Section III: Evidence

Outcome Assessment and Evidence on the Clinical Performance of Orthodontic Aligners

Spyridon N. Papageorgiou and Theodore Eliades

Summary

In the last decade several systematic reviews of clinical studies comparing orthodontic aligners with fixed appliances have emerged. However, they all present methodological issues that can introduce bias and hamper their ability to draw robust evidence-based recommendations, including among others: assessment of oral hygiene but not efficacy, lack of an a priori design/pre-registered protocol, language bias, inclusion of nonrandomized studies with uncontrolled confounding, inadequate handing of the studies’ risk of bias, lack of quantitative data synthesis (meta-analysis), improper data synthesis methods, and being outdated. This chapter presents a critical appraisal on the clinical performance of orthodontic aligners based on currently available studies founded on the principles of evidence-based medicine. According to currently existing clinical evidence from randomized trials and matched nonrandomized studies on mostly adult patients with mild to severe malocclusions treated with or without extractions it seems that orthodontic treatment with aligners is associated with inferior treatment outcome compared to fixed appliances. Treatment duration is not directly influenced by choice of appliance alone and patient-related or treatment-related factors might come into play.

Background

The use of sequential clear aligners has seen a remarkable surge in the last decades and, following considerable technical developments, has been widely adopted by both orthodontic specialists and general dentists alike. Fueled by aggressive marketing campaigns from various manufacturers of aligner systems, a growing interest has been reported, especially among adult patients, for such methods of invisible orthodontics.1 ,​ 2 A survey of Australian orthodontists in 2013 indicated that 73% of responders had used aligners to treat at least one case in the last year, with a median of 8 aligner cases/year.3 A similar survey among Irish orthodontists in 2014 reported that 19% of them often used aligners to treat adult patients.4 A large 2014 survey among orthodontist in the United States5 revealed that 89% of them had treated at least one case with aligners (compared to 76% in 2008) and treated a median of 22 cases/year with aligners (compared to 12 cases/year in 2008). Responding orthodontists who used aligners employed them in a variety of cases, with the most common diagnostic category for aligner treatment being: Class I with moderate crowding (94%), space closure (78%), Class II (68%), lower incisor extraction (47%), Class I with severe crowding (37%), and Class III (49%), while only few orthodontists used aligners for premolar extraction cases (9–18%). Interestingly, responders in 2014 considered 90% of their aligner cases successful (compared to 80% in 2008), but also saw about 10% of aligner cases with relapse (same as in 2008). Additionally, another survey among members of the European Aligner Society indicated that 45% of orthodontists believed that aligners limit orthodontic treatment outcomes (even though the respective percentage among general dentists was only 5%).6 These data might indicate that the initial surge of aligner treatment during its early years of fame might have now given its place to a more mature evaluation of this treatment modality, based on long-term evaluations of previously treated patients.

In any case, it is imperative that any treatment modality offered to orthodontic patients as an alternative is based on both the doctor’s clinical expertise and solid evidence on the clinical performance on this modality. Unfortunately, contrary to many medical fields, it is commonly seen in orthodontics that novel marketed products and treatment approaches are clinically adopted based on advertisement without the appropriate clinical evidence to back any claims made by the manufacturers.7 ,​ 8 Good clinical practice, however, obligates that any treatment decision between the treating orthodontist and the patient is done after meticulous discussion of all available treatment options and evidence-based notions about their efficacy and adverse effects. Ideally, these should be based on well-designed and well-reported comparative clinical trials on human patients and systematic reviews/meta-analyses thereof.9 ,​ 10 Ample empirical evidence has now been gathered about the importance of proper study design and the role that various methodological characteristics can play in introducing bias.11 17

In the last decade, several systematic reviews of clinical studies comparing orthodontic aligners with fixed appliances have emerged.18 27 However, they all present methodological issues that can introduce bias and hamper their ability to draw robust evidence-based recommendations, including among others: assessment of oral hygiene but not efficacy,18 ,​ 23 ,​ 24 lack of an a priori design/preregistered protocol,19 22 ,​ 26 ,​ 27 language bias,20 ,​ 22 ,​ 25 inclusion of nonrandomized studies with uncontrolled confounding,19 ,​ 20 ,​ 22 ,​ 25 27 inadequate handing of the studies’ risk of bias,19 22 ,​ 25 27 lack of quantitative data synthesis (meta-analysis),19 ,​ 20 ,​ 22 ,​ 25 ,​ 27 outdated data synthesis methods,21 ,​ 26 and outdated literature searches.19 21 Therefore, clinical practice ought to be informed by a critical appraisal of currently available studies according to the principles of evidence-based medicine.

Appraisal of Evidence from Existing Clinical Studies

To this end, a systematic review was designed a priori based on the Cochrane guidelines,28 registered in PROSPERO (CRD42019131589), and is reported according to the PRISMA statement.29 Eight databases (MEDLINE through PubMed, Cochrane Database of Systematic Reviews, Cochrane Central Register of Controlled Trials, Cochrane Database of Abstracts of Reviews of Effects, Scopus, Virtual Health Library, and Web of Knowledge) were searched up to April 25, 2019, without any restrictions for publication date, language, or type with the following search strategy: (orthodon* OR malocclusion* OR “tooth movement” OR “fixed appliances”) AND (aligner* OR “clear aligner” OR “clear aligners” OR “ClearCorrect” OR “Invisalign” OR “Orthocaps” OR “TwinAligner”).

