A 48-year-old woman from Mumbai undergoes a left superficial parotidectomy for a benign mixed tumor. During surgery, the surgeon carefully identifies the facial nerve trunk at the stylomastoid foramen and preserves all five terminal branches. However, 2 weeks postoperatively, the patient develops weakness of the lower lip with inability to depress the lower lip and pull it downward and laterally. The remaining facial nerve branches function normally. Which nerve branch was likely injured during the procedure?

Explanation

## Facial Nerve Branches and Motor Functions ### Clinical Presentation Analysis The patient has selective weakness of the lower lip with inability to depress and pull it downward and laterally. This specific motor deficit points to injury of a single facial nerve terminal branch. ### Facial Nerve Terminal Branches and Their Functions | Branch | Motor Innervation | Clinical Deficit if Injured | |--------|-------------------|-----------------------------| | **Temporal** | Frontalis, orbicularis oris (upper), corrugator | Loss of forehead wrinkling, eyebrow elevation | | **Zygomatic** | Orbicularis oculi, zygomaticus major/minor | Loss of eye closure, smile asymmetry | | **Buccal** | Buccinator, orbicularis oris (upper), levator labii | Loss of cheek puffing, upper lip elevation | | **Marginal Mandibular** | Depressor anguli oris, depressor labii inferioris, mentalis | **Loss of lower lip depression, inability to pull lip down and laterally** | | **Cervical** | Platysma | Loss of neck skin wrinkling, neck flexion weakness | **Key Point:** The **marginal mandibular branch** innervates the depressor muscles of the lower lip and is the most commonly injured branch during parotid surgery because of its superficial course along the inferior border of the mandible. ### Anatomical Vulnerability of the Marginal Mandibular Branch **High-Yield:** The marginal mandibular branch: - Exits the parotid gland at or below the angle of the mandible - Runs superficially along the inferior border of the mandible - Crosses over the facial artery and vein - Is at risk during dissection of the inferior and anterior aspects of the parotid gland - Is the most frequently injured facial nerve branch in parotidectomy (up to 25% of cases, though most recover within 3–6 months) **Clinical Pearl:** Injury to the marginal mandibular branch produces a characteristic clinical sign: the patient cannot depress the lower lip or pull it downward and laterally. The smile may appear asymmetrical with drooping of the lower lip on the affected side. This is distinct from the broader facial paralysis seen with proximal facial nerve injury. ### Why This Branch Is Vulnerable During Surgery ```mermaid flowchart TD A[Superficial Parotidectomy]:::action --> B[Dissection of inferior parotid pole]:::action B --> C[Marginal mandibular branch at risk]:::urgent C --> D{Branch injured?}:::decision D -->|Yes| E[Lower lip depression weakness]:::outcome D -->|No| F[Normal lower lip function]:::outcome E --> G[Depressor anguli oris paralysis]:::outcome ``` **Mnemonic:** **TZBC** — Terminal branches of facial nerve in order of emergence from stylomastoid foramen: **T**emporal, **Z**ygomatic, **B**uccal, **C**ervical. The **Marginal Mandibular** branch is often considered separately as it arises from the buccal/cervical complex and is the most superficial and vulnerable. ### Distinguishing Features - **Temporal branch injury:** Loss of forehead wrinkling and eyebrow elevation - **Zygomatic branch injury:** Loss of eye closure (lagophthalmos) and smile asymmetry - **Buccal branch injury:** Loss of cheek puffing and upper lip elevation - **Marginal mandibular injury:** Loss of lower lip depression (as in this case) - **Cervical branch injury:** Loss of platysma contraction and neck skin wrinkling