To compare the effects of two common methods of overbite reduction on smile esthetics.
A prospective clinical trial was conducted with 32 patients in whom overbite reduction was achieved using a maxillary incisor intrusion arch (18 patients) or flat anterior bite plate (14 patients). Clinical and cephalometric records were compared pretreatment (T1), after overbite reduction (T2), and posttreatment (T3).
Both treatment groups experienced a reduction in overbite and maxillary and mandibular incisor proclination during treatment (T1–T3). The center of resistance of the maxillary incisor and the incisal edge was significantly intruded in the intrusion arch group during overbite reduction (T1–T2). However, most of the intrusion of the center of resistance was lost by the end of treatment (T2–T3). Both treatment groups experienced a reduction in maxillary incisor display and flattening of the smile arc during overbite reduction.
Both overbite reduction methods caused a decrease in incisor display and flattening of the smile arc. Smiles were improved in some patients by the end of treatment. However, reduction in incisor display persisted. Clinicians should take precautions to prevent negative effects of overbite reduction.
The primary goals of orthodontics are to establish occlusal harmony within and between the maxillary and mandibular dentition while maintaining or enhancing facial esthetics.1 An attractive smile is considered one of the most important attributes of facial esthetics.2,3 Esthetic considerations in selecting orthodontic treatment goals have become increasingly important.4 Attempts have been made to define and quantify the smile characteristics that are considered ideal both from orthodontists' and laypersons' perspectives.5–7 Vertical positioning of upper incisors and configuration of the smile arc are two significant factors that influence smile attractiveness.
Angle8 stated that if teeth were in proper occlusion, optimal facial esthetics would be produced. Burstone9 indicated that facial esthetics, perioral function, and stability were influenced by soft tissues. He defined anterior tooth display or “lip-to-tooth” as the vertical length of maxillary incisors showing below the lip at rest when both lips were unstrained and the teeth were together. Peck et al.10 introduced the concept of smile esthetics and sought to examine the nature of the gingival smile line. Ackerman and Ackerman11 popularized the term “smile arc”12,13 to describe the relationship between the upper anterior teeth and the contour of the lower lip. Hulsey,12 Rigsbee et al,14 and Ackerman and Ackerman11 found that the unstrained, posed smile could be reproduced consistently.
Sarver15 described the smile arc as “the relationship of the curvature of the incisal edges of the maxillary incisors and canines to the curvature of the lower lip in the posed smile.” The ideal smile arc has maxillary incisal edges parallel to the curve of the lower lip upon smile (consonant). A smile is flat if the incisal edges are straight, and reverse if the incisal edges are aligned in an arc opposite to the contour of the lower lip. A consonant smile is considered more youthful and attractive in appearance.12,16
Hulsey12 evaluated the “smile line” and showed that harmony between an arc of curvature connecting the incisal edges of upper incisors and the upper border of the lower lip was an important characteristic of an attractive smile. Attractive smiles also displayed symmetry with the upper lip at the height of the gingival margin of the upper central incisors on smile.
Mackley17 attempted to determine effects of orthodontics on the smile and found that the most attractive smiles had a smiling lip line close to the gingival margin of the upper incisors. Patients who showed the greatest improvements in smile appearance had a decreased vertical lip-to-tooth relationship with an increase in maxillary incisor torque.
Sarver and Ackerman18 showed that careful leveling without intrusion of the maxillary incisors was important to preserve a favorable smile arc. To control overbite and maintain smile esthetics, intrusion of mandibular incisors, rather than maxillary incisors was suggested.18–20
Orthodontic mechanics may cause the smile arc to flatten during treatment. These include broadening of the maxillary arch9,17 and maxillary intrusion arches or maxillary archwires with accentuated curve.15 Ackerman and Ackerman11 indicated that use of the segmented-arch technique offered better control of leveling compared with a continuous arch, which may cause flattening of the smile arc. Zachrisson20 cautioned against overintrusion of maxillary incisors in patients with low lip lines because it decreased the lip-to-tooth relationship. Despite these recommendations, few studies have examined the effects of orthodontic interventions on smile esthetics.
The purpose of this study was to compare the effects of two methods of overbite reduction, namely, maxillary incisor intrusion and posterior tooth eruption, on smile esthetics.
MATERIALS AND METHODS
Institutional Review Board approval was granted by Virginia Commonwealth University (VCU) to conduct the study, and data were collected over a 4-year period from patients attending the VCU Orthodontic Clinic. This was a prospective study using a convenience sample ,and patients were asked to participate in the study if they had at least 50% overbite pretreatment and were older than 10 years. Patients with Sella-Nasion to mandibular plane angles greater than 40°; patients with extractions planned as part of treatment were excluded. The treatment method for each patient was determined by the orthodontic resident and attending orthodontist as the best treatment to reduce overbite for that particular patient. However, the procedure used was largely dependent on the day of the week the patient chose to be treated because different attending orthodontists consistently implemented their own preferred overbite correction method.
