The osteochondral interface is known to smoothly mitigate the stress concentration over the surface area of the subchondral bone, with a structural transition from the cartilage to bone. These sites in the body are exposed to high forces, which makes them prone to failure. To improve the repair strategies, a better understanding of the mechanical property-structure relationship of osteochondral interface is crucial. The aim of this study was to observe the structure and map the stiffness of the equine osteochondral interface during maturation and detect a possible mechanical property-structure relationship.
Osteochondral samples from the metacarpo- and metatarsophalangeal joint of 9 days, 12 days, 7 weeks, 13 months old and three mature horses were investigated in this study. Histology was performed to observe the structure. The local elastic modulus was determined using nanoindentation and the strain distribution with compression tests combined with digital image correlation (DIC).
The cartilage thickness decreased with age and an emerging tidemark was detected in the 13 months old joint, indicating calcification of the cartilage starts at this age. The local elastic modulus showed a clear transitional region between low and high values for the mature interface, indicating the articular cartilage and subchondral bone region was indented, respectively. For the immature samples, a less clear transition was found and in the 9 days and 12 days old interface the entire matrix consisted of low elastic modulus values. The strain distribution from the compression tests over the interface confirmed the decreasing cartilage thickness with age. The transition location from high to low strain decreased with age.
The mechanical transition from cartilage to bone showed significant changes within the interface and during maturation for the local elastic modulus and the strain distribution. These changes can be compared to the structural changes demonstrated in the histology images. The data obtained in this research provides new insights into the maturing osteochondral interface and the mechanical and structural changes within this complex biological tissue.