Study selection and flow diagram.The search identified 1246 preliminary references related to changes in the gummy smile following BTX-A therapy, of which 236 were found in Pubmed, 26 in Scopus, 63 in Cochrane, 506 in Embase, 362 in Web of Science, 53 in the grey literature search (OpenGrey) and 4 through hand-searching based on the references cited in the articles included. After excluding 508 duplicates, the remaining 738 were screened. Of these, 392 were excluded on reading the title and abstract as they were unrelated to the research question. After examining the full text of the resulting 49 articles, 35 were excluded for the following reasons: 17 did not answer the PICO question, 11 literature review, narratives or letter to editor and 1 had a sample size of fewer than 10 patients. Finally, 26 articles met the inclusion criteria and were included in the qualitative review, and 23 were included in the quantitative review (meta-analysis). The level of agreement between investigators was κ= 0.74 (95% CI, 0.56–0.93). Finally, there were 15 articles that met the inclusion criteria and were included in the qualitative synthesis and 11 were included in the quantitative synthesis (meta-analysis). The PRISMA flowchart (Figure 1) provides an overview of the item selection process.
Evaluation of quality.Risk of bias assessment of the included studies using the ROBINS tool did not show a high risk of bias in any of the studies, however most studies (n=12, 92.3%) had moderate risk of bias. Moderate risk of bias was reported for study participant selection, classification of interventions, deviations from intended interventions, missing data, outcome measurements, and selection of reported outcomes in most studies. Only one study with low risk of bias was found.15
Characteristics of the studies included.Of the 15 studies, 14 were prospective and 1 retrospective. 25 All studies were controlled clinical trials except for 1 randomized study. 26 The selected patients were of adult age, diagnosed with a gummy smile with an exposure greater than 2 millimeters and treated with BTX-A infiltration with or without previous or additional treatments; providing pre- and post-treatment
Qualitative synthesis of the studies included.The mean pre-treatment gingival exposure ranged between 3.5 and 7.2 millimeters, reaching a reduction of up to 6 millimeters after infiltration with botulinum toxin at 12 weeks. 5
The amount of gingiva present in the smile was a parameter studied in all the studies, its post-operative changes through photographs and/or video in the smile were carried out in all the research works. The method used frequently to evaluate the reduction of gummy smile after treatment was the one used by Polo in 2005 11, using the upper right central incisor to calibrate the images of each patient from the highest point of the gingival zenith to the incisal edge. 5,8,12,14-15,26-28 However, other authors 9 took the upper lateral incisor as a reference, measuring the distance between its central cervical area to the lower part of the upper lip. Mazzuco and Hexsel., 2010 1 in two of their four study groups used the upper central incisors and in another two upper premolar groups when evaluating a more posterior gummy smile, however Sucupira and Abramovitz., 201216used only the upper central incisors, Suber et al.,13and Dutra etal.,25also used the upper central incisor although its gingival measurement was the distance between the stomion point of the upper lip and the incisal edge of the upper central incisor subtracting the length from it. Gupta and Kohli., 201929did not mention their anatomic reference.
Results of the individual studies. The muscles par excellence selected for blockade with BTX-A were: levator labii superioris, levator labii superioris ala nasalis, and zygomaticus minor; infiltrating from 1.25 to 7.5 units per side. Many of the studies 5, 11,28-29 refer to the infiltration site as “Yonsei Point” described by Hwang et al .,in 200930 located in the center of the anatomical confluence of the three afore-mentioned muscles that form an imaginary triangle where its center would be the point of infiltration. Other authors1,8,14,16,27 infiltrated only the levator labii superioris muscle.
The follow-up time of the patients in each study was analyzed to assess the surgical process over time; differentiating 4 stages: T0 (Pre-surgical), T1 (Post-surgical), T2 (1st Revision), T3 (2nd Revision), T4 (3rd Revision), T5 (4th Revision), T6 (5th Revision), T7 (6th Revision) . The most common follow-up times were at two weeks, one month, 3 and 6 months. The studies that took into account a longer follow-up time was that of Al Waily et al., 5 reaching 9 months and Rajogoapal et al. 28 with a follow-up at T4 that ranged between 7 and 14 months, however in this review, botulinum toxin was infiltrated again as the second cycle of treatment.
Nine studies provided information on the reduction of the gummy smile after infiltration with botulinum toxin at two weeks, representing a global sample of 213 patients. The decrease in the parameter is significant for all authors (p<0.001) except Hexsel et al.,26and Sucupira and Abramovitz 16, with mean values between -1.22 31 and -5, 1115 millimeters.
Five studies are included in the meta-analysis estimates regarding the reduction of gummy smile 3 months after infiltration, grouping a total of 182 patients. The mean reduction difference ranges from 1 millimeter estimated by Somaiah et al.,12to -5.95 millimeters in the work of Al Wayli et al.,5In all cases the reduction is significant (p<0.001) except from Somaiah et al.,.12
Quantitative synthesis of included studies. A total of 11 studies evaluate the changes in millimeters of the gummy smile after infiltration with BTX-A at two weeks and three months.
Changes in gummy smile at two weeks. To estimate the reduction in gummy smile at two weeks, 9 studies have been combined using a random effects model, estimating a reduction of -3.22 millimeters (95% CI between -4.43 and -2.01) (Figure 2). The heterogeneity between the included studies was high (Q Test= 170.04; p value <0.05; I2 95.3%). The infiltrated botulinum toxin units after performing a meta-regression analysis does not affect significantly (p=0.781) (Figure 3), the number of millimeters that the gummy smile is reduced by the injected BTX-A units being independent.
Changes in gummy smile at three months. 5 studies that present results of the measurement of the gummy smile 3 months after the first infiltration with BTX-A have been combined, estimating a reduction of -2.70 millimeters (95% CI between -4.52 and -0.88 ) (Figure 4). Like the previous one, the meta-analysis has shown heterogeneity (Q test=193.35; p value <0.05; I2=97.9%). The reduction of gummy smile at 3 months is not influenced by the number of infiltrated BTX-A units. The meta-regression analysis has not shown a significant effect (p=0.357) (Figure 5).
Comparison between two weeks and three months. Through a subgroup analysis, no significant differences were found between the estimates of gummy smile reduction at two weeks and three months (Q test for intergroup heterogeneity = 0.221; p value = 0.638). Similarly, through a meta-regression in which the 11 studies have been included, no significant effect of the follow-up time on the reduction of the gummy smile was found (p value=0.630). (Figure 6)
Publication bias. The two meta-analyses do not appear to be affected by publication bias. Using the Duval and Tweedie's trim and fill method, no differences were observed in the estimation of the reduction between the meta-analyses at 2 weeks and 3 months with the observed studies and the meta-analyses with the imputed studies (Figures 7, 8). In addition, using the classic fail safe number, the number of non-significant studies necessary to include so that the significant estimate of the reduction ceased to be significant, presented high values, specifically 1156 in the meta-analysis at two weeks and 530 at three months.