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Illustration of the piriformis anatomy
Beaton and Anson classification of different anatomical relationships between the sciatic nerve and piriformis muscle and their estimated prevalence. (a) Type 1: undivided sciatic nerve passing anterior and below the piriformis. (b) Type 2: common peroneal nerve component piercing a bifid piriformis, tibial component running in normal position anterior and inferior to piriformis. (c) Type 3: one division posterior to and the other anterior to the piriformis. (d) Type 4: undivided sciatic nerve piercing bifid piriformis. (e) Type 5: one division through and the other posterior to the piriformis. (f) Type 6: undivided nerve posterior to piriformis.[1]
  • Anterior surface of the Sacrum
  • Gluteal surface of Ilium
  • Sacrotuberous ligament



  • External rotation of hip when in extension
  • Abduction of hip when in flexion
  • Stabilizes Femur in acetabulum

Vascular Supply

  • Superior Gluteal Artery
  • Inferior Gluteal Artery
  • Internal Pudendal Artery (gemellar branches)


  • Piriformis Nerve (S1-S2)
    • Also referred to as 'nerve to piriformis'

Sciatic Nerve

  • Beaton et al cadaveric study[2]
    • 90% of cadavers had traditional anatomy with an undivided sciatic nerve emerging below the piriformis muscle
    • Divided sciatic nerve passing through and below the piriformis muscle
    • Divided sciatic nerve passing above and below the piriformis muscle
    • Undivided nerve passing through the piriformis muscle
  • Smoll et al systematic review and meta-analysis[3]
    • 16.9% of cadavers: abnormal relationship between the sciatic nerve and piriformis muscle
  • Bartet et al MRI study[4]
    • 19.2% of MRI revealed an abnormal relationship between the sciatic nerve and piriformis muscle

Clinical Significance

See Also


  1. Bharadwaj, Upasana Upadhyay, et al. "Variant Sciatic Nerve Anatomy in Relation to the Piriformis Muscle on Magnetic Resonance Neurography: A Potential Etiology for Extraspinal Sciatica." Tomography 9.2 (2023): 475-484.
  2. Beaton LA, Anson BJ. The sciatic nerve and the piriformis muscle: their interrelation a possible cause of coccygodynia. J Bone Joint Surg. 1938;20(3):686‐688.
  3. Smoll NR. Variations of the piriformis and sciatic nerve with clinical consequence: a review. Clin Anat. 2010;23(1):8‐17.
  4. Bartret AL, Beaulieu CF, Lutz AM. Is it painful to be different? Sciatic nerve anatomical variants on MRI and their relationship to piriformis syndrome. Eur Radiol. 2018;28(11):4681‐4686.
Created by:
Chris Hauglid on 7 July 2020 18:28:55
Last edited:
15 May 2023 04:11:13