Data sourcesElectronic search was conducted up to September 2021 in three electronic databases including Scopus, Web of Science, and EMBASE. Only studies in English language were selected.Study selectionProspective and retrospective studies including cohort, cross-sectional, randomized control trials, and qualitative studies were included. Both the inclusion and exclusion criteria were reported. The search in the databases and the selection of the studies were performed independently by two reviewers. The included studies assessed the effects of clear aligner therapy on the speech difficulty.Data extraction and synthesisData extraction was performed independently by two reviewers. The data from the relevant studies were extracted into a customized Template. The systematic review was carried out and reported based on the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) reporting guidelines. Details regarding the authors, year of publication, sample size, included participants, age, groups, outcome assessment, results, and inferences were extracted from the selected studies. The quality of the selected studies was assessed based on the relevant guidelines from Cochrane Handbook for Systematic Reviews. The criteria examined sequence generation, allocation concealment, blinding, outcome data, selective reporting, and other sources of bias. Non-randomized studies were assessed using the ROBINS-I tool (Risk of Bias in Non-randomized studies). The strength of evidence was assessed by the evidence grading system developed by the GRADE collaboration as described in the Cochrane Handbook for Systematic Reviews of Interventions.ResultsThe search identified 283 results. 269 articles were excluded based on their title and abstract after evaluation against the inclusion criteria. 14 articles were assessed in full text. Finally, 7 papers were included in the qualitative analysis. The included studies were 5 cohort studies and 2 randomized clinical studies. 332 patients were examined in the included studies. There were 195 females and 137 males. 191 patients were treated using clear aligner therapy (CAT) while 122 patients were treated using labial fixed appliances and 19 patients were treated using lingual orthodontic treatment. Out of the 191 patients who were treated using CAT, 179 patients were treated using Invisalign (Santa Clara, CA, USA) while 12 patients were treated using Smile Align (Mumbai, India). All seven selected studies examined speech dysfunction. The speech difficulty was assessed through two tools - semiobjective assessment by speech pathologists and professionals and subjective evaluations by the patients themselves through a patient questionnaire that assess the effects of aligners on speech. The risk of bias assessment revealed that two studies had moderate risk of bias while five articles had serious risk of bias. Meta-analysis was not performed due to the high heterogeneity of the included studies. The level of evidence was assessed as low due to the methodological insufficiencies and risk of bias in the included studies. The results showed that both CAT and fixed appliances (FA) resulted in speech difficulties in terms of clarity and delivery of speech that include speech alteration, slurring of words, lisping, and nasality. Some patients who were treated using CAT reported impairment in the lingual space that affected their speech. Lingual FA resulted in more speech difficulty when compared to labial FA and CAT. Aligners caused errors in the articulation of /s/,/z/,/zh/,/sh/,/th/,/ch/. Acoustic analysis revealed /s/ sound appeared most affected. Aligners had an effect on speech while reading, with patients slowing down to their speed to better articulate. The above-mentioned speech difficulties were transient. The included patients normalized their speech within 7-14 days from start of treatment while few patients took 30-60 days to recover.ConclusionsAlthough the likelihood of speech difficulties would be high with CAT, the current evidence states that speech difficulties shown with CAT are similar to those found with FA. However, the patients who were treated using CAT adapt quickly and speech recovers within a few weeks. Time to recovery varied greatly, ranging from a week to two months in certain cases.
