The structure and mechanical properties of articular cartilage are highly resilient towards transient dehydration

Structure, composition and anisotropic swelling of the bovine acetabular labrum

The fibro-cartilaginous labrum surrounds the acetabular rim and is important for hip joint stability and sealing. Sealing may be enhanced by swelling pressure within the normal labrum. Swelling of the degenerated or torn labrum might occur and potentially contribute to the development of osteoarthritis, through altered load transmission. This study aimed to characterize the three-dimensional swelling behaviour, the collagen fiber orientation and spatial proteoglycan distribution of the bovine acetabular labrum. Specimens were harvested from bovine donors (192-652 days, male, n = 6 donors). Structure was analyzed by scanning electron microscopy, histology, and dimethylmethylene blue assay. Specimen dimensions were measured before and after incubation in phosphate buffered saline to assess the swelling. Results showed that the articulating surface is composed of a collagen mesh network. Collagen fiber bundles showed a low degree of alignment close to the surface and were circumferentially aligned in the deep tissue. Proteoglycans were identified clustered between the collagen bundles. Glycosaminoglycan content was 10 x lower than that of cartilage (23.1 ± 6.4 compared to 299.5 ± 19.1 μg/mg dry weight) with minor regional differences. Specimens swelled significantly more in the orthogonal direction (swelling ratio 124.7 ± 10.2%) compared to the swelling parallel to the articulating surface (108.8 ± 6.1% and 102.8 ± 4.1%). In the deep tissue, swelling was also restricted in the main collagen fiber bundle direction (circumferentially), with a swelling ratio of 109.5 ± 4.0% in the main fiber bundle direction compared to 126.8 ± 7.3 % and 122.3 ± 5.8% radially. The findings demonstrate that the labrum shows anisotropic swelling properties, which reflect the anisotropy in the tissue structure and inter-fiber localisation of proteoglycans.

The effect of attending rehabilitation after traumatic knee joint injury on femoral articular cartilage morphology in collegiate rugby players with a history of intracapsular knee joint injury during two-year consecutive rugby seasons

Introduction

This present study aimed to compare ultrasonographic measures of femoral articular cartilage during two-year seasons between collegiate rugby players who have attended supervised rehabilitation following intracapsular knee joint injury and those without a history of knee injury.

Methods

Using a prospective observational study design, 12 male collegiate rugby players with a previous history of intracapsular knee joint injury who have received and completed supervised rehabilitation following their injury and 44 players without knee joint injury participated in this study. Ultrasonographic images were used to verify changes in femoral articular cartilage thickness and cross-sectional area (CSA) with or without a previous history of knee joint injury over two consecutive rugby seasons.

Results

Significant time main effects were observed for the lateral condylar thickness (p < 0.001), the intercondylar thickness (p = 0.001), the medial condylar thickness (p < 0.001), and CSA (p < 0.001). No significant interactions nor group main effects were identified for all femoral articular cartilage (p < 0.05).

Conclusions

Collegiate rugby players demonstrated a decrease in femoral articular cartilage thickness and CSA over two-year consecutive rugby seasons. These findings indicate that engaging in collegiate rugby induces alterations in femoral articular cartilage structure. Furthermore, there were no differences in all femoral cartilage outcome measures between rugby players with and without a previous history of traumatic knee joint injury. Therefore, attending supervised rehabilitation at the time of their knee joint injury appeared to reduce the impact of a previous history of intracapsular knee joint injury on the change in femoral articular cartilage thickness and CSA among active rugby players.

Human Cartilage Biomechanics: Experimental and Theoretical Approaches towards the Identification of Mechanical Properties in Healthy and Osteoarthritic Conditions

Articular cartilage is a complex connective tissue with the fundamental functions of load bearing, shock absorption and lubrication in joints. However, traumatic events, aging and degenerative pathologies may affect its structural integrity and function, causing pain and long-term disability. Osteoarthritis represents a health issue, which concerns an increasing number of people worldwide. Moreover, it has been observed that this pathology also affects the mechanical behavior of the articular cartilage. To better understand this correlation, the here proposed review analyzes the physiological aspects that influence cartilage microstructure and biomechanics, with a special focus on the pathological changes caused by osteoarthritis. Particularly, the experimental data on human articular cartilage are presented with reference to different techniques adopted for mechanical testing and the related theoretical mechanical models usually applied to articular cartilage are briefly discussed.