Subjects and Measurements
Sixty patients agreed to participate in the study: 31 in the intrusion arch group and 29 in the bite plate group. Forty had data collected at the pretreatment stage and after the overbite correction: 20 intrusion arch and 20 bite plate patients. Seventeen patients never received the planned treatment, two patients moved during treatment, and one patient had incomplete records. Of the 40 patients who had post–overbite correction records, 32 had their data collected for final analysis at the end of full treatment: 18 from the intrusion arch and 14 from the bite plate groups. Six patients received other means of overbite correction during treatment and were excluded, one patient had incomplete records, and one patient had not completed treatment at the time of final data collection.
Extraoral posed smile photographs and cephalometric radiographs were taken pretreatment (T1), after overbite reduction (T2), and posttreatment (T3). The cephalometric measurements used are described in Table 1. Clinically determined lip to tooth measurements were also made at each timepoint to the nearest 0.5 mm. The middle of the right central incisor incisal edge was used as a reference point. Smile arc (consonant, flat, or reverse) was clinically determined, as recommended by Sarver and Ackerman,21 by the same examiner at each time point.
The intrusion arch used was that advocated by Burstone22 or Isaacson et al.23 and was used in the maxillary arch only. Bite plate patients received either a removable or fixed maxillary acrylic bite plate that contacted the lower incisors to prevent posterior occlusal contact. Aligning archwires in addition to overbite correction appliances were used in most patients during overbite correction.
Skeletal and dental variables were measured on cephalograms at T1, T2, and T3. Maxillary and mandibular superimpositions were used to evaluate the vertical position of the incisors. Superimpositions were accomplished using Bjork's structural method.24 The center of resistance of the maxillary and mandibular incisors was identified at half the root length in the alveolar process on pretreatment cephalometric films and carried forward to the post–overbite correction incisors. An incisor template was used to standardize this process. The pretreatment functional occlusal plane and mandibular plane were transferred to the post–overbite correction and posttreatment radiographs to serve as stable reference planes for describing tooth movements. Linear measurements were made perpendicular to the functional occlusal plane and were assigned a negative value if they were below this plane.
Intraexaminer reliability of the cephalometric and clinical measurements was assessed by comparing repeated measurements of 10 randomly selected radiographs/photographs at a 4-week interval. Cephalometric and clinical measurement changes as a result of overbite correction and treatment were evaluated within and between groups using repeated measures analysis of variance; significance was set at P = .05. Smile arc changes within and between groups were evaluated using χ2 analysis with the threshold for significance set at P = .05. The current individual group sample size would detect a clinically significant difference of 1.0 mm or 1.0° with a power of 80% at the 5% level.
There was a significant correlation (P < .001) between the original and repeated cephalometric measurements. Correlations ranged from 0.94 to 0.99 for upper incisor angulation and occlusal plane to upper incisor incisal edge, respectively. The greatest mean difference between initial and repeated measurements was (1°) for upper incisor angulation (SN-U1). Reliability of smile arc assessment was very good: 9 of the 10 repeated photos received the same assessment.
Characteristics of Treatment Groups
There were no significant differences between the groups in mean age at the start of treatment or in total treatment time (P > .05, Table 2). However, mean time taken for overbite correction was 1.6 months shorter for the bite plate group (P < .05).
Intrusion Arch Group
Means and standard deviations (SDs) of the clinical and cephalometric measurements for patients treated using an intrusion arch to reduce the overbite are illustrated in Table 3. All clinical and cephalometric measurements were significantly reduced during the overbite reduction phase (T1–T2) except upper and lower incisor angulations (SN-U1 and MP-L1) and SN-MP (Table 3). The only significant change between T2 and T3 was an increase in the upper incisor angulation (SN-U1). At the completion of treatment (T3), all measured parameters were significantly different from pretreatment values (T1) except the upper incisor centroid to functional occlusal plane distance (OP-CRU1) and SN-MP (Table 3).
Bite Plate Group
Means and SDs of the clinical and cephalometric measurements for patients treated using a bite plate to reduce the overbite are illustrated in Table 4. All clinical and cephalometric measurements were significantly reduced during the overbite reduction phase (T1–T2) except the upper incisor centroid to functional occlusal plane distance (OP-CRU1) and SN-MP (Table 4). There were no significant changes between T2 and T3 (Table 4). At T3, all measured parameters were significantly different compared with T1 except the upper and lower incisor centroid to functional occlusal plane distances (OP-CRU1 and OP-CRL1) and SN-MP (Table 4).
Comparisons of Different Treatment Groups
Mean (SD) clinical and cephalometric measurements of the two treatment groups at each time point are compared in Table 5. There were no significant pretreatment (T1) differences between groups except for the mandibular plane angle (MP-SN), which was increased in the intrusion arch group by 3.4° compared with the bite plate group (Table 5).
