Article Study Designa Follow-up period (years) Study Groups; Intervention vs control Patients (n) Mean Age (years) Men/women Outcome variables Bondemark et al. [5] Systematic review ≥ 5 years Various different 38 studiesb nr nr Treatment stability Littlewood et al. [2] Systematic review > 3 months Various different 5 studies nr nr Treatment stability Retainer failure 442 Edman Tynelius et al. [13] RCT 2 years Removable upper retainer and fixed lower retainer 25 14 30/45 Treatment stability Retainer failure     Removable upper retainer and stripping lower anterior teeth 25  Removable upper and lower retainer (positioner) 25   Artun et al. [11] RCT 3 years Fixed retainer 35 nr nr Calculus Dental caries Gingivitis Incisor irregularity Loss of attachment Plaque Retainer failure Removable retainer 14 Cerny et al. [12] CT > 15 years Fixed retainer (mixed groups) 41 nr nr Treatment stability Alveolar bone level Calculus Dental caries Periodontitis (Gingival recession) Gingivitis Plaque Retainer failure ≤ 2 years Removable retainer 18 Levin et al. [16] CT 4.6 years Fixed retainer No retainer 48 arches 72 arches 21 46/46 Gingival pocket depth Gingival recession Gingivitis Plaque Rody et al. [15] CT ≥ 4 years Fixed retainer 10 28 7-Mar Gingivitis Removable lower retainer 11 24 2-Sep Periodontitis (Gingival pocket depth) No retainer 10 27 5-May Plaque Artun [14] CT Approx. Fixed 3-3 retainer 49 17-19 nr Calculus 1-9 years   No 3-3 retainer 25 17 Dental caries Fixed maxillary retainer 14 22 Gingivitis Removable maxillary retainer 20 16 Periodontitis (Gingival pocket depth) Plaque Andrén et al. [18] Case-series ≥ 5 years Fixed retainer 103 35 22/81 Complications Dahl and Zachrisson [17] Case-series 3-6 years Fixed retainer 142 nr nr Retainer failure Renkema et al. [21] Case-series 5 years Fixed retainer 221 nr 146/75 Other complications Retainer failure Störmann and Ehmer [19] Case-series 2 years Fixed retainer 103 13-17   Patient discomfort Retainer failure Tacken et al. [20] Case-seriesc 2 years Fixed retainer 184 14 90/94 Gingivitis Plaque Retainer failure aCase-series were only used to record complications. bNumber of included patients, not reported. cCT, in this context considered as case-series regarding fixed retainer (control group without orthodontic treatment). CT = non-randomised, controlled study. nr = not reported.

Table 1: Included studies and patient characteristics (alphabetically, according to study design).