Internal Obturator
| |
| Internal Obturator | |
|---|---|
| Muscle Type | |
| Origin | Internal surface of obturator membrane and edges of obturator foramen (ischium and pubis) |
| Insertion | Medial aspect of greater trochanter (trochanteric fossa) |
| Action | Externally rotates extended thigh; abducts flexed thigh; stabilizes femoral head |
| Synergists | |
| Antagonists | |
| Spinal innervation | |
| Peripheral Innervation | Nerve to obturator internus (L5–S2) |
| Vasculature | Inferior gluteal artery; internal pudendal artery |
The obturator internus is a fan-shaped muscle that originates deep within the pelvis and exits to the gluteal region by making a sharp turn through the lesser sciatic foramen. It, along with the gemelli, forms a functional unit in the lateral rotator group. Obturator internus laterally rotates the extended thigh, abducts the flexed thigh, and helps hold the femoral head in the acetabulum. Notably, its tendon is visible in the gluteal region and serves as a central tendon for the gemelli (earning the trio the nickname “triceps coxae”). Because it arises intrapelvically, it also has roles in pelvic structure and is sometimes considered in pelvic floor context.
Structure
Obturator internus is a thick, triangular muscle that is largely within the pelvis. It has a unique path: it leaves the pelvis via the lesser sciatic notch and becomes tendinous, using the lesser sciatic notch as a pulley to redirect its force. It is part of the deep lateral rotator group and is covered posteriorly by the gemelli and gluteus maximus.
Origin
It originates from the pelvic (internal) surface of the obturator membrane and the surrounding bony margins of the obturator foramen on the pubis and ischium. Essentially, it fills the internal aspect of the anterolateral pelvic wall. Specifically, it arises from the ilium’s inner surface near the rim of the acetabulum, the pubic bone inferior to the obturator canal, the ischium (ramus) near the obturator foramen, and the obturator membrane that spans the foramen.
Insertion
After exiting the pelvis and making a 90° turn, the obturator internus’s tendon inserts on the trochanteric fossa on the medial surface of the greater trochanter. This is the same insertion as the gemelli, as they fuse with its tendon. The muscle becomes tendinous around the lesser sciatic foramen, and that tendon angles across the hip to the femur. The lesser sciatic notch is lined with cartilage to facilitate this tendon glide, and a bursa often reduces friction.
Action
Obturator internus laterally rotates the thigh, particularly when the hip is extended (pulling the back of the femur medially). When the hip is flexed (e.g., sitting), its angle of pull allows it to abduct the thigh. Additionally, it is a strong stabilizer of the hip; by spanning the hip posteriorly, it resists excessive internal rotation and helps keep the femoral head aligned (much like rotator cuff of shoulder).
Because of its pelvic origin, when the femur is fixed, contraction of obturator internus can also produce a slight lateral rotation of the pelvis and possibly assist in pelvic floor support (it’s adjacent to pelvic floor muscles and fascially connected). However, its primary functional contributions are at the hip joint.
Synergists
Obturator internus works intimately with the superior and inferior gemellus, which insert onto its tendon. Together, they function as a unit in lateral rotation and abduction (triceps coxae). It also synergizes with piriformis (above) and quadratus femoris (below) for lateral rotation. During hip stabilization, obturator internus co-contracts with other short rotators and gluteus maximus to steady the joint. In thigh abduction (flexed hip), it synergizes with piriformis and the gluteus medius/minimus to move the thigh outward. Considering pelvic influence, it may work with pelvic floor muscles during certain maneuvers (though indirectly).
Antagonists
The gluteus medius (anterior fibers), gluteus minimus, and tensor fasciae latae antagonize its lateral rotation by internally rotating the hip. Also, the adductors antagonize its abducting role when the hip is flexed. For example, with the hip flexed 90°, obturator internus tries to abduct, but adductor longus/brevis would oppose that. Functionally, internal rotators are the main antagonists in normal stance. If obturator internus is tight, it could limit internal rotation; if weak, internal rotators might dominate leading to a toe-in posture (though usually multiple muscles contribute, not just one).
Spinal Innervation
L5, S1, S2 (via nerve to obturator internus).
Peripheral Innervation
Nerve to Obturator Internus. This nerve also innervates the superior gemellus. It exits the pelvis via the greater sciatic foramen, sends a branch to superior gemellus, then enters the perineum via the lesser sciatic foramen to reach obturator internus on its inside (medial) surface. Because the bulk of obturator internus is inside the pelvis, the nerve supplies it before it exits (on the muscle’s pelvic side). Damage to this nerve (e.g., entrapped in the ischioanal fossa or injured in deep pelvic surgery) would affect obturator internus and gemellus superior.
