Introduction
Today, more and more people are striving to realize [4, 14] in their own life the motto: “everything in a human being should be perfect” [15].

Mostly, we pay a lot of attention to our appearance and special importance is attached to our face [5,6]. Every unique human face has important personal significance for each of us. Smile and lips are the leading factors in the perception of face aesthetics.

Many authors like Pererverzev (1978), Khoroshilkina (1979), Persin (1988), Polma L (1996, 2010), Arsemima (1998), Ricketts (1981), Bishara (1985), Bacceti (2000), Sarver (2001), and Ackerman (2004) made great contributions to the study of the issue of face aesthetics and its violations.

The motivation of patients for dental treatment in recent years is increasingly aimed at obtaining an aesthetic result. Analysis of the reasons why patients turn to dentists showed [7,11], that 23.0% of patients want to improve the aesthetic appearance of their teeth, 71.2% orthodontic patients want to improve face and teeth aesthetics such as reconstruction of the teeth rows.

During the last years, facial aesthetics is especially associated with the smile zone. The smile zone is a special structural, a functional and aesthetically significant area. It consists of different macro- (facial and labial) and micro (gingival and tooth crowning) parameters [3].

The smile formation process divides into 4 stages:

1. stage – lips are closed
   2. stage – lips are ajar (half-opened)
   3. stage – natural smile (three quarters opened lips)
2. stage – broad smile.

Currently, smile analysis is the key to diagnosis and planning of rehabilitation of dental patients. A smile can destroy or emphasize the face harmony. Therefore, an attractive smile becomes an important indicator of successful dental rehabilitation. The clinical value of a harmonious balance is determined by the limits of the possible effect on soft tissues and the direction of orthopedic treatment, which allows to achieve the best aesthetic result.

What is “the perfect smile”? Are there any clear criteria for this concept? We can easily tell which smile is beautiful, but it is difficult for us to describe those characteristics, that creates it.

Many investigations of the problem of aesthetic facial disorders of young patients indicates the presence of “gingival smile” in 10 – 15% of cases.

Actually, smile aesthetics depends on: ratio parameters between teeth and gums, their compliance with the rules of structural beauty, the ratio between the teeth and lips parameters and their harmonious integration with the components of the face. Mimic muscles are the main component of a smile [32, 33].

Approximately 7% of men and 14% of women have excessive visualization of gums with a smile. Excessive gum visualization is a descriptive term rather than diagnosis that involves the mandatory conduct of a specific treatment.

Gummy smile (or gingival smile) – it’s a kind of structure of tissues of the oral cavity, wherein smile occurs during displacement of the upper lip exposing the gums. For the correct diagnosis doctor requires knowledge in the field of facial aesthetics. The main parameter of the estimation is the height of the face oval. The height of the middle part of the face should be equal to the height of the lower part with a relaxed state of mimic muscles.

A “gummy smile” may be a symptom of a disorder in structures of the facial skeleton and hyperactive facial muscles (Fig. 1).

fFg. 1: Aetiology and pathogenesis of gummy smile.

Anatomical reference point of the middle part of the face is glabella – the most prominent point of the frontal bone between the superciliary arches and the lower point of the nasal septum. The lower part is measured from the bottom point of the nasal septum to the lowest point of the soft tissues of the mandibula, i.e. the lower edge of the chin.

Fig. 2a + b: Measurements of lip harmony (see text).

It is necessary to measure the length of the upper lip after making assessment of the height of the face. In the state of relaxation of the facial muscles length from the bottom of the nasal septum up to the lower edge of the upper lip is 20-22 mm on average for young women (Fig. 2a) and 22-24 mm for young men. In this case for women, 3-4 mm maxillary central incisors are usually visualized (Fig. 2b), and 2 mm less for men. Over time, there is a trend to upper lip lengthening.

Short or hyperactive upper lip is one of the factors forming a “gummy smile”. Usually, with a broad smile, teeth crowns 10-11 mm long are completely visible. However, for patient with a hyperactive upper lip these parameters can be raised in 1.5-2 times (Fig. 3).

Fig. 3: Clinical presentation of a gummy smile.

In addition, excessive lengthening of the teeth of the frontal group of the maxilla [20] leads to a displacement of the gingiva together with the underlying bone, and their lower position leads to the appearance of a “gummy smile”.

The reason of excessive visualization of the gum also might be an increase in the height of the mandible, which makes the lower part of the face longer relatively to the middle part [16,18,22]. According to Jiao Wei et al. (2015), one of the aetiological factors of the “gingival smile” may be dysplasia of the nasal septum [21]. The most complicated case that require special attention – is a combination of several factors.

Smile is unique for each person. There are several classifications of smiles. According to the Rubin and Philips classification, there are three basic types of smile [12, 30, 31].

The first type is a commissural smile (“La Gioconda’s smile”) – occurs in 67% of people. When smiling, the corners of the lips move laterally upwards by 7-22 mm. The corners of the lips (commissures) occupy a position above the upper lip and the lateral part of the lips forms an angle of 40 ° (more often 24-38 °) to the horizon. Lips form two curved arcs, in the gleam of them only the upper teeth are visible, sometimes even wisdom teeth. Zygomaticus major and minor muscles are involved in the formation of this type of smile.

The second type of smile – “canine” or “labial” – is observed at 31% of people. It is formed without a significant shift up the corners of the mouth. Upper lip rises upwards, exposing 6-8 upper teeth, lower teeth are closed with lower lip. The lower lip takes the form of an arch, the upper one has curves, in one of which the canines are exposed (this is the reason of the characteristic name). Muscles lifting the upper lip and the one lifting the upper lip and the wing of the nose are involved in the formation of this type of smile.

