Proteoglycan and collagen contribution to the strain-rate-dependent mechanical behaviour of knee and shoulder cartilage

The contribution of the proteoglycan to the strain-rate-dependent mechanical behaviour of cartilage tissues has been suggested to decrease with an increase in the strain-rate. On the other hand, the contribution from the collagen network has been suggested to increase as the strain-rate increases. These conclusions are drawn mainly based on numerical studies conducted on high-load-bearing knee cartilage tissues, while experimental evidence of these behaviours have not been demonstrated previously. Further, in contrast to the reported findings on highload bearing knee cartilage, our previous study on the low-load-bearing kangaroo shoulder cartilage indicated that proteoglycan and collagen contribution remained steady as the strain-rate increases. Therefore, in the present study, we experimentally investigate the contribution of proteoglycan and collagen network to the strainrate-dependent behaviour of the kangaroo knee cartilage, and plausible reasons for the differences observed in relation to the kangaroo shoulder cartilage. Firstly, in order to quantify the contribution of proteoglycans and collagen network, the indentation testings on normal, proteoglycan, and collagen-degraded kangaroo knee cartilage were conducted at different strain-rates. Then, structural and compositional differences between the kangaroo knee and shoulder cartilage were assessed qualitatively through polarised light microscopy (PLM) imaging and histological staining. Identified differences in the collagen architecture and proteoglycan composition were incorporated in a fibril-reinforced porohyperelastic Finite Element (FE) model with the objective of explaining the mechanisms underlying differences observed between the two tissues. Experimental results on knee cartilage indicated that when the strain-rate increases, proteoglycan contribution decreases while collagen contribution increases, where statistically significant differences were identified at each strain-rate (p < 0.05). PLM images revealed a sizable deep zone in the kangaroo knee cartilage where collagen fibrils were oriented perpendicular to the subchondral bone. On the other hand, no such apparent deep zone was observed in the shoulder cartilage. FE model confirmed that the biomechanical differences observed in the knee and shoulder cartilage are due to the differences in the collagen fibril arrangement in the deep zone. From these results, it can be concluded that in high-load-bearing cartilage tissues, the collagen network in the deep zone assists in increasing the stiffness of tissue with strain-rate and plays a significant role in supporting transient loads. This, in turn, helps protect the solid matrix against large distortions and strains at the subchondral junction, pointing to the importance of the collagen network in deep zone in assisting high-load-bearing cartilage tissues.