Post-treatment changes in permanent retention

While permanent retention is today the method of choice to stabilize orthodontic treatment outcomes, recent studies have increasingly reported posttreatment changes in tooth position during permanent retention. We conducted this study to analyze changes in the anterior mandible, whether the changes follow an underlying movement pattern, and, aiming for a preventive strategy, whether any risk factors could be identified comparing findings with the pretreatment situations.

We included 30 patients who had worn fixed Twistflex retainers (UK 3-3) extending from canine to canine in the mandible. Casts reflecting the intraoral situations before orthodontic treatment (T0), directly after completion of active therapy (T1), and 6 months later (T2) were scanned and superimposed using Imageware Surfacer software. Posttreatment changes (T2-T1) of tooth position within the retainer block were analyzed on 3D virtual models and were compared to pretreatment (T0) and treatment-related (T1-T0) findings to identify potential risk factors.

Almost all analyzed patients revealed three-dimensional changes in tooth position within the retainer block. Comparing these movements, we repeatedly found rotated retainer blocks in labio-oral direction, while the center of rotation was located at the first incisors. This pattern was associated with intercanine expansion and excessive overjet correction during orthodontic treatment. The canines underwent the most pronounced (rotational and translational) movements.

In general permanent lingual retainers are safe but in special clinical cases retainers can induce undesired tooth movement. Risk factors seem to be intercanine expansion and excessive overjet correction during orthodontic treatment. In specific cases an additional retention device might be needed.