic stress levels were studied. Cortical bone Raman spectral profiles from wavenumber ranges of 800-1750cm^-1 and 2700-3800cm^-1 were obtained and compared from: a) non-fatigue vs fatigue fracture sites and b) radiated vs. unirradiated states. Raman biomarker ratios 1670/1640 and 3220/2949, which reflect collagen denaturation and organic matrix (mainly collagen)-bound water, respectively, were assessed. One- and two-way ANOVA analyses were utilized to identify differences between groups along with interaction effects between cyclic fatigue and radiation-induced damage.

RESULTS

Cyclic fatigue damage resulted in increases in collagen denaturation (1670/1640: 1.517 ± 0.043 vs 1.579 ± 0.021, p < 0.001) and organic matrix-bound water (3220/2949: 0.109 ± 0.012 vs 0.131 ± 0.008, p < 0.001). Organic matrix-bound water increased secondary to 25kGy irradiation (3220/2949: 0.105 ± 0.010 vs 0.1161 ± 0.009, p = 0.003). Organic matrix-bound water was correlated positively with collagen denaturation (r = 0.514, p < 0.001).

CONCLUSIONS

Raman spectroscopy can detect the effects of cyclic fatigue damage and 25kGy irradiation via increases in organic matrix (mainly collagen)-bound water. A Raman measure of collagen denaturation was sensitive to cyclic fatigue damage but not 25kGy irradiation. Collagen denaturation was correlated with organic matrix-bound water, suggesting that denaturation of collagen to gelatinous form may expose more binding sites to water by unwinding the triple alpha chains. This research may eventually be useful to help identify allograft quality and more appropriately match donors to recipients.

Rib biomechanical properties exhibit diagnostic potential for accurate ageing in forensic investigations

Age estimation remains one of the most challenging tasks in forensic practice when establishing a biological profile of unknown skeletonised remains. Morphological methods based on developmental markers of bones can provide accurate age estimates at a young age, but become highly unreliable for ages over 35 when all developmental markers disappear. This study explores the changes in the biomechanical properties of bone tissue and matrix, which continue to change with age even after skeletal maturity, and their potential value for age estimation. As a proof of concept we investigated the relationship of 28 variables at the macroscopic and microscopic level in rib autopsy samples from 24 individuals. Stepwise regression analysis produced a number of equations one of which with seven variables showed an R2 = 0.949; a mean residual error of 2.13 yrs ±0.4 (SD) and a maximum residual error value of 2.88 yrs. For forensic purposes, by using only bench top machines in tests which can be carried out within 36 hrs, a set of just 3 variables produced an equation with an R2 = 0.902 a mean residual error of 3.38 yrs ±2.6 (SD) and a maximum observed residual error 9.26yrs. This method outstrips all existing age-at-death methods based on ribs, thus providing a novel lab based accurate tool in the forensic investigation of human remains. The present application is optimised for fresh (uncompromised by taphonomic conditions) remains, but the potential of the principle and method is vast once the trends of the biomechanical variables are established for other environmental conditions and circumstances.

Variability of in vivo linear microcrack accumulation in the cortex of elderly human ribs

Excessive accumulation of microdamage in the skeleton in vivo is believed to contribute to fragility and risk of fracture, particularly in the elderly. Current knowledge of how much in vivo damage accrual varies between individuals, if at all, is lacking. In this study, paired sixth ribs from five male and five female elderly individuals (76–92 years, mean age = 84.7 years) were examined using en bloc staining and fluorescent microcopy to quantify linear microcracks present at the time of death (i.e. in vivo microdamage). Crack number, crack length, crack density, and crack surface density were measured for each complete cross-section, with densities calculated using the variable of bone area (which accounts for the influence of porosity on the cortex, unlike the more frequently used cortical area), and analyzed using a two-way mixed model analysis of variance. Results indicate that while microcracks between individuals differ significantly, differences between the left and right corresponding pairs within individuals and the pleural and cutaneous cortices within each rib did not. These results suggest that systemic influences, such as differential metabolic activity, affect the accumulation of linear microcracks. Furthermore, variation in remodeling rates between individuals may be a major factor contributing to differential fracture risk in the elderly. Future work should expand to include a wider age range to examine differences in in vivo microdamage accumulation across the lifespan, as well as considering the influence of bisphosphonates on microdamage accumulation in the context of compromised remodeling rates in the elderly.

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Linear microcrack accumulation varies significantly between 6th ribs of elderly individuals.

