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Smile outcome comparison of Invisalign and traditional fixed-appliance treatment: A case-control study
ORIGINAL ARTICLE
Smile outcome comparison of Invisalign and traditional fixed-appliance treatment: A case-control study Terpsithea Christou,a Roberto Abarca,b Vasileios Christou,c and Chung How Kaua Birmingham, Ala, and Bethlehem, Pa
Introduction: The purpose of this study was to evaluate and compare smile treatment outcomes between patients treated with Invisalign clear aligners (Align Technology, Santa Clara, Calif) and those treated with traditional fixed appliances by integrating variables such as lip symmetry, smile index, smile cant, buccal corridors, and gingival display into smile outcome evaluation. Methods: Records from 58 patients, 29 of whom received Invisalign treatment (mean age 19.03 years) and 29 of whom received traditional fixedappliance treatment (mean age 13.83 years), were compared for their smile outcome. Pretreatment scores, posttreatment scores, and differences between within-group smile score before and after treatment were determined for each group. Fifteen variables comprised the smile outcome, and the data were analyzed with the Mann-Whitney U test and the Wilcoxon t test for 2 dependent samples. Results: Six variables within the fixed-appliance group presented with better smile scores than those within the Invisalign group; buccal corridors (%) (mean difference 5 8.42%), buccal corridors (mm) (5.35 mm), smile cant (0.42 ), maxillary dental midline (0.21 mm), gingival display (0.56 mm), and smile index (1.09%) for P \0.05. Invisalign performed better on 2 variables that determined maxillary incisor position (1.26 mm) and inclination (2.09 ). No significant difference (for P 5 0.05) was shown between pretreatment and posttreatment scores for either of the 2 groups. Conclusions: Using the 15 variables in this study, the results suggested that for patients with Class I nonextraction, treatment with traditional fixed appliances changes the patient's smile more than Invisalign treatment, and fixed appliances appear to be more effective in improving the variables that quantify posttreatment smile outcome. (Am J Orthod Dentofacial Orthop 2020;157:357-64)
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hroughout history, the smile has been an important determinant for others' perception of social attractiveness, intelligence, and popularity.1-3 Nevertheless, smile evaluation is challenging,4,5 and it has not been incorporated into the current grading systems used for orthodontic treatment outcome assessment.6-8
a Department of Orthodontics, School of Dentistry, University of Alabama at Birmingham, Birmingham, Ala. b School of Health Professions, University of Alabama at Birmingham, Birmingham, Ala. c Department of Civil and Environmental Engineering, Advanced Technology for Large Structural Systems Engineering Research Center, Lehigh University, Bethlehem, Pa. All authors have completed and submitted the ICMJE Form for Disclosure of Potential Conflicts of Interest, and none were reported. Address correspondence to: Terpsithea Christou, Department of Orthodontics, School of Dentistry, University of Alabama at Birmingham, 1919 7th Ave S, SDB 314, Birmingham, AL 35294-0007; e-mail, [email protected]. Submitted, November 2018; revised and accepted, March 2019. 0889-5406/$36.00 Ó 2019 by the American Association of Orthodontists. All rights reserved. https://doi.org/10.1016/j.ajodo.2019.03.030
Multiple studies have separately assessed smile width (mm),9-11 lip symmetry (%),12 smile index (%),13-16 smile cant ( ),17-20 maxillary dental midline (mm),21-24 buccal corridors (%),23 buccal corridors (mm),24-26 upper lip thickness (mm),27 lower lip thickness (mm),27 gingival display (mm),28 maxillary incisor position (mm),29-31 maxillary incisor inclination ( ),29-31 lip shape,32 smile arc,33 and lip line.34 However, research has not yet integrated all the aforementioned variables to evaluate the smile treatment outcome.35 The evaluation of orthodontic treatment on smile outcome variables is an important area of study because smile improvement is an important motivator for those who seek orthodontic treatment.36 Therefore, it is important to better understand how treatment type affects the smile outcome. Although traditional fixed appliances have been known as the gold standard of orthodontic treatment for a long time, the Invisalign system (Align Technology, Santa Clara, Calif) has recently emerged as an alternative to traditional braces. The Invisalign appliance is a commercially available medical device. Treatment with 357
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Invisalign involves custom-designing a number of vacuum-formed removable trays that, when applied according to the treatment protocol, shift and move the teeth. Since the company's initiation in 1997, the Invisalign system has been used to treat more than 6 million people. Moreover, Invisalign is now available internationally and is sought with increasing frequency.37-43 As more patients seek Invisalign treatment, the need for outcome comparison between fixed appliances and this alternative treatment modality becomes apparent. A few studies have compared the 2 treatment modalities, and no study has focused on smile evaluation yet. Thus, the purpose of the present study is 2-fold: to compare both treatment modalities while focusing on smile evaluation, and to incorporate all the aforementioned variables in an attempt to quantify smile evaluation. MATERIAL AND METHODS
The study was designed as a retrospective casecontrol study and was approved by the Institutional Review Board of the University of Alabama at Birmingham (UAB) (Protocol No. IRB-300000815). This study's protocol is in accordance with the statement by Strengthening the Reporting of Observational Studies in Epidemiology.44-46 Patient data were collected from the orthodontic department of UAB after generating 2 separate reports through Dolphin Software (Dolphin Imaging & Management Solutions, Chatsworth, Calif) with the words “Invisalign” and “ABO cases” used as search keys, respectively. A total of 453 Invisalign cases and 83 American Board of Orthodontics (ABO) cases (N 5 536) were retrieved from the computer search within the UAB clinical database. Two groups were established from these records. The Invisalign group comprised patients treated with Invisalign, and the fixed-appliance (ABO) group comprised patients treated with conventional fixed appliances. The following inclusion and exclusion criteria were then applied to the potential participants. Inclusion criteria (both groups): males and females aged 12-30 years; Class I molar classification; nonextraction treatment; minor crowding (1-4 mm) in each dental arch; treated in both dental arches; and pretreatment and posttreatment records available, including intraoral and extraoral composite photographs and lateral cephalometric radiographs taken within 6 months of initiation and completion of the treatment. An additional inclusion criterion for the ABO group was that patients should have presented to the ABO in St Louis and successfully passed the clinical examination during the past 3 years (2015-2018).
