Superior Gemellus Anatomy

Superior gemellus is the upper of the two small, flat, triangular muscles, the gemelli (Latin for twins), that flank the tendon of obturator internus as it emerges from the lesser sciatic foramen. Together the three form the triceps coxae, and their fibres fuse into a single conjoint tendon inserting on the medial surface of the greater trochanter.

On its own, superior gemellus is a small and functionally trivial muscle. Its importance is entirely relational, and there are four relationships an examiner will test:

1. It is part of the conjoint tendon, the structure divided and repaired in the posterior approach to the hip.
2. Its tendon lies posterior to the deep branch of the medial circumflex femoral artery, which is what makes it safe to divide provided obturator externus is preserved.
3. Its nerve, the nerve to obturator internus, crosses the ischial spine immediately lateral to the pudendal nerve, the anatomical fact that underpins pudendal nerve blocks and explains combined injuries at the spine.
4. It arises from the ischial spine, the key palpable landmark in pelvic and deep gluteal surgery and the site of the sacrospinous ligament attachment.

Why the gemelli exist. Obturator internus arises inside the pelvis, on the inner surface of the obturator membrane and the surrounding pelvic bone. Its tendon must leave through the lesser sciatic foramen, turning through roughly 90 degrees around the smooth, cartilage-covered groove on the posterior surface of the ischium at the lesser sciatic notch.

The pulley and its price. The notch acts as a trochlea, with a bursa, the bursa of obturator internus, to reduce friction and grooves in the bone corresponding to the tendon's slips. Turning a tendon through a right angle wastes force and reduces its effective excursion. The gemelli arise outside the pelvis on either side of the tendon and add muscle fibres to it distal to the pulley, restoring the power lost at the turn.

Accessory muscles. That is why the classical description calls the gemelli "accessory to obturator internus". Restoring force at the pulley, not any independent action, is what they are for.

Actions. Superior gemellus externally rotates the extended hip as one component of the triceps coxae, and abducts the flexed hip because its line of pull changes as the hip flexes. With the other short rotators it forms the posterior dynamic sleeve, which compresses the femoral head into the acetabulum and resists posterior translation, the hip's functional analogue of the rotator cuff.

The flexion reversal. In extension the short external rotators are efficient external rotators. Beyond about 60-70 degrees of hip flexion the line of pull of piriformis and the obturator group swings so that they become internal rotators and abductors. The reversal explains:

• why testing rotation in flexion gives different information from testing in extension
• why the FAIR position (flexion, adduction, internal rotation) tensions piriformis and the obturator group against the sciatic nerve
• why deep gluteal syndrome is provoked in flexion

Its share of the torque. Superior gemellus contributes a very small fraction of total external rotation torque. Gluteus maximus is by far the largest external rotator, with the posterior fibres of gluteus medius and the obturator group contributing the remainder.

What its loss costs. Isolated loss is silent: no measurable deficit in strength or function follows division of superior gemellus alone, which is why it is divided routinely without hesitation. Loss of the whole posterior sleeve, the situation after an unrepaired posterior approach, removes a passive tether and a dynamic compressive contribution, and is one mechanism behind the increased dislocation rate.

Palpation. Superior gemellus is not palpable: it lies deep to gluteus maximus and the sciatic nerve, on the ischial spine. What you palpate are the bony landmarks around it.

• The greater trochanter laterally, whose posterior and medial surfaces carry the rotator insertions
• The ischial tuberosity inferomedially
• The posterior superior iliac spine, from which a line to the greater trochanter approximates the upper border of piriformis, and a line to the ischial tuberosity crosses the field of the rotators

The ischial spine itself is palpable per rectum or per vaginam, not externally, and this is how it is located for a pudendal nerve block.

Named clinical tests

No test isolates it. External rotation strength is graded 0-5 for the group, and an examiner asking for an isolation test for superior gemellus is testing whether you know that one does not exist. The clinically useful discrimination is between the deep external rotator group (deep, posterior, provoked in flexion and internal rotation) and the abductor mechanism (lateral, provoked by single-leg stance), and between both of those and the hip joint itself (groin pain, C-sign, restricted internal rotation in flexion).

