Mandibular alignment changes after full-fixed orthodontic treatment: a systematic review and meta-analysis

Background: Although post-treatment mandibular alignment has been extensively investigated, the findings remain controversial. Objectives: The objective was to assess mandibular alignment changes, as measured by the irregularity index, of patients who underwent full-fixed orthodontic treatment and were followed up at least 1 year after retention. Search methods: MEDLINE, EMBASE, and Cochrane library, in addition, the reference lists of included studies, were screened. The search was conducted up to April 2018. Selection criteria: The study designs included both interventional and observational studies of orthodontic patients who received either extraction or non-extraction treatment. Data collection and analysis: The interventional studies were assessed using the Cochrane Collaboration's risk of bias assessment tool. The quality of the observational studies was evaluated using National Institution of Health quality assessment tools. The first two authors independently applied the eligibility criteria, extracted the data, and assessed the risk of bias. Any conflicts were resolved with consensus discussion with the third author. Results: The search retrieved 11 326 articles, 170 of which were assessed for eligibility. There were 44 studies included in the qualitative assessments and 30 in the meta-analyses.The studies included 1 randomized control trial (RCT) and 43 observational studies. The RCT was judged to have a high risk of bias and all of the observational studies had either fair or poor quality. The meta-analysis was based on studies judged to be of fair quality, including a total of 1859 patients. All meta-analyses were performed using random-effect models. The standardized mean difference between post-treatment and post-retention irregularity was 1.22 (95% CI, 1.04-1.40) and 0.85 (95% CI, 0.63-1.07) after extraction and non-extraction treatments, respectively. There was a substantial heterogeneity for the extraction (I-2 = 75.2%) and non-extraction (I-2 = 70.1%) studies.The follow-up duration (1-10 versus 10-20 years) explained 33% of the heterogeneity, with longer follow-up studies showing more irregularity. Limitations: The quality of evidence provided by the studies was low.There was a risk of publication bias, and the search was limited to English language. Conclusions and implications: Post-treatment mandibular irregularity increases are limited. Irregularity increases are slightly greater in patients treated with mandibular premolars extractions, and in patients followed up over longer periods of time.