Conceptual fracture parameters for articular cartilage

Background: Superficial cracks can occur in articular cartilage due to trauma or wear and tear. Our understanding of the behaviour of such cracks in a loaded matrix is limited. A notable study investigated the growth of cracks induced in the bottom layer of the matrix. This paper extends existing studies, characterizing the propagation of superficial cracks and matrix resistance under tension at varying rates of loading.

Methods: Cartilage strips with artificially induced superficial cracks were subjected to tensile loading at different loading velocities using a miniature tensile testing device. Load–displacement data, video and still images were recorded for analysis.

Findings: The propagation of superficial cracks in articular cartilage does not follow the classical crack tip advance that is characteristic of most engineering materials. Instead, the crack tip exhibited a negligible movement while the side edges of the crack rotated about it, accompanied by matrix stretching and an upward pull (necking) of the bottom layer of the sample. As loading progresses, the crack edges stretch and rotate to assume a position parallel to the articular surface, followed by the final fracture of the matrix at a point just below the crack tip. Using the recorded mechanical data and images, an analogous poroelastic fracture toughness, Kp_Ic = 1.83 MPa. square root mm (SD 0.8) is introduced.

Interpretation: It is extremely difficult for a superficial crack to propagate through articular cartilage. This may be because of the energy dissipation from the crack due to the movement and exudation of water, and large stretching of the matrix.