Principles of Smile Design

GOLDEN RULE (Golden Proportion)nnnnThe golden rule is an ancient principle used in mathematics, art and architecture to provide a guide for esthetic pleasing proportion. A line is divided into two parts such that one part is the mean and the other part is the extreme. The ratio of proportion is 1 to 1.61'8, mean to extreme. Interestingly, the same progression of numbers can be achieved by multiplying by 1.618 or dividing by 0.618. The principle of golden proportion to evaluate facial and smile esthetics was described in the following article. Levin El. Dental Esthetics and the Golden Proportion J Prosthetic Dentistry 40:244-252 1978 Several books and articles describe the following proportions as golden proportion mean to extreme. It should be noted that these measurements are taken face on only as if done on a photograph and not three dimensionally as would be done on the face.nnnnPROPORTIONS MEAN TO EXTREMEnn nn - The lower edge of the nose to the incisal edge of the maxillary incisorsn n - The incisal edge of the maxillary incisors to the bottom of the chin.nn - The inner canthus to the outer canthus of each eyennn - The inner canthus of the right eye to the inner canthus of the left eye.nn - The pupil of the eye to the inner canthus of the eyen n - The midline between the eyes to the inner canthus of the eyenn - The cervical apex of a tooth to the height of the gingival papillaen n - The height of the gingival papillae to the incisal edge of a tooth.nn - The mesial of the central incisor to the distal of the cuspidnn - The distal of the cuspid to the distal of the last posterior tooth.nnn - The mesial distal width of a tooth nn - The mesial distal width of the adjacent, mesial tooth.nn nnGENERAL PRINCIPLESnnnnFACIAL DIMENSIONnnnnA line drawn through the pupils of the eyes should be perpendicular to the midline. The lip line and overall incisal edges of teeth within an arch should be parallel to the line drawn through the pupils.nnnnA line drawn through the pupils of the eyes and a line drawn through the lips at rest should divide the face into three equal portions. The upper third is considered the cerebral ox intellectual portion. The middle third is considered the sentimental or social portion. The lower third is considered the sensual or physical portion. Increased dimension of any area increases perceived personality of a person.nnnnOVERALL INCISAL CONTOURnnnnA line drawn following the outline formed by the incisal edges of the maxillary teeth should be 1 to 3 millimeters parallel/equidistant to the lower lip line. There will be some variation as aging occurs. Old individuals loose elasticity in the lips which results in sagging. The result is prominence of the mandibular teeth and diminution of the maxillary teeth. A masculine smile is a straight line. A feminine smile forms a curved smile.nnnnOVERALL GINGIVAL CONTOURSnnnnA line drawn following the gingival contours of the maxillary teeth should follow the line of the upper lip in what is referred to as a medium (average) smileline. There will be less correlation with a high or low lip line.nnnnSYMMETRYnnnnThere should be symmetry of color, shape and position of teeth about the midline. The central incisors should be dominant with perspective such that each tooth posteriorly appears to get smaller. Key words to achieve this are dominance, proportion, symmetry and balance.nnnnNEGATIVE SPACEnnnnThe back or the mouth is considered a dark space as no light enters when standing. A negative space is an area within an ideal smile which shows through the back of the mouth and therefore darkness.nn nnAXIAL INCLINATION Axial inclination of teeth, anterior and posterior are tilted to the mesial. Posterior axial inclination appears parallel to each other. The incisal edge is perpendicular to the long axis of a tooth. Incisors incline to the facial while maxillary cuspids appear to have a lingual tilt with the height of contour to the facial at the gingival third.nnnnCONTACTSnnnnContacts of maxillary incisors and cuspids can be point or long in an incisal gingival direction. Contact starts (and can extend gingivally) at the incisal third central incisor to central incisor, the junction of the incisal to middle third central incisor to lateral incisor, and middle third lateral incisor to cuspid.nnnnGINGIVAL CONTOURSnnnnGingival contours form a silhouette around the lower section of a tooth. The highest, most gingival peak is referred to as an apex. The apex of maxillary central incisors and cuspids is distal to a line drawn through the midline/long axis of the tooth. The maxillary lateral incisor apex is coincidental to the midline/long axis of the tooth.nnnnThe gingival apex of a lateral incisor is 1 millimeter short of the central incisor and cuspid apex heights. The cuspid and central incisor gingival apex height are equal in height.nnnnFACE AND SILHOUETTE OF A TOOTHnnnnTeeth mesial and distal line angles, gingival curvature of tooth structure at the height of contour and incisal curvature defines the face of a tooth. Altering placement and shape of these defines the face of a tooth and the perceived size.nnnnThe mesial face and silhouette of a tooth is more angled off vertical than the distal aspect of a tooth.