X-ray with finite element analysis is a viable alternative for MRI to predict knee osteoarthritis: Data from the Osteoarthritis Initiative.

Magnetic resonance imaging (MRI) offers superior soft tissue contrast compared to clinical X-ray imaging methods, while also providing accurate three-dimensional (3D) geometries, it could be reasoned to be the best imaging modality to create 3D finite element (FE) geometries of the knee joint. However, MRI may not necessarily be superior for making tissue-level FE simulations of internal stress distributions within knee joint, which can be utilized to calculate subject-specific risk for the onset and development of knee osteoarthritis (KOA). Specifically, MRI does not provide any information about tissue stiffness, as the imaging is usually performed with the patient lying on their back. In contrast, native X-rays taken while the patient is standing indirectly reveal information of the overall health of the knee that is not seen in MRI. To determine the feasibility of X-ray workflow to generate FE models based on the baseline information (clinical image data and subject characteristics), we compared MRI and X-ray-based simulations of volumetric cartilage degenerations (N = 1213) against 8-year follow-up data. The results suggest that X-ray-based predictions of KOA are at least as good as MRI-based predictions for subjects with no previous knee injuries. This finding may have important implications for preventive care, as X-ray imaging is much more accessible than MRI.

Atlas-based finite element analyses with simpler constitutive models predict personalized progression of knee osteoarthritis: data from the osteoarthritis initiative.

New technologies are required to support a radical shift towards preventive healthcare. Here we focus on evaluating the possibility of finite element (FE) analysis-aided prevention of knee osteoarthritis (OA), a disease that affects 100 million citizens in the US and EU and this number is estimated to increase drastically. Current clinical methods to diagnose or predict joint health status relies on symptoms and tissue failures obtained from clinical imaging. In a joint with no detectable injuries, the diagnosis of the future health of the knee can be assumed to be very subjective. Quantitative approaches are therefore needed to assess the personalized risk for the onset and development of knee OA. FE analysis utilizing an atlas-based modeling approach has shown a preliminary capability for simulating subject-specific cartilage mechanical responses. However, it has been verified with a very limited subject number. Thus, the aim of this study is to verify the real capability of the atlas-based approach to simulate cartilage degeneration utilizing different material descriptions for cartilage. A fibril reinforced poroviscoelastic (FRPVE) material formulation was considered as state-of-the-art material behavior, since it has been preliminary validated against real clinical follow-up data. Simulated mechanical tissue responses and predicted cartilage degenerations within knee joint with FRPVE material were compared against simpler constitutive models for cartilage. The capability of the atlas-based modeling to offer a feasible approach with quantitative evaluation for the risk for the OA development (healthy vs osteoarthritic knee, p < 0.01, AUC ~ 0.7) was verified with 214 knees. Furthermore, the results suggest that accuracy for simulation of cartilage degeneration with simpler material models is similar to models using FPRVE materials if the material parameters are chosen properly.

Molecular Quantity Variations in Human-Mandibular-Bone Osteoid.

Osteoid is a layer of new-formed bone that is deposited on the bone border during the process of new bone formation. This deposition process is crucial for bone tissue, and flaws in it can lead to bone diseases. Certain bone diseases, i.e. medication related osteonecrosis, are overexpressed in mandibular bone. Because mandibular bone presents different properties than other bone types, the data concerning osteoid formation in other bones are inapplicable for human-mandibular bone. Previously, the molecular distribution of other bone types has been presented using Fourier-transform infrared (FTIR) spectroscopy. However, the spatial distribution of molecular components of healthy-human-mandibular-bone osteoid in relation to histologic landmarks has not been previously presented and needs to be studied in order to understand diseases that occur human-mandibular bone. This study presents for the first time the variation in molecular distribution inside healthy-human-mandibular-bone osteoid by juxtaposing FTIR data with its corresponding histologic image obtained by autofluorescence imaging of its same bone section. During new bone formation, bone-forming cells produce an osteoid constituted primarily of type I collagen. It was observed that in mandibular bone, the collagen type I increases from the osteoblast line with the distance from the osteoblasts, indicating progressive accumulation of collagen during osteoid formation. Only later inside the collagen matrix, the osteoid starts to mineralize. When the mineralization starts, the collagen accumulation diminishes whereas the collagen maturation still continues. This chemical-apposition process in healthy mandibular bone will be used in future as a reference to understand different pathologic conditions that occur in human-mandibular bone.

