INTRODUCTION

Using intraoral appliances, maxillary molars can routinely be moved distally with little or no patient cooperation. A distal movement rate of approximately 1 mm per month of the first molars' crowns has been reported, but there is marked individual variation.1–9 One factor that influences the movement rate is the type of movement and another factor is the timing of treatment.4 Usually faster movement occurs when the molars are tipped whereas bodily movement takes longer time.2

A favorable time to move molars distally appears to be in the mixed dentition before the eruption of the second molars.4 Furthermore, when molars are moved distally by intraoral mechanisms, anchorage loss will be evident as an increase in overjet of 1 to 2 mm.24689 The problem of increased overjet can be totally reversed and eliminated in most instances by subsequent multibracket appliance and intermaxillary Class II elastics.10 However, the problem with anchorage loss is regarded as less before second molar eruption when compared with treatment after the eruption of the second molars.

Consequently, the usual recommended time to move maxillary molars distally with intraoral appliances is in the mixed or late mixed dentition.411 However, good treatment results have been presented when second maxillary molars have erupted,23568 and obviously, there is need for more investigation on this topic. Thus, using an intraoral nickel titanium (NiTi) coil appliance for distal movement of maxillary molars, the aim of this study was to evaluate distal molar movement, including anchorage loss, before and after eruption of second maxillary molars. It was hypothesized that intraoral movement of maxillary first molars before eruption of second maxillary molars will result in more effective molar movement and less anchorage loss than after eruption of second molars.

MATERIALS AND METHODS

In this study, subjects were divided into two groups. In one group, distal movement was performed on maxillary first molars, ie, before eruption of second maxillary molars (MD 1 group), and in the other group both first and second maxillary molars were simultaneously moved distally (MD 2 group). The sample size for each group was calculated and based on an alpha significance level of 0.05 and a beta of 0.1 to achieve 90% power to detect a mean difference of 0.2 mm per month (±0.15 mm per month) in distal molar movement rate between the MD 1 and MD 2 groups. The sample size calculation showed that 12 patients in each group were needed, and to increase the power even more it was decided to select 20 patients for each group.

The patients were retrospectively recruited from the Orthodontic Clinic in Malmoe and Hassleholm, both at the National Health Service, County Council Skane, Sweden. Two experienced orthodontic specialists had treated all the patients in the National Health Service system, in which the specialists are salaried and the treatment provided at no costs to the patient and the parents. The ethics committee of Lund/Malmoe University, Sweden, which follows the guidelines of the Declaration of Helsinki, had approved the protocol and informed consent form.

The inclusion criteria for all patients were: (1) the use of an intra-arch NiTi coil appliance with a Nance appliance to provide anchorage; (2) a nonextraction treatment plan; (3) a Class II molar relationship, defined by at least an end-to-end molar relationship; (4) a space deficiency in the maxilla; and (5) no orthodontic treatment before distal molar movement.

Besides the criteria above, the patients in the MD 1 group had to have all their maxillary first permanent molars in occlusion and no erupted maxillary second permanent molars during the distalization period, whereas in the MD 2 group both the first and second maxillary molars had to be in occlusion at the start of treatment. According to panoramic radiographs, all maxillary second and third molars were present in the alveolar bone in the MD 1 group. In the MD 2 group, the maxillary third molars were present in both the right and left sides in 17 of the 20 patients.

The 20 patients (10 boys and 10 girls) in the MD 1 group were identical to a group of patients earlier randomized for treatment using an intraoral NiTi coil appliance for distal movement of maxillary molars and had a mean age of 11.4 years (SD 1.37).9 The MD 2 group included 20 patients randomly selected (through a random table) among 87 patients earlier treated using an intraoral NiTi coil appliance for simultaneous distal movement of maxillary first and second molars. The patients were matched to the patients in the MD 1 group regarding gender and had an average age of 14.6 years (SD 1.10).

The main 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.

• Skeletal sagittal position changes of the maxilla and the mandible.

• Bite opening effect.

Appliance Design

The appliance in the MD 1 group (Figure 1) consisted of bands placed bilaterally on the maxillary first molars and either the second deciduous molars or the first or second premolars (this because some of the patients still had deciduous teeth left). There were nine patients with bands on second premolars, two with bands on first premolars, and nine with bands on second deciduous molars. The MD 2 group had their bands placed bilaterally on the first maxillary molars and on the second premolars (Figure 2). In both groups a 1.1-mm-diameter tube, approximately 10 mm in length, was soldered on the lingual side of the molar band. A 0.9 mm lingual arch wire that united a Nance acrylic button was soldered on the lingual side of the second deciduous molar or to the first or second premolar in the MD 1 group and to the lingual of the second premolar in the MD 2 group. The lingual arch wire also provided two distal pistons that passed bilaterally through the palatal tubes of the maxillary molar bands (Figures 1 and 2). The tubes and the pistons were required to be parallel in both the occlusal and the sagittal views.

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A NiTi coil (GAC Int Inc, Central Islip, 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 pistons and compressed to half its length when the molar band with its lingual tube was adapted to the distal piston of the lingual arch wire (Figures 1 and 2).6 When the coil was compressed, two forces were produced, one distally directed force to move the molars distally and one reciprocal mesially directed force against which the Nance button provided anchorage. NiTi coils demonstrate a wide range of superelastic activity with a small fluctuation of load in spite of a large deflection. They also exhibit small increments of deactivation over time, and therefore the number of reactivation appointments can be reduced.12

Because the compression of the NiTi coil to half its length provided about 200 g of maximal force, and because of the small fluctuation of load in spite of a large deflection of the coil, the force fell from approximately 200 g to 180 g as the molars moved distally. Thus, after the appliances were inserted with the compressed NiTi coils, there was no need for further activation of the coils during the molar distalization period.6 Two orthodontic technicians made the appliances and efforts were made to construct the Nance button with equal size and dimension for all patients.

