Human Tooth Movement by Continuous High and Low Stresses

The magnitude of stress applied by an orthodontic appliance is thought to be critical for velocity of tooth movement; however, an optimum stress for maximum rate has yet to be defined. Objectives: The purpose of this study was to measure and compare three-dimensional tooth movements resulting from constant stresses of relatively higher and lower magnitudes while also measuring the torque, tip, and rotation to test appliance effectiveness in producing translational tooth movement. Methods: By split-mouth design, maxillary canines were retracted in eight individuals, whose maxillary first premolars were removed as part of their orthodontic treatment, using nickel-titanium closed coil springs with constant stresses of 78 kPa and 4 kPa randomly assigned to right and left sides. Ten appointments were scheduled over 84 days to make impressions and dental casts. Templates for the casts were constructed that contained reference markers for measurement with a three-axis microscope to determine linear and rotational movements over time. Results: Teeth moved by 78 kPa had higher mean distal velocity: 0.66 (±0.020) mm/day, compared to 4 kPa: 0.031 (±0.012) mm/day. Lateral movement and distopalatal rotation also increased progressively with time at higher rates (4-fold and 10-fold, respectively) with the higher stress. Average extrusion, crown torque, and crown tip were small (≤│0.25│mm, │2.29│°, and │1.98│°, respectively) and fluctuated with time. Mixed linear modeling demonstrated that both stress and time had significant positive effects on distal, lateral, and distopalatal rotation movements (p<0.0001) but no significant effects on extrusion, labial crown torque, or distal crown tip (p>0.05). Conclusions: Maxillary canines were retracted at higher velocities by 78 kPa than 4 kPa. Controlled translation is possible with 4 kPa, but 78 kPa outstripped appliance constraints and caused distopalatal rotation. Funded in part by UMKC Summer Scholars Program and the AAOF.