Eligible for inclusion were randomized trials comparing adolescent/adult patients with any kind of malocclusion receiving full-arch comprehensive treatment with either orthodontic aligners or any kind of fixed appliances. Due to the scarcity of randomized trials on the subject, nonrandomized studies were also included, with the requirement that the populations to be compared were matched regarding baseline malocclusion severity with objective measures such as the Peer Assessment Rating (PAR) index30 or the Discrepancy Index (DI)31 from the American Board of Orthodontics (ABO). The PAR index uses seven criteria: tooth alignment (referring to dental crowding), right and left buccal segment relationship (sagittal, vertical, and transverse assessments), overjet, overbite, and centerline (midline discrepancies). Each difference from the norm has points attributed to the severity of the discrepancy. Once tabulated and weighted according to the United States or United Kingdom weightings, an overall score for the malocclusion is calculated. The ABO DI scores 12 target disorders: overjet, overbite, anterior open bite, lateral open bite, crowding, occlusal relationship, lingual posterior cross-bite, buccal posterior cross-bite, ANB angle, mandibular plane inclination, lower incisor inclination, and a category “other” that includes complexes such as Bolton discrepancy, shortened roots, deep curve of Spee, traumatic injuries, bimaxillary protrusion cases with critical anchorage need, and craniofacial dysmorphologies. Similarly to the PAR index, scores are assigned and tabulated to reflect the malocclusion severity. Matching was judged adequate when the Cohen’s d for PAR or ABO DI between aligner and fixed appliance group at baseline was up to 0.3. The primary outcome for this review was the outcome of comprehensive orthodontic treatment judged with objective and reliable measures such as the PAR index and the ABO’s Objective Grading System (ABO-OGS) for dental casts and panoramic radiographs.32 The ABO-OGS rates the final occlusion after appliance removal with eight criteria that contribute to ideal intercuspation and function: alignment, marginal ridges, buccolingual inclination, overjet, occlusal contacts, occlusal relationships, interproximal contacts, and root angulation. Best occlusion and alignment receive a score of 0 points, while for each parameter that deviates from the ideal, 1 or 2 penalty points are added. The greater the posttreatment ABO-OGS score, the more the final treatment result deviates from ideal occlusion, while a case can also be classified as “successful” or “failed” according to their ABO criteria for score lower or higher than 30 points. Secondary outcomes included treatment duration, as well as adverse effects such as loss of periodontal support, external apical root resorption (EARR), gingival recession, and uncontrolled proclination of the lower incisors during treatment.

Study selection, data extraction, and risk of bias assessment were performed by three independent assessors. The risk of bias of included studies was assessed according to Cochrane guidelines with the RoB 2.0 tool for randomized trials33 and the ROBINS-I (“Risk Of Bias In Nonrandomised Studies-of Interventions”) tool for nonrandomized studies.34 Mean differences (MDs) for continuous outcomes and relative risks (RRs) for binary outcomes and their corresponding 95% confidence intervals (CIs) were pooled with random-effects meta-analysis (using a restricted maximum likelihood variance estimator35), with p < 0.05 considered significant, and presented in contour-enhanced forest plots.36 Relative/absolute heterogeneity was assessed with I 2 and tau,2 respectively, and incorporated into random-effects 95% predictions to quantify expected treatment effects in a future clinical setting.37 The overall quality of clinical recommendations (confidence in effects estimates) for each of the main outcomes was rated using the Grades of Recommendation, Assessment, Development, and Evaluation (GRADE) approach38 using an improved Summary of Findings table format39 and guidance on how to combine randomized and nonrandomized studies.40

Characteristics of Existing Clinical Studies Comparing Aligners to Fixed Appliances

The electronic literature search up to April 2019 yielded 1,376 hits, while 7 additional studies were manually identified through checking of the reference or citation lists of identified reports (Fig. 9.1). After applying the review’s eligibility criteria, 11 publications pertaining to 11 unique studies (4 randomized and 7 retrospective nonrandomized)41 51 published as journal papers or dissertation/theses were included (Table 9.1). The included studies were conducted in university clinics (n = 6; 55%), private practices (n = 4; 36%), or hospitals (n = 1; 9%) and originated from six different countries (Canada, China, Ireland, Italy, South Korea, and the United States). A total of 446 and 443 patients were treated with aligners and fixed appliances, respectively, with a median total sample of 66 patients per included study (range 19–200 patients per study). Out of the 7 studies reporting on patient sex, 215 of the 661 patients in total were male (33%), while the mean patient age out of the 9 studies reporting this was 28.0 years.

No Image Available!

Fig. 9.1 PRISMA flow diagram for the identification and selection of eligible studies in this review.

Table 9.1 Characteristics of included studies

Study

Design; setting; countrya

Patients (M/F); ageb

Malocclusion/Tx

Appliance

Aligners/refinement/IPR

FU (mo)

Outcome

Abbate 2015

RCT; Uni; ITA

AL: 25 (NR); (10–18)

FX: 22 (NR); (10–18)

Non-Ex

AL: Invisalign

FX: Labial CLB

NR/NR/NR

BL, 3.0, 6.0, 9.0, 12.0 mo in Tx

PPD

Djeu 2005

rNRS; Pract; USA

AL: 48 (NR); 33.6

FX: 48 (NR); 23.7

DI: 19.3; Ex/Non-Ex

AL: Invisalign

FX: Labial CLB (TE)

NR/NR/allowed

BL, END

ABO-OGS8; TxDur

Fetouh 2008

rNRS; Pract; USA

AL: 33 (NR); NR

FX: 33 (NR); NR

Mild crowding; DI: 3.22/Non-Ex

AL: Invisalign

FX: Labial CLB

NR/NR/NR

BL, END

ABO-OGS7

Gu 2017

rNRS; Pract; USA

AL: 48 (16/32); 26.0

FX: 48 (18/30); 22.1

PAR: 21.8; compliant/Non-Ex

AL: Invisalign

FX: Labial CLB (SW)

