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

In this study, optical coherence tomography (OCT) with an axial resolution of 15 μm 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 successfully prepared. Each disk was cut into two halves and three horizontal cartilage slices (n = 18 × 2 × 3) with an approximately equal thickness of 0.5 mm were further prepared from each half disk. The cartilage slices were digested by two different enzymes, collagenase and trypsin, to disturb collagen fibrils and proteoglycans, respectively. The samples were submerged in the physiological saline and tested using OCT before and after the enzyme digestion and the RI for each specimen was calculated. The RI of articular cartilage from the superficial to deep regions was 1.361 ± 0.032 (mean ± SD), 1.338 ± 0.036, and 1.371 ± 0.041 for normal specimens; 1.357 ± 0.036, 1.331 ± 0.030, and 1.392 ± 0.037 for trypsin digested specimens; and 1.361 ± 0.032, 1.336 ± 0.048, and 1.376 ± 0.043 for those treated by collagenase, respectively. Two-factor repeated measure ANOVA revealed that for all the three groups of specimens, the RI in different depths was significantly different (p < 0.05). However, we found 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.