Introduction
The purpose of this retrospective case-control study was to compare the treatment effectiveness and efficiency of the Invisalign system with conventional fixed appliances in treating orthodontic patients with mild to moderate malocclusion in a graduate orthodontic clinic.
Methods
Using the peer assessment rating (PAR) index, we evaluated pretreatment and posttreatment records of 48 Invisalign patients and 48 fixed appliances patients. The 2 groups of patients were controlled for general characteristics and initial severity of malocclusion. We analyzed treatment outcome, duration, and improvement between the Invisalign and fixed appliances groups.
Results
The average pretreatment PAR scores (United Kingdom weighting) were 20.81 for Invisalign and 22.79 for fixed appliances ( P = 1.0000). Posttreatment weighted PAR scores between Invisalign and fixed appliances were not statistically different ( P = 0.7420). On average, the Invisalign patients finished 5.7 months faster than did those with fixed appliances ( P = 0.0040). The weighted PAR score reduction with treatment was not statistically different between the Invisalign and fixed appliances groups ( P = 0.4573). All patients in both groups had more than a 30% reduction in the PAR scores. Logistic regression analysis indicated that the odds of achieving “great improvement” in the Invisalign group were 0.329 times the odds of achieving “great improvement” in the fixed appliances group after controlling for age ( P = 0.0150).
Conclusions
Our data showed that both Invisalign and fixed appliances were able to improve the malocclusion. Invisalign patients finished treatment faster than did those with fixed appliances. However, it appears that Invisalign may not be as effective as fixed appliances in achieving “great improvement” in a malocclusion. This study might help clinicians to determine appropriate patients for Invisalign treatment.
Highlights
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Both Invisalign and fixed appliances improved mild to moderate nonextraction patients.
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Invisalign finished treatment faster than fixed appliances.
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Fixed appliances improved malocclusion more effectively than Invisalign.
The Invisalign system, introduced by Align Technology (Santa Clara, Calif) in 1999, involves moving teeth in increments with a series of removable clear polyurethane trays (aligners). Over the past few years, Align Technology has seen significant growth, with more than 3 million patients treated with Invisalign worldwide. Patients prefer Invisalign treatment over conventional fixed appliances because of its superior esthetics and comfort.
However, in the era of evidence-based dentistry, the scientific evidence on which to choose the treatment of more than 3 million patients is limited. The most recent systematic review of clear aligners only identified 11 relevant scientific articles. Of those, 6 were published more than 5 years ago, and no evidence-based conclusions can be drawn from those studies due to poor quality levels.
Randomized clinical trials have been conducted by a research group to evaluate the effects of aligner material stiffness and activation frequency on Invisalign treatment completion and outcome. The authors concluded that patients with a 2-week activation protocol, no extractions, and a low initial Peer Assessment Rating (PAR) score were more likely to complete their initial series of aligners. This study supports Align Technology’s 2-week activation time recommendation and also suggests that Invisalign is not suitable for extraction patients and those with complex treatment plans. Furthermore, the authors reported that fixed appliances will be needed in premolar extraction patients treated with aligners to correct dental tipping. They also concluded that the aligners were most successful in improving anterior alignment, transverse relationships, and overbite; moderately successful in improving midline and overjet; and least successful in improving buccal occlusion.
Two retrospective cohort studies compared the treatment results of Invisalign patients with those with fixed appliances using the American Board of Orthodontics objective grading system. The authors reported that Invisalign patients lost 13 more objective grading system points on average than did fixed appliances patients and achieved a passing rate 27% lower than for fixed appliances. The study indicated that Invisalign and fixed appliances are similar in correcting rotations, marginal ridge heights, space closure, and root alignment, but fixed appliances are superior in correcting occlusal contacts, posterior torque, and anteroposterior discrepancies. In their follow-up study on postretention dental changes of treated Invisalign patients, the authors reported that patients treated with Invisalign relapsed more than did those treated with fixed appliances, particularly in maxillary anterior alignment.
