Anatomy of the Piriformis Muscle

The piriformis is a small, flat, and pear-shaped muscle located deep in the buttock. It lies in the center of the buttock and directly below the large gluteus maximus muscle, ¹ Chang C, Jeno SH, Varacallo M. Anatomy, Bony Pelvis and Lower Limb, Piriformis Muscle. [Updated 2022 Oct 3]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2022 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK519497/  which provides shape and form to the buttock and hip. ² Elzanie A, Borger J. Anatomy, Bony Pelvis and Lower Limb, Gluteus Maximus Muscle. [Updated 2022 Mar 28]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2022 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK538193/  The piriformis muscle originates near/just below the lower spine, on the front side of the tailbone (sacrum), and inserts into the top of the thighbone (femur). 

The piriformis is a type of external rotator muscle. It helps rotate the hip in an outward direction, which allows the thigh and knee to rotate or move away from the body, such as when stepping to the side from a standing position or getting out of a car.

A healthy piriformis muscle is critical to the stability of the hip joint. Its attachment to the top part of the thighbone helps maintain balance while walking by shifting the body's weight to the opposite side.

The large, thick sciatic nerve, which originates at the base of the spine, typically passes right below the piriformis  but may also pass above the muscle or pierce through it in some cases. ³ Pan J, Vasudevan J. Piriformis Syndrome. In Challenging Neuropathic Pain Syndromes (pp. 205–215). Elsevier; 2018. https://doi.org/10.1016/B978-0-323-48566-1.00024-3  

When the piriformis muscle is overused or fatigued, it can spasm and irritate the sciatic nerve, causing pain, numbness, and weakness in the leg. This condition is called piriformis syndrome and can result in similar pain and discomfort as sciatica. ⁴ Probst D, Stout A, Hunt D. Piriformis Syndrome: A Narrative Review of the Anatomy, Diagnosis, and Treatment. PM & R, 11 Suppl 1. https://doi.org/10.1002/PMRJ.12189

Piriformis Muscle Origin and Insertion

The piriformis is the only muscle in the entire pelvic complex that originates from the anterior surface of the body.

Specifically, the piriformis muscle originates from 4 main anatomical locations in the pelvic region, including ¹ Chang C, Jeno SH, Varacallo M. Anatomy, Bony Pelvis and Lower Limb, Piriformis Muscle. [Updated 2022 Oct 3]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2022 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK519497/ ^{, 5} Essentials of Pain Medicine. Elsevier; 2018. doi:10.1016/C2014-0-03837-3 :

• The upper part of the sacrotuberous ligament (ligament connecting the spine to the pelvis) 
• The front part of the sacral spine between S2 and S4 (base of the spine)
• The outer capsule of the sacroiliac (SI) joint (joint between the spine and pelvis)
• The upper part of the ilium (the large bone that forms the upper part of the pelvis)

The muscle runs sideways and passes through a large bony opening called the greater sciatic foramen. At this point, the muscle fibers become more tendon-like, and the tendinous fibers finally insert into the upper part of the thighbone. ⁵ Essentials of Pain Medicine. Elsevier; 2018. doi:10.1016/C2014-0-03837-3

The site of piriformis muscle insertion in the thigh bone is called the greater trochanter, which forms the most prominent part of the hip. The greater trochanter also serves as a site of attachment for other pelvic muscles. ⁴ Probst D, Stout A, Hunt D. Piriformis Syndrome: A Narrative Review of the Anatomy, Diagnosis, and Treatment. PM & R, 11 Suppl 1. https://doi.org/10.1002/PMRJ.12189 ^{, 6} Windisch G, Braun EM, Anderhuber F. Piriformis muscle: clinical anatomy and consideration of the piriformis Syndrome. Surg Radiol Anat. 2007;29:37-45. https://doi.org/10.1007/s00276-006-0169-x

The piriformis is surrounded by several other external rotator muscles, and these muscles work together to aid in thigh and leg movements and help to keep the hip stable and maintain balance while walking. ⁷ Jenkins DB. Hollinshead’s Functional Anatomy of the Limbs and Back. 9th ed. Elsevier; 2009.

