Patients and method: In order to assess stability after orthodontic treatment, 132 patients who had been treated by one orthodontist in private practice were examined 6 years on average after completion of their treatment. Therapeutically induced movements and post-therapeutic changes were measured using dental casts, and the number of cases which had relapsed was calculated as a percentage of the individual measurements. The extent of post-therapeutic changes in patients with and without long-term retention was analyzed together with possible correlations with retention time and retainer type. Furthermore, the influence of gender, Angle classification, treatment-induced changes, initiation, type and duration of therapy, and extraction of premolars on the development of relapse was also investigated.
Results and conclusions: Some degree of relapse was recorded in 13% of the measurements on average, even in patients with some form of long-term retention. However, it may be possible to reduce the relapse rate by taking account of the following criteria: Retention time should be increased in cases of short-term therapy, since relapse was found to occur more often on average (19%) when the treatment time was less than 3 years than when it was longer than 4 years (13%). As the highest relapse rate registered was 24% when therapy was started between the ages of 9 and 12 years, but was up to 42% in younger and older patients, there should be more extensive retention in these latter cases. Where there is a risk of relapse in the anterior arch, fixed lingual retainers should be given preference over removable ones. This is especially applicable to male patients and to non-extraction patients, as relapses in the anterior segment occurred more often or were more marked in these cases. The use of removable retainers is necessary when the transverse stability of the buccal segment is at risk. When only fixed retainers were used in the anterior area, relapses were recorded 6-31% more frequently in the interpremolar distance in the maxilla (21%) and the mandible (35%), and in the lower intermolar distance (27%). In particular, the use of a removable mandibular retainer should not be dispensed with after bicuspid extractions, transverse expansion and, in female patients, in the lower jaw, as relapse in the buccal segment was more marked or more common in such cases. On the basis of our clinical findings and of earlier studies, the retention time should be at least 2 years. The retainer type used has been found to be just as important as the retention time. If optimum relapse prevention is aimed at, fixed maxillary and mandibular retainers in the anterior region should be combined with a removable retainer and worn until the patients reach their late twenties.