This study was perfomed on twenty five teeth which showed radiographic lesions( 6mm over 25 mm). the teeth were arranged in seven different groups and root canal therapy ( by warm Gutta-percha technique) as surgical intervention were performed at various time intervals. In this investigation, histologic studies of the periapical tissues begin shortly after elimination of the root canal system with or without root  canal obturation. This healing demonstrated with a) replacement of granulation tissue by connective tissue, inflammatory cells diminish in number and density
b) fibroblastic activity and their differentiation of osteoblast
c) osteoblastic activity forming osteoid and trabecular bone. The newly formed trabeculae extend from the periphery of the lesion to the center and root surfaces
d) nonorganized periodontal ligament fibers return to their original orientation soon after the newly formed trabeculae reach the apical root surface and form lamina dura
Inflammatory responses may continue simultaneously with periapical tissue regeneration and bone formation. Chronic inflammatory cells seem to be present occasionally in marrow spaces of the newly formed bone. Cellular activity and bone formation are demonstrated in the presence of the lining epithelium. This is seen between newly formed bone and apical root tip and might be suggestive of healing of the periapical cyst.