The third type – “full denture” or “complex” smile – found at 2% of people. With a smile, both upper and lower teeth are exposed and the lips have the form of two practically parallel lines. The maximum number of antagonist muscles of middle and lower third of the face is involved in the formation of this type of smile. Therefore, the key characteristic of this smile – strong muscular tension and displacement of the lower lip down and backward [13].

Multifactor analysis [10] (2009) carried out by Polma allowed to reveal a syndrome, accompanying unaesthetic types of smiles, for which patients complain. A high type of smile (“gummy” smile) is common not only due to vertical enlargement of the upper jaw, or the increase in the height of the lower part of the face and the prevalence of the vertical type of growth, but also as a result of soft tissue anatomy. In 90% of cases a straight or downward bending of the upper lip is noted. Large percentage of “gummy” smiles (90%) arises due to anterior rotation of the upper jaw, and in 75% of cases it is accompanied by a retraction of incisors of the upper jaw.

fig. 4: an interdisciplinary approach to gummy smile.

There are five variations for exposing teeth and gums in a smile:

Type 1 – only the upper teeth;
Type 2 – upper teeth and more than 3 mm of gum;
Type 3 – only lower teeth;
Type 4 – upper and lower teeth;
Type 5 – neither upper nor lower teeth.

With aging, there is an elongation of the upper lip occurs with simultaneous reduction of the alveolar processes of the upper jaw and the maxillary bone in general. Against this background, the exposure of the gum with a smile is leveled.

Mazzuco and Hexsel suggested an aesthetic-functional classification of “gingival smile” [23,24] (Tab. 1):

Multifactorial analysis of the smile and its consistent design are the key stages of diagnosis and planning aesthetic correction. Diagnosis of smile aesthetics disorders should be conducted on an interdisciplinary basis, acknowledging the standards of harmonious smile, professionally installed for different age-sex and ethnic groups.

The plan for aesthetic correction of the “gummy smile” is developed after an accurate diagnosis and includes the use of both orthodontic correction and maxillofacial surgery, also the use of botulinum neuroprotein, for mimic muscle relaxation [21] (Fig. 4).

According to various authors injections of botulinum neuroprotein (or botulinum toxin) are necessary for patients with gingival smile for reduce hypermobility of the upper lip [8, 12, 25, 29]. The main target muscle is the one that lifts the upper lip (m. levator labii superioris) together with m. zygomaticus minor, m. zygomaticus major, m. depressor septi nasi, m. orbicularis oris [17, 28] (Fig. 5).

Injection of botulinum toxin into the muscle lifting the upper lip may be accompanied sometimes by ptosis of the upper lip and its excessive elongation, protrusion of the lower lip and its asymmetry [19, 32].

The mechanism of action of the botulinum neuroprotein is due to the progress of chemodenervation – direct peripheral influence on motor fibers (neuromuscular transmission), binding to the presynaptic terminal and blockade of transport protein, that takes from 1 to 3 days, so the effect of muscle relaxation begins to manifest a few days after the injection of botulinum toxin into the muscles [1]. The use botulinum toxin for the correction of muscle hypertonicity is based on the following positions [3]:

1. Botulinum toxin provides long lasting muscle relaxation, that allows to break the vicious circle of muscle tension and pain, and to eliminate nerve compression by the tense muscle, if the last exists.
2. Important advantages of treating with botulinum toxin are its local, predictable, dose- dependent effect and a low risk of systemic side effects.

Fig. 5: Target muscles for botulinum toxin to correct gummy smile [2].

Materials and methods
Criteria for selection were complaints about aesthetic dissatisfaction with a smile, exposure of the gum more than 3 mm. The research included 18 patients, men – 22%, women 78%, the average age is 29.5 years.

Attention was drawn to the following clinical signs: facial expression, the condition of the mimic musculature with a smile, degree and nature of exposure of the gum. Gingival distance (GD) Is estimated as the distance between the area of the upper lip and the border of the cutting edge of the teeth.

Depending on the type of “gummy smile”, all patients were divided into four groups (Table 2)

The examination included: analysis of data of anamnesis, Clinical examination (examination of the face, oral cavity, Functional tests of mimic muscles), photoanalysis (portrait and intraoral photos). Depending on the type of gingival smile all patients were classified into four groups (see Table 2).

All patients were injected by botulinum toxin, injection points were selected depending on the nature of muscular hyperactivity.

Methodology
In the apical area of the nasolabial fold 5-10 ED of Dysport® were injected symmetrically on both sides. It is possible to determine the injection point of the drug by placing the tip of the index finger on the edge of the pear-shaped hole direct under the nasal-maxillary suture (Fig. 6).

Injection sites and dose of botulinum toxin were defined depending on the type of “gingival smile” (Table 3).

Fig. 6: The injection technique of botulinum toxin.

Results
Analysis of clinical patient’s data demonstrated, that initial positive dynamics in the form of reduced gingival distance was noted on 7th day, and the maximum – on the 14th day after the injection. All the patients noted improvement of smile aesthetics. After 6 months, there was a return of muscular hyperactivity and a “gummy smile”, that required a repeat of injection. In subsequent injections, the doses of botulinum toxin depend on the result of the treatment. Therapy should be conducted very carefully, starting from the lowest dosage. The key to the success of the “gummy smile” correction is the careful selection of patients and use of minimally necessary dosages of botulinum toxin

Conclusion
Since injections of the botulinum toxin are safe, reliable and reproducible [29], and the effect is reversible [26]. Botulinum therapy is an independent therapeutic effect for the temporary correction of gingival smiles, also it gives the ability to supply or postpone surgical interventions for a later period [27].