INTRODUCTION

To correct a Class II dental malocclusion or to create space in the maxillary arch by a nonextraction protocol, maxillary molars can be moved distally and thereby gain space and convert the Class II molar relationship to a Class I. Then, the molars are held in place whereas the premolars, canines, and incisors usually are retracted by conventional multibracket techniques. A variety of modes of distal molar movement have been suggested including extraoral traction12 and extraoral traction in combination with removable appliances.3 Despite their efficacy in tooth movement, these treatments are highly dependent on patient cooperation. Therefore, various intraoral devices that have almost eliminated the reliance on the patient have been introduced. These techniques include Wilson arches,4 Hilgers pendulum appliances,5–7 repelling magnets, and superelastic coils.8–15 However, in a literature review, it was reported that the quality of evidence for any method of moving maxillary molars distally was not high.16

So far, there is no randomized controlled trial (RCT) comparing the effectiveness of extraoral appliance (EOA) and intraoral appliance (IOA) as methods of distalizing maxillary first permanent molars as part of a course of orthodontic treatment. Thus, using RCT methodology, the aim of this study was to evaluate and compare the treatment effects of extraoral traction (cervical headgear) and an IOA using superelastic coils for distal movement of maxillary first molars.

MATERIALS AND METHODS

The sample size for each group was calculated based on an alpha significance level of 0.05 and a beta of 0.1 to achieve 90% power to detect a clinically meaningful difference of two mm (±1.5 mm) distal molar movement between the EOA and the IOA groups. The power analysis showed that 13 patients in each group were needed, and to compensate for conceivable withdrawal or dropouts during the trial, it was judged to enroll at least 20 patients in each group.

The patients were recruited from one orthodontic clinic at the National Health Service, County Council Skane, Malmö, Sweden. Two experienced orthodontic specialists treated all the patients. In the system of the National Health Service, the specialists are salaried and the treatment provided at no cost to the patient and parents. The patient inclusion criteria for this study were:

• No orthodontic treatment before molar distalization;

• A nonextraction treatment plan;

• Maxillary first permanent molars in occlusion and no erupted maxillary second permanent molars;

• Class II molar relationship, defined by at least end-to-end molar relationship.

The ethic committee of Lund/Malmö University, Sweden, which follows the guidelines of the Declaration of Helsinki, approved the protocol and the informed consent form.

When a patient who satisfied the inclusion criteria attended the orthodontic clinic, he or she was invited to enter the trial, and the orthodontist supplied the patient and parent both oral and written information of details to the study. After written consent was obtained from the patient and parent, the patient was randomized to receive treatment with either the EOA or IOA. A restricted randomization method was used in blocks of 10 to ensure that equal numbers of patients were allocated to each of the two treatment groups. During the molar distalization time, no other appliances were placed.

Outcome measures to be assessed in the trial were:

• Treatment time, ie, the time in months to achieve a normal molar relation;

• Distal movement and distal tipping of maxillary first permanent molars;

• Anterior movement and inclination of maxillary central incisors, ie, anchorage loss;

• Movement of mandibular first permanent molars;

• Movement and inclination of mandibular central incisors;

• Skeletal sagittal position changes of the maxilla and mandible;

• Bite-opening effect.

Design of the IOA

The appliance consisted of bands placed bilaterally on the maxillary first molars and on either the second deciduous molars or first or second permanent premolars. A tube, 1.1 mm in diameter and approximately 10 mm in length, was soldered on the lingual side of the molar band. A 0.9-mm lingual archwire that united a Nance acrylic button was soldered on to the lingual of the second deciduous molar or to the first or second permanent premolar band (Figure 1). The lingual archwire also provided two distal pistons that passed bilaterally through the palatal tubes of the maxillary molar bands. The tubes and pistons were required to be parallel in both the occlusal and sagittal views.