Creep-recovery behaviors of articular cartilage under uniaxial and biaxial tensile loadings

Creep deformation in cartilage can be observed under physiological loads in daily activities such as standing, single-leg lunge, the stance phase of gait. If not fully recovered in time, it may induce irreversible damage in cartilage and further lead to early osteoarthritis. In this study, 36 cruciform-shape samples in total from 18 bulls were employed to conduct the uniaxial and biaxial creep-recovery tests by using a biaxial cyclic testing system. Effects of stress level (σ = .5, 1.0, 1.5 MPa) and biaxial stress ratio (B = 0, .3, .5, 1.0) on creep-recovery behaviors of cartilage were characterized. And then, a viscoelastic constitutive model was employed to predict its creep-recovery behaviors. The results showed that the creep strain and its three components, namely instantaneous elastic strain, delayed elastic strain and viscous flow strain, increase with the increasing stress level or with the decreasing biaxial stress ratio. Compared with uniaxial creep-recovery, biaxial creep-recovery exhibits a smaller creep strain, a faster recovery rate of creep strain and a smaller residual strain. Besides, the built viscoelastic model can be used to describe the uniaxial creep-recovery behaviors of cartilage as a good correlation between the fitted results and test results is achieved. The findings are expected to provide new insights into understanding normal joint function and cartilage pathology.

Coefficient of Friction and Height Loss: Two Criteria Used to Determine the Mechanical Property and Stability of Regenerated Versus Natural Articular Cartilage

Background: The coefficient of friction (CoF) serves as an indicator for the mechanical properties of natural and regenerated articular cartilage (AC). After tribological exposure, a height loss (HL) of the cartilage pair specimens can be measured. Our aim was to determine the CoF and HL of regenerated AC tissue and compare them with those of natural AC from non-operated joints and AC from joints where the regenerated tissues had been created after different treatments. Methods: In partial-thickness defects of the trochleae of the stifle joints of 60 Göttingen Minipigs, regenerated AC was created. In total, 40 animals received a Col I matrix, 20 laden with autologous chondrocytes, and 20 without. The defects of 20 animals were left empty. The healing periods were 24 and 48 weeks. A total of 10 not-operated animals, delivered the “external” control specimens. Osteochondral pins were harvested from defect and non-defect areas, the latter serving as “internal” controls. Using a pin-on-plate tribometer, we measured the CoF and the HL. Results: The CoF of the regenerated AC ranged from 0.0393 to 0.0688, and the HL, from 0.22 mm to 0.3 mm. The differences between the regenerated AC of the six groups and the “external” controls were significant. The comparison with the “internal” controls revealed four significant differences for the CoF and one for the HL in the operated groups. No differences were seen within the operated groups. Conclusions: The mechanical quality of the regenerated AC tissue showed inferior behavior with regard to the CoF and HL in comparison with natural AC. The comparison of regenerated AC tissue with AC from untreated joints was more promising than with AC from the treated joints.

Creep behavior of human knee joint determined with high-speed biplanar video-radiography and finite element simulation

Creep and relaxation of knee cartilage and meniscus have been extensively studied at the tissue level with constitutive laws well established. At the joint level, however, both experimental and model studies have been focused on either elastic or kinematic responses of the knee, where the time-dependent response is typically neglected for simplicity. The objectives of this study were to quantify the in-vivo creep behavior of human knee joints produced by the cartilaginous tissues and to use the relevant data to validate a previously proposed poromechanical model. Two participants with no history of leg injury volunteered for 3T magnetic resonance imaging (MRI) of their unloaded right knees and for biplanar video-radiography (BVR) of the same knees during standing on an instrumented treadmill for 10 min. Approximately 550 temporal data points were obtained for the in-vivo displacement of the right femur relative to the tibia of the knee. Models of the bones and soft tissues were derived from the MRI. The bone models were used to reconstruct the 3D bone kinematics measured using BVR. Ground reaction forces were simultaneously recorded for the right leg, which were used as input for the subject-specific finite element knee models. Cartilaginous tissues were modeled as fluid-saturated fibril-reinforced materials. In-vivo creep of the knee was experimentally observed for both participants, i.e., the joint displacement increased with time while the reaction forces at the foot were approximately constant. The creep displacements obtained from the finite element models compared well with the experimental data when the tissue properties were calibrated (Pearson correlation coefficient = 0.99). The results showed the capacity of the poromechanical knee model to capture the creep response of the joint. The combined experimental and model study may be used to understand the fluid-pressure load support and contact mechanics of the joint using material properties calibrated from the displacement data, which enhance the fidelity of model results.

