Piriformis – Functional Anatomy

Overview

Piriformis is the largest, muscle in the lateral hip rotator group. Also, it is the only lateral rotator that attaches to the sacrum.

Origin

• medial margin of the greater sciatic notch
• anterior sacrum from the Level of S2-S4
• superior portion of the sacrotuberous ligament

Insertion

• superior greater trochanter

Function

• lateral rotation of the femur
• abduction of the femur when flexed

Nerve

• Sacral plexus (L5-S2)

Functional Considerations

Piriformis assists in tilting the pelvis. Also, the muscle stabilizes the pelvis when weight shifts to swing the opposite limb.

Piriformis has a different origin than other lateral rotators. Like the deep fibers of the gluteus maximus, this traps the os coxae between the femur and sacrum.

The anterior attachment skews the os coxae when this muscle is tight. On the other hand, the sacrospinous ligament works to stabilize this rotation of the os coxae on the sacrum.

Anomalies, Etc.

Piriformis varies a great deal in size. It can be a small, thin muscle or a very thick, powerful muscle. It may fuse with gluteus medius or gluteus minimus.

In many cadavers, it appears to be more triangular. Compared to the posterior view, this view shows how the sacrum’s tilt makes it appear wider at the insertion.

The femoral attachment is usually above the triceps coxae. However, it may blend with the tendons of the gemellus superior or obturator internus.

There are variations in the arrangement of the sciatic nerve as it passes the piriformis. Studies vary but basically, the sciatic nerve passes below the piriformis in about 80% of cases. In 15-20% of cases, the sciatic nerve pierces the piriformis. One study states that the sections of the nerve may pass through or above the muscle. However, none of them report the nerve as passing entirely above piriformis.

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