The effect of premolar extraction on lip's curvatures in borderline orthodontic patients

Although orthodontists have long recognized that the extraction (Ext) of premolars often is accompanied by changes in the soft tissue profile, investigations indicate that the soft tissue does not always respond favorably to hard-tissue retraction. This study was designed to evaluate the effects of treatment, with and without premolar Ext, on the lateral facial profiles of borderline patients with particular reference to the curvature of the upper and lower lips. Eighty-nine border line patients were included in this study. 41 patients were treated with a non-extraction (Non-Ext) protocol, 16 patients were treated with extractions of 5/5, 18 patients with 4/4 and 14 patients with 4/5. All pre-and posttreatment lateral cephalograms were traced and analysis by one examiner. To quantify the soft tissue effects of growth and treatment, the depths of the upper and lower lip curves were calculated in two ways relative to the PM reference line and anterior soft tissue reference line. The changes in depth of upper and lower lip curves were statistically significant (P<0.001). The Non-Ext group showed an increase in depth of upper and lower lip curves, whereas the Ext group showed a decrease in depth of upper and lower lip curves. No significant differences were found between the various extraction sequences (5/5, 4/4, 4/5) for depth of the upper and lower lip curves changes. 1) The upper and lower lip changes were significant more retrusive in the Ext group and more protrusive in the Non-Ext group. 2) There were no significant post treatment differences in the depths of upper and lower lip curves between premolar extraction sequences.