To investigate long-term stability 20 years after orthodontic treatment and the association with arch width changes during treatment.

Materials and Methods

This retrospective study investigated 103 patients with Class I and II malocclusions treated with fixed appliances with and without extractions. The sample was treated by one experienced orthodontist and collected from a private orthodontic office. Dental casts were obtained pretreatment (T1), posttreatment (T2), and long-term postretention (T3); they were scanned and converted to STL files. Measurements were evaluated in for the upper and lower arch: intercanine width (IC), intermolar (IM) width, Little's irregularity index (LII).


There were 73 female and 30 male patients. Class I was present in 74 patients and Class II in 29. Average postretention time was 17.2 (±6.5) years after an average active retention time of 3.4 (±1.17) years. Extraction was performed in 55 patients while 48 received nonextraction treatment. Bonferroni Post Hoc test showed that LII in the upper and lower arches at T1 was significantly higher in the extraction group (P < .001). Upper and lower arch LII at T3 was slightly higher in extraction cases but remained under 2.05 mm. LII at T3 in the upper and lower arches showed negative correlation with IM T3 in the upper arch (Pearson, N = 103, P = .047), while IC in the upper and lower arches at T3 correlated with IM T3 in the upper and lower (N = 103, P < .001).


Clinically relevant long-term stability in both arches was found in extraction and nonextraction cases. Intermolar width and its change during orthodontic treatment was an influential factor on long-term stability in extraction cases.

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Author notes


Research Assistant, Department of Orthodontics, Faculty of Dental Medicine, University of Rijeka, Rijeka, Croatia.


Private Practice, Arlington, TX, USA.


Private Practice, Waco, TX, USA.


Assistant Professor, Faculty of Engineering, Laboratory for Precision Engineering, University of Rijeka, Rijeka, Croatia.


Professor, Department of Orthodontics, School of Dental Medicine, University of Zagreb, Zagreb, Croatia.