INTRODUCTION

Several treatment approaches may be considered for the management of lower anterior crowding, including the extraction of a lower incisor. The orthodontic extraction of a mandibular incisor is uncommon and remains controversial in the orthodontic profession due to possible unwanted side effects (see Table 1 for a summary of the literature),^1–6 although some of these adverse effects may arise in any orthodontic case. Within the literature, however, there have been many case reports demonstrating excellent treatment outcomes following orthodontic mandibular incisor extraction (MIE).^7–10

Table 1 Summary of Potential Side Effects Associated With the Orthodontic Extraction of a Mandibular Incisor

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The rationale for extracting a mandibular incisor in orthodontic cases includes a reduction in treatment time and cost as well as the maintenance of a harmonious profile.⁴ There are contradicting reports in the literature regarding posttreatment stability in the anterior region with some studies reporting the recurrence of crowding in the mandibular anterior region after MIE,¹⁰ and others reporting a more stable result in the anterior region after treatment.⁴ In light of the potential benefits of MIE, this treatment modality is supported by many case reports and case series in the literature as, with careful case selection, an esthetic and functional result can be achieved with minimal orthodontic intervention.^3,6,11–14

The use of diagnostic wax setups is recommended to aid in the planning of potential MIE treatment plans to assess the predicted final overbite and overjet.^{3,6,7,9,12,13} Some of the undesirable outcomes after MIE treatment, such as an increased overbite and overjet, may be detected or visualized prior to treatment through this method, and setups are particularly useful for borderline cases, which may be treated by other modalities. Diagnostic wax setups are the best way to visually predict the final treatment objective as opposed to only undertaking a Bolton's analysis, which may not be as accurate.¹³ Although such setups are a good method to visualize final treatment objectives (e.g. achieving an acceptable overbite and overjet), the value of diagnostic setups to estimate final overbite and overjet has not been evaluated previously.

To have desirable outcomes in MIE cases, the initial case selection is crucial, and the decision to extract should ideally be supported by initial records, a diagnostic wax setup, and clinical experience.^8,12,13 Due to the ability of laypeople, dental students, and dentists being able to perceive the absence of a mandibular incisor, some advocate that the extraction of a mandibular incisor should not be the treatment of choice when there are other available treatment options.¹⁵

Accordingly, the objectives of the study were: (1) to compare the pre- and posttreatment change in Peer Assessment Rating (PAR) score between MIE cases and nonextraction (NE) controls; (2) to determine the usefulness of a wax setup in treatment planning of MIE cases, by determining the correlation of occlusal characteristics of posttreatment study casts and diagnostic wax setups; and (3) to compare the pre- and posttreatment change in dental attractiveness between cases and controls between laypeople and dental students.

MATERIALS AND METHODS

This study was undertaken as a retrospective cross-sectional study on clinical records (pre- and posttreatment study casts, wax setups and intraoral photographs) of patients treated by orthodontic postgraduate students in the Discipline of Orthodontics, Faculty of Dentistry, University of Otago. Ethical approval was obtained from the University of Otago Human Ethics Committee.

The sample size was estimated a priori using G*Power Software (Heinrich Heine University of Düsseldorf, Düsseldorf, Germany),¹⁶ based on the primary objective of the study. By setting the alpha level to 0.05, power to 0.80, and an allocation ratio to 1:1, it was estimated that a minimum of 20 participants (10 cases and 10 controls) were required to detect a difference of 10 units in PAR score between cases and controls.

The study sample consisted of records of 14 patients who had previously been treated with MIE and full fixed appliances. Twenty-three MIE cases from 2000 to 2015 were screened, with 14 cases meeting all the inclusion criteria of this study (Table 2). All cases were carefully selected and the decision to treat with a MIE were based according to the MIE indications where cases had moderate crowding (4–8 mm) in the lower arch, a Class I malocclusion or a mild tendency toward Class III, an anterior Bolton's discrepancy greater than 2 mm and mild crowding in the upper arch. A control group (n = 14) treated without extractions was sought from the same source population and pairwise matched on age, gender, and skeletal classification (ANB [A point, nasion, B point] angle value within 2°, as assessed on a lateral cephalogram). The inclusion and exclusion criteria are presented in Table 2. The Dental Aesthetic Index (DAI) was applied to all 28 pretreatment dental casts to assess the baseline characteristics of the two groups.¹⁷

Table 2 Inclusion and Exclusion Criteria of the Sample

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Data Collection

Comparing the change in occlusal characteristics, assessed by the PAR Index, between pre- and posttreatment study casts in cases and controls