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An additional inclusion criterion for the Invisalign group was that treatment should have been completed solely with Invisalign clear aligners and patients should have had excellent treatment cooperation and strictly followed treatment instructions (eg, wear aligner 22.5 h/d). No filter on the period of recruitment was applied for this group. Exclusion criteria were as follows: craniofacial discrepancies or syndromes; previous orthognathic or cosmetic treatment; previous esthetic treatment of lips or previous traumas involving oral soft tissue; patients with incomplete or poor quality of records; and unnatural posed smiles47 (ie, a smile that is not reproducible, and therefore cannot be used as a reference for further measurements). From the 536 total cases identified, 58 met the preselected criteria, including 29 treated with Invisalign and 29 with fixed appliances (ABO). The study's strategy followed individual matching by having 1 control (ABO) per case (Invisalign).48 All eligible cases were exported from Dolphin Software database and imported into Adobe Photoshop 5.0 software for further analyses. Each frame was opened in Adobe Photoshop 5.0 and adjusted by using the millimeter ruler in the frame. Calibration of all images in the software was done according to Bray et al.17 Fifteen variables (12 continuous and 3 categorical) were used to assess the smile outcome. Patients were measured on all 15 variables at pretreatment and posttreatment. All variables with their definitions are presented in Table I. Patient demographics, including age, sex, and treatment duration, are included in Table II. The protocol used for the Invisalign group in this study's sample was such that patients changed their aligners every 2 weeks and were instructed to wear their aligners 22.5 hours per day. Ellipsoid, beveled, and rectangle-shaped attachments were used as needed. There was no filter on the period of recruitment for the Invisalign group during the initial search. The final group selected for inclusion in this study received treatment between the years 2012 and 2018. One faculty member planned and supervised all treatments. The protocol used for the ABO group in this study's sample was the Edgewise technique; McLaughlin, Bennett, and Trevisi prescription49; and brackets with 0.22-inch slots. With the exception of settling vertical elastics during the finishing stage of fixed-appliance treatment, no additional biomechanical appliances were used. For both groups, interproximal reduction (IPR) could have been used between the 6 mandibular anterior teeth (5 sites of IPR), if needed. In that case, 0.2 mm of IPR was performed on each side, equaling 0.1 mm of enamel
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Table I. Definitions of variables used as measurements in this study Variable Smile width (mm) Lip symmetry (%) Smile index (%) Smile cant ( ) Buccal corridor (%) Buccal corridor (mm) Upper lip thickness (mm) Lower lip thickness (mm) Gingival display (mm) Maxillary dental midline (mm) Maxillary incisor position (mm) Maxillary incisor inclination ( ) Lip shape Smile arc Lip line
Definition Linear distance between the most distal point of the last visible posterior tooth on the right and the most distal point of the last visible posterior tooth on the left The sum of the upper and lower left lip length divided by the upper and lower right lip length Smile width to interlabial gap ratio The angle between the interpupillary line and a line from the right to the left corner of the mouth The ratio of the intercommissure width divided by the distance from first premolar on right side to first premolar on left side Dark space (negative space) visible during smile formation between the corners of the mouth and the buccal surfaces of the maxillary teeth The vertical distance from the most superior peak of the lip to the most inferior portion of the tubercle of the upper lip The vertical distance from the deepest midline point on the superior margin of the lower lip to the most inferior portion of the lower lip Area of maxillary gingival exposure between the inferior border of the upper lip and the marginal gingiva of the maxillary teeth on the right side Refers to the vertical contact interface between 2 maxillary centrals Distance from upper incisor incisal point to N-A line The angle between maxillary incisor axial line and S-N line The relationship of the center of the upper lip to the corners of the mouth in smiling 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 amount of vertical tooth exposure in smiling
Table II. Demographics ABO group Variable Age, y Treatment duration, mo
Median 13.00 24.00
Mean (SD) 13.83 (2.05) 23.62 (3.41)
reduction on each side of each mandibular incisor and on the mesial side of the mandibular canines. Data analysis was performed by a nonorthodontic research assistant who was trained by an orthodontic faculty member. All measurements were taken twice by the same operator with a 2-month period difference. If the first and the second measurements did not match, the mean of the 2 was calculated and recorded as the final score. Subsequent statistical analyses were performed with Statistical Package for the Social Sciences software (version 24; IBM, Armonk, NY) and were run using the Mann-Whitney U test to determine if pretreatment and posttreatment outcome measurements differed significantly between both conditions (ie, Invisalign vs ABO) in patients. The Wilcoxon t test was used to learn whether significant differences existed in pretreatment and posttreatment variables within each group. RESULTS
Data from 58 patients (29 Invisalign and 29 ABO) were included in the study. Within the sample, 39 females and 19 males were included (Invisalign:
Invisalign group Range 10 13
Median 17.00 19.03
Mean (SD) 19.03 (5.49) 19.52 (6.49)
Range 17 23
P value 0.000 0.005
22 females, 7 males; ABO: 17 females, 12 males). The Invisalign group had an average pretreatment age of 19.3 years (standard deviation [SD] 5 5.49), whereas the ABO group's mean pretreatment age was 13.83 years (SD 5 2.05). The mean treatment times for the Invisalign and ABO patients were 19.52 6 6.49 months and 23.62 6 3.41 months, respectively. An alpha error of 0.05 was used as the level of statistical significance for all analyses. No statistically significant differences were found between the ABO and Invisalign groups at pretreatment for the 15 variables studied (P \0.05) (Table III). Invisalign and ABO posttreatment smile scores are presented in Table IV, along with exact P values and SDs. The smile scores within the ABO group demonstrated a statistically significant change from pretreatment to posttreatment for 9 of 12 continuous variables, including smile width (mean difference 5 15.37 mm), smile index (mean difference 5 10.54%), smile cant (mean difference 5 0.68 ), maxillary dental midline (mean difference 5 0.61 mm), buccal corridors (%) (mean difference 5 5.43%), buccal corridors (mm)
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Table III. Pretreatment smile scores of the ABO and Invisalign groups of patients ABO group Variable Smile width, mm Lip symmetry, % Smile index, % Smile cant, Maxillary dental midline, mm Buccal corridors, % Buccal corridors, mm Upper lip thickness, mm Lower lip thickness, mm Gingival display, mm Maxillary central incisor position, mm Maxillary central incisor inclination,
Median 47.1 0.9 5.03 0.89 0.60 21.91 13.13 4.93 7.75 1.15 4.10 103.0
Invisalign group
Mean (SD) 46.9 (4.64) 0.9 (0.08) 4.9 (1.15) 1.20 (1.11) 0.82 (0.61) 21.78 (7.03) 13.32 (0.29) 5.40 (2.05) 7.93 (1.66) 1.42 (1.45) 3.54 (3.27) 102.13 (9.49)
Median 47.17 0.99 5.02 0.87 0.57 22.43 13.92 5.18 8.23 1.32 4.23 104.42
Mean (SD) 47.04 (4.48) 0.99 (0.77) 5.06 (1.31) 1.11 (0.98) 0.76 (0.60) 22.83 (6.15) 13.96 (4.33) 5.42 (1.70) 8.23 (1.63) 1.32 (1.60) 4.23 (3.04) 104.42 (8.30)
P value 0.50 0.12 0.88 0.53 0.33 0.11 0.24 0.66 0.56 0.34 0.18 0.05
Table IV. Pretreatment and posttreatment scores within groups ABO group Variable Smile width, mm Lip symmetry, % Smile index, % Smile cant, Maxillary dental midline, mm Buccal corridors, % Buccal corridors, mm Upper lip thickness, mm Lower lip thickness, mm Gingival display, mm Maxillary incisor position, mm Maxillary incisor inclination,
IN mean (SD) 46.94 (4.64) 0.98 (0.08) 4.98 (1.15) 1.20 (1.11) 0.82 (0.61) 21.78 (7.03) 13.32 (4.94) 5.40 (2.05) 7.93 (1.66) 1.42 (1.45) 3.54 (3.27) 102.13 (9.49)
FN mean (SD) 52.31 (3.45) 0.98 (0.69) 5.52 (0.96) 0.52 (0.85) 0.21 (0.57) 16.35 (4.71) 10.35 (3.53) 4.98 (1.65) 8.11 (1.63) 2.17 (1.69) 5.33 (2.45) 106.77 (6.31)
Invisalign group Mean difference 15.37 0 10.54 0.68 0.61 5.43 2.97 0.42 10.18 10.75 11.79 14.64
P value 0.000 0.90 0.031 0.001 0.002 0.000 0.000 0.38 0.43 0.011 0.009 0.021
IN mean (SD) 47.04 (4.48) 0.99 (0.77) 5.06 (1.31) 1.11 (0.98) 0.76 (0.60) 22.83 (6.15) 13.96 (4.33) 5.42 (1.70) 8.23 (1.63) 1.32 (1.60) 4.23 (3.04) 104.42 (8.36)
FN mean (SD) 48.79 (4.53) 0.98 (0.60) 4.43 (0.93) 0.94 (0.74) 0.42 (0.38) 24.82 (6.45) 15.70 (4.65) 5.15 (1.36) 8.57 (1.74) 1.61 (1.87) 4.07 (2.48) 104.68 (4.98)
Mean difference 11.75 0.01 0.63 0.17 0.34 1.99 11.74 10.27 10.34 10.29 0.16 10.26
P value 0.030 0.10 0.003 0.57 0.012 0.52 0.22 0.10 0.80 0.15 0.09 0.031
IN, pretreatment; FN, posttreatment.