NR/38%/NR

BL, END

PAR; TxDur

Han 2015

rNRS; Uni; KOR

AL: 10 (NR); 51.2
FX: 9 (NR); 47.3

Previous PerioDis; DI: 4.4/Non-Ex

AL: NR

FX: Labial CLB

NR/NR/allowed

BL, END

PB; ABL TxDur

Hennessy 2016

RCT; Hosp; IRL

AL: 20 (6/14); 29.1

FX: 20 (7/13); 23.7

Mild crowding/Non-Ex

AL: Invisalign

FX: Labial SLB (MBT)

18 ALs/allowed/AL:FX 1.9:1.5

BL, END

IMPA; TxDur

Lanteri 2018

rNRS; Pract; ITA

AL: 100 (30/70); 28.0

FX: 100 (30/70); 25.0

PAR: 23.3/Non-Ex

AL: Invisalign

FX: Labial SLB (MBT)

43 ALsc/37%/AL:FX 1.3:1.5

BL, END, 24.0 mo Post-Tx

PAR; RetFail; GingRec

Li 2015

RCT; Uni; CHN

AL: 76 (27/45); 35.2

FX: 76 (27/45); 32.2

DI: 27.4/Ex

AL: Invisalign

FX: Labial CLB

NR/NR/allowed (AL)

BL, END

ABO-OGS8; TxDur

Preston 2017

RCT; Uni; USA

AL: 22 (10/12); 27.8

FX: 22 (7/15); 25.4

Mild crowding/Non-Ex

AL: Invisalign

FX: Labial CLB (ALX)

100% (2 refinements)

BL, END, 1.0, 6.0 mo Post-Tx

ABO-OGS2; TxDur; contact areas

Robitaille 2016

rNRS; Uni; CAN

AL: 24 (11/13); 29.8

FX: 25 (6/19); 23.4

DI: 31.5/orthognathic surgery

AL: Invisalign

FX: Labial CLB

NR/NR/NR

BL, END

ABO-OGS8; TxDur

Yi 2018

rNRS; Uni; CHN

AL: 40 (9/31); 21.8

FX: 40 (11/29); 23.3

PAR: 22.6/Non-Ex

AL: NR

FX: Labial CLB

NR/65%/NR

BL, END

PAR; TxDur; EARR

Abbreviations: ABL, alveolar bone level; ABO-OGS, American Board of Orthodontics Objective Grading System (number of components assessed given in subscript); AL, aligner; ALX, Alexander technique; BL, baseline; CLB, conventionally ligated brackets; DI, discrepancy index; EARR, external apical root resorption; END, end of comprehensive treatment; Ex, extraction; FU, follow-up; FX, fixed appliance; GingRec, gingival recession; Hosp, hospital; IMPA, inclination of lower incisors to mandibular plane; IPR, interproximal enamel reduction; M/F, male/female; MBT, MacLaughlin-Bennet-Trevisi prescription; mo, month; NR, not reported; PAR, peer assessment rating; PPD, periodontal probing depth; Pract, private practice/clinic; RCT, randomized clinical trial; SLB, self-ligating bracket; SW, straightwire; TE, Tip-Edge; Tx, treatment; TxDur, treatment duration; Uni, university clinic.

aCountries given with their alpha-3 codes.

bPatient age is given either as mean (one value in without parenthesis) or, if mean is not reported, as range (two values in parenthesis).

cIncluding refinement aligners

As far as complexity of the treated cases is concerned, only six studies (55%) reported this with either the PAR index (n = 3; 27%) or the ABO DI (n = 3; 27%). Eight of the studies (73%) performed nonextraction treatment, one study (9%) both extraction and nonextraction treatment, and one study (9%) extraction treatment. The majority of studies (9/11 studies; 82%) reported on conventional comprehensive treatment, while 1 study (9%) reported on orthodontic treatment of patients with history of periodontal disease and 1 study (9%) reported on combined orthodontic/orthognathic treatment. Details of the aligner treatment were only partly reported among the included studies, with only two studies (18%) reporting the number of aligners, four studies (36%) reporting on “refinement” rate (i.e., the midcourse re-evaluation and planning of additional aligners), and two studies (18%) on the actual amount of interproximal enamel reduction performed during treatment in both groups.

The included randomized trials presented several issues that increased their risk for bias (Table 9.2). Two trials were in high risk of bias due to problems in the randomization process, deviations from intended interventions, missing outcome data, and outcome measurement. The remaining two trials were in low risk of bias, except for the fact that no a priori trial protocol could be found to rule out selective reporting. The included nonrandomized studies were in considerably higher risk of bias, with five of them presenting moderate risk of bias, one of them serious risk of bias, and one of them critical risk of bias (Table 9.3). Their main shortcomings pertained to confounding, selection of participants into the study, deviations from intended interventions, outcome measurement, and selection of the reported result.

Table 9.2 Risk of bias of included randomized clinical trials with the RoB 2.0 tool

Study

Randomization process

Deviations from intended interventions

Mising outcome data

Measurement of the outcome

Selection of the reported result

Overall

Comments

Abbate 2015

High

High

Low

Low

Some concerns

High

Additionally, incomplete reporting is seen for all continuous outcomes

Hennessy 2016

Low

High

High

High

Some concerns

High

Also, incomplete reporting of treatment duration

Li 2015

Low

Low

Low

Low

Some concerns

Some concerns

Incomplete reporting for treatment duration, due to missing standard deviations, but these were provided by the authors after contacting them

Preston 2017

Low

Low

Low

Low

Some concerns

Some concerns

Table 9.3 Risk of bias of included nonrandomized studies with the ROBINS-I tool

Bias due to/in

Confounding

Selection of participants into the study

Classification of interventions

Deviations from intended interventions

Missing data

Measurement of outcomes

Selection of the reported result

Overall

Djeu 2005

Moderate

NI

Low

NI

NI

Moderate

Low

Moderate

Fetouh 2008

Moderate

NI

Low

NI

NI

Moderate

Low

Moderate

Gu 2017

Moderate

Critical

Low

NI

NI

Low

Low

Critical

Han 2015

Moderate

NI

Low

NI

NI

Low

Moderate

Moderate

Lanteri 2018

Moderate

NI

Low

Low

NI

Moderate

Low

Moderate

Robitaille 2016

Moderate

NI

Low

NI

NI

Moderate

Low

Moderate

Yi 2018

Moderate

NI

Low

Serious

NI

Low

Moderate

Serious

Abbreviation: NI, no information.

