Photoelastic stress analysis of mandibular canine distalization via different retraction methods
Altemose, Mindy L.
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Objective: The purpose of this research is to evaluate the biomechanics of canine retraction for eight unique retraction methods via optical photoelastic stress analysis. Rationale: Extraction of a first premolar is common in orthodontic treatment where there exists a mismatch in arch perimeter and tooth size/position. Clinically, this space is often closed via canine retraction, in a controlled manner and in a concise amount of time. Understanding differences in the force distributions along the canine root for various retraction modalities provides insight into the biomechanics of tooth movement and potentially guides an orthodontist considering retraction during treatment planning. Method: A photoelastic mandibular dentition model is designed and fabricated using a urethane polymer material in an effort to simulate dentition post-leveling with no spacing, other than at the first premolar extraction site. Eight total retraction methods or groups are investigated, via two forms of anchorage; dental anchorage utilizing the second premolar and utilizing the first molar, as well as skeletal retraction utilizing a simulated temporary anchorage device (TAD). Within each anchorage method, elastomeric power chains (EPC) and nickel titanium (NiTi) coil springs are considered. Each retraction method is measured using three different retraction force values, 100 grams, 150 grams, and 200 grams, respectively. In collaboration with the teaching laboratory at the University of Rochester, Institute of Optics, Rochester, NY, an optical polariscope is constructed and aligned to measure, both qualitatively and quantitatively, the stress distribution within the mandible model for each retraction method. Results: For dental anchorage retraction methods, as the amount of retraction force increases from 100 grams to 200 grams, stress increases in the apical mesial region, and the entire distal side of the canine root. For skeletal anchorage retraction methods, as the amount of retraction force increases from 100 grams to 200 grams, stress increases along the entire canine root. As the angle between the temporary anchorage device and the occlusal plane increases, so does the force on the apical distal region of the canine. Conclusions: The use of dental anchorage provides a controlled tipping distal movement, whereas the use of skeletal anchorage provides an intrusion component to the stress along the canine root. When the extrinsic applied force is positioned from the distal wing of the canine to the mesial wing of the second premolar or to the molar hook, the resulting movement resembles controlled tipping. When the extrinsic applied force is positioned from canine hook to first molar hook, the resulting movement begins to resemble a translational movement. For skeletal anchorage, the increase of stress surrounding the entire canine root corresponds to the rotational force felt by the canine. As the angle between the temporary anchorage device and the occlusal plane increases, so does the amount of intrusion on the canine.