|Origin||Anterior surface of the sacrum (around the level of S2 to S4)
Gluteal surface of the ilium
|Insertion||Greater trochanter of the femur (medial side of the superior aspect)|
|Action||Laterally rotates the thigh at hip joint
Abducts thigh at hip joint (when hip is flexed past 60⁰)
Stabilises head of femur in acetabulum
|Nerve||Superior gluteal nerve
Ventral rami of S1 and S2
|Artery||Superior and inferior gluteal arteries
Internal pudendal artery
Location & Overview
The piriformis muscle is located in the gluteal region of the hip and proximal thigh. It is a somewhat pear shaped and flat. It runs parallel to the posterior margin of the gluteus medius and deep to the gluteus maximus. In Latin, the word ‘piriformis’ refers to its pear-shaped appearance. Pirum = pear and forma = shape in Latin. The piriformis muscle is commonly associated with a condition called piriformis syndrome with common symptoms involving the patient having hip pain, buttock pain, or pain down the back of the leg. This may present as shooting, burning, or aching feelings; or even numbness and tingling sensations  .
The piriformis is one of the six short external rotator muscles in the gluteal region. The other five are the obturator internus, quadratus femoris, obturator externus, superior gemellus and inferior gemellus muscles. The piriformis muscle’s tendon joins the tendons of the obturator internus, inferior gemellus, and the superior gemellus (forming the conjoint tendon) and then inserts on the femur. .
Origin & Insertion
The piriformis originates on the anterior surface of the sacrum at around the level of S2 to S4, the spinal region of the gluteal muscles, the capsule of the adjacent sacroiliac joint, the gluteal surface of the ilium near the margin of the greater sciatic notch, and sometimes the sacrotuberous ligament. It then inserts onto the greater trochanter of the femur on the medial side of the superior aspect .
The piriformis muscle is a lateral (or external) rotator of the hip. The quadratus femoris, inferior gemellus, obturator internus, and obturator externus can also assist in this movement. When the hip is extended, this is when the piriformis is in the most advantageous position to assist in lateral rotation of the femur. When the hip joint is flexed (past around 60⁰ of hip flexion) this is when the piriformis muscle is then in a more advantageous position for hip abduction.   .
There is some variation in the piriformis muscle’s body, the tendon and the tendon insertion. Variations in the muscle impact the actions of the muscle. Therefore, although the piriformis muscle has been suggested to be an internal rotator of the hip, the ability to perform this action is dependent on the individual’s own individual anatomical structure. Consequently, the piriformis muscle has been subject to controversy with regards to its muscle actions. The simple answer to if the piriformis is an internal rotator or not is that it actually just depends on the individual person’s anatomy and where their piriformis inserts. In most people, the piriformis has a variation that is going to prevent it assisting in internal rotation of the hip   .
Previously the piriformis muscle was said to be innervated by the ‘nerve to piriformis’. Though more recent cadaver research suggests that “a single ‘nerve to piriformis’ does not exist in the majority of specimens thus this term should be abandoned” . Instead, the most common innervations of the piriformis are the superior gluteal nerve and the ventral rami of S1 and S2.   
Blood is supplied to the piriformis via the superior gluteal artery, inferior gluteal artery, and internal pudendal artery which are branches of the internal iliac artery  .
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|↑1, ↑5, ↑9, ↑12, ↑16||Chang C, Jeno SH, Varacallo M. Anatomy, Bony Pelvis and Lower Limb, Piriformis Muscle. [Updated 2021 Oct 11]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2022 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK519497/|
|↑2||Hicks BL, Lam JC, Varacallo M. Piriformis Syndrome. [Updated 2022 Feb 12]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2022 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK448172/|
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|↑7||Lung K, Lui F. Anatomy, Abdomen and Pelvis, Superior Gluteal Nerve. 2021 Aug 11. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2022 Jan–. PMID: 30571029.|
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|↑11, ↑13||Iwanaga J, Eid S, Simonds E, Schumacher M, Loukas M, Tubbs RS. The majority of piriformis muscles are innervated by the superior gluteal nerve. Clin Anat. 2019 Mar;32(2):282-286. doi: 10.1002/ca.23311. Epub 2018 Dec 21. PMID: 30408241.|
|↑14||Fishman LM, Hosseini M. Piriformis syndrome – a diagnosis comes into its own. Muscle Nerve. 2019 Apr;59(4):395-396. doi: 10.1002/mus.26417. Epub 2019 Feb 4. PMID: 30623985.|
|↑15||Goidescu OC, Enyedi M, Tulin AD, et al. Overview of the anatomical basis of the piriformis syndrome-dissection with magnetic resonance correlation. Exp Ther Med. 2022;23(2):113. doi:10.3892/etm.2021.11036|