Post-treatment development of the curve of Spee

The aim of this study was to investigate the post-treatment development of the curve of Spee (CS) and to predict its post-treatment stability on the basis of cephalometric parameters. Lateral cephalograms and study models of 135 subjects (50 males and 85 females) were taken before orthodontic treatment (T1; 12.0 +/- 1.5 years), at the end of orthodontic treatment (T2; 14.6 +/- 1.5 years), and at least 3 years out of retention (T3; 26.6 +/- 5.0 years); the curve depth (CD), location of the deepest point (LDP) of the curve, and eight cephalometric parameters were assessed. The sample was divided into a treated and an untreated lower arch group. The upper arch was treated in all patients. The sample consisted of 25 per cent Class I, 73 per cent Class II, and 2 per cent Class-III-treated malocclusions. The results showed that the post-treatment CD was frequently unstable and unexpected changes were relatively common. The LDP was displaced distally during T1-T2 and showed mesial relocation during T2-T3. Assessment of potential predictors of the post-treatment changes in CD and LDP using stepwise regression analysis showed that a deep curve at T2 was associated with a decrease of the CD during T2-T3. A combination of distal location of the LDP with proclination of the lower incisors at T2 and extraction treatment was associated with mesial relocation of the LDP during T2-T3. The results also suggest that an optimal CD of about 2.0 mm at T2 was associated with the least amount of post-treatment change.