Treatment change comparisons between skeletal Class I and II white adolescents with 3 different vertical divergencies—part 1: Frankfort-Mandibular Incisor Angle (FMIA) and soft-tissue profile

Abstract

This study investigated the interrelationship between FMIA and soft-tissue profile changes in skeletal Class I and II white adolescents with 3 different vertical skeletal patterns. Two hundred seventy cephalometric images were constructed from pretreatment and posttreatment cone-beam computed tomography images of 135 white adolescents (69 females and 66 males with a mean age of 12.8 ± 1.4 years pretreatment and 15.0 ± 1.4 years posttreatment). SNA, SNB, ANB, Frankfort Mandibular Angle [FMA], Incisor Mandibular Plane Angle [IMPA], Frankfort-Mandibular Incisor Angle [FMIA], and Z angle were measured. A mixed-model analysis of variance was performed for patients with an increased posttreatment Z angle to evaluate within-subject and between-subject effects of variables in relation to horizontal and vertical skeletal patterns. Post-hoc tests were conducted to identify statistical significance among the 3 different divergent subgroups. Patients with a skeletal Class I relationship had straighter facial profiles and a larger FMIA than patients with a skeletal Class II relationship before and after treatment. FMA, FMIA, IMPA, and Z angle treatment changes were similar between the skeletal Class I and II groups. The final FMIA means of the 3 divergent subgroups converged at 65° in the skeletal Class I group and 60° in the skeletal Class II group. The mandibular incisors were uprighted during treatment in the hyperdivergent patients whose Z angle values increased after treatment. Horizontal skeletal relationships seem more suitable for determining the desired FMIA in Tweed's total space analysis than vertical skeletal divergencies.