75 research outputs found
Multi-scale physico-chemical phenomena in articular cartilage and subchondral bone
No full textArticular cartilage and its connecting subchondral bone plate are main compartments that play an important role in proper mechanical functioning of diarthrodial joints. However, in ageing and osteoarthritis structural changes propagate in these tissues, which impairs them for proper functioning. One way to characterize the structural alterations during different stages of degeneration and disease is to perform solute transport tests in an experimental set-up and quantify the parameters such as diffusivity and permeability that represent the quality of the tissue. Therefore, common imaging techniques to investigate diffusion across cartilage have been reviewed and suggestions to select appropriate tools to determine diffusion parameters were made. The primary aim of this thesis is to setup a set of experiments that can capture the transport properties of cartilage and its underlying bone. To this end we designed experiments that used contrast-enhanced micro-computed tomography to measure diffusion through cartilage and subchondral bone and applied computational models of these experiments to quantify the physical parameters such as diffusivity and permeability of the tissues concerned. The individual role of the bath osmolality, concentration and solute’s charge on the diffusion in articular cartilage was successfully studied and solute’s charge was identified as the dominant factor. Biphasic-solute and multiphasic finite element models were subsequently developed to precisely simulate the transport of neutral and charged solutes in various cartilage zones, respectively. As the cartilage-bone interface experiences morphological alterations after joint degeneration and during progression of osteoarthritis, characterization of the transport properties of the interface becomes of paramount importance. Therefore, experiments based on contrast-enhanced micro-computed tomography were first conducted and a correlation was found between the extent of diffusion and the micro-architecture of the subchondral bone plate and articular cartilage. Multi-zone biphasic-solute finite element models then assisted in determination of diffusion coefficients in different cartilage layers and the subchondral bone plate. To identify the pathways between cartilage and bone responsible for transport, focused-ion-beam scanning electron microscopy imaging was employed. Using the 3D data of the pore structure at the osteochondral interface and with the aid of advanced pore-network modeling, the diffusive and permeability attributes of the extracellular matrices were successfully determined. The next theme of this thesis was to assess the effects of collagen fibril conformation under different environmental osmotic pressure on advanced non-enzymatic glycation, a process, responsible for cartilage deterioration during ageing. Sugars were added under different external bath osmotic pressures to study the glycation process when collagen fibrils are either stretched or shrunk. Using micro-indentation, biochemical assays, contrast-enhanced micro-computed tomography and cartilage surface colorimetry, the stretching of the collagen fibrils was found to minimize the degenerative effects of sugar-induced glycation. The author believes that the current findings contribute to solve the yet-challenging physico-chemical problems involved in the cartilage and its related joint disease. The current work will lead to new techniques that aim to not only understand the fundamental physico-chemical aspects of cartilage, but also might suggest methods for efficient delivery of drugs into the cartilage tissue and visualizing agents that could better follow and diagnose joint disease
A Multi-Scale Approach to Implications of the Preferred Vertebral Trabecular Orientation on Spine Biomechanics
No full textKnowledge of the influence of loading directions on trabecular bone remodeling in spine is of significant value in understanding the development of spine deformities and vertebral bone quality across different scales. Information on the constitution of a preferred trabecular orientation and mechanical properties of trabecular bone are important indicators in this respect. The current thesis aimed at exploring these aspects across multiple length scales in the spine. The thesis is divided in two parts. The influence of loadings less dominant than compression, i.e. shear, on the constitution of a preferred trabecular orientation in the spine on the macro-tissue level (>10 mm) was investigated in the first part (Part I). This influence was related to mechanical characteristics of