Changes in anterior femoral articular cartilage structure in collegiate rugby athletes with and without a history of traumatic knee joint injury following a five-month competitive season

This study aimed to examine anterior femoral cartilage morphology before (pre-season) and after (post-season) a 5-month competitive season in collegiate ruby players with and without a previous history of traumatic injury to ligamentous, meniscus, and/or cartilage structures at the knee joint. Using a prospective cohort design, 42 male collegiate rugby players with a previous history of traumatic intracapsular knee joint injury and 124 players without knee injury history were included in this study. Ultrasonography assessments of anterior femoral cartilage were performed before (pre-season) and following a 5-month athletic season (post-season). Rugby players with a history of traumatic knee joint injury had greater lateral condylar thickness (2.37 ± 0.35 mm, p = 0.03), intercondylar thickness (2.51 ± 0.47 mm, p = 0.03), and partial area (44.67 ± 7.28mm², p = 0.02) compared to control players (lateral = 2.23 ± 0.35 mm, intercondylar = 2.32 ± 0.47 mm, partial area = 41.60 ± 7.26 mm²), regardless of pre-and post-season assessment time points. Pre-season ultrasonography assessment of lateral condylar thickness (2.34 ± 0.47 mm, p = 0.02), medial condylar thickness (2.05 ± 0.43 mm, p = 0.03), and partial area (44.10 ± 9.23 mm², p = 0.001) were significantly greater than the post-season ultrasonography assessment time point (lateral = 2.26 ± 0.43 mm, medial = 1.98 ± 0.43 mm, partial area = 42.17 ± 8.82 mm²), regardless of group membership. Rugby players with a history of intracapsular knee joint injury displayed altered anterior femoral cartilage size via ultrasonography assessments. Regardless of a presence of injury history, collegiate rugby players showed a decrease in cartilage thickness and partial area following a 5-month competitive season.

Resilience to height loss of articular cartilage of osteoarthritic stifle joints of old pigs, compared with healthy cartilage from young pigs in a tribological pin-on-plate exposure, revealing similar friction forces

Introduction

We saw a lack of data on the biomechanical behavior of degenerated articular cartilage (OA) compared with that of healthy cartilage, even though the susceptibility to wear and tear of articular cartilage plays a key role in the progression of osteoarthritis (OA). Therefore, we performed a comparison between naturally occurring OA and healthy cartilage from pigs, before and after tribological stress.

Aim

The aim of the study was to compare OA-cartilage with healthy cartilage and to analyze the resilience to tribological shear stress, which will be measured as height loss (HL), and to friction forces of the cartilage layers. The findings will be substantiated in macro- and microscopical evaluations before and after tribological exposure.

Methods

We assessed stifle joints of fifteen old and sixteen young pigs from the local abattoir radiologically, macroscopically and histologically to determine possible OA alterations. We put pins from the femoral part of the joints and plates from the corresponding tibial plateaus in a pin-on-plate tribometer under stress for about two hours with about 1108 reciprocating cycles under a pressure of approximately 1 MPa. As a surrogate criterion of wear and tear, the HL was recorded in the tribometer. The heights of the cartilage layers measured before and after the tribological exposure were compared histologically. The condition of the cartilage before and after the tribological exposure was analyzed both macroscopically with an adapted ICRS score and microscopically according to Little et al. (2010). We assessed the friction forces acting between the surfaces of the cartilage pair–specimens.

Results

Articular cartilage taken from old pigs showed significant degenerative changes compared to that taken from the young animals. The macroscopic and microscopic scores showed strong alterations of the cartilage after the tribological exposure. There was a noticeable HL of the cartilage specimens after the first 100 to 300 cycles. The HL after tribological exposure was lower in the group of the old animals with 0.52 mm ± 0.23 mm than in the group of the young animals with 0.86 mm ± 0.26 mm (p < 0.0001). The data for the HL was validated by the histological height measurements with 0.50 mm ± 0.82 mm for the old and 0.79 mm ±0.53 mm for the young animals (p = 0.133). The friction forces measured at the cartilage of the old animals were 2.25 N ± 1.15 N and 1.89 N ± 1.45 N of the young animals (p = 0.3225).

