The changing role of the superficial region in determining the dynamic compressive properties of articular cartilage during postnatal development.

Objective: To explore how changes to the superficial region (SR) of articular cartilage during skeletal development impact its functional properties. It was hypothesised that a functional superficial region is not present in skeletally immature articular cartilage, and removal of this zone of the tissue would only negatively impact the dynamic modulus of the tissue with the attainment of skeletal maturity.

Methods: Porcine osteochondral cores were mechanically tested statically and dynamically with and without their respective superficial regions in confined and unconfined compression at different stages of postnatal development and maturation. A novel combination of histological, biochemical and imaging techniques were utilised to accurately describe changes to the superficial region during postnatal development.

Results: Articular cartilage was found to become stiffer and less permeable with age. The confined and unconfined dynamic modulus significantly decreased after removal of the superficial region in skeletally mature cartilage, whilst no significant change was observed in the 4 week old tissue. Biochemical analysis revealed a significant decrease in overall sGAG content with age (as % dry weight), whilst collagen content significantly increased with age, although the composition of the superficial region relative to the remainder of the tissue did not significantly change with age. Helium ion microscopy (HIM) revealed dramatic changes to the organization of the superficial region with age.

Conclusions: The findings demonstrate that the superficial region of articular cartilage undergoes dramatic structural adaptation with age, which in turn plays a key role in determining the dynamic compressive properties of the tissue.