Time course changes to structural, mechanical and material properties of bone in rats after complete spinal cord injury.

OBJECTIVE: Characterise the spatiotemporal trabecular and cortical bone responses to complete spinal cord injury (SCI) in young rats. METHODS: 8-week-old male Wistar rats received T9-transection SCI and were euthanised 2-, 6-, 10- or 16-weeks post-surgery. Outcome measures were assessed using micro-computed tomography, mechanical testing, serum markers and Fourier-transform infrared spectroscopy. RESULTS: The trabecular and cortical bone responses to SCI are site-specific. Metaphyseal trabecular BV/TV was 59% lower, characterised by fewer and thinner trabeculae at 2-weeks post-SCI, while epiphyseal BV/TV was 23% lower with maintained connectivity. At later-time points, metaphyseal BV/TV remained unchanged, while epiphyseal BV/TV increased. The total area of metaphyseal and mid-diaphyseal cortical bone were lower from 2-weeks and between 6- and 10-weeks post-SCI, respectively. This suggested that SCI-induced bone changes observed in the rat model were not solely attributable to bone loss, but also to suppressed bone growth. No tissue mineral density differences were observed at any time-point, suggesting that decreased whole-bone mechanical properties were primarily the result of changes to the spatial distribution of bone. CONCLUSION: Young SCI rat trabecular bone changes resemble those observed clinically in adult and paediatric SCI, while cortical bone changes resemble paediatric SCI only.

Mineral Crystal Thickness in Calcified Cartilage and Subchondral Bone in Healthy and Osteoarthritic Human Knees.

Osteoarthritis (OA) is the most common joint disease, where articular cartilage degradation is often accompanied with sclerosis of the subchondral bone. However, the association between OA and tissue mineralization at the nanostructural level is currently not understood. In particular, it is technically challenging to study calcified cartilage, where relevant but poorly understood pathological processes such as tidemark multiplication and advancement occur. Here, we used state-of-the-art microfocus small-angle X-ray scattering with a 5-µm spatial resolution to determine the size and organization of the mineral crystals at the nanostructural level in human subchondral bone and calcified cartilage. Specimens with a wide spectrum of OA severities were acquired from both medial and lateral compartments of medial compartment knee OA patients (n = 15) and cadaver knees (n = 10). Opposing the common notion, we found that calcified cartilage has thicker and more mutually aligned mineral crystals than adjoining bone. In addition, we, for the first time, identified a well-defined layer of calcified cartilage associated with pathological tidemark multiplication, containing 0.32 nm thicker crystals compared to the rest of calcified cartilage. Finally, we found 0.2 nm thicker mineral crystals in both tissues of the lateral compartment in OA compared with healthy knees, indicating a loading-related disease process because the lateral compartment is typically less loaded in medial compartment knee OA. In summary, we report novel changes in mineral crystal thickness during OA. Our data suggest that unloading in the knee might be involved with the growth of mineral crystals, which is especially evident in the calcified cartilage. © 2022 The Authors. Journal of Bone and Mineral Research published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research (ASBMR).

Generalized Uncoupled Bone Remodeling Associated With Delayed Healing of Fatigue Fractures.

Fatigue fractures in bones are common injuries with load-bearing activities, during which the remodeling aimed at removing microdamage has been suggested to play a role in increasing related fracture risk. Much attention has been given to the uncoupling between osteoclastic bone resorption and osteoblastic osteogenesis in fatigue fracture cases; however, the underlying pathophysiologic mechanisms of impaired fracture healing are yet unknown. Here we report multiple fatigue fractures in a physically active woman receiving contraceptive pills for years. Her fracture healing was remarkably slow, although she has been otherwise healthy. The patient underwent bone biopsy of the iliac crest that showed remarkable peritrabecular fibrosis with increased osteoclastic bone resorption combined with relatively low bone formation. Analysis of bone biochemical composition revealed a more complex picture: First, notable declines in bone mineral content-based parameters indicating abnormal mineralization were evident in both cancellous and cortical bone. Second, there was elevation in mineral crystal size, perfection, and collagen maturity in her bone tissues from different anatomical sites. To our knowledge, this is the first report showing generalized uncoupling in bone remodeling, increased peritrabecular fibrosis, and bone compositional changes associated with delayed healing of fatigue fractures. These results may explain delayed healing of fatigue and stress fractures. © 2021 The Authors. JBMR Plus published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research.