Conclusion

Unlike articular cartilage from young pigs, articular cartilage from old pigs showed OA alterations. Tribological shear stress exposure revealed that OA cartilage showed less HL than healthy articular cartilage. Tribological stress exposure in a pin–on–plate tribometer seemed to be an appropriate way to analyze the mechanical stability of articular cartilage, and the applied protocol could reveal weaknesses of the assessed cartilage tissue. Friction and HL seemed to be independent parameters when degenerated and healthy articular cartilage were assessed under tribological exposure in a pin–on- plate tribometer.

The Effects of Crosslinking on the Rheology and Cellular Behavior of Polymer-Based 3D-Multilayered Scaffolds for Restoring Articular Cartilage

Polymer-based tri-layered (bone, intermediate and top layers) scaffolds used for the restoration of articular cartilage were prepared and characterized in this study to emulate the concentration gradient of cartilage. The scaffolds were physically or chemically crosslinked. In order to obtain adequate scaffolds for the intended application, the impact of the type of calcium phosphate used in the bone layer, the polymer used in the intermediate layer and the interlayer crosslinking process were analyzed. The correlation among SEM micrographs, physical-chemical characterization, swelling behavior, rheological measurements and cell studies were examined. Storage moduli at 1 Hz were 0.3-1.7 kPa for physically crosslinked scaffolds, and 4-5 kPa (EDC/NHS system) and 15-20 kPa (glutaraldehyde) for chemically crosslinked scaffolds. Intrinsic viscoelasticity and poroelasticity were considered in discussing the physical mechanism dominating in different time/frequency scales. Cell evaluation showed that all samples are available as alternatives to repair and/or substitute cartilage in articular osteoarthritis.

Establishment of a New Device for Electrical Stimulation of Non-Degenerative Cartilage Cells In Vitro

In cell-based therapies for cartilage lesions, the main problem is still the formation of fibrous cartilage, caused by underlying de-differentiation processes ex vivo. Biophysical stimulation is a promising approach to optimize cell-based procedures and to adapt them more closely to physiological conditions. The occurrence of mechano-electrical transduction phenomena within cartilage tissue is physiological and based on streaming and diffusion potentials. The application of exogenous electric fields can be used to mimic endogenous fields and, thus, support the differentiation of chondrocytes in vitro. For this purpose, we have developed a new device for electrical stimulation of chondrocytes, which operates on the basis of capacitive coupling of alternating electric fields. The reusable and sterilizable stimulation device allows the simultaneous use of 12 cavities with independently applicable fields using only one main supply. The first parameter settings for the stimulation of human non-degenerative chondrocytes, seeded on collagen type I elastin-based scaffolds, were derived from numerical electric field simulations. Our first results suggest that applied alternating electric fields induce chondrogenic re-differentiation at the gene and especially at the protein level of human de-differentiated chondrocytes in a frequency-dependent manner. In future studies, further parameter optimizations will be performed to improve the differentiation capacity of human cartilage cells.

Transient stiffening of cartilage during joint articulation: A microindentation study

As a mechanoactive tissue, articular cartilage undergoes compression and shear on a daily basis. With the advent of high resolution and sensitive mechanical testing methods, such as micro- and nanoindentation, it has become possible to assess changes in small-scale mechanical properties due to compression and shear of the tissue. However, investigations on the changes of these properties before and after joint articulation have been limited. To simulate articular loading of cartilage in the context of human gait, a previously developed bioreactor system was used. Immediately after bioreactor testing, the stiffness was measured using microindentation. Specifically, we investigated whether the mechanical response of the tissue was transient or permanent, dependent on counterface material, and an effect limited to the superficial zone of cartilage. We found that cartilage surface stiffness increases immediately after articular loading and returns to baseline values within 3 hr. Cartilage-on-cartilage stiffening was found to be higher compared to both alumina- and cobalt chromium-on-cartilage stiffening, which were not significantly different from each other. This stiffening response was found to be unique to the superficial zone, as articular loading on cartilage with the superficial zone removed showed no changes in stiffness. The findings of this study suggest that the cartilage superficial zone may adapt its stiffness as a response to articular loading. As the superficial zone is often compromised during the course of osteoarthritic disease, this finding is of clinical relevance, suggesting that the load-bearing function deteriorates over time.

