Introduction: Vacuum-formed retainers (VFRs) are often prescribed by orthodontists in the British National Health Service (NHS). There is no good evidence that VFRs are more effective than Hawley retainers. The aim of this study was to compare the clinical effectiveness of Hawley and VFRs over a 6-month period of retention. The study design was a randomized clinical trial, performed in a single orthodontic practice.
Methods: Eligible patients treated by a specialist orthodontist were randomly allocated to either Hawley retainers (n = 196) or VFRs (n = 201). Two technicians fabricated the retainers to standardized designs. A blinded, dentally qualified examiner analyzed the records. Maxillary and mandibular dental casts at debond and 6 months into retention were assessed for tooth rotations mesial to the first permanent molars, intercanine and intermolar widths, and Little's index of irregularity.
Results: The results showed significantly greater changes in irregularity of the incisors in the Hawley group than in the VFR group at 6 months. There were otherwise no statistically significant differences.
Conclusions: VFRs are more effective than Hawley retainers at holding the correction of the maxillary and mandibular labial segments. The median differences were 0.56 mm in the mandibular arch and 0.25 mm in the maxillary arch. Although this difference is unlikely to be clinically significant in the maxillary arch, it could be considered clinically significant in the mandibular arch if located to a single tooth displacement.