Anatomy of the Piriformis Muscle

Courtesy: Prof Nabil Ebraheim ,University of Toledo, Ohio, USA

Overview

The piriformis muscle is one of the six short lateral rotators of the hip. It occupies a key anatomical position in the deep gluteal region and serves as an important landmark for neurovascular structures exiting the pelvis through the greater sciatic foramen.

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Origin

The piriformis muscle arises from:

• The anterior (pelvic) surface of the sacrum, usually from the second to fourth sacral segments

• The capsule of the sacroiliac joint

• The margin of the greater sciatic notch

Because of its origin from the sacrum and sacroiliac joint capsule, there is a recognized association between piriformis syndrome and sacroiliac joint–related pain.

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Insertion

• Inserts onto the superior border of the greater trochanter of the femur

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Function

• Lateral (external) rotation of the hip

• Abduction of the hip when the hip is flexed

• Acts as a dynamic stabilizer of the hip joint

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Innervation

• Nerve to piriformis

• Root values: L5, S1, S2

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Relations

Superior Relations

• Superior gluteal nerve

• Superior gluteal artery

Inferior Relations

• Inferior gluteal nerve

• Inferior gluteal artery

The piriformis muscle effectively divides the greater sciatic foramen into:

• Suprapiriform foramen

• Infrapiriform foramen

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Relationship of the Sciatic Nerve to the Piriformis Muscle

The sciatic nerve is composed of:

• Tibial division

• Common peroneal (fibular) division

These divisions are usually bound together but may separate as they pass the piriformis muscle. Several anatomical variations are described.

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Anatomical Variations of the Sciatic Nerve–Piriformis Relationship

Type 1 (Most Common)

• The entire sciatic nerve passes inferior to the undivided piriformis muscle

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Type 2

• The piriformis muscle is divided

• The common peroneal division passes between the two heads of the piriformis

• The tibial division passes below the muscle

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Type 3

• The common peroneal division passes above the undivided piriformis

• The tibial division passes below the muscle

• The two divisions appear to “straddle” the piriformis

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Type 4 (Rare)

• The entire sciatic nerve passes through a divided piriformis muscle

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Clinical Significance

• Variations in the relationship between the sciatic nerve and piriformis muscle are important in:

  • Piriformis syndrome

  • Sciatica without spinal pathology

  • Hip surgery and posterior approaches

  • Image-guided injections and nerve blocks

• Hypertrophy, spasm, or inflammation of the piriformis may compress the sciatic nerve, particularly in variant anatomies

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Key Take-Home Points

• Piriformis is a key deep gluteal landmark muscle

• It externally rotates the hip and abducts the hip in flexion

• Innervated by L5–S2

• Closely related to major neurovascular structures

• Anatomical variations of the sciatic nerve are clinically important and common