Differential diagnosis of deep buttock pain

Sciatic nerve. Sciatic nerve injury is the commonest neurological complication of the posterior approach. Its mechanisms are direct trauma during the release, retractor pressure, traction from limb lengthening and thermal injury, so limiting lengthening belongs with protecting the nerve. The peroneal division is more vulnerable because of its more lateral position, larger fascicles and less protective connective tissue.

Pudendal nerve. Injury presents as perineal numbness and, in severe cases, sexual and continence dysfunction. The commonest cause is traction against a perineal post during hip arthroscopy or fracture table use. The others are medial dissection beyond the ischial spine in a Kocher-Langenbeck approach, and retractor placement on the quadrilateral surface or over the ischial spine.

Preventing pudendal injury. Prevention is the answer. Use a well-padded, large-diameter post, or a post-free or lateral traction system where available, minimise traction force, limit traction time to the minimum and document it, and stop dissecting when you reach the spine.

Nerve to obturator internus. It is injured during deep dissection at the ischial spine, posterior column acetabular fixation, pelvic and gynaecological surgery, and sacrospinous ligament fixation. Isolated injury is functionally silent, since the muscles it supplies are expendable, but its proximity to the pudendal nerve makes it a warning marker: an injury to it should prompt a careful assessment of pudendal function.

Other nerves. Deep capsular dissection can injure the nerve to quadratus femoris, producing selective quadratus femoris and inferior gemellus atrophy. Extending the dissection above piriformis can injure the superior gluteal nerve, with abductor weakness and a Trendelenburg gait.

Vascular. Femoral head osteonecrosis follows injury to the MFCA deep branch if obturator externus is divided or the release is taken flush against the femur. An avulsed inferior gluteal artery retracts into the pelvis and cannot be controlled through the posterior wound: it needs packing and, if uncontrolled, angiographic embolisation or a pelvic approach. The superior gluteal artery can be injured at the notch with the same problem of proximal retraction, and the internal pudendal vessels at the ischial spine by medial dissection.

The repair. Dislocation is the risk the repair addresses, and follows if the repair is omitted or fails. Repairs fail when sutures are placed in poor-quality tendon, when the tendon is divided flush at the bone so there is no cuff, or when the repair is tensioned in internal rotation. Suture knots can irritate over the posterior trochanter, and heterotopic ossification can form in the split gluteus maximus and the rotator bed, particularly after acetabular fracture surgery.

Deep gluteal syndrome

The space. Deep gluteal syndrome is extrapelvic entrapment of the sciatic nerve in the space bounded by gluteus maximus posteriorly, the posterior acetabular column and hip capsule anteriorly, the sacrotuberous and falciform fascia medially, the linea aspera laterally, the sciatic notch superiorly and the hamstring origin inferiorly. Superior gemellus and the conjoint tendon form part of the floor of this space, immediately deep to the nerve.

Causes. The classical named cause is piriformis syndrome, but the term has broadened because the nerve can be tethered by fibrous bands, the obturator internus and gemelli complex, the hamstring origin, vascular structures and post-traumatic scar. The obturator internus and gemellus complex is a recognised cause distinct from piriformis: the nerve passes over the conjoint tendon, and fibrous adhesions or a hypertrophied complex can tether it.

Presentation. Buttock pain is worse on sitting and radiates down the posterior thigh, without a clear dermatomal pattern and with a normal lumbar spine. The obturator complex is distinguished from piriformis by pain reproduced on resisted external rotation with the hip flexed and on direct palpation lower and more lateral than the piriformis point, closer to the lesser sciatic notch.

Investigation. Four tests contribute, and of them a positive response to image-guided diagnostic injection is the most useful single test.

