Influence of orthodontic treatment, midline position, buccal corridor and smile arc on smile attractivenessA systematic review
To assess the scientific evidence of the influence of some variables on smile attractiveness: orthodontic treatment, midline position, axial midline angulation, buccal corridor, and smile arc. Literature was searched through PubMed, Web of Science, Embase, and All EBM Reviews. The inclusion criteria consisted of studies written in English; published in the past three decades; concerning the influence of orthodontic treatment, midline position, axial midline angulation, buccal corridor, and smile arc on smile esthetics; and judged by a minimum of 10 raters. Quality features evaluated were adequate description of samples, absence of confounding factors, and description of methods used to evaluate the smiles and statistical analyses. Initially, 203 articles were retrieved. Of these, 20 abstracts met the initial inclusion criteria and were selected. Thirteen articles were classified as high quality, seven as average, and none as low quality. Four-premolar extraction or nonextraction treatment protocols seem to have no predictable effect on overall smile esthetics, meaning that if well indicated, extraction in orthodontics does not necessarily have a deleterious effect on facial esthetics. The selected articles recommend that a small dental midline deviation of 2.2 mm can be considered acceptable by both orthodontists and laypeople, whereas an axial midline angulation of 10° (2 mm measured from the midline papilla and the incisal edges of the incisors) is already very apparent, and considering studies dealing with real smiles, buccal corridor sizes and smile arc alone do not seem to affect smile attractiveness.Abstract
Objective:
Materials and Methods:
Results:
Conclusion:
INTRODUCTION
For a long time, orthodontic treatment was primarily based on occlusal relationship results.1 Currently, modern orthodontics also requires a harmonious balance between soft tissues and occlusion. The influence of smile attractiveness components is important because it allows the professional to identify the hierarchy of esthetic preference.2 Therefore, knowledge of the influence of orthodontic treatment on smile attractiveness is very important, and recently, some smile components such as midline position, axial midline angulation, buccal corridor, and smile arc have received greater attention.1–20
A symmetrical dental arrangement is thought to be a fundamental component of an attractive smile, so that facial and dental midline coordination are basic to appreciation of facial harmony and balance.21–24 Although subtle dental to facial midline asymmetry within normal limits is acceptable, significant midline discrepancies can be quite detrimental to dentofacial esthetics.25 The axial midline angulation is also important. This irregularity can be easily detected and affects the final orthodontic esthetic results.4,9,13
Another essential smile feature is the transverse dimension of the smile. This characteristic refers to the buccal corridors. Their importance on smile attractiveness is well reported in the 1958 study of Frush and Fischer,26 whose results demonstrated that the presence of buccal corridors added the illusion of a natural dentition, whereas its absence gave the patient an artificial appearance.
The smile arc is the relationship between the curvature of the incisal edges of the maxillary incisors and canines to the curvature of the lower lip in the posed smile.27 Frush and Fisher26 proposed that there should be harmony between the curvature of the incisal edges of the maxillary anterior teeth and the curvature of the upper border of the lower lip.
Understanding smile attractiveness is important because it is what most laypeople use as a parameter to judge whether treatment is successful or not.2 Therefore, the purpose of this systematic review was to analyze the influence of orthodontic treatment, midline position, axial midline angulation, buccal corridor, and smile arc on smile attractiveness.
MATERIAL AND METHODS
Using the main terms smile, orthodontics, esthetic, aesthetic, and attractive, a computerized search was conducted in the following electronic databases: PubMed, Web of Science, Embase, and All Evidence-Based Medicine Reviews (EBM Reviews).
To identify potential articles, the initial search was performed by title and abstract. The initial four inclusion criteria were studies written in English; published between 1979 and 2009; quoting at least one of the following smile features: influence of orthodontic treatment, midline position, axial midline angulation, buccal corridor, and smile arc on smile esthetics; and judged by a minimum of 10 raters. This selection process was independently conducted by two researchers. Thereafter, the articles from the selected abstracts were independently evaluated by three researchers. Interexaminer conflicts were resolved by discussion on each article to reach a consensus regarding which articles fulfilled the main selection criteria.
The articles ultimately selected were then classified based on the following quality features: sample description, consideration of confounding factors, validity of the method, and statistical analyses.
Sample description was considered adequate when the author clearly established the initial aspect of the occlusion, whether actual or digital. Also, the treatment protocol chosen to be applied had to be stated as well as whether the features of the occlusion were digitally altered and to what extent.
To consider whether control of the confounding factors was adequate, distractors such as part of the nose, cheeks, skin color, and any blemishes or facial hair had to have been minimized.
Validity of the method was adequate when the selected studies with digital alteration showed natural modification of the smile features. For those not digitally altered, the photographs should have been standardized. The statistical analyses were judged by a statistician.
Afterwards, the application of these qualification features was used to classify the articles based on their scientific weight. Articles with all qualification features were classified as high, articles with one or two absences were classified as average, and those with three or more absences were classified as low.
