The effects of mandibular retractive force on the growing rat mandible.

The purpose of this study was to investigate the three-dimensional effects of a mandibular retractive force on the growing rat mandible and mandibular growth after the force was removed. Eighty 4-week-old male Wistar rats served as experimental animals and 100 4-week-old male Wistar rats served as the control group. The experimental rats were divided into four groups. Thirty grams of retractive force was applied to each mandible for 4 weeks. Each group was then killed after 4-, 8-, 10-, 12-, or 16-week periods. At the time of death, tibia, skull, and mandible were removed, measurements of those bone specimens were taken, and histologic examinations were performed. Compared with the control group, the experimental group demonstrated no significant differences in body weight nor in the length of measurements of the tibia, skull, and proximal end of the tibia. The mandibles of the experimental group demonstrated shorter anteroposterior length, higher coronoid processes, and a thicker corpus in the retromolar region and condylar neck. The condylar cartilages of the experimental group showed moderate adaptation in the histologic sections. However, mandibles of the experimental animals did not show any catch-up growth behavior after removal of the force. It was concluded that the mandibular retractive force could produce overall growth retardation and transformation of growing rat mandibles, but the force had no effect on the growth behavior after the appliance was removed.