(mean difference 5 2.97 mm), gingival display (mean difference 5 10.75 mm), maxillary incisor position (mean difference 5 11.79 mm), and maxillary incisor inclination (mean difference 5 14.64 ). The changes in lip symmetry (mean difference 5 0%, P 5 0.91), upper lip thickness (mean difference 5 0.42 mm, P 5 0.39), and lower lip thickness (mean difference 5 10.18 mm, P 5 0.43) from pretreatment to posttreatment did not reach statistical significance. Conversely, in the Invisalign group, changes in 4 variables reached statistical significance: smile width (mean difference 5 11.75 mm), smile index (mean difference 5 0.63%), maxillary dental midline (mean difference 5 0.34 mm), and maxillary incisor inclination (mean difference 5 12.26 ). Again, the changes in lip symmetry (mean difference 5 0.01%, P 5 0.10), upper lip thickness (mean difference 5 10.27 mm, P 5 0.10), and lower lip thickness (mean difference 5 10.34 mm, P 5 0.80) did not reach statistical significance.
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The following pretreatment-to-posttreatment differences emerged as significantly different between the ABO and Invisalign groups (Table V): smile width (P \0.05, mean difference 5 13.52), smile index (P \0.05, mean difference 5 11.09), smile cant (P \0.05, mean difference 5 0.42), maxillary dental midline (U 5 288.00, P \0.05, mean difference 5 0.21), buccal corridors (%) (U 5 193.00, P \0.05, mean difference 5 8.42), buccal corridors (mm) (U 5 215.50, P \0.05, mean difference 5 5.35), gingival display (U 5 632.50, P \0.05, mean difference 5 0.56), maxillary incisor position (U 5 632.50, P\0.05, mean difference 5 1.26), and maxillary incisor inclination (U 5 621.50, P \0.05, mean difference 5 12.09). The differences between groups in lip symmetry (U 5 482.00, P 5 0.34), upper lip thickness (U 5 0.448.00, P 5 0.67), and lower lip thickness (U 5 470.50, P 5 0.44) did not reach significance. This study also sought to investigate changes between and within groups in smile arc, lip shape, and
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Table V. Pretreatment and posttreatment smile score difference comparison between the ABO and Invisalign groups ABO group Variable Smile width, mm Lip symmetry, % Smile index, % Smile cant, Maxillary dental midline, mm Buccal corridors, % Buccal corridors, mm Upper lip thickness, mm Lower lip thickness, mm Gingival display, mm Maxillary incisor position, mm Maxillary incisor inclination,
Median 52.13 0.08 5.49 0.40 0.00 16.45 10.00 4.68 7.69 1.95 5.40 106.20
Mean (SD) 52.31 (3.45) 0.98 (0.69) 5.52 (0.96) 0.52 (0.85) 0.21 (0.57) 16.35 (4.71) 10.35 (3.53) 4.98 (1.65) 8.11 (1.63) 2.17 (1.69) 5.33 (2.45) 106.77 (6.31)
Invisalign group Range 13.15 0.31 3.62 4.55 3.01 19.42 15.29 6.40 6.25 5.01 11.20 29.80
lip line variables, which are categorical and can each have 3 possible outcomes. Specifically, the smile arc can be reverse, flat, or consonant; lip shape can be upward, straight, or downward; and, finally, lip line can be either optimal or high. After using contingency tables to elucidate the number of patients in each possible category for each variable, the authors determined that a larger sample size would be needed to analyze the data for all possible conditions. Therefore, these variables were excluded from the study. DISCUSSION
The current study compared smile outcome evaluation for 2 different orthodontic treatment modalities— Invisalign and traditional fixed appliances. With regard to posttreatment outcomes of the 2 groups, there were significant differences in 9 of 12 outcome variables of interest, with mean differences trending in favor of the ABO group. In terms of treatment duration, our results indicated that Invisalign treatment duration was, on average, about 4 months shorter than ABO treatment duration (Table II), despite previous studies being suggestive of no differences between the 2 approaches.50 Li et al51 added nuance to previous studies by stating that Invisalign treatment is expected to be shorter only in nonextraction patients, as was the case for our sample. In patients who needed complex extractions, Invisalign treatment was 44% longer than ABO treatment.51 The ABO group outperformed the Invisalign group on 6 posttreatment variables that comprise smile outcome evaluation: acceptable values for buccal corridors in % and millimeters with ranges close to 17% and 11.5 mm.5 Our study showed that the ABO group was within the acceptable range with a mean value of 16.35% or 10.35 mm, whereas the Invisalign group was not within the acceptable range (mean 5 24.82% and 15.70 mm).
Median 48.17 1.01 4.45 0.94 0.28 23.17 14.88 4.78 8.65 0.80 4.00 105.20
Mean (SD) 48.79 (4.53) 0.98 (0.60) 4.43 (0.93) 0.94 (0.74) 0.42 (0.38) 24.82 (6.45) 15.70 (4.65) 5.15 (1.36) 8.57 (1.74) 1.61 (1.87) 4.07 (2.48) 104.68 (4.98)
Range 16.89 0.23 3.44 2.67 1.56 24.66 17.70 5.29 8.03 5.88 12.10 23.80
Mean difference 13.52 10.00 11.09 0.42 0.21 8.42 5.35 0.31 0.46 0.56 1.26 12.09
P value 0.00 0.33 0.00 0.02 0.03 0.00 0.01 0.66 0.43 0.01 0.01 0.02
Smile cant and maxillary dental midline should be 0 at the end of treatment, with cutoff values of 4 18-20 and 2.38 mm,5 respectively. In our study, smile cant for the Invisalign group (mean 5 0.94 ) was farther from 0 than smile cant for the ABO group (mean 5 0.52 ). In addition, maxillary dental midline improved by 74.4% of the initial value with our study's ABO group and only 45% with the Invisalign group. Perceptions of gingival display have been evaluated extensively in the past, and there is consensus that the range of acceptability is from 1.5 to 4.0 mm,5 with an ideal value of 2.1 mm. In our study, mean gingival display was 2.17 mm in the ABO group, whereas it was 1.61 mm in the Invisalign group. Several studies have agreed that a high smile index is ideal and that a lower smile index could add an “aging effect” to the smile.14 Two studies found the mean value of smile index to be 6.012%15 and 6.15% 6 1.31%16 with sample sizes of 100 and 50 patients, respectively. Even if there is not yet an agreed-on range that quantifies the ideal smile index, these results suggest that our ABO group is a closer match with these previous samples than the Invisalign group. In the Invisalign group, the mean smile index value (4.43%) was smaller than the mean smile index value in the ABO group (5.52%). The Invisalign group outperformed the ABO group on 2 posttreatment variables that comprise smile outcome evaluation. Regarding maxillary incisor position, the cephalometric analysis by Ricketts31 in 1961 showed the clinical norm for the maxillary incisor position to be 3.5 6 2.3 mm. In our study, maxillary incisors appeared to be less protrusive in the Invisalign group (mean 5 4.07 mm) than in the ABO group (mean 5 5.33 mm). In addition, according to the values presented by Bumann30 in 1994, the norm range of maxillary incisor
