Smile outcome comparison of Invisalign and traditional fixed-appliance treatment: A case-control study


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.


Records from 58 patients, 29 of whom received Invisalign treatment (mean age 19.03 years) and 29 of whom received traditional fixed-appliance 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.


Six variables within the fixed-appliance group presented with better smile scores than those within the Invisalign group; buccal corridors (%) (mean difference = 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 = 0.05) was shown between pretreatment and posttreatment scores for either of the 2 groups.


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.


  • Fixed appliances may be more effective in improving variables that quantify posttreatment smiles.

  • Neither treatment modality seems to change the lips at rest or on smiling.

  • Invisalign treatment may be of shorter duration than traditional fixed braces.

Throughout history, the smile has been an important determinant for others’ perception of social attractiveness, intelligence, and popularity. Nevertheless, smile evaluation is challenging, , and it has not been incorporated into the current grading systems used for orthodontic treatment outcome assessment.

Multiple studies have separately assessed 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 (°), lip shape, smile arc, and lip line. However, research has not yet integrated all the aforementioned variables to evaluate the smile treatment outcome.

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. 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 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.

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 case-control 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.

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 = 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).

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 smiles (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). 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.

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 .

Table I
Definitions of variables used as measurements in this study
Variable Definition
Smile width (mm) 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
Lip symmetry (%) The sum of the upper and lower left lip length divided by the upper and lower right lip length
Smile index (%) Smile width to interlabial gap ratio
Smile cant (°) The angle between the interpupillary line and a line from the right to the left corner of the mouth
Buccal corridor (%) The ratio of the intercommissure width divided by the distance from first premolar on right side to first premolar on left side
Buccal corridor (mm) Dark space (negative space) visible during smile formation between the corners of the mouth and the buccal surfaces of the maxillary teeth
Upper lip thickness (mm) The vertical distance from the most superior peak of the lip to the most inferior portion of the tubercle of the upper lip
Lower lip thickness (mm) 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
Gingival display (mm) 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
Maxillary dental midline (mm) Refers to the vertical contact interface between 2 maxillary centrals
Maxillary incisor position (mm) Distance from upper incisor incisal point to N-A line
Maxillary incisor inclination (°) The angle between maxillary incisor axial line and S-N line
Lip shape The relationship of the center of the upper lip to the corners of the mouth in smiling
Smile arc 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
Lip line The amount of vertical tooth exposure in smiling

Table II
Variable ABO group Invisalign group
Median Mean (SD) Range Median Mean (SD) Range P value
Age, y 13.00 13.83 (2.05) 10 17.00 19.03 (5.49) 17 0.000
Treatment duration, mo 24.00 23.62 (3.41) 13 19.03 19.52 (6.49) 23 0.005

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 prescription ; 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 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.


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: 22 females, 7 males; ABO: 17 females, 12 males). The Invisalign group had an average pretreatment age of 19.3 years (standard deviation [SD] = 5.49), whereas the ABO group’s mean pretreatment age was 13.83 years (SD = 2.05). The mean treatment times for the Invisalign and ABO patients were 19.52 ± 6.49 months and 23.62 ± 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 ).

Table III
Pretreatment smile scores of the ABO and Invisalign groups of patients
Variable ABO group Invisalign group P value
Median Mean (SD) Median Mean (SD)
Smile width, mm 47.1 46.9 (4.64) 47.17 47.04 (4.48) 0.50
Lip symmetry, % 0.9 0.9 (0.08) 0.99 0.99 (0.77) 0.12
Smile index, % 5.03 4.9 (1.15) 5.02 5.06 (1.31) 0.88
Smile cant, ° 0.89 1.20 (1.11) 0.87 1.11 (0.98) 0.53
Maxillary dental midline, mm 0.60 0.82 (0.61) 0.57 0.76 (0.60) 0.33
Buccal corridors, % 21.91 21.78 (7.03) 22.43 22.83 (6.15) 0.11
Buccal corridors, mm 13.13 13.32 (0.29) 13.92 13.96 (4.33) 0.24
Upper lip thickness, mm 4.93 5.40 (2.05) 5.18 5.42 (1.70) 0.66
Lower lip thickness, mm 7.75 7.93 (1.66) 8.23 8.23 (1.63) 0.56
Gingival display, mm 1.15 1.42 (1.45) 1.32 1.32 (1.60) 0.34
Maxillary central incisor position, mm 4.10 3.54 (3.27) 4.23 4.23 (3.04) 0.18
Maxillary central incisor inclination, ° 103.0 102.13 (9.49) 104.42 104.42 (8.30) 0.05

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 = +5.37 mm), smile index (mean difference = +0.54%), smile cant (mean difference = −0.68°), maxillary dental midline (mean difference = −0.61 mm), buccal corridors (%) (mean difference = −5.43%), buccal corridors (mm) (mean difference = −2.97 mm), gingival display (mean difference = +0.75 mm), maxillary incisor position (mean difference = +1.79 mm), and maxillary incisor inclination (mean difference = +4.64°). The changes in lip symmetry (mean difference = 0%, P = 0.91), upper lip thickness (mean difference = −0.42 mm, P = 0.39), and lower lip thickness (mean difference = +0.18 mm, P = 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 = +1.75 mm), smile index (mean difference = −0.63%), maxillary dental midline (mean difference = −0.34 mm), and maxillary incisor inclination (mean difference = +2.26°). Again, the changes in lip symmetry (mean difference = −0.01%, P = 0.10), upper lip thickness (mean difference = +0.27 mm, P = 0.10), and lower lip thickness (mean difference = +0.34 mm, P = 0.80) did not reach statistical significance.

Mar 9, 2020 | Posted by in Orthodontics | Comments Off on Smile outcome comparison of Invisalign and traditional fixed-appliance treatment: A case-control study
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