External Apical Root Resorption in Clear Aligner Vs. Fixed Orthodontic Appliances: Systematic Review and Meta-Analysis

Introduction

Since clear aligner therapy (CAT) has been introduced as a treatment method, an increasing number of patients are opting for this more esthetical and comfortable alternative treatment modality, rather than fixed orthodontic appliances. Theoretically, from the mechanical point of view, clear aligners differ from fixed orthodontic appliances in the number of orthodontic forces applied to teeth [1]. Studies have shown that orthodontic forces have a significant effect on the development of external apical root resorption (EARR) [2]. Even to this day, the exact nature of orthodontically induced root resorption remains unclear [3]. It is known that this phenomenon presents with multifactorial etiology. Patient-related (age, gender, genetics, nutrition) or treatment-related factors (the amount of force used during treatment, duration of the treatment, the type of appliance, use of elastics, extraction treatment, etc.) have contributing effects on root resorption [4]. It has also been demonstrated that heavy orthodontic forces produced significantly higher levels of EARR than light forces [3]. Therefore, heavy orthodontic forces generated by orthodontic appliances are reported as a clear cause of EARR by numerous scientific articles [5-7]. 

EARR is a common inflammatory response, encountered during treatment using fixed orthodontic appliances with 27,7% of cases [5]. The process is associated with the elimination of the hyalinization zone triggered by microphage-like cells from periodontal ligament blood vessels which expose the cementum and accelerate root resorption [3]. Studies have shown that the maxillary incisors are more susceptible to EARR than other teeth, due to the longer distance of orthodontic tooth movement [8]. Forces applied to the teeth by aligners are intermittent whereas during treatment with fixed orthodontic appliances forces are produced continuously. In addition, the magnitude of forces applied to the teeth by aligners is lower than by fixed orthodontic appliances [9]. These characteristics could have an impact on EARR as they cause stress in the radicular-apical area [10]. An orthodontist must assess root resorption in patients undergoing orthodontic treatment [11]. Several studies have examined EARR during CAT, however, the results are controversial – while some report decreased EARR in CAT compared to fixed orthodontic appliances, others show that CAT could lead to increased EARR [4, 9, 12-16]. Panoramic, periapical radiographs or cone-beam computed tomography is used to calculate the root-crown ratio and evaluate the longevity of the teeth [17].

The purpose of this meta-analysis was to update the current literature and compare the severity of EARR during treatment with clear aligners and fixed orthodontic appliances.

Materials and Methods

Protocol and registration

This systematic review was prepared to adhere to PRISMA guidelines. The protocol for the systematic review was registered in the PROSPERO (International Prospective Register of Systematic Reviews) database. Registration number: CRD42021240269.

Focused question

The question was raised using the PICOS model:

• Population (P) – patients with malocclusion requiring orthodontic treatment.
• Intervention/Exposure to risk factor (I) – clear aligner therapy.
• Control (C) – fixed orthodontic appliance therapy.
• Outcome (O) – lower external apical root resorption level.

Does clear aligner therapy induce lower levels of external apical root resorption compared to fixed orthodontic appliances?

Search strategy

On November 6, 2022, a systematic search in the medical literature was carried out to identify all peer-reviewed papers, published from 2016 to 2022, evaluating the amount of EARR in patients who received CAT or treatment with fixed orthodontic appliances. Combinations of keywords “Clear Aligner Appliances”, “Fixed Orthodontic Appliances”, and” Root resorption” were used in MEDLINE (searched via PubMed), EMBASE (searched via ScienceDirect), System for Information on Grey Literature in Europe, The Cochrane Library (Cochrane Central Register of Controlled Trials) and LILACS electronic bibliographic databases. Furthermore, the search was broadened by checking for possible articles in the references of the included articles.

Study selection and data collection process

Three authors (K.J., M.V., and A.Var.) performed an electronic database search and selected articles that appeared to have appropriate titles and abstracts for the review and met the criteria. After reviewing the full-text articles, the final selection was made. The fourth reviewer (A.Vas.) would have tried to resolve the conflict, if there were any discrepancies.

