Long-term stability of the leveling of the curve of Spee

The aim of the study was to investigate whether the orthodontic leveling of the curve of Spee is a treatment procedure with a stable result on a long-term basis. Measurements were made on the plaster casts of 149 orthodontically treated patients (57 males and 92 females). The mean age before treatment was 12.8 years (range, 8-25 years). Study casts were taken before treatment (T1), at the completion of orthodontic therapy (T2), and 6.7 years (mean) posttreatment (T3). Inclusion criteria were no extractions, all Angle classifications except Class III malocclusions, and all permanent teeth fully erupted except second and third molars. The curve of Spee and the irregularity index were measured on standardized digital photographs of the casts. Overjet and overbite were assessed with a ruler. Changes in the curve of Spee were correlated with changes in irregularity index, overjet, and overbite from T1 to T3. The following results were noted: (1) leveling of the curve of Spee is a relatively stable treatment procedure compared with a return of incisor crowding and deepening of the bite; (2) neither the initial depth of the curve of Spee nor the initial irregularity index is an indicator for the amount of relapse; (3) the amount of leveling is not correlated with the relapse of the 4 tested parameters (curve of Spee, irregularity index, overjet, and overbite); and (4) there is a mild correlation between the relapse of the curve of Spee and the relapse of the irregularity index, overjet, and overbite. According to the results of the study, leveling the curve of Spee during orthodontic treatment seems to be very stable on a long-term basis; it was weakly correlated with the other variables tested.