Do changes of incisal inclination during orthodontic treatment influence the size of the buccal corridors?

Purpose: The buccal so-called corridors are important aspects in smile aesthetics. Changes in the inclination of incisors may influence dental arch display and lip position during smile, with possible consequences on the buccal corridor size. Thus, in this study the effects of changes in the incisal inclination on smile aesthetics after orthodontic treatment were analysed. Methods: A total of 77 patients (44 females and 33 males, mean age 14.4 ± 2.7 years), treated with orthodontic fixed appliance with passive self-ligating brackets were retrospectively selected. Changes in the upper incisors inclination (UII), lower incisors inclination (LII), and various dentoskeletal parameters (SNA, SNB, ANB, SN/AnsPns, AnsPns/GoGn, Fh/GoGn, SN/Occl and IIA) were measured on lateral cephalograms. Smile measurements were normalised by using relative values. Thus, changes in the ratio between the intercanine distance and the intercommissure distance (named "relative intercanine amplitude" [RICaAm]), in the ratio between the dental arch width and the intercommissure distance (named "relative dental arch amplitude" [RDAAm]) and in the ratio between the buccal corridor area and the smile area (named "relative buccal corridor area" [RBCAr]) were measured on smile photographs. Linear regression analysis was used to estimate the effects of ∆UII and ∆LII on ∆RBCAr (by adjusting for age, sex, ANB, FH/GoGn, and ∆RICaAm; significance level α = 0.05). Results: The RBCAr showed a significant reduction of 3.2% after treatment (p < 0.001), which was independent from changes in UII and LII, but rather influenced by an increased RICaAm (p = 0.015), and by the age at the beginning of orthodontic treatment (p = 0.013). Conclusion: Beginning orthodontic treatment at younger age and widening the intercanine amplitude may lead to smaller buccal corridors after treatment, while incisal inclination seems to have no important effect on the buccal corridors.