Mean treatment changes are compared between the intrusion arch and bite plate groups in Table 6. Both the intrusion arch and bite plate groups had significant decreases in lip-to-tooth relationship from T1 to T2 and from T1 to T3 (Figure 1; P < .05) with the intrusion arch group experiencing a significantly greater decrease than the bite plate group from T1 to T2 (P < .05). There were no significant differences between the groups for change in lip-to-tooth relationship from T2 to T3 or from T1 to T3 (P > .05; Table 6).
Both the intrusion arch and bite plate groups had significant apical movement of the incisal edge of the upper incisor from T1 to T2 and from T1 to T3 (Figure 2; P < .05). The intrusion arch group had significantly greater apical movement than the bite plate group from T1 to T2, from T1 to T3, and at T3 (Figure 2; P < .05).
The intrusion arch group had significantly greater intrusion of the upper incisor center of resistance from T1 to T2 compared with the bite plate group (Figure 3; P < .05). However, the intrusion arch group experienced significant extrusion of the upper incisor from T2 to T3 (Figure 3; P < .05). There were no significant changes in the position of the center of resistance of the upper incisor for the bite plate group throughout treatment and no significant differences between groups at T3.
Smile Arc Assessment
One patient in each group was excluded from this portion of the analysis due to anterior tooth anomalies. The intrusion arch group had 15 patients with a consonant smile at T1, decreased to seven at T2. and then increased to 10 at T3 (Table 7). The bite plate group started with 11 patients with a consonant smile at T1, decreased to only two at T2, and then increased to six at T3 (Table 7). Both groups started with 1 patient each with a flat smile at T1, increasing to 10 each at T2 then reducing back to 7 each at T3 (Table 7). Both groups started with one patient each with a reverse smile at T1 and had none at T3. There were no statistically significant differences between groups at any time point (P > .05). The overall patient population and the bite plate group experienced significant flattening of the smile arc during overbite correction (Table 7; P < .05). Of the 30 patients evaluated in the present study, 14 had flat smiles at the conclusion of treatment.
The present study expanded on a previous study examining overbite correction and smile esthetics using a subset of the present sample and only evaluating changes immediately after the overbite correction stage.25 Maxillary incisor center of resistance was significantly intruded during overbite correction in the intrusion arch group. This was accompanied by a significant decrease in anterior tooth display and apical movement of the maxillary incisor incisal edge (Table 6). Interestingly, around half the apical movement of the center of resistance of the maxillary central incisor was lost from T2 to T3 (Table 6). This was likely due to extrusive mechanics associated with the use of continuous flat arch wires after segmental overbite correction. Clinicians should be aware that most overbite reduction achieved by an intrusion arch may be lost if precautions are not taken to maintain it later in treatment. Intrusion at the conclusion of treatment was similar to that reported in earlier studies with similar patient demographics.26,27
Significant decreases in lip-to-tooth relationship and apical movement of the upper incisor incisal edge were recorded during overbite correction in the bite plate group (Table 6). Relative apical movement of the incisal edge of upper incisors was likely due to a change in the position of the functional occlusal plane as a result of overeruption of posterior teeth as well as significant proclination of upper incisors during treatment. This may also explain the decrease in lip-to-tooth relationship observed.
Many authors have claimed that maxillary incisor intrusion can lead to an unesthetic flattening of the smile arc and have recommended other methods of overbite correction to avoid this deleterious outcome.9,11,17 There was an increase in patients assessed as having a flat smile in the intrusion arch group from one patient at T1 to 10 at T2; an identical increase was seen in patients treated with a bite plate: one patient at T1 and 10 at T2 (Table 7). The number of patients assessed as having a flat smile was similar in both treatment groups at all time points (Table 7). Flattening of the smile arc in the intrusion arch group may have been a result of a combination of factors: intrusion of the upper incisors, upper arch leveling, and bracket positioning. Similarly, flattening of the smile in the bite plate group may have been a result of upper arch leveling and bracket positioning. The present study showed that flattening of the smile arc was unrelated to the method of overbite reduction. Mackley17 indicated that one of the most important factors associated with improvement of the smile was a decrease in maxillary incisor show during treatment. Conversely, Zachrisson20 recommended avoiding excessively decreasing lip-to-tooth distance. Final vertical anterior tooth positioning goals must be made on an individual patient basis. If decreasing the lip-to-tooth relationship is an objective of treatment, then either method of overbite correction can produce a favorable outcome.
Both treatment groups experienced a reduction in overbite and maxillary and mandibular incisor proclination during treatment (T1–T3).
The center of resistance of the maxillary incisor and the incisal edge were intruded in patients treated with an intrusion arch during overbite reduction (T1–T2; P < .05). However, most of the actual intrusion was lost by the end of treatment (T2–T3, P > .05)
Both overbite reduction methods caused a decrease in incisor display and flattening of the smile arc (T1–T2). Smile arc improved in some patients by the end of treatment (T2–T3). However, reduction in incisor display persisted.
Clinicians should take precautions to prevent negative effects of overbite reduction in individual patients.
This work was supported in part by a grant from the Virginia Orthodontic Education and Research Foundation.