Vasculature
Supplied by the inferior gluteal artery once it exits (branches to its tendon) and by the internal pudendal artery and obturator artery on its internal aspect (as it originates on the obturator membrane). The inferior gluteal artery provides blood as it passes below piriformis, and a small branch of the internal pudendal may supply the muscle near the ischial spine. There is also likely contribution from the obturator artery inside the pelvis. The dual supply (internal and external) ensures the muscle gets blood both in the pelvic portion and near its insertion.
Clinical Relevance
Obturator Internus Abscess/Bursitis: Because the obturator internus is adjacent to pelvic organs, an inflammatory process (like appendicitis or diverticulitis) can lead to an obturator internus abscess, which can cause pain on internal rotation of the hip (positive obturator sign in appendicitis). The obturator sign is tested by flexing the hip and knee and then internally rotating the hip – stretching obturator internus, causing pain if an inflamed appendix is in contact. This underscores the muscle’s anatomical proximity to pelvic viscera. Obturator internus bursitis (between its tendon and ischium) can cause deep gluteal pain, though it’s relatively rare.
Deep Gluteal Syndrome (DGS): A tight obturator internus (with gemelli) can irritate the sciatic nerve (as part of DGS). If piriformis syndrome treatment isn’t helping, sometimes attention is turned to the obturator internus. Stretching targeted at obturator internus (e.g., hip flexion, adduction, and internal rotation stretch) might relieve such pain. In refractory cases, Botox injections into obturator internus have been tried to reduce muscle spasm and thereby nerve compression. However, isolating it is challenging (Botox often given to piriformis in such syndromes but could diffuse to internus).
Pelvic Floor: The obturator internus forms part of the lateral pelvic wall and provides attachment to pelvic floor fascia (the arcus tendineus). Thus, dysfunction in obturator internus (like trigger points) can refer pain to the pelvic floor or mimic levator ani issues. Pelvic floor physical therapists sometimes address obturator internus by internal (vaginal/rectal) trigger point release to alleviate pelvic pain or urinary symptoms. Because of its innervation (nerve to obturator internus), which has roots similar to pudendal nerve, there’s some interplay in syndromes like pudendal neuralgia vs obturator internus myofascial pain.
Ischiofemoral Impingement Connection: While quadratus femoris is the main muscle affected in IFI, a severely narrowed ischiofemoral space can also compress obturator internus (as it exits the lesser sciatic foramen). That could potentially cause buttock pain and imaging might show obturator internus tendon thickening or edema. But it’s less common to specifically cite obturator internus in IFI; usually by the time it’s that narrow, quadratus femoris is already significantly impacted.
Entrapment of Obturator Nerve vs. Obturator Internus: Not to confuse – obturator internus is innervated by nerve to obturator internus (sacral plexus) and is distinct from the obturator nerve (lumbar plexus) which innervates medial thigh muscles (and obturator externus). However, pathology in the obturator canal (obturator nerve entrapment) wouldn’t affect obturator internus, but pelvic trauma could injure both nerve to obturator internus and obturator nerve due to proximity along pelvic wall. That would present as weakness in both lateral rotation (obturator internus) and thigh adduction (adductors), which is a rare combination.
Surgery & Landmarks: In a transischiorectal fossa approach (used to reach certain pelvic tumors or abscesses), the obturator internus is a key landmark – the pudendal canal runs on its fascia (Alcock’s canal). Surgeons identify obturator internus to locate the internal pudendal vessels and nerve. In treating pudendal nerve entrapment, part of the decompression involves releasing the fascia of obturator internus (which forms Alcock’s canal). So obturator internus is directly involved in that neuropathy – a tight fascia or spasm in obturator internus can contribute to pudendal nerve compression. Therefore, obturator internus relaxation (via Botox) is one treatment approach for pudendal neuralgia in some cases.
Rehabilitation: After hip surgeries (like total hip arthroplasty), sometimes the short external rotators (including obturator internus) are taken down and repaired. Strengthening the lateral rotators is part of rehab to regain stability – though no exercise isolates obturator internus, exercises like prone hip extension with external rotation (the “clam shell” in slight extension) and resisted external rotation (with bands) will target these deep rotators. Also, balance and proprioceptive exercises train them to co-contract for stability. If obturator internus is weak, one might have subtle difficulty controlling internal rotation of the femur (leading to knee valgus or foot pronation).
Piriformis vs. Obturator Internus Stretch: To stretch deep rotators: hip flexion, adduction, and internal rotation – often done supine by bringing the flexed knee toward opposite shoulder (piriformis stretch). This actually stretches piriformis and obturator internus/gemelli together (since those muscles all externally rotate and some abduct). Distinguishing between them clinically is not typically necessary as they usually become tight or weak as a group.