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inclination is 102 -105 . According to these guidelines, the maxillary incisor in the Invisalign group was positioned ideally (mean 5 104.68 ), whereas in the ABO group, it was slightly overproclined (mean 5 106.77 ). Both study groups performed comparably at posttreatment evaluation on smile width. A previous study that compared the linear distance between the center of facial axis of the most visible tooth points of the most visible tooth to the left and right of a nonextraction group found that smile width values from 46 to 52.6 mm seemed to remain stable at least 4 years after retention.11 Despite the fact that smile width value for the Invisalign group appeared to be about 4 mm smaller than that of the ABO group (mean difference 5 3.52 mm), both groups performed within the acceptable range. Some variables showed no significant differences from pretreatment to posttreatment for either of the 2 study groups. For example, lip symmetry and upper and lower lip thickness did not change. Several studies addressed the importance of lips in improving the overall smile,25,52-54 but thus far, little is known about acceptable threshold values that can serve as clinical guides. The study by Machado28 suggests that voluminous lips could significantly improve smile and lip esthetics and even make the patient look younger, whereas thin lips are negatively associated with the latter variables (ie, thin lips may have an aging effect).Therefore, the orthodontic treatment should be performed carefully to prevent a decrease in lip volume, especially in cases that involve teeth retraction. Furthermore, some patients will benefit from complementing orthodontic biomechanics with lip fillers for lip augmentation purposes. The sample size of this study was small and retrospective in nature; there were 58 patients who were equally distributed between Invisalign (n 5 29) and ABO (n 5 29) groups. Therefore, the results should be interpreted with caution. For example, our sample did not include all possible outcomes from each variable (eg, upward, straight, and downward for lip shape), and we were, thus, unable to representatively analyze the categorical parameters (smile arc, lip shape, and lip line).This limited sample also means that our study's results may not be generalizable to the population; studies with larger samples may include individuals that represent all possible outcomes for variables. Thus, this study may better serve as preliminary insight for higherpowered studies in the future. Because of the small number of cases that met the inclusion criteria for the study, the authors were unable to age-match cases. Thus, there was a statistically significant difference between the ages of the 2 groups. The mean age for the Invisalign group was, on average, 5 years greater than that of the ABO
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group (Table II). Despite this, Djeu et al55 found that statistically significant age discrepancies between groups do not have observable clinical effects on results. Vig et al,56 however, posited that age differences do indeed influence clinical results, especially when the clinical outcome being assessed is related to a soft-tissue parameter. Furthermore, approximately 75% of the Invisalign group identified as female, whereas only approximately 58% identified as such in the ABO group. As suggested by Vig et al,56 this may lead to sexual dimorphism bias. Owing to the retrospective nature of case-control studies, matching participants on age and sex is not always feasible. To avoid possible confounders, future studies using experimental data should match participants between both experimental groups on both age and sex. To assess the homogeneity of the comparison groups, given the inherent characteristic differences in our sample (ie, age and sex), pretreatment mean scores for all 12 variables assessed were obtained and were shown to be similar, suggesting that all participants' smiles were quantified similarly before treatment. CONCLUSIONS
The data suggest that orthodontic treatment with fixed appliances in patients with Class I nonextraction produces greater changes in the patient's smile than Invisalign treatment. Fixed appliances appear to be more effective in improving the variables that quantify posttreatment–smile outcome. However, Invisalign treatment may be of shorter duration than ABO treatment. Neither of the 2 treatment modalities have any significant effect on the lips at rest or on smiling. ACKNOWLEDGMENTS
The authors thank the American Association of Orthodontists Foundation for funding this project. In addition, T.C. would like to thank her American Association of Orthodontists Foundation mentors: Drs Chung How Kau, Peter Ngan, and Veerasathpurush Allareddy for their continuous support and guidance. REFERENCES 1. Miller AG. Role of physical attractiveness in impression formation. Psychon Sci 1970;19:241-3. 2. Howells DJ, Shaw WC. The validity and reliability of ratings of dental and facial attractiveness for epidemiologic use. Am J Orthod 1985;88:402-8. 3. Zhang K, Huang L, Yang L, Xu L, Xue C, Xiang Z, et al. Effects of transverse relationships between maxillary arch, mouth, and face on smile esthetics. Angle Orthod 2016;86:135-41.
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4. Sarver DM, Ackerman MB. Dynamic smile visualization and quantification: part 1. Evolution of the concept and dynamic records for smile capture. Am J Orthod Dentofacial Orthop 2003;124:4-12. 5. Parrini S, Rossini G, Castroflorio T, Fortini A, Deregibus A, Debernardi C. Laypeople's perceptions of frontal smile esthetics: a systematic review. Am J Orthod Dentofacial Orthop 2016;150: 740-50. 6. Eismann D. A method of evaluating the efficiency of orthodontic treatment. Trans Eur Orthod Soc 1974;223-32. 7. Gottlieb EL. Grading your orthodontic treatment results. J Clin Orthod 1975;9:155-61. 8. Casko JS, Vaden JL, Kokich VG, Damone J, James RD, Cangialosi TJ, et al. Objective grading system for dental casts and panoramic radiographs. American Board of Orthodontics. Am J Orthod Dentofacial Orthop 1998;114:589-99. 9. Grover N, Kapoor DN, Verma S, Bharadwaj P. Smile analysis in different facial patterns and its correlation with underlying hard tissues. Prog Orthod 2015;16:28. 10. Ghaffar F, Fida M. Effect of extraction of first four premolars on smile aesthetics. Eur J Orthod 2011;33:679-83. 11. Akyalcin S, Misner K, English JD, Alexander WG, Alexander JM, Gallerano R. Smile esthetics: evaluation of long-term changes in the transverse dimension. Korean J Orthod 2017;47:100-7. 12. Hulsey CM. An esthetic evaluation of lip-teeth relationships present in the smile. Am J Orthod 1970;57:132-44. 13. Sarver DM, Ackerman MB. Dynamic smile visualization and quantification: part 2. Smile analysis and treatment strategies. Am J Orthod Dentofacial Orthop 2003;124:116-27. 14. Maganzini AL, Schroetter SB, Freeman K. Improvement in smile esthetics following orthodontic treatment: a retrospective study utilizing standardized smile analysis. Angle Orthod 2014;84: 492-9. 15. Parekh H, Patel D, Mehta F, Joshi N, Bhattacharya A. Smile–A Diagnostic Tool: photographic analysis in Adult Gujarati Population. IOSR-JDMS 2013;12:39-46. 16. Ackerman MB, Brensinger C, Landis JR. An evaluation of dynamic lip-tooth characteristics during speech and smile in adolescents. Angle Orthod 2004;74:43-50. 17. Bray D, Henstrom DK, Cheney ML, Hadlock TA. Assessing outcomes in facial reanimation: evaluation and validation of the SMILE system for measuring lip excursion during smiling. Arch Facial Plast Surg 2010;12:352-4. 18. Ker AJ, Chan R, Fields HW, Beck M, Rosenstiel S. Esthetics and smile characteristics from the layperson’s perspective: a computer-based survey study. J Am Dent Assoc 2008;139:1318-27. 19. Silva BP, Jimenez-Castellanos E, Martinez-de-Fuentes R, Greenberg JR, Chu S. Laypersons’ perception of facial and dental asymmetries. Int J Periodontics Restorative Dent 2013;33: e162-71. 20. Wang C, Hu WJ, Liang LZ, Zhang YL, Chung KH. Esthetics and smile-related characteristics assessed by laypersons. J Esthet Restor Dent 2018;30:136-45. 21. Sharma PK, Sharma P. Dental smile esthetics: the assessment and creation of the ideal smile. Semin Orthod 2012;18: 193-201. 22. Miller EL, Bodden WR Jr, Jamison HC. A study of the relationship of the dental midline to the facial median line. J Prosthet Dent 1979;41:657-60. 23. Cheng HC, Cheng PC. Factors affecting smile esthetics in adults with different types of anterior overjet malocclusion. Korean J Orthod 2017;47:31-8. 24. Bhuvaneswaran M. Principles of smile design. J Conserv Dent 2010;13:225-32.