The pre- and posttreatment study casts of the 14 cases and 14 controls were analyzed by a calibrated clinician using the PAR Index^18,19 and PAR ruler designed to aid with recording of measurements. The examiner was blinded as to the time point (T0 [pretreatment] or T1 [posttreatment]) of the dental casts, as well as their grouping by combining all the casts together and presenting them for analysis in a random order. However, MIE cases were identifiable at T1 and thus true blinding was not possible. All dental casts were scored for alignment of maxillary and mandibular anterior segments, buccal occlusion, overjet, overbite, and dental midlines.¹⁸ Recommended weightings were applied to the raw scores.¹⁸

Comparing the occlusal characteristics of posttreatment study casts and diagnostic wax setups

The “resemblance” of the posttreatment cast to that of the diagnostic wax setup was assessed by evaluating the following variables: overjet, overbite, and canine relationship. Differences in the overjet, overbite, and canine relationship were compared between the diagnostic wax setup and posttreatment study cast. For the overjet (measured using a ruler parallel to the occlusal plane) and overbite (the maximum vertical overlap) variables, measurements were obtained by recording the average measurement from the two central incisors. Canine relationship was recorded separately for the left and right sides. All measurements were made in millimeters using an orthodontic clinical ruler and a Williams probe.

Comparing the pre- and posttreatment change in dental attractiveness between MIE and NE controls between laypeople and dental students

Pre- and posttreatment intraoral photographs (left buccal, frontal, and right buccal), as shown in Figure 1, were presented as a slideshow and rated by participants using a visual analogue scale (VAS). Two groups of assessors: laypeople (n = 10) and fifth-year dental students at the Faculty of Dentistry, University of Otago, (n = 76) subjectively assessed the attractiveness of the intraoral photographs. Parents of children who attended the Orthodontic Clinic (Department of Oral Sciences, Faculty of Dentistry, University of Otago) were recruited as laypeople. Assessments were carried out in a seminar room with each slide showing the pre- and posttreatment intraoral photographs side by side (Figure 1). Each slide was shown via a digital projector for 30 seconds. A total of 12 sets of pre- and posttreatment photographs (six cases and six controls) were displayed in a random order and participants were asked to rate the attractiveness of the photographs on a VAS (100 mm) (Figure 2), which was anchored by “not attractive” on the left and “very attractive” on the right.

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RESULTS

The sociodemographic and clinical characteristics of the study sample are presented in Table 3. The majority of the sample were female (64%), with an average age of 15.9 years (SD = 4.1). The average DAI score in the overall sample was 27.7 (SD = 6.1), indicating the presence of a definite malocclusion. There were no significant differences (P > .05) in either the sociodemographic or clinical characteristics between the study groups.

Table 3 Baseline Characteristics of Study Sample

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The mean values for the pre- and posttreatment weighted PAR scores are summarized in Table 4. Following treatment, the PAR score reduced in both cases and controls by approximately 73% and 76%, respectively. There were no significant differences in either the pre- or posttreatment weighted PAR scores between the groups. Intrarater agreement was very good for both the pretreatment weighted (r = 0.750; P = .025) and posttreatment weighted PAR scores (r = 0.936; P = 0).

Table 4 PAR Outcomes by Study Group

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In comparison between wax setups and posttreatment study models, there were moderate, but nonsignificant, correlations in the overjet (r = 0.461; P = .057), overbite (r = 0.439; P = .067), and the right canine classification (r = 0.409; P = .165). There was a weak, nonsignificant correlation for the left canine classification between the two sets of study models (r = 0.245; P = .421).

Posttreatment dental attractiveness was rated higher than pretreatment dental attractiveness in both the MIE (mean VAS score change of 49.8 ± 4.3 % [SE]) and NE group (mean VAS score change of 40.8 [SE = 4.3]). There was no significant difference in the percentage change in pre- and posttreatment VAS scores between the two groups (F = 2.3; P = .158). Overall, however, there was a significant difference (F = 81.2; P < .001) between the observer groups with a mean change of 36.0 (SE = 2.9) reported in the dental student group and 54.5 (SE = 3.4) in the layperson group.

DISCUSSION

The extraction of a mandibular incisor for orthodontic purposes remains controversial because of concerns about its effects on the occlusion. Indeed, several unwanted side effects have been reported to occur in orthodontic cases treated with MIE. Some concerns also exist about the dental esthetics of having one less mandibular incisor and non-coincident midlines. The primary aim of this case control study was, therefore, to assess the objective and subjective outcomes of MIE cases compared to matched controls treated without extractions. The findings suggested that the extraction of a mandibular incisor, in carefully selected cases, can result in good treatment outcomes that are perceived to be as attractive as those treated without extractions.