Many of those studies were conducted several years ago before Align Technology introduced changes to the tray material, attachments, and treatment algorithms. After 2008, improved technologies such as Precision Cuts, Precision Bite Ramps, and Smart Force Attachments led to innovations of Invisalign G3, G4, and G5 that resulted in a possibly wider range and more precision for tooth movement. It seems reasonable to reevaluate the effectiveness and efficiency of this popular treatment system after the introduction of what the company calls significant advances in materials. Therefore, well-designed clinical trials are needed to provide evidence for contemporary Invisalign treatment.
An assessment of orthodontic treatment outcomes with a quantitative index helps to establish goals, evaluate effectiveness, and achieve a measureable finish for completed patients. Several quantitative indexes have been developed to evaluate the malocclusion severity and orthodontic treatment need or treatment outcome. One index, the PAR, has been used widely for evaluating the effects of treatment in a variety of circumstances. The PAR is an occlusal index that not only measures how much a patient deviates from ideal occlusion, but also quantitatively evaluates orthodontic treatment outcomes by comparing pretreatment and posttreatment casts.
The purpose of this retrospective case-control study was to determine the effectiveness and efficiency of the Invisalign system compared with conventional fixed appliances in treating orthodontic patients with mild to moderate malocclusion using the PAR index. The specific aims were to compare patients treated with Invisalign and fixed appliances for (1) posttreatment PAR scores, (2) posttreatment reduction in PAR scores, (3) treatment duration, and (4) malocclusion improvement between Invisalign and fixed appliances patients after establishing 2 groups of Invisalign and fixed appliances patients with comparable pretreatment characteristics. See Supplemental Materials for a short video presentation about this study.
Material and methods
The study protocol was reviewed and approved by the Ohio State University Institutional Review Board.
The sample for this retrospective case-control study was selected from approximately 1500 conventional orthodontic patients and 250 Invisalign patients in the archives of the Division of Orthodontics at Ohio State University College of Dentistry. All patients were started and completed by orthodontic faculty and residents between 2009 and 2014. The patients were chosen without regard to their history or final treatment results. Selection was based on the following criteria: (1) available pretreatment and posttreatment records including digital models (OrthoCad) and photos; (2) age, 16 years or older when treatment started; (3) no auxiliary appliances other than elastics used during treatment; (4) no extraction patients; (5) no orthognathic surgery or syndromic patients; and (6) full permanent dentition except third molars. The only patients not included were those who were debonded early or terminated (Invisalign patients, because of compliance, hygiene, or transfer). After the initial review, 62 fixed appliances patients and 61 Invisalign patients met the criteria. To match the pretreatment malocclusions between the 2 groups and eliminate any early termination patients, 48 subjects from each group were selected. With a nondirectional alpha risk of 0.05 and an estimated standard deviation of 4.316, our power to detect a difference of ±3 units of weighted post-PAR score was 0.92.
Patients in the fixed appliances group were treated with fixed orthodontic appliances with various prescriptions, but all appliances were straight-wire edgewise appliances.
The PAR index (United Kingdom weighted PAR, which includes the mandibular anterior ) was used in this study to assess 8 components: maxillary anterior segment alignment, mandibular anterior segment alignment, anteroposterior discrepancy, transverse discrepancy, vertical discrepancy, overjet, overbite, and midline.
Digital models were used to determine the PAR scores. Previous research has demonstrated that PAR index scores derived from digital models are valid and reliable measures of malocclusion. One investigator (J.S.T.) was trained and calibrated for the PAR index and performed all the PAR measurements. This investigator was blinded to the group assignment (Invisalign or fixed appliances) to which the models belonged. Intraexaminer reliability was assessed with intraclass correlation coefficients determined by duplicate scoring of 12 randomly selected subjects from each category (total of 24) 2 months after initial data collection.
In their original article to evaluate the PAR index, Richmond et al determined that “at least a 30 percent reduction in PAR score was required for a case to be considered as improved and a change of 22 PAR points brought about great improvement.” However, not every patient starts with s PAR score above 22. To include all subjects, we redefined “great improvement” as either (1)weighted PAR score reduction of 22 points or more or (2), if the initial PAR score was less than 22, a weighted PAR score after treatment equal to 0.