The included studies reported on a wide spectrum of treatment outcomes, with only three studies41 ,​ 45 ,​ 47 reporting on the complete ABO-OGS score including all eight components, as well as failure of the case to pass the ABO criteria for adequate occlusal results (ABO-OGS score < 30 points). One study reported on the ABO-OGS score of seven out of eight components (excluding root angulation)42 and also excluded scoring the second molars without any justification. One study also reported solely on two of the eight ABO-OGS components49—namely, marginal ridges and buccolingual inclination. Three studies used the PAR index48 ,​ 50 ,​ 51 and reported either posttreatment PAR scores or PAR reductions. Eight studies reported on treatment duration,41 ,​ 46 51 though considerable variation in the reported results was seen. Finally, single studies reported on periodontal probing depth, alveolar bone loss, EARR, lower incisor inclination, and gingival recessions.

Treatment Efficacy in Terms of Occlusal Outcome

The ABO-OGS enables the objective and precise evaluation of the outcome of comprehensive orthodontic treatment including the fine details of a balanced ideal occlusion. The meta-analysis combining the results of the three existing studies measuring the total ABO-OGS score at debond is presented in Fig. 9.2. On average, meta-analysis of three studies indicated that orthodontic treatment with aligners was associated with a statistically significant reduction in the finishing quality according to ABO-OGS compared to fixed appliances (MD: 9.9 points; 95% CI: 3.6–16.2 points; p = 0.002). Considerable heterogeneity was seen among the three included studies (I 2 = 84%), which meant that several patient- or treatment-related factors might play a role in the actual final occlusal result. However, existing heterogeneity influenced only the precise calculation of the difference between aligners and fixed appliances, as one study indicated a moderate difference and the other two indicated a large one. It did not, however, influence the direction of the effect, as all three studies showed that fixed appliances were significantly associated with better treatment results than aligners.

No Image Available!

Fig. 9.2 Contour-enhanced forest plot on the comparison of total ABO-OGS scores posttreatment between aligners and fixed appliances. ABO-OGS, American Board of Orthodontics Objective Grading System; AL, aligner; CI, confidence interval; FX, fixed appliance; M, mean; MD, mean difference; N, number of patients; SD, standard deviation. Contours correspond to different effect magnitude and the red dotted line corresponds to 95% random-effects prediction.

The same conclusion was drawn when looking at the proportion of cases being finished to an acceptable quality according to the ABO standards—i.e., the proportion of patients having less than 30 ABO-OGS points at debond (Fig. 9.3). Meta-analysis of three studies indicated that treatment with aligners was associated with significantly increased probability for the case to be considered a “failure” according to the ABO standards (i.e., that the case has an ABO-OGS score more than 30) compared to fixed appliances (RR: 1.6; 95% CI: 1.2–2.0; p < 0.001). No considerable heterogeneity across studies was seen, which reported a small to moderate increase in the rate of suboptimal finishing quality. On absolute terms, this is translated to a 60.6% “failing” grade according to ABO for the aligner group compared to a 38.9% rate for the fixed appliance group (Fig. 9.4). This is in turn translated to a number needed to treat of 5, which means that every fifth case treated with aligners instead of fixed appliances would fail the ABO examination, but would get a “passing” grade if it was treated with fixed appliances, which is a potentially clinically relevant effect.

No Image Available!

Fig. 9.3 Contour-enhanced forest plot on the comparison of proportion of “passing” cases according to the ABO examination (cases with ABO-OGS score lower than 30 points) posttreatment between aligners and fixed appliances. ABO-OGS, American Board of Orthodontics Objective Grading System; AL, aligner; CI, confidence interval; FX, fixed appliance; N, number of patients; RR, relative risk. Contours correspond to different effect magnitude and the red dotted line corresponds to 95% random-effects prediction.

No Image Available!

Fig. 9.4 Illustration of the expected absolute risk for a case to have an ABO-OGS score of over 30 post debond when treated with aligners or fixed appliances, according to the results of the meta-analysis. ABO, American Board of Orthodontics.

Looking at the comparative performance for each separate component of ABO-OGS between aligners and fixed appliances gives a more precise image about the occlusal aspects mostly affected by the treatment modality (Fig. 9.5). Overall, meta-analyses of three studies indicated that five of the eight aspects of the occlusion were significantly better finished with fixed appliances than with aligners: buccolingual inclination (MD: 0.8 point; 95% CI: 0.5–1.1 point; p < 0.001), occlusal contacts (MD: 3.1 points; 95% CI: 0.6–5.6 points; p = 0.02), occlusal relationship (MD: 1.0 point; 95% CI: 0.6–1.4 points; p < 0.001), overjet (MD: 1.8 points; 95% CI: 0.6–3.0 points; p = 0.002), and root angulation (MD: 0.8 point; 95% CI: 0.5–.1 point; p < 0.001). It has been reported that it is considerably more difficult to control root movement with aligners compared to fixed appliances, especially without the use of attachments.2 ,​ 51 ,​ 52 The third generation of aligners presumably allows for improved movement of this type by adding ellipsoid precision attachments that are able to produce couples creating root movement,2 which remains to be tested experimentally. On the other side, three ABO-OGS components (alignment, marginal ridges, and interproximal contacts) gave very similar results for both modalities. This is not surprising, since aligners are known to consistently produce adequate space closure of up to 6 mm by progressively tipping teeth into spaces in small increments and can successfully straighten dental arches by derotating teeth, especially when composite attachments are bonded.52 54 Looking carefully at the effect magnitude, it is obvious that the clinical relevance for each separate criterion is questionable, as small to moderate differences between aligners and fixed appliances are seen on average. However, when adding all these differences for each criterion, a clinically relevant worse finishing outcome is seen with aligners overall (as seen in Fig. 9.2 and Fig. 9.3).

