Résumé
Tooth extraction is one of the most controversial subjects in orthodontics due to its inevitable effects on facial esthetics, and smile characteristics. The aim of this study was to evaluate lip position in relation to S-line and Snpog', Merrifield Z angle, as evaluating well as upper and lower arch widths in canine, premolar, and molar regions following four premolar extraction in class I malocclusion patients. Pre and post lateral cephalograms and dental casts of 47 class one malocclusion patients including 43 girls and four boys with the mean age of 12.7 +/- 1.86, treated by the same practitioner were used in this study. All patients had four first premolar extraction, and orthodontic treatment was performed with 0.22 slot standard edgewise appliance. Convenient improbable sampling was used in this cross sectional study. The distance of the most prominent points of lips from S-line and Sn-Pog', and Z-angle values were measured on lateral cephalograms. Upper and lower arch widths in canine, premolar and molar regions were also measured by a digital caliper on dental casts before and after treatment. To compare the changes, paired t-test with 0.05 significance level was used. Following orthodontic treatment with four premolar extraction, dental arch widths in upper and lower arch increased in canine region [p<0.001], but it was decreased in premolar and molar regions significantly [p<0.001]. It was also observed that, the lips were retracted significantly with reference to S-line and Sn-Pog' [p<0.001], and Z-angle was also increased [p<0.05]. Orthodontic treatment with four first premolar extractions in class one malocclusion patients resulted in: 1. Increase in upper and lower arch width in anterior region, and decrease in posterior region. This means that, first premolars extraction in Class one malocclusions does not increase buccal corridor width, and consequently have no adverse effects on smile esthetics. 2. More posterior position of the lips related to S-line and Sn-Pog' as well as increase in Zangle leading to more straight profiles in patients