Glycosaminoglycan depletion increases energy dissipation in articular cartilage under high-frequency loading

High-frequency material behavior of cartilage at macroscopic lengths is not widely understood, despite a wide range of frequencies and contact lengths experienced in vivo. For example, cartilage at different stages of matrix integrity can experience high-frequency loading during traumatic impact, making high-frequency behavior relevant in the context of structural failure. Therefore, this study examined macroscopic dissipative and mechanical responses of intact and glycosaminoglycan (GAG)-depleted cartilage under previously unexplored high-frequency loading. These dynamic responses were complemented with the evaluation of quasi-static responses. A custom dynamic mechanical analyzer was used to obtain dynamic behavior, and stress relaxation testing was performed to obtain quasi-static behavior. Under high-frequency loading, cartilage energy dissipation increased with GAG depletion and decreased with strain; dynamic modulus exhibited opposite trends. Similarly, under quasi-static loading, equilibrium modulus and relaxation time of cartilage decreased with GAG depletion. The increased energy dissipation after GAG depletion under high-frequency loading was likely due to increased viscoelastic dissipation. These findings broaden our understanding of fundamental properties of cartilage as a function of solid matrix integrity in an unprecedented loading regime. They also provide a foundation for analyzing energy dissipation associated with cartilage failure induced by traumatic impact.

Evaluation of cartilage biomechanics and knee joint microenvironment after different cell-based treatments in a sheep model of early osteoarthritis

PURPOSE

Aiming to prevent cartilage damage during early osteoarthritis (OA), the therapeutic challenge is to restore and maintain the physiological and functional properties of such a tissue with minimally invasive therapeutic strategies.

METHODS

Accordingly, an in vivo model of early OA in sheep was here treated through three different cell therapies (culture expanded ADSCs, SVF, and culture expanded AECs) thus to preserve the joint surface from the progression of the pathology. Three months after the treatment injections, their performance was assessed through mechanical automated mapping (Young's modulus and cartilage thickness), gross evaluation of articular surfaces, and biochemical analysis of the synovial fluid.

RESULTS

No severe degeneration was observed after three months from OA induction. Cartilage mechanical properties were crucial to identify early degeneration. All the treatments improved the macroscopic cartilage surface aspect and reduced pro-inflammatory cytokines in the synovial fluid. Among the three treatments, SVF highlighted the best performance while ADSCs the worst.

CONCLUSION

Despite that the evaluated experimental time is an early follow-up and, thus, longer trial is mandatory to properly assess treatments effectiveness, the proposed multidisciplinary approach allowed to obtain preliminary, but also crucial, results concerning the reduction in OA signs on cartilage properties, in osteophyte development and in all the inflammatory markers.

Full-Field Strain Uncertainties and Residuals at the Cartilage-Bone Interface in Unstained Tissues Using Propagation-Based Phase-Contrast XCT and Digital Volume Correlation

A deeper understanding of the cartilage-bone mechanics is fundamental to unravel onset and progression of osteoarthritis, enabling better diagnosis and treatment. The aim of this study is therefore to explore the capability of X-ray computed (XCT) phase-contrast imaging in a lab-based system to enable digital volume correlation (DVC) measurements of unstained cartilage-bone plugs from healthy adult bovines. DVC strain uncertainties were computed for both articular cartilage and mineralized tissue (calcified cartilage and subchondral bone) in the specimens at increasing propagation distances, ranging from absorption up to four times (4× such effective distance. In addition, a process of dehydration and rehydration was proposed to improve feature recognition in XCT of articular cartilage and mechanical properties of this tissue during the process were assessed via micromechanical probing (indentation), which was also used to determine the effect of long X-ray exposure. Finally, full-field strain from DVC was computed to quantify residual strain distribution at the cartilage-bone interface following unconfined compression test (ex situ). It was found that enhanced gray-scale feature recognition at the cartilage-bone interface was achieved using phase-contrast, resulting in reduced DVC strain uncertainties compared to absorption. Residual strains up to ~7000 µε in the articular cartilage were transferred to subchondral bone via the calcified cartilage and micromechanics revealed the predominant effect of long phase-contrast X-ray exposure in reducing both stiffness and hardness of the articular cartilage. The results of this study will pave the way for further development and refinement of the techniques, improving XCT-based strain measurements in cartilage-bone and other soft-hard tissue interfaces.

