Soboutia 2016.

Study characteristics
Methods	3‐arm parallel RCT
Participants	Number recruited: 150 randomised, 128 analysed in statistical analysis (22 participants dropped out due to failure to regularly attend the follow‐up sessions) Sex (M:F): 60:68 Mean age (years): 18.0 (SD 3.6) (range 13‐25 years) Inclusion criteria: healthy people who had no history of previous dental extraction or orthognathic surgery, with an original indication for non‐extraction treatment (Class I crowding, incisor mandibular plane angle [IMPA] < 92º, vertically normal or horizontal and no pattern of vertical excess, and with a crowding extent and soft tissue characteristics appropriate for non‐extraction treatment), with ideally aligned dentition, a Class I relationship with an overbite/overjet between 1 and 3 mm, no issues contraindicating or interfering with retention, bruxism or clenching, advanced dental abrasion/attrition/erosion, and a need for mandibular canine‐to‐canine fixed retention. In addition, patients who had good oral hygiene, healthy periodontal condition and no previous history of using bonded retainers Exclusion criteria: patients with widespread probing depths > 3 mm and radiographic evidence of periodontal bone loss Setting: private office in Iran
Interventions	Three types of lower fixed lingual retainers from right to left canine: Group 1: fibre‐reinforced composite retainer (Everstick Ortho; Stick Tech, Turku, Finland) Group B: flexible multistranded stainless steel spiral wire (Ortosmail, Krugg spa, Milan, Italy; Ø 0.0175 inch) retainer Group C: twisting 2 ⌀ 0.009 inch dead soft wires (Ligature wire; 3M Unitek, Monrovia, CA, USA) retainer
Outcomes	Failure rate in terms of first detachment of any composite pad or retainer breakage/distortion Survival of retainers (in months). Kaplan‐Meier survival plots and hazard ratio Time point: 24 months
Notes
Risk of bias
Bias	Authors' judgement	Support for judgement
Random sequence generation (selection bias)	Low risk	"Using a random number table"
Allocation concealment (selection bias)	Unclear risk	Not reported
Blinding of outcome assessment (detection bias) All outcomes	Unclear risk	Not reported
Incomplete outcome data (attrition bias) All outcomes	Low risk	Number of dropouts clearly reported and described
Selective reporting (reporting bias)	Low risk	All outcomes were reported according to its material and method section
Other bias	Low risk	No other area of bias detected