Quantitative Assessment of Optical Properties in Healthy Cartilage and Repair Tissue by Optical Coherence Tomography and Histology

Abstract

Quantification of the OCT signal is an important step toward clinical implementation of a diagnostic tool in cartilage imaging. Discrimination of structural cartilage differences in patients with osteoarthritis is critical, yet challenging. This study assesses the variation in the optical attenuation coefficient ( $\mu_{{\rm OCT}}$ ) between healthy cartilage, repair tissue, bone, and layers within repair tissue in a controlled setting. OCT and histology were used to assess goat talus articular surfaces in which central osteochondral defects were created. Exact matches of OCT and histology were selected for research. $\mu_{{\rm OCT}}$ measurements were taken from healthy cartilage, repair tissue, and bone. Measured $\mu_{{\rm OCT}}$ in healthy cartilage was higher compared to both repair tissue and bone tissue. Two possible mechanisms for the difference in attenuation were investigated. We studied morphological parameters in terms of nucleus count, nucleus size, and inter-nucleus distance. Collagen content in healthy cartilage and repair tissue was assessed using polarization microscopy. Quantitative analysis of the nuclei did not demonstrate a difference in nucleus size and nucleus count between healthy cartilage and repair tissue. In healthy cartilage, cells were spaced farther apart and had a lower variation in local nuclear density compared to the repair tissue. Polarization microscopy suggested higher collagen content in the healthy cartilage compared to the repair tissue. $\mu_{{\rm OCT}}$ measurements can distinguish between healthy cartilage, repair tissue, and bone. Results suggest that cartilage OCT attenuation measurements could be of great impact in clinical diagnostics of osteoarthritis.