Prevalence of chronic chikungunya and associated risks factors in the French West Indies (La Martinique): A prospective cohort study

BACKGROUND

The chikungunya virus (CHIKV) is a re-emerging alphavirus that can cause chronic and potentially incapacitating rheumatic musculoskeletal disorders known as chronic chikungunya arthritis (CCA). We conducted a prospective cohort study of CHIKV-infected subjects during the 2013 chikungunya outbreak in Martinique. The aim of this study was to assess the prevalence of CCA at 12 months and to search for acute phase factors significantly associated with chronicity.

METHODOLOGY/PRINCIPAL FINDINGS

A total of 193 patients who tested positive for CHIKV RNA via qRT-PCR underwent clinical investigations in the acute phase (<21 days), and then 3, 6, and 12 months after inclusion. The Asian lineage was identified as the circulating genotype. A total of 167 participants were classified as either with or without CCA, and were analyzed using logistic regression models. The overall prevalence of CCA at 12 months was 52.1% (95%CI: 44.5-59.7). In univariate analysis, age (RD 9.62, 95% CI, 4.87;14.38, p<0.0001), female sex (RD 15.5, 95% CI, 1.03;30.0, p = 0.04), headache (RD 15.42, 95% CI, 0.65;30.18 p = 0.04), vertigo (RD 15.33, 95% CI, 1.47;29.19, p = 0.03), vomiting (RD 12.89, 95% CI, 1.54;24.24, p = 0.03), dyspnea (RD 13.53, 95% CI, 0.73;26.33, p = 0.04), intravenous rehydration (RD -16.12, 95% CI, -31.58; -0.66 p = 0.04) and urea (RD 0.66, 95% CI, 0.12;1.20, p = 0.02) were significantly associated with the development of CCA. For the subpopulation with data on joint involvement in the acute phase, the risk factors significantly associated with CCA were at least one 1 enthesitis (RD 16.7, 95%CI, 2.8; 30.7, p = 0.02) and at least one tenosynovitis (RD 16.8, 95% CI, 1.4-32.2, p = 0.04).

CONCLUSIONS

This cohort study conducted in Martinique confirms that CCA is a common complication of acute chikungunya disease. Our analysis emphasized the importance of age and female sex for CCA occurrence, and highlighted the aggravating role of dehydration during the acute phase. Early and adequate hydration were found to reduce the risk chronic chikungunya disorders.

TRIAL REGISTRATION

clinicaltrials.gov (NCT01099852).

Cartilage and collagen mechanics under large-strain shear within in vivo and at supraphysiogical temperatures

Human joints, particularly those of extremities, experience a significant range of temperatures in vivo. Joint temperature influences the mechanics of both joint and cartilage, and the mechanics of cartilage can affect the temperature of both joint and cartilage. Thermal treatments and tissue repairs, such as thermal chondroplasty, and ex vivo tissue engineering may also expose cartilage to supraphysiological temperatures. Furthermore, although cartilage undergoes principally compressive loads in vivo, shear strain plays a significant role at larger compressive strains. Thus, we aimed to determine whether and how the bulk mechanical responses of cartilage undergoing large-strain shear change (1) within the range of temperatures relevant in vivo, and (2) both during and after supraphysiological thermal treatments. We completed large-strain shear tests (10 and 15%) at four thermal conditions: 24^∘C and 40^∘C to span the in vivo range, and 70^∘C and 24^∘C repeated after 70^∘C to explore mechanics during and after potential treatments. We calculated the bulk mechanical responses (strain-energy dissipation densities, peak-to-peak shear stresses, and peak-effective shear moduli) as of function of temperature and used statistical methods to probe significant differences. To probe the mechanisms underlying differences we assessed specimens, principally the type II collagen, with imaging (second harmonic generation and transmission electron microscopies, and histology) and assessed the temperature-dependent mechanics of type II collagen molecules within cartilage using steered molecular dynamics simulations. Our results suggest that the bulk mechanical responses of cartilage depend significantly on temperature both within the in vivo range and at supraphysiological temperatures, showing significant reductions in all mechanical measures with increasing temperature. Using imaging and simulations we determined that one underlying mechanism explaining our results may be changes in the molecular deformation profiles of collagen molecules versus temperature, likely compounded at larger length scales. These new insights into the mechanics of cartilage and collagen may suggest new treatment targets for damaged or osteoarthritic cartilage.

