The piriformis muscle originates from the anterior (front) part of the sacrum, the part of the spine in the gluteal region, and from the superior margin of the greater sciatic notch (as well as the sacroiliac joint capsule and the sacrotuberous ligament). It exits the pelvis through the greater sciatic foramen to insert on the greater trochanter of the femur. Its tendon often joins with the tendons of the superior gemellus, inferior gemellus, and obturator internus muscles prior to insertion.
The piriformis is a flat muscle, pyramidal in shape, lying almost parallel with the posterior margin of the gluteus medius.
It is situated partly within the pelvis against its posterior wall, and partly at the back of the hip-joint. It arises from the front of the sacrum by three fleshy digitations, attached to the portions of bone between the first, second, third, and fourth anterior sacral foramina, and to the grooves leading from the foramina: a few fibers also arise from the margin of the greater sciatic foramen, and from the anterior surface of the sacrotuberous ligament. The muscle passes out of the pelvis through the greater sciatic foramen, the upper part of which it fills, and is inserted by a rounded tendon into the upper border of the greater trochanter behind, but often partly blended with, the common tendon of the obturator internus and superior and inferior gemellus muscles.
In 17% of people, the piriformis muscle is pierced by parts or all of the sciatic nerve. Several variations occur, but the most common type of anomaly (81% of anomalies) is the Beaton's type B which is when the common peroneal nerve pierces the piriformis muscle.
It may be united with the gluteus medius, send fibers to the gluteus minimus, or receive fibers from the superior gemellus. It may have one or two sacral attachments; or it may be inserted into the capsule of the hip joint.
Palpation of the piriformis muscle is done through the gluteus maximus muscle using the fingers or thumb. When using the fingers, the top hand pushes perpendicularly to the piriformis while the bottom hand through and under the gluteus maximus.
Tension may be noted in the outer upper edge of the muscle.
Because the piriformis muscle may be tender under normal conditions, it is important to remember that repeated palpation of this area can lead to increased tenderness.
The following grading system may also be used.
Grade 0 - No tenderness
Grade I - Tenderness to palpation without grimace or flinch
Grade II - Tenderness with grimace or flinch
Grade III - Tenderness with withdrawl ( + Jump sign)
Grade iV - Withdrawl to non-noxious stimuli (superficial palpation, skin prick, gentle percussion)