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The classic work of Sandstedt1 in 1904 marked the start of scientific research based on experimental data to account for tissue changes associated with orthodontic tooth movement. Schwarz2 acknowledged the scientific merit of Sandstedt's effort and in addition developed a semiquantitative correlation between forces and histologic changes. Gottlieb and Orban3 also investigated the changes observed after a force had been active on dogs' teeth for periods varying from 12 to 72 hours. Reitan4 over the years has published extensive histological studies concerned with the early tissue reactions resulting from the use of forces of various intensities and duration. Waldo and Rothblatt,5 Macapanpan, Weinmann, and Brodie,6 Yen,7 and Storey8 have also investigated the early tissue changes following tooth movement.It is the purpose of this study to examine the sequence of early histologic changes in the periodontal tissues following the application of force to the molar teeth of the albino rat. Materials and MethodsTwenty-four male albino rats of the Wistar strain averaging 220 gm. in weight were anesthetized.t Strips of dental rubber dam 2 mm. wide and 0.14 mm. thick were stretched and inserted between the crowns of the upper left first and second molars using the technique described by Waldo and Rothblatt.5 The first molar was thus forced in a mesial direction by the recoil of the rubber strip. The upper right quadrant was used as a control. The lower jaw was left intact. Six rats were sacrificed after each of the following periods: 6, 12, 24, and 72 hours. In each instance the upper three molars with their surrounding bone and gingival tissues, on both the experimental and control sides, were cut out and fixed in 10 per cent Formalin. These specimens were decalcified in 5 per cent formic acid and, following dehydration, were blocked in paraffin. Serial histologic sections 7-10 1i thick were prepared from the tissue quadrants obtained in each of the four experimental time-interval groups. The molar quadrants of three rats were cut in a mesiodistal direction, the quadrants of two rats were cut in a horizontal or occlusal plane, and those of one rat were cut in a buccoBased on a thesis submitted to the graduate faculty, Indiana University School of Dentistry, in partial fulfilment of the requirements for the M.S.D. degree.

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