Oxytetracycline microfluorescent comparison of orthodontic retraction into recent and healed extraction sites

The effects of recent versus healed extraction sites on orthodontic retraction were evaluated histologically by employing oxytetracycline vital staining. Six female Macaca rhesus monkeys with permanent dentitions and unerupted third molars were selected. The mandibular left first molar of each animal was extracted, and the site was allowed to heal for 7 weeks. After healing, the mandibular right first molar was extracted. Left and right three-tooth sectional orthodontic appliances were placed immediately, and the second premolars were activated distally. All animals were injected intramuscularly with oxytetracycline once each week during the activation period in order to mark the calcifying alveolar bone. Appliances were reactivated after 2 and 4 weeks. Experimental animals were killed at 1-, 3-, and 6-week intervals. The control animal with unactivated appliances was killed after 6 weeks. After the animals were killed, the formalin-fixed undecalcified alveolar bone and tooth material was prepared by a combination machine-sectioning, handgrinding, and staining technique. Fluorescent microscopic evaluation showed that in the 1- and 3-week animals the osteoclastic widening of the periodontal space in areas of compression was greater on the healed side than on the recent extraction side. The differences in widening were less apparent in the 6-week experimental animals. In tension areas, the healed side showed an increased amount of new bone and spicule formation when compared to the recent extraction side. Movement, therefore, was greater on the healed side than on the recent extraction side. This difference was probably caused indirectly by the previous reversal of second premolar mesial drift during the initial 7-week healing period before appliance placement. The reversal was exhibited by the control animal after approximately 5 weeks of healing and coincided with the demonstrated reorganization of transseptal fibers across the extraction site. The recent extraction side showed a relative absence of new bone formation mesial to the second premolar in the 1- and 3-week animals. Retraction was unable to produce bending of the distal interseptal bone. Rather, the osteoclastic resorption of alveolar bone in areas of compression was necessary to initiate significant movement. The simultaneous independent activation of both mandibular second premolars showed that no advantage resulted from immediate retraction into the site of a recent extraction.