Analysis of the mechanical effects of defect shape on damage evolution of articular cartilage under rolling load.

To study the mechanical effects of defect shape on the damage evolution of knee cartilage and find the causes of fragments, so as to obtain damage evolution rules and determine the most appropriate shape used in a clinical repair. A porous viscoelasticity fiber-reinforced 2D numerical model with different micro-defect shapes was established which considered the depth-dependent Young's modulus, fiber distribution, porosity and permeability. The stress-strain relationship, interstitial hydraulic and interstitial flow velocity was obtained under rolling load. The results showed that damage developed at the bottom corner of the defect, preferentially deep within the cartilage tangential to the fibers direction, and then extended to the surface along adjacent fibers, finally forming fragments. In the early stages of damage, the shear stress and interstitial flow velocity within cartilage with a rectangular cross-sectional defect were the lowest, while interstitial hydraulic pressure was the highest, followed by 100° trapezoid and semicircle, and finally 80° trapezoid defects. In the later stage of damage, the results were very similar. The shear strain, interstitial flow velocity and interstitial hydraulic pressure decreased with increasing defect depth. Therefore, defect shape only affected damage evolution in the early stages. The fragments in cartilage were the result of the damage evolution which sizes were correlated with the initial defect depth. The damage velocity of cartilage with a rectangular section-incision was the slowest. Finally, we concluded that cylindrical incisions are optimal in clinical surgery. These results provide a theoretical basis for the clinical interpretation of pathological degeneration and repair therapy.

Experimental Study on the Mechanical Properties of Porcine Cartilage with Microdefect under Rolling Load

Objectives: To investigate the mechanical responses of microdefect articular cartilage under rolling load and find out the failure rule. Methods: Rolling load was applied to the porcine articular cartilage samples with rectangular notches of different depths. The displacement and strain near the notches were obtained by the noncontact digital image correlation technique. Results: The strain value and peak frequency around the notch increased; the maximum equivalent strain value could be observed at both bottom corners of the notch; the equivalent strain value first increased and then decreased at the points in the superficial and middle layers with the increase of rolling velocity; the points in the deep layer were less affected by rolling velocity; the equivalent strain value of the points in the superficial layer declined after rising with the increase of defect depth, while a decreased trend could be found for the points in the middle and deep layers. Conclusions: The shear strain, which rose with the increase in defect depth, was the main factor in cartilage destruction. The cartilage tended to be destructed firstly at the bottom corner of the defect. Rolling velocity showed significant effects on superficial and middle layers. Cartilage had the ability to resist destruction.

Experimental Study on the Mechanical Properties of Porcine Cartilage with Microdefect under Rolling Load.

OBJECTIVES: To investigate the mechanical responses of microdefect articular cartilage under rolling load and find out the failure rule. METHODS: Rolling load was applied to the porcine articular cartilage samples with rectangular notches of different depths. The displacement and strain near the notches were obtained by the noncontact digital image correlation technique. RESULTS: The strain value and peak frequency around the notch increased; the maximum equivalent strain value could be observed at both bottom corners of the notch; the equivalent strain value first increased and then decreased at the points in the superficial and middle layers with the increase of rolling velocity; the points in the deep layer were less affected by rolling velocity; the equivalent strain value of the points in the superficial layer declined after rising with the increase of defect depth, while a decreased trend could be found for the points in the middle and deep layers. CONCLUSIONS: The shear strain, which rose with the increase in defect depth, was the main factor in cartilage destruction. The cartilage tended to be destructed firstly at the bottom corner of the defect. Rolling velocity showed significant effects on superficial and middle layers. Cartilage had the ability to resist destruction.

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.