The effects of force magnitude on a sutural model: a quantitative approach

In an effort to quantify the biologic effects of an orthodontic tensile force, the rat interpremaxillary suture was investigated as a model for the periodontal ligament and expanded in vivo with a helical spring across the maxillary incisors. Three levels of force were used: light (50 to 75 g), medium (150 to 175 g), and heavy (250 to 300 g). Thymidine labeling and histologic studies after 12 hours and 1, 2, and 4 days of force delivery are described (n = 48 rats), as are biochemical studies after 2 and 4 days including a 6-hour organ culture (n = 32). The percentage of labeled cells increased significantly in all force groups at 1 day, followed by a rapid decline at 2 days, to a value at 4 days not significantly different from the controls. Biochemical studies showed significant increases in proline incorporation and alkaline phosphatase activity after 2 days of heavy force application. Histologic examinations showed obvious tissue changes beginning by day 1 and involving increases in suture width, vascularity, size and number of cells, amount of osteoid production, and changes in suture morphology. The experimental system was convenient, inflammation-free, and appeared to be reliable as evidenced by characteristic, synchronous tissue and autoradiographic changes in all experimental sutures through 4 days.