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25. McNamara L, McNamara JA, Ackerman MB, Baccetti T. Hard- and soft-tissue contributions to the esthetics of the posed smile in growing patients seeking orthodontic treatment. Am J Orthod Dentofacial Orthop 2008;133:491-9. 26. Krishnan V, Daniel ST, Lazar D, Asok A. Characterization of posed smile by using visual analog scale, smile arc, buccal corridor measures, and modified smile index. Am J Orthod Dentofacial Orthop 2008;133:515-23. 27. Rai D, Janardhanam P, Rai A. Esthetic factors of smile in vertical dimensions: a comparative evaluation. J Indian Orthod Soc 2015;49:25-31. 28. Machado AW. 10 commandments of smile esthetics. Dent Press J Orthod 2014;19:136-57. 29. Ghaleb N, Bouserhal J, Bassil-Nassif N. Aesthetic evaluation of profile incisor inclination. Eur J Orthod 2011;33:228-35. 30. Bumann A. References cephalometriques de la position des incisives. Orthod Fr 1994;65:11-49. 31. Ricketts RM. Cephalometric analysis and synthesis. Angle Orthod 1961;31:141-56. 32. Ackerman MB, Ackerman JL. Smile analysis and design in the digital era. J Clin Orthod 2002;36:221-36. 33. Sarver DM. The importance of incisor positioning in the esthetic smile: the smile arc. Am J Orthod Dentofacial Orthop 2001;120: 98-111. 34. Sabri R. The eight components of a balanced smile. J Clin Orthod 2005;39:155-67;quiz 154. 35. Schabel BJ, McNamara JA, Baccetti T, Franchi L, Jamieson SA. The relationship between posttreatment smile esthetics and the ABO Objective Grading System. Angle Orthod 2008;78: 579-84. 36. Prabakaran R, Seymour S, Moles DR, Cunningham SJ. Motivation for orthodontic treatment investigated with Q-methodology: patients’ and parents’ perspectives. Am J Orthod Dentofacial Orthop 2012;142:213-20. 37. Coughlan A, Hennessy J, Najjar A, Auyang E, Batanghari W, Cartwright C. Invisalign: Orthodontics unwired. Kellogg School of Management Cases 2017;20:1-8. 38. Tuncay OC. The Invisalign system. London: Quintessence Publishing; 2006. 39. Kuncio DA. Invisalign: current guidelines for effective treatment. NY State Dent J 2014;80:11-4. 40. Weir T. Clear aligners in orthodontic treatment. Aust Dent J 2017; 62(Suppl 1):58-62. 41. Kravitz ND, Kusnoto B, Agran B, Viana G. Influence of attachments and interproximal reduction on the accuracy of canine rotation with Invisalign. A prospective clinical study. Angle Orthod 2008; 78:682-7. 42. Kravitz ND, Kusnoto B, BeGole E, Obrez A, Agran B. How well does Invisalign work? A prospective clinical study evaluating the efficacy of tooth movement with Invisalign. Am J Orthod Dentofacial Orthop 2009;135:27-35. 43. Plunkett JW. Plunkett's companion to the Almanac of American Employers 2009 Mid-Size Firms 2009. Houston, TX: Plunkett Research Ltd; 2009. 44. Vandenbroucke JP. The making of STROBE. Epidemiology 2007; 18:797-9. 45. von Elm E, Altman DG, Egger M, Pocock SJ, Gøtzsche PC, Vandenbroucke JP. The strengthening the reporting of observational studies in epidemiology (STROBE) statement: guidelines for reporting observational studies. Int J Surg 2014;12: 1495-9. 46. Vandenbroucke JP, von Elm E, Altman DG, Gøtzsche PC, Mulrow CD, Pocock SJ, et al. Strengthening the Reporting of
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Observational Studies in Epidemiology (STROBE): explanation and elaboration. PLoS Med 2007;4:e297. Ackerman JL, Ackerman MB, Brensinger CM, Landis JR. A morphometric analysis of the posed smile. Clin Orthod Res 1998;1:2-11. Ury HK. Efficiency of case-control studies with multiple controls per case: continuous or dichotomous data. Biometrics 1975;31:643-9. McLaughlin RP, Bennett JC, Trevisi HJ. Systemized Orthodontic Treatment Mechanics. Maryland Heights, MO: Mosby International Ltd; 2001. Kuncio D, Maganzini A, Shelton C, Freeman K. Invisalign and traditional orthodontic treatment postretention outcomes compared using the American Board of Orthodontics objective grading system. Angle Orthod 2007;77:864-9. Li W, Wang S, Zhang Y. The effectiveness of the Invisalign appliance in extraction cases using the the ABO model grading system: a multicenter randomized controlled trial. Int J Clin Exp Med 2015; 8:8276-82.
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52. De-Marchi LM, Pini NIP, Pascotto RC. The relationship between smile attractiveness and esthetic parameters of patients with lateral agenesis treated with tooth recontouring or implants. Clin Cosmet Investig Dent 2012;4:43-9. 53. Springer NC, Chang C, Fields HW, Beck FM, Firestone AR, Rosenstiel S, et al. Smile esthetics from the layperson's perspective. Am J Orthod Dentofacial Orthop 2011;139:e91-101. 54. Chang CA, Fields HW, Beck FM, Springer NC, Firestone AR, Rosenstiel S, et al. Smile esthetics from patients' perspectives for faces of varying attractiveness. Am J Orthod Dentofacial Orthop 2011;140:e171-80. 55. Djeu G, Shelton C, Maganzini A. Outcome assessment of Invisalign and traditional orthodontic treatment compared with the American Board of Orthodontics objective grading system. Am J Orthod Dentofacial Orthop 2005;128:292-8;discussion 298. 56. Vig RG, Brundo GC. The kinetics of anterior tooth display. J Prosthet Dent 1978;39:502-4.
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Smile outcome comparison of Invisalign and traditional fixed-appliance treatment: A case-control study Terpsithea Christou,a Roberto Abarca,b Vasileios Christou,c and Chung How Kaua Birmingham, Ala, and Bethlehem, Pa
Introduction: The purpose of this study was to evaluate and compare smile treatment outcomes between patients treated with Invisalign clear aligners (Align Technology, Santa Clara, Calif) and those treated with traditional fixed appliances by integrating variables such as lip symmetry, smile index, smile cant, buccal corridors, and gingival display into smile outcome evaluation. Methods: Records from 58 patients, 29 of whom received Invisalign treatment (mean age 19.03 years) and 29 of whom received traditional fixedappliance treatment (mean age 13.83 years), were compared for their smile outcome. Pretreatment scores, posttreatment scores, and differences between within-group smile score before and after treatment were determined for each group. Fifteen variables comprised the smile outcome, and the data were analyzed with the Mann-Whitney U test and the Wilcoxon t test for 2 dependent samples. Results: Six variables within the fixed-appliance group presented with better smile scores than those within the Invisalign group; buccal corridors (%) (mean difference 5 8.42%), buccal corridors (mm) (5.35 mm), smile cant (0.42 ), maxillary dental midline (0.21 mm), gingival display (0.56 mm), and smile index (1.09%) for P \0.05. Invisalign performed better on 2 variables that determined maxillary incisor position (1.26 mm) and inclination (2.09 ). No significant difference (for P 5 0.05) was shown between pretreatment and posttreatment scores for either of the 2 groups. Conclusions: Using the 15 variables in this study, the results suggested that for patients with Class I nonextraction, treatment with traditional fixed appliances changes the patient's smile more than Invisalign treatment, and fixed appliances appear to be more effective in improving the variables that quantify posttreatment smile outcome. (Am J Orthod Dentofacial Orthop 2020;157:357-64)
T
hroughout history, the smile has been an important determinant for others' perception of social attractiveness, intelligence, and popularity.1-3 Nevertheless, smile evaluation is challenging,4,5 and it has not been incorporated into the current grading systems used for orthodontic treatment outcome assessment.6-8
a Department of Orthodontics, School of Dentistry, University of Alabama at Birmingham, Birmingham, Ala. b School of Health Professions, University of Alabama at Birmingham, Birmingham, Ala. c Department of Civil and Environmental Engineering, Advanced Technology for Large Structural Systems Engineering Research Center, Lehigh University, Bethlehem, Pa. All authors have completed and submitted the ICMJE Form for Disclosure of Potential Conflicts of Interest, and none were reported. Address correspondence to: Terpsithea Christou, Department of Orthodontics, School of Dentistry, University of Alabama at Birmingham, 1919 7th Ave S, SDB 314, Birmingham, AL 35294-0007; e-mail, [email protected]. Submitted, November 2018; revised and accepted, March 2019. 0889-5406/$36.00 Ó 2019 by the American Association of Orthodontists. All rights reserved. https://doi.org/10.1016/j.ajodo.2019.03.030
