A 32-year-old man sustained a crush injury to the posterior thigh. Clinical examination reveals weakness of knee flexion and ankle plantarflexion, with loss of sensation in the lateral leg and dorsum of the foot. All of the following nerves are likely damaged EXCEPT:

Question

Accepted Answer

B. Obturator nerve — causing weakness of hip adduction and medial thigh sensory loss. ## Clinical Analysis: Crush Injury to Posterior Thigh

### Clinical Presentation Breakdown

**Key Point:** The patient has:
1. Weakness of knee flexion (hamstrings — tibial nerve)
2. Weakness of ankle plantarflexion (gastrocnemius, soleus — tibial nerve)
3. Sensory loss in lateral leg and dorsum of foot (common fibular nerve distribution)

These findings are consistent with **sciatic nerve injury** affecting both its terminal branches.

### Nerve Involvement in Posterior Thigh Injuries

| Nerve | Motor Supply | Sensory Distribution | Injury Pattern |
|-------|--------------|----------------------|----------------|
| **Tibial nerve** | Knee flexion, ankle plantarflexion, toe flexion | Sole of foot, medial leg | Present in this case |
| **Common fibular nerve** | Ankle dorsiflexion, toe extension, foot eversion | Lateral leg, dorsum of foot | Present in this case |
| **Obturator nerve** | Hip adduction | Medial thigh | NOT involved |
| **Sciatic nerve (parent)** | Both tibial and fibular functions | Both distributions | Affected as trunk |

**High-Yield:** The obturator nerve arises from the **lumbar plexus** (L2–L4) and exits the pelvis through the obturator foramen. It does NOT travel through the posterior thigh and therefore is NOT damaged in posterior thigh crush injuries.

### Why Obturator Nerve Is NOT Involved

**Clinical Pearl:** The obturator nerve has a completely different course:
- Exits pelvis via obturator foramen (anterior pelvic wall)
- Supplies medial thigh muscles (adductors)
- Does not pass through the posterior thigh compartment

A crush injury to the posterior thigh cannot damage the obturator nerve because it is anatomically separated from the injury site.

**Mnemonic:** **LUMBOSACRAL PLEXUS ORIGIN** — Obturator = Lumbar plexus (L2–L4); Sciatic = Sacral plexus (L4–S3). Different origins → different courses → different injury patterns.

Answer

A. Sciatic nerve proper — as the parent trunk before its terminal branches

Answer

C. Common fibular nerve — causing foot drop and sensory loss over the lateral leg and dorsum

Answer

D. Tibial nerve — causing weakness of plantarflexion and loss of sole sensation

A 32-year-old man sustained a crush injury to the posterior thigh. Clinical examination reveals weakness of knee flexion and ankle plantarflexion, with loss of sensation in the lateral leg and dorsum of the foot. All of the following nerves are likely damaged EXCEPT:

Explanation

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