Protocol
The study protocol followed the PRISMA (Preferred Reporting Items for Systematic Review and Meta-Analyses) guidelines of the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) statement8 and the recommendations of the Cochrane collaboration.9 The PICO question (population [P], intervention [I], comparison [C], and outcome [O]) was developed as follow: In patients treated with fixed orthodontic (P), does the self-ligating brackets (I) compared to conventional brackets (C) make less degree of external apical root resorption (O) based on randomized control trials (RCTs) and clinical controlled trials (CCTs)?
Search Strategy
An electronic literature search was conducted through databases including PubMed, OVID, the Cochrane Library, EMBASE, FMRS, CBM, CNKI, VIP and WanFang Data. Furthermore, related articles were hand-searched in NLM (National Library of Medicine), SIGLE (System for Information on Grey Literature in Europe), Campbell library, WHOLIS (World Health Organization's library database), Chinese Journal of Evidence-based Medicine and the Journal of Orthodontics. Terms used in the search included root resorption, root shortening, root alternation, self-ligating and self-ligating bracket. Retrieval time was set from database establishment to November 2019. No language restriction was applied during the literature search.
Criteria for included studies
The inclusion criteria were as follows: (1) Types of studies: randomized controlled trials (RCTs) or controlled clinical trials (CCTs), prospective and retrospective controlled studies; (2) Types of participants: healthy patients who required fixed orthodontic treatment; (3) Types of intervention: the experimental group and the control group used self-ligating brackets and conventional brackets; (4) Outcomes: maxillary and/or mandibular incisors root lengths before and after treatment were measured, and then calculated the reduction of root length (in millimeters and in percentage).
The exclusion criteria were as follows: (1) Studies involving patients with systemic diseases or primary periodontal disease before orthodontic treatment; (2) The reduction of root length been measured was not comparing before treatment with after treatment but in other treatment time; (3) Not the long-term (after finishing treatment) studies; (4) Animal experiments; (5) Reviews, case reports or abstracts.
Data Extraction and quality analysis
Studies were extracted and recorded independently by two reviewers (J.X and X.W) and in duplicate using a customized data collection form, on the following items: author and year of publication, study design, comparisons, participants (numbers, gender, age) and outcomes (EARR, measurement approach, evaluated teeth).
The quality assessment of RCT articles was performed referring to Cochrane Handbook for Systematic Reviews of Interventions(version 5.1.0).10 Meanwhile, quality scores were calculated by a modified version of the method described by Jadad.9 Otherwise, the quality evaluation of CCT articles were reformed according to the standards described in the Cochrane Handbook for Systematic Reviews Interventions and Liu.12 The methods and results sections of each article were read and scored by two readers (J.X and X.W) independently. Then evaluators discussed their extracted data, and when discrepancy occurred, it was solved by discussing with a third reviewer (K.Y).
Data Analysis
In this systematic review, evaluation of EARR mainly includes maxillary central incisors, maxillary lateral incisors, mandibular central incisors and mandibular lateral incisors. The Review Manager 5.3 software was used for the meta-analysis of quantitative data. Statistical heterogeneity within or between groups was assessed by the Cochrane’s test. The outcome measure was standardized mean difference (SMD).The heterogeneity of recruited studies was explored by I2 statistic. If there was no substantial heterogeneity (I2<50%), outcomes would be pooled for the meta-analysis with the fixed effect model. Otherwise, the random effect model would be adopted. Descriptive assessment was done when the statistical synthesis of data failed.