INTRODUCTION

Recently, with the development of the orthodontic miniscrew, there have been reports of the orthodontic closure of space caused by a missing mandibular first molar (ML-6) or a missing deciduous mandibular second molar with a missing succedaneous premolar (ML-E).^1–6 These treatments were more difficult to conduct on mandibles, where the bone is more dense than in the maxilla. Usually space caused by missing posterior teeth is closed by reciprocal traction, which is a combination of distal movement of the anterior teeth and mesial movement of the posterior teeth. Because the space of the missing first molar or deciduous second molar is larger than that of an extracted premolar, the third molar is often considered a substitute.

Until now, studies regarding the eruption of third molars when posterior teeth are missing were mostly concerned with either the spontaneous eruption of the third molars after second molar extraction^7–9 or premolar extraction.^10–12 Cases of space closure of the missing posterior teeth other than second molars or premolars are few, and research regarding using third molar eruption in ML-6 or ML-E is rare. Although there have been reports about the effects of the loss of first molars on the development of third molars,^13,14 these reports have been based on patients missing first molars without space closure.

The objectives of this study are to investigate (1) whether impacted third molars vertically erupt better after mesialization of second molars and (2) what factors affect the vertical eruption of impacted third molars in cases in which space caused by ML-6 or ML-E can be successfully closed by mesialization of the second molar with the use of miniscrews.

MATERIALS AND METHODS

This retrospective study sample included two groups. The treatment group (Group 1) comprised 52 patients (13 males and 39 females). The average age was 19.39 ± 4.54 years. The inclusion criteria of Group 1 were (1) impacted mandibular third molars at the start of treatment, (2) 2+ years after the pubertal growth peak¹⁵ (older than 13 years for females and 15 years for males), (3) space caused by ML-6 or ML-E that had been successfully closed by mesialization of the second molars with the use of miniscrews, and (4) second molar roots arranged in parallel with adjacent teeth at the time of missing space closure. Cases that were under suspicion of ankylosis and malformation of the impacted third molar were excluded. For each patient, panoramic radiographs were collected at pretreatment (T1) and at the time of missing space closure (T2). Figure 1 shows examples of Group 1.

View Figure

• Download as Powerpoint
• Download Figure

The control group (Group 2) comprised 46 patients (8 males and 38 females). The average age was 18.07 ± 3.06 years. The inclusion criteria for Group 2 were (1) impacted mandibular third molars at T1, (2) comparable gender and age to Group 1, and (3) either no orthodontic treatment or nonextraction orthodontic treatment (because there were not enough cases with only no orthodontic treatment for the control group). In Group 2, panoramic radiographs were selected with a comparable time gap to Group 1 between T1 and T2. Figure 2 shows examples of Group 2. Approval for this study was obtained from the institutional review board of The Catholic University of Korea.

View Figure

• Download as Powerpoint
• Download Figure

The panoramic views at T1 and T2 were digitized by a single operator (Dr Baik). Figure 3 and Table 1 show the landmarks and measurements used in this study. First, the occlusal plane (OP) and the mandibular plane (MP) were established. The landmarks used in this study were the intersection between the ramus and OP (J), the distal surface of the second molar (D7), perpendicular point from D to OP (D7'), the center of the occlusal surface of the third molar (C8), and the most superior point of the third molar (S8). Measurements on the panoramic radiographs were as follows: (1) three measurements of horizontal available space in millimeters (J-D7', 8MD, J-D7'/8MD), (2) three measurements of vertical eruption in millimeters (C8-OP, C8-MP, S8-OP), and (3) three measurements of the angle of the third molar in degrees (∠78, ∠8OP, ∠8MP).

View Figure

• Download as Powerpoint
• Download Figure

Table 1.  Summary of Measured Parameters and Associated Landmarks

View Fullscreen

For the root development of the impacted third molars at T1, the Nolla developmental stage¹⁶ was recorded. The mesiodistal width of the second molars of each patient was measured on casts using a digital caliper. The ratio between the size of the second molars on the cast and those on the panoramic view were calculated to compensate for the error of magnification. Similar to a previous study,⁷ in this study we used panoramic radiographs in conjunction with study model measurements of molar size to eliminate distortion. Tracing and measurement errors of panoramic radiographs and the dental casts were determined by remeasurement at least twice on two separate occasions, 2 weeks apart. A total of 15 randomly selected participants from each group were measured at least twice on two separate occasions, 2 weeks apart, by the same investigator. All intraclass correlation coefficients exceeded 0.85, thus providing an indication of very high intrarater reliability.¹⁷

Statistical evaluations were performed using SAS software (version 9.3; SAS Institute, Cary, NC). Student’s t-test and chi-square test were used to assess the difference between the groups at baseline. The difference at the final measurements and the amount of change between the two panoramic radiographs was assessed by Student’s t-test. Linear regression analysis was performed on the treatment group to assess the factors affecting the vertical eruption of the third molars. The significance level was set at P < .05. Bonferroni correction was applied when applicable.

