Human tooth movement by continuous high and low stresses.

Objective: To compare three-dimensional tooth movements resulting from relatively higher and lower stresses in a split-mouth design. Materials and Methods: Eight volunteers whose maxillary first premolars were removed for orthodontic treatment participated. Each subject's maxillary canines were retracted by randomly assigned constant stresses of 78 kPa and 4 kPa via segmental mechanics. Dental casts depicting 8-10 visits per subject over 84 days and a three-axis microscope were used to measure movements serially. Descriptive statistics and mixed linear modeling were applied for data analyses (alpha = .05). Results: Teeth moved by 78 kPa had significantly faster (P = .0005) distal movement (0.066 +/- 0.020 mm/day) compared to teeth moved by 4 kPa (0.031 +/-. 0.012 mm/day). Lateral movement and distopalatal rotation were also significantly faster (fourfold and 10-fold, respectively) with higher than with lower stress (P < .0001). Average extrusion-intrusion, crown torque, and tip were small (<= vertical bar 0.25 vertical bar degrees mm, vertical bar 2.29 vertical bar degrees, and vertical bar 1.98 vertical bar degrees, respectively), fluctuated, and not significantly different between high and low stresses. No lag phase of tooth movement was evident. Conclusions: Maxillary canines were retracted faster by 78 kPa than by 4 kPa. Controlled translation was possible with 4 kPa, but 78 kPa outstripped appliance constraints, causing distopalatal rotation.