Evaluation of the botulinum toxin effects in the correction of gummy smile 32 weeks after application

Carlos Eduardo Alonso Vieira, Wilson Rodrigues de Almeida, Paula Cotrin, Renata Cristina Gobbi de Oliveira, Ricardo Cesar Gobbi de Oliveira, Fabrício Pinelli Valarelli, Julie Watanabe Zamuner, Karina Maria Salvatore de Freitas Departamento de Ortodontia, Centro Universitário Inga (UNINGÁ) – Maringá (PR), Brazil Departamento de Ortodontia, Faculdade de Odontologia de Bauru, Universidade de São Paulo (USP) – Bauru (SP), Brazil


INTRODUCTION
We are living in a connected world. The internet has become one of the main tools for exchanging information and entertainment. In this internet era, social media have gained enormous popularity, and consequently, people began to have self-promoting behavior, leaning on to post selfies and self-presented photographs 1 . Everyone wants to look beautiful in the photographs.
With the advent of the internet and social media, patients are increasingly attentive to the beauty of their faces 2 . Nowadays, new techniques of smile design are being developed. Esthetic procedures performed by dentists have gained special attention, such as dental whitening, dental contact lenses, gingivoplasty, and lately, facial harmonization, like botulinum toxin and dermal fillers [3][4][5] . Patients undergoing single cosmetic procedures report overall improvements in quality of life 6,7 .
The facial esthetic harmony comprises the equal proportion of the sizes of the three facial segments, the width of the nose (narrow in women and average in men), and the soft tissue profile 8 . In addition to that, it is also directly correlated with the union of three components of the smile: teeth, gum and lips 9 . Some characteristics are considered essential for the attractiveness of the smile, like smile arc, maxillary dental midline coincident with the facial midsagittal plane and gingival display at smiling. The gingival exposure, when in excess, is one of the factors that most displeases patients [10][11][12][13] . Gingival display of more than 2 mm is rated as progressively less attractive 14 .
The etiology of the gummy smile can be: dentogingival, due to an abnormal dental eruption, with a short clinical crown; muscular, caused by hyperactivity of the main muscles involved in gingival exposure, like levator muscle of the upper lip, levator labii superioris alaeque nasi, risorius and the zygomatic muscles (major and minor); dentoalveolar (skeletal), due to excessive vertical growth of the maxilla, and due to a combination of more than one of the above-described factors [15][16][17] . A good diagnosis could be done, and the right treatment plan could be set up only after a careful analysis based upon the etiopathogenetic factors 16 . The most common therapeutic modalities proposed for the treatment of gingival smile include gingivectomy or gingivoplasty, orthodontic intrusion of the incisors, orthognathic surgery, and recently a less invasive approach, the botulinum toxin 15,[18][19][20][21] .
Botulinum neurotoxin type A (BTX-A) is a neurotoxic protein produced by the Gram-positive strictly anaerobic bacterium Clostridium botulinum. The BTX-A exhibits transient, nondestructive, dose-dependent and localized actions, with minimal side effects. The BTX-A inhibits the release of acetylcholine, which is the neurotransmitter responsible for the activation of muscle contraction. This inhibition process reduces the muscle tone at the site of application 22 . Its cosmetic facial application is safe, predictable and without serious complications when following the recommended guidelines 23-25 . There are several studies in the literature evaluating the application of botulinum toxin for the correction of gummy smile with follow-up of its effects from 2 to 24 weeks after injection 20,24-26 . However, no published studies evaluate its effects for more than 24 weeks after application. This way, this study aimed to evaluate the effects of BTX-A in the correction of the gummy smile with 32 weeks follow-up. The sample size calculation was performed based on an alpha significance level of 5% and a beta of 20% to detect a minimum difference of 1.75 mm with a standard deviation of 2.58 for the measurement of the upper lip stomion to the incisal border of the maxillary central incisor 27 . Thus, the sample size calculation resulted in the need for 36 subjects.

This prospective study was approved by the Ethics
The data were collected according to the following inclusion criteria: the presence of gingival display greater than 2 mm in the maxillary anterior region due to muscle hyperfunction; no vertical maxillary excess, as determined by lateral cephalometric analysis, presence of good periodontal health, no previous esthetic or surgical procedures to correct the gummy smile. Patients with more than 5 mm of gingival display were excluded from the study. For calibration and to correct the photograph magnification, the real size of the right maxillary central incisor was obtained and then transferred to each photograph, and a rule of three was applied to calculate the real value of the gingival display.

Error study
One month after the first measurement, 30% of the photographs were randomly selected and re-measured by the same examiner (CEAV). The random errors were calculated according to Dahlberg's formula 28 and the systematic errors were evaluated with dependent t tests 29 .

Statistical analysis
The normal distribution of the data was evaluated with Kolmogorov-Smirnov tests. Descriptive statistics were performed to evaluate the age of the patients. The

RESULTS
The random error was 0.63 and was within the acceptable range 30 (Table 1). There was no significant systematic error.
The gingival display decreased significantly 2 weeks after application and significantly increased after 32 weeks but did not return to the baseline value (Table 2).

DISCUSSION
The excessive gingival display is a disharmonious periodontal condition that brings esthetic and social disadvantages to the patients. Its treatment should be based on a correct diagnosis. According to the current literature, when this excessive gingival display was due to a muscle hyperfunction, this condition could be treated with surgery 26,31 . Surgical treatments can often be invasive and painful to the patient. In this present study, the choice of applying botulinum toxin for the treatment of excessive gingival display was due to the simple, safe, comfortable and less invasive technique 17,23,24,32,33 .
The dosages of BTX-A used for cosmetic purposes are usually less than 100 U 34 .
There is no consensus in the literature on the number of units that should be applied in the region for the correction of gingival smile 24,26,35 . It is suggested that the dosage and sites of application must be customized according to the severity of each case 23 . In the present study, 2 U per side was applied because the amount of gingival display was moderate. This dosage is in agreement with some previous studies 21,24 .
Only 5 of the 35 patients in the sample were male. This can be justified since women care more about their beauty and esthetics and also seek more cosmetic treatments than men 36,37 . Besides that, it is known that the upper lip of the female subjects is in a more superior position at maximum smile than male subjects 38 . Furthermore, men exhibit a longer upper lip than women 38 . Therefore, authors 27,39 found more female subjects in their study as well.
The levator labii superioris alaeque nasi is the ideal muscle for injection 33 , however, other authors performed injections in different sites, like levator labii superioris 39 . In the present study, the applications were performed only in this area because it offers fewer complications and more predictable results 32 .
In this study, in order to quantify the decrease in the gummy smile, photographs of spontaneous smiles were taken with the same camera, lenses and distance from the patient to the camera. Also, to correct magnification of the image, the real measurement Several studies showed that BTX has a significant effect in reducing gingival smile, progressively decreasing over time. They also showed that some results could be maintained and did not return to pre-injection values after 12 weeks, depending on the muscle thickness and anatomy 20,26,35,39 . However, there is no known study showing these effects after more than 24 weeks of BTX injection. Even though the reported effects of BTX last about 24 weeks, we decided to evaluate its duration for 8 more weeks, to reduce the number of injections in the patient. In our study, a significant decrease in the gingival display was observed 2 weeks after application. Then, gingival display increased significantly 32 weeks after BTX application, but did not return to baseline values. Polo 27 found related results, but he evaluated only 24 weeks after BTX injection. A longer follow-up is needed to determine if the relapse of the gingival display returns to the baseline value over time.