Long term studies have shown that relapse occurs  in around 70%  of  orthodontic treatment.  However it is difficult to predict either those cases that will relapse or the degree to which it will occur. The aim of the this study was to compare the effectiveness of vacuum-formed and Hawley retainers in maintaining arch width as well as incisor alignment post treatment.

The authors randomised 82 patients  who had received treatment with upper and lower fixed appliances to either a Hawley or a vacuum-formed retainer.   Outcomes were assessed on study models taken on the day of appliance removal and at two, six, and 12 months. The outcome variables were upper and lower intermolar widths, intercanine widths, arch length, and a modified Little’s index of irregularity. Method error was determined by repeating the measurements on 10 sets of models.

They  found that for each of the variables under test and at each of the four time periods, there were no statistically significant differences between each of the two retainers.

They concluded:-

The degree of relapse that is likely to occur following a course of fixed appliance therapy is unlikely to be affected by the choice of retainer, vacuum-formed or Hawley. Therefore, when deciding on the type of retainer to be fitted following fixed appliance therapy, other factors such as cost may play a more significant role.

Barlin S, Smith R, Reed R, Sandy J, Ireland AJ. A retrospective randomized double-blind comparison study of the effectiveness of Hawley vs vacuum-formed retainers. Angle Orthod. 2011 May;81(3):404-9. Epub 2011 Jan 24. PubMed PMID:1261482.