The observation that the upper lip covers a significant portion, or all, of the teeth during a full smile is a common concern related to facial anatomy. An aesthetically pleasing smile is characterized by a harmonious balance between the lips, gums, and teeth. The “smile line” describes the relationship of the lower edge of the upper lip to the upper teeth when smiling. When the lip obscures the teeth, it suggests a variation in the vertical relationships of the soft tissues, the underlying skeletal structure, or the dental components. This phenomenon results from a combination of factors involving the soft tissues of the face and the hard tissues of the jaw and teeth.
Soft Tissue Factors Determining Lip Coverage
The primary soft tissue element influencing tooth coverage is the vertical length of the upper lip at rest. A naturally longer upper lip, measured from the base of the nose to the border of the lip, covers more of the upper incisors even before smiling. The average upper lip length at rest is shorter in females than in males, which is why females often display more of their upper teeth at rest. With increasing age, the upper lip tends to lengthen and droop, further decreasing the visible tooth area.
The muscles responsible for lifting the lip during smiling also play a role in tooth visibility. The Levator labii superioris and the Levator labii superioris alaeque nasi are the main muscles that contract to elevate the upper lip. If the vertical movement from the resting position to a full smile is less than the average range (7 to 8 millimeters), the upper lip may not fully clear the teeth. This reduced elevation means the lip remains low relative to the incisal edge, leading to increased coverage.
The contraction of the Zygomaticus major and minor muscles pulls the corners of the mouth up and outward, contributing to the final smile shape. Although these muscles widen the smile, their vertical pull must be sufficient to lift the lip fully away from the teeth. If muscle activity or the inherent length of the lip limits upward movement, the teeth remain partially or fully obscured. The total vertical elevation of the upper lip during a smile varies significantly among individuals, ranging from 2 to 12 millimeters.
Hard Tissue Contributions from Teeth and Jaw Structure
The underlying dental and skeletal framework determines the vertical position of the teeth relative to surrounding structures. The vertical dimension of the maxilla, the upper jawbone, is a significant factor in how the teeth are presented. If the maxilla is positioned higher than average, the teeth are set higher in the face, closer to the upper lip’s resting line. This skeletal positioning contributes to the teeth being easily hidden by the lip when smiling.
Dentoalveolar discrepancies, such as a deep bite, can cause lip coverage. A deep bite is an excessive vertical overlap of the upper teeth over the lower teeth, often involving upper incisors that are over-erupted or positioned too low in the jaw. When the upper incisors are positioned too far down, the lip has less distance to travel to cover them completely upon smiling, leading to reduced tooth display.
The shape and condition of the teeth contribute to the visible surface area. Tooth wear, known as attrition, reduces the length of the clinical crown (the portion of the tooth visible above the gum line). Over time, friction from chewing or grinding shortens the upper incisors, making them appear smaller and more easily covered by the lip. If the upper incisors are retroclined (tilted inward), their visible vertical height is diminished, increasing the likelihood of lip coverage during a smile.
Professional Treatment Pathways for Adjustment
Addressing reduced tooth display requires a diagnosis that identifies whether the cause is muscular, dental, or skeletal. For issues related to muscle function, neuromodulators, such as botulinum toxin (Botox), are used. Small, precise injections are administered into the lip elevator muscles. This temporarily reduces the muscle’s ability to contract, preventing the lip from descending too far and allowing for greater tooth visibility during a smile. The effect lasts approximately three to six months and requires repeat treatments.
Orthodontic Solutions
If the issue is primarily dental, such as a deep bite or retroclined incisors, orthodontic treatment is often the solution. Orthodontists use braces or clear aligners to intrude the upper incisors, effectively pushing them upward into the jawbone. This procedure repositions the teeth vertically, increasing the distance between the incisal edge and the lower border of the upper lip, thus improving tooth display.
Cosmetic Restoration
In cases where the teeth have been shortened due to wear, cosmetic dentistry offers restorative solutions like bonding or porcelain veneers. These procedures rebuild the worn incisal edge, increasing the clinical crown length to a more harmonious proportion. This helps the teeth extend past the lip line.
Lip Repositioning Surgery
When the problem is rooted in a severely high-set maxilla or an inability to achieve sufficient lip elevation, surgical options may be considered. Lip repositioning surgery is a conservative procedure performed by periodontists or oral surgeons. It involves removing a strip of mucosa inside the upper lip, which creates an internal scar line. This scar restricts the upward pull of the lip elevator muscles, permanently limiting the lip’s vertical movement and preventing it from fully covering the teeth.
Orthognathic Surgery
For the most severe skeletal discrepancies, such as true vertical maxillary excess, orthognathic surgery, specifically a Le Fort I osteotomy, may be necessary. This procedure moves the entire upper jaw upward into the facial structure. It permanently resolves the vertical skeletal imbalance and ensures a proper tooth-to-lip relationship.