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Linear microcrack accumulation is symmetrical (no differences) within 6th rib pairs.

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No differences exist in microcrack accumulation between pleural and cutaneous rib cortices.

Finite Element-Based Mechanical Assessment of Bone Quality on the Basis of In Vivo Images

Beyond bone mineral density (BMD), bone quality designates the mechanical integrity of bone tissue. In vivo images based on X-ray attenuation, such as CT reconstructions, provide size, shape, and local BMD distribution and may be exploited as input for finite element analysis (FEA) to assess bone fragility. Further key input parameters of FEA are the material properties of bone tissue. This review discusses the main determinants of bone mechanical properties and emphasizes the added value, as well as the important assumptions underlying finite element analysis. Bone tissue is a sophisticated, multiscale composite material that undergoes remodeling but exhibits a rather narrow band of tissue mineralization. Mechanically, bone tissue behaves elastically under physiologic loads and yields by cracking beyond critical strain levels. Through adequate cell-orchestrated modeling, trabecular bone tunes its mechanical properties by volume fraction and fabric. With proper calibration, these mechanical properties may be incorporated in quantitative CT-based finite element analysis that has been validated extensively with ex vivo experiments and has been applied increasingly in clinical trials to assess treatment efficacy against osteoporosis.

Mechanical properties of cortical bone and their relationships with age, gender, composition and microindentation properties in the elderly

The growing incidence of skeletal fractures poses a significant challenge to ageing societies. Since a major part of physiological loading in the lower limbs is carried by cortical bone, it would be desirable to better understand the structure-mechanical property relationships and scale effects in this tissue. This study aimed at assessing whether microindentation properties combined with chemical and morphological information are usable to predict macroscopic elastic and strength properties in a donor- and site-matched manner. Specimens for quasi-static macroscopic tests in tension, compression, and torsion and microindentation were prepared from a cohort of 19 male and 20 female donors (46 to 99 years). All tests were performed under fully hydrated conditions. The chemical composition of the extra-cellular matrix was investigated with Raman spectroscopy. The results of the micro-mechanical tests were combined with morphological and compositional properties using a power law relationship to predict the macro-mechanical results. Microindentation properties were not gender dependent, remarkably constant over age, and showed an overall small variation with standard deviations of approximately 10 %. Similar results were obtained for chemical tissue composition. Macro-mechanical stiffness and strength were significantly related to porosity for all load cases (p<0.05). In case of macroscopic yield strain and work-to-failure this was only true in torsion and compression, respectively. The correlations of macro-mechanical with micro-mechanical, morphological, and chemical properties showed no significance for cement line density, mineralisation, or variations in the microindentation results and were dominated by porosity with a moderate explanatory power of predominately less than 50 %. The results confirm that age, with minor exceptions gender, and small variations in average mineralisation have negligible effect on the tissue microindentation properties of human lamellar bone in the elderly. Furthermore, our findings suggest that microindentation experiments are suitable to predict macroscopic mechanical properties in the elderly only on average and not on a one to one basis. The presented data may help to form a better understanding of the mechanisms of ageing in bone tissue and of the length scale at which they are active. This may be used for future prediction of fracture risk in the elderly.

Intraskeletal variation in human cortical osteocyte lacunar density: Implications for bone quality assessment

