Association of Mandibular Incisor Proclination and Mandibular Arch Widths with The Levelling of Curve of Spee

Background: In this study, we aimed to determine the changes in mandibular incisor proclination and mandibular arch widths after leveling the curve of Spee (COS). Methods: Patients included in this quasi-experimental study were scheduled for fixed orthodontic treatment with a non-extraction therapy in the mandibular arch, no posterior crowding, eruption of all mandibular teeth except third molars, no spacing and Little’s irregularity index greater than ۲. The COS depth and mandibular arch widths (inter-canine and inter-molar widths) were measured on plaster casts and recorded in millimeters. Mandibular incisors inclinations were measured on a cephalometric radiograph according to the incisor mandibular plane angel (IMPA) and lower incisor to NB (L۱) angle. The pre-treatment and post-treatment values were compared using paired sample T-test. Pearson correlation was used to analyze the association between COS changes and other variables. P-value <۰.۰۵ was considered statistically significant. Results: A total of ۵۸ subjects with an average age of ۲۵.۱۰±۶.۸۱ years participated in this study. COS depth values significantly reduced after treatment (۲.۳۴±۰.۷۸mm vs ۱.۷۵±۰.۵۶mm), (P<۰.۰۰۱). Average IMPA and L۱-NB values significantly increased following COS flattening (P<۰.۰۰۱). Mandibular inter-canine and inter-molar widths also showed significant increase after treatment (P<۰.۰۰۱). Pearson’s correlation revealed a significant and inverse correlation between COS changes and mandibular arch width and mandibular incisor proclination values (P<۰.۰۰۱ for both). Patient’s age and gender did not have a significant effect on COS depth. Conclusion: Flattening of COS leads to an increase in lower incisor proclination and mandibular arch width.

Keywords:

Cephalometry , Orthodontics , Interceptive , Orthodontic Wires , Treatment outcome