[Histological investigations on the effect of interrupted expansion force applied to the midpalatal suture in the rat].

The purpose of this study is to find out the most suitable expansion speed in the rapid palatal expansion method in the rat. A total of 96 male rats of Wistar strain were used. The midpalatal suture of the rat was expanded by means of distal displacement of two maxillary incisors. Circular metal wedges of different thickness were inserted between the two incisors. The animals were divided into three groups according to the expansion speeds applied; slow, intermediate and rapid groups, respectively. It took 5 days in the slow group, 3 days in the intermediate, and 1 day in the rapid to achieve 2 mm interval between the two incisors. 1. Although the body weight of the rats was temporarily lost when an interval reached 2 mm, but it turned to gain after 48 hrs in the slow group, 72 hrs in the intermediate, and 96 hrs in the rapid. 2. A cephalography revealed that the width of the midpalatal suture in the control rats was 0.13 mm on the average. On the 6th day of the experiment, the width of the suture increased into 0.26 mm in the slow group, 0.34 mm in the intermediate, and 0.55 mm in the rapid. On the 26th day, however, the width of the suture in the slow and the intermediate group recovered to normal, while the width remained at 0.46 mm in the rapid group. 3. The width of premaxilla increased in the intermediate and rapid group. 4. Histologically, in the slow group, most fibers in the sutural tissue ran along the force line, the osteoid seam was conspicuous, and a few Howship's lacunae were found on the 6th day of the experiment. On the 26th day, the fibers were recumbent, the osteoid seam decreased in width, and Howship's lacunae were not found. It was assumed that the restoration of the expanded suture was almost complete. 5. On the 6th day of the intermediate group, the osteoid seam was conspicuous and some Howship's lacunae were found. The cellular infiltration and the impletion of capillaries were also found in the sutural tissue.(ABSTRACT TRUNCATED AT 400 WORDS)