Multiple studies have separately assessed smile width (mm),9-11 lip symmetry (%),12 smile index (%),13-16 smile cant ( ),17-20 maxillary dental midline (mm),21-24 buccal corridors (%),23 buccal corridors (mm),24-26 upper lip thickness (mm),27 lower lip thickness (mm),27 gingival display (mm),28 maxillary incisor position (mm),29-31 maxillary incisor inclination ( ),29-31 lip shape,32 smile arc,33 and lip line.34 However, research has not yet integrated all the aforementioned variables to evaluate the smile treatment outcome.35 The evaluation of orthodontic treatment on smile outcome variables is an important area of study because smile improvement is an important motivator for those who seek orthodontic treatment.36 Therefore, it is important to better understand how treatment type affects the smile outcome. Although traditional fixed appliances have been known as the gold standard of orthodontic treatment for a long time, the Invisalign system (Align Technology, Santa Clara, Calif) has recently emerged as an alternative to traditional braces. The Invisalign appliance is a commercially available medical device. Treatment with 357
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Invisalign involves custom-designing a number of vacuum-formed removable trays that, when applied according to the treatment protocol, shift and move the teeth. Since the company's initiation in 1997, the Invisalign system has been used to treat more than 6 million people. Moreover, Invisalign is now available internationally and is sought with increasing frequency.37-43 As more patients seek Invisalign treatment, the need for outcome comparison between fixed appliances and this alternative treatment modality becomes apparent. A few studies have compared the 2 treatment modalities, and no study has focused on smile evaluation yet. Thus, the purpose of the present study is 2-fold: to compare both treatment modalities while focusing on smile evaluation, and to incorporate all the aforementioned variables in an attempt to quantify smile evaluation. MATERIAL AND METHODS
The study was designed as a retrospective casecontrol study and was approved by the Institutional Review Board of the University of Alabama at Birmingham (UAB) (Protocol No. IRB-300000815). This study's protocol is in accordance with the statement by Strengthening the Reporting of Observational Studies in Epidemiology.44-46 Patient data were collected from the orthodontic department of UAB after generating 2 separate reports through Dolphin Software (Dolphin Imaging & Management Solutions, Chatsworth, Calif) with the words “Invisalign” and “ABO cases” used as search keys, respectively. A total of 453 Invisalign cases and 83 American Board of Orthodontics (ABO) cases (N 5 536) were retrieved from the computer search within the UAB clinical database. Two groups were established from these records. The Invisalign group comprised patients treated with Invisalign, and the fixed-appliance (ABO) group comprised patients treated with conventional fixed appliances. The following inclusion and exclusion criteria were then applied to the potential participants. Inclusion criteria (both groups): males and females aged 12-30 years; Class I molar classification; nonextraction treatment; minor crowding (1-4 mm) in each dental arch; treated in both dental arches; and pretreatment and posttreatment records available, including intraoral and extraoral composite photographs and lateral cephalometric radiographs taken within 6 months of initiation and completion of the treatment. An additional inclusion criterion for the ABO group was that patients should have presented to the ABO in St Louis and successfully passed the clinical examination during the past 3 years (2015-2018).
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An additional inclusion criterion for the Invisalign group was that treatment should have been completed solely with Invisalign clear aligners and patients should have had excellent treatment cooperation and strictly followed treatment instructions (eg, wear aligner 22.5 h/d). No filter on the period of recruitment was applied for this group. Exclusion criteria were as follows: craniofacial discrepancies or syndromes; previous orthognathic or cosmetic treatment; previous esthetic treatment of lips or previous traumas involving oral soft tissue; patients with incomplete or poor quality of records; and unnatural posed smiles47 (ie, a smile that is not reproducible, and therefore cannot be used as a reference for further measurements). From the 536 total cases identified, 58 met the preselected criteria, including 29 treated with Invisalign and 29 with fixed appliances (ABO). The study's strategy followed individual matching by having 1 control (ABO) per case (Invisalign).48 All eligible cases were exported from Dolphin Software database and imported into Adobe Photoshop 5.0 software for further analyses. Each frame was opened in Adobe Photoshop 5.0 and adjusted by using the millimeter ruler in the frame. Calibration of all images in the software was done according to Bray et al.17 Fifteen variables (12 continuous and 3 categorical) were used to assess the smile outcome. Patients were measured on all 15 variables at pretreatment and posttreatment. All variables with their definitions are presented in Table I. Patient demographics, including age, sex, and treatment duration, are included in Table II. The protocol used for the Invisalign group in this study's sample was such that patients changed their aligners every 2 weeks and were instructed to wear their aligners 22.5 hours per day. Ellipsoid, beveled, and rectangle-shaped attachments were used as needed. There was no filter on the period of recruitment for the Invisalign group during the initial search. The final group selected for inclusion in this study received treatment between the years 2012 and 2018. One faculty member planned and supervised all treatments. The protocol used for the ABO group in this study's sample was the Edgewise technique; McLaughlin, Bennett, and Trevisi prescription49; and brackets with 0.22-inch slots. With the exception of settling vertical elastics during the finishing stage of fixed-appliance treatment, no additional biomechanical appliances were used. For both groups, interproximal reduction (IPR) could have been used between the 6 mandibular anterior teeth (5 sites of IPR), if needed. In that case, 0.2 mm of IPR was performed on each side, equaling 0.1 mm of enamel
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Table I. Definitions of variables used as measurements in this study Variable Smile width (mm) Lip symmetry (%) Smile index (%) Smile cant ( ) Buccal corridor (%) Buccal corridor (mm) Upper lip thickness (mm) Lower lip thickness (mm) Gingival display (mm) Maxillary dental midline (mm) Maxillary incisor position (mm) Maxillary incisor inclination ( ) Lip shape Smile arc Lip line
Definition Linear distance between the most distal point of the last visible posterior tooth on the right and the most distal point of the last visible posterior tooth on the left The sum of the upper and lower left lip length divided by the upper and lower right lip length Smile width to interlabial gap ratio The angle between the interpupillary line and a line from the right to the left corner of the mouth The ratio of the intercommissure width divided by the distance from first premolar on right side to first premolar on left side Dark space (negative space) visible during smile formation between the corners of the mouth and the buccal surfaces of the maxillary teeth The vertical distance from the most superior peak of the lip to the most inferior portion of the tubercle of the upper lip The vertical distance from the deepest midline point on the superior margin of the lower lip to the most inferior portion of the lower lip Area of maxillary gingival exposure between the inferior border of the upper lip and the marginal gingiva of the maxillary teeth on the right side Refers to the vertical contact interface between 2 maxillary centrals Distance from upper incisor incisal point to N-A line The angle between maxillary incisor axial line and S-N line The relationship of the center of the upper lip to the corners of the mouth in smiling 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 amount of vertical tooth exposure in smiling
Table II. Demographics ABO group Variable Age, y Treatment duration, mo
Median 13.00 24.00
Mean (SD) 13.83 (2.05) 23.62 (3.41)
reduction on each side of each mandibular incisor and on the mesial side of the mandibular canines. Data analysis was performed by a nonorthodontic research assistant who was trained by an orthodontic faculty member. All measurements were taken twice by the same operator with a 2-month period difference. If the first and the second measurements did not match, the mean of the 2 was calculated and recorded as the final score. Subsequent statistical analyses were performed with Statistical Package for the Social Sciences software (version 24; IBM, Armonk, NY) and were run using the Mann-Whitney U test to determine if pretreatment and posttreatment outcome measurements differed significantly between both conditions (ie, Invisalign vs ABO) in patients. The Wilcoxon t test was used to learn whether significant differences existed in pretreatment and posttreatment variables within each group. RESULTS