Previous studies investigating MIE outcomes have frequently relied on traditional clinical variables such as overjet and overbite measurements, intercanine, and intermolar distances, mandibular arch length, molar classification, and anterior crowding.^6–10 Treatment outcomes were deemed to be good when an optimal overjet and overbite were achieved, maxillary and mandibular arches were aligned in good intercuspation, anterior crowding had been eliminated, and Class I molar and canine relationships had been achieved at the end of treatment. In the present study, however, the PAR index¹⁸ was used to assess clinical-based outcomes. Few previous studies have used the PAR Index to assess MIE outcomes; however, these studies indicated that cases treated with MIE had over 70% reduction in PAR scores.^4,20,21 This study found that both groups experienced a PAR score reduction of over 70%, indicative of a high standard of treatment.¹⁸ These results were in agreement with a recently published study, and indicated that MIE can be considered as a valid treatment option in objective outcomes as rated by the PAR Index.²¹ Although some studies have presented results indicating that cases treated with NE had slightly higher percentage reductions in PAR scores than cases treated with MIE, the current study showed that there was no significant difference between the two groups.^20,21 Differences in the selection criteria for both cases and controls between the studies may partly explain this finding.

A secondary aim of the study was to explore similarities between a diagnostic wax setup and posttreatment models of MIE cases. Despite recommendations in the literature on the use of diagnostic wax setups in MIE cases, this is an area that lacks scientific evidence.^5,7,8,12,13 The results of the current study showed a moderate correlation in the overjet, overbite, and right canine classification between diagnostic setups and posttreatment study casts, and a low correlation for the left canine classification. The overjet and overbite were assessed in this study as it is commonly accepted that these parameters can increase after MIE treatment.¹ However, a criticism of using these two measurements to explore the usefulness of a diagnostic wax setup is that this gave no information on where the teeth have been set up in relation to the bone.

In addition to objective outcomes, this study evaluated perceived attractiveness between dental students and laypeople. Interestingly, both groups of assessors did not perceive any significant difference in attractiveness between the MIE and NE photographs, indicating that MIE treatment can result in esthetically acceptable results. However, a strong effect was seen in regard to the observer groups, with laypeople tending to rate posttreatment photographs as being more attractive than pretreatment photographs regardless of the treatment group allocation. Dental students, on the other hand, were more critical at rating the attractiveness of posttreatment photographs, with the mean change in VAS score being smaller in this group. Differences in the extent of these subjective assessments between professionals and laypeople have been documented previously.²²

So far, two other studies have subjectively assessed dental attractiveness after MIE.^15,23 Rather than using pre- and posttreatment photographs of MIE cases, however, the authors used computer-generated images and found that, after modifying the mesiodistal width of lower incisors, dental professionals and dental students were more skillful than laypeople at identifying deviations from normal.¹⁵ In that study, however, there were no significant differences in the percentage change of pre- and posttreatment VAS scores between the two observer groups.¹⁵ In addition, the group evaluated smile esthetics based on the presence of black triangles in the mandibular anterior region. They found that all groups were able to differentiate the presence and absence of black spaces, with the presence having a negative impact on dental esthetics.²³ A similar protocol was not used in this study as this would have sensitized the assessors to specific features, thereby preventing them from making a general assessment of dental esthetics.

This study had several limitations that deserve some discussion. Although the study's sample size was determined a priori, recruiting a larger number of participants would have increased statistical power and reduced the risk of type II error. The PAR index was used as the outcome measure in this study; however, it may not be the optimal tool for the evaluation of treatment. There are important limitations associated with this instrument such as the utility of a weighting system where some features (e.g. centerline, overjet, overbite) are given more weight than others (e.g. buccal segments) as well as it only measuring the occlusal aspect of treatment outcomes.¹⁸ Additionally, it does not consider all factors that contribute to treatment quality, including the psychosocial attitudes of patients, changes in facial profile, periodontal health, and patient compliance.^19,24,25 The issue of external validity also exists; a number of factors were involved in creating individualized treatment plans for both the case and control patients, and a case with a large overbite or overjet for example would have been less likely to be selected as a MIE case. In addition, set criteria for the decision to extract a mandibular incisor in all selected cases was not provided.

CONCLUSIONS

• There were no significant differences in the outcomes of orthodontic cases treated with MIE or NE, indicating that carefully selected cases treated with MIE is a valid orthodontic treatment option.

• Laypeople and dental students both rated posttreatment dental attractiveness higher than pretreatment dental attractiveness in both MIE and NE groups, although dental students tended to be more critical than laypeople.