INTRODUCTION

Facial attractiveness plays a key role on modern society and it can influence not only self-esteem but also social opportunities, professional performance and employment prospects. In face-to-face situation a person’s eyes primarily observe the other person’s eyes and the area of the mouth. This means that smile aesthetics is becoming a dominant concern for patients, in particular when a dental treatment is required (^{Rotundo et al., 2015}).

In recent years, aesthetic dentistry has become a major focus for the public. This trend was initiated by prosthodontists and, recently, by orthodontists. It can be said that the smile is the sum of features which comprise the lips, soft tissue (gingiva) and hard tissue (teeth). Numerous factors are related to dental aesthetics, such as the color, the shape, the size, the proportions of the teeth and the shape of the dental arch. They are influenced by individual preferences, cultural factors, and sociodemographic factors. The selection of teeth for an edentulous patient can be difficult especially when no pre-extraction records are available. The size, form and color of the teeth must be in harmony with surrounding oral and facial structures (^{Frush & Fisher, 1956}). One of the primary concerns in denture aesthetics is the selection of suitable sized maxillary anterior artificial teeth.

Maxillary central incisors are considered to be the key teeth in smile since they are the most visible teeth during facial activity. Their size, shape, and arrangement are the most influential factor for harmonious appearance. On the other hand, the mesiodistal dimensions of lateral incisors also play an important role in facial appearance. One of the essential elements in the aesthetic complex is exactly the contrast between the width of the central incisor and the smaller lateral incisor. Dental anomalies in number, size and shape affect upper lateral incisors very often (^{Kotsomitis & Freer, 1997}). Some studies prove that these deformities are even interconnected. They can be observed in cases with asymmetric hypodontia, microdontia or peg-shaped tooth crown of the opposing lateral incisor (^{Bot & Salmon, 1977}; ^{Alvesalo & Portin, 1969}). This impedes not only the aesthetics but also occlusal relationship. Success of the treatment option connected with an orthodontic open missing tooth space and subsequent prosthetic or implant restoration depends on proper determination of medio-distal diameter of the missing and mis-shaped incisor. The inter-incisive index is the mathematical expression of this ratio between both incisors. Geometrical or mathematical relationship between teeth is an important determinant to achieve an aesthetic restorative result (^{Tarvade & Agrawal, 2015}), therefore this index can be very helpful in aesthetic dentistry, prosthodontics and orthodontics.

The aim of this study was to define certain values of the inter-incisive index in Bulgarians, to assess its degree of sexual dimorphism and bilateral asymmetry and to verify statistically significant differences in this index between Bulgarians and other Balkan populations.

MATERIAL AND METHOD

The present study included 121 males and 111 females of Bulgarian origin in the age group 20-40 years. Before starting the study, subjects were informed about the nature of the study and written informed consents were obtained. Patients were included based on the following criteria:

1. Presence of complete set of fully erupted and periodontally healthy maxillary teeth.

2. No periodontal disease.

3. No spacing and crowding in anterior maxillary teeth.

4. No history of orthodontic treatments.

5. No intruded, extruded or rotated teeth in the anterior region.

Mesiodistal dimensions of maxillary central and lateral incisors were measured by Dentistry Sliding Vernier Caliper. We used the technique of direct anthropometry, modified by ^{Yordanov et al. (2012}). According to Yordanov the mesiodistal dimension is the greatest distance between the most convex approximal points on the mesial and distal sides of the crowns, usually it is in the upper or middle third of coronal height. It is also termed the dental width. We calculated the inter-incisive index as ratio of the mesiodistal dimensions of maxillary lateral incisor to the mesiodistal dimensions of the maxillary central incisor. [(MD12/MD11) x100].

An ethical approval was taken for this study by the Ethics committee in Medical University-Plovdiv. Informed consents were obtained from all patients involved in the study. All methods were performed in accordance with the relevant guidelines and regulations.

The measurements were analyzed with SPSS 23.0 using Student’s t-test. The level of statistical significance was set at P< 0.05. The degree of significance was considered weak (P<0.05), moderate (0.01>P>0.001) or high (P<0.001). Z-test was used to compare the proportions.

An ethical approval was taken for this study by the Ethics committee in Medical University-Plovdiv.

RESULTS

1. We did not find statistically significant differences in the inter-incisive index between left and right sides in Bulgarian men and women (Table I).

2. There were no statistically significant differences in the inter-incisive index between Bulgarian males and females (Table II).