Osteocytes and their lacunocanalicular network have been identified as the regulator of bone quality and function by exerting extensive influence over metabolic processes, mechanical adaptation, and mineral homeostasis. Recent research has shown that osteocyte apoptosis leads to a decrease in bone quality and increase in bone fragility mediated through its effects on remodeling. The purpose of this study is to investigate variation in cortical bone osteocyte lacunar density with respect to major factors including sex, age, and intracortical porosity to establish both regional and systemic trends. Samples from the midshaft femur, midshaft rib and distal one-third diaphysis of the radius were recovered from 30 modern cadaveric individuals (15 males and 15 females) ranging from 49 to 100 years old. Thick ground undecalcified histological (80 μm) cross-sections were made and imaged under bright field microscopy. Osteocyte lacunar density (Ot.Lc.N/B.Ar) and intracortical porosity (%Po.Ar) were quantified. No significant sex differences in Ot.Lc.N/B.Ar or %Po.Ar were found in any element. Linear regressions demonstrated a significant decrease in osteocyte lacunar density (Ot.Lc.N/B.Ar) and increase in intracortical porosity (%Po.Ar) with age for the sex-pooled sample in the femur (R² = 0.208, 0.297 respectively) and radius (R² = 0.108, 0.545 respectively). Age was unable to significantly predict osteocyte lacunar density or intracortical porosity in the rib (R² = 0.058, 0.114 respectively). Comparisons of regression coefficients demonstrated a systemic trend in the decrease in osteocyte lacunar density (Ot.Lc.N/B.Ar) and increase in intracortical porosity (%Po.Ar) with age. In each element, intracortical porosity was significantly negatively correlated with lacunar density for which the radius demonstrated the strongest relationship (r = - 0.746). Using pore number (Po.N) as a proxy for available vascularity to support the osteocyte population, Po.N was able to predict 61.8% of variation in osteocyte lacunar number (Ot.Lc.N) in the rib. The femur and radius also demonstrated significant relationships between these variables (R² = 0.560 and 0.397 respectively). The results from this study indicate that although the femur, radius and rib may be experiencing systemically influenced declines in osteocyte lacunar density, there may be differential effects at each anatomical site potentially due to age related changes in mechanical loading. With decreasing osteocyte lacunar density in each element, intracortical porosity increased with likely direct impacts on gross bone strength. This study provides a foundation upon which to build interpretations of osteocyte lacunar density values and their effect on differential fracture risk for aging individuals.

Elastic-plastic fracture toughness and rising JR-curve behavior of cortical bone is partially protected from irradiation-sterilization-induced degradation by ribose protectant

OBJECTIVE

This study tested the hypothesis that pre-treating cortical bone with ribose would protect the rising fracture resistance curve behavior and crack initiation fracture toughness of both bovine and human cortical bone from the degrading effects of γ-irradiation sterilization.

MATERIALS AND METHODS

A ribose pre-treatment (1.8 M for bovine, and 1.2 M for human, in PBS at 60 °C for 24 h) was applied to single-edge notched bending fracture specimens prior to sterilization with a 33 kGy dose of γ-irradiation. Fracture resistance curves were generated with a single specimen method using an optical crack length measurement technique. The effect of the treatment on overall fracture resistance behavior, crack initiation fracture toughness, and tearing modulus was compared with non-irradiated and conventionally irradiation sterilized controls. Hydrothermal isometric tension testing was used to examine collagen network connectivity and thermal stability to explore relationships between collagen network quality and fracture resistance.

RESULTS

The ribose pre-treatment successfully protected the crack growth initiation fracture toughness of bovine and human bone by 32% and 63%, respectively. The rising JR-curve behavior was also partially protected. Furthermore, collagen connectivity and thermal stability followed similar patterns to those displayed by fracture toughness.

CONCLUSIONS

This paper demonstrates that the fracture toughness of irradiation-sterilized bone tissue can be partially protected with a ribose pre-treatment. This new approach shows potential for the production and clinical application of sterilized allografts with improved mechanical performance and durability.

Bone Material Properties and Skeletal Fragility

Deformations of vertebrae and sudden fractures of long bones caused by essentially normal loading are a characteristic problem in osteoporosis. If the loading is normal, then the explanation for and prediction of unexpected bone failure lies in understanding the mechanical properties of the whole bone-which come from its internal and external geometry, the mechanical properties of the hard tissue, and from how well the tissue repairs damage. Modern QCT and MRI imaging systems can measure the geometry of the mineralized tissue quite well in vivo-leaving the mechanical properties of the hard tissue and the ability of bone to repair damage as important unknown factors in predicting fractures. This review explains which material properties must be measured to understand why some bones fail unexpectedly despite our current ability to determine bone geometry and bone mineral content in vivo. Examples of how to measure the important mechanical properties are presented along with some analysis of potential drawbacks of each method. Particular attention is given to methods useful to characterize the loss of bone toughness caused by mechanical fatigue, drug side effects, and damage to the bone matrix.

Fragility of Bone Material Controlled by Internal Interfaces

Bone material is built in a complex multiscale arrangement of mineralized collagen fibrils containing water, proteoglycans and some noncollagenous proteins. This organization is not static as bone is constantly remodeled and thus able to repair damaged tissue and adapt to the loading situation. In preventing fractures, the most important mechanical property is toughness, which is the ability to absorb impact energy without reaching complete failure. There is no simple explanation for the origin of the toughness of bone material, and this property depends in a complex way on the internal architecture of the material on all scales from nanometers to millimeters. Hence, fragility may have different mechanical origins, depending on which toughening mechanism is not working properly. This article reviews the toughening mechanisms described for bone material and attempts to put them in a clinical context, with the hope that future analysis of bone fragility may be guided by this collection of possible mechanistic origins.