Data from 58 patients (29 Invisalign and 29 ABO) were included in the study. Within the sample, 39 females and 19 males were included (Invisalign:
Invisalign group Range 10 13
Median 17.00 19.03
Mean (SD) 19.03 (5.49) 19.52 (6.49)
Range 17 23
P value 0.000 0.005
22 females, 7 males; ABO: 17 females, 12 males). The Invisalign group had an average pretreatment age of 19.3 years (standard deviation [SD] 5 5.49), whereas the ABO group's mean pretreatment age was 13.83 years (SD 5 2.05). The mean treatment times for the Invisalign and ABO patients were 19.52 6 6.49 months and 23.62 6 3.41 months, respectively. An alpha error of 0.05 was used as the level of statistical significance for all analyses. No statistically significant differences were found between the ABO and Invisalign groups at pretreatment for the 15 variables studied (P \0.05) (Table III). Invisalign and ABO posttreatment smile scores are presented in Table IV, along with exact P values and SDs. The smile scores within the ABO group demonstrated a statistically significant change from pretreatment to posttreatment for 9 of 12 continuous variables, including smile width (mean difference 5 15.37 mm), smile index (mean difference 5 10.54%), smile cant (mean difference 5 0.68 ), maxillary dental midline (mean difference 5 0.61 mm), buccal corridors (%) (mean difference 5 5.43%), buccal corridors (mm)
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Table III. Pretreatment smile scores of the ABO and Invisalign groups of patients ABO group Variable Smile width, mm Lip symmetry, % Smile index, % Smile cant, Maxillary dental midline, mm Buccal corridors, % Buccal corridors, mm Upper lip thickness, mm Lower lip thickness, mm Gingival display, mm Maxillary central incisor position, mm Maxillary central incisor inclination,
Median 47.1 0.9 5.03 0.89 0.60 21.91 13.13 4.93 7.75 1.15 4.10 103.0
Invisalign group
Mean (SD) 46.9 (4.64) 0.9 (0.08) 4.9 (1.15) 1.20 (1.11) 0.82 (0.61) 21.78 (7.03) 13.32 (0.29) 5.40 (2.05) 7.93 (1.66) 1.42 (1.45) 3.54 (3.27) 102.13 (9.49)
Median 47.17 0.99 5.02 0.87 0.57 22.43 13.92 5.18 8.23 1.32 4.23 104.42
Mean (SD) 47.04 (4.48) 0.99 (0.77) 5.06 (1.31) 1.11 (0.98) 0.76 (0.60) 22.83 (6.15) 13.96 (4.33) 5.42 (1.70) 8.23 (1.63) 1.32 (1.60) 4.23 (3.04) 104.42 (8.30)
P value 0.50 0.12 0.88 0.53 0.33 0.11 0.24 0.66 0.56 0.34 0.18 0.05
Table IV. Pretreatment and posttreatment scores within groups ABO group Variable Smile width, mm Lip symmetry, % Smile index, % Smile cant, Maxillary dental midline, mm Buccal corridors, % Buccal corridors, mm Upper lip thickness, mm Lower lip thickness, mm Gingival display, mm Maxillary incisor position, mm Maxillary incisor inclination,
IN mean (SD) 46.94 (4.64) 0.98 (0.08) 4.98 (1.15) 1.20 (1.11) 0.82 (0.61) 21.78 (7.03) 13.32 (4.94) 5.40 (2.05) 7.93 (1.66) 1.42 (1.45) 3.54 (3.27) 102.13 (9.49)
FN mean (SD) 52.31 (3.45) 0.98 (0.69) 5.52 (0.96) 0.52 (0.85) 0.21 (0.57) 16.35 (4.71) 10.35 (3.53) 4.98 (1.65) 8.11 (1.63) 2.17 (1.69) 5.33 (2.45) 106.77 (6.31)
Invisalign group Mean difference 15.37 0 10.54 0.68 0.61 5.43 2.97 0.42 10.18 10.75 11.79 14.64
P value 0.000 0.90 0.031 0.001 0.002 0.000 0.000 0.38 0.43 0.011 0.009 0.021
IN mean (SD) 47.04 (4.48) 0.99 (0.77) 5.06 (1.31) 1.11 (0.98) 0.76 (0.60) 22.83 (6.15) 13.96 (4.33) 5.42 (1.70) 8.23 (1.63) 1.32 (1.60) 4.23 (3.04) 104.42 (8.36)
FN mean (SD) 48.79 (4.53) 0.98 (0.60) 4.43 (0.93) 0.94 (0.74) 0.42 (0.38) 24.82 (6.45) 15.70 (4.65) 5.15 (1.36) 8.57 (1.74) 1.61 (1.87) 4.07 (2.48) 104.68 (4.98)
Mean difference 11.75 0.01 0.63 0.17 0.34 1.99 11.74 10.27 10.34 10.29 0.16 10.26
P value 0.030 0.10 0.003 0.57 0.012 0.52 0.22 0.10 0.80 0.15 0.09 0.031
IN, pretreatment; FN, posttreatment.
(mean difference 5 2.97 mm), gingival display (mean difference 5 10.75 mm), maxillary incisor position (mean difference 5 11.79 mm), and maxillary incisor inclination (mean difference 5 14.64 ). The changes in lip symmetry (mean difference 5 0%, P 5 0.91), upper lip thickness (mean difference 5 0.42 mm, P 5 0.39), and lower lip thickness (mean difference 5 10.18 mm, P 5 0.43) from pretreatment to posttreatment did not reach statistical significance. Conversely, in the Invisalign group, changes in 4 variables reached statistical significance: smile width (mean difference 5 11.75 mm), smile index (mean difference 5 0.63%), maxillary dental midline (mean difference 5 0.34 mm), and maxillary incisor inclination (mean difference 5 12.26 ). Again, the changes in lip symmetry (mean difference 5 0.01%, P 5 0.10), upper lip thickness (mean difference 5 10.27 mm, P 5 0.10), and lower lip thickness (mean difference 5 10.34 mm, P 5 0.80) did not reach statistical significance.
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The following pretreatment-to-posttreatment differences emerged as significantly different between the ABO and Invisalign groups (Table V): smile width (P \0.05, mean difference 5 13.52), smile index (P \0.05, mean difference 5 11.09), smile cant (P \0.05, mean difference 5 0.42), maxillary dental midline (U 5 288.00, P \0.05, mean difference 5 0.21), buccal corridors (%) (U 5 193.00, P \0.05, mean difference 5 8.42), buccal corridors (mm) (U 5 215.50, P \0.05, mean difference 5 5.35), gingival display (U 5 632.50, P \0.05, mean difference 5 0.56), maxillary incisor position (U 5 632.50, P\0.05, mean difference 5 1.26), and maxillary incisor inclination (U 5 621.50, P \0.05, mean difference 5 12.09). The differences between groups in lip symmetry (U 5 482.00, P 5 0.34), upper lip thickness (U 5 0.448.00, P 5 0.67), and lower lip thickness (U 5 470.50, P 5 0.44) did not reach significance. This study also sought to investigate changes between and within groups in smile arc, lip shape, and
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Table V. Pretreatment and posttreatment smile score difference comparison between the ABO and Invisalign groups ABO group Variable Smile width, mm Lip symmetry, % Smile index, % Smile cant, Maxillary dental midline, mm Buccal corridors, % Buccal corridors, mm Upper lip thickness, mm Lower lip thickness, mm Gingival display, mm Maxillary incisor position, mm Maxillary incisor inclination,
Median 52.13 0.08 5.49 0.40 0.00 16.45 10.00 4.68 7.69 1.95 5.40 106.20
Mean (SD) 52.31 (3.45) 0.98 (0.69) 5.52 (0.96) 0.52 (0.85) 0.21 (0.57) 16.35 (4.71) 10.35 (3.53) 4.98 (1.65) 8.11 (1.63) 2.17 (1.69) 5.33 (2.45) 106.77 (6.31)
Invisalign group Range 13.15 0.31 3.62 4.55 3.01 19.42 15.29 6.40 6.25 5.01 11.20 29.80
lip line variables, which are categorical and can each have 3 possible outcomes. Specifically, the smile arc can be reverse, flat, or consonant; lip shape can be upward, straight, or downward; and, finally, lip line can be either optimal or high. After using contingency tables to elucidate the number of patients in each possible category for each variable, the authors determined that a larger sample size would be needed to analyze the data for all possible conditions. Therefore, these variables were excluded from the study. DISCUSSION
The current study compared smile outcome evaluation for 2 different orthodontic treatment modalities— Invisalign and traditional fixed appliances. With regard to posttreatment outcomes of the 2 groups, there were significant differences in 9 of 12 outcome variables of interest, with mean differences trending in favor of the ABO group. In terms of treatment duration, our results indicated that Invisalign treatment duration was, on average, about 4 months shorter than ABO treatment duration (Table II), despite previous studies being suggestive of no differences between the 2 approaches.50 Li et al51 added nuance to previous studies by stating that Invisalign treatment is expected to be shorter only in nonextraction patients, as was the case for our sample. In patients who needed complex extractions, Invisalign treatment was 44% longer than ABO treatment.51 The ABO group outperformed the Invisalign group on 6 posttreatment variables that comprise smile outcome evaluation: acceptable values for buccal corridors in % and millimeters with ranges close to 17% and 11.5 mm.5 Our study showed that the ABO group was within the acceptable range with a mean value of 16.35% or 10.35 mm, whereas the Invisalign group was not within the acceptable range (mean 5 24.82% and 15.70 mm).