Inclusion criteria

1. Clinical studies that included orthodontic patients.
2. Studies published in English.
3. Studies that performed 2D panoramic radiographs, periapical radiographs, or cone beam computed tomography (CBCT) before and after orthodontic treatment.
4. No radiographic evidence of EARR before orthodontic treatment.
5. Retrospective studies, randomized control trials.

Exclusion criteria

1. Systematic reviews, case series, meta-analyses, and case reports.
2. In vitro studies or animal studies.
3. Articles that did not compare fixed orthodontic appliances with clear aligners.
4. Articles published more than 5 years ago.

Methodological quality

The Cochrane Collaboration's ROBINS-I tool [18] and the Cochrane risk-of-bias tool for randomized trials (RoB 2) were used to assess the overall quality and risk of bias in this systematic review, evaluating confounding, study participant selection, intervention classifications, deviation from intended interventions, missing data, outcome measurement, and reported result selection.

Statistical analysis

To perform meta-analysis, the means, standard deviations, and sizes of study samples were extracted. An inverse variance method with random effect model meta-analysis was conducted to compare the standard mean difference (SMD) in EARR between patients who received fixed orthodontic appliance therapy and clear aligner therapy with a 95% confidence interval (CI). Cochran’s Q and I2 tests were used to evaluate the heterogeneity of the studies. The heterogeneity was considered to be significant when P<0,05 for Q statistics or I2>50%. Summarized quantitative data were graphically presented in forest plots using Review Manager 5.4.1 software [19]. Sensitivity analysis was conducted to test the stability of the results, and whether the exclusion of some studies had any impact on the outcome of the results.

Results and Discussion

Study selection

The procedure of study selection is presented in Figure 1. The initial electronic database search yielded 1276 articles. After removing the articles that were not relevant based on their title and abstracts, ten full-text papers were retrieved and reviewed for their suitability. Four studies were excluded for the following reasons: two studies were excluded because they were theses [20, 21], one study was considered not suitable as it was a case-control genetic association study [22], and another study was excluded since it was published in Mandarin [23]. Hence, 6 publications [9, 12, 14, 16, 24, 25], were included in the qualitative and quantitative data analysis.

Study characteristics

Table 1 presents characteristics of all 6 articles that were included in the review. 5 of the included publications were retrospective cohort studies and 1 was a randomized clinical trial. The publication dates of the articles were between the years 2017 and 2022. Several patients ranged from 33 to 110 (a total of 391 patients were included in all 6 studies) with ages ranging from 14 to 31 years. Treatment duration did not significantly differ between different treatment modalities. All of the included studies compared clear aligner therapy with fixed orthodontic appliances. Two studies compared clear aligners with two different types of fixed orthodontic appliances [12, 25]. All 6 publications evaluated maxillary incisors, and four of them assessed mandibular incisors [9, 14, 16, 24]. Two studies evaluated canines additionally [14, 16]. Four studies assessed EARR by using CBCT [9, 12, 16, 25], one study performed measurements using a 2D panoramic radiograph [14] and one study performed measurements using a 2D periapical radiograph [24].

Table 1. General characteristics of the selected studies

Note. G – treatment group, SD – standard deviation, %= percentage, CBCT – cone beam computed tomography

Quality assessment

By the ROBINS-I tool, all five publications had a low risk of confounding, participant selection, missing data, and reported results selection bias. Two out of five studies had moderate bias due to deviations from intended interventions [9, 12], one had a moderate risk of measurement of outcomes [9] and one had a moderate risk classification on intervention bias [16]. Two studies had a low risk of bias for all of the analyzed factors [14, 25]. According to the Cochrane risk-of-bias tool for randomized trials (RoB 2), one study had a moderate risk of bias in blinding participants and personnel [24]. Overall, three studies had moderate [9, 12, 16], and three had low risk of bias [14, 24, 25] (Figure 2).

Quantitative synthesis of the results

When the EARR data was evaluated, it was observed that, even with different scales, the CAT significantly benefited this outcome (SMD = 0.76, 95% CI = -1.17, -0.34; p < 0.00001). However, due to the significant heterogeneity among studies (p = 0.004, I2 = 60%), subgroup analyses depending on the group of the teeth (maxillary central incisors, maxillary lateral incisors, mandibular central incisors, and mandibular lateral incisors) were performed (Figure 3).