The materials science of collagen

Collagen is the principal biopolymer in the extracellular matrix of both vertebrates and invertebrates. It is produced in specialized cells (fibroblasts) and extracted into the body by a series of intra and extracellular steps. It is prevalent in connective tissues, and the arrangement of collagen determines the mechanical response. In biomineralized materials, its fraction and spatial distribution provide the necessary toughness and anisotropy. We review the structure of collagen, with emphasis on its hierarchical arrangement, and present constitutive equations that describe its mechanical response, classified into three groups: hyperelastic macroscopic models based on strain energy in which strain energy functions are developed; macroscopic mathematical fits with a nonlinear constitutive response; structurally and physically based models where a constitutive equation of a linear elastic material is modified by geometric characteristics. Viscoelasticity is incorporated into the existing constitutive models and the effect of hydration is discussed. We illustrate the importance of collagen with descriptions of its organization and properties in skin, fish scales, and bone, focusing on the findings of our group.

Bone fracture toughness and strength correlate with collagen cross-link maturity in a dose-controlled lathyrism mouse model

Collagen cross-linking is altered in many diseases of bone, and enzymatic collagen cross-links are important to bone quality, as evidenced by losses of strength after lysyl oxidase inhibition (lathyrism). We hypothesized that cross-links also contribute directly to bone fracture toughness. A mouse model of lathyrism using subcutaneous injection of up to 500 mg/kg β-aminopropionitrile (BAPN) was developed and characterized (60 animals across 4 dosage groups). Three weeks of 150 or 350 mg/kg BAPN treatment in young, growing mice significantly reduced cortical bone fracture toughness, strength, and pyridinoline cross-link content. Ratios reflecting relative cross-link maturity were positive regressors of fracture toughness (HP/[DHLNL + HLNL] r(2)  = 0.208, p < 0.05; [HP + LP]/[DHNL + HLNL] r(2)  = 0.196, p < 0.1), whereas quantities of mature pyridinoline cross-links were significant positive regressors of tissue strength (lysyl pyridinoline r(2)  = 0.159, p = 0.014; hydroxylysyl pyridinoline r(2)  = 0.112, p < 0.05). Immature and pyrrole cross-links, which were not significantly reduced by BAPN, did not correlate with mechanical properties. The effect of BAPN treatment on mechanical properties was dose specific, with the greatest impact found at the intermediate (350 mg/kg) dose. Calcein labeling was used to define locations of new bone formation, allowing for the identification of regions of normally cross-linked (preexisting) and BAPN-treated (newly formed, cross-link-deficient) bone. Raman spectroscopy revealed spatial differences attributable to relative tissue age and effects of cross-link inhibition. Newly deposited tissues had lower mineral/matrix, carbonate/phosphate, and Amide I cross-link (matrix maturity) ratios compared with preexisting tissues. BAPN treatment did not affect mineral measures but significantly increased the cross-link (matrix maturity) ratio compared with newly formed control tissue. Our study reveals that spatially localized effects of short-term BAPN cross-link inhibition can alter the whole-bone collagen cross-link profile to a measureable degree, and this cross-link profile correlates with bone fracture toughness and strength. Thus, cross-link profile perturbations associated with bone disease may provide insight into bone mechanical quality and fracture risk.

γ-Irradiation sterilized bone strengthened and toughened by ribose pre-treatment

OBJECTIVE

This study tested the hypothesis that a ribose-based pre-treatment would protect the strength, ductility and toughness of γ-irradiation sterilized cortical bone.

METHODS

Experiment 1: The effects of ribose pre-treatment (1.8M in PBS at 60°C for 24h) prior to 33 kGy of irradiation on strength, ductility and toughness (beams in three-point bending) and fracture toughness (J-integral at instability in single edge notched (bending)) were tested against matched non-irradiated and irradiated controls from bovine tibiae. Experiment 2: Three-point bending tests were conducted using beams from human femora (males, 59-67 years). Bone collagen thermal stability and network connectivity were examined using hydrothermal isometric tension testing.

RESULTS

Ribose pre-treatment protected the strength, ductility and toughness of irradiation sterilized bovine and human specimens to differing degrees. Their ultimate strength was not detectably different from non-irradiated control levels; toughness in bovine and human specimens was protected by 57 and 76%, respectively. Untreated human bone was less affected by irradiation and ribose pre-treatment was more effective in human bone than bovine bone.