High-resolution infrared microspectroscopic characterization of cartilage cell microenvironment.

The lateral resolution of infrared spectroscopy has been inadequate for accurate biochemical characterization of the cell microenvironment, a region regulating biochemical and biomechanical signals to cells. In this study, we demonstrate the capacity of a high-resolution Fourier transform infrared microspectroscopy (HR-FTIR-MS) to characterize the collagen content of this region. Specifically, we focus on the collagen content in the cartilage cell (chondrocyte) microenvironment of healthy and osteoarthritic (OA) cartilage. Human tibial cartilage samples (N = 28) were harvested from 7 cadaveric donors and graded for OA severity (healthy, early OA, advanced OA). HR-FTIR-MS was used to analyze the collagen content of the chondrocyte microenvironment of five distinct zones across the tissue depth. HR-FTIR-MS successfully showed collagen content distribution across chondrocytes and their environment. In zones 2 and 3 (10 - 50% of the tissue thickness), we observed that collagen content was smaller (P < 0.05) in early OA compared to the healthy tissue in the vicinity of cells (pericellular region). The collagen content loss was extended to the extracellular matrix in advanced OA tissue. No significant differences in the collagen content of the chondrocyte microenvironment were observed between the groups in the most superficial (0-10%) and deep zones (50-100%). HR-FTIR-MS revealed collagen loss in the early OA cartilage pericellular region before detectable changes in the extracellular matrix in advanced OA. HR-FTIR-MS-based compositional assessment enables a better understanding of OA-related changes in tissues. This technique can be used to identify new disease mechanisms enabling better intervention strategies. STATEMENT OF SIGNIFICANCE: Osteoarthritis (OA) is the most common degenerative joint disease causing pain and disability. While significant progress has been made in OA research, OA pathogenesis is still poorly understood and current OA treatments are mainly palliative. This study demonstrates that high-resolution FTIR microspectroscopy (HR-FTIR-MS) can characterize OA-induced compositional changes in the cell microenvironment (pericellular matrix) during the early disease stages before tissue changes in the extracellular matrix become apparent. This technique may further enable the identification of new OA mechanisms and improve our current understanding of OA pathogenesis, thus, enabling the development of better treatment methods.

Effects of human articular cartilage constituents on simultaneous diffusion of cationic and nonionic contrast agents.

Contrast-enhanced computed tomography is an emerging diagnostic technique for osteoarthritis. However, the effects of increased water content, as well as decreased collagen and proteoglycan concentrations due to cartilage degeneration, on the diffusion of cationic and nonionic agents, are not fully understood. We hypothesize that for a cationic agent, these variations increase the diffusion rate while decreasing partition, whereas, for a nonionic agent, these changes increase both the rate of diffusion and partition. Thus, we examine the diffusion of cationic and nonionic contrast agents within degraded tissue in time- and depth-dependent manners. Osteochondral plugs (N = 15, d = 8 mm) were extracted from human cadaver knee joints, immersed in a mixture of cationic CA4+ and nonionic gadoteridol contrast agents, and imaged at multiple time-points, using the dual-contrast method. Water content, and collagen and proteoglycan concentrations were determined using lyophilization, infrared spectroscopy, and digital densitometry, respectively. Superficial to mid (0%-60% depth) cartilage CA4+ partitions correlated with water content (R < -0.521, P < .05), whereas in deeper (40%-100%) cartilage, CA4+ correlated only with proteoglycans (R > 0.671, P < .01). Gadoteridol partition correlated inversely with collagen concentration (0%-100%, R < -0.514, P < .05). Cartilage degeneration substantially increased the time for CA4+ compared with healthy tissue (248 ± 171 vs 175 ± 95 minute) to reach the bone-cartilage interface, whereas for gadoteridol the time (111 ± 63 vs 179 ± 163 minute) decreased. The work clarifies the diffusion mechanisms of two different contrast agents and presents depth and time-dependent effects resulting from articular cartilage constituents. The results will inform the development of new contrast agents and optimal timing between agent administration and joint imaging.

