Piriformis: Origin, Insertion, Nerve, Action & Sciatica

The Piriformis is a small, pear-shaped muscle located deep in the buttock, behind the Gluteus Maximus. Despite its small size, it is critically important clinically because of its close relationship with the Sciatic Nerve, often being a primary cause of "pseudo-sciatica" or buttock pain.

[Image of Piriformis muscle anatomy]

Quick Anatomy Snapshot

Origin (Proximal)	Anterior surface of the Sacrum (between S2-S4 sacral foramina) and the Sacrotuberous Ligament.
Insertion (Distal)	Superior border of the Greater Trochanter of the Femur.
Nerve Supply	Nerve to Piriformis (Anterior rami of S1, S2).
Blood Supply	Superior and Inferior Gluteal Arteries.
Primary Actions	External Rotation: Rotates the thigh outward (when hip is extended). Abduction: Moves thigh away from midline (when hip is flexed). Stabilization: Secures the femoral head in the acetabulum.

Deep Dive: The Key to the Gluteal Region

The Piriformis is considered the most important anatomical landmark in the gluteal region because it divides the Greater Sciatic Foramen into two parts.

1. The Gatekeeper

Structures leaving the pelvis to enter the gluteal region pass either above or below the Piriformis muscle belly:
• Superior to Piriformis: Superior Gluteal Nerve and Artery.
• Inferior to Piriformis: Inferior Gluteal Nerve/Artery, Pudendal Nerve, and most importantly, the Sciatic Nerve.

2. Action Reversal

The Piriformis is a "functional changeling."
• In neutral or extension, it is an External Rotator.
• When the hip is flexed past 60-90 degrees, its line of pull shifts, and it becomes an Internal Rotator and Abductor. This is why we stretch it by flexing and adducting the hip.

Physio Corner: Clinical Relevance

💪 Functional Fact: During the stance phase of walking, the Piriformis contracts to prevent the opposite side of the pelvis from dropping (working with Glute Medius) and controls internal rotation of the femur.

Palpation

Locate the PSIS (Posterior Superior Iliac Spine) and the Greater Trochanter. The Piriformis lies along the line connecting these two points. Palpation requires deep pressure through the Gluteus Maximus. Tenderness here often reproduces "sciatic" symptoms down the leg.

⚠️ Clinical Pathology: Piriformis Syndrome
Spasm or hypertrophy of the Piriformis can compress the Sciatic Nerve beneath it. This causes pain, tingling, and numbness down the back of the leg, mimicking a herniated disc.
Anatomical Variation: In about 15-20% of the population, the Sciatic nerve (or part of it) pierces through the muscle belly, predisposing them to this syndrome.

Manual Muscle Testing (MMT)

We test external rotation strength to assess the Piriformis and other deep rotators.

Testing Tip: To bias the Piriformis, test in a seated position (hip flexed). This puts the muscle in a more horizontal line of pull for abduction/rotation.

Step-by-Step Procedure (Oxford Scale)

Grade	Patient Action & Resistance
Grade 3, 4, 5 (Against Gravity)	Position: Short sitting (legs hanging off table). Action: Patient moves the foot medially (External Rotation of the hip). Resistance: Applied just proximal to the medial malleolus, pushing the foot laterally (into internal rotation). Grade 3: Full active range. Grade 4/5: Holds against moderate/strong resistance.
Grade 2 (Gravity Eliminated)	Position: Supine. Leg extended and internally rotated. Action: Patient rolls the leg outward (toes point out). Result: Full range of external rotation.
Grade 0, 1 (Palpation)	Action: Deep palpation between Sacrum and Greater Trochanter. Cue: "Try to turn your thigh out." Grade 1: Deep contraction felt. Grade 0: No activity.

Frequently Asked Questions

How do you stretch the Piriformis?

The "Figure 4" stretch or "Pigeon Pose" places the hip in flexion, adduction, and external rotation (or internal rotation depending on the angle), lengthening the muscle.

Does it originate inside the pelvis?

Yes. It originates on the anterior (front) surface of the sacrum, inside the pelvic bowl. It then exits through the Greater Sciatic Foramen to reach the hip.

Is it the only muscle that compresses the Sciatic nerve?

No. While it is the most common soft-tissue cause (Piriformis Syndrome), the "Deep Gluteal Syndrome" umbrella includes compression by the Gemelli, Obturator Internus, or proximal hamstrings as well.

Test Your Knowledge: Piriformis Quiz

1. The Piriformis originates from which surface of the sacrum?

2. Which nerve typically passes inferior to the Piriformis?

3. Where does the Piriformis insert?

4. The Piriformis exits the pelvis through which opening?

5. Which action does the Piriformis perform when the hip is flexed to 90 degrees?

6. What shape is the Piriformis?

7. Which nerve supplies the Piriformis?

8. Which structure passes SUPERIOR to the Piriformis?

9. Piriformis Syndrome mimics which condition?

10. True or False: The Piriformis is superficial to the Gluteus Maximus.

References

• Moore, K. L., Dalley, A. F., & Agur, A. M. R. (2018). Clinically Oriented Anatomy. 8th ed. Philadelphia: Wolters Kluwer.
• Standring, S. (2016). Gray's Anatomy: The Anatomical Basis of Clinical Practice. 41st ed. Elsevier.
• Travell, J. G., & Simons, D. G. (1999). Myofascial Pain and Dysfunction: The Trigger Point Manual. 2nd ed. Williams & Wilkins.