Assessment of depth and degeneration dependences of articular cartilage refractive index using optical coherence tomography in vitro

Measuring the depth and degeneration dependences of articular cartilage is important for the investigation of cartilage structure and the reason behind its degeneration. In this study, optical coherence tomography (OCT) was used to investigate the depth and degeneration dependences of the refractive index (RI) of articular cartilage collected from bovine patellae in vitro. Eighteen disks of articular cartilage with a diameter of 6.35 mm harvested from different patellae were prepared. Each disk was cut into two halves and three horizontal cartilage slices (n=18×2×3) with approximately equal thickness were further prepared from each half disk. The cartilage slices were digested by two different enzymes, collagenase and trypsin, to remove collagen fibres and proteoglycans, respectively. The samples were tested using OCT before and after the enzyme digestion and the RI for each specimen was calculated. Two-factor repeated measure ANOVA showed that for all the three groups of specimens, the RI in different depths was significantly different (p<0.05). However, it was revealed that the trypsin and collagenase treatments did not exert a significant effect on the RI (p>0.05). The results suggested that the depth dependence of articular cartilage should be taken into account when OCT is used for related measurement.