Individual Factors of Botulinum Type A in Treatment of Gummy Smile: A Prospective Study

BACKGROUND Botulinum type A (BTX-A) injection is a promising corrective method for gummy smile (GS). However, its effect among patients is varied and inconsistent. OBJECTIVE To explore the effect of individual factors on BTX-A treatment for GS and the degree of their influence, and to establish the indications of average-dose BTX-A injection for GS treatment. METHODS In this prospective clinical study, a standardized BTX-A injection technique comprising bilateral single-point injections of 2 U BTX-A (total, 4 U) was administered to all GS patients. Data were collected at baseline and 4, 12, and 32 weeks of follow-up. Twenty-nine potential individual factors were analyzed using correlation and regression analysis to exclude confounding bias. RESULTS In all, 94 patients completed the BTX-A injection. After adjusting for potential confounding factors such as exposed medial incisor, medial incisor length, width-to-length ratio of the medial incisor length, overbite and overjet of the anterior teeth, the correlation and regression analysis confirmed the following formula (adjusted R 2 = 0.617, P ≤ 0.001): anterior gingival exposure (GE) at 4 weeks = 1.44 + (0.94 × baseline anterior gingival exposure) - (1.88 × sex) (where male = 1 and female = 2). The confidence interval(CI) of the prediction showed that for all female participants with baseline anterior GE <5.3 mm, the 95%CI of anterior GE was 0.3-3.0 mm after 4 weeks of this average dose of BTX-A treatment, and it was 3.0-8.9 mm for all female participants with baseline anterior GE ≥6 mm. This value would likely be between 1.5 mm and 3.3 mm for male patients with a baseline anterior GE of 3 mm, which was between 3.2 mm and 8.9 mm for male patients with baseline anterior GE ≥4.6 mm. CONCLUSION The individual effect of the average dose of BTX-A treatment for GS was GS severity and patient's sex, rather than GS etiology and other individual factors. Further, female participants with baseline anterior GE<5.3 mm were more likely to show complete improvement after 4 weeks of this average dose of BTX-A treatment. However, female participants with baseline anterior GE ≥6.0 mm or male participants were less likely to show complete improvement at 4 weeks.