Long-term effects of Herbst treatment on the mandibular incisor segment: a cephalometric and biometric investigation.

The purpose of this study was to analyze mandibular incisor changes during and after Herbst treatment with respect to tooth inclination and anterior crowding. The sample consisted of 24 Class II, Division 1 subjects (15 boys and 9 girls) treated with Herbst appliance. Dental casts and lateral head films from before and after treatment, 6 months after treatment and at the end of the growth period (at least 5 years after treatment) were analyzed. During treatment, the lower incisors were proclined (ILi/ML) an average of 10.8 degrees and the incisal edge (li) moved anteriorly by 3.2 mm. The available space and the irregularity index in the lower anterior region were in general unaffected by therapy. During the first posttreatment period of 6 months, the lower incisor inclination (ILi/ML) recovered an average of 7.9 degrees and the incisal edge (li) moved posteriorly by 2.5 mm. However, the available space was almost unchanged. During the second posttreatment period, i.e., from 6 months after treatment to the end of growth, the lower incisor inclination remained on average unchanged in relation to the mandibular plane (ILi/ML) but the teeth retroclined in relation to the nasion-sella line (ILi/NSL). The available space decreased (mean 0.8 mm, p < 0.01) and the irregularity index increased (mean 2.0 mm, p < 0.01). The correlation between changes in the ILi/NSL and in the NSL/ML angles was moderate (r = -0.57, p < 0.01), indicating that the reduction in the ILi/NSL angle was partly a result of anterior mandibular growth rotational changes. In conclusion, it can be said that the proclination of the lower front teeth during Herbst treatment did not result in incisor crowding after treatment. In a long-term perspective, the development of incisor crowding was thought to be associated with normal craniofacial growth changes.