INTRODUCTION

Orthodontists strive to resolve malocclusions efficiently. The orthodontic diagnosis and associated treatment plan often require retraction of anterior teeth, and in these cases, premolar extractions are normally performed. The tooth movement rate is important to enable the orthodontist to anticipate treatment duration, and a good understanding of the rates of maxillary and mandibular tooth movement as well as the amount of anchorage loss is the basis for making treatment more efficient.¹

Canine retraction is a common treatment procedure in orthodontics and can be performed by different techniques (eg, sliding mechanics or closing loops).^2–6 In vitro studies have investigated the frictional resistance between self-ligating (SL) and conventional (CV) bracket systems and have shown lower friction associated with sliding mechanics for SL brackets due to passive configurations between the arch wire and the bracket slots.^7–10 However, in the oral environment, studies have shown no difference between the two types of brackets.^11–19 So far, only Burrow²⁰ has shown differences in the movement rate in the maxillary canines between the SL and CV brackets. The studies have shown that canine retraction rates in the maxilla were greater than the rates observed in the mandible, but the results were not consistent because of a small sample size (≤12 subjects) and the clinical study design.^2,4,21

Several studies have evaluated maxillary canine retraction,^{3,4,11,20,22–30} but smaller numbers of studies have analyzed mandibular canine retraction.^{2,4,5,12,31–33} Therefore, few studies have been done comparing the rates of maxillary and mandibular tooth movements.^2,4,5,12,21 To reconcile existing inconsistencies and the lack of information, a larger sample size compared with previous studies is necessary. The aim of this split-mouth randomized clinical trial study was to investigate and compare the tooth movement rates with sliding mechanics using SL brackets and CV brackets detecting differences between upper and lower jaws. The second aim was to measure the mesial movement of the first molars during maxillary and mandibular canine retraction.

MATERIALS AND METHODS

In this split-mouth randomized clinical trial, 25 adult subjects were treated orthodontically (Table 1). Subjects were selected according to the following criteria: Class I molar relationship, maxillary and mandibular crowding equal or smaller than 4 mm, bimaxillary dental protrusion requiring four first premolar extractions, no missing teeth except third molars, good hygiene, and healthy dentition. This study was reviewed and approved by the Institutional Review Board from the Araraquara Dental School, Sao Paulo State University/UNESP, Araraquara, Brazil. All patients gave informed consent, as required by the human subjects committee.

Table 1 Summary of Characteristics of the 25 Subjects

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The patients had stainless steel fixed appliances placed from second molar to second molar in the maxilla and mandible. All first molars were banded, and patients had the second molars either bonded or banded. CV brackets and tubes (Ovation brackets, 0.022-inch slot, GAC, Bohemia, NY, USA), were used. In a split-mouth design, SL brackets (In-Ovation brackets, 0.022-inch slot, GAC), were randomly bonded to one maxillary and one mandibular canine in all patients. Therefore, randomly, each patient had one maxillary canine and one mandibular canine bonded with SL brackets but never on the same side (Figure 1).

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Leveling and alignment of the arches were performed using 0.014-inch NiTi superelastic archwire, 0.020-inch NiTi superelastic archwire, and 0.020-inch stainless steel archwire with omega loops flush and tied to the mesial of the buccal tube on the first molars. After 4 weeks of stainless steel archwires, the posterior segment (second molar, first molar, and second premolar) were tied together using a 0.010-inch ligature wire forming the anchorage segment, and extractions were performed. No additional anchorage system was used in any patient. Performing the leveling and alignment before the extractions was necessary to prevent the malocclusion from influencing the canines.³⁴ Canine retraction began between 7 and 14 days after extractions using GAC Sentalloy retraction springs (100 g). Using a Correx gauge (Haag-Streit AG, Koeniz, Switzerland), the retraction springs were stretched to 17 mm (approximately 2.5 times the initial length) to deliver the correct force necessary to retract the canines. Retraction springs were tied from the first molar tube hook to the canine bracket hook using a 0.010-inch ligature wire when required to achieve the essential amount of force. Every 4 or 5 weeks, the force delivered by the closed-coil spring was measured and adjusted to maintain 100 g of retraction force. Ligature wire (0.012-inch) was used to tie the CV brackets to the archwire.

Oblique lateral cephalometric radiographs (45° exposure) of both sides were taken immediately before the starting of canine retraction (T1) and at the end of canine retraction when there was no space remaining between the canine and second premolar (T2). Rotating the patient 45° toward the radiographic film allowed the image to be focused on one side without superimposing the opposite side, but two radiographs were necessary to evaluate the left and right sides separately. All patients completed the retraction phase and had a total of two right (T1 and T2) and two left (T1 and T2) oblique radiographs. The cephalograms were traced using mechanical pencil with a 0.3-mm tip on acetate paper, and the eight landmarks and the three fiducial points (Table 2; Figure 2) were digitized by one operator (Dr. Monini). A horizontal reference line used the fiducial points one and two that were marked over the occlusal plane line (using first molars and incisors), and a third fiducial point was marked posterior and superior in the cephalograms. A vertical reference line, perpendicular to the occlusal plane line, was drawn using the third fiducial point.

Table 2 Landmarks Used on Oblique Lateral Cephalograms

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Partial superimpositions were performed on the best fit of the stable structures.³⁵ Maxillary superimposition was done on the contour of the inner cortical bone of the anterior part in the canine region of the maxilla from the contralateral side, posterior contour of the infrazygomatic crest, and orbital contour and nasal floor (Figure 3). Mandibular superimposition was done on the inner cortical structure of the inferior border of the symphysis and the mandibular corpus of the opposite side and detail structures of the mandibular canal and foramen (Figure 4). Structures and fiducial points were transferred from initial cephalograms (T1) to final cephalograms (T2).

