Nanoscale mechanics of collagen in articular cartilage

Abstract

Articular cartilage is a mechanically important soft tissue whose organisation at the micro- and nanoscale is critical for healthy joint function and where degeneration is associated with widespread disorders such as osteoarthritis. The tissue possesses a complex, graded and depth-dependent structure and at the nanoscale, cartilage mechanical functionality is dependent on the collagen and hydrated proteoglycans that form the extracellular matrix. The structure and in situ dynamic response of the collagen fibrils at the nanoscale, however, remain unclear. Here we utilise small angle X-ray diffraction to measure the depth-wise structure of the fibrillar architecture whilst performing time-resolved measurements during compression of bovine and human cartilage explants. We demonstrate the existence of a depth-dependent fibrillar pre-strain as determined by the D-periodicity, estimated at approximately 1-2%, due to osmotic swelling pressure from the proteoglycans. Furthermore, we reveal a rapid reduction and recovery of this pre-strain during stress relaxation, approximately 60 seconds after onset of peak load. Selective proteoglycan removal disrupts both collagen fibril pre-strain and transient responses during stress relaxation. Additionally, we show that IL-1β induced tissue inflammation also results in a reduction in fibrillar pre-strain and altered fibrillar mechanics. Cyclic loading induces a dynamic reduction and recovery in the D-period that is present regardless of loading rate or treatment, along with changes in diffraction peak intensities and widths. These findings suggest that the fibrils respond to loading via intra- and inter-fibrillar disordering alongside a transient response that is mediated by changes in hydration. These are the first studies to highlight previously unknown transient and cyclic responses to loading at the fibrillar level, and are likely to transform our understanding of the role of collagen fibril nano-mechanics in cartilage and other hydrated soft tissues. These methods can now be used to better understand cartilage in aging and other muscoskeletal diseases.