trabecular structures on the micro-tissue scale (1-10 mm) in the second part (Part II). In Part I, primary trabecular orientations (PTOsmacro) near the superior and inferior vertebral endplates of L1 and L5 of 6 human spine cadavers were determined on the macro level using micro computed tomography imaging (voxel size = 120 m3), by calculating the dominant fabric principal vector. Their relative deviations to the axial compression vectors in the spines, quantified by the normals to the endplate (NEs), were determined afterwards. The average deviation between the PTOmacro and NEs was 6.24⁰ (±4.34⁰). The PTOsmacro did not show a preference towards the anterior or posterior direction relative to the NE. From the deviations, it was concluded that trabecular bone in the spine predominantly adapts to compression loads. However, secondary loading directions, such as shear, are of additional influence. In Part II, 13 small cubes (6.0x6.0 mm) from the volumes of interest in Part I were analysed on the micro level with regard to elasticity. Components, component ratios and primary elastic orientations (PEOmicro) of elasticity tensors, computed by the simulation of mechanical tests in finite element (FE) models, were calculated. PTOs of the cubes (PTOsmicro) were compared to the PEOsmicro and related to the PTOsmacro and NEs (Part I) qualitatively. Elasticity tensor components were within a reasonable range (approximately 1-250 MPa, excluding outliers) and no material symmetry was found, i.e. the structures were mechanically anisotropic. PTOsmicro deviated 13.90⁰ (±8.04⁰) with respect to the PEOsmicro on average. 10 out of 13 PEOsmicro had similar anterior or posterior tendencies as the PTOsmacro with respect to the NEs. 11 out of 13 PTOsmicro had similar anterior or posterior tendencies as PTOsmacro with respect to the NEs. Elastic properties of typical trabecular structures in the vertebral bodies were successfully determined. Due to a relatively low resolution, PEOsmicro deviated strongly with the PTOsmicro. Such deviations could function as indicators for bone quality in skeletal disease diagnostics using low resolution imaging. PTOsmicro and PEOsmicro agreed relatively well to the PTOsmacro on the macro-tissue level, in terms of anteriorly or posterior tendencies relative to axial loading in the spine. This outcome shows promise for multi-scalar biomechanical analysis of trabecular bone.A Multi-Scale Approach to Implications of the Preferred Vertebral Trabecular Orientation on Spine BiomechanicsBiomedical Engineering | Tissue Biomechanics and Implant
Non-enzymatic cross-linking of collagen type II fibrils is tuned via osmolality switch
No full textAn important aspect in cartilage ageing is accumulation of advanced glycation end products (AGEs) after exposure to sugars. Advanced glycation results in cross-links formation between the collagen fibrils in articular cartilage, hampering their flexibility and making cartilage more brittle. In the current study, we investigate whether collagen cross-linking after exposure to sugars depends on the stretching condition of the collagen fibrils. Healthy equine cartilage specimens were exposed to l-threose sugar and placed in hypo-, iso-, or hyper-osmolal conditions that expanded or shrank the tissue and changed the 3D conformation of collagen fibrils. We applied micro-indentation tests, contrast enhanced micro-computed tomography, biochemical measurement of pentosidine cross-links, and cartilage surface color analysis to assess the effects of advanced glycation cross-linking under these different conditions. Swelling of extracellular matrix due to hypo-osmolality made cartilage less susceptible to advanced glycation, namely, the increase in effective Young's modulus was approximately 80% lower in hypo-osmolality compared to hyper-osmolality and pentosidine content per collagen was 47% lower. These results indicate that healthy levels of glycosaminoglycans not only keep cartilage stiffness at appropriate levels by swelling and pre-stressed collagen fibrils, but also protect collagen fibrils from adverse effects of advanced glycation. These findings highlight the fact that collagen fibrils and therefore cartilage can be protected from further advanced glycation ("ageing") by maintaining the joint environment at sufficiently low osmolality. Understanding of mechanochemistry of collagen fibrils provided here might evoke potential ageing prohibiting strategies against cartilage deterioration.Biomaterials & Tissue Biomechanic
Isolated effects of external bath osmolality, solute concentration, and electrical charge on solute transport across articular cartilage