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DFPlus software (DentoFacial Planner Software 2.0, Toronto, Canada) was used to digitize the radiographs and to make the measurements. The digitization was performed twice, with a 30-day interval between the first and second digitization, by the same investigator (Dr. Monini), and measurements were averaged to reduce error. Changes of the landmarks, amount of canine retraction, and anchorage loss were measured by the horizontal distance perpendicular to the vertical reference line. Differences between the initial and final cephalograms (T2–T1) were used to calculate the amount of change during space closure. Monthly rate changes were divided by the time necessary to close the space between canine and premolar completely.

Power analysis was performed (G-Power software, version 3.0.22.). Based on an estimated difference between groups of 0.35 mm/month for lower canine retraction rates and a standard deviation of 0.6 mm taken from a previous study,¹² a sample size of 25 patients/group was needed (5% significance level and a power of 80%). A priori sample size calculation for a paired t-test to detect a medium size effect (which would be clinically relevant) requested 20 pairs for comparison. Dahlberg's formula³⁶ was used to determine the error and standard deviation of the variables. The linear measurement error was found to be less than 0.43 mm, while the angular measurement error was less than 1.67°. The measurements were transferred to SSPS software (version 16.0, SPSS, Chicago, IL, USA) for statistical analyses. The skewness and kurtosis statistics indicated normal distributions. Paired t-tests were used to compare side and jaw effects.

RESULTS

SL and CV brackets did not show differences related to monthly canine movement and anchorage loss rates in the maxilla and in the mandible (Table 3). Since no significant different characteristics were observed between bracket groups, they were combined in the same jaw to evaluate differences between upper and lower arches (Table 3).

Table 3 Descriptive Statistics and Statistical Comparisons Between Bracket Types

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Maxillary canines showed a greater amount of tooth movement per month than mandibular canines (0.71 mm and 0.57 mm, respectively). Another difference was noticed in the duration of total canine retraction (Table 4). Upper canines were retracted 3 months faster (10.78 months) than lower canines (13.74 months). The anchorage loss between the maxilla and mandible was not significant (Table 4).

Table 4 Descriptive Statistics and Statistical Comparisons Between Maxillary and Mandibular Canine Retraction^a

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DISCUSSION

The SL and CV brackets had no effect on the rate and treatment time of canine retraction. Even though no difference was found between the bracket systems, the SL bracket rate movement was up to 10% slower than that of the CV brackets, requiring approximately 1.5% and 2.8% more time than the CV brackets to retract the canines completely in the maxillary and mandibular premolar spaces, respectively. Although studies performed in vitro showed that SL brackets had smaller coefficients of friction, clinical studies have shown that bracket type had no influence on rate of tooth movement between SL and CV brackets.^{11,12,20,37,38} It is important to note that canine retraction was performed with the force occlusal to the center of resistance, allowing tipping that may have caused binding. The binding-release phenomenon is about the same independent of bracket type.³⁹

Maxillary canines moved faster than mandibular canines. Maxillary canines had a 25% greater rate of tooth movement than mandibular canines. Previous studies have shown inconsistencies.^2,4,5,12 Those studies had small sample sizes that led to insufficient power to rule out a difference between upper and lower jaws. Some were not able to show differences between maxillary and mandibular canine movement rates,^2,4,5 although Dinçer and Işcan²¹ showed greater lower canine movement rates. Differences in bone density and remodeling rate between the maxilla and mandible may explain the smaller tooth movement rate in the lower arch.^40,41 Also, the occlusion could have interfered with canine movement. All patients were Class I, and the lower canine could have been blocked by the occlusal contact of the upper canine. Using a bite raiser could be an option to relieve the occlusion, but this procedure does not seem to be clinically necessary.

Anchorage loss was the same between SL and CV brackets during canine retraction, and no difference was found between the upper and lower jaws. Maxillary and mandibular anchorage losses represented approximately 15% of the premolar space. Less anchorage loss is expected when smaller forces are applied.^30,33,42 The present study showed smaller anchorage loss than other studies that used only teeth as anchorage.^2,4,5,12 Previous studies showed greater amounts of anchorage loss ranging from 1.6 to 2.5 mm but no difference between maxillary and mandibular posterior segments.^21,43,44 The current study design, which included second molars in the posterior anchorage segment combined with the incisors,⁴⁵ as well as lighter forces applied to retract the canines,^30,33,42 could explain the results found. Orthodontists must control the amount of mesial movement of the posterior segment according to the patient's treatment plan.

Although the literature has used different amounts of force to provide canine retraction, there is no consensus about the optimal force required for retraction. In this study, canine retraction was obtained using a NiTi coil spring that provided 100 g of force. Studies have shown canine retraction using forces as low as 18 g.⁴⁶ Hixon and coworkers^32,46 did not demonstrate an optimal force, but the rate of tooth movement increased as the force increased until 300 g. Heavier forces were used, but no difference between maxillary canine and mandibular canine rates of movement was shown.^2,5 Because reports have shown effective tooth movement with light forces,^2,4,42 100 g force was applied with nickel-titanium closed-coil springs in this study for retraction of the canines. Another reason to have used light force in this study was that higher force magnitudes can lead to anchorage loss in the posterior segment, because the force per unit area (stress) may be too high to move the canine but could be optimum to move the posterior teeth due to root area differences.

CONCLUSIONS

• Canine retraction with SL brackets and CV brackets showed the same monthly rates of tooth movement.

• Maxillary canine retraction showed greater monthly rates of tooth movement then mandibular canine retraction.

• There was no difference in anchorage loss between maxillary and mandibular posterior segments during canine retraction.