Date Published: October 01, 2019
Author(s): Sheetal R. Inamdar, Ettore Barbieri, Nicholas J. Terrill, Martin M. Knight, Himadri S. Gupta.
Structural and associated biomechanical gradients within biological tissues are important for tissue functionality and preventing damaging interfacial stress concentrations. Articular cartilage possesses an inhomogeneous structure throughout its thickness, driving the associated variation in the biomechanical strain profile within the tissue under physiological compressive loading. However, little is known experimentally about the nanostructural mechanical role of the collagen fibrils and how this varies with depth. Utilising a high-brilliance synchrotron X-ray source, we have measured the depth-wise nanostructural parameters of the collagen network in terms of the periodic fibrillar banding (D-period) and associated parameters. We show that there is a depth dependent variation in D-period reflecting the pre-strain and concurrent with changes in the level of intrafibrillar order. Further, prolonged static compression leads to fibrillar changes mirroring those caused by removal of extrafibrillar proteoglycans (as may occur in aging or disease). We suggest that fibrillar D-period is a sensitive indicator of localised changes to the mechanical environment at the nanoscale in soft connective tissues.
Collagen plays a significant role in both the structural and mechanical integrity of articular cartilage, allowing the tissue to withstand highly repetitive loading. However, the fibrillar mechanics of the collagen network in cartilage are not clear. Here we find that cartilage has a spatial gradient in the nanostructural collagen fibril pre-strain, with an increase in the fibrillar pre-strain with depth. Further, the fibrillar gradient changes similarly under compression when compared to an enzymatically degraded tissue which mimics age-related changes. Given that the fibrils potentially have a finite capacity to mechanically respond and alter their configuration, these findings are significant in understanding how collagen may alter in structure and gradient in diseased cartilage, and in informing the design of cartilage replacements.
Many connective tissues are found to possess a structural gradient in terms of their composition and architecture in order to fulfil their functional role . Such gradients are found both at the interfaces between different tissue types, as well as – at different length scales – within individual tissues. For example, the interface between tendon and bone is crucial for the anchorage of muscle, whilst bone itself possesses a three dimensional structure (lamellar osteons) that are graded by the varying orientations of fibrils and mineral platelets in 3D , . As a result, such tissues often have complex, multi-phase and spatially varying interactions between the components that comprise the hierarchical structure. In particular, articular cartilage (AC) is a case where both types of gradients can be observed . The primary role of AC is to provide a smooth lubricated surface between contacting bones, which allows both frictionless sliding as well as a reduction in contact stresses to the underlying bone . The depth-dependent variation in structure in AC is believed to be necessary for its mechanical role of a sliding on one side and firm anchorage to bone on the other whilst reducing the interfacial shear stresses between joint surfaces . However, whilst the tissue- and micro-level architectural gradients in AC are well understood , the corresponding spatial variation in the nanoscale and molecular structural parameters are much less investigated. Understanding these ultrastructural gradients and their biomechanical significance will be critical in developing new biomaterials for articular cartilage repair and replacement.
In summary, we demonstrate that there is a clear gradient in fibrillar pre-strain and intrafibrillar order through the thickness of articular cartilage, likely mediated by the localised changes in PG-derived hydration and associated swelling of the tissue. The measured gradient in the zonal fibrils are a combination of both an absolute change in the D-period as well as a disordering amongst the sub-units of the fibrils. Further, we show that under compression of cartilage, there is a reduction in the fibrillar pre-strain present within the ECM. Interestingly, this effect is closely mimicked by the response of the tissue to PG removal alone, highlighting both the importance of the internal hydration of the tissue, as well as the way in which nanoscale mechanical homeostasis in soft tissues can be disrupted by small variations in component matrix profiles. Our results thus provide both insight into understanding how age-related alterations in the matrisome of musculoskeletal, vascular and other collagen-rich tissues can significantly disrupt their biomechanical function, and may help develop, in future, avenues to detect and ameliorate such changes.