Superior Gemellus Anatomy

Origin

• Site: the dorsal (external) surface of the ischial spine, and the adjacent margin of the greater sciatic notch.
• Character: a small, flat, triangular muscle, typically 2-3 cm in its longest dimension, arising by short fibres.
• Landmark value: the ischial spine is the key reference point. It gives attachment to the sacrospinous ligament medially, forms the boundary between the greater and lesser sciatic foramina, and is the point around which the pudendal neurovascular bundle and the nerve to obturator internus hook.

Insertion

• Site: the fibres run laterally and fuse with the upper surface of the obturator internus tendon, and with the inferior gemellus below it, forming the conjoint tendon.
• The conjoint tendon inserts on the medial surface of the greater trochