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.