The smile arc: review and synthesis

ABSTRACT Introduction: The smile arc is an esthetic parameter that has been better investigated by Orthodontics after the “new esthetic paradigm”. Its diagnostic evaluation and inclusion in the objectives of orthodontic planning has become fundamental for professionals seeking for more beautiful and youthful natural esthetic outcomes. Objectives: To review concepts related to the smile arc, analyze the determinants of its appearance, understanding how the possible variations can affect the esthetic perception of smile.


INTRODUCTION
The smile is the most pleasant expression on the face, translating the beauty, youth and personality of people. From this understanding, raises one of the objectives of orthodontic treatment: the ability to restore smiles adapted to the face, age and lifestyle of patients, enhancing their positive esthetic characteristics and increasing the self-esteem and self-confidence when smiling. 1,2,3 The components that define the maximum esthetic potential have been extensively discussed in the orthodontic literature, many originated from concepts of complete dentures. One of the most important, the "Smile arc" (also known as "Smile curve"), was defined as the relationship between the curvature of the incisal edges of maxillary anterior teeth (upper incisal line) and the curvature formed by the lower lip when smiling (Fig 1). 4,5 This issue has been extensively studied by restorative specialties, [6][7][8][9] and has gained the attention of orthodontists, based on the "new esthetic paradigm", 10,11 which has been guiding treatments since the 1990s.
In August 2001, David Sarver published one of the most read papers of AJODO: "The importance of incisor positioning in the esthetic smile: The smile arc." 5 His contribution was mainly based on the findings of some studies that confirmed the lower attractiveness of flatter upper incisal lines, and in studies reporting that this flattening was more common in orthodontically treated patients, compared to individuals with "normal" untreated occlusion. 3,12,13,14 This publication by Saver 5 became popular by recognizing the importance and impact of this esthetic parameter on the orthodontic outcomes, in both short and long terms, besides evaluating the resources and mechanical decisions that harm, maintain or optimize its design and appearance. Then, clinical orthodontists began to dedicate more attention to this aspect, since it had been scarcely observed, understood and addressed for a long time.  4,5 Several other scientific publications also provided well-known and important contributions on this topic. [15][16][17][18]   » It has been observed that, during the aging process, the lips assume a lower position, contributing to the reduction of exposure of the maxillary anterior teeth and increased exposure of mandibular anterior teeth.
» Orthodontics acknowledges its limitations when facing problems related to the morphology and behavior of the lips when smiling. The lower lip does not always form a concave curvature to serve as parameter for the convexity of the upper incisal design. 2,16 Since the smile arc is formed by the consonance of two curved lines, the isolated establishment of a beautiful curvature of the upper incisal line is insufficient to define its formation and aspect. However, this is the only site of action whose change is within the scope of Orthodontics.
Considering previous publications and using some clinical perception on the issue, this paper aims to organize and summarize the subject, facilitating the diagnosis and understanding of factors that can determine the achievement of different esthetic perceptions in relation to the smile arc.
The determining factors for its formation and aspect can be listed into three main groups, described in Figure 3.

-TECHNICAL DETERMINANTS
The aspects related to planning and accomplishment of orthodontic or orthodontic-surgical treatment.
» Appliance placement/Digital setup: Figuring as the initial step of active treatment, this should be performed aiming at achieving an individualized incisal design, according to the patient's gender and age. 15,[19][20][21][22] In the recent past, with planning focused almost exclusively on occlusion, the concern with the role of canines in laterality movements defined a more occlusal positioning of their cusp tips, when compared to the incisal edges of upper central incisors ( Fig 4A).
Currently, the appliances position the edges of maxillary central incisors leveled with the canine cusp tips (Fig 4B), since Concerning factors related to gender and age, female smiles accept an incisal design exhibiting greater height discrepancy between maxillary central and lateral incisors (from 1.0 to 1.5mm), while older patients require less marked incisal designs, with smaller height discrepancies between the upper incisal edges. Figure 6A shows a situation in which the upper incisal line was flattened due to an inadequate appliance placement, and Figure 6B shows its correction after retreatment using a more favorable placement. Also, the perception of canine tips in frontal view is influenced by the distortion of perspective, the known parallax effect (what is more distant seems smaller) and by the occlusal plane inclination, as shown later. 2 In digital planning, it is possible to use tools to guide the upper incisal design, such as the "Smile Curves" template. 15 Seixas MR, Câmara CA -The smile arc: review and synthesis  intrusion performed with "steps" or marked curves of Spee in the dental arches, were responsible for very undesirable esthetic results regarding the beauty and youthfulness of smile (Fig 7). Therefore, in the search for patients with pleasant and youthful smile arcs, planning the leveling of dental arches becomes absolutely essential.     Seixas MR, Câmara CA -The smile arc: review and synthesis Figure 17: The behavior of lips, during their "unveiling", has a favorable influence on the formation of smile arc of types "Monalisa" and "Canine" (A, B). In smiles of the "Complex" and "Infinite or mirror" type, it is more difficult to achieve harmony between the upper incisal line and the lower lip. In these cases, Orthodontics faces its major limitation: alteration of the muscle and tissue pattern of the lips and perioral regions.  By inclining the patient's head slightly up or down, it is also inclined in relation to the camera and, consequently, the visual perception of the smile arc curvature is changed.
The described movement modifies the angle of the dental occlusal plane in relation to the observer, leading to a perception of flatter or more curved upper incisal line (Fig 19).
The downward inclination of the head, together with the parallax factor, potentiates the formation of the smile arc.
This condition needs to be carefully evaluated to avoid masking structural problems. The opposite is also true; the perception of the esthetic effect of correct dental leveling can be impaired by inclining the head upwards.  (Fig 20).
To record the smiles as accurately and reproducibly as possible, it is necessary to pay attention to this aspect and attempt to achieve the photographs in a manner as careful and standardized as possible.

Carlos Alexandre Câmara (CAC)
Conception or design of the study:

MRS, CAC.
Writing the article:

MRS.
Critical revision of the article:

MRS, CAC.
Final approval of the article:

MRS.
Patients displayed in this article previously approved the use of their facial and intraoral photographs.
The authors report no commercial, proprietary or financial interest in the products or companies described in this article.

FINAL CONSIDERATIONS
The achievement of beautiful, natural and pleasant smiles has a direct correlation with the dental, skeletal and facial characteristics of each individual. Also, the artistic perception of the orthodontist, the individualization of the appliance placement/ digital setup, as well as the knowledge on orthodontic mechanics, can favor or impair the treatment outcomes. The smile arc is one of the most important esthetic parameters for dentistry and should receive special attention in contemporary orthodontic planning. Understanding the factors that determine its appearance is essential for maximum use of its esthetic potential.