Diminishing effects of mechanical loading over time during rat Achilles tendon healing.

Mechanical loading affects tendon healing and recovery. However, our understanding about how physical loading affects recovery of viscoelastic functions, collagen production and tissue organisation is limited. The objective of this study was to investigate how different magnitudes of loading affects biomechanical and collagen properties of healing Achilles tendons over time. Achilles tendon from female Sprague Dawley rats were cut transversely and divided into two groups; normal loading (control) and reduced loading by Botox (unloading). The rats were sacrificed at 1, 2- and 4-weeks post-injury and mechanical testing (creep test and load to failure), small angle x-ray scattering (SAXS) and histological analysis were performed. The effect of unloading was primarily seen at the early time points, with inferior mechanical and collagen properties (SAXS), and reduced histological maturation of the tissue in unloaded compared to loaded tendons. However, by 4 weeks no differences remained. SAXS and histology revealed heterogeneous tissue maturation with more mature tissue at the peripheral region compared to the center of the callus. Thus, mechanical loading advances Achilles tendon biomechanical and collagen properties earlier compared to unloaded tendons, and the spatial variation in tissue maturation and collagen organization across the callus suggests important regional (mechano-) biological activities that require more investigation.

Multiscale Characterization of Embryonic Long Bone Mineralization in Mice.

Long bone mineralization occurs through endochondral ossification, where a cartilage template mineralizes into bone-like tissue with a hierarchical organization from the whole bone-scale down to sub-nano scale. Whereas this process has been extensively studied at the larger length scales, it remains unexplored at some of the smaller length scales. In this study, the changes in morphology, composition, and structure during embryonic mineralization of murine humeri are investigated using a range of high-resolution synchrotron-based imaging techniques at several length scales. With micro- and nanometer spatial resolution, the deposition of elements and the shaping of mineral platelets are followed. Rapid mineralization of the humeri occurs over approximately four days, where mineral to matrix ratio and calcium content in the most mineralized zone reach adult values shortly before birth. Interestingly, zinc is consistently found to be localized at the sites of ongoing new mineralization. The mineral platelets in the most recently mineralized regions are thicker, longer, narrower, and less aligned compared to those further into the mineralized region. In summary, this study demonstrates a specific spatial distribution of zinc, with highest concentration where new mineral is being deposited and that the newly formed mineral platelets undergo slight reshaping and reorganization during embryonic development.

Sub-trabecular strain evolution in human trabecular bone.

To comprehend the most detrimental characteristics behind bone fractures, it is key to understand the material and tissue level strain limits and their relation to failure sites. The aim of this study was to investigate the three-dimensional strain distribution and its evolution during loading at the sub-trabecular level in trabecular bone tissue. Human cadaver trabecular bone samples were compressed in situ until failure, while imaging with high-resolution synchrotron radiation X-ray tomography. Digital volume correlation was used to determine the strains inside the trabeculae. Regions without emerging damage were compared to those about to crack. Local strains in close vicinity of developing cracks were higher than previously reported for a whole trabecular structure and similar to those reported for single isolated trabeculae. Early literature on bone fracture strain thresholds at the tissue level seem to underestimate the maximum strain magnitudes in trabecular bone. Furthermore, we found lower strain levels and a reduced ability to capture detailed crack-paths with increased image voxel size. This highlights the dependence between the observed strain levels and the voxel size and that high-resolution is needed to investigate behavior of individual trabeculae. Furthermore, low trabecular thickness appears to be one predictor of developing cracks. In summary, this study investigated the local strains in whole trabecular structure at sub-trabecular resolution in human bone and confirmed the high strain magnitudes reported for single trabeculae under loading and, importantly extends its translation to the whole trabecular structure.

Structure-Function Relationships of Healthy and Osteoarthritic Human Tibial Cartilage: Experimental and Numerical Investigation.

