The ANG, OPN, and OPG were found to be significantly higher in the OPLL group than in the NOPLL group (3.0 vs. 1.94, 1.81 vs. 1.27, and 1.35 vs. 0.75, p < 0.05, respectively, Table 2). Here, SPP1 is linked to ossification of the posterior longitudinal ligament of the spine.