TNFRSF11B and periodontitis: Elevated levels of receptor activator of nuclear factor kappa-Β ligand (RANKL), osteoprotegerin (OPG), and matrix metalloproteinases (MMP-8, MMP-9) in GCF are indicative of periodontitis and reflect increased bone resorption, which is also relevant in the context of systemic bone diseases such as osteoporosis [10, 135].