Early tissue reaction in the tension zone of PDL during orthodontic tooth movement

• Tissue reaction in the tension zone of PDL during tooth movement consist of two phases.
• First phase is inflammation.
• Second phase is rapid recovery and renovation of the PDL with bone formation.

ObjectiveThe aim of the study was to examine the early tissue reaction in the tension zone of periodontal ligament (PDL) during orthodontic tooth movement.DesignUpper first molars of rats were moved buccally with fixed appliances. The PDL in the tension zone was examined histologically, immunohistochemically and at a molecular level after 24 h, 3 days and 7 days.ResultsAfter 24 h of orthodontic force loading, the periodontal space appeared considerably expanded. The periodontal fibers were stretched between the bone and the root. Three days after loading, the expanded periodontal space had slightly narrowed, the periodontal fiber arrangement was relaxed, and the blood vessels did not appear elongated. A considerable layer of osteoid was formed on the bone surface.The total cross-sectional areas of the PDL in experimental groups were significantly larger than control group. The total cross-sectional areas of the blood vessels were not significantly different among the groups.Significantly high expressions of IL-1β and PTX3 were characteristically observed not only in the endothelial cells and cells around the blood vessel, but also in fibroblasts throughout the PDL of the tension zone 24 h after orthodontic force loading. Three and 7 days after loading, these showed tendencies to return to control levels.ConclusionsThe present results suggest that the early reaction in the tension zone of the PDL during tooth movement consists of two phases: first, inflammation and second, rapid recovery and renovation of the PDL with bone formation.