COL9A2 and intervertebral disk degenerative disorder: Fifteen studies (2292 cases and 2089 controls) investigated the correlation between COL9A2 Trp2 and disc degeneration susceptibility, 13 studies (1623 cases and 1606 controls) for COL9A3 Trp3, 4 studies (310 cases and 812 controls) for COL1A1 sp1, and 5 studies (8 datasets, 1817 cases and 1728 controls) for COL11A1 C4603T.