Objective To investigate the expression of receptor activator of nuclear factor κB ligand（RANKL） and osteoprotegerin（OPG） in periapical cyst and periapical granuloma by comparison with the expression in the normal periodontal tissue as control, and to identify their functional mechanism in the bone destruction of periapical cyst and granuloma. Methods 20 periapical cyst tissues（cyst group）, 20 periapical granuloma tissues（granuloma group）, and 20 normal periodontal tissues（control group）were collected respectively. Immunohistochemical technology was performed to detect the expression of RANKL and OPG in above three groups. Results In cyst group, granuloma group and control group, the expression of RANKL were 75.00±7.54, 68.40±6.74 and 29.40±2.46, respectively. The expression of OPG were 38.10±7.09, 47.65±13.85 and 58.60±5.88, respectively. The differences among the three groups were statistically
significant（P0.05）. Conclusion RANKL and OPG play roles in the bone absorption of periapical disease. In periapical disease, abnormal expression of RANKL and OPG are detected, RANKL significantly increase, OPG decrease, bone absorption accelerate and osteolytic lesion are observed. In periapical cyst, the
bone absorption is more active compared with periapical granuloma.