CONCLUSIONS

This paper presents the first proof-of-principle that irradiation-sterilized bone with improved mechanical properties can be produced through the application of a ribose pre-irradiation treatment, which provides a more stable and connected collagen network than found in conventionally irradiated controls.

Quantitative relationships between microdamage and cancellous bone strength and stiffness

Microscopic tissue damage (microdamage) is an aspect of bone quality associated with impaired bone mechanical performance. While it is clear that bone tissue submitted to more severe loading has greater amounts of microdamage (as measured through staining), how microdamage influences future mechanical performance of the bone has not been well studied, yet is necessary for understanding the mechanical consequences of the presence of microdamage. Here we determine how stained microdamage generated by a single compressive overload affects subsequent biomechanical performance of cancellous bone. Human vertebral cancellous bone specimens (n=47) from 23 donors (14 males, 9 females, 64-92years of age) were submitted to a compressive overload, stained for microdamage, then reloaded in compression to determine the relationship between the amount of microdamage caused by the initial load and reductions in mechanical performance during the reload. Damage volume fraction (DV/BV) caused by the initial overload was related to reductions in Young's modulus, yield strength, ultimate strength, and yield strain upon reloading (p<0.05, R(2)=0.18-0.34). The regression models suggest that, on average, relatively small amounts of microdamage are associated with large reductions in reload mechanical properties: a 1.50% DV/BV caused by a compressive overload was associated with an average reduction in Young's modulus of 41.0±3.2% (mean±SE), an average reduction in yield strength of 63.1±4.5% and an average reduction in ultimate strength of 52.7±4.0%. Specimens loaded beyond 1.2% (1.2-4.0% apparent strain) demonstrated a single relationship between reload mechanical properties (Young's modulus, yield strength, and ultimate strength) and bone volume fraction despite a large range in amounts of microdamage. Hence, estimates of future mechanical performance of cancellous bone can be achieved using the bone volume fraction and whether or not a specimen was previously loaded beyond ultimate strain. The empirical relationships provided in this study make it possible to estimate the degree of impaired mechanical performance resulting from an observed amount of stained microdamage.

Timescales of self-healing in human bone tissue and polymeric ionic liquids

Strain (stress-free) relaxation in mechanically prestrained bone has a time constant of 75 s. It occurs by a reorganization of the proteoglycan-glycoprotein matrix between collagen fibers, which requires ionic interactions. Dissolving and relinking the ionic bonds is thus an important tool of nature to enable plastic deformation and to develop self-healing tissues. A way to transfer this approach to technical materials is the attachment of ionic end groups to polymeric chains. In these classes of materials, the so-called polymeric ionic liquids, structural recovery of thermally disorganized material is observed. A time constant between minutes and a week could be achieved, also by ionic rearrangement. The same mechanism, rearrangement of ionic bonds, can lead to vastly different relaxation times when the ionic interaction is varied by exchange of the cationic end groups or the anions.

Modeling of femoral neck cortical bone for the numerical simulation of ultrasound propagation

Quantitative ultrasound assessment of the cortical compartment of the femur neck (FN) is investigated with the goal of achieving enhanced fracture risk prediction. Measurements at the FN are influenced by bone size, shape and material properties. The work described here was aimed at determining which FN material properties have a significant impact on ultrasound propagation around 0.5 MHz and assessing the relevancy of different models. A methodology for the modeling of ultrasound propagation in the FN, with a focus on the modeling of bone elastic properties based on scanning acoustic microscopy data, is introduced. It is found that the first-arriving ultrasound signal measured in through-transmission at the FN is not influenced by trabecular bone properties or by the heterogeneities of the cortical bone mineralized matrix. In contrast, the signal is sensitive to variations in cortical porosity, which can, to a certain extent, be accounted for by effective properties calculated with the Mori-Tanaka method.

Effects of model Maillard compounds on bone characteristics and functionality

BACKGROUND

Physical and biomechanical properties of bone can be affected by non-enzymatic crosslinks, which are implicated in bone pathologies such as osteoporosis. The purpose of this study was to analyse the effects of the consumption of model Maillard reaction product (MRP) from glucose-lysine heated for 90 min at 150 °C (GL90) on bone composition and features. Rats were fed either a control diet or a diet containing 30 g kg(-1) GL90 for 88 days. Food consumption and the animals' body weights were monitored. After sacrifice, the femur, pelvic bone and tibia were removed for analysis of their composition and physical and biomechanical properties.

