TNFSF11 and periodontitis: Collectively, no significant difference in OPG expression was found between the two groups, which is not consistent with some previous reports (Oliveira et al., 2015; Ozaki et al., 2017), but both higher RANKL levels and higher RANKL/OPG ratios in gingival tissues in the CP group suggested that progressive bone resorption occurs in severe periodontitis.