External Obturator

External Obturator
Muscle Type
Origin	External surface of obturator membrane; rim of pubis and ischium bordering obturator foramen
Insertion	Trochanteric fossa on medial surface of greater trochanter
Action	Externally rotates thigh; assists in adduction of hip; stabilizes pelvis (by pulling femoral head into acetabulum)
Synergists
Antagonists	Gluteus Medius, Gluteus Minimus, Tensor Fascia Lata, Gluteus Medius
Spinal innervation
Peripheral Innervation	Posterior division of obturator nerve (L3–L4)
Vasculature	Obturator artery; medial circumflex femoral artery

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The obturator externus is a flat, triangular muscle situated in the anterior (medial) aspect of the hip, deep in the upper thigh. It covers the outer surface of the obturator foramen. Functionally, it is a lateral rotator of the hip and also helps in adduction and stabilization of the femoral head. Unique among the short hip rotators, it lies in the medial compartment and is innervated by the obturator nerve (like the adductors). Obturator externus forms part of the posterior wall of the femoral triangle and plays a role in controlling thigh rotation during gait.

Structure

Obturator externus is a deep triangular muscle in the upper medial thigh. It is part of the medial compartment of the thigh due to innervation and location, though functionally it's a lateral rotator. It has a broad origin and a narrow insertion forming a conical shape flattened against the bone.

Origin

It originates from the external (anterior) surface of the obturator membrane and the bony margins of the obturator foramen on the pubis and ischium. Specifically, from the superior and inferior pubic rami and the ramus of the ischium around the obturator foramen. This broad origin covers the front of the lower pelvis between the adductors and the pelvic floor muscles.

Insertion

The muscle fibers pass posterolaterally under the hip joint and insert into the trochanteric fossa (the depression on the medial side of the greater trochanter). The tendon lies deep to the neck of the femur (between the neck and the joint capsule). This insertion is in common with the obturator internus/gemelli; thus, obturator externus essentially attaches at the upper border of quadratus femoris insertion area. Its tendon is short and strong.

Action

Obturator externus laterally rotates the thigh at the hip joint. Because of its more anterior position, it is especially active in external rotation when the hip is flexed or in neutral (it’s less effective in full extension due to line of pull). It also can assist in hip adduction – since it arises from the pubis/ischium, it lies inferior to the joint axis in some positions and can contribute to pulling the thigh inward (particularly from a flexed or externally rotated position). Additionally, it acts as a stabilizer: by pulling the femoral head into the acetabulum, it helps stabilize the pelvis on the femur, especially during weight-bearing. It’s often active during stance phase of gait to control internal rotation (i.e., it eccentrically controls the hip’s tendency to internally rotate due to pronation of the foot).

Synergists

For lateral rotation, obturator externus works with the posterior gluteus medius and gluteus maximus (which are broader rotators) and with the deep rotators posteriorly (piriformis, obturator internus, gemelli, quadratus femoris). However, it is unique in being in the anterior plane; it particularly synergizes with quadratus femoris in external rotation and adduction (both insert near each other and reinforce the posterior capsule). It also joins the adductor magnus in providing external rotation when the hip is near extension (as the adductor magnus also has an external rotation component in extension). During gait, as the leg is in stance, obturator externus synergizes with the short rotators to resist internal rotation caused by the ground reaction force, thus working with piriformis and quadratus femoris in that stabilizing role.

Antagonists

The gluteus medius (anterior part) and gluteus minimus and TFL are antagonists by internally rotating the femur, opposite to obturator externus’s external rotation. Additionally, the gluteus medius (overall) is antagonistic in that it abducts the hip, whereas obturator externus can assist in adduction. However, obturator externus is not a primary adductor, so main antagonism is considered in the rotation aspect. In stance, if obturator externus is weak, the internal rotators might dominate causing a slight inward collapse (knee valgus) – showing the antagonistic functional relationship.

Spinal Innervation

L3, L4 (via obturator nerve).

Peripheral Innervation

The Posterior branch of the Obturator Nerve innervates obturator externus. This is distinct from all other short rotators (which are sacral plexus). The obturator nerve comes from lumbar plexus, runs through the obturator canal, splits into anterior and posterior divisions, the latter of which pierces obturator externus and then innervates the adductor magnus (adductor portion). This means a high obturator nerve injury (like in pelvic trauma or compression at obturator canal) will weaken obturator externus along with the adductors.