RESULTS
After the database search, 168 articles were retrieved from PubMed, 59 from Web of Science, and 63 from Embase. All EBM Reviews identified only three papers. From hand search, two studies were identified (Table 1). The entire search strategy resulted in 203 abstracts, 65.5% of which were published between 2003 and August 2009. These results demonstrate that studies about the influence of smile attributes on smile attractiveness have considerably increased in the past few years.
In this systematic review, 20 articles met the initial inclusion criteria. The influence of orthodontic treatment, midline position, axial midline angulation, buccal corridors, and smile arc on smile attractiveness was stated in 3, 8, 3, 10, and 6 articles, respectively (Table 2). The summarized data of the 20 articles included in the review are shown in Tables 3 and 4. After quality feature analysis, 13 articles were classified as high-quality level, seven as average-quality level, and no article classified as a low-quality level (Table 5).
Influence of Orthodontic Treatment on Smile Attractiveness
Improvement in smile attractiveness by orthodontic treatment was assessed in three studies.1,5,8 All studies compared smile attractiveness among nonextraction and four-premolar extraction patients.1,5,8 One of them also compared the smile attractiveness between treated and untreated subjects.1
After quality features evaluation, the three articles fulfilled all selection criteria and were classified as high quality and were used for this systematic review.1,5,8 One study concluded that there was no difference with regard to smile attractiveness between orthodontically treated subjects and subjects with well-balanced faces and good occlusion.1 Furthermore, treatment modality alone had no predictable effect on the overall esthetic assessment of the smile, and subjects with ideal occlusions and Class I malocclusions, when treated with or without extractions, did not show a difference in smile esthetics.1,5,8
Influence of Midline Position on Smile Attractiveness
The influence of midline position on smile esthetics was investigated in eight studies3,4,6,7,9–12 and the influence of axial midline angulation in three studies.4,9,13 Only two articles were classified as average quality,4,12 and the others3,6,7,9–11,13 were classified as high quality.
Comparing the perception of the dental midline deviation judged by orthodontists and other groups of dentists and laypeople, orthodontists had the greater perception of midline deviation9,10 and dentists in general are more discerning than laypeople.3,6,10 When analyzing the amount of acceptable dental deviation judged by orthodontists and laypeople, there was not a general agreement. According to one study, 1 mm of deviation is the maximum accepted by the orthodontists without decreasing smile esthetics.10 Three articles agreed that deviations of up to 2 mm are acceptable by orthodontists,3,6,12 whereas another study stated that only dental midline deviation greater than 4 mm is rated to be less esthetic by orthodontists.9 One article stated that laypeople found maxillary to mandibular midline deviation acceptable until it exceeded 2.1 mm, and one-third of the raters accepted the maxillary to facial midline maximal deviation of 2.9 mm.7 Two articles considered that the dental to facial midline deviation of 2 mm is considered normal.4,12 In another article, laypeople were unable to detect 3 mm of dental deviation,11 and in two articles, laypeople did not detect 4 mm of dental deviation.9,10 In contrast, another study concluded that 4 mm of deviation can be noticed by 96% of laypeople.6
One study analyzed the perception of axial midline angulation, stating that orthodontists, general dentists, and laypeople were able to identify a 2-mm discrepancy in incisor angulation, measured from the midline papilla to the most gingival portion of the incisal embrasure between the maxillary central incisor crowns.9 A similar study stated that discrepancies of 10° were unacceptable by 68% of orthodontists and 41% of laypeople,13 whereas another study stated that 5° is perceived by orthodontists and laypeople.4
Influence of Buccal Corridor on Smile Attractiveness
In 10 studies, the impact of buccal corridors on smile attractiveness was analyzed.2,4,7,14–20 Six studies were classified as average quality2,4,15,16,18,19 and four as high quality.7,14,17,20
There was no agreement between the results from the selected articles. Two articles found no correlation between buccal corridors and smile esthetics.17,20 The other articles concluded that large buccal corridors are considered less attractive.2,4,7,14–16,18,19
Influence of Smile Arc on Smile Attractiveness
The effects of smile arc on smile esthetics were evaluated in six studies.2,4,7,11,17,19 Three articles were classified as high quality7,11,17 and the other three as average quality.2,4,19 An ideal smile arc or a consonant smile arc with the lower lip was considered the most acceptable smile arc variation in three articles.2,7,19 On the other hand, one article stated that a flat smile is more attractive in males and a flat/consonant smile in females.4 Two articles did not find a correlation between smile arc and smile esthetics.11,17
DISCUSSION
Quality Evaluation
A detailed sample description is an important component for a correct interpretation in smile attractiveness studies. Articles that did not report the sample description and their characteristics when required were considered inadequate because sample size and selection are the major source of bias in a large amount of studies.28
Elimination of the confounding factors is necessary to minimize the influence of background on facial attractiveness, such as part of the nose, cheeks, and chin. It is important to focus on the esthetics of the smile without distracting the interviewees.5,8,10,14,17,20,29 Conversion of color photographs to black and white is also important in the articles that judge smiles of different subjects, because this procedure evens the skin shades of the sample and reduces the number of confounding factors.1,5,20,29,30
Most studies digitally altered the smile images to create variations of buccal corridors, midline deviations, and smile arcs. To validate the method, a natural smile appearance should be preserved. However, these characteristics were not found in many studies, especially those on buccal corridors.2,4,15,16,18,19
In this systematic review, two studies had deficiencies in their statistical analyses.4,12 The statistical analysis is an important criterion to be considered in an article because the complete presentation of the data gives transparency to the investigation. For this systematic review, a statistician evaluated the statistical analyses of the selected articles.
