Piriformis

The piriformis is a flat, pyramid-shaped muscle of the gluteal region that lies deep to the gluteus maximus. It belongs to a group of six short external rotators of the hip: gemellus superior, obturator internus, gemellus inferior, obturator externus and quadratus femoris. It is the most superficial of these six muscles. This muscle is important for the stabilisation of the hip joint and moving the hip in multiple directions as it attaches from the sacrum on one end and to the greater trochanter of the hip at the other.

The piriformis originates from the anterior surface of the sacrum and the gluteal surface of the ilium, and inserts into the greater trochanter of the femur. Its main action is to externally rotate the hip joint and abduct the thigh when the hip is flexed. The piriformis also stabilizes the head of the femur in the acetabulum of the hip joint.

This muscle also divides the greater sciatic foramen into two foramina known as the suprapiriform foramen and infrapiriform foramen. It should be noted that various neurovascular structures exit the pelvis into the groin and lower limb through these foramina.

As stated above, a number of neurovascular structures passes through the piriformis as it exits the pelvis, and of note is the sciatic nerve. The sciatic nerve is formed by the L4-S3 ventral roots and usually exits the pelvis via a single trunk, through the greater sciatic foramen below the piriformis (infrapiriform foramen). The sciatic nerve makes it course through the back of the thigh and divides into the tibial and the common peroneal branches at the popliteal fossa. The tibial and common peroneal nerves are surrounded by a common epineural sheath from the main sciatic nerve trunk. Therefore, the sciatic nerve innervates the muscles of the back of the thigh as well as all the lower leg and foot muscles.

Nevertheless, because the development of the tibial and peroneal divisions is separate from one another, this could explain why the sciatic nerve variants develop during embryonic development. The possible relationships between the sciatic nerve and piriformis were first categorised by Beaton and Anson into the following six morphological types as shown in the diagram above:

• Type A: typical pattern, with the sciatic nerve passing below the piriformis, undivided
• Type B: the common peroneal nerve exits through the piriformis muscle belly and tibial nerve exits below the piriformis
• Type C: the common peroneal nerve exits above the piriformis while the tibial nerve exits below the piriformis
• Type D: the sciatic nerve exits through the piriformis muscle belly, as a single trunk
• Type E: the common peroneal nerve exits above the piriformis and tibial nerve through the piriformis
• Type F: the sciatic nerve passes undivided above the piriformis

The sciatic nerve can get irritated if the external rotators of the hip are tight, leading to pain radiating into the lower limb. Hence, it is important to be clinically aware of the sciatic nerve variants since they may account for the cause of piriformis syndrome. For more details regarding this syndrome, click here.

Piriformis trigger points can transmit pain to the back the thigh so the pain is not mainly concentrated in the buttocks and hip area. The medial piriformis trigger point (closer to the midline) lies along the piriformis line about an inch outside the edge of the sacrum. The lateral piriformis trigger point (away from the midline) is a few inches to the inside of the greater trochanter of the hip, along the piriformis muscle belly. These trigger points in the buttocks are capable of referring pain to the buttock region, sacroiliac joint and back of the thigh.

The psoas and piriformis are important muscles that connect the spine to the lower limbs. With the psoas from the front and piriformis in back, these muscles lend support to the spine by keeping it afloat above the pelvis. The psoas pulls down from the front and the piriformis pulls in the same direction from behind. Problems arise when this relationship is disrupted. When the psoas is tight, it pulls the lumbar spine into a deeper lordotic curve. This lifts the sacrum and ilium into an anterior tilt leading to strain on the piriformis.

If you suspect that your buttock and/or back of thigh pain may be related to your piriformis, try this variation of a stretching exercise at home. However, if you are suffering from persisting buttock and/or back of thigh pain, please do consult a healthcare professional for a more detailed assessment.

Reference

Beaton, LE., Anson, BJ., 2005. The relation of the sciatic nerve and of its subdivisions to the piriformis muscle. The Anatomical Record [online], 70, 1-5.
Keibel, F., Mall, FP, 1910. Manual of Human Embryology. Philadelphia: J.B. Lippincott Company.
Poutoglidou, F., Piagkou, M., Totlis, T., Tzika, M., Natsis, K., 2020. Sciatic Nerve Variants and the Piriformis Muscle: A Systematic Review and Meta-Analysis. Cureus [online], 12 (11), e11531.