SFRP1 and ossification of the posterior longitudinal ligament of the spine: Although the serum levels of OSC, DKK-1, SFRP-1, SOST, and OPG were not significantly different between patients with one of these diseases (Table 5), we found that OSC levels were higher in OPLL (7.95 ± 3.91 ng/mL) than in AS (3.96 ± 1.03 ng/mL) patients (**p < 0.01).