A 35-year-old man sustains a posterior hip dislocation in a road traffic accident. On arrival, the limb is shortened, internally rotated, and adducted. The dislocation is reduced within 4 hours. Two weeks later, MRI shows signal abnormality in the femoral head consistent with early avascular necrosis. Which factor most significantly determines the risk of developing AVN after posterior hip dislocation?

• A Age of the patient at the time of injury
• B Presence of associated acetabular rim fracture
• C Gender of the patient
• D Duration of dislocation before reduction (time-to-reduction) ✓

Explanation

Time-to-reduction is the most critical determinant of AVN risk after hip dislocation. The posterior capsule and retinacular vessels supplying the femoral head (branches of the medial femoral circumflex artery) are stretched/kinked in the dislocated position. Reduction within 6 hours reduces AVN risk to ~5-10%; delay beyond 6-12 hours increases risk to 25-50% or more. The widely cited threshold is reduction within 6 hours. Age and associated fractures contribute but time-to-reduction has the strongest independent association with AVN in epidemiological studies.

Reference: Maheshwari Essential Orthopaedics, 6th ed.

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Written and medically reviewed by the StethoPrep medical team.