Lip changes after premolar extraction in Class I bimaxillary protrusion: A retrospective study in Thai female adults

Purpose

To determine the ratios of dental and lip changes and develop the predictive equations of lip changes in Class I bimaxillary protrusion in Thai female adults after orthodontic treatment with 4 premolars extraction.

Materials and methods

The sample consisted of pre- and post- treatment lateral cephalograms of 91 Thai subjects with Class I bimaxillary protrusion. The ages of the samples were between 17–39 years old (22.57 ± 4.7). These patients had orthodontic treatment with four premolars extraction and retraction of upper and lower incisors. Pre-and post-treatment lateral cephalograms were traced and superimposed by using SN-7° plane. The parameters from cephalograms were measured with digital vernier caliper and cephalometric protractor. Statistical analysis was performed to analyze the dental and lip changes by paired t-test. Pearson’s correlation and stepwise multiple regression were made to determine factors that related with lip changes.

Results

Significance antero-posterior changes after treatment were found both in dental and lip analysis but vertical changes were not significant. The correlations were found low to moderate relationship between dental and lip changes in both sagittal and vertical directions. The equations of upper and lower lip changes in sagittal and vertical dimensions were constructed.

Conclusion

The ratios of upper and lower anterior teeth retraction and upper and lower lip changes were 1:0.56 and 1:0.87, respectively. The predictive equations of upper and lower lip reductions were ΔH-U-lip = 0.34(ΔH-tUI) + 1.19 and ΔH-L-lip = 0.57(ΔH-cLI) + 2.11, respectively.