Topography and course of the obturator internus muscle
The anatomy of the obturator internus muscle (m. obturatorius internus) features an interesting course of muscle fibers that change direction as they exit the pelvic cavity.
- Origin: The fibers originate on the inner surface of the obturator membrane, as well as on the bone structures directly surrounding the obturator foramen (foramen obturatum).
- Course of fibers: The muscle extends outward from the pelvic cavity. It passes through the lesser sciatic foramen (foramen ischiadicum minus), smoothly curving around the lesser sciatic notch.
- Insertion site: Having completed its path, the muscle tendon reaches the trochanteric fossa (fossa trochanterica) on the femur, where it firmly attaches.
Function, innervation, and blood supply
From a biomechanical perspective, the main task of the obturator internus muscle is to perform external rotation of the lower limb. By contracting, it laterally rotates the thigh.
Neural regulation and blood supply of the muscle are provided by the following structures:
- Innervation: Provided by the nerve of the same name — n. obturatorius internus. This nerve arises from spinal nerve roots at the lumbar and sacral levels (L₅–S₂).
- Blood supply: Arterial blood flow comes from two sources: the obturator artery (a. obturatoria) and the inferior gluteal artery (a. glutea inferior).
Articular muscle of the knee (m. articularis genus)
As part of studying the muscles of the lower limb, it is also worth mentioning the articular muscle of the knee (m. articularis genus), which performs a specific protective function for the knee joint.
- Origin: The muscle starts from the anterior surface of the lower third of the femur.
- Insertion: Its fibers blend directly into the knee joint capsule.
- Function: When the leg is extended, the muscle contracts and pulls on the joint capsule. This action (retracting the capsule) prevents it from getting pinched between the articular surfaces of the bones.
- Innervation: Provided by the femoral nerve (n. femoralis).
- Blood supply: Nutrition is provided by the genicular arteries — a. genus, which are branches of the arterial network of the knee joint.
Briefly: answers to topical questions
What structures pass through the lesser sciatic foramen?
The obturator internus muscle and the pudendal neurovascular bundle pass through the lesser sciatic foramen.
- Obturator internus muscle (m. obturatorius internus) — exits the pelvic cavity to the posterior surface;
- Pudendal neurovascular bundle — curves around the ischial spine and returns to the pelvic cavity (into the ischioanal fossa). It includes:
- Internal pudendal artery (a. pudenda interna);
- Internal pudendal veins (vv. pudendae internae);
- Pudendal nerve (n. pudendus).
Which fascia covers the obturator internus muscle from the pelvic cavity side?
The obturator internus muscle from the lesser pelvis cavity side is covered by the parietal pelvic fascia (fascia pelvis parietalis).
This fascia covers the parietal muscles and pelvic floor muscles, including m. obturatorius internus. In the area of the obturator internus muscle, it is most prominent.
To remember the course of m. obturatorius internus, imagine that it «locks» the pelvis from the inside, and then exits outward through the «small» door (foramen ischiadicum minus) to rotate the thigh.
Class and exam questions
On which surface of the femur does the articular muscle of the knee begin?
The articular muscle of the knee originates on the anterior surface of the lower third of the femur.
Through which foramen does the obturator internus muscle leave the pelvic cavity?
It exits the pelvis through the lesser sciatic foramen (foramen ischiadicum minus).
Where does m. obturatorius internus attach?
Its tendon attaches to the trochanteric fossa (fossa trochanterica) of the femur.
What else to study on this topic
- Structure of the obturator membrane and boundaries of the obturator foramen
- Boundaries and contents of the lesser sciatic foramen
- Anatomy of the trochanteric fossa (fossa trochanterica)
- Formation and branching of the obturator nerve (L₅–S₂)
- Arterial network of the knee joint and branches of a. genus
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