RESULTS

There were no significant differences between the two groups in mean age, gender distribution, or Nolla stage of the measurements at T1 (Table 2). At T2, the space between the ramus and the second molar (J-D7') and its ratio to the third molar (J-D7'/8MD) were greater in Group 1 due to a large amount of mesialization of the second molar. The vertical measurements showed smaller distance from the center of the third molars to the occlusal plane (C8-OP) and greater distance from the center of the third molars and the mandibular plane (C8-MP) than in Group 2, which indicates greater vertical eruption of the impacted third molars in Group 1. The angle between the axis of second and third molars (∠78) at T2 was significantly smaller than those of T1 in Group 1 (Table 3).

Table 2.  Baseline Characteristics of Patients by Group^a

View Fullscreen

Table 3.  Measurements at T1 and T2^a

View Fullscreen

For the changes from T1 to T2, the available space of Group 1 (J-D7') showed a 5.63 mm increase, whereas Group 2 showed only a 0.3 mm increase. The treatment effect showed a significant increase in the vertical eruption of the third molars (C8-OP: 2.54 mm) in Group 1 when compared with a 0.41-mm increase in Group 2. Also, the vertical distance from the third molars to the mandibular plane (C8-MP) increased by 3.55 mm in Group 1, but decreased by 0.52 mm in Group 2 (P < .0001). The angles between the long axes of the third and the second molars (∠78), occlusal plane (∠8OP), and mandibular plane (∠8MP) were significantly different in the two groups (Table 4).

Table 4.  Changes From T1 to T2 by Group^a

View Fullscreen

Of the factors associated with the vertical change of impacted third molars, age and the Nolla stage did not show significant correlations. Likewise, the angle of the third molar at T1 (∠78, ∠8OP, ∠8MP) revealed no significant correlation (Table 5). However, depth of the impaction of the third molars (C8-OP, S8-OP, and C8-MP at T1) and change of available space (J-D7') showed significant correlations with the vertical eruption of the third molars.

Table 5.  Factors at T1 Associated With Vertical Eruption in Group 1 (n  =  52)^a

View Fullscreen

Table 6 reflects the univariate analyses of factors potentially associated with successful eruption of the impacted third molars. Table 7 reflects the results of stepwise logistic regression of all factors. Only treatment duration (Txdur) and C8-OP remained in the equation and were significant.

Table 6.  Baseline Characteristics of Patients in the Treatment Group by Eruption of the Impacted Third Molars^a

View Fullscreen

Table 7.  Prognostic Factors for Eruption in Treatment Group (n  =  52) (no change)

View Fullscreen

DISCUSSION

Mandibular third molars are known to present wider variation in size, shape, position, development, and eruption patterns than any other teeth. They are also known to be impacted the most frequently.^11,18 For the eruption of mandibular third molars, the available space between the ascending ramus and the second molar is an important factor; therefore, securing space for third molars to erupt can reduce the incidence of impaction.^10–12 In orthodontics, healthy premolars are usually extracted to posteriorly move anterior teeth. Such extraction space is partially closed by the mesial movement of the posterior teeth. The greater the movement, the greater the possibility that the third molars will erupt as more space is obtained. Kim¹² reported that premolar extraction therapy reduces the frequency of third molar impaction because of the increased eruption space concomitant with mesial movement of the molars during space closure. Other studies on premolar extraction also revealed similar results.^10,11 Third molar eruption after second molar extraction has been studied by many researchers.

Orton-Gibbs et al.⁷ found that 96% of mandibular third molars erupted into a good or acceptable position after the extraction of second molars, and during eruption, mandibular third molar crowns continued to move significantly to an upright position after active treatment. De-la-Rosa-Gay et al.⁹ studied 74 mandibular second molar extraction cases and reported a predictive logistic regression model of the probability of correct eruption of the third molar by using initial angle, jaw, gender, age, and the developmental stage of the third molar as variables. However, research of third molar eruption related to ML-6 or ML-E is rare. To close such space, miniscrews are necessary, but there are not many studies about molar protraction using miniscrews. It is possible to consider that the case of mesialization of the second molar facilitating third molar eruption is similar to the case of aforementioned premolar extraction, but the range of second molar protraction in ML-6 and ML-E is much greater than that of premolar extraction. During the movement of a tooth through a long distance of missing posterior tooth space, tipping is one of the biggest concerns. With the miniscrew anchorage system, the biomechanics are somewhat different. To prevent mesial tipping, long buccal hooks attached to the second molar brackets were also used to protract the teeth through the center of resistance.^4–6 All of the cases selected in this study had good root parallelism. In this study, there were no significant differences between ML-6 and ML-E in the amount of second molar mesialization or in the vertical eruption patterns of impacted third molars (data not shown).

