Elucidation of the mechanism of post-repantation replacement resorption as a basis for the development of periodontal ligament regeneration therapy

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External replacement resorption, characterized by the fusion of the bone and tooth root (ankylosis), is known as an uncontrollable complication after tooth replantation. This study was conducted as an initial exploration of the pathogenic mechanisms of post-replantation replacement resorption. Experimental replantation of rat molars were carried out in two different conditions (immediate replantation and replantation after sodium hypochlorite immersion), and pathological changes of the pulpal and periodontalligament (PDL) tissues were examined at 1 to 3 weeks postooperatively by means of histology and immunohistochemistry. Results demonstrated that (i) ankylotic changes in the PDL, (ii) bone-like tissue formation in the pulp, and (iii) CD68-expressing macrophage infiltration in the pulp and PDL were more prominent in the NaClO-immersed group than in the immediately replanted group. These findings suggest that severe damage to the PDL cells remaining on the root surface triggers the development of the above-mentioned pathological changes.