No full textAn Experimental and Finite Element Protocol to Investigate the Transport of Neutral and Charged Solutes across Articular Cartilage
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Application of multiphysics models to efficient design of experiments of solute transport across articular cartilage
No full textTransport of solutes helps to regulate normal physiology and proper function of cartilage in diarthrodial joints. Multiple studies have shown the effects of characteristic parameters such as concentration of proteoglycans and collagens and the orientation of collagen fibrils on the diffusion process. However, not much quantitative information and accurate models are available to help understand how the characteristics of the fluid surrounding articular cartilage influence the diffusion process. In this study, we used a combination of micro-computed tomography experiments and biphasic-solute finite element models to study the effects of three parameters of the overlying bath on the diffusion of neutral solutes across cartilage zones. Those parameters include bath size, degree of stirring of the bath, and the size and concentration of the stagnant layer that forms at the interface of cartilage and bath. Parametric studies determined the minimum of the finite bath size for which the diffusion behavior reduces to that of an infinite bath. Stirring of the bath proved to remarkably influence neutral solute transport across cartilage zones. The well-stirred condition was achieved only when the ratio of the diffusivity of bath to that of cartilage was greater than ≈1000. While the thickness of the stagnant layer at the cartilage-bath interface did not significantly influence the diffusion behavior, increase in its concentration substantially elevated solute concentration in cartilage. Sufficient stirring attenuated the effects of the stagnant layer. Our findings could be used for efficient design of experimental protocols aimed at understanding the transport of molecules across articular cartilage
Effects of applied stress ratio on the fatigue behavior of additively manufactured porous biomaterials under compressive loading
No full textAdditively manufactured (AM) porous metallic biomaterials are considered promising candidates for bone substitution. In particular, AM porous titanium can be designed to exhibit mechanical properties similar to bone. There is some experimental data available in the literature regarding the fatigue behavior of AM porous titanium, but the effect of stress ratio on the fatigue behavior of those materials has not been studied before. In this paper, we study the effect of applied stress ratio on the compression-compression fatigue behavior of selective laser melted porous titanium (Ti-6Al-4V) based on the diamond unit cell. The porous titanium biomaterial is treated as a meta-material in the context of this work, meaning that R-ratios are calculated based on the applied stresses acting on a homogenized volume. After morphological characterization using micro computed tomography and quasi-static mechanical testing, the porous structures were tested under cyclic loading using five different stress ratios, i.e + R = 0.1, 0.3, 0.5, 0.7 and 0.8, to determine their S-N curves. Feature tracking algorithms were used for full-field deformation measurements during the fatigue tests. It was observed that the S-N curves of the porous structures shift upwards as the stress ratio increases. The stress amplitude was the most important factor determining the fatigue life. Constant fatigue life diagrams were constructed and compared with similar diagrams for bulk Ti-6Al-4V. Contrary to the bulk material, there was limited dependency of the constant life diagrams to mean stress. The notches present in the AM biomaterials were the sites of crack initiation. This observation and other evidence suggest that the notches created by the AM process cause the insensitivity of the fatigue life diagrams to mean stress. Feature tracking algorithms visualized the deformation during fatigue tests and demonstrated the root cause of inclined (45°) planes of specimen failure. In conclusion, the R-ratio behavior of AM porous biomaterials is both quantitatively and qualitatively different from that of bulk materials.Accepted Author ManuscriptStructural Integrity & CompositesBiomaterials & Tissue Biomechanic
The Effect of Mechanical Strain on Non-enzymatic Cross-linking of Collagen type II Fibrils in Articular Cartilage
