What is the best soft-tissue reference plane to quantify lip change in bimaxillary protrusion cases? A retrospective cohort study

Abstract

Objectives: The objective of the study was to evaluate the validity of five soft-tissue profile planes to actual horizontal lower lip changes following treatment of severe bimaxillary protrusion patients with vertical maxillary excess using extra-alveolar miniscrews. The null hypothesis was no differences in the incremental changes of horizontal lower lip changes from pre-treatment to post-treatment of the five methods compared to actual changes. Materials and Methods: Seventy adults were treated orthodontically with extractions for bimaxillary protrusion and “gummy” smile using extra-alveolar miniscrews. Lower lip horizontal position was assessed with pre- and post-treatment lateral cephalograms and five commonly used soft-tissue reference lines were used to measure horizontal lower lip treatment change. Results: Compared to actual therapeutic lower lip horizontal retraction (4.38 mm), soft-tissue references Ricketts’ E-line (3.89 mm) and Steiner’s S-line (3.88 mm) demonstrated no statistical difference (P > 0.05) from actual change. The five profile plane measures showed moderately high to high intercorrelations among themselves, but none of them were related to the actual amount of anteroposterior lip change that occurred. None of the five soft-tissue measurements showed a statistically significant difference (P > 0.05) between subgroups with least and greatest lower lip retraction. Conclusion: Under conditions of maximum lower lip retraction, Rickett’s E-line and Steiner’s S-line were fair measures of horizontal lower lip change. Although actual lower lip change and soft-tissue reference plane changes were correlated poorly, intercorrelations among the five soft-tissue references planes were moderately high. None of the five soft-tissue measurements was able to discriminate (P > 0.05) between treatments with least and greatest lower lip retraction. It may be concluded that Rickett’s E-line and Steiner’s S-line soft-tissue profile references are valid when there is considerable therapeutic retraction (4+ mm) of the lower lip.