No Image Available!

Fig. 9.5 Composite contour-enhanced forest plot illustrating the summary results of eight meta-analyses (each with three studies and 297 patients) for the comparison of each separate ABO-OGS component between orthodontic aligners and fixed appliances. ABO-OGS, American Board of Orthodontics Objective Grading System; CI, confidence interval; MD, mean difference. Contours correspond to different effect magnitude and the red dotted lines correspond to 95% random-effects predictions.

Looking at the occlusal outcome of treatment through meta-analyses of two studies using the PAR index gives a slightly different picture (Table 9‑4). Overall, no statistically significant difference in the posttreatment PAR scores between aligners and fixed appliances was seen (MD: 0 points; 95% CI: –2.0 to 2.0 points; p = 0.98). Contrary to that, treatment with orthodontic aligners was associated with a significantly smaller reduction in PAR scores (significantly worse treatment efficacy) compared to fixed appliances (MD: –2.9 points; 95% CI: –5.0 to –0.8 points; p = 0.007). However, even though the effect was statistically significant, the magnitude of this difference was small, which makes its clinical relevance questionable. Results of a single study48 indicated that aligners were worse in terms of reduction for the PAR component for upper anteriors (MD: –1.0 point; 95% CI: –1.9 to –0.1 point; p = 0.02) and overbite (MD: –1.0 point; 95% CI: –1.9 to –0.2 points; p = 0.02) compared to fixed appliances (Table 9.5). Again, differences between aligners and fixed appliances for these outcomes might be statistically significant, but probably are not clinically relevant. On the other side, the proportion of patients experiencing a great improvement in their PAR scores through treatment (PAR reduction of at least 22 points or PAR score of 0 posttreatment) was significantly smaller with aligners than with fixed appliances (RR: 0.5; 95% CI: 0.3–0.9; p = 0.02; Table 9.5). This corresponds to absolute risk for great PAR improvement of 22.9 and 45.8% for aligners and fixed appliance, respectively (Fig. 9.6). The number needed to treat is again 5, which is translated as every fifth case treated with aligners instead of fixed appliances not experiencing a great reduction in PAR scores through treatment, which would be seen if the case had been treated with fixed appliances, and denotes a potentially clinically relevant effect. This discrepancy between the results of the ABO-OGS and the PAR index can be explained by obvious differences between the two tools. The PAR index was developed to assess in a systematic manner the outcome of orthodontic treatment in order to be incorporated in both quality assessment measures of orthodontic care and scientific research. It, however, provides a vague assessment of the occlusion and disregards aspects such as tooth inclination, remaining spaces, and alignment of the posterior dental arch, which are important variable for board examination cases.32 It does not provide a detailed assessment of the position of each tooth and relationship with its neighbors within an ideal dental arch as the ABO-OGS does, which was developed in order to assess the fine details expected to be seen in a meticulously finished case in all three planes (first, second, and third order). Reported limitations of the PAR index55 include, among others, a low weighting for overbite scores and high weighting for overjet scores.56 Indeed, posttreatment PAR scores do not correlate significantly with posttreatment ABO-OGS scores.57 ,​ 58 Subsequently, the PAR index has been widely used to also assess the baseline severity of a case. However, the PAR index to this end does not take into account aspects such as skeletal discrepancies/cephalometric values, developmental tooth anomalies, ectopic teeth, or soft-tissues relationships and again does not correlate well with the ABO DI.57

No Image Available!

Fig. 9.6 Illustration of the expected absolute risk for a case to experience a great improvement in its PAR score (PAR reduction of at least 22 points or PAR score of 0 posttreatment) when treated with aligners or fixed appliances, according to the results of a single included study. PAR, peer assessment rating.

Table 9.4 Results of random-effects meta-analyses for eligible outcomes with at least two contributing studies

Outcome

n

Effect

p

I 2 (95% CI)

tau2 (95% CI)

95% prediction

ABO-OGS total score

3

MD: 9.91 (3.62, 16.21)

0.002a

84% (38%, 99%)

25.52 (3.01, 507.80)

–66.15, 85.98

ABO-OGS failure (score>30)

3

RR: 1.56 (1.23, 1.98)

<0.001a

0% (0%, 91%)

0 (0, 0.55)

0.33, 7.32

ABO-OGS component 1: alignment

3

MD: 1.59 (–1.05, 4.22)

0.24

91% (60%, 100%)

4.93 (0.71, 95.19)

–31.38, 34.55

ABO-OGS component 2: marginal ridges

3

MD: 0.46 (–0.18, 1.10)

0.16

0% (0%, 88%)

0 (0, 2.52)

–3.68, 4.61

ABO-OGS component 3: buccolingual inclination

3

MD: 0.78 (0.46,1.09)

<0.001a

0% (0%, 94%)

0 (0, 3.77)

–1.26, 2.81

ABO-OGS component 4: occlusal contacts

3

MD: 3.07 (0.57, 5.57)

0.02a

79% (19%, 99%)

3.78 (0.24, 79.63)

–26.47, 32.61

ABO-OGS component 5: occlusal relationship

3

MD: 0.99 (0.58, 1.40)

