To determine the potential correlation between RI distribution and OA progress, a quantitative technique was adopted in the estimation of PG loss. For normal AC (lower left corner of Fig. 4), the coefficient of RI fluctuation was low and insensitive to the PG concentration. This is because the PG loss in normal AC is maintained in similar levels and hence, the RI fluctuations measured from these samples are similar. However, other factors, such as the development of calcification and the proportions of different matrix compositions, could induce slight diversity in RI fluctuations. In the upper right region of Fig. 4, the 10 AC plugs were detected with waveform of RI distribution and distributed as two separated groups (circled as Group A and Group B). Despite the similar average RI of these two groups, the average PG loss of Group B is 37.50% higher than Group A, but the coefficient of RI fluctuation measured from Group B is averagely 43.28% higher than Group A (Table 2). Comparing the PG loss and RI fluctuation in normal AC, Group A and Group B, it is reasonable to conclude that the severity of RI fluctuation increases with the proportion of PG loss. However, the coefficient of RI fluctuation may still have difficulty in distinguishing the pathological progress of two AC samples in which the PG loss does not sufficiently differ. There is only one AC plug which has very low RI fluctuation as well as a relative high level of PG loss (Fig. 4), but the single exception is insufficient to deny the correlation between RI fluctuation and PG loss.