ABSTRACT. As a common human expression that conveys a variety of emotions in intentional and unintentional ways, the smile must be characterized by a series of features that allow identifying all possible alterations in its physiological, anatomical, and functional aspects. This article will approach the smile from different perspectives, classifications, and standards for its assessment. By means of a thorough analysis of the smile and its elements, formal and functional alterations in the esthetic zone will be identified in order to suggest the best therapeutic options to provide patients with comprehensive treatment.

Smiling is a common human expression that communicates pleasure or amusement, but it may also be an involuntary expression of anxiety or several other emotions such as anger or irony. It is considered to be a normal reaction to certain stimuli, innate to individuals (we are born with it), and not linked to sociocultural aspects. The smile is an important part of an individual’s physical stereotype and perception, and it is also important in the judgment that the others have of our appearance and personality; therefore, smile asymmetry plays an important role in the perception of beauty.

From a physiological point of view, a smile is a facial expression produced by flexing 17 muscles located around the mouth and the eyes. Considering muscle functions, the smile is produced in two different stages. During the first one, the upper lip and the nasolabial folds are contracted, with participation of the levator labii superioris, the zygomatic major muscle, and some upper fibers of the buccinator. In the second stage, or the final one, there is contraction of the periocular muscles so as to support maximum elevation of the upper lip, thus producing half-closed eyes.

Depending on the lips raising direction and the muscle group involved in the smile, it is classified into three categories: The cuspid smile, characterized by participation of all the levator labii superioris, thus exposing both teeth and gingival tissue. The complex smile, characterized by the simultaneous action of levator labii superioris and lower lip depressors, and the commissure smile or Mona Lisa smile, in which the zygomatic major muscles bring the commissures up and outwards, followed by a gradual elevation of the upper lip as in an arch shape, so that the center of the lip becomes lower than its lateral portions.

Another way of classifying the smile considers the level of consciousness involved. The voluntary smile may or may not be motivated by an emotion; the static one is extendable and reproducible, and the involuntary one, which is induced by gladness and has a dynamic nature, expresses authentic human emotions but cannot be sustained for long periods of time.

From an anatomic point of view, it may be classified, according to gingival line localization, as high, medium, and low, depending on the relation between the upper lip inferior edge and the upper incisors and their gingiva. If the gingival line when smiling displays 100% of the anterior tooth and even a portion of the gum, it is considered as a high smile; if the smile line exposes between 75 and 100% of the tooth it is called a medium smile, and if it only shows 50% or less of the incisor it is considered to be a lower smile.

An ideal smile depends on the symmetry and balance of facial and dental characteristics such as color, shape, and teeth position, considering that shape determines function and that the anterior teeth play a critical role in the patient’s oral health. Estimation of a harmonic smile includes assessment and analysis of the “smile zone”, which, depending on its shape, may be: straight, curved, elliptical, arched, rectangular, or inverted.

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