Fig. 4

Tukey’s box-and-whisker plots showing the differences in the levels of serum COMP (a), HA (b), and PIICP (c) between the age 40–54 years and age ≥ 55 years groups in healthy participants and early osteoarthritis (EOA) patients and the age 50–69 years and age ≥ 70 years groups in patients with late osteoarthritis (LOA). Patients with LOA were also divided into the mild (KL of 2 in at least one knee) and severe (KL of 3 or 4 in at least one knee) groups. Data are presented as medians (IQR). Mann-Whitney U test was used for pairwise statistical analysis. Only statistically significant differences are shown and indicated with asterisks: * p < 0.05