Establishing standards for Yonsei point in a White South African population for the treatment of gummy smile

Abstract Introduction The purpose of this study is to establish the efficacy of Yonsei point in the treatment of a gummy smile in a White South African population. The accurate surface anatomy criteria in relation to the underlying musculature for the administration of Botulinum toxin injections in the treatment of gummy smile was determined. Materials and Methods Nineteen (10 males and 9 females) cadavers were selected for facial dissection. Facial profile photographs were taken before and after dissection. The before and after photographs were overlayed to determine where the pin positions should be on the dissected cadaver to determine the Yonsei point. The levator labii superioris (LLS), LLS alaeque nasi (LLSAN), zygomaticus minor, and zygomaticus major muscles were measured using a protractor and ruler, which accounted for the manual measurements. Digital measurements were measured by importing dissected images into ImageJ. Circles with a 2 cm diameter (1 cm radius) were constructed to determine whether the Yonsei point could successfully influence muscles fibers. Results Digital and manual measurements show comparable results with high correlation and reliability. Results showed that the White South African population had narrower facial musculature angles as compared with the Korean population. Conclusion Based on the selected sample, the Yonsei point is an ineffective injection site for the successful treatment of gummy smile in a White South African population.

as a result of skeletal, gingival, and muscular anomalies (Moura et al., 2017). These anomalies are related to the abnormal gingival movement away from the crown of the teeth (altered passive eruption), overeruption of teeth (dentoalveolar extrusion), excessive maxillary growth in the vertical plane (vertical maxillary excess), and short or hyperactive lip elevator muscles (Mostafa, 2018). Orthodontists have previously tailored their treatment to target hard tissue structures such as the maxilla. As a result, very minimal treatment and attention is given to hyperactive lip elevator muscles (Moura et al., 2017). Although orthognathic surgery is invasive, this treatment option is used for patients with gummy smiles that are not mainly caused by hyperactive lip elevator muscles. The treatment for hyperactive lip elevator muscles is minimally invasive and can be performed with administration of Botulinum toxin (Botox) injections into the muscles (Mostafa, 2018).
The muscles that are responsible for the elevation of the lip are the levator labii superioris (LLS), LLS alaeque nasi (LLSAN), zygomaticus minor (ZMi), and zygomaticus major (ZMj) (Agur et al., 2019). Injection of Botox intramuscularly causes the cleavage of synaptosome-associated protein . As a result, acetylcholine is blocked from exocytosis and causes partial denervation of the muscles (Mostafa, 2018). This effectively causes the lip elevator muscles to depress during smiling and helps to conceal the gum.
Several studies have been conducted to determine the efficacy and effectiveness of Botox injections at certain points in the face to treat a gummy smile (Mazzuco & Hexsel, 2010;Polo, 2008;Sucupira & Abramovitz, 2012). The safest and most effective site for Botox treatment as proposed by Hwang et al. (2009) is located in the face at a place known as "Yonsei point." The administered dose of the Botox injection should be based on the severity of the gummy smile; however, a maximum of 5 IU with initial injections is a safe approach to treatment (Duruel et al., 2019). Yonsei point is an area lateral to the nose in which the LLS, LLSAN, and ZMi all pass through a triangular space created by three points. The three points are the lateral point of the ala of the nose, midpoints of nasolabial fold in between the ala and commissure, and the maxillary point about onequarter between the ala and tragus of the ear (Moura et al., 2017).
With one Botox injection at this point, the LLS, LLSAN, and ZMi are successfully targeted and opposes the need to administer one injection into all three muscles.
As established above, Yonsei point is a generalized point used by clinicians to determine the site of Botox injections to effectively treat gummy smile. However, due to the vast number of variations between populations and ancestry groups around the world, the generalized Yonsei point might not be accurate for every population group. This is due to slight facial morphological differences between population groups. Results show that the average Korean male and female have a broader and more prominent malar and zygomatic region compared with Houstonian White faces (Kim et al., 2016). Other differences found were more protrusion in the glabella, nasion, and pogonion regions. This suggests that different population groups might have slightly different Yonsei points, although reference data for other populations is limited. Therefore, it is important to establish population standards for the Yonsei point to ensure that the treatment thereof is safe and effective.
In South Africa, there is limited research and treatment standards regarding a gummy smile. Establishing South African standards for the location of the Yonsei point is vital in ensuring safe and effective treatment for a gummy smile using Botox injections.
Thus, the aim of the study is to determine the accurate surface anatomy criteria for the administration of Botox injections to treat a gummy smile in the South African population. The objectives of this study are to establish accurate surface landmarks pertaining to the lip elevator muscles based on muscle morphology, establish a point (Yonsei point) in which the administered Botox has the greatest effect on the lip elevator muscles to treat a gummy smile, and, lastly, to determine the variability between lip elevator muscles in the South African population so that Botox injections can be more safely and accurately administered.

| Materials
Thirty-eight hemifaces from 19 adult, White South African embalmed cadavers (10 males and 9 females) from the Department of Anatomy, University of Pretoria, were included. All cadavers were embalmed with a 4% formaldehyde solution. The White South African population are historically descendants from German, Dutch, French, and British populations (Davenport & Saunders, 2000).

| Methods
Before dissection commenced, pins were placed at various surface landmarks of the face. These identification points represent surface landmarks for individual muscle Botox injection sites ( Figure 1 and Table 1).
From these pin positions, a triangular area was constructed using pin positions 1 and 2, 3 and 4, and 5 and 6 on both sides of the face  of Botox spread of each injection landmark was divided into three areas/zones surrounding the pin. The zones were divided into thirds: the inner one-third, middle one-third, and outer one-third (Figure 4a).
For the Yonsei point to be a viable injection site, its radius of effectiveness would need to pass through the inner one-third of the landmark injection radii. The prevalence of the overlaps of circular areas of each landmark was determined and recorded ( Figure 4b).

| Statistics
Statistical analyses were conducted in SPSS Statistics 28.0.1.0(142) (IBM) to express the distribution of each muscle according to the vector directions in relation to the midline of the face. Basic descriptive statistics was used to determine parameters such as minimum, maximum, mean, median, and quartiles.
Independent t tests were used to assess the differences between the male and female specimens, as well as between the left and right measurements. The significance level was adjusted to p ≤ .05. The correlation between digital and manual measurements were evaluated by determining the inter-class correlation (ICC) values for all measurements. Interobserver repeatability was assessed by re-evaluating 10 of the cadaver measurements via ICC.  (Bobak et al., 2018). Manual or digital measurements can be successfully used to determine the angle measurements for facial muscles.

| RESULTS
In this study, it was found that there was a lack of efficacy of the  Note: X represents where Yonsei point overlapped with the injection landmark radii on each cadaver. A dash represents no overlapping of Yonsei point with the injection landmark radii at all. The colours in table represent the overlapping injection sites in Figure 6.