Obturator Internus: Origin, Insertion, Nerve & Hip Stability

The Obturator Internus is a large, fan-shaped muscle originating inside the pelvis. It makes a unique 90-degree turn as it exits the pelvis to insert onto the femur. It is the central component of the "Triceps Coxae" and is a key stabilizer of the hip joint.

[Image of Gluteus Maximus muscle anatomy]

Quick Anatomy Snapshot

Origin (Proximal)	Internal surface of the Obturator Membrane and surrounding bony margins (Ischiopubic ramus).
Insertion (Distal)	Medial surface of the Greater Trochanter of the Femur (trochanteric fossa).
Nerve Supply	Nerve to Obturator Internus (L5, S1).
Blood Supply	Internal Pudendal and Obturator Arteries.
Primary Actions	External Rotation: Rotates the extended thigh laterally. Abduction: Abducts the flexed thigh. Stabilization: Holds the femoral head in the acetabulum.

Deep Dive: The Right-Angle Turn

The Obturator Internus is structurally unique because of its pathway out of the pelvis.

1. The Pulley System

The muscle belly lies inside the pelvis, covering the obturator foramen. Its tendon exits the pelvis through the Lesser Sciatic Foramen. As it passes over the ischium (specifically the lesser sciatic notch), it makes a sharp 90-degree turn to head toward the femur. The bone here is covered by cartilage and a bursa to reduce friction.

2. The "Triceps Coxae" Core

Once the tendon exits the pelvis, it is joined by two smaller muscles:
• Gemellus Superior (from above)
• Gemellus Inferior (from below)
These three muscles insert together via a common tendon. The Obturator Internus is the powerful core of this "three-headed" hip rotator.

Physio Corner: Clinical Relevance

💪 Functional Fact: The Obturator Internus acts as a "hammock" for the pelvic floor. While not a primary pelvic floor muscle, it shares fascial connections with the Levator Ani. Dysfunction here can sometimes mimic pelvic floor pain or pudendal neuralgia.

Palpation

External Palpation: Locate the Ischial Tuberosity and the Greater Trochanter. The tendon lies deep in the midpoint of the line connecting them.
Internal Palpation: Pelvic floor therapists can palpate the muscle belly directly via an internal vaginal or rectal exam, located on the lateral walls of the pelvis.

⚠️ Clinical Pathology: Obturator Internus Syndrome
This is a form of deep gluteal syndrome. The Sciatic Nerve and Pudendal Nerve run very close to this muscle. Hypertonicity (spasm) or thickening of the Obturator Internus can entrap the pudendal nerve (causing pelvic/genital pain) or irritate the sciatic nerve (causing sciatica-like symptoms).

Manual Muscle Testing (MMT)

We test the short external rotators as a group.

Testing Tip: The Obturator Internus is a more effective abductor when the hip is flexed. Testing in a seated position biases this muscle group.

Step-by-Step Procedure (Oxford Scale)

Grade	Patient Action & Resistance
Grade 3, 4, 5 (Against Gravity)	Position: Short sitting (legs dangling). Action: Patient moves the foot medially (rotating the hip outward). Resistance: Applied just proximal to the medial malleolus, pushing the foot laterally (into internal rotation). Grade 3: Full range against gravity. Grade 4/5: Holds against moderate/max 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: Palpate deep, posterior to the Greater Trochanter. Cue: "Try to turn your thigh out." Grade 1: Deep contraction felt. Grade 0: No activity.

Frequently Asked Questions

Does it pass through the Greater Sciatic Foramen?

No. The Piriformis passes through the Greater Sciatic Foramen. The Obturator Internus passes through the Lesser Sciatic Foramen.

Why is it called "Internus"?

Because it originates on the Internal (pelvic) surface of the obturator membrane. The Obturator Externus originates on the External (outer) surface.

Can it cause hip snapping?

Yes. "Posterior Snapping Hip Syndrome" can involve the tendon of the Obturator Internus snapping over the posterior acetabulum or ischial spine, though snapping of the Iliopsoas or IT band is more common.

Test Your Knowledge: Obturator Internus Quiz

1. Which foramen does the Obturator Internus tendon pass through?

2. Which nerve supplies the Obturator Internus?

3. Where does the Obturator Internus originate?

4. What is the primary action of the Obturator Internus?

5. Which muscles fuse with the Obturator Internus tendon?

6. Where does the Obturator Internus insert?

7. Which fascia is continuous with the Obturator Internus fascia?

8. The Obturator Internus creates a 90-degree turn around which bony landmark?

9. Obturator Internus dysfunction can mimic which condition?

10. True or False: The Obturator Internus muscle belly is located in the thigh.

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.