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A Ni-Ti coil (GAC Int Inc, Central Inslip, NY), 0.012 inches in diameter, with a lumen of 0.045 inches and cut to 10 to 14 mm in length, was inserted on the distal piston and compressed to half of its length when the molar band with its lingual tube was adapted to the distal piston of the lingual arch wire (Figure 1).13 When the coil was compressed, two forces were produced, one distally directed to move the molars distally and a reciprocal mesially directed force against which the Nance button provided anchorage. Ni-Ti coils demonstrate a wide range of superelastic activity with a small fluctuation of load despite a large deflection and exhibit small increments of deactivation with time, and therefore the number of reactivation appointments can be reduced.17 Because the compression of the Ni-Ti coil to half its length provided about 200 g of maximal force and because of the small fluctuation of load despite a large deflection of the coil, the force fell from approximately 200 to 180 g as the molars moved distally. Thus, after the appliance was inserted with the compressed Ni-Ti coils, there was no need for further activation of the coils during the molar distalization period.13

RESULTS

A total of 44 patients were enrolled, and four of these refused to enter the study. Thus, 40 patients were randomized, 20 (10 girls and 10 boys) were allocated to receive treatment with the IOA, and 20 (12 girls and eight boys) with the EOA. All 40 patients completed the trial (Figure 2). The mean age was 11.4 (SD 1.37) and 11.5 (SD 1.25) years for the IOA and EOA groups, respectively. No significant difference in any of the variables used in the study was found between girls and boys, and consequently, the data for girls and boys were pooled and analyzed together.

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Pretreatment cephalometric records are summarized in Table 1. Cephalometrically the two groups were in good accordance with each other because no significant between-group difference was found for the variables measured.

TABLE 1.  Pretreatment Cephalometric Records for the IOA Group and the EOA Group^a

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All patients in the EOA group submitted the form showing how many hours per day the appliance had been used during the treatment period. It was found that the appliance had been used for an average time of 10.8 h/d (SD 0.72).

The average molar distalization time for the IOA group was 5.2 months (SD 1.00) whereas in the EOA group the corresponding time was 6.4 months (SD 0.97). Thus, the treatment time for the distal molar movement was significantly shorter for the IOA than the EOA group (P < .01). The mean amount of distal molar movement within the maxilla was significantly greater in the IOA than in the EOA group (P < .001), three mm (SD 0.64) vs 1.7 mm (SD 0.91) (Table 2). In the EOA group, no interdependence was found between the time of use of the appliance per day and distal molar movement (r = 0.23). The amount of distal molar tipping was small in both groups and no significant difference existed between the groups (Table 2).

TABLE 2.  Changes in Cephalometric Variables Within and Between the Two Groups After Distal Movement of Maxillary Molars. Changes were Calculated as the Difference After-Minus-Before Position^a

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The total molar relation correction was 3.3 mm in the IOA group and 2.4 mm in the EOA group (Figure 3). The molar relation was corrected mainly by distal movement of the maxillary first molars in the IOA group corrected, whereas in the EOA group the molar relation was corrected by an equal amount of distal movement of the maxillary molars and mesial movement of mandibular first molars (Figure 3).

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Because of anchorage loss, the maxillary incisors in the IOA group proclined and moved forward 0.8 mm (SD 0.88) and the overjet was increased by an average of 0.9 mm (SD 0.88) (Figure 3; Table 2). However, in the EOA group, the maxillary incisors retroclined and moved distally one mm (SD 0.99) and the overjet decreased by a mean of 0.9 mm (SD 0.63) (Figure 3; Table 2).

In both groups, the overbite was significantly reduced, 0.8 mm (SD 0.80) in the IOA group and 0.7 mm (SD 0.72) in the EOA group (Table 2). During the trial period, the maxilla and mandible in both groups both moved forward small amounts and the mandibular plane angle increased (Table 2).

DISCUSSION

In any scientific study, it is important that the power is high and the characteristics of any withdrawal subjects are known. The power analysis revealed that a sample size of 13 patients per group was sufficient. Because 20 patients per group were enrolled in this study and the number of withdrawals after randomization was zero, no loss of information biased the data. Moreover, because the measurement analysis of the cephalograms was performed in a blinded manner, ie, the examiner was unaware of the group to which the patient had been allocated, and the risk of measurements being affected by the researcher was low.

