Proteoglycan and collagen contribution to the strain-rate-dependent mechanical behaviour of knee and shoulder cartilage.

The contribution of the proteoglycan to the strain-rate-dependent mechanical behaviour of cartilage tissues has been suggested to decrease with an increase in the strain-rate. On the other hand, the contribution from the collagen network has been suggested to increase as the strain-rate increases. These conclusions are drawn mainly based on numerical studies conducted on high-load-bearing knee cartilage tissues, while experimental evidence of these behaviours have not been demonstrated previously. Further, in contrast to the reported findings on high-load bearing knee cartilage, our previous study on the low-load-bearing kangaroo shoulder cartilage indicated that proteoglycan and collagen contribution remained steady as the strain-rate increases. Therefore, in the present study, we experimentally investigate the contribution of proteoglycan and collagen network to the strain-rate-dependent behaviour of the kangaroo knee cartilage, and plausible reasons for the differences observed in relation to the kangaroo shoulder cartilage. Firstly, in order to quantify the contribution of proteoglycans and collagen network, the indentation testings on normal, proteoglycan, and collagen-degraded kangaroo knee cartilage were conducted at different strain-rates. Then, structural and compositional differences between the kangaroo knee and shoulder cartilage were assessed qualitatively through polarised light microscopy (PLM) imaging and histological staining. Identified differences in the collagen architecture and proteoglycan composition were incorporated in a fibril-reinforced porohyperelastic Finite Element (FE) model with the objective of explaining the mechanisms underlying differences observed between the two tissues. Experimental results on knee cartilage indicated that when the strain-rate increases, proteoglycan contribution decreases while collagen contribution increases, where statistically significant differences were identified at each strain-rate (p < 0.05). PLM images revealed a sizable deep zone in the kangaroo knee cartilage where collagen fibrils were oriented perpendicular to the subchondral bone. On the other hand, no such apparent deep zone was observed in the shoulder cartilage. FE model confirmed that the biomechanical differences observed in the knee and shoulder cartilage are due to the differences in the collagen fibril arrangement in the deep zone. From these results, it can be concluded that in high-load-bearing cartilage tissues, the collagen network in the deep zone assists in increasing the stiffness of tissue with strain-rate and plays a significant role in supporting transient loads. This, in turn, helps protect the solid matrix against large distortions and strains at the subchondral junction, pointing to the importance of the collagen network in deep zone in assisting high-load-bearing cartilage tissues.

The investigation of fluid flow in cartilage contact gap.

Synovial fluid flow in articular joint capsule plays an important role during mixed mode lubrication. However, the actual fluid flow behaviour during cartilage contact has not been fully understood so far. This is due to the difficulties in measuring the gap permeability using conventional experimental techniques. The problem becomes further complicated with consideration of the cartilage surface roughness. Here a validated numerical study was developed to quantify the gap permeability of lateral synovial fluid flow. Both macro- and micro-scale gap flow models were created based on Darcy's law at the macro-scale and the Navier-stokes equation at the micro-scale. To generate model inputs, the cartilage topography was numerically synthesised based on the experimental measurements of bovine medial tibia cartilage surface roughness using Dektak Stylus Profilers. The experimental results show that the average roughness height Ra is 1.97 µm and root-mean-square roughness height Rq is 2.44 µm, while the correlation lengths of the secondary and tertiary undulations are round 100 µm and 20 µm, respectively. The numerical results indicate that the contact gap height and fluid pressure gradient are two critical parameters which significantly affect the gap permeability. As the contact gap closes, there is a decrease in gap permeability, and most importantly, the gap permeability is also very sensitive to the fluid pressure gradient. Furthermore, with gap closure, the permeability of the contact gap gradually approaches that of the cartilage tissue, at which point the contact gap is functional closed. This occurs at a contact gap height around 1 µm and fluid pressure gradient below 5 × 105 Pa/m in this study.

MRI magic-angle effect in femorotibial cartilages of the red kangaroo.