Experimental Study on Creep Characteristics of Microdefect Articular Cartilages in the Damaged Early Stage

Traumatic joint injury is known to cause cartilage deterioration and osteoarthritis. In order to study the mechanical mechanism of damage evolution on articular cartilage, taking the fresh porcine articular cartilage as the experimental samples, the creep experiments of the intact cartilages and the cartilages with different depth defect were carried out by using the noncontact digital image correlation technology. And then, the creep constitutive equations of cartilages were established. The results showed that the creep curves of different layers changed exponentially and were not coincident for the cartilage sample. The defect affected the strain values of the creep curves. The creep behavior of cartilage was dependent on defect depth. The deeper the defect was, the larger the strain value was. The built three-parameter viscoelastic constitutive equation had a good correlation with the experimental results and could predict the creep performance of the articular cartilage. The creep values of the microdefective cartilage in the damaged early stage were different from the diseased articular cartilage. These findings pointed out that defect could accelerate the damage of cartilage. It was helpful to study the mechanical mechanism of damage evolution.

Rate-dependent crack nucleation in cartilage under microindentation

This study investigates rate-dependent crack nucleation in cartilage under microindentation using a poroviscoelastic framework and nano/microscopic images. Localized crack failure was induced at known locations and at different loading rates via microindentation with an axisymmetric sphero-conical indenter. Finite element (FE) modeling was used to reproduce results of microindentation tests within a poroviscoelastic framework. Scanning electron microscopy (SEM) was used to examine nano- and microscale structural features of crack surfaces. Microindentation results showed rate-dependent crack nucleation in cartilage. In particular, critical total work required for crack nucleation was larger at the slow loading rate compared to the fast loading rate. FE results suggested that viscoelastic relaxation of cartilage was a major contributor to the rate dependency and that tensile stresses localized at the indenter tip was a governing factor in crack nucleation. SEM images combined with microindentation and FE results suggested that the solid matrix in the vicinity of the tip experienced relatively large relaxation and kinematic fiber rearrangement at the slow loading rate in comparison to the fast loading rate. These findings extend current understanding of rate-dependent failure mechanisms in cartilage.

Effect of relaxation-dependent adhesion on pre-sliding response of cartilage

Possible links between adhesive properties and the pre-sliding (static) friction response of cartilage are not fully understood in the literature. The aims of this study are to investigate the relation between adhesion and relaxation time in articular cartilage, and the effect of relaxation-dependent adhesion on the pre-sliding response of cartilage. Adhesion tests were performed to evaluate the work of adhesion of cartilage at different relaxation times. Friction tests were conducted to identify the pre-sliding friction response of cartilage at relaxation times corresponding to adhesion tests. The pre-sliding friction response of cartilage was systematically linked to the work of adhesion and contact conditions by a slip-based failure model. It was found that the work of adhesion increases with relaxation time. Also, the work of adhesion is linearly correlated to the resistance to slip-based failure. In addition, as the work of adhesion increases, the adhered (stick) area at the moment of failure increases, and the propagation rate of the annular slip (crack) area towards its centre increases. These findings offer a mechanistic explanation of the pre-sliding friction behaviour and stick-slip response of soft hydrated interfaces such as articular cartilage and hydrogels. In addition, the linear correlation between adhesion and threshold to slip-based failure enables estimation of the adhesive strength of such interfaces directly from the pre-sliding friction response (e.g. shear wave elastography).

A DAMAGE MECHANISM OF MICRO-PARTICLES ON ARTICULAR CARTILAGE OF KNEE BY NANOINDENTATION

Articular cartilage plays an important role in organism due to its excellent shock absorbing and buffering functions. Increasing problems about damages of articular cartilage are making a great deal of trouble to human beings. The damage mechanism of articular cartilage is very complicated and keeps unclear. In this research, the damage mechanism was investigated from the perspective of micro-particle attrition by nanoindentation experiments. The micro-particle was simulated by the indenter in experiments. The experimental results demonstrated that the load from micro-particle could not maintain when water content was adequate. However, the load could maintain and increase after dehydration. It was found that the partial surface of articular cartilage was crushed and adhered to the indenter. The plastic energy was bigger than elastic energy in the nanoindentation process. Therefore, water content was the crucial factor to protect the articular cartilage from damage. And the recurring partial dehydration owing to ongoing compression enhanced the damage of micro-particle to articular cartilage. This research may provide a new understanding to the damage mechanism of articular cartilage.

