The Depressor Labii Inferioris Muscle

The depressor labii inferioris (DLI) is the primary muscle responsible for depressing the lower lip. 1, 2

Anatomical Characteristics

The depressor labii inferioris originates from the mandible and inserts into the skin and mucosa of the lower lip, functioning to pull the lower lip downward and laterally 2
The DLI works in conjunction with other facial muscles, particularly the incisivus labii inferioris muscle (ILI), which has inferior bundles that connect with the DLI 2
The DLI receives innervation from the marginal mandibular branch of the facial nerve (cranial nerve VII) 1

Paralysis of the DLI causes asymmetry of the lower lip, particularly noticeable during smiling and facial expressions 1, 3
DLI dysfunction can result in difficulty speaking clearly and impaired lower lip movement 1
Patients with unilateral DLI paralysis often show asymmetry when showing their lower teeth during smiling 3

DLI palsy can occur as a complication of various maxillofacial surgeries, including chin augmentation procedures 1
For patients with unilateral marginal mandibular nerve paralysis affecting the DLI, treatment options include:
- Resection of the DLI on the non-paralyzed side to create symmetry 3
- Chemodenervation (botulinum toxin injection) of the contralateral healthy DLI to balance facial appearance 4
In cases of synkinesis (abnormal facial muscle movements after facial nerve injury), depressor anguli oris myectomy can improve facial symmetry by addressing the imbalance with the DLI 5

The DLI has close relationships with the platysma muscle and the inferior bundle of the incisivus labii inferioris 2
The transverse and inferolateral slips of the inferior bundle of the ILI typically attach to the deep fibers of both the platysma lateral to the DLI 2
Understanding these anatomical relationships is crucial for procedures involving the lower lip region, including botulinum toxin injections and facial surgeries 2

When DLI palsy occurs as a complication of procedures like chin augmentation, patients may recover completely within approximately three months with appropriate management 1
Facial exercises may be beneficial in the recovery process 1

Careful surgical technique and thorough knowledge of facial anatomy are essential to prevent complications involving the DLI during maxillofacial procedures 1
Awareness of the course of the marginal mandibular branch of the facial nerve is particularly important to avoid injury during procedures in the submandibular region 1