Statistical analysis
All statistical analyses were performed with software (version 9.3 SAS, Cary, NC). Subject characteristics were compared for the 2 treatment groups using chi-square or Fisher exact tests for categorical variables and randomization tests for continuous variables. Multiple comparisons were adjusted using the step-down Bonferroni method of Holm. The Fisher exact test was also used to compare the distribution of percent reduction of weighted PAR scores between the 2 groups.
Logistic regression was used to evaluate differences in great improvement outcome due to treatment group after controlling for age. The level of statistical significance for all analyses was set at α = 0.05.
Results
The calibrated examiner demonstrated good intraexaminer reliability. (The intraclass correlation coefficient scores ranged from 0.66 for posttreatment overbite to ≥0.98 for the remaining variables.)
The basic sample description and characteristics are presented in Table I . The Invisalign group had a mean pretreatment age of 26.0 ± 9.7 years (average ± standard deviation), and the fixed appliances group had a pretreatment age of 22.1 ± 7.9 years. This difference was statistically significant ( P = 0.0374). The differences between the 2 groups for the following variables were not statistically different: sex, race/ethnicity, percentage of Class I molar patients, or percentage of retreatment patients (previous orthodontic treatment) ( P >0.05; Table I ).
| Variable | Treatment groups | |||
|---|---|---|---|---|
| All | Invisalign | Fixed appliances | P value | |
| (n = 96) | (n = 48) | (n = 48) | ||
| Age (y) | 24.0 (SD, 9.0) | 26.0 (SD, 9.7) | 22.1 (SD, 7.9) | 0.0374 ∗ |
| Sex | 0.6695 | |||
| Female | 62 (65%) | 32 (67%) | 30 (63%) | |
| Male | 34 (35%) | 16 (33%) | 18 (37%) | |
| Race/ethnicity | 0.8813 | |||
| African American | 16 (17%) | 7 (15%) | 9 (19%) | |
| Asian | 4 (4%) | 2 (4%) | 2 (4%) | |
| White | 73 (76%) | 38 (79%) | 35 (73%) | |
| Hispanic | 3 (3%) | 1 (2%) | 2 (4%) | |
| Class I molars | 65 (68%) | 35 (73%) | 30 (63%) | 0.2751 |
| Retreatment | 7 (7%) | 4 (8%) | 3 (6%) | 1.0000 |
The mean weighted pretreatment PAR scores between the Invisalign and fixed appliances groups were not statistically different ( P = 1.0000; Table II ). There were no statistically significant differences between the 2 groups for the 8 individual components of pretreatment PAR scores ( Table II ).
| Variable | Invisalign (n = 48) | Fixed appliances (n = 48) | P value | ||||||
|---|---|---|---|---|---|---|---|---|---|
| Mean | SD | Minimum | Maximum | Mean | SD | Minimum | Maximum | ||
| UANT | 5.25 | 2.07 | 1 | 10 | 6.23 | 2.09 | 1 | 11 | 0.2313 |
| LANT | 4.65 | 2.41 | 0 | 10 | 5.13 | 2.45 | 0 | 9 | 1.0000 |
| AP | 2.48 | 1.34 | 0 | 4 | 2.44 | 1.29 | 0 | 4 | 1.0000 |
| TRANS | 0.17 | 0.66 | 0 | 3 | 0.33 | 0.81 | 0 | 3 | 1.0000 |
| VERT | 0.02 | 0.14 | 0 | 1 | 0 | 0 | 0 | 0 | 1.0000 |
| OJ | 5.75 | 5.25 | 0 | 24 | 4.88 | 6.27 | 0 | 24 | 1.0000 |
| OB | 1.92 | 1.65 | 0 | 8 | 2.54 | 2.36 | 0 | 10 | 1.0000 |
| MID | 0.58 | 1.65 | 0 | 8 | 1.25 | 2.05 | 0 | 8 | 1.0000 |
| PREWPAR | 20.81 | 6.79 | 9 | 43 | 22.79 | 7.72 | 9 | 38 | 1.0000 |
After treatment, weighted PAR scores for both groups were less than 5 and did not differ significantly ( P = 0.7420; Table III ). None of the 8 individual components of posttreatment PAR scores differed significantly between the groups ( Table III ).
