An investigation of biomechanical properties of collagen fibrils extracted from osteoarthritic and osteoporotic cartilages

Abstract

Osteoarthritis (OA) is one of the most concerned diseases in the field of orthopaedics. During the process of this disease, articular cartilages are degenerated and worn out at the end stage, which create pain and disabilities to the patients. Although multiple mechanical and biochemical factors may initiate and/or enhance the progression of OA, alternation of biomechanical properties of articular cartilage is one of the products. There are several major components in articular cartilage; it is believed that each of their contribution cannot be entirely neglected. Superficial zone is mainly consisted with collagen and it was found that the biomechanical properties of this part of cartilage are also alternated significantly as a result of OA. Hence, degeneration of collagen network also occurs. Alternatively, osteoporosis (OP) is another common disease, which is associated to the decrease of bone mineral density; the effect of OP on articular cartilage is limited. In reverse, increasing bone mineral density in subchondral plate alters the loads distribution on articular cartilage and possibly leads to OA. This current study investigated the morphological and biomechanical properties of individual collagen fibrils extracted from OA, OP and healthy cartilages.

A total of ten joint specimens were recruited, 3 OA joints were from 3 OA patients, 3 OP joints were from 3 OP patients, and 4 joints were from 2 healthy individuals. All cartilages were harvested from non weight-bearing zone, and average diameters were calculated from 350 fibrils’ measurements. In addition, 50 fibrils were randomly selected for nano-indentation under ambient condition. However, the representation of biomechanical properties tested at low humidity may be questionable. This current study also investigated the stiffness of hydrated fibrils.

The results showed that the collagen fibrils extracted from OA cartilages were thinner than the ones extracted from OP and healthy cartilages. It was believe that the fibrillation and derangement of collagen network spread from superficial zone towards deeper zones. However, the number of thinner collagen fibrils increased in OA specimens could be the reason of the loss of larger fibrils and/or fragmentation occurred in the superficial zone, where contains thinner fibrils. The biomechanical tests showed that fibrils extracted from OA cartilages owned the highest elastic modulus, while the ones from OP had the lowest; significant differences were found between all groups when tested under ambient condition. Alternatively, the same pattern of results could also be found when hydrated fibrils were tested; however, due to the limited amount of samples, only the difference between OA and OP were considered significant. In addition, no individual difference was found; no significant difference between samples within each group could be observed. Since nano-indentations were performed at the center of each fibril, the elastic moduli measured represented the stiffness of the crosslinks and molecules within fibrils. Assuming the triple helix structure of collagen is relatively tough, the decrease of tensile modulus of superficial zone in OA cartilages could due to the changes of the crosslinks between fibrils in collagen network.