Relationships between composition, structure and constituent-specific functional properties of human articular cartilage at different stages of osteoarthritis (OA) are poorly known. We established these relationships by comparison of elastic, viscoelastic and fibril-reinforced poroelastic mechanical properties with microscopic and spectroscopic analysis of structure and composition of healthy and osteoarthritic human tibial cartilage (n = 27). At a low frequency (0.005 Hz), proteoglycan content correlated negatively and collagen content correlated positively with the phase difference (i.e. tissue viscosity). At a high-frequency regime (> 0.05 Hz), proteoglycan content correlated negatively and collagen orientation angle correlated positively with the phase difference. Proteoglycans were lost in the early and advanced OA groups compared to the healthy group, while the superficial collagen orientation angle was greater only in the advanced OA group compared to the healthy group. Simultaneously, the initial fibril network modulus (fibril pretension) was smaller in the early and advanced OA groups compared to the healthy group. These findings suggest different mechanisms contribute to cartilage viscosity in low and high frequencies, and that the loss of superficial collagen pretension during early OA is due to lower tissue swelling (PG loss), while in advanced OA, both collagen disorganization and lower swelling modulate the collagen fibril pretension.

Bright ultrashort echo time SWIFT MRI signal at the osteochondral junction is not located in the calcified cartilage.

In this study, we aimed to precisely localize the hyperintense signal that is generated at the osteochondral junction when using ultrashort echo time magnetic resonance imaging (MRI) and to investigate the osteochondral junction using sweep imaging with Fourier transformation (SWIFT) MRI. Furthermore, we seek to evaluate what compositional properties of the osteochondral junction are the sources of this signal. In the study, we obtained eight samples from a tibial plateau dissected from a 68-year-old male donor, and one additional osteochondral sample of bovine origin. The samples were imaged using high-resolution ultrashort echo time SWIFT MRI and microcomputed tomography (µCT) scans. Localization of the bright signal in the osteochondral junction was performed using coregistered data sets. Potential sources of the signal feature were examined by imaging the bovine specimen with variable receiver bandwidths and by performing variable flip angle T1 relaxation time mapping. The results of the study showed that the hyperintense signal was found to be located entirely in the deep noncalcified articular cartilage. The intensity of this signal at the interface varied between the specimens. Further tests with bovine specimens indicated that the imaging bandwidth and T1 relaxation affect the properties of the signal. Based on the present results, the calcified cartilage has low signal intensity even in SWIFT imaging. Concomitantly, it appears that the bright signal seen in ultrashort echo time imaging resides within the noncalcified cartilage. Furthermore, the most likely sources of this signal are the rapid T1 relaxation of the deep cartilage and the susceptibility-induced effects arising from the calcified tissues.

Synchrotron MicroCT Reveals the Potential of the Dual Contrast Technique for Quantitative Assessment of Human Articular Cartilage Composition.

Dual contrast micro computed tomography (CT) shows potential for detecting articular cartilage degeneration. However, the performance of conventional CT systems is limited by beam hardening, low image resolution (full-body CT), and long acquisition times (conventional microCT). Therefore, to reveal the full potential of the dual contrast technique for imaging cartilage composition we employ the technique using synchrotron microCT. We hypothesize that the above-mentioned limitations are overcome with synchrotron microCT utilizing monochromatic X-ray beam and fast image acquisition. Human osteochondral samples (n = 41, four cadavers) were immersed in a contrast agent solution containing two agents (cationic CA4+ and non-ionic gadoteridol) and imaged with synchrotron microCT at an early diffusion time point (2 h) and at diffusion equilibrium (72 h) using two monochromatic X-ray energies (32 and 34 keV). The dual contrast technique enabled simultaneous determination of CA4+ (i.e., proteoglycan content) and gadoteridol (i.e., water content) partitions within cartilage. Cartilage proteoglycan content and biomechanical properties correlated significantly (0.327 < r < 0.736, p < 0.05) with CA4+ partition in superficial and middle zones at both diffusion time points. Normalization of the CA4+ partition with gadoteridol partition within the cartilage significantly (p < 0.05) improved the detection sensitivity for human osteoarthritic cartilage proteoglycan content, biomechanical properties, and overall condition (Mankin, Osteoarthritis Research Society International, and International Cartilage Repair Society grading systems). The dual energy technique combined with the dual contrast agent enables assessment of human articular cartilage proteoglycan content and biomechanical properties based on CA4+ partition determined using synchrotron microCT. Additionally, the dual contrast technique is not limited by the beam hardening artifact of conventional CT systems. © 2019 The Authors. Journal of Orthopaedic Research® published by Wiley Periodicals, Inc. on behalf of Orthopaedic Research Society. J Orthop Res 38:563-573, 2020.