The most important finding of this study was that the amount of distal movement of the maxillary first molars was significantly higher and more rapid with the IOA than the EOA. It is not possible to make comparisons with previous studies because of a lack of RCTs between IOA and EOA for molar distalization. Nevertheless, a RCT regarding two IOAs, a Jones Jig and an upper removable appliance, has recently reported that the amount of distal movement was small (one to two mm) and that both appliances were equally effective.22 Furthermore, a systematic review has revealed that the amount of distal molar movement that may be achieved is approximately two mm.16 In this study, the amount of distal molar movement produced by the EOA was comparable with the Jones Jig and the upper removable appliance,22 whereas the amount of distal molar movement produced with the IOA was higher (three mm). The distal molar tipping was small in both groups, and thus, the molar distalization consisted of mainly bodily movement.

In the IOA group, the molar correction consisted of 66% distal movement of maxillary molars and 34% of mesial movement of mandibular molars. This is contradictory to the findings by Paul et al,22 who reported that significant correction of the molar relation was by mesial movement of mandibular molars. However, in the EOA group, the correction in molar relation was in accordance with Paul et al,22 ie, 45% distal movement of maxillary molars and 55% mesial movement of mandibular molars.

The likelihood of patient cooperation is one of the most important factors influencing the effectiveness of a treatment. The patients in the EOA group used the cervical headgear for an average time of 10.8 h/d. This time of use was judged as acceptable or good and quite normal for orthodontic patients in Scandinavian countries where treatment is provided at no cost to the patient and parents. Because the duration of the force per day on the molars of course was greater in the IOA than in the EOA group, this was the major explanation for the higher effectiveness of the IOA in distal molar movement. Another advantage of the IOA lies in its single activation because the Ni-Ti coils demonstrate a wide range of superelastic activity with a small fluctuation of load despite a large deflection.17 Furthermore, the Ni-Ti coils exhibit small increments of deactivation with time, thus reducing the number of reactivation appointments during the molar distalization period.

An advantage with the EOA (cervical headgear) is that during molar distalization this appliance also creates distal movement of the maxillary incisors implying decreased overjet, and this decrease in overjet is of course desired when Class II division 1 occlusions are treated. However, when maxillary molars are moved distally by an intraoral appliance, an anchorage loss or forward movement of one to two mm of the anterior teeth occurs.67913–15 In this study, a similar amount of forward movement of maxillary incisors was observed for the IOA. In most instances, the problem of forward movement of the incisors can be controlled with modest intervention. It has been shown that forward movement of the maxillary incisors associated with distal molar movement was totally reversed and eliminated by the subsequent multibracket appliances and intermaxillary Class II elastics.23 On the other hand, in cases with retroclined maxillary incisors, for example in subjects with a Class II division 2 occlusion, the reciprocal effect of forces can be used for proclination of the incisors.

In both groups, only small skeletal changes were shown and these changes were mainly assigned to normal growth changes. Because the two appliances had a small or negligible corrective effect on the Class II skeletal relationships, these appliances should only be used in cases of moderate dental discrepancies and arch-length deficiencies. Of course skeletal effects such as restricted forward growth of the maxilla can be produced by the EOA, but then much higher forces than 500 g have to be used.

Even if cost effectiveness as well as the patients' perceptions of pain and discomfort of the appliances were not evaluated in this investigation, it seems natural to claim that the IOA is a more favorable method than the EOA to create distal molar movement. To accomplish a complete comparison, studies regarding cost effectiveness and patients' perceptions of the two appliances have been commenced and will be presented later.

CONCLUSIONS

• It was clearly demonstrated that the IOA was more effective than the EOA to create distal movement of maxillary first molars.

• Moderate and acceptable anchorage loss was produced with the IOA implying increased overjet whereas the EOA created decreased overjet.

• For the clinician, the IOA is a more favorable method than the EOA to create distal molar movement.

• The two appliances did not have any considerable corrective effect on Class II skeletal relationships and these appliances shall therefore only be used in cases of moderate dental sagittal discrepancies and arch-length deficiencies.