In a posterior approach to total hip arthroplasty, the sciatic nerve is at risk. In what percentage of individuals does the common fibular division of the sciatic nerve pass through the piriformis muscle rather than below it, making it more vulnerable?

• A Approximately 12–15% of individuals have a common fibular division piercing the piriformis (Beaton-Anson Type B anomaly) ✓
• B The sciatic nerve always exits inferior to the piriformis in 100% of individuals
• C About 50% of individuals show this anatomical variant bilaterally
• D The entire sciatic nerve pierces the piriformis in about 30% of cases

Explanation

The Beaton-Anson classification describes sciatic nerve-piriformis relationships. In the most common type (Type A), the entire sciatic nerve exits below piriformis. In Type B (approximately 12–15%), the common fibular division pierces the piriformis while the tibial division exits below it. This anomaly predisposes to piriformis syndrome and increases the risk of sciatic nerve injury during posterior hip surgery and intramuscular gluteal injections. The entire nerve piercing through the piriformis is a much rarer variant. This has surgical significance for hip arthroplasty and piriformis release procedures.

Reference: BD Chaurasia's Human Anatomy, 8th ed.

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Written and medically reviewed by the StethoPrep medical team.