• MRI of the pelvis, to exclude a mass, assess the rotators for asymmetry, hypertrophy or atrophy, and evaluate the hamstring origin
• MR neurography where available, looking for sciatic nerve signal change, calibre change and loss of the normal perineural fat plane
• Electrophysiology, which is usually normal and is used mainly to exclude a radiculopathy or a more proximal lesion
• Image-guided diagnostic injection into the deep gluteal space or the obturator internus bursa

Management. Treatment is non-operative first: activity and sitting modification, neural mobilisation, hip external rotator and abductor rehabilitation, and image-guided injection. Refractory cases go to endoscopic sciatic nerve decompression, in which the nerve is followed from the sciatic notch distally over piriformis, the conjoint tendon and quadratus femoris and all tethering bands are released. Piriformis and, if implicated, the conjoint tendon may be released, which is well tolerated because these muscles are expendable.

Outcome. Outcomes from endoscopic decompression series are generally good, but selection is everything. Patients with a clear positive diagnostic injection and no significant lumbar pathology do best.

Obturator internus bursitis

The lesion. Inflammation of the bursa between the obturator internus tendon and the lesser sciatic notch produces deep buttock pain, worse on resisted external rotation and on sitting. It is a recognised and often-missed cause of posterior hip pain, occasionally associated with a snapping sensation as the tendon moves over the notch.

Diagnosis and treatment. MRI shows fluid distending the bursa, best seen on axial fluid-sensitive sequences. Image-guided corticosteroid injection into the bursa is usually effective, and surgery is rarely needed.

The mimic. An infected obturator internus bursa or a pyomyositis of obturator internus presents in children and young adults with fever, limp and hip pain, and is a genuine mimic of septic arthritis. MRI distinguishes them, and missing it delays treatment of a septic collection.

Denervation patterns

Reading the atrophy. Recognising the pattern of selective atrophy on MRI is a high-yield way to localise a pelvic or deep gluteal lesion, and to distinguish a focal nerve injury from a radiculopathy.

• Nerve to obturator internus: obturator internus and superior gemellus, sparing inferior gemellus and quadratus femoris (nerve to quadratus femoris) and piriformis (its own nerve)
• Nerve to quadratus femoris: quadratus femoris and inferior gemellus
• Nerve to piriformis: isolated piriformis atrophy
• All of them plus the hamstrings: a sacral plexus or proximal sciatic lesion, not an individual nerve

The conjoint tendon in the posterior approach. In the posterior (Moore or Southern) approach to the hip, the conjoint tendon is the principal structure released, along with piriformis. The release and repair run in this order.

1. Position the patient lateral decubitus with the pelvis rigidly held, and confirm the pelvis has not rolled before assessing component version.
2. Split the iliotibial band and gluteus maximus fascia, and split gluteus maximus bluntly along its fibres.
3. Internally rotate the hip to bring the rotators into view and tension their tendons.
4. Palpate the sciatic nerve, which lies superficial to the rotators and deep to gluteus maximus, rather than dissecting it out or skeletonising it.
5. Tag piriformis and the conjoint tendon with heavy braided non-absorbable suture before dividing them.
6. Divide them a short distance medial to the trochanteric footprint, leaving a cuff on bone. Obturator externus should be preserved, and quadratus femoris is usually left intact.
7. Reflect the tagged sleeve posteriorly, where it protects the sciatic nerve for the remainder of the case.
8. Perform the capsulotomy, tagging its corners.
9. At closure, repair the capsule first, then the tendon sleeve, through transosseous drill holes in the posterior greater trochanter or with suture anchors. Hold the hip in extension with neutral or slight external rotation, so the repair is not tensioned in the position that will later be avoided.
10. Check stability through the dislocating arc of flexion, adduction and internal rotation before closing, and document the safe range.

Why the repair matters. The posterior capsule and the short external rotators together act as a passive checkrein against the flexion, adduction and internal rotation position that dislocates a posterior-approach hip, and as a dynamic compressor centring the head. Their repair is the best-evidenced technical step against posterior dislocation: pooled series report a dislocation rate of roughly 4 per cent without repair, falling to under 1 per cent with a formal transosseous repair.

