Effects of Power Arm Application in Maxillary Canine Distalization

Malocclusion, Angle Class II, Tooth Movement Techniques, Orthodontic Anchorage Procedures

canine retraction, power arm, anchorage loss, split-mouth, closed-coil spring, center of resistance

The aim of this study is to investigate the effects of a canine distalization force applied between power arms placed on the maxillary canine and first molar, in comparison with the classical method without power arms. Following maxillary first premolar extraction, canine distalization is commonly accompanied by undesirable effects such as distal tipping and distopalatal rotation of the canine and mesial movement (anchorage loss) of the first molar. By bringing the applied force closer to the teeth's centers of resistance through power arms, the study seeks to move the canine into the extraction space as close to parallel (bodily) movement as possible. The hypothesis is that applying force with Ni-Ti closed-coil springs between power arms placed on the molar and canine teeth reduces undesirable dental effects such as rotation, tipping and anchorage loss during distalization compared with the classical method. In a split-mouth design, 16 patients with maxillary first premolar extractions and moderate anchorage requirement receive power-arm mechanics on one randomly selected side (study side) and conventional mechanics on the other side (control side), with a 150 g force on both sides, over three months of canine distalization.

Design and setting: Prospective, randomized, controlled, split-mouth in vivo clinical study conducted as a specialty (residency) thesis at the Department of Orthodontics, Faculty of Dentistry, Ondokuz Mayis University, Samsun, Turkey. Sixteen patients (11 female, 5 male; mean age at treatment start 16 years 1 month) with Angle Class II malocclusion requiring extraction of the maxillary right and left first premolars and fixed appliance treatment with moderate anchorage were included. The right and left maxillary quadrants of each patient form the study and control groups (16 sides per group). Randomization: At bracket bonding, the side receiving the molar tube with a power arm was selected at random using a computer program; the other side received a standard molar tube. The side with the power arm forms the study group and the side without it the control group. Treatment: Brackets were bonded one week after extraction of the maxillary first premolars (37% orthophosphoric acid, Transbond XT primer and adhesive, 3M) using 0.022 x 0.028-inch vertical-slot metal brackets (Mini Master V-Slot, American Orthodontics). Leveling and alignment were completed with 0.014-, 0.016- and 0.019 x 0.025-inch copper Ni-Ti archwires at 4-week intervals; no laceback ligatures were used. A 0.019 x 0.025-inch stainless steel archwire was then placed and left for one month without active force to allow bracket torque expression, after which baseline (T0) impressions and photographs were taken. On the study side, a U-shaped canine power arm was bent from 0.016 x 0.022-inch stainless steel wire with its gingival opening facing mesially, its height matched to the molar power arm. One end of a 9-mm Ni-Ti closed-coil spring (American Orthodontics) was attached to the molar power-arm hook and the other to the canine power-arm hook with a 0.010-inch long ligature wire, at the level at which a force gauge (Loyka ATG-500) read 150 g. On the control side, the spring was attached in the same way between the molar tube hook and the canine bracket hook, activated to 150 g. Records and measurements: Patients were recalled every 4 weeks for three months. At each visit the maxillary archwire was removed and intraoral photographs and alginate impressions were taken at the start of distalization (T0) and at the 1st (T1), 2nd (T2) and 3rd (T3) months. Plaster casts were scanned with a 3D model scanner (3Shape) and the digital models were superimposed (T0-T1, T0-T2, T0-T3) in OrthoAnalyzer (3Shape) using the medial and lateral points of the third palatal ruga and the medial point of the second palatal ruga. Canine distalization (mm), canine rotation (degrees), canine tipping (degrees) and first molar anchorage loss (mm) were measured separately for each side relative to the occlusal plane and a sagittal plane through and parallel to the midpalatal suture. Impressions, casts, scanning and all measurements were performed by the same investigator. Method error was assessed by repeating 104 measurements after 2 weeks (Dahlberg formula). Sample size: Based on data from Leethanakul et al., a power analysis in G\*Power 3.1.9.7 (alpha = 0.05, power = 95%) indicated 6 subjects per group; considering possible attrition during active orthodontic treatment, 8 per group (16 individuals in total) were included. This record was registered retrospectively after completion of the trial.

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Turkey (Türkiye)