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.

Figure 1:. The ideal Smile Arc occurs when the upper incisal line has a similar curvature as that formed by the lower lip when smiling. ^,

Figure 2:. Types of smile arcs: A) pleasant and youthful; B) straight or flat, and C) reverse or inverted.

Figure 3:. Determinants of the formation and aspect of the Smile Arc can have a technical nature, morphofunctional origin and those related to posture and photographic recording issues.

Figure 4:. A) Placement of upper appliance with attention focused on achieving the canine guidance: the heights of brackets tend to flatten the smile arc. B) Appliance placement based on the contemporary esthetic paradigm: decreased dominance of cusp tips of maxillary canines and formation of a smooth convex upper incisal line (shaped as a “deep dish”), which allows the formation of an esthetically pleasant smile arc .

Figure 5:. Upper incisal line shaped as a “deep dish”.

Figure 6:. A-C) Intraoral and extraoral photographs of the upper incisal line flattened by orthodontic treatment, forming a straight and not so youthful smile arc for a 15-year-old girl. B) Improvement of the upper incisal design, after a new appliance placement, promoting greater curvature of the upper incisal line and forming a more pleasant, feminine and youthful smile arc.

Figure 7:. Intraoral and extraoral photographs of the inverted upper incisal line after overbite correction by a sharp curve of Spee in the upper arch (A-C). D,E) Retreatment of the case, with good control of overbite, reestablishment of the curvature of the upper incisal line and formation of a more pleasant and youthful smile arc.

Figure 8:. A-E) Patient treated by projection of incisors, presenting dentoalveolar biprotrusion, anterior open bite, flat upper incisal line, and change of light reflection on the buccal surfaces of maxillary incisors. F-J) Retreatment performed by tooth extractions and retraction of anterior teeth: improved light reflection from the smile and achievement of an upper incisal line shaped as a “deep dish”. The correct appliance placement and tooth movement performed enabled the formation of a more esthetically acceptable smile arc.

Figure 9:. Elastic mechanics applied in Class II (A) and Class III (B) directions, and their possible side effects in the inclination of upper and lower occlusal planes.

Figure 10:. Relationship between facial archetypes and the smile arc. A) Short faces, in which there is greater tendency to counterclockwise rotation of the maxillomandibular complex, usually present occlusal plane with upward inclination, little exposure of maxillary anterior teeth when smiling and in lip rest position, more buccally inclined maxillary incisors, and a flatter or even inverted incisal line. B) Long faces, in which there is a greater tendency to clockwise rotation of the maxillomandibular complex, have opposite characteristics to those previously described and greater possibilities to achieve more pleasant smile arcs.

Figure 11:. A, B) Young patient, with anterior open bite, concave upper incisal line and inverted smile arc. C, D) Upper leveling performed by the combination of posterior intrusive and anterior extrusive movements, achieving a better upper anterior incisal design. Retreatment of this case provided a pleasant smile arc, compatible with the patient’s age and personal preference.

Figure 12:. A, B) Facial photographs of patient with a slightly convex profile, lower lip protrusion and buccal inclinations of maxillary incisors. C) Initial lateral cephalogram and some Steiner cephalometric measurements, describing a skeletal Class III pattern and dentoalveolar bimaxillary protrusion. D-F) Intraoral photographs showing mild Angle Class III malocclusion, compensated by the projection of maxillary incisors, presenting acceptable posterior intercuspation, minimum values of overbite and overjet, flattening of the upper incisal line, and tooth-size discrepancy with 2-mm excess of mandibular anterior teeth width.

Figure 13:. Final intraoral photographs of orthodontic retreatment, showing good overjet and overbite, and good coordination of dental arches. The desired tooth movement was allowed by proximal stripping performed in the maxillary and mandibular anterior regions, aiming at correcting the Bolton discrepancy, and improving the maxillary anterior dental proportions (increased dominance of the maxillary central incisors). Also observe the greater convexity of the upper incisal line.

Figure 14:. A, B) Final facial photographs, showing the reduction of lower lip protrusion and the improvement of maxillary teeth inclination. C) Final lateral cephalogram and some Steiner cephalometric measurements, showing the reduction of dentoalveolar bimaxillary protrusion. Close-up photographs of the initial (D) and final (E) smile, showing the esthetic improvement obtained after disinclination and extrusion of maxillary incisors, increase of gingival exposure and upper incisal line curvature, with consequent achievement of a more beautiful and youthful smile arc.

Figure 15:. Intraoral and smile photographs, initial (A-D) and final (E-H), showing a case of marked overbite and upper dentoalveolar protrusion. The improvement in upper incisal line curvature was obtained by extrusion and displacement of maxillary incisors. For that purpose, it was necessary to perform an excellent leveling of the mandibular arch.

Figure 16:. Initial (A, B; D, E) and final (C, F) intraoral and smile photographs of two young patients with anterior “edge-to-edge” relationship, incisal wear, changes in volume and dental proportions and gingival contour of the maxillary anterior region, inverted curvature of the upper incisal line and reverse smile arc. Both patients were orthodontically treated to obtain adequate levels of overjet and overbite and restoration of worn teeth.

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.

Figure 18:. Natural head position (NHP) and imaginary frontal line parallel to it. The camera should be positioned perpendicular to the latter.

Figure 19:. Changes in inclination of the patient’s head during photographic recording: A,B) By inclining it downwards, the occlusal plane is angulated inferiorly, and the curvature of the upper incisal line is increased. C, D) By inclining it backwards, the occlusal plane is angulated superiorly, and the curvature of the upper incisal line is flattened or even inverted.

Figure 20:. Changes in perception of the smile arc, achieved with different camera inclinations in relation to the occlusal plane. A) Camera inclined upwards, planning the upper incisal line curvature. B) Camera parallel to the occlusal plane, allowing correct analysis of the smile arc; and C) Camera inclined downwards, increasing the upper incisal line curvature.