A perpetually sad expression exists independent of mood for some people. Their mouth corners naturally turn downward, creating the appearance of unhappiness even at rest. The Depressor Anguli Oris muscle, known as the DAO, bears responsibility for this downturn. When overactive or dominant, it pulls the corners of the mouth toward the jaw, creating what injectors sometimes call “resting sad face.” Botox relaxes this muscle and allows the mouth corners to return to neutral or even slightly upturn.
Anatomy of Sorrow
The DAO originates from the mandible near the chin and inserts into the modiolus, the fibromuscular junction at the corner of the mouth. When it contracts, it pulls the oral commissure downward and slightly backward.
This muscle serves legitimate functions in expression. Frowning, expressing disgust, and communicating disapproval all involve DAO contraction. The problem arises when the muscle maintains excessive tone at rest, pulling the corners down even without emotional intent.
Contributing factors include genetics, habitual expression patterns, and aging-related volume loss in the perioral region. As the cheeks and lips lose fat and collagen with age, the downward pull of the DAO becomes more visible because there is less structural support to oppose it.
The clinical appearance varies in severity. Mild downturn shows subtle downward angulation of the corners. Moderate downturn creates visible marionette lines extending from the corners toward the jaw. Severe downturn produces a persistent frown that others comment on.
Locating the Muscle (The Clench Test)
Finding the DAO precisely requires palpation and observation. The clench test helps: ask the patient to clench their teeth and pull their mouth corners downward as if frowning. The DAO will contract and become palpable as a band running from the mandible toward the mouth corner.
The injection point lies approximately 1 cm lateral to the oral commissure and 1 cm below it, over the body of the contracted muscle. This location is close to, but not directly at, the mouth corner.
Feeling for the muscle provides more reliable targeting than memorized landmarks. Anatomy varies between individuals, and the DAO’s exact position shifts with facial structure. An injector who locates the muscle by palpation in each patient produces more consistent results than one who injects the same coordinates every time.
The depth should be intramuscular, not superficial. The DAO lies beneath the skin and subcutaneous tissue. Superficial injection wastes product; appropriate depth reaches the muscle belly.
Differentiating DAO from DLI
The Depressor Labii Inferioris (DLI) sits medial to the DAO and performs a different function. It pulls the lower lip downward, exposing the lower teeth. Hitting the DLI instead of or in addition to the DAO creates asymmetric lower lip movement, particularly visible when speaking.
The two muscles lie close together. The DLI is more medial, closer to the midline. The DAO is more lateral, closer to the corner of the mouth. Injecting too far medially risks affecting the DLI and creating a crooked appearance when the patient speaks or smiles.
| Muscle | Location | Action | Complication if Overtreated |
|---|---|---|---|
| DAO | Lateral, near commissure | Pulls corner down | Asymmetric smile |
| DLI | Medial, near chin | Pulls lower lip down | Asymmetric lower lip, speech changes |
The DLI complication is more noticeable and bothersome than minor asymmetry from DAO treatment alone. Patients notice something wrong with their mouth when speaking, and others perceive their expression as odd. This makes precision in DAO targeting essential.
Conservative dosing provides additional safety margin. Starting with 3-5 units per side and assessing at two weeks allows adjustment before any asymmetry becomes pronounced.
Combining with Marionette Filler
The marionette lines extending downward from the mouth corners result from two factors: the DAO pulling the corners down and volume loss creating a fold in the skin. Botox addresses the muscle; filler addresses the volume.
Combination treatment produces better results than either modality alone for most patients with established marionette lines. The Botox relaxes the downward pull. The filler fills the groove and provides structural support that further opposes the remaining muscle action.
The sequence matters less than thoroughness, but most injectors prefer Botox first, filler second. Placing filler first requires accounting for the swelling it causes when positioning Botox injections. Placing Botox first allows the filler to be positioned precisely once the immediate injection effects resolve.
For patients who cannot afford combination treatment, Botox alone provides partial improvement by addressing the dynamic component. The mouth corners upturn, but the static crease remains. As the patient ages or seeks further correction, adding filler becomes the next step.
Risks: The Asymmetrical Smile
The most common complication of DAO Botox involves asymmetric movement of the mouth during animation. If the toxin diffuses beyond the intended area or if dosing differs between sides, the smile becomes crooked.
Mild asymmetry is common and often acceptable. Faces are naturally asymmetric, and small differences in movement usually go unnoticed by casual observers. The patient may notice in the mirror, but the effect is subtle.
Moderate to severe asymmetry affects social interaction. Others perceive something wrong with the expression. The patient feels self-conscious about smiling in photographs or during conversation.
Risk factors for asymmetry include excessive dosing, injection too close to the modiolus, and unintentional diffusion to adjacent muscles. The risorius, depressor labii inferioris, and mentalis all lie near the DAO. Affecting any of them distorts normal perioral movement.
Recovery from asymmetry requires waiting for the Botox to wear off. No reversal agent exists. The duration is typically 2-3 months, shorter than upper face treatments due to the smaller doses used and the active movement of the perioral region.
Longevity in the Lower Face
Lower face Botox generally wears off faster than upper face treatments. While forehead and glabella treatments last 3-4 months for most patients, DAO treatment often provides 2-3 months of effect.
The perioral region sees constant movement from speaking, eating, and expression. This activity may accelerate toxin clearance compared to less dynamic areas. The smaller doses used for safety also contribute to shorter duration.
Patients seeking sustained improvement need to budget for more frequent treatments. Four to six appointments per year maintains continuous effect, compared to three or four for forehead maintenance. The cost per year exceeds upper face treatment for comparable smoothness.
Some patients prefer the shorter duration as a safety feature. Any unwanted effect resolves faster. For first-time lower face treatment, the relatively quick offset provides reassurance that problems will not persist indefinitely.
Sources:
- DAO anatomy and function: Anatomical Record, “Muscles of Facial Expression”
- DAO vs DLI differentiation: Journal of Cosmetic Dermatology, “Lower Face Botulinum Toxin Injection Techniques”
- Duration of perioral treatment: Aesthetic Surgery Journal, “Longevity of Botulinum Toxin A in Different Facial Regions”