We'll start by looking at the piriformis muscle itself as it is, unsurprisingly, the key factor in piriformis syndrome.



Taking it's name from it's pear-like shape, the pirifromis muscle is a fairly small muscle deep to the gluteus muscles across the upper buttock. It originates from the anterior surface of the sacrum and the superior margin of the greater sciatic notch. It also has attachment to the capsule of the sacro-iliac joint and also the sacrotuberous ligament.



Exiting the pelvis through the greater sciatic notch, it's fibres run almost horizontally to insert into the superior aspect of the greater trochanter of the femur, just above the insertion of obturator internus. As many muscles insert into the greater trochanter in this area, the tendon of piriformis is said to often blend with the tendons of obturator internus itself and the superior and inferior gemellus muscles.



Superficial to the piriformis is first the gluteus medius muscle followed ay the larger gluteus maximus, which covers the entire muscle as it sweeps across the posterior leg to insert into the ilio-tibial band, between vastus lateralis and adductor magnus.



The sciatic nerve, which we shall cover in more depth later, passes deep to piriformis in most cases (approximately 85% of people) but can infact pierce the piriformis itself, predisposing to piriformis syndrome and subsequent sciatica. Even if the sciatic nerve runs deep to piriformis, spasm in this muscle put direct pressure on the nerve, causing the resultant pain and discomfort.