Various ligation techniques and materials have been shown to affect the frictional resistance and the rate of tooth movement with sliding mechanics for space closure. The aim of this study was to evaluate the clinical efficiency of nonconventional elastomeric ligatures and conventional elastomeric ligatures during the canine retraction phase by comparing the rates of canine retraction.
The 20 patients (12 female, 8 male) in our sample had individual canine retraction (in the first premolar extraction space) in each quadrant (2 maxillary, 2 mandibular) with nonconventional elastomeric ligatures and conventional elastomeric ligatures on either side of the arch. The amount of canine retraction in each interval of 1 month was determined. The rate of canine retraction was calculated and subjected to statistical calculations.
The rates of canine retraction were higher with the nonconventional elastomeric ligatures. However, no statistically significant difference was observed in relation to the maxillary arch. Clinically, in most instances, canine retraction was completed in the same interval in both groups.
No significant difference in the rate of canine retraction was observed between the nonconventional elastomeric ligature and conventional elastomeric ligature groups in the maxillary arch. Clinically, the nonconventional elastomeric ligature group showed no reduction in time required for complete canine retraction in the maxillary and mandibular arches.
In orthodontic tooth movement, when an archwire slides through the bracket slot (eg, en-masse retraction of anterior teeth) or the bracket slides over the archwire (eg, individual canine retraction), some resistance to tooth movement is encountered as a result of friction between the interacting components of the orthodontic appliances such as brackets and archwires (when a bracket slides over an archwire). Individual retraction of canines through the first premolar extraction space is usually required during the correction of a protrusive malocclusion and represents a critical stage in orthodontic treatment.
The factors that have been suggested to influence frictional resistance are bracket-wire interaction, bracket material and wear of the wire, bracket width and interbracket distance, archwire material, archwire diameter and cross-sectional shape, wire stiffness, active torque, bracket-wire angulations, method of ligation, surface roughness of the wire, sliding velocity, and saliva. It has been proven that ligatures significantly contribute to friction when they exert a force against the archwire, pressing the wire against the bracket. Elastomeric ligatures are believed to exert 50 to 150 g of force at the time of seating, thereby contributing to the friction. To reduce frictional force from ligation, various ligation methods have evolved to an extent of incorporation of ligation in bracket systems; these are known as self-ligating brackets. Ligation material and methods have also evolved with time in an attempt to reduce friction: eg, slackened stainless steel ligatures, stainless steel ligatures coated with fluorinc-containing resins, modified elastomeric modules, Super Slick ligatures (TP Orthodontics, LaPorte, Ind), and Slide ligatures (Leone, Sesto Fiorentino, Italy) (nonconventional elastomeric modules).
Slide ligatures are an innovative ligature made of a special polyurethane mix for medical use, manufactured by the injection molding technique. They were introduced in 2005, with a claim of lowering the levels of friction in treatment mechanics with preadjusted edgewise appliances. The ligature interacts with the bracket slot to form a tube-like structure; the combination takes the shape of a passive self-ligating bracket. The ligature forms the fourth wall of the slot with its incisal and occlusal edge resting along the buccolabial surface of the tie wings, permitting the bracket to slide over the archwire freely while transmitting most of the tooth-moving forces to the surrounding dentoalveolar structures. Previous in-vitro studies and a few clinical studies have shown that nonconventional elastomeric ligatures can reduce frictional resistance compared with conventional ligatures, both during initial leveling and aligning, and in the retraction phase of orthodontic treatment. Clinically, the efficiency of the ligation material in reducing friction was evaluated by comparing the rates of canine retraction with nonconventional elastomeric ligatures and conventional elastomeric ligatures during the space-closure phase with preadjusted edgewise appliances.
Material and methods
The objective of this study was to evaluate the effectiveness of 2 ligation systems in the retraction of canines in preadjusted edgewise appliance therapy, by calculating the rates of canine retraction (amount of retraction in millimeters divided by the time required to complete the retraction).
