Objectives: Experimentally evaluate the effects of different intrusive forces, applied using mini-implants as anchorage, and compare (1) the amount of tooth intrusion and (2) the amount of root resorption of multiradicular teeth. Methods: Using a randomized split-mouth repeated measures design, 8 mature beagle dogs (20-24 mo) were evaluated every 2 weeks for 14 weeks. Intrusive forces were applied to the second (LPM2), third (LPM3) and fourth (LPM4) mandibular premolars. Immediately loaded mini-implants (12 per dog) were placed perpendicular to the mandibular lingual and buccal cortical plates. Constant forces were applied using one coil spring per mini-implant (1 buccal and 1 lingual) with forces of 25, 50 and 100 g. The intrusive forces were assigned randomly across the arch, with LPM2 having 50 or 100 g, LPM3 having 100 or 200 g and LPM4 having 50 or 200 g. Standardized radiographs were used to assess tooth movements, relative to superimposed bone markers, and root resorption. Repeated measures were statistically modeled to estimate changes over time. Results: Clinically significant amounts of intrusion, ranging from 1.2 mm (LPM4) to 3.3 mm (LPM3) were obtained, with differences depending primarily on root surface area. Both linear and curvilinear patterns of intrusion were observed. Paired comparisons of different force magnitudes showed no statistically significant differences in the amounts of intrusive movement. Clinically insignificant amounts of root resorption occurred (maximum 0.1 mm) at the apices and furcations; there was no consistent pattern of root resorption. Conclusions: (1) variable forces within the 50-200 g range did not differentially affect the amounts of intrusion observed, (2) little or no root resorption occurred after the application of intrusive forces ranging between 50 to 200 g, (3) root resorption was not related with the amounts of tooth movement.

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