RESULTS

No significant difference in treatment effects was found between girls and boys, and consequently, the data for girls and boys were pooled and analyzed together.

Pretreatment cephalometric records are summarized in Table 1. Cephalometrically the two groups were in good accordance with each other. There were no significant between-group difference for the variables measured except for the maxillary incisor inclination and the maxillary first molar inclination.

TABLE 1. Pretreatment Cephalometric Records

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The average molar distalization time for the MD 1 group was 5.2 months (SD 1.00) and in the MD 2 group the corresponding time was 6.5 months (SD 0.83). Accordingly, the treatment time for the distal molar movement was significantly shorter for the MD 1 than for the MD 2 group (P < .001). The mean amount of distal molar movement within the maxilla was significantly greater in the MD 1 group than in the MD 2 group (P < .01), 3.0 mm (SD 0.64) vs 2.2 mm (SD 0.84; Table 2). Thus, the movement rate was almost two times higher in the MD 1 than in the MD 2 group, 0.63 vs 0.34 mm per month (P < .001). The average correction of molar relation was 3.3 mm in the MD 1 group and 2.5 mm in the MD 2 group (Figure 3).

TABLE 2. Changes in Cephalometric Variables within and between the Two Groups after Distal Movement of Maxillary Molars

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The mean amount of distal tipping was small (below 3°) in both groups, with no significant difference between the groups (Table 2).

Because of anchorage loss, the maxillary incisors in both the MD 1 group and the MD 2 group proclined and moved forward 0.8 mm (SD 0.88) vs 1.8 mm (SD 0.97). Hence, a significantly greater anchorage loss occurred in the MD 2 group and, for every millimeter of distal molar movement, the anchorage loss was 0.27 mm in the MD 1 group and 0.82 mm in the MD 2 group. Also, the overjet was significantly increased in both groups, although no significant difference was found between the groups (Table 2).

Overbite was significantly reduced, by 0.8 mm (SD 0.80) in the MD 1 group and 1.2 mm (SD 1.01) in the MD 2 group (Table 2). During the trial period, the maxilla and mandible in both groups moved forward small amounts, and the mandibular as well as the maxillary inclination increased (Table 2; Figure 3). During the treatment period, the occlusal plane inclination was stable in the MD 1 group, but in the MD 2 group a change of 1.2° in a counterclockwise direction was found (Table 2).

DISCUSSION

The main findings of this study were that the amount of distal movement of the maxillary first molars was significantly greater and the anchorage loss was significantly lower in the group with no second molars erupted. The molar distalization time was also significantly shorter in this group, and thus the movement rate was two times higher. This leads to the conclusion that intraoral movement of maxillary first molars before eruption of second maxillary molars will result in more effective molar movement and less anchorage loss, as our opening hypothesis suggested. Thus, if there is an option to choose to move the maxillary molars distally in the mixed dentition or in the permanent dentition, it is an advantage to make this intervention as an early treatment.

The reason why it is more effective to move the maxillary first molars distally before the second molars have erupted is, of course, that there is one more tooth, and thus, a larger area of root surface to be moved when the second molars have erupted. Conceivably, this also implies that the strain on the anchorage teeth will increase when the first and second molars are moved simultaneously. Thus, the anchorage loss (forward movement of the maxillary incisors) will be lower if the molars are moved before eruption of the second molars. Even if the anchorage loss can be corrected with modest intervention,410 the amount of lower anchorage loss will result in less time-consuming correction.

The findings in this study are similar to and are supported by another study, in which the efficiency of a pendulum appliance for distal molar movement was related to second and third molar eruption stage.11

It has been reported that the first molar crowns can be moved distally at the rate of approximately 1 mm per month, although there is marked individual variation.1–9 In this study the mean rate of distal molar movement was 0.63 mm/mo in the MD 1 group. The lesser movement rate found in this study was because of the bodily movement of the molars, whereas in those studies that showed higher movement rates, the movement consisted in considerable part of distal tipping.17–19

However, it can be pointed out that even if bodily movement takes a longer time, it seems more appropriate to move the molars bodily during a somewhat longer treatment time than move them quickly involving a distal tipping component, which will result in questionable stability and will later require another 4 to 6 months of rather difficult orthodontic molar uprighting.

The ultimate methodological design to evaluate the efficiency of distal molar movement before and after eruption of second molars has been to randomize the patients into two groups—one that started treatment in the mixed dentition with no erupted second molars, and one for which the intervention began later, when the second molars had erupted. The consequence of this strategy has been that the later group (ie, the MD 2 group) had to wait some years before treatment could start. In such circumstances, there is risk that new malocclusions will occur during the “waiting period” implying that the later group (MD 2 group) will not be comparable with the early intervention group (MD 1 group). Moreover, it can be claimed that postponement of the intervention when indicated will be unethical to the patients. Thus, for that reason a study design was used in which patients were retrospectively selected into two groups according to predefined inclusion criteria, except that the second molars were erupted in one group.

From an evidence-based view, it can be argued that the scientific evidence drawn from results of a retrospective study can be ranked only as low. However, even if a randomized controlled trial is the “gold standard,” it has been claimed that sound methodology in a well-designed prospective or retrospective studies shall not be ignored when assessing scientific literature.20

CONCLUSIONS

• Intraoral movement of maxillary first molars before eruption of second maxillary molars will result in more effective molar movement and less anchorage loss than movement after eruption of second molars.

• Consequently, the most opportune time to move maxillary first molars distally is before eruption of the second molars.