Our objective was to determine whether total posttreatment mandibular rotation is associated with mandibular incisor crowding after retention.
Mandibular incisor irregularity (II) at least 10 years out of retention (T3) measured on dental casts from the postretention database at the University of Washington in Seattle was used to define subjects (II ≥6 mm, relapse group) and controls (II ≤3.5 mm, stable group). After matching patients and controls for the pretreatment II, the relapse group consisted of 33 subjects (17 boys, 16 girls; age at the end of treatment [T2], 15.0 years; follow-up, 16.8 years), and the stable group comprised 36 subjects (11 boys, 25 girls; age at T2, 16.0 years; follow-up, 15.1 years). On cephalograms taken at T2 and T3, facial morphology was evaluated, and total posttreatment mandibular rotation was measured with a structural method. Logistic regression analyses were used to determine the association between relapse and amount of total mandibular rotation. The models were adjusted for potentially confounding variables (age at T2, sex, length of follow-up, and intercanine width change during treatment).
No association between total posttreatment mandibular rotation and relapse was found ( P = 0.515). Total mandibular rotations were comparable in the relapse and stable groups ( P = 0.386). Age at T2 was found to be correlated with long-term incisor stability ( P = 0.030). Sex and intercanine width change during treatment were not related to relapse ( P = 0.225 and P = 0.264, respectively).
Total mandibular rotation is not associated with relapse of the mandibular incisors. Posttreatment skeletal and dental growth changes were comparable in the relapse and stable groups.
The relapse of mandibular incisors after comprehensive orthodontic treatment is so common that some orthodontists automatically plan for lengthy retention of these teeth. But if this change is truly expected, why is our specialty unaware of specific predictors for crowding? The predictors of age, sex, tooth size, anjd initial crowding have been evaluated in the past with moderate success. These authors looked at the association between mandibular incisor crowding and posttreatment mandibular rotation in the long term. While noting that vertical changes take place during late adolescent growth, they wondered whether mandibular rotation is correlated with arch width, which in turn would also impact mandibular incisor alignment.
To locate a sample, the postretention collection in the Department of Orthodontics at the University of Washington was screened, and the incisor irregularity (II) index of Little was measured on study models taken at least 10 years out of retention. Depending on the index value, 2 groups were established: relapse (patients) with II ≥6 mm and stable (controls) with II ≤3 mm. Both study casts and cephalometric films were measured. The authors found that total mandibular rotation is not associated with relapse of the mandibular incisors. The findings show no association between posttreatment mandibular rotation and relapse of mandibular dental arch alignment. This investigation had some limitations that might affect the results. Treatment notes were unavailable to the investigators, so the subjects from both groups might have differed in the details of therapy. These potential confounding variables were not controlled for. The power of the sample to detect differences in total mandibular rotation between the relapse and stable subjects was 41%.