OBJECTIVE: Kangaroo knee cartilages are robust tissues that can support knee flexion and endure high levels of compressive stress. This study aimed to develop a detailed understanding of the collagen architecture in kangaroo knee cartilages and thus obtain insights into the biophysical basis of their function. DESIGN: Cylindrical/square plugs from femoral and tibial hyaline cartilage and tibial fibrocartilage were excised from the knees of three adult red kangaroos. Multi-slice, multi-echo MR images were acquired at the sample orientations 0° and 55° ("magic angle") with respect to the static magnetic field. Maps of the transverse relaxation rate constant (R2) and depth profiles of R2 and its anisotropic component (R2A) were constructed from the data. RESULTS: The R2A profiles confirmed the classic three-zone organisation of all cartilage samples. Femoral hyaline cartilage possessed a well-developed, thick superficial zone. Tibial hyaline cartilage possessed a very thick radial zone (80% relative thickness) that exhibited large R2A values consistent with highly ordered collagen. The R2A profile of tibial fibrocartilage exhibited a unique region near the bone (bottom 5-10%) consistent with elevated proteoglycan content ("attachment sub-zone"). CONCLUSIONS: Our observations suggest that the well-developed superficial zone of femoral hyaline cartilage is suitable for supporting knee flexion; the thick and well-aligned radial zone of tibial hyaline cartilage is adapted to endure high compressive stress; while the innermost part of the radial zone of tibial fibrocartilage may facilitate anchoring of the collagen fibres to withstand high shear deformation. These findings may inspire new designs for cartilage tissue engineering.

Investigation of the mechanical behavior of kangaroo humeral head cartilage tissue by a porohyperelastic model based on the strain-rate-dependent permeability.

Solid-interstitial fluid interaction, which depends on tissue permeability, is significant to the strain-rate-dependent mechanical behavior of humeral head (shoulder) cartilage. Due to anatomical and biomechanical similarities to that of the human shoulder, kangaroos present a suitable animal model. Therefore, indentation experiments were conducted on kangaroo shoulder cartilage tissues from low (10(-4)/s) to moderately high (10(-2)/s) strain-rates. A porohyperelastic model was developed based on the experimental characterization; and a permeability function that takes into account the effect of strain-rate on permeability (strain-rate-dependent permeability) was introduced into the model to investigate the effect of rate-dependent fluid flow on tissue response. The prediction of the model with the strain-rate-dependent permeability was compared with those of the models using constant permeability and strain-dependent permeability. Compared to the model with constant permeability, the models with strain-dependent and strain-rate-dependent permeability were able to better capture the experimental variation at all strain-rates (p < 0.05). Significant differences were not identified between models with strain-dependent and strain-rate-dependent permeability at strain-rate of 5 × 10(-3)/s (p = 0.179). However, at strain-rate of 10(-2)/s, the model with strain-rate-dependent permeability was significantly better at capturing the experimental results (p < 0.005). The findings thus revealed the significance of rate-dependent fluid flow on tissue behavior at large strain-rates, which provides insights into the mechanical deformation mechanisms of cartilage tissues.

Constrained path tracking at varying angles in a mouse tracking task.

OBJECTIVE: This study was aimed to determine the effects of direction and path length on movement time when traversing a constrained path of width, W, in a mouse tracking task. BACKGROUND: Tracking within constrained paths has been demonstrated to hold in many applications. Movement time and velocity of movement have shown very similar relationships, possibly because of the lack of extreme testing conditions. Most previous research evaluated conditions with only constant path length (A) of movement. METHOD: A total of 15 participants performed a mouse steering task within a constrained path at various angles. The independent variables were track width (W), path length, and path angle. Movement time was the dependent variable. RESULTS: Analyses showed a significant effect of movement direction on movement time, and the relationship was approximately sinusoidal and symmetrical about the horizontal axis. Path length had a significant effect on speed of movement, which was not that apparent on movement time. At low A/W values, movements appeared to be ballistic. CONCLUSION: Tracking within constrained paths can be modeled to account for the effect of path angle. APPLICATION: Vertical hand movements, especially within constrained paths, may not be ideal from a performance and biomechanical standpoint. The performance curve gradients are a good way to evaluate and standardize the testing of input devices and to define acceptable speeds for various tolerances in computer and industrial tasks that involve angular motions. The results of this experiment will help designers to optimize products and training programs.

Open-loop and feedback-controlled mouse cursor movements in linear paths.

Path length (A), path width (W) and movement direction (θ) are identified as the main factors affecting visually-controlled movement times in linear paths. Effects of A and W are well described by Drury's ( 1971 . Movements with lateral constraint. Ergonomics, l4 (2), 293-305.) model in which movement time is linearly related to the ratio of A/W. At low A/W values, departure from linearity has been identified but not investigated in detail. Data are presented for both open-loop and feedback-controlled movements in linear paths at 0, 60 and 150° movement directions. Movement amplitude and path width were varied over a wide range to determine the effects of A and (A/W) on movement time. Movements were found to be made ballistically or in open-loop mode when the ratio (A/W) was less than about 8 to 10 and the movement times were linearly related to √A for all angles of movement. Feedback-controlled movements followed Drury's law; ballistic movements had movement speed linear with √A. PRACTITIONER SUMMARY: Many tasks require manoeuvring equipment or devices through a path of limited width. These movements can be made with or without feedback control, depending on the path constraints. The conditions for the two forms of movement are determined in this research.