<0.001a

0% (0%, 94%)

0 (0, 7.24)

–1.66, 3.64

ABO-OGS component 6: overjet

3

MD: 1.81 (0.64, 2.98)

0.002a

50% (0%, 97%)

0.54 (0, 17.35)

–10.25, 13.87

ABO-OGS component 7: interproximal contacts

3

MD: 0.02 (–0.16, 0.21)

0.82

0% (0%, 89%)

0 (0, 0.74)

–1.18, 1.22

ABO-OGS component 8: root angulation

3

MD: 0.79 (0.49, 1.10)

<0.001a

0% (0%, 89%)

0 (0, 0.65)

–1.18, 2.76

PAR post-Tx

2

MD: –0.03 (–2.02, 1.96)

0.98

83% (0%, 100%)

1.72 (0, 258.55)

NC

PAR reduction via Tx

2

MD: –2.92 (–5.02, –0.81)

0.007a

0% (0%, 98%)

0 (0, 126.05)

NC

Treatment duration (months)

7

MD: –0.55 (–3.73, 2.63)

0.73

94% (82%, 99%)

16.25 (4.74, 73.67)

–11.72, 10.62

Abbreviations: ABO-OGS, American Board of Orthodontics Objective Grading System; CI, Confidence Interval; MD, Mean Difference; n, number of contributing studies; NC, Noncalculable; PAR, Peer Assessment Rating; RR, Relative Risk.

Note: Meta-analyses that are both statistically significant and clinically relevant are given in bold, judged as having an effect being at least equal to the average standard deviation of the control (fixed appliance) group across included studies.

aStatistically significant findings at the 5% level.

Table 9.5 Results of eligible outcomes assessed by only single studies

Outcome

Effect

P

PAR reduction per month

MD: 0.39 (0.09,0.69)

0.01a

PAR component 1: upper anteriors

MD: –1.00 (–1.86, –0.14)

0.02a

PAR component 2: lower anteriors

MD: –0.4 (–1.42,0.53)

0.38

PAR component 3: anteroposterior relationship

MD: –0.33 (–0.84,0.18)

0.20

PAR component 4: transverse relationship

MD: –0.17 (–0.42,0.08)

0.18

PAR component 5: vertical relationship

MD:0.04 (–0.02,0.10)

0.16

PAR component 6: overjet

MD:0.12 (–2.12,2.36)

0.92

PAR component 7: overbite

MD: –1.03 (–1.90, –0.16)

0.02a

PAR component 8: midline deviation

MD: –0.58 (–1.34,0.18)

0.14

PAR great improvement (reduction>30)

RR:0.50 (0.27,0.91)

0.02 a

Mandibular alignment not perfect

RR:0.67 (0.29,1.56)

0.35

EARR (total)

MD: –1.84 (–2.35, –1.33)

<0.001a

EARR (maxillary central incisors)

MD: –1.13 (–2.20, –0.06)

0.04a

EARR (maxillary lateral incisors)

MD: –1.76 (–2.84, –0.68)

0.001a

EARR (mandibular central incisors)

MD: –1.15 (–2.07, –0.23)

0.02a

EARR (mandibular lateral incisors)

MD:3.30 (4.24,2.36)

<0.001 a

Lower incisor inclination to mandibular plane

MD: –1.90 (–4.14,0.34)

0.10

Gingival recession

RR:0.90 (0.31,2.68)

0.86

Abbreviations: CI, confidence interval; EARR, external apical root resorption; MD, mean difference; NC, noncalculable; PAR, Peer Assessment Rating; RR, relative risk.

Note: Results that are both statistically significant and clinically relevant are given in bold, judged as having an effect being at least equal to the average standard deviation of the control (fixed appliance) group of the included study.

aStatistically significant findings at the 5% level.

Treatment Efficiency in Terms of Duration and Adverse Effects

Considerable variation was seen in the effect of treatment modality on treatment duration. Meta-analysis of seven studies indicated that on average no definite conclusions can be drawn regarding treatment duration with either aligners or fixed appliances (MD: –0.6 month; 95% CI: –3.7 to 2.6 months; p = 0.73). Extreme heterogeneity was seen across studies (I 2 = 94%), which makes the ability to synthesize existing studies questionable (Fig. 9.7). Specifically, two studies reported statistically significant reduction in treatment duration with aligners and two studies reported statistically significant increase in treatment duration with aligners, while the remaining three studies did not find statistically significant differences. Furthermore, exclusion of a study assessing combined orthodontic/orthognathic treatment47 instead of only orthodontic treatment did not improve the results (six studies; MD: –0.1 month; 95% CI: –3.5 to 3.4 months; I 2 = 95%). Nor was the situation improved by limiting the meta-analysis to only randomized trials (two studies; MD: 2.69 months; 95% CI: –5.0 to 10.4 months; I 2 = 96%) or to only studies with nonextraction treatment (five studies; MD: 0.6 month; 95% CI: –3.2 to 4.4 months; I 2 = 96%). Therefore, it is logical to assume that treatment duration is influenced by many other confounding variables and that the choice of appliance alone does not show a consistent effect on treatment duration.

No Image Available!

Fig. 9.7 Contour-enhanced forest plot on the comparison of treatment duration in months between aligners and fixed appliances. AL, aligner; CI, confidence interval; FX, fixed appliance; M, mean; MD, mean difference; N, number of patients; SD, standard deviation. Contours correspond to different effect magnitude and the red dotted line corresponds to 95% random-effects prediction.

Additionally, results of a single study48 indicated that aligners were more efficient in terms of PAR reduction/month of treatment compared to fixed appliances (MD: 0.4 point/month; 95% CI: 0.1–0.7 point/month; p = 0.01). However, as the same study reported that aligners were overall associated with smaller reductions in the PAR scores than fixed appliances, looking at the PAR reduction/month outcome might be misleading.

