Using polarisation-sensitive optical coherence tomography to unravel the complex mechano-structural properties of articular cartilage and joint tissue degeneration

Abstract

Osteoarthritis (OA) is estimated to affect more than 303 million people globally. The is no cure and the disease results in debilitating joint pain, significantly reducing the quality of life of anyone affected. Early detection of changes in the mechano-structural properties of articular cartilage is a key challenge that has largely remained elusive due to the limited imaging techniques readily available. What is required by the OA community is a real-time, high-resolution and non-destructive imaging system that can interrogate both the structural and functional integrity of the tissue. Polarisation-sensitive optical coherence tomography (PS-OCT) is one such modality with great potential to fill the gap in the imaging domain. The aim of this thesis is to evaluate the ability of PS-OCT to detect early stage degeneration, with a particular emphasis on understanding the response of cartilage to mechanical stimuli. Firstly, we explored the optical response of articular cartilage to a single impact event. Articular cartilage samples were imaged before-, immediately following-, and three hours after impact. Changes in the optical properties of healthy and degenerate tissue were found to vary significantly depending on impact energy applied. Degenerate tissue exhibited larger changes in optical birefringence following impact, highlighting the increased vulnerability and predisposition of such cartilage to damage. Secondly, we evaluated the ability of PS-OCT to detect the subtle degenerative features that are often difficult, if not impossible, to detect using traditional techniques. Multiple degeneration anomalies were qualitatively observed in both the articular cartilage and underlying bone demonstrating the capacity of PS-OCT to provide insights into the extent and significance of degeneration through non-destructive means. A quantitative classification algorithm was implemented and achieved an accuracy of 93% to objectively classify healthy and degenerate tissue. Finally, an innovative mechanical PS-OCT system was developed to simultaneously compress and image soft-tissue. The system enables changes in optical and mechanical properties of articular cartilage to be visualised as the tissue is compressed. For the first time, spatial changes in optical properties were measured in an in situ manner, revealing insights into the tissue ultrastructure and mechanisms of matrix function. Combined with the ability to extract the mechanical properties of the tissue, the imaging platform provides a convenient way to interrogate the structural and functional integrity of articular cartilage.