Speech and orthodontic appliances: a systematic literature review

Background: Various types of orthodontic appliances can lead to speech difficulties. However, speech difficulties caused by orthodontic appliances have not been sufficiently investigated by an evidence-based method. Objectives: The aim of this study is to outline the scientific evidence and mechanism of the speech difficulties caused by orthodontic appliances. Search methods and selection criteria: Randomized-controlled clinical trials (RCT), controlled clinical trials, and cohort studies focusing on the effect of orthodontic appliances on speech were included. A systematic search was conducted by an electronic search in PubMed, EMBASE, and the Cochrane Library databases, complemented by a manual search. Data collection and analysis: The types of orthodontic appliances, the affected sounds, and duration period of the speech disturbances were extracted. The ROBINS-I tool was applied to evaluate the quality of non-randomized studies, and the bias of RCT was assessed based on the Cochrane Handbook for Systematic Reviews of Interventions. No meta-analyses could be performed due to the heterogeneity in the study designs and treatment modalities. Results: Among 448 screened articles, 13 studies were included (n = 297 patients). Different types of orthodontic appliances such as fixed appliances, orthodontic retainers and palatal expanders could influence the clarity of speech. The /i/, /a/, and /e/ vowels as well as /s/, /z/, /l/, /t/, /d/, /r/, and /j/ consonants could be distorted by appliances. Although most speech impairments could return to normal within weeks, speech distortion of the /s/ sound might last for more than 3 months. The low evidence level grading and heterogeneity were the two main limitations in this systematic review. Conclusions: Lingual fixed appliances, palatal expanders, and Hawley retainers have an evident influence on speech production. The /i/, / s/, /t/, and /d/ sounds are the primarily affected ones. The results of this systematic review should be interpreted with caution and more high-quality RCTs with larger sample sizes and longer follow-up periods are needed.