How subtle structural changes associated with maturity and mild degeneration influence the impact-induced failure...

Implicit structural changes in the joint tissues, not apparent in gross appearance and related to age and mild degeneration, represent potentially important biomechanical factors that could influence the vulnerability of the joint to trauma. The hypothesis of this study was that micro-level structural differences in the cartilage tissue matrix, and its interface with the underlying bone, would result in different fracture responses to single impact loading.

For this study a range of cartilage-on-bone samples, from intact to mildly degenerate, were obtained from bovine patellae. These samples were subjected to a single impact, via a cylindrical 6-mm diameter plane-ended indenter, sufficient to create a visible fracture on the articular surface. Microstructural assessment of the region of failure was carried out using differential interference contrast optical imaging. Distinct differences in the modes of fracture propagation were correlated with microstructural changes.

It was found that the intact tissues required impact energies of approximately 2.3J to induce surface rupture. These ruptures advanced to a variable radial depth that depended on the age of the animal from which the tissue was obtained. In the intact tissues from adult animals, the ruptures were largely confined to the upper third of the cartilage thickness. In the intact tissues from adolescent animals the ruptures progressed into the deep matrix zone and crossed the underdeveloped calcified cartilage region and underlying bone. For the mildly degenerate tissue cohort, lower impact energies of approximately 1.6J was sufficient to cause extensive detachment of the articular cartilage at or near the osteochondral junction.

The subtle microstructural differences in intact cartilage-bone tissue obtained from adolescent versus mature animals are important as they correlate with the observed differences in impact response. Any mechanical model or structural analogue of cartilage should consider such implicit structural variations and their implications for overall joint function during weight-bearing.