Anatomic Variants of the Piriformis Muscle

Rarely, the piriformis muscle may have specific variations in its anatomy. Common variations reported in the literature include ⁶ Windisch G, Braun EM, Anderhuber F. Piriformis muscle: clinical anatomy and consideration of the piriformis Syndrome. Surg Radiol Anat. 2007;29:37-45. https://doi.org/10.1007/s00276-006-0169-x ^{, 8} Natis K, Totlis T, Konstantinidis GA, Paraskevas G, Piagkou M, Koebke J. Anatomical variations between the sciatic nerve and the piriformis muscle: a contribution to surgical anatomy in piriformis syndrome. Surg Radiol Anat. 2014.36:273-280. https://doi.org/10.1007/s00276-013-1180-7 :

• Fusion with other buttock muscles. The piriformis may completely or partially fuse with other muscles of the buttock, including the gluteus medius, glutes minimus, and superior gemellus muscles.
• Fusion with other tendons. The piriformis tendon may fuse with one or more tendons in the pelvic region, including the superior gemellus, obturator internus, and gluteus medius.
• Different sites of insertion. The piriformis may insert into the capsule surrounding the hip joint instead of the greater trochanter.
• Relationship between the sciatic nerve and piriformis muscle. The sciatic nerve may be positioned above or below the piriformis or even penetrate the piriformis muscle. ⁸ Natis K, Totlis T, Konstantinidis GA, Paraskevas G, Piagkou M, Koebke J. Anatomical variations between the sciatic nerve and the piriformis muscle: a contribution to surgical anatomy in piriformis syndrome. Surg Radiol Anat. 2014.36:273-280. https://doi.org/10.1007/s00276-013-1180-7 ^{, 9} Beaton LE, Anson JB. The relation of the sciatic nerve and of its subdivisions to the piriformis muscle. Anat Rec. 1937.70:1-5. http://doi.org/10.1002/ar.1090700102

Piriformis muscle variations, particularly fusions with other buttock muscles and tendons typically require a magnetic resonance imaging (MRI) scan to confirm the anatomic changes.

In multiple studies, it was shown that anatomical variation does not significantly contribute to the likelihood of developing complications or pain in the muscle, such as piriformis syndrome. ¹⁰ Smoll NR. Variations of the piriformis and sciatic nerve with clinical consequence: a review. Clin Anat. 2010.23(1):8-17. http://doi.org/10.1002/ca.20893 ^{, 11} Bartret Al, Beaulieu CF, Lutz AM. Is it painful to be different? Sciatic nerve anatomical variants on MRI and their relationship to piriformis syndrome. European radiology. 2018.28(11):4681-4686. http://doi.org/10.1007/s00330-018-5447-6 However, in general, the wider pelvic bone in women makes them more prone to developing piriformis syndrome compared to men. ¹² Boyajian-O'Neill LA, McClain RL, Coleman MK, Thomas PP. Diagnosis and Management of Piriformis Syndrome: An Osteopathic Approach. Journal of Osteopathic Medicine. 2008.108(11):657-664. http://doi.org/10.7556/jaoa.2008.108.11.657

Core Functions of the Piriformis Muscle

The piriformis muscle works in combination with the other hip muscles to keep the head of the thigh bone steady and stabilize the hip joint.

The origin and insertion of this muscle allow it to perform the following actions ¹³ Hopayian K, Danielyan A. Four Symptoms define the piriformis syndrome: an updated systematic review of its clinical features. Eur J Orthop Surg Traumatol. 2018.28(2):155-164. https://doi.org/10.1007/s00590-017-2031-8 :

