Three-dimensional analysis of posttreatment tooth movements despite bonded retainers: part II-lower jaw.

Purpose: Complications of bonded lingual retainers in terms of unexpected tooth movements have been reported with increasing frequency during the last decade, but the vast majority of the literature comprises case reports. The purpose of the present retrospective case-control study was to analyze the amount and direction of unwanted tooth movements despite lower bonded retainers, to analyze possible predisposing pretreatment- and treatment-related factors, and to seek for movement thresholds which could enhance the rapid and objective identification of critical cases.

Methods: Plaster casts of 1026 patients who completed orthodontic treatment and a subsequent retention phase of 2 years were screened for unintentional tooth movements. The study group comprised 39 patients with tooth movements in the lower jaw, while 39 randomly selected patients without visible tooth movements served as the control group. For all patients, plaster casts after debonding of multibracket appliances (T1) and after supervised retention (T2) were digitized, and a three-dimensional (3D) digital superimposition based on the best fit of premolars and molars was undertaken. Thereafter, translational as well as rotational movements were measured in all three planes of space. Pretreatment- and treatment-related factors of the study and control groups were compared. A severity classification based on rotational movement thresholds was applied to seek a critical amount of translational movements.

Results: The mean translational movements ranged between 0 and 0.4 mm and the average rotational movements between 0 and 1.6°. Large individual movements up to 1.9 mm translation and 16° rotation were seen. A twist-effect with opposite movements of the canines along the Y‑axis could be confirmed. Compared to the control group, patients of the study group had a smaller intercanine distance at all timepoints. In addition, study group patients presented a slightly larger intercanine expansion during treatment and were more often affected by retainer bonding site detachments. Applying the severity classification based on rotational thresholds, translational movements of 0.5-1.0 mm along the X‑ and Y‑axis could serve as a critical threshold. It can be assumed that extrusive translational movements along the Z‑axis seem to be of specific nature and perhaps do not reflect a retainer complication in terms of unwanted tooth movements.

Conclusions: Patients with a larger intercanine distance after active treatment and those with more frequent retainer bonding site detachments could be at higher risk for unwanted tooth movements during fixed retention. Sagittal and transverse movements of 0.5-1.0 mm should sensitize the practitioner for further measures.