Vasculature

The obturator artery (branch of internal iliac) supplies the muscle from the anterior side. Also, the medial circumflex femoral artery (branch of deep femoral) sends branches to this region. There’s an anastomosis between obturator and medial circumflex around the hip. The muscle’s position in the medial compartment ensures it gets blood alongside the adductors. If the obturator artery is compromised (as can happen in pelvic surgeries if the artery is ligated or obstructed), the medial circumflex usually suffices to perfuse obturator externus via trochanteric anastomosis.

Clinical Relevance

Groin Pain & Obturator Externus: A strain or tendinopathy of obturator externus can cause deep groin pain that might mimic hip joint pathology. Because it is deep, it’s not commonly injured, but in sports requiring quick changes of direction (soccer, fencing), there can be strain at its insertion or origin. Pain would be elicited on resisted external rotation and possibly passive internal rotation stretch of the hip. It’s often managed with rest and therapy focusing on rotator cuff strengthening of the hip.

Obturator Nerve Palsy: In an obturator nerve lesion (e.g., due to pelvic lymph node dissection or an obturator hernia compressing the nerve), patients lose adductor strength and also obturator externus function. Clinically, they present with inability to cross legs and some external rotation weakness. However, overall external rotation is usually preserved by the sacral plexus rotators. Patients might have an externally rotated limb at rest (as adductors also contribute to internal rotation in flexion), but the main deficits are adduction and perhaps subtle gait instability.

Gait Mechanics: Obturator externus is active in late stance of walking and in pivoting maneuvers. A weak obturator externus (like in obturator nerve neuropathy) might contribute to excessive internal rotation of the limb during stance (leading to knee valgus and foot pronation). Conversely, a tight or overactive obturator externus might limit internal rotation and contribute to toe-out gait or impingement of the hip. Stretching the muscle can improve internal rotation if it’s short (this is done by flexing, abducting, and internally rotating the hip – basically the reverse of its actions).

Posterior Hip Stability: Surgeons sometimes note that obturator externus forms part of the “fold” of the joint capsule externally. In posterior approaches to the hip, after dislocating the hip, the obturator externus tendon can be seen crossing under the neck of the femur. It’s generally preserved (no need to cut it in posterior approach). If the femoral head is replaced (as in hip arthroplasty), a preserved obturator externus helps maintain anteromedial stability. If it’s accidentally damaged, one theoretical risk is increased tendency for anterior dislocation or loss of fine stability, but usually other structures compensate.

Impingement and Arthritis: In some cases of hip arthritis or impingement (FAI), the obturator externus tendon can ossify or form a “popeye” deformity due to repetitive stress. This can be seen on imaging as calcifications near the trochanteric fossa. It’s usually not addressed directly unless causing mechanical irritation.

Obturator Externus Bursa: A bursa lies between obturator externus and the femoral neck. Rarely, this can distend (as with certain hip joint effusions or synovial cysts) and cause a mass effect. It might present as a palpable fullness in the groin or on imaging as a cystic lesion. This is usually secondary to hip joint pathology (like synovitis). Treatment would target the primary cause; rarely would one need to aspirate this bursa specifically.

Obturator Hernia Sign: Howship–Romberg sign (inner thigh pain on internal rotation, extension, or abduction of the hip) is classically due to an obturator hernia compressing the obturator nerve. The pain distribution is along the obturator nerve (medial thigh). While that’s a nerve issue, one could confuse it with obturator externus strain since those motions also stretch obturator externus. However, in obturator nerve issues, there is numbness in medial thigh or adductor weakness. In obturator externus strain, it’s more localized deep pain with normal sensation and nerve function.

Rehabilitation: Strengthening obturator externus specifically is done through resisted external rotation movements. It’s also engaged in multi-directional hip strengthening often given to athletes (like “ballerina” exercise with theraband or multi-hip machine in external rotation mode). People with patellofemoral syndrome sometimes benefit from strengthening hip external rotators (including obturator externus) to reduce knee valgus, thereby proving its role in kinetic chain. Strengthening is often via clam shells (though those target gluteus medius more) or prone foot rotations outwards against resistance. Closed chain exercises like single-leg Romanian deadlifts also train the external rotators eccentrically (controlling internal rotation).

Palpation/Needling: Unlike the internal rotators, obturator externus is hard to palpate due to its deep location under pectineus and quadratus femoris. It cannot be directly palpated externally. It’s sometimes accessed via deep anterior hip needling under image guidance for trigger point release, but that’s uncommon. It’s more approached indirectly by addressing overall adductor and rotator flexibility.