ABSTRACT Objective: To evaluate the maxillary molar distalization and anchorage loss in two groups, one before (MD 1 group) and one after (MD 2 group) eruption of second maxillary molars.
Materials and Methods: After a sample size calculation, 20 patients were recruited for each group from patients who fulfilled the following criteria: no orthodontic treatment before distal molar movement, Class II molar relationship defined by at least end-to-end molar relationship, space deficiency in the maxilla, and use of an intra-arch NiTi coil appliance with a Nance appliance to provide anchorage. Patients in the MD 1 group were without any erupted second molars during the distalization period, whereas in the MD 2 group both the first and second molars were in occlusion at start of treatment. The main outcome measures to be assessed were: treatment time, time in months to achieve a normal molar relation, distal movement of maxillary first molars, and anterior movement of maxillary incisors (anchorage loss). The mean age in the MD 1 group was 11.4 years; in the MD 2 group, 14.6 years.
Results: The amount of distal movement of the first molars was significantly greater (P .01) and the anchorage loss was significantly lower (P .01) in the group with no second molars erupted. The molar distalization time was also significantly shorter (P .001) in this group, and thus the movement rate was two times higher.
Conclusions: It is more effective to distalize the first maxillary molars before the second molars have erupted.
KEY WORDS: Distal molar movement; Intraoral appliance; First and second molars
INTRODUCTION Using intraoral appliances, maxillary molars can rou- tinely be moved distally with little or no patient coop- eration. A distal movement rate of approximately 1 mm per month of the first molars crowns has been reported, but there is marked individual variation. One factor that influences the movement rate is the type of movement and another factor is the timing of treatment. Usually faster movement occurs when the molars are tipped whereas bodily movement takes longer time. A favorable time to move molars distally appears to be in the mixed dentition before the eruption of the second molars. Furthermore, when molars are moved distally by intraoral mechanisms, anchorage loss will be evident as an increase in overjet of 1 to 2 mm. The problem of increased overjet can be totally reversed and eliminated in most instances by subse- quent multibracket appliance and intermaxillary Class II elastics. 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. However, good treatment results have been presented when second maxillary molars have erupted, 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 max- illary 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 signifi- cance 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, de- fined 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 ap- pliance for distal movement of maxillary molars and had a mean age of 11.4 years (SD 1.37). The MD 2 group included 20 patients randomly selected (through a random table) among 87 patients earlier treated us- ing 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.
Figure 1. (a) Occlusal view before treatment of one patient in the MD 1 group with no second molars erupted. (b) Occlusal view of the intraoral appliance inserted in the same patient.
Figure 2. Occlusal view of the intraoral appliance in one patient in the MD 2 group with both second molars erupted.
Appliance Design The appliance in the MD 1 group (Figure 1) con- sisted 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 pre- molar in the MD 1 group and to the lingual of the sec- ond 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. 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). 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. 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. Two orthodontic technicians made the appliances and efforts were made to construct the Nance button with equal size and dimension for all patients.
Data Collection The time in months to achieve normal molar relation by distal molar movement was recorded. Lateral head radiographs in centric occlusion were obtained at the start and after completion of molar distalization. The radiographs were taken with the same kind of equip- ment (Proline PM 2002 cc cephalostat, Planmeca OY, Finland) and under equivalent conditions. The measuring points, reference lines, and measurements used were based on those defined and described by Bjoš rk and Pancherz. Dental and skeletal changes as well as dental changes within maxilla and mandible were obtained by the Pancherz analysis. Measurements were made manually and by one author (Dr Bondemark) to the nearest 0.5 mm or 0.5 . Images of bilateral structures were bisected. No correction was made for linear enlargement (10%). Changes in the different measuring points during the treatment were calculated as differences between positions before and after treatment (after before). An intention-to-treat approach was performed, and results from all patients were analyzed regardless of the outcome of treatment.