Transition and determinants of orthodontic root resorption-repair sequence.

When severe root resorption is observed during orthodontic treatment the clinician needs to know how the active process can be stopped. It is generally assumed that if active force is discontinued, the root resorption will end. The determinants of the resorption/repair sequence are, however, not well understood. The aim of the present study was to register and analyse determinants that may influence the extent of resorbed root surface, as well as the transition of a process of active root resorption into a process of tissue deposition/repair in the resorption lacunae. Using an experimental model that would simulate the first force-activation cycle, the upper first molars of rats were moved mesially by a fixed orthodontic appliance for periods between 2 and 21 days. The results revealed that (1) the extent of root resorption after 21 days corresponded with the maximal extent of the hyalinized zone; (2) a process of repair started from the periphery in the resorbed lacunae where the periodontal ligament (PDL) had been re-established, while ongoing active resorption was observed beneath the existing hyalinized tissue; and (3) root resorption continued in the area where hyalinized tissue persisted even after active force had terminated. It is hypothesized that determinants of continued resorption/repair generally seem to be associated with the persistence and removal of the necrotic tissue.