In the present study, there were no significant differences in age, gender distribution, or Nolla stage between the two groups because cases with similar baseline characteristics were selected as Group 2 samples (Table 2). The emergence variable indicates whether a tooth will emerge into the oral cavity spontaneously at T2. Usually the eruption of an impacted third molar is highly likely after a second molar extraction. Orton-Gibbs et al.⁷ reported that 96% erupted. In our study, however, Group 1 showed 75% of the initially impacted third molars emerged, including horizontal impaction at the time of space closure. This is because our study designated T2 as the time of space closure, not debonding.

In Group 1, there was a large amount of mesialization of the second molars, which increased the available space for the third molars. This resulted in the vertical eruption of the third molars (C8-OP, C8-MP) (Table 4). On the other hand, the shortest distance from the occlusal plane to the third molars (S8-OP) did not show much change. Perhaps this is because the most superior point of the third molars (S8) can be variable and might still decrease if the angle of the third molar changes when the tooth erupts.

Many cases of ML-6 and ML-E showed spontaneous eruption of the initially impacted third molars while closing the missing space (Figure 1A). In some cases, however, the third molars did not emerge to the oral cavity even after the space closure was complete (Figure 1B). The present study also investigated which factors are associated with the vertical eruption of the impacted third molars as space is secured with mesialization of the second molars. Among the many variables, depth of the impaction of the third molars (C8-OP, S8-OP, and C8-MP at T1) and change of available space (J-D7') showed significant correlations. Meanwhile, age and Nolla stage showed no correlations (Table 5). Regarding age and Nolla stage, a study by De-la-Rosa-Gay et al.,⁸ which focused on the change of third molars after second molar extraction in patients ranging in age from 11 to 23 years, reported that increased age and high Nolla stage prevented spontaneous eruption of impacted third molars. The difference in findings of the two studies can be attributed to mesialization of the second molars being the main focus as compared to second molar extraction alone. In addition, to avoid the confounding of the data because of the normal development and eruption of the third molars, the current study excluded female patients younger than age 13 and male patients younger than age 15.

For the angulation of the impacted third molars, at first it was assumed that if the initial horizontal angulation was severe, the tooth would not erupt well. According to previous reports on first premolar extraction and third molar angulation,¹¹ when the third molar is angulated horizontally, the third molar may remain impacted. In this study, however, the angulation of the third molar (∠78, ∠8OP, ∠8MP) at T1 did not show a significant correlation with eruption (Table 5). This might be because the participants in this study had greater protraction of the second molars than participants who had first premolar extraction. Therefore, it can be concluded that if the horizontal angulation of a third molar is severe, small amounts of mesialization of the second molar by premolar extraction might not provide enough space for spontaneous eruption of the impacted third molar; but if a large space can be secured by second molar protraction in ML-6 and ML-E cases, eruption can take place. In fact, in Figure 1B, where horizontal angulation is severe, vertical eruption was still observed as demonstrated by the distance between the ramus and third molar. This ultimately indicates that initial vertical location and available space are important factors for the eruption of impacted third molars.

In our study, the width of the third molar changed (however, not a statistically significant change) from T1 to T2 by about 0.3 mm in each group. This can be explained by the mean error in landmark identification. Baumrind and Frantz¹⁹ reported mean estimating errors of 0.39 mm in identifying porion and 0.37 mm in identifying the central incisor edge. In their study, other landmarks showed higher estimating errors ranging up to a mean of 3.75 mm at gonion. Interestingly, the mandibular first molar cusp had a mean estimating error of 1.32 mm. Therefore, in our study, the 0.3-mm error in the width of the third molar between T1 and T2 seems to be clinically acceptable.

The most common problems impacted third molars can cause are infection (pericoronitis), surgical trauma during extraction, distal caries of second molars, and distal periodontal defects of second molars. In ML-6 and ML-E cases, eruption of initially impacted third molars will solve most of these problems. If the first molar is hopeless and the third molar is impacted by mesialization of the second molar, the impacted third molar can be brought into occlusion without the need for dental implants. This study focused on the vertical eruption of impacted third molars and will, in the future, be extended to analyze horizontal movements and angular change. However, this study has limitations associated with panoramic radiographs because of their two-dimensional characteristics. More studies using three-dimensional computed tomography would be beneficial.

CONCLUSIONS

• Impacted mandibular third molars vertically erupt as a result of uprighting with mesialization of the second molar.

• Initial vertical location of the impacted third molar and available space affect vertical eruption.