A 32-year-old man sustains a posterior hip dislocation in a road traffic accident. Closed reduction is achieved under general anaesthesia at 4 hours post-injury. Despite successful reduction, the patient develops avascular necrosis (AVN) of the femoral head at 18 months. Which pathophysiological mechanism is the PRIMARY determinant of AVN in this case?

• A Intraosseous fat embolism following trauma
• B Increased intra-articular pressure from haemarthrosis compressing epiphyseal vessels
• C Disruption of the retinacular blood vessels (lateral epiphyseal vessels) at the time of dislocation ✓
• D Damage to the artery of ligamentum teres causing isolated femoral head ischaemia

Explanation

The lateral epiphyseal (retinacular) vessels — branches of the medial femoral circumflex artery that travel along the femoral neck under the capsule — supply the weight-bearing portion of the femoral head and are torn or kinked at the moment of dislocation. This primary vascular disruption is the dominant cause of AVN. The artery of ligamentum teres contributes only minimally in adults and is insufficient to prevent AVN alone. Haemarthrosis-related pressure and fat embolism are not primary mechanisms for post-dislocation AVN.

Reference: Maheshwari Essential Orthopaedics, 6th ed.

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Written and medically reviewed by the StethoPrep medical team.