Triple Contrast CT Method Enables Simultaneous Evaluation of Articular Cartilage Composition and Segmentation.

Early degenerative changes of articular cartilage are detected using contrast-enhanced computed tomography (CT) with a cationic contrast agent (CA). However, cationic CA diffusion into degenerated cartilage decreases with proteoglycan depletion and increases with elevated water content, thus hampering tissue evaluation at early diffusion time points. Furthermore, the contrast at synovial fluid-cartilage interface diminishes as a function of diffusion time hindering accurate cartilage segmentation. For the first time, we employ quantitative dual-energy CT (QDECT) imaging utilizing a mixture of three CAs (cationic CA4+ and non-ionic gadoteridol which are sensitive to proteoglycan and water contents, respectively, and bismuth nanoparticles which highlight the cartilage surface) to simultaneously segment the articulating surfaces and determine of the cartilage condition. Intact healthy, proteoglycan-depleted, and mechanically injured bovine cartilage samples (n = 27) were halved and imaged with synchrotron microCT 2-h post immersion in triple CA or in dual CA (CA4+ and gadoteridol). CA4+ and gadoteridol partitions were determined using QDECT, and pairwise evaluation of these partitions was conducted for samples immersed in dual and triple CAs. In conclusion, the triple CA method is sensitive to proteoglycan depletion while maintaining sufficient contrast at the articular surface to enable detection of cartilage lesions caused by mechanical impact.

Mineralization of cortical bone during maturation and growth in rabbits.

INTRODUCTION: The composite nature of bone as a material governs its structure and mechanical behavior. How the collagenous matrix mineralizes, in terms of both mineral deposition and structure of the mineral crystals, is highly interesting when trying to elucidate the complex structural changes that occur during bone growth and maturation. We have previously looked at mineral deposition and structural evolution of the collagenous matrix, linking both to changes in mechanics. The purpose of this study was to provide specific information on changes in crystal size and organization as a function of growth and maturation. MATERIALS AND METHODS: Using micro-computed tomography (µCT) and micro-focused scanning small-angle X-ray scattering (SAXS) we investigated cortical bone in two orthogonal directions relative to the long axis of the humeri of New Zealand White rabbits spanning from new-born to 6-months of age. We also investigated the changes with tissue age by looking at radial profiles of osteonal structures in the 6-months old rabbits. The findings were compared to our previous compositional, structural and mechanical data on the same sample cohort. RESULTS: µCT showed a continuous mineral deposition up until 3-months of age, whilst the SAXS data showed an increase in both crystal thickness and degree of orientation up until 6-months of age. The osteonal profiles showed no statistically significant changes in crystal thickness. CONCLUSIONS: Comparison to previously collected mechanical data suggests that changes are not only explained by amount of mineral in the tissue but also by the crystal dimensions.

Simultaneous Quantitation of Cationic and Non-ionic Contrast Agents in Articular Cartilage Using Synchrotron MicroCT Imaging.

Early diagnosis of acute cartilage injuries enables monitoring of disease progression and improved treatment option planning to prevent post-traumatic osteoarthritis. In contrast-enhanced computed tomography (CECT), the changes in cationic agent diffusion within the tissue reflect cartilage degeneration. The diffusion in degenerated cartilage depends on proteoglycan (PG) content and water content, but each having an opposite effect on diffusion, thus compromising the diagnostic sensitivity. To overcome this limitation, we propose the simultaneous imaging of cationic (sensitive to PG and water contents) and non-ionic (sensitive to water content) agents. In this study, quantitative dual-energy CT (QDECT) imaging of two agents is reported for the first time at clinically feasible imaging time points. Furthermore, this is the first time synchrotron microCT with monochromatic X-rays is employed in cartilage CECT. Imaging was conducted at 1 and 2 h post contrast agent immersion. Intact, PG-depleted, and mechanically injured + PG-depleted cartilage samples (n = 33) were imaged in a mixture of cationic (iodine-based CA4+) and non-ionic (gadolinium-based gadoteridol) agents. Concurrent evaluation of CA4+ and gadoteridol partitions in cartilage is accomplished using QDECT. Subsequent normalization of the CA4+ partition with that of the gadoteridol affords CA4+ attenuations that significantly correlate with PG content - a key marker of OA.

