A 32-year-old woman from Delhi presents with acute left hip pain and inability to bear weight following a motor vehicle accident. On examination, the left hip is held in flexion, adduction, and internal rotation; the left leg appears shortened. There is severe pain on any attempt at passive hip movement. Neurovascular examination reveals intact distal pulses and normal sensation. Which structure is most likely injured, and what is the anatomical basis for the characteristic posture?

Explanation

## Diagnosis: Posterior Hip Dislocation ### Clinical Presentation The classic triad of **posterior hip dislocation** is: 1. **Flexion, adduction, and internal rotation (FAIR) posture** — the hip is "locked" in this position 2. **Apparent shortening** of the affected limb (flexion causes relative shortening) 3. **Severe pain** on any movement, especially attempts at external rotation or abduction **Key Point:** Posterior dislocation accounts for 80–90% of hip dislocations and is classically associated with **flexed knee + flexed hip** (dashboard injury, high-energy trauma). ### Anatomical Basis of the FAIR Posture ```mermaid flowchart TD A[Posterior Hip Dislocation]:::outcome --> B[Femoral head displaced posteriorly]:::outcome B --> C{Capsular stretch?}:::decision C -->|Yes| D[MFCA stretched posteriorly]:::urgent C -->|Yes| E[Capsule under tension]:::urgent D --> F[Pain from vascular compromise]:::outcome E --> F F --> G[Protective FAIR posture adopted]:::action G --> H[Flexion + adduction + IR<br/>relieves capsular tension]:::action H --> I[Pain reduced, muscle spasm minimized]:::outcome ``` ### Why FAIR Relieves Pain | Movement | Effect on Posterior Capsule | Effect on MFCA | |----------|----------------------------|----------------| | Flexion | Relaxes posterior capsule | Reduces stretch | | Adduction | Brings femoral head medially | Reduces posterior displacement | | Internal rotation | Internally rotates femur | Reduces external rotator tension | **Clinical Pearl:** The **medial femoral circumflex artery (MFCA)** enters the hip joint via the posterior capsule. In posterior dislocation, the femoral head is displaced posteriorly, stretching the MFCA and posterior capsule. The FAIR posture is a protective reflex that minimizes this stretch and reduces pain. ### Distinguishing Features: Posterior vs. Anterior Dislocation | Feature | Posterior Dislocation | Anterior Dislocation | |---------|----------------------|---------------------| | Hip position | Flexion, adduction, IR | Flexion, abduction, ER | | Apparent length | Shortened | Lengthened | | Mechanism | Flexed knee + flexed hip (dashboard) | Forced abduction + ER | | Frequency | 80–90% | 10–20% | | Sciatic nerve injury | 10–15% | Rare | **High-Yield:** Always check for **sciatic nerve injury** (foot drop, sensory loss in S1 distribution) in posterior dislocation — it is the most common neurological complication. ### Management 1. **Immediate:** Neurovascular assessment (this patient has intact pulses and sensation — good prognostic sign) 2. **Reduction:** Closed reduction under anesthesia (Allis maneuver for posterior dislocation) 3. **Imaging:** Post-reduction CT to rule out associated acetabular or femoral head fractures 4. **Follow-up:** Monitor for late complications (avascular necrosis of femoral head, post-traumatic arthritis) **Warning:** Delay in reduction >12 hours significantly increases risk of AVN due to prolonged vascular compromise.