The Depressor Anguli Oris (DAO) muscle is a small, paired muscle located in the lower third of the face. Its function is closely associated with expressions of sadness, frowning, or displeasure due to the downward pull it exerts on the mouth’s corners. The DAO muscle is often implicated in the formation of lines and wrinkles that affect the appearance of the lower face.
Where the DAO Muscle Is Located
The DAO muscle is a thin, superficial facial muscle with a generally triangular shape. It originates along the outer surface of the mandible (lower jawbone). Specifically, it attaches to the oblique line of the mandible, an area situated below the mental tubercle near the chin.
From this anchor point, the muscle fibers converge as they travel upward toward the mouth. The muscle then inserts into the modiolus, a collection of muscle fibers and connective tissue found at the corner of the mouth. This strategic position allows it to exert force on the oral commissure (corner of the lip), despite its small size. Its inferior and medial borders are typically located about 1.5 centimeters lateral to the midline of the chin.
Its Role in Facial Expression
The primary mechanical action of the DAO muscle is to depress the angle of the mouth, pulling the corners of the lips downward and slightly outward. This movement is fundamental to certain non-verbal communication signals. When the muscle contracts, it creates the facial expression commonly interpreted as a frown or a look of unhappiness.
The DAO is known as the “muscle of sadness” because its contraction contributes significantly to conveying emotions such as sadness or distress. Constant or frequent use of this muscle can cause the mouth corners to remain in a slightly downturned position, even when the face is at rest. This resting state can inadvertently suggest a sad, tired, or angry disposition to others.
Targeting the DAO Muscle for Facial Aesthetics
The hyperactivity of the DAO muscle is a common concern in medical aesthetics because its constant downward pull can lead to aesthetic issues. The contraction of this muscle contributes directly to the formation of marionette lines, which are creases that run vertically from the corners of the mouth down toward the chin. The sustained depression of the mouth corners can also create the appearance of a perpetually sad or drooping mouth.
A common non-surgical treatment to address this issue involves the use of neuromodulators, such as botulinum toxin. When injected directly into the DAO muscle, the neuromodulator temporarily blocks the release of acetylcholine, a chemical messenger that signals the muscle to contract. This process results in a temporary weakening, or flaccid paralysis, of the targeted muscle fibers.
By relaxing the DAO muscle, the downward pull on the corners of the mouth is reduced, allowing the opposing, lifting muscles to exert a greater influence. This shift in muscular balance can result in a temporary elevation of the mouth corners and a softening of the marionette lines. Anatomical knowledge is necessary for this procedure, as incorrect placement can affect adjacent muscles, such as those that move the lower lip. The effects of the treatment are temporary, typically lasting for several months before a repeat injection is necessary.