Variations involving the relationship between the sciatic nerve and the piriformis muscle are commonly classified based on Beaton and Anson’s classification.
Mnemonic: I Prefer Smart Anatomy For Perfect nerve-sparing
| Type | Mnemonic | Meaning | Description | Prevalence |
| I | I | Inferior | Undivided sciatic nerve passes below piriformis | 87% |
| II | Prefer | Piercing | Peroneal pierces, tibial passes below | 13% |
| III | Smart | Superior | Peroneal above, tibial below | <1% |
| IV | Anatomy | All through | Entire sciatic pierces piriformis | <1% |
| V | For | Forked/Above and Through | Peroneal above, tibial pierces piriformis | <1% |
| VI | Perfect | Pierced both | Both divisions pierces piriformis | <1% |
Clinical Significance
- Piriformis Syndrome: Variations where nerve pierces or splits through the piriformis may predispose to entrapment neuropathy.
- Sciatic Nerve Block: Anomalies affect landmark-based regional anesthesia.
- Hip and Gluteal Surgery: Important for avoiding iatrogenic injury during posterior hip approaches.
- Imaging Interpretation: Variations may mimic masses or pathology on MRI/CT.