Can vacuum-formed retainers maintain arch dimensions and alignment compared to Hawley and fixed bonded retainers after treatment with fixed appliances? A systematic review and meta-analysis

Abstract Background Different types of retention appliances have been proposed over the years, but their effectiveness in maintaining arch dimensions and alignment after orthodontic treatment is still unclear. Aim To assess the efficacy of vacuum-formed retainers (VFRs) in preserving arch widths, arch length, and anterior alignment in maxillary and mandibular arches, compared to removable Hawley retainers (HRs) or fixed bonded retainers (FBRs). Search methods: unrestricted literature search of five major databases up to March 2024. Selection criteria randomized/non-randomized clinical studies comparing VFRs to removable HRs or FBRs. Data collection and analysis after duplicate study selection, data extraction, and risk of bias assessment, random effects meta-analyses of standardized mean differences and their 95% confidence intervals were performed, followed by meta-regressions, sensitivity analyses, and assessment of the quality of evidence with GRADE. Results Twenty-two prospective studies (4 non-randomized and 18 randomized controlled trials) involving 1797 patients (mean age 17.01 years, 38.3% males) were included. No significant differences were found in the intercanine width, intermolar width, and arch length between VFRs and HRs, in both arches (P > 0.05). However, VFRs were statistically more effective than HRs in terms of Little’s irregularity scores (LII) in the maxilla (eight studies; SMD = −0.42; 95% CI: −1.03 to −0.09; P = 0.02; I2 = 73.4%) but not in the mandible (P = 0.12). No significant differences were reported for all considered outcomes between VFRs and FBRs in in both arches (P > 0.05), except for lower LII, where VFRs were significantly less efficient (eight studies; SMD = 1.49; 95% CI = 0.26–2.7; P = 0.02; I2 = 93%). Follow-up times, risk of bias, and wire type (of FBRs) did not show statistically significant effects on outcome variables. Sensitivity analyses showed robustness of the findings for including non-randomized and postretention studies. The certainty in these estimates was from moderate to low due to the risk of bias and inconsistency. Conclusions Low to moderate quality evidence indicates that VFRs are as effective as HRs in maintaining arch widths, length, and alignment. Low-quality evidence found similar efficacy between VFRs and FBRs, with FBRs being statistically more effective at maintaining lower arch alignment, but the difference was not clinically significant. Registration PROSPERO registration (CRD42024518433).