What the repair does not fix. Component malposition. A posterior repair does not rescue a retroverted cup or inadequate offset, so correct version, offset and head size and a formal repair are complementary, not alternatives.

Piriformis-sparing modification. The approach works through the piriformis interval, dividing only the conjoint tendon, or in some descriptions only a portion of it. Superior gemellus is therefore the first structure divided, the upper border of the released sleeve. The justification is soft tissue preservation, better repair quality and possibly faster early recovery; it is not a vascular one.

Kocher-Langenbeck approach. The same rotator release exposes the posterior column and posterior wall of the acetabulum. Keep the hip extended and the knee flexed throughout to protect the sciatic nerve, and palpate the ischial spine as the medial landmark for the quadrilateral surface rather than dissecting it aggressively or medial to it. Some surgeons use a trochanteric flip osteotomy rather than a rotator release, which preserves the entire posterior sleeve including obturator externus and gives better access to the superior acetabulum.

Ganz surgical hip dislocation. The external rotators are not divided at all: the entire posterior sleeve, including superior gemellus and obturator externus, is preserved, access is gained through the trochanteric flip osteotomy instead, and the MFCA is shielded by the intact obturator externus. This is why the technique has such a low osteonecrosis rate.

Antegrade femoral nailing. The piriformis fossa entry point sits just medial to the tip of the greater trochanter and superior to the conjoint tendon footprint. A malpositioned entry point damages the rotator insertions and can breach the medial trochanteric cortex.

Structures at risk

Anatomical Variation Across Populations

• Superior gemellus is the most variable of the short external rotators. It is reported as hypoplastic or absent in a minority of cadaveric specimens across European, North American and Asian series, with wide variation in reported prevalence reflecting differing definitions of what counts as absent. The inferior gemellus is more consistently present and larger.
• Sciatic nerve variants relative to piriformis are described in all populations. A meta-analysis of 44 cadaveric studies gave a pooled prevalence of 90 per cent for the typical pattern, with variants more common in East Asian populations (31 per cent pooled prevalence of total variants) and no significant difference by sex or laterality. The surgical message is to expect a variant in roughly one in ten patients, and more often in East Asian populations.
• The extracapsular course of the deep branch of the medial circumflex femoral artery was constant in all 24 hips in the defining cadaveric study, which is what permits the Ganz principles to be taught as rules.
• Ischial spine morphology varies, and a prominent spine is associated with acetabular retroversion — the basis of the radiographic ischial spine sign.

Differences in Described Practice

Registry and Practice Signals

• National joint registries (NJR, AOANJRR, the Nordic registries and the AJRR) consistently list dislocation among the leading indications for early revision after primary total hip arthroplasty, which is the practice-level argument for the posterior repair.
• Registries also show declining dislocation rates over time across all approaches, attributable to larger femoral heads, dual-mobility bearings in high-risk patients, improved component positioning and the widespread adoption of posterior soft tissue repair.
• There is no registry for deep gluteal syndrome or for endoscopic sciatic nerve decompression. The evidence base is single-centre case series, and the consistent signal is that outcomes depend on selection — a positive diagnostic injection and the absence of significant lumbar pathology.

High- and Limited-Resource Practice

• Well-resourced settings: MR neurography for suspected deep gluteal syndrome, image-guided diagnostic blocks, endoscopic sciatic nerve decompression, dual-mobility bearings and navigation or robotics for component positioning.
• Limited-resource settings: the anatomical rules cost nothing and matter most. Tagging the conjoint tendon before dividing it, dividing short of the femur to leave a cuff, preserving obturator externus, and performing a transosseous repair with a drill and a suture are available in every operating theatre in the world and deliver the single largest reduction in dislocation risk. Diagnosis of deep gluteal syndrome can be made clinically with the FAIR and active piriformis tests plus a landmark-guided injection.