As far as adverse effects of treatment are concerned, a single identified study on EARR51 reported that significantly smaller percentage of the incisors’ root was resorbed during aligner treatment compared to fixed appliances (MD: –1.8%; 95% CI: –2.4 to –1.3%; p < 0.001; Table 9.5). The same was seen for the various subgroups according to tooth type (central vs. lateral incisor) and jaw (maxilla vs. mandible), but the effect magnitude was on average very small and probably of no clinical relevance. It must be stressed here also that evaluation of EARR during treatment is complicated, since many risk factors come into play, including the patient’s genetic predisposition toward EARR,59 the chosen mechanotherapy,60 the duration of treatment,61 and the actual amount of tooth movement (and especially apical movement).59 A carefully conducted retrospective nonrandomized study taking confounders such as baseline severity through ABO DI, genetic polymorphisms, and absolute apical displacement into account concluded that treatment with orthodontic aligners results in similar amounts of EARR compared to fixed appliances. Therefore, it might be prudent to check if any significant differences in EARR reported in the literature are not rather due to actually teeth being moved less around with aligners.

Additionally, treatment with aligners was not associated, in a single included study46 with significantly lower proclination of the lower incisors compared to fixed appliances (MD: –1.9°; 95% CI: –4.1 to 0.3°; p = 0.10). However, it must be noted that a very small sample was included, which makes the study probably underpowered to identify such a small difference of 1.9° between groups, if this really exists.

Furthermore, no significant difference in the development of gingival recessions 2 years after treatment with aligners or fixed appliances was seen in another single study (MD: 0.9; 95% CI: 0.3–2.7; p = 0.86).50 It might be expected that choice of appliance alone might not directly influence the development of gingival recession. Even if appliance choice was associated with increased anterior anchorage loss/incisor proclination (which was not seen), this would not necessarily translate to increased risk of gingival recession.62 ,​ 63 Although orthodontic treatment on average increases the risk for gingival recessions,64 its precise etiology is multifactorial with risk factors including periodontal disease, mechanical trauma, patient age, smoking, and induction of bone dehiscences by positioning the teeth beyond the alveolar plate.65 72

Finally, limited evidence on the effect of appliance choice on loss of periodontal attachment was provided by a single identified study,44 which assessed orthodontic alignment of anterior teeth in adult patients with previous history of treated periodontal disease. After retrieving raw data from the author and matching the study’s groups for baseline status, no differences between aligners and fixed appliances were seen for periodontal probing depth (MD: 0 mm; 95% CI: –0.4 to 0.4 mm; p = 1.00) or alveolar bone levels (MD: 0.1 mm; 95% CI: –0.4 to 0.6 mm; p = 0.69). On the other side, fixed appliances were significantly quicker in repositioning the patients’ migrated anterior teeth compared to aligners (3.9 vs. 6.0 months; MD: –2.1 months; 95% CI: –3.7 to –0.5 months; p = 0.01). It must be noted here that although previous systematic reviews of mostly compromised studies have reported that aligners might be associated with facilitation of better oral hygiene than fixed appliances,18 ,​ 23 ,​ 45 a recent randomized clinical trial73 found no significant consistent advantage in terms of plaque index, gingival index, or periodontal bleeding index between patients treated with aligners and fixed appliances. It seems, therefore, that fixed appliances also can be compatible with proper oral hygiene.

Strength of Current Recommendations and Threats to Their Validity

Our confidence in the clinical recommendations that can be formulated based on the quality of evidence using the GRADE framework is presented in Table 9.6. We can say with moderate certainty that compared to treatment with fixed appliances, treatment with orthodontic aligners: (1) leads to worse finishing quality (higher ABO-OGS scores), (2) leads to greater proportion of treated cases that would not pass the ABO examination criteria (ABO-OGS score > 30 points), and (3) makes little to no difference in the development of gingival recessions. This means that future research could have an important impact, which might change current estimates of effect. The main reason for downgrading the quality of evidence pertained to the inclusion of nonrandomized studies that (although being matched) had methodological issues that could introduce some bias. Therefore, even though a potentially large clinical difference in posttreatment ABO-OGS scores between aligners and fixed appliances was seen, this cannot be used as basis to upgrade our confidence in these estimates, as heterogeneity across studies precludes precise effect quantification (Fig. 9.2).

Table 9.6 Summary of findings table according to the GRADE approach

Relative effect (95% CI)

Anticipated absolute effects (95% CI)

Outcome [follow-up]

Studies (patients)

Fixed appliancea

Aligners

Difference with aligners

Quality of the

evidence (GRADE)b

What happens with aligners

ABO-OGS score [post Tx]

297 patients (3 studies)

26.7 pts

9.9 pts greater

(3.6 to 16.2 greater)

⊕⊕⊕O moderatec,d,e

due to bias

Probably leads to worse finishing quality (higher ABO-OGS scores)

Unacceptable finishing quality (ABO-OGS score>30 pts) [post Tx]

297 patients (3 studies)

RR 1.6

(1.23 to 1.98)

38.9%

60.6%

(47.8%–77.0%)

21.7% more

(8.9% to 38.0% more)

⊕⊕⊕O moderatec

due to bias

Probably leads to more patients with unacceptable finishing quality

PAR reduction [post Tx]

176 patients (2 studies)

19.6 pts

2.9 pts less

(0.8 to 5.0 less)

⊕⊕OO lowf

due to bias

Might lead to lower treatment efficacy (smaller reduction in PAR scores)

Great improvement in PAR (PAR reduction>30 pts) [post Tx]

96 patients (1 study)

RR 0.5

(0.27 to 0.91)