• Allow external and lateral rotation of the hip. The piriformis muscle externally rotates the hip when the leg is extended (straightened out) and abducts the hip when the leg is flexed (bending the knee).
  • Hip abduction allows the thigh and knee to rotate outward, away from the midline of the body, and is important while walking.
  • Actions that use external hip rotation include stepping to the side, getting in and out of a car, pitching a baseball, and any other movement that requires a person to rotate the pelvis while placing most of the body’s weight on one leg.
  • The piriformis muscle also helps in the lateral rotation of the hip—and through abduction and lateral rotation, it also limits internal rotation to protect the lower extremity in motion. 
• Maintain balance while walking. The piriformis supports the lower body by stabilizing the pelvis and the base of the spine while walking.
  • This muscle helps shift the body’s weight to the opposite side when the hip and leg perform a combination of rotation and forward movement during each step, preventing one from falling. ¹ Chang C, Jeno SH, Varacallo M. Anatomy, Bony Pelvis and Lower Limb, Piriformis Muscle. [Updated 2022 Oct 3]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2022 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK519497/

The strategic location of the piriformis muscle connecting the lower spine to the pelvis makes it essential to almost every movement of the lower body and is frequently used in daily activities. 

When the Piriformis Muscle May Become Painful

If the piriformis muscle becomes injured or inflamed, it may begin to spasm and cause pain deep in the buttock. This condition is called piriformis syndrome. The spasming may also irritate the sciatic nerve, which runs in close proximity to the piriformis muscle, sending sciatica-like symptoms down the thigh and leg along the course of the nerve.

The piriformis may spasm due to a direct injury to the muscle or an injury to the nearby structures, such as ⁴ Probst D, Stout A, Hunt D. Piriformis Syndrome: A Narrative Review of the Anatomy, Diagnosis, and Treatment. PM & R, 11 Suppl 1. https://doi.org/10.1002/PMRJ.12189 ^{, 6} Windisch G, Braun EM, Anderhuber F. Piriformis muscle: clinical anatomy and consideration of the piriformis Syndrome. Surg Radiol Anat. 2007;29:37-45. https://doi.org/10.1007/s00276-006-0169-x ^{, 13} Hopayian K, Danielyan A. Four Symptoms define the piriformis syndrome: an updated systematic review of its clinical features. Eur J Orthop Surg Traumatol. 2018.28(2):155-164. https://doi.org/10.1007/s00590-017-2031-8 :

• The irritation of the piriformis muscle due to a problem in the SI joint
• A direct injury to the buttock that causes the piriformis muscle to tighten
• An injury to the buttock or hip that causes the piriformis muscle to swell
• Bleeding in the area around the piriformis muscle
• Infection of the piriformis muscle

Piriformis syndrome is often overlooked due to its symptoms and similarities with other lower back problems. Research suggests that up to 17% of patients with chronic low back pain and leg pain have piriformis syndrome. ¹⁴ Fahmi A, Rahmadhan MA, Aprianto DR, Subianto H, Turchan A. Complete resolution of recurrent piriformis syndrome after piriformis resection with 3 years’ follow up: A case report. International Journal of Surgery Case Reports. 2020;77:576-579. doi:10.1016/j.ijscr.2020.11.099  

Piriformis syndrome is more common in athletes and laborers who overuse their back muscles. The condition may also develop due to prolonged strain from occupations that require sitting for long periods of time, such as desk jobs and driving. ¹⁵ Chang A, Ly N, Varacallo M. Piriformis Injection. [Updated 2022 Sept 4]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2022 Jan-. Available from: https://pubmed.ncbi.nlm.nih.gov/28846327/

Piriformis syndrome typically affects individuals in the age group of 30-40 years. ¹³ Hopayian K, Danielyan A. Four Symptoms define the piriformis syndrome: an updated systematic review of its clinical features. Eur J Orthop Surg Traumatol. 2018.28(2):155-164. https://doi.org/10.1007/s00590-017-2031-8

See Symptoms and Diagnosis of Piriformis Syndrome

The piriformis muscle performs a major role in the lower back and pelvis, stabilizing the hip and assisting in everyday leg and hip movements. This small but essential muscle can get overused or injured, causing debilitating pain in the buttock, which may also affect nearby structures, such as the sciatic nerve, and result in leg and thigh weakness and pain.