Objectives: External apical root resorption (EARR) is a common complication of orthodontic treatment. Recent work indicates genetic factors may play a role in determining susceptibility to EARR. Siblings are more likely to have similar EARR than those who are not related. Variability in histological root resorption associated with orthodontic force among inbred mouse strains has also been reported. RANKL is an essential factor for osteoclast differentiation through its receptor RANK, which is expressed on the osteoclast membrane. OPG (osteoprotegerin) is a soluble decoy receptor that inhibits osteoclast differentiation through its binding to RANKL. We investigated the difference in expression of RANKL and OPG during orthodontic movement of teeth in mice susceptible to root resorption compared to mice resistant to root resorption. Methods: Male DBA/2J (susceptible to EARR) and A/J (resistant to EARR) were obtained and either treated with an orthodontic appliance to tip the maxillary first molar mesially (N=9 per strain) or used as untreated controls, (N=9 per strain). Maxillae were collected, fixed, decalcified, paraffin imbedded and sectioned. Immunohistochemistry was performed using anti-RANKL and anti-OPG antibodies. Results: Increased localization of RANKL in the tissues surrounding the root of the susceptible strain was detected compared to the resistant strain and the controls. In contrast, increased localization of OPG was found in the tissues surrounding the roots in the resistant strain compared to the susceptible strain. Conclusion: These results suggest that the difference in the expression of these key bone resorption mediators may play a major role in determining the susceptibility to root resorption during orthodontic treatment.

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