RESUMEN
<p><b>OBJECTIVE</b>The purpose of this retrospective cephalometric study was to compare changes of the upper lip soft tissue in patients with Class I malocclusion after extraction of either first premolars or second premolars.</p><p><b>METHODS</b>The pretreatment and posttreatment records of 50 patients were used in the study. And 25 patients involved extraction of first premolars and 25 patients involved the extraction of second premolars. The lateral cephalometric radiographs taken before and after treatment were traced and measured. 15 typical values of soft tissue were measured in Winceph 7.0 software and statistical analysis was carried by SPSS 10.0 software.</p><p><b>RESULTS</b>1) The horizontal and vertical lines were not significant different in the two extraction group before the treatment. But after the treatment the vertical changes of upper lip (A'-X, UL-X) were more apparent in the second premolar extraction group (P < 0.05). 2)In the soft tissue measurements, the values of Sn-A'-UL in the first premolar extraction group were less than those in the second premolar extraction group significantly before the treatment. After the treatment all the patients had a significant increase in length of upper lip. At the same time, UL-EP, A'-EP distances were diminished obviously (P < 0.05). But the changes were not significant difference between the two premolar extraction groups (P > 0.05).</p><p><b>CONCLUSION</b>The upper lips both changed remarkably after the treatment and there was no significant difference between the two premolar extraction groups.</p>