| Variable | Invisalign (n = 48) | Fixed appliances (n = 48) | P value | ||||||
|---|---|---|---|---|---|---|---|---|---|
| Mean | SD | Minimum | Maximum | Mean | SD | Minimum | Maximum | ||
| PUANT | 0.44 | 0.94 | 0 | 5 | 0.42 | 0.79 | 0 | 3 | 1.0000 |
| PLANT | 0.04 | 0.2 | 0 | 1 | 0.08 | 0.35 | 0 | 2 | 1.0000 |
| PAP | 2.06 | 1.29 | 0 | 4 | 1.69 | 1.27 | 0 | 4 | 1.0000 |
| PTRANS | 0.06 | 0.43 | 0 | 3 | 0.06 | 0.43 | 0 | 3 | 1.0000 |
| PVERT | 0 | 0 | 0 | 0 | 0.02 | 0.14 | 0 | 1 | 1.0000 |
| POJ | 0.75 | 2.94 | 0 | 12 | 0 | 0 | 0 | 0 | 1.0000 |
| POB | 0.56 | 0.9 | 0 | 2 | 0.17 | 0.56 | 0 | 2 | 0.3060 |
| PMID | 0.17 | 0.81 | 0 | 4 | 0.25 | 0.98 | 0 | 4 | 1.0000 |
| POSTWPAR | 4.08 | 4.35 | 0 | 19 | 2.69 | 2.23 | 0 | 9 | 0.7420 |
Neither of the weighted PAR reduction scores after treatment or the reduction scores of the 8 individual PAR components was statistically different between the 2 groups ( Table IV ). The treatment time for the Invisalign group (13.35 months) was significantly shorter than that for the fixed appliances group (19.08 months) ( P = 0.0040; Table IV ). To compare the treatment efficiency of the groups, we evaluated the weighted PAR reduction per month. The Invisalign group was not significantly different compared with the fixed appliances group in reducing the weighted PAR score per month of treatment ( P = 0.2318; Table IV ).
| Variable | Invisalign (n = 48) | Fixed appliances (n = 48) | P value | ||||||
|---|---|---|---|---|---|---|---|---|---|
| Mean | SD | Minimum | Maximum | Mean | SD | Minimum | Maximum | ||
| ΔWPAR | 16.73 | 6.78 | 5 | 38 | 20.1 | 8.06 | 7 | 38 | 0.4573 |
| MOS | 13.35 | 8.63 | 4 | 48 | 19.08 | 5.92 | 6 | 31 | 0.0040 ∗ |
| ΔWPAR/MOS | 1.55 | 0.8 | 0.25 | 3.5 | 1.16 | 0.69 | 0.43 | 4.33 | 0.2318 |
| ΔUANT | 4.81 | 2.09 | 1 | 10 | 5.81 | 2.21 | 1 | 11 | 0.4573 |
| ΔLANT | 4.6 | 2.44 | 0 | 10 | 5.04 | 2.42 | 0 | 9 | 1.0000 |
| ΔAP | 0.42 | 1.18 | −2 | 3 | 0.75 | 1.34 | −2 | 4 | 1.0000 |
| ΔTRANS | 0.1 | 0.52 | 0 | 3 | 0.27 | 0.71 | 0 | 3 | 1.0000 |
| ΔVERT | 0.02 | 0.14 | 0 | 1 | −0.02 | 0.14 | −1 | 0 | 1.0000 |
| ΔOJ | 5 | 4.85 | 0 | 24 | 4.88 | 6.27 | 0 | 24 | 1.0000 |
| ΔOB | 1.35 | 1.76 | −2 | 8 | 2.38 | 2.53 | −2 | 10 | 0.4573 |
| ΔMID | 0.42 | 1.7 | −4 | 8 | 1 | 2.1 | −4 | 8 | 1.0000 |
| Improved | 48/48 (100%) | 48/48 (100%) | 1.0000 | ||||||
| Great improvement | 11/48 (22.9%) | 22/48 (45.8%) | 0.0150 ∗ | ||||||
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