The sample for this in-vivo study consisted of 20 patients (12 female, 8 male) with a mean age of 18.3 ± 3.4 years, who visited the Department of Orthodontics of Nair Hospital Dental College in Mumbai, India. These subjects had Angle Class I malocclusions with bimaxillary proclination of the anterior teeth with no history of previous orthodontic treatment; they required extraction of all first premolars for correction of the malocclusion. All patients had completely established permanent dentitions. All teeth were anatomically normal and vital. Informed consent was obtained from all patients, according to the recommendations of our ethics committee at Nair Hospital Dental College and the guidelines for main dissertation under Maharashtra University of Health Sciences, Nashik, Maharashtra, India.
In each patient, both maxillary and mandibular arches were included. The split-mouth design was used, with the rate of canine retraction on 1 side of the arch compared to that of other side of the same arch by using 2 types of elastomeric ligation systems on the canines. Thus, in 20 patients, considering both arches, the sample consisted of 80 quadrants.
The subjects were divided into 2 groups: group A received nonconventional elastomeric ligatures (Slide), used to ligate canine brackets to the archwire (ie, 20 quadrants in the maxillary arch and 20 quadrants in the mandibular arch); and group B received conventional elastomeric ligatures (Libral Traders, Delhi, India), used to ligate canine brackets to the archwire (ie, 20 quadrants in the maxillary arch and 20 quadrants in the mandibular arch).
The materials used in the study were (1) 0.018-in slot stainless steel straight wire bracket kit with a Roth prescription (American Orthodontics, Sheboygan, Wis); (2) conventional elastomeric ligatures of grey color (Rabbit Force Ligature Ties; Libral Traders, New Delhi, India) to ligate all the teeth, except for the canines in group A; (3) nonconventional elastomeric ligatures (Slide) of small size to ligate the canines in group A; (4) clear elastomeric chain (Rabbit Force Elastic Chain; Libral Traders) of short filament configuration to retract the canines; (5) a tension gauge (Dontrix, TP Orthodontics) used to calibrate the force of the elastic chain to retract the canines; and (6) a digital vernier caliper (digimatic caliper; Mitutoyo, Kawasaki, Japan) of 0.01-mm sensitivity, to measure the amounts of canine retraction on the study models.
Extraction of all first premolars was performed in 2 stages by an experienced oral surgeon. The first premolars of the same side, maxillary and mandibular, were extracted in the same appointment, followed by extraction on the contralateral side after 3 days. Extractions of all first premolars were completed about 1 week before starting the orthodontic treatment to ensure the patient’s comfort. Brackets with 0.018-in slots with the Roth prescription were bonded. Initial alignment and leveling of both arches were completed by using 0.016-in nickel-titanium and 0.016 × 0.022-in nickel-titanium archwires. The nickel-titanium wires were replaced by 0.016 × 0.022-in stainless steel archwires, and arch leveling continued for 1 month. After this, maxillary and mandibular study models were made. After alignment and leveling of the arches, the distances between the central pit of the first molar and the canine tip were measured with the digital vernier caliper. This recording was considered the base reading to evaluate the amount of canine retraction in subsequent intervals.
In 10 patients (selected randomly from the 20 patients), on the right side of the maxillary and mandibular arches (first and fourth quadrants), the canine bracket was ligated to the 0.016 × 0.022-in stainless steel archwire with the nonconventional elastomeric ligature (Slide). All other teeth were ligated with conventional elastomeric ligatures. The ligation method in the remaining 10 patients was the opposite: the canines on the left side of the maxillary and mandibular arches (second and third quadrants) were ligated by using nonconventional elastomeric ligatures (Slide). This setup ensured elimination of side bias. The canines ligated with Slide ligatures (nonconventional elastomeric ligatures) were included in group A of the sample, and the canines ligated with conventional elastomeric ligatures were included in group B. In an ideal case of individual canine retraction, only the canine bracket slides over the archwire. The archwire does not slide through premolar brackets and molar tubes, unlike in en-masse retraction of anterior teeth. Therefore, all teeth other than the canines in group A were ligated with conventional elastomeric ligatures, since the archwire was presumed to be stationary with respect to these teeth during individual canine retraction.
Canine retraction was initiated with a clear elastomeric chain. Considering the force decay of elastomeric ligatures by approximately 40% in the first 24 hours, approximately 8 oz of force was applied by the elastomeric chain (checked by the Dontrix tension gauge). The elastomeric chains were prestretched according to the instantaneous prestretching technique of Young and Sandrik to reduce the amount of force decay over a period of time.