Biomimetic design and fabrication of multilayered osteochondral scaffolds by low-temperature deposition manufacturing and thermal-induced phase-separation techniques

Integrative osteochondral repair is a useful strategy for cartilage-defect repair. To mimic the microenvironment, it is necessary that scaffolds effectively mimic the extracellular matrix of natural cartilage and subchondral bone. In this study, biomimetic osteochondral scaffolds containing an oriented cartilage layer, a compact layer, and a three-dimensional (3D)-printed core-sheath structured-bone layer were developed. The oriented cartilage layer was designed to mimic the structural and material characteristics of native cartilage tissue and was fabricated with cartilage matrix-chitosan materials, using thermal-induced phase-separation technology. The 3D-printed core-sheath structured-bone layer was fabricated with poly(L-lactide-co-glycolide)/β-tricalcium phosphate-collagen materials by low-temperature deposition technology, using a specially designed core-sheath nozzle, and was designed to mimic the mechanical characteristics of subchondral bone and improve scaffold hydrophilicity. The compact layer was designed to mimic the calcified-layer structure of natural cartilage to ensure the presence of different suitable microenvironments for the regeneration of bone and cartilage. A dissolving-bonding process was developed to effectively combine the three parts together, after which the bone and cartilage scaffolds exhibited good mechanical properties and hydrophilicity. Additionally, goat autologous bone mesenchymal stem cells (BMSCs) were isolated and then seeded into the bone and cartilage layers, respectively, and following a 1 week culture in vitro, the BMSC-scaffold constructs were implanted into a goat articular-defect model. Our results indicated that the scaffolds exhibited good biocompatibility, and 24 weeks after implantation, the femoral condyle surface was relatively flat and consisted of a large quantity of hyaloid cartilage. Furthermore, histological staining revealed regenerated trabecular bone formed in the subchondral bone-defect area. These results provided a new method to fabricate biomimetic osteochondral scaffolds and demonstrated their effectiveness for future clinical applications in cartilage-defect repair.

On mechanical mechanism of damage evolution in articular cartilage

Superficial lesions of cartilage are the direct indication of osteoarthritis. To investigate the mechanical mechanism of cartilage with micro-defect under external loading, a new plain strain numerical model with micro-defect was proposed and damage evolution progression in cartilage over time has been simulated, the parameter were studied including load style, velocity of load and degree of damage. The new model consists of the hierarchical structure of cartilage and depth-dependent arched fibers. The numerical results have shown that not only damage of the cartilage altered the distribution of the stress but also matrix and fiber had distinct roles in affecting cartilage damage, and damage in either matrix or fiber could promote each other. It has been found that the superficial cracks in cartilage spread preferentially along the tangent direction of the fibers. It is the arched distribution form of fibers that affects the crack spread of cartilage, which has been verified by experiment. During the process of damage evolution, its extension direction and velocity varied constantly with the damage degree. The rolling load could cause larger stress and strain than sliding load. Strain values of the matrix initially increased and then decreased gradually with the increase of velocity, and velocity had a greater effect on matrix than fibers. Damage increased steadily before reaching 50%, sharply within 50 to 85%, and smoothly and slowly after 85%. The finding of the paper may help to understand the mechanical mechanism why the cracks in cartilage spread preferentially along the tangent direction of the fibers.

Shear deformations of human articular cartilage: Certain mechanical anisotropies apparent at large but not small shear strains

Articular cartilage has pronounced through-the-thickness heterogeneity in both ultrastructure and mechanical function. The tissue undergoes a combination of large deformations in vivo, where shear is critical in both failure and chondrocyte death. Yet the microstructure mechanical response of cartilage to multi-axial large shear deformations is unknown. We harvested a total of 42 cartilage specimens from seven matched locations across the lateral femoral condyles and patellofemoral grooves of six human male donors (30.2±8.8yrs old, M±SD). With each specimen we applied a range of quasi-static, multi-axial large (simple) shear displacements both parallel and perpendicular to the local split-line direction (SLD). Shear stresses in cartilage specimens from the patellofemoral grooves were higher, and more energy was dissipated, at all applied strains under loading parallel to the local SLD versus perpendicular, while specimens from the lateral condyles were mechanically anisotropic only under larger strains of 20% and 25%. Cartilage also showed significant intra-donor variability at larger shear strains but no significant inter-donor variability. Overall, shear strain-energy dissipation was almost constant at 5% applied shear strain and increased nonlinearly with increasing shear magnitude. Our results suggest that full understanding of cartilage mechanics requires large-strain analyses to account for nonlinear, anisotropic and location-dependent effects not fully realized at small strains.