Statistical Analysis The arithmetic mean and standard deviation were calculated for each variable. Differences in means within samples/groups were tested by paired t-tests and between samples/groups by unpaired t-tests. Differences with probabilities of less than 5% (P .05) were considered statistically significant.
Error of the Method Twenty randomly selected cephalograms were traced on two separate occasions. No significant mean differences between the two series of records were found by employing paired t-tests. The method error ranged from 0.5 to 1.0 and 0.5 to 0.8 mm, except for the variables inclination of lower incisors and first max- illary molar inclination, for which errors were 1.5 and 1.4 respectively. The coefficients of reliability ranged from .92 to .97 and from .94 to .98, respectively.
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
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). The mean amount of distal tipping was small (below 3 ) in both groups, with no significant difference between the groups (Table 2).
TABLE 2. Changes in Cephalometric Variables within and between the Two Groups after Distal Movement of Maxillary Molars
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 max- illa 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).
Figure 3.Skeletal and dental mean changes (in mm) and standard deviations contributing to alterations in sagittal molar relationship and overjet. N 20 in each group. *P .05; **P .01; ***P .001.
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. Con- ceivably, 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, 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. 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 varia- tion. 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. 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 retrospective- ly selected into two groups according to predefined inclusion criteria, except that the second molars were erupted in one group. From an evidencebased 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 welldesigned prospective or retrospective studies shall not be ignored when assessing scientific literature.
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.
ACKNOWLEDGMENTS The Swedish Dental Society and Skane County Council, Sweden, supported the research.
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