The number of raters in these studies is also an important issue because they are representing part of a population and a misguided sample may have sufficient power to determine an effect for one particular parameter.28 All selected studies did not calculate the appropriate rater group size, and consequently, a minimum of 10 raters in each group was considered as an inclusion criterion in this review.
Influence of Orthodontic Treatment on Smile Attractiveness
The orthodontist must be aware of the advantages and disadvantages of each treatment protocol. The analysis of the articles on the influence of orthodontic treatment suggests that a treatment modality alone cannot influence smile esthetics.1,8 These results are expected because, in any group of subjects, there is individual variability—shape of the teeth, curl of the lips, and mouth expression—that could influence the smile perception as esthetically pleasing or not.1 Therefore, it is not the fact of extracting or not that influences facial harmony but rather the correct indication of extractions.
Influence of Midline Deviation and Axial Midline Angulation on Smile Attractiveness
There was an agreement that orthodontists were more perceptive to midline discrepancies than laypeople.3,6,9,10 Some studies suggest that, by virtue of their formal training and experience, orthodontists are more sensitive to aberrations in dentofacial appearance than the general public.6,20 An important result is that all selected articles agree that a small dental midline deviation does not compromise the smile esthetics and is not perceived by laypeople.3,6,7,9–11
On average, orthodontists were able to detect midline deviations greater than 2.2 mm while laypeople were able to detect only midline deviations greater than 3 mm. Considering that either laypeople or another more accurate observer can be part of our patient's environment, it is advisable to use the most restricted limit of 2.2 mm as a boundary of acceptable midline deviation. Studies that used this average calculation were only those that established an acceptable limit of deviation. Those that only quoted the amount of deviation that could be detected without establishing the acceptable limit of deviation were not included. The limits are tighter regarding the axial midline angulation. Most of the articles agree that slight angulations such as 10° (2 mm measured from the midline papilla and the incisal edges of the incisors) are very apparent.9,13
Influence of Buccal Corridors on Smile Attractiveness
A controversial result concerns the influence of buccal corridors on smile attractiveness. It is interesting to note that one of the major differences between these articles is the type of samples. Almost all articles that used only digitally altered smiles as a sample suggest that the size of the buccal corridor has an influence on smile esthetics.2,4,7,14–16,18,19 On the contrary, in articles in which the raters judged smiling photographs from different subjects, the results found no correlations between buccal corridor sizes and smile esthetics.17,20 According to Kokich et al.,9 there is a threshold level that a digital alteration must exceed for the viewer to detect it because both dentists and laypeople notice only the extremes.14 If enough teeth are deleted from the lateral aspects on the smile, there would be some detraction from smile esthetics, and the smile would probably appear unnatural.9 The size differences of the buccal corridors in patients may be more subtle, whereas the changes performed in the computer are more dramatic.14
It became clear that those studies that used digitally altered images brought out opposite results than those studies with actual images. Unfortunately, image alteration in those studies was too poor to be used as the variations that exist on actual patients. Hence, based on the articles that used actual subjects instead of digitally altered smiles, it is very acceptable to state that buccal corridor sizes alone do not affect smile esthetics, considering that those digital alterations do not represent natural buccal corridor diversity.
Influence of the Smile Arc on Smile Attractiveness
In reference to the smile arc influence, it is possible to state that the results are also related to the sample type, as observed in buccal corridor studies.2,4,7,14–20 When the digitally modified smile arc was judged in its condition on influencing smile attractiveness, in almost all articles, the smile arc interfered in smile esthetics.2,4,7,19 When natural smiles were on trial, results showed that the smile arc did not interfere in smile attractiveness.17 The tendency was to favor those articles that dealt with actual subjects and accept their conclusion that the smile arc alone is not able to influence the smile esthetics.
Finally, smile attractiveness is a set of features that must be considered in orthodontic treatment planning with the understanding that every aspect of the set is important in the final score of smile beauty.
CONCLUSIONS
-
Nonextraction and four-premolar extraction treatment protocols seem to have no predictable effect on the overall esthetic assessment of the smile. This means that the simple fact of extracting teeth or not does not necessarily have a detrimental facial esthetic effect.
-
A limit of 2.2 mm can be considered acceptable for midline deviation. Concerning the axial midline angulation, 10° (2 mm measured from the midline papilla and the incisal edges of the incisors) is already very apparent.
-
Based on studies with actual subjects, neither buccal corridor sizes nor smile arc alone seem to affect smile attractiveness.
Contributor Notes