3. Our study did not show statistically significant differences in the inter-incisive index between Bulgarian and Serbian populations. There were statistically significant differences in mesiodistal dimension of the maxillary central incisors in favor of Serbians (Table III).

4. We did not find statistically significant differences in the inter-incisive index between Bulgarian and Greek populations. Mesiodistal dimension of the maxillary lateral incisors showed statistically significant differences in favor of Bulgarians (Table IV).

Table I Comparison of inter-incisive index in Bulgarians between left and right side. 

^{* Inter Incisive Index}.

Table II Comparison of inter-incisive index between Bulgarian men and women. 

^{*Inter Incisive Index}.

Table III Comparison of inter-incisive index between Bulgarians and Serbians. 

^{*Inter Incisive Index}.

Table IV Comparison of inter-incisive index between Bulgarians and Greeks. 

^{* Inter Incisive Index}.

DISCUSSION

Our results showed no statistically significant differences in the inter-incisive index between right and left sides of both sexes. This means that the ratio of the width of the lateral incisor towards the width of the central incisor is similar in both sides of the dental arch. Insignificant bilateral asymmetries in the antimeric teeth were reported by others as well (^{Lysell & Myrberg, 1982}; ^{Axelsson & Kirveskari, 1983}; ^{Buschang et al., 1988}). Lack of significant asymmetries was reported by Keith et al. who assessed the differences in the odontometric dimensions between left and right members of antimeric teeth in South Chinese, ^{Yuen et al. (1997}). This justifies the use of average values for the dimensions of left and right maxillary central and lateral incisors. We did not find statistically significant differences in the inter-incisive index between Bulgarian males and females which indicated no sexual dimorphism. These results were in accordance with ^{Seipel (1946}) and ^{Jensen et al. (1957}) who reported that the most dimorphic teeth were the canines, while the maxillary incisors were the least. Similar results were observed by ^{Garn et al. (1964}) who claimed that the most dimorphic teeth were the molars, while the upper incisors were the least dimorphic.

We believe that our findings can be useful for the prosthodontists because in treating patients with missing maxillary anterior teeth, they must determine tooth size and shape to achieve an optimal aesthetic result. If the size and shape of a replaced tooth are not in harmony with patients’ face and other teeth, psychological and social problems might arise (^{Parnia et al., 2010}). Using average values for left and right inter-incisive index and for males and females would considerably facilitates dentists in esthetic treatment with facets and manufactures of artificial teeth for prosthesis.

Inter-incisive index values can be used by the orthodontists for treating hypodontia of the upper lateral incisor when a decision should be taken for how much space is needed to be opened for an implant. In all cases with microdontia of upper incisors or deep bite correction by stripping of the crowns of maxillary incisor, mean values for the size of the crowns or inter-incisive index can be applied.

In our study we did not find statistically significant differences in the inter-incisive index neither between Bulgarians and Serbians nor between Bulgarians and Greeks. Mesiodistal dimensions of central incisors between Bulgarians and Serbians were significantly different though. Similar significant difference we found in the mesiodistal dimensions of lateral incisors between Bulgarians and Greeks.

^{Yordanov et al. (2012}) reported that values of the inter-incisive index are between 75- 78 % in Europeans, while in Mongoloids they are between 82- 84 %. He observed that the index in women is lower. This is in contrast with the results of our study according to which women show higher values (80.13 %) than men (79.05 %). Our results showed that values of inter-incisive index were closer to those of the Europeans (79.05 %). This is in contrast with the results from Abadzhiev (2017) who observed closer values of the index in North- Eastern Bulgarians to those of the Mongoloids. Examples of ethnic differences and geographic variability in tooth size have been documented and they could be related to the degree of ethnic mixing, ^{Brook et al. (2014}). The fact that there were no significant differences in the inter-incisive index between Bulgarians and the two Balkan nations can be explained by geographical proximity of the countries, a decrease in racial differences due to increase of racial mixture and the common origin. Both Bulgarians and Serbians belong to the South - Slavic ethnic group.

CONCLUSION

Inter-incisive index can be very useful in esthetic dentistry in the treatment with facets, in prosthodontics in the treatment of edentulous patients with prosthesis and in orthodontic treatment planning. The fact that it showed no sexual dimorphism and bilateral asymmetry facilitates the dentists with use of averaged size between mesiodistal dimensions of both the incisors. Since it did not show population- specificity the index might be applicable for Serbians and Greeks as well.