Median 48.17 1.01 4.45 0.94 0.28 23.17 14.88 4.78 8.65 0.80 4.00 105.20
Mean (SD) 48.79 (4.53) 0.98 (0.60) 4.43 (0.93) 0.94 (0.74) 0.42 (0.38) 24.82 (6.45) 15.70 (4.65) 5.15 (1.36) 8.57 (1.74) 1.61 (1.87) 4.07 (2.48) 104.68 (4.98)
Range 16.89 0.23 3.44 2.67 1.56 24.66 17.70 5.29 8.03 5.88 12.10 23.80
Mean difference 13.52 10.00 11.09 0.42 0.21 8.42 5.35 0.31 0.46 0.56 1.26 12.09
P value 0.00 0.33 0.00 0.02 0.03 0.00 0.01 0.66 0.43 0.01 0.01 0.02
Smile cant and maxillary dental midline should be 0 at the end of treatment, with cutoff values of 4 18-20 and 2.38 mm,5 respectively. In our study, smile cant for the Invisalign group (mean 5 0.94 ) was farther from 0 than smile cant for the ABO group (mean 5 0.52 ). In addition, maxillary dental midline improved by 74.4% of the initial value with our study's ABO group and only 45% with the Invisalign group. Perceptions of gingival display have been evaluated extensively in the past, and there is consensus that the range of acceptability is from 1.5 to 4.0 mm,5 with an ideal value of 2.1 mm. In our study, mean gingival display was 2.17 mm in the ABO group, whereas it was 1.61 mm in the Invisalign group. Several studies have agreed that a high smile index is ideal and that a lower smile index could add an “aging effect” to the smile.14 Two studies found the mean value of smile index to be 6.012%15 and 6.15% 6 1.31%16 with sample sizes of 100 and 50 patients, respectively. Even if there is not yet an agreed-on range that quantifies the ideal smile index, these results suggest that our ABO group is a closer match with these previous samples than the Invisalign group. In the Invisalign group, the mean smile index value (4.43%) was smaller than the mean smile index value in the ABO group (5.52%). The Invisalign group outperformed the ABO group on 2 posttreatment variables that comprise smile outcome evaluation. Regarding maxillary incisor position, the cephalometric analysis by Ricketts31 in 1961 showed the clinical norm for the maxillary incisor position to be 3.5 6 2.3 mm. In our study, maxillary incisors appeared to be less protrusive in the Invisalign group (mean 5 4.07 mm) than in the ABO group (mean 5 5.33 mm). In addition, according to the values presented by Bumann30 in 1994, the norm range of maxillary incisor
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inclination is 102 -105 . According to these guidelines, the maxillary incisor in the Invisalign group was positioned ideally (mean 5 104.68 ), whereas in the ABO group, it was slightly overproclined (mean 5 106.77 ). Both study groups performed comparably at posttreatment evaluation on smile width. A previous study that compared the linear distance between the center of facial axis of the most visible tooth points of the most visible tooth to the left and right of a nonextraction group found that smile width values from 46 to 52.6 mm seemed to remain stable at least 4 years after retention.11 Despite the fact that smile width value for the Invisalign group appeared to be about 4 mm smaller than that of the ABO group (mean difference 5 3.52 mm), both groups performed within the acceptable range. Some variables showed no significant differences from pretreatment to posttreatment for either of the 2 study groups. For example, lip symmetry and upper and lower lip thickness did not change. Several studies addressed the importance of lips in improving the overall smile,25,52-54 but thus far, little is known about acceptable threshold values that can serve as clinical guides. The study by Machado28 suggests that voluminous lips could significantly improve smile and lip esthetics and even make the patient look younger, whereas thin lips are negatively associated with the latter variables (ie, thin lips may have an aging effect).Therefore, the orthodontic treatment should be performed carefully to prevent a decrease in lip volume, especially in cases that involve teeth retraction. Furthermore, some patients will benefit from complementing orthodontic biomechanics with lip fillers for lip augmentation purposes. The sample size of this study was small and retrospective in nature; there were 58 patients who were equally distributed between Invisalign (n 5 29) and ABO (n 5 29) groups. Therefore, the results should be interpreted with caution. For example, our sample did not include all possible outcomes from each variable (eg, upward, straight, and downward for lip shape), and we were, thus, unable to representatively analyze the categorical parameters (smile arc, lip shape, and lip line).This limited sample also means that our study's results may not be generalizable to the population; studies with larger samples may include individuals that represent all possible outcomes for variables. Thus, this study may better serve as preliminary insight for higherpowered studies in the future. Because of the small number of cases that met the inclusion criteria for the study, the authors were unable to age-match cases. Thus, there was a statistically significant difference between the ages of the 2 groups. The mean age for the Invisalign group was, on average, 5 years greater than that of the ABO
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group (Table II). Despite this, Djeu et al55 found that statistically significant age discrepancies between groups do not have observable clinical effects on results. Vig et al,56 however, posited that age differences do indeed influence clinical results, especially when the clinical outcome being assessed is related to a soft-tissue parameter. Furthermore, approximately 75% of the Invisalign group identified as female, whereas only approximately 58% identified as such in the ABO group. As suggested by Vig et al,56 this may lead to sexual dimorphism bias. Owing to the retrospective nature of case-control studies, matching participants on age and sex is not always feasible. To avoid possible confounders, future studies using experimental data should match participants between both experimental groups on both age and sex. To assess the homogeneity of the comparison groups, given the inherent characteristic differences in our sample (ie, age and sex), pretreatment mean scores for all 12 variables assessed were obtained and were shown to be similar, suggesting that all participants' smiles were quantified similarly before treatment. CONCLUSIONS
The data suggest that orthodontic treatment with fixed appliances in patients with Class I nonextraction produces greater changes in the patient's smile than Invisalign treatment. Fixed appliances appear to be more effective in improving the variables that quantify posttreatment–smile outcome. However, Invisalign treatment may be of shorter duration than ABO treatment. Neither of the 2 treatment modalities have any significant effect on the lips at rest or on smiling. ACKNOWLEDGMENTS
The authors thank the American Association of Orthodontists Foundation for funding this project. In addition, T.C. would like to thank her American Association of Orthodontists Foundation mentors: Drs Chung How Kau, Peter Ngan, and Veerasathpurush Allareddy for their continuous support and guidance. REFERENCES 1. Miller AG. Role of physical attractiveness in impression formation. Psychon Sci 1970;19:241-3. 2. Howells DJ, Shaw WC. The validity and reliability of ratings of dental and facial attractiveness for epidemiologic use. Am J Orthod 1985;88:402-8. 3. Zhang K, Huang L, Yang L, Xu L, Xue C, Xiang Z, et al. Effects of transverse relationships between maxillary arch, mouth, and face on smile esthetics. Angle Orthod 2016;86:135-41.
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