Optical properties of articular cartilage in the near-Infrared spectral range are related to its proteoglycan content

Articular cartilage is a connective tissue that enables smooth movements between bones in articulating joints. Cartilage consists of extracellular matrix (ECM) and chondrocytes – the cells responsible for synthesis of the ECM. The ECM consists of type II collagen, proteoglycans, water, and some other minor components. Cartilage is prone to degenerative joint conditions, such as osteoarthritis, due to its weak repair capacity resulting from a lack of vascular, neural, and lymphatic networks. Osteoarthritis causes erosion of the cartilage matrix and therefore inhibits its function, resulting in joint pain, loss of mobility, and significant global socioeconomic burden. Currently, surgical treatment of cartilage pathologies is carried out during arthroscopy with variable outcomes. This variability occurs due to the subjective nature of arthroscopy, which relies on manual palpation and visual evaluation of the tissue surface. Diffuse optical spectroscopy in the near-infrared spectral region probes tissue structure and composition via a relationship with its optical properties (the absorption and reduced scattering coefficients). Due to its avascular nature, healthy cartilage is translucent. It thus has low absorption in the near-infrared region, providing the necessary conditions for light to traverse deep into the tissue. This research reports, for the first time, cartilage absorption and reduced scattering coefficients in the near-infrared spectral range and assess their capacity for characterizing the depth-wise profile of cartilage proteoglycan content. The results revealed that cartilage optical properties are strong predictors of its proteoglycan content. The best performance was observed with the prediction of the proteoglycan content by the absorption coefficient.