Introduction: The aim of the present study was to evaluate the variability in posttreatment arch shapes, and investigate its relation to the long-term stability of dentition. Materials and Methods: The sample consisted of 100 previously treated orthodontic patients from 3 private practices, evaluated at posttreatment (mean age 15.6 +- 3.1 years old; 51 extraction, 49 nonextraction) and again at postretention (mean age 33.4 +- 7.7 years old). Three dimensional orthodontic models were digitized to determine arch shape, dimensions, and malalignment. Arch shape was determined using fourth-order polynomials best fit to the digitized arch. Malalignment was based on TSALD and incisor irregularity. Results: Malalignment changes were not significantly different between groups. TSALD and incisor irregularity were significantly correlated. Contact angles were smallest between canines and lateral incisors. They were significantly smaller in extraction cases than nonextraction cases. Extraction cases also had significantly smaller posttreatment arch dimensions. Factor analyses demonstrated that both males and nonextraction arch shapes were broader at posttreatment than female and extraction arches, respectively. Broader posterior arch shapes were significantly correlated with fewer changes in posttreatment crowding. Conclusions: Orthodontic treatment can be very stable long term. Sex and treatment modality are related to arch shape. Broader posterior arches, together with larger contact angles, indicate more stable arches.