RESULTS

The organic matrix of the femur and the density of the pelvic bone decreased after MRP intake, whereas pentosidine content increased greatly with respect to the control group (41.7 ± 9.9 vs 171.4 ± 3.3 mmol mol(-1) collagen). The rising level of C-telopeptide degradation products from type I collagen (β-CTX) suggested a possible situation of increased bone resorption and/or higher turnover.

CONCLUSION

In conjunction, the detrimental effect on the organic matrix, the situation of higher resorption and/or bone turnover indicated by the β-CTX values and the high pentosidine content in bone provoked negative consequences on certain mechanical properties such as the ability to withstand force and absorb energy without failure.

In vitro bone exposure to strontium improves bone material level properties

In rats treated with strontium ranelate, the ultimate load of intact bone is increased and associated with changes in microstructure and material level properties. Evaluation by micro-computed-tomography-based finite element analysis has shown that these changes independently contribute to the improvement of bone strength induced by strontium ranelate treatment. However, the mechanism by which Sr ion acts on bone material level properties remains unknown. The vertebrae of intact female rats were exposed overnight to 0.5, 1 or 2M chloride salt solutions of Sr, Ca and Ba. The latter two were used to assess the specificity of Sr. Bone material level properties were evaluated by measuring hardness, elastic modulus and working energy in a nanoindentation test. Wavelength dispersive X-ray spectroscopy provided semi-quantitative elemental analysis and mapping. Incubation with Sr rendered bone stiffer, harder and tougher. Among the divalent ions tested, Sr had the greatest effect. Sr affinity was also assessed on in vivo treated bone specimens. After in vitro exposure, the highest improvements were observed in ovariectomized rats. However, anti-osteoporotic treatments did not influence the capacity of Sr to modify bone material level properties. Our findings demonstrated that in vitro incubation with Sr selectively improved bone material level properties, which may contribute to the macroscopic increase of bone properties observed under Sr therapy.

Longitudinal elastic properties and porosity of cortical bone tissue vary with age in human proximal femur

Tissue level structural and mechanical properties are important determinants of bone strength. As an individual ages, microstructural changes occur in bone, e.g., trabeculae and cortex become thinner and porosity increases. However, it is not known how the elastic properties of bone change during aging. Bone tissue may lose its elasticity and become more brittle and prone to fractures as it ages. In the present study the age-dependent variation in the spatial distributions of microstructural and microelastic properties of the human femoral neck and shaft were evaluated by using acoustic microscopy. Although these properties may not be directly measured in vivo, there is a major interest to investigate their relationships with the linear elastic measurements obtained by diagnostic ultrasound at the most severe fracture sites, e.g., the femoral neck. However, before the validity of novel in vivo techniques can be established, it is essential to understand the age-dependent variation in tissue elastic properties and porosity at different skeletal sites. A total of 42 transverse cross-sectional bone samples were obtained from the femoral neck (Fn) and proximal femoral shaft (Ps) of 21 men (mean±SD age 47.1±17.8, range 17-82years). Samples were quantitatively imaged using a scanning acoustic microscope (SAM) equipped with a 50MHz ultrasound transducer. Distributions of the elastic coefficient (c33) of cortical (Ct) and trabecular (Tr) tissues and microstructure of cortex (cortical thickness Ct.Th and porosity Ct.Po) were determined. Variations in c33 were observed with respect to tissue type (c33Tr<c33Ct), location (c33(Ct.Ps)=37.7GPa>c33(Ct.Fn)=35.3GPa>c33(Tr.Ps)=33.8GPa>c33(Tr.Fn)=31.9GPa), and cadaver age (R(2)=0.28-0.46, p<0.05). Regional variations in porosity were found in the neck (superior 13.1%; inferior 6.1%; anterior 10.1%; posterior 8.6%) and in the shaft (medial 9.5%; lateral 7.7%; anterior 8.6%; posterior 12.0%). In conclusion, significant variations in elastic coefficients were detected between femoral neck and shaft as well as between the quadrants of the cross-sections of neck and shaft. Moreover, an age-related increase in cortical porosity and a stiffening of the bone tissue were observed. These findings may explain in part the increase in susceptibility to suffer low energy fractures during aging and highlight the potential of ultrasound in clinical osteoporosis diagnostics.