The visibility of upper teeth when a person smiles, known as incisal display, is a significant component of facial aesthetics. Low tooth visibility during a smile is a common cosmetic concern. It is generally not due to a single issue but rather a combination of underlying anatomical structures and functional dynamics. The amount of tooth visible is determined by the static relationship between the upper lip and the teeth at rest, combined with the dynamic movement of the lips during a full smile.
Facial Anatomy and Lip Position
The length of the upper lip is a primary static predictor of how much of the upper teeth is visible. A longer upper lip creates a greater vertical distance between the nose and the gum line, naturally covering more of the tooth structure. In adult females, the average upper lip length is between 20 and 22 millimeters, and in males, it averages slightly longer, between 22 and 24 millimeters.
If the lip is longer, it requires more vertical lift from the facial muscles to expose the teeth fully. The vertical positioning of the maxilla, or upper jaw bone, also plays a role. If the maxilla is positioned higher or flatter, the teeth are vertically recessed in the face, which contributes to a low incisal display.
The thickness and fullness of the upper lip also influence the degree of tooth coverage. A thicker upper lip may cover more of the upper incisors, especially during a relaxed or less animated smile. This structural arrangement acts like a frame, naturally obscuring the teeth when the lip is long.
The Role of Tooth Dimensions and Wear
The visible height of the teeth, referred to as the clinical crown length, directly influences incisal display. Teeth that are petite or have short clinical crowns are easily covered by an average-length lip. The average visible length for a maxillary central incisor is approximately 10 millimeters, and anything shorter reduces visibility.
Dental attrition, or the progressive wear of the teeth, is a major factor that shortens the visible length of the teeth. Habits like tooth grinding (bruxism) or decades of chewing cause the biting edges to wear down. This gradual shortening means the teeth do not extend far enough vertically to drop below the upper lip line.
An excessive vertical overlap of the upper teeth over the lower teeth, called a deep overbite, also contributes to a low smile line. In this situation, the upper teeth are positioned far down over the lower teeth, which can push the lower lip down or cause the upper lip to settle lower. Misaligned or crowded teeth may also hide certain teeth behind others, reducing the visual impact of the smile.
Muscle Action and the Effects of Aging
The dynamic movement of the lips during a smile is governed by facial muscles, including the levator labii superioris and the zygomaticus major. Some individuals have decreased lip mobility, meaning the muscles do not achieve the necessary vertical displacement to fully reveal the teeth. The typical range of upper lip movement from rest to a full smile is approximately 6 to 8 millimeters.
If a person’s muscles only lift the lip minimally, they exhibit a low smile line, regardless of underlying tooth or jaw position. Chronic tension in the facial muscles can also restrict the natural movement required for a full vertical lip lift. This tension creates a tighter, guarded smile that shows less of the tooth structure.
The process of aging compounds this issue by causing measurable changes to the soft tissues of the face. As people age, the upper lip tends to lengthen and loses elasticity due to gravity and tissue changes. This increased length means the lip hangs lower over the teeth, leading to a progressive decrease in upper tooth display. This phenomenon, where older adults show less of their upper teeth and more of their lower teeth, is known as reverse incisal display and is a characteristic change that occurs after the age of 30.
When to Consult a Dental Professional
If you are concerned about low tooth visibility, consulting a dental professional, such as a cosmetic dentist or an orthodontist, is the appropriate next step. These specialists perform a detailed smile analysis that goes beyond just looking at the teeth. The diagnostic process involves measuring the upper lip length at rest, the incisal display at rest, and the maximum vertical movement of the lip during a full smile.
Understanding the cause allows the professional to recommend a solution tailored to the specific issue. If the cause relates to tooth dimension, treatments may include placing porcelain veneers or cosmetic bonding to lengthen the teeth. If the gum tissue covers too much of the tooth, a procedure like crown lengthening or gum contouring can expose more of the natural tooth. When skeletal or tooth position is the primary factor, orthodontic treatment can reposition the teeth vertically. If the lip is the main issue, non-surgical options like Botox or minor surgical lip repositioning may be considered to increase the vertical lip lift.