No full textInvestigations into mechanotransduction in connective tissue extracellular matrix (ECM) have demonstrated that collagen networks show cell-independent mechanosensitive behavior. It has been suggested that mechanical strain could lead to conformational changes in the molecular structure of collagen, thereby influencing the susceptibility to other molecules. In the process of normal aging, the collagen fibrils in cartilage undergo a non-enzymatic process known as glycation. It involves the accumulation of advanced glycation end products (AGEs) after exposure to sugars, resulting in the formation of cross-links between the collagen fibrils. This process is correlated with increased stiffness and brittleness of the cartilage, making it more prone to mechanical damage. The goal of this thesis was to assess whether mechanical compression has any effects on the formation of non-enzymatic cross-links during the aging of articular cartilage. Two different models have been developed to mimic aging knees that undergo static and dynamic compression. Healthy cartilage explants were exposed to L-threose sugar to induce artificial aging. During incubation, these explants were submitted to either static or dynamic unconfined compression. Treatment with static compression consisted of a 5, 10 or 15\% strain throughout the whole incubation period, using a custom-made bioreactor. Treatment with dynamic compression consisted of multiple loading cycles at a frequency of either 0.01 Hertz (Hz) or 1 Hz, using a Dynamic Mechanical Analyzer (DMA). We conducted cartilage surface color analyses, micro‐indentation tests, dynamic mechanical analyses and biochemical measurements of pentosidine cross‐links to assess the effects of advanced glycation cross‐linking under these different conditions. Dynamic compression at a frequency of 1 Hz was found to affect the formation of non-enzymatic cross-links. Biomechanical and biochemical data showed a similar trend, namely, the average values for equilibrium modulus, dynamic moduli, phase shifts and pentosidine per collagen level were noticeably higher (or lower in case of phase shift) for the 1 Hz treated samples compared to samples of other treatment groups. The results of these studies suggest that compression at the physiological frequency of walking does affect the formation of cross-links in the articular cartilage during aging. These findings contribute to a better understanding of the mechanochemistry of collagen fibrils, which is necessary to develop future strategies against cartilage aging and deterioration.Biomedical Engineerin
Effects of Collagen Post-Translational Modifications on Bone Density and Mechanical Properties in Osteogenesis Imperfecta
No full textOsteogenesis imperfecta (OI), also known as brittle bone disease, affects 1 in 10.000 births. This disease is characterized by skeletal dysplasias, bone fragility and many secondary health issues. It is a heritable disorder affecting collagen type I, which is an essential protein in bone tissue. Post-translational overmodifications in collagen biosynthesis disturb the collagen packing and alter the mineralization process. This results in bone with poor structural parameters and inferior mechanical properties. In the current study, we have tested the hypothesis that the structural and mechanical properties in bone samples from OI patients are related to altered post-translational modifications in the collagen molecules.Hydroxylysine (Hyl) and the crosslinks hydroxylysyl-pyridinoline (HP) and lysyl-pyridinoline (LP) in bone samples from OI patients were measured with liquid chromatography. Bone quantity, in terms of volumetric bone mineral density (vBMD) and volumetric tissue mineral density (vTMD), was measured with micro-computed tomography. Hardness and Young’s moduli were derived from indentation experiments. Lysine was overhydroxylated in OI bone (p < 0.001), which led to increased HP values (p < 0.001) and higher HP/LP ratios (p = 0.013) compared to controls. OI tissue mineral density was more heterogeneously distributed, although average vTMD values were similar in both groups. The vBMD was significantly higher in controls compared to OI bone samples. Elevated Hyl levels were significantly related to decreased vBMD and vTMD in bone samples from OI patients. In both OI and control samples, an increase in LP crosslinks was associated with elevated vTMD values. Indentation with a spherical tip (r = 0.25mm) showed no altered mechanical properties in OI compared with controls. Unfortunately, we could not determine if impaired overmodifications in collagen are related to the poor mechanical properties of OI bone. Young’s moduli and hardness were not related to HP levels or HP/LP ratios. In control bone samples a negative correlation between Hyl residues and mechanical properties is observed. More mechanical experiments are necessary to address the hypothesis appropriately.Biomedical Engineerin
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