Date of Award


Degree Type


Degree Name

Master of Clinical Dentistry




Dr. Lesley Short

Second Advisor

Dr. David Banting


The purpose of this study was to compare the soft tissue changes in subjects with orthodontic treatment involving two different extraction patterns: four first premolars or four second premolars. Pretreatment and posttreatment orthodontic treatment records of 81 four premolar extraction patients were obtained and divided into two groups: (PM1) four first premolar extraction group (n= 48) and, (PM2) four second premolar extraction group (n=33). A comparison was made of the changes between the pre- and posttreatment measurements of nasolabial angle (NLA), upper and lower lips to E-plane(mm) using age, sex, upper lip thickness, convexity, and facial axis as variables. Correlation tests were also performed between changes in dental and soft tissue outcomes. The results showed that the NLA increased in both treatment groups indicating a reduction in soft tissue lip protrusion. However, there was no statistically significant difference between the two groups (p=0.99). The average change in lip position between the two groups was not statistically significantly different (p=0.68 and p=0.27 for the upper and lower lip to E-plane, respectively). The upper lip position was strongly correlated with the position of the upper incisor and the lower lip. Conclusion: There was no statistically significant difference between the change in the nasolabial angle or the retraction of the upper and lower lips between four first and four second premolar extraction orthodontic treatments. Furthermore, the change in the nasolabial angle was not significantly correlated with any dental or skeletal changes in either group. Clinically significant difference was found between the groups in some dental and soft tissue outcomes. Extraction pattern alone was not a significant predictor of the changes in soft tissues.



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