45.8%

22.9%

(12.4%–41.7%)

22.9% less
(4.1% to 33.4% less)

⊕⊕OO lowf

due to bias

Might lead to less patients with great improvement in PAR scores

Treatment duration [post Tx]

759 patients (7 studies)

19.6 mo

0.6 mo shorter

(3.7 shorter to 2.6 longer)

⊕OOO very lowg,h

due to bias, inconsistency

Too heterogenous response to synthesize across studies

EARR as % of anteriors’ root length [post Tx]

80 patients/640 teeth (1 study)

7.0%

1.8% less

(1.3% to 2.4% less)

⊕⊕OO lowf

due to bias

Might lead to greater EARR

Inclination of lower incisors [near Tx end]

44 patients (1 study)

5.3°

1.9° less

(4.1° less to 0.3° more)

⊕⊕OO lowi,j

due to bias, imprecision

Little to no difference in lower incisor inclination

Gingival recession [2 years post Tx]

158 patients (1 study)

RR 0.9

(0.31 to 2.68)

8.0%

7.2%

(2.5%–21.4%)

0.8% less
(5.5% less to 13.4% more)

⊕⊕⊕O moderatec

due to bias

Little to no difference in gingival recession

Abbreviations: ABO-OGS, American Board of Orthodontics Objective Grading System; CI, confidence interval; EARR, external apical root resorption; GRADE, Grading of Recommendations Assessment, Development and Evaluation; PAR, peer assessment rating; pt, point; Tx, treatment.

Note: Intervention: comprehensive orthodontic treatment with thermoplastic aligners versus fixed appliances. Population: adolescent or adult patients with any kind of malocclusion. Setting: university clinics, private practice, hospital (Canada, China, Ireland, Italy, United States).

aResponse in the control group is based on average response of included studies (random-effects meta-analysis).

bStarts from “high.”

cDowngraded by one level for bias due to the inclusion of nonrandomized studies with moderate risk of bias.

dNo downgrading for inconsistency (even though I2 > 75%), as it affects only our estimate about the difference between treatment modalities, but not our decision (all studies are on the right side of forest plot and show significant effects).

ePotentially great effect observed (larger than one average standard deviation), but no upgrading due to residual confounding.

fDowngraded by two levels for bias due to the inclusion of nonrandomized studies with critical/serious risk of bias.

gDowngraded by two levels for bias due to the inclusion of randomized trials with high risk of bias and nonrandomized studies with serious/critical risk of bias.

hDowngraded by one level due to inconsistency; great variability is seen among included studies with significant studies arranged on both sides of the forest plot (confident signs of heterogeneity that influence our decision about which treatment is shorter, which precludes calculating an average effect).

iDowngraded by one level for bias due to the inclusion of a randomized trial with high risk of bias.

jDowngraded by one level for imprecision due to the inclusion of an inadequate sample.

We can say with low confidence that compared to treatment with fixed appliances treatment with orthodontic aligners: (1) leads to lower treatment efficacy (smaller PAR reduction through treatment), (2) leads to greater proportion of treated cases seeing a great improvement (PAR reduction of at least 22 points or PAR score of 0 posttreatment), (3) leads to greater EARR, and (4) makes little to no difference in proclination of the lower incisors during treatment. The main reason for downgrading the quality of evidence pertained to the inclusion of nonrandomized studies with serious/critical methodological issues that most probably introduce bias. This was especially seen in the retrospective study of Gu et al48 that selectively reported data from what might be regarded as “good” cases, while excluding patients with issues of compliance or oral hygiene. This means that further research in terms of well-designed studies is very likely to have an important impact, which is likely to change our current estimates of effect.

Finally, we have very low confidence on the currently observed effect of treatment modality choice (aligners vs. fixed appliances) on treatment duration. This has to do with the fact that very heterogeneous results were seen across existing studies that could not be explained by either clinical heterogeneity, study design, or incorporation of extractions in the treatment plan. Therefore, any estimate about the average difference in duration between aligners and fixed appliances is very uncertain and future well-designed studies should be based on careful selection of cases matched for baseline severity and take into account potential confounders such as case severity, incorporation of extractions, amount of interproximal enamel reduction, and quality of the final occlusal outcome.

Nevertheless, it is important to point out that several threats to the validity of currently available clinical recommendations exist. For one, methodological issues existed for all included studies that might influence conclusions, and this is especially the case for included retrospective nonrandomized studies.11 14 Furthermore, most meta-analyses were based predominantly on small trials, which might affect their results.74 Additionally, the small number of trials that were ultimately included in the meta-analyses and their incomplete reporting of results and potential confounders such as level of case severity, oral hygiene, compliance, use of bonded attachments, number of aligners, rate of refinement need, or amount of interproximal enamel reduction precluded the conduct of many analyses for subgroups and meta-regressions that might enable identification of patient subgroups for which aligners might be equally or even more appropriate treatment alternative compared to fixed appliances.

Conclusion

According to currently existing clinical evidence from randomized trials and matched nonrandomized studies on mostly adult patients with mild to severe malocclusions treated with or without extractions, it seems that orthodontic treatment with aligners is associated with worse treatment outcome compared to fixed appliances. On the other side, aligners might be associated with a small decrease in EARR during treatment, while there seems to be little to no difference on proclination of lower incisors and development of gingival recessions. Treatment duration is not defined by choice of appliance alone and patient- or treatment-related factors might come into play.

Note

Since the preparation and submission of this chapter, additional data were made available, which explains small differences in the results of this chapter compared to the subsequent journal paper,74 and a publication notice was issued.76 However, the study’s results and final conclusions in all instances remain practically the same with no threat to their validity.

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Dec 4, 2021 | Posted by in Orthodontics | Comments Off on Section III: Evidence
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