Experimental mechanical strain measurement of tissues.

Strain, an important biomechanical factor, occurs at different scales from molecules and cells to tissues and organs in physiological conditions. Under mechanical strain, the strength of tissues and their micro- and nanocomponents, the structure, proliferation, differentiation and apoptosis of cells and even the cytokines expressed by cells probably shift. Thus, the measurement of mechanical strain (i.e., relative displacement or deformation) is critical to understand functional changes in tissues, and to elucidate basic relationships between mechanical loading and tissue response. In the last decades, a great number of methods have been developed and applied to measure the deformations and mechanical strains in tissues comprising bone, tendon, ligament, muscle and brain as well as blood vessels. In this article, we have reviewed the mechanical strain measurement from six aspects: electro-based, light-based, ultrasound-based, magnetic resonance-based and computed tomography-based techniques, and the texture correlation-based image processing method. The review may help solving the problems of experimental and mechanical strain measurement of tissues under different measurement environments.

Linking multiscale deformation to microstructure in cortical bone using in situ loading, digital image correlation and synchrotron X-ray scattering.

The incidence of fragility fractures is expected to increase in the near future due to an aging population. Therefore, improved tools for fracture prediction are required to treat and prevent these injuries efficiently. For such tools to succeed, a better understanding of the deformation mechanisms in bone over different length scales is needed. In this study, an experimental setup including mechanical tensile testing in combination with digital image correlation (DIC) and small/wide angle X-ray scattering (SAXS/WAXS) was used to study deformation at multiple length scales in bovine cortical bone. Furthermore, micro-CT imaging provided detailed information about tissue microstructure. The combination of these techniques enabled measurements of local deformations at the tissue- and nanoscales. The orientation of the microstructure relative to the tensile loading was found to influence the strain magnitude on all length scales. Strains in the collagen fibers were 2-3 times as high as the strains found in the mineral crystals for samples with microstructure oriented parallel to the loading. The local tissue strain at fracture was found to be around 0.5%, independent of tissue orientation. However, the maximum force and the irregularity of the crack path were higher when the load was applied parallel to the tissue orientation. This study clearly shows the potential of combining these different experimental techniques concurrently with mechanical testing to gain a better understanding of bone damage and fracture over multiple length scales in cortical bone. STATEMENT OF SIGNIFICANCE: To understand the pathophysiology of bone, it is important to improve our knowledge about the deformation and fracture mechanisms in bone. In this study, we combine several recently available experimental techniques with mechanical loading to investigate the deformation mechanisms in compact bone tissue on several length scales simultaneously. The experimental setup included mechanical tensile testing in combination with digital image correlation, microCT imaging, and small/wide angle X-ray scattering. The combination of techniques enabled measurements of local deformations at the tissue- and nanoscales. The study clearly shows the potential of combining different experimental techniques concurrently with mechanical testing to gain a better understanding of structure-property-function relationships in bone tissue.

The compositional and nano-structural basis of fracture healing in healthy and osteoporotic bone.

Osteoporosis, a prevalent metabolic bone disorder, predisposes individuals to increased susceptibility to fractures. It is also, somewhat controversially, thought to delay or impair the regenerative response. Using high-resolution Fourier-transform infrared spectroscopy and small/wide-angle X-ray scattering we sought to answer the following questions: Does the molecular composition and the nano-structure in the newly regenerated bone differ between healthy and osteoporotic environments? And how do pharmacological treatments, such as bone morphogenetic protein 7 (BMP-7) alone or synergistically combined with zoledronate (ZA), alter callus composition and nano-structure in such environments? Cumulatively, on the basis of compositional and nano-structural characterizations of newly formed bone in an open-osteotomy rat model, the healing response in untreated healthy and ovariectomy-induced osteoporotic environments was fundamentally the same. However, the BMP-7 induced osteogenic response resulted in greater heterogeneity in the nano-structural crystal dimensions and this effect was more pronounced with osteoporosis. ZA mitigated the effects of the upregulated catabolism induced by both BMP-7 and an osteoporotic bone environment. The findings contribute to our understanding of how the repair processes in healthy and osteoporotic bone differ in both untreated and treated contexts and the data presented represents the most comprehensive study of fracture healing at the nanoscale undertaken to date.