The patients were recalled 1 month after the start of canine retraction to make impressions for the study models, and change the elastomeric ligatures and elastic chains to maintain the force of retraction. This process was repeated until the canine retraction in each quadrant was completed.
Distances were measured on the study models that were made at the end of each interval. The distance was measured (by using the digital vernier caliper) from the central pit of first permanent molar to the cusp tip of the canine. The readings at the end of each interval were subtracted from the readings of the previous interval or the base reading to obtain the amount of canine retraction in that interval. These measurements were made by 2 operators (K.D.D. and S.R.B.) at different times, and the averages of the 2 readings were calculated and tabulated.
The rate of canine retraction was calculated as the amount of extraction space closed divided by the time (number of intervals) required for complete space closure. This was recorded in millimeters per interval (eg, if the canine was retracted by 3 mm at the end of 4 months, the rate of retraction will be 3/4, or 0.75 mm per month). Since the patients were recalled at intervals of 1 month, it was not known when exactly the canines were completely retracted in the last interval. As the tooth movement is a slow process, patient reporting of complete canine retraction after 1 month might also indicate that the canines could have been retracted some days previously. Therefore, for all quadrants and samples, the midpoint of the last interval was declared the endpoint of the retraction to minimize the error of overestimation of time required for complete canine retraction. The last interval was thus recorded as a half interval for each canine and noted as the endpoint.
The above data along with the rate of canine retraction were tabulated. According to the 1-sample Kolmogorov-Smirnov test, the data obtained were parametric data and therefore subjected to dependent or paired t tests to compare the rates of canine retraction in the 2 groups: group A (Slide ligatures) and group B (conventional elastomeric ligatures). Comparison of the amounts of canine retraction in each interval of each group was performed by using 1-way analysis of variance (ANOVA).
The total sample size was 20 patients (80 quadrants) consisting of 12 female and 8 male subjects with an age range of 14 to 23 years (mean age, 18.3 ± 3.41 years).
Table I shows that, in the maxillary arch, the mean rates of canine retraction were 1.7 ± 0.6 mm per month for group A with Slide ligatures and 1.6 ± 0.5 mm per month for group B with conventional elastomeric ligatures. Table II shows that, in the mandibular arch, the mean rates of canine retraction were 1.8 ± 0.9 mm per month for group A and 1.5 ± 0.5 mm per month for group B.
Table III shows the differences between the mean rates of canine retraction for groups A and B: maxillary arch, 0.05 ± 0.2 mm per month, and mandibular arch, 0.34 ± 0.6 mm per month.
|Arch||Groups compared||Mean difference (mm)||SD||SEM|
|Maxillary||A – B||0.05317||0.16061||0.03591|
|Mandibular||A – B||0.34710||0.59326||0.13266|
Tables IV and V show that, for the maxillary arch, with a 95% CI, the mean difference in the rate of canine retraction between groups A and B would be 0.03 to 0.2 mm per month (0.05 – 0.02 to 0.05 + 0.12). For the mandibular arch with a 95% CI, the mean difference in the rates of canine retraction between groups A and B would be 0.4 to 1 mm per month (0.34 + 0.07 to 0.34 + 0.6). For the above mentioned t test, the P value for the maxillary arch was 0.16 (>0.05), indicating no statistically significant difference between the mean rates of canine retraction of groups A (Slide ligatures) and B (conventional elastomeric ligatures). For the mandibular arch, the P value obtained was 0.017 (<0.05), indicating sa tatistically significant difference between the mean rates of canine retraction of groups A and B.
|Arch||Groups compared||t||df (n – 1)|
|Maxillary||A – B||1.480||19|
|Mandibular||A – B||2.617||19|
|Arch||Groups compared||95% CI of the difference||Significance (2-tailed)|
|Maxillary||A – B||0.02200||0.12834||0.155|
|Mandibular||A – B||0.06944||0.62475||0.017 ∗|
Tables VI and VII , for the maxillary and mandibular arches, respectively, the P value obtained for both groups explains that no statistically significant difference was observed between the amounts of canine retraction in each interval.
|Arch||Comparison||Sum of squares||df||Mean square||F||Significance|
|Group A||Between intervals||5.225||6||0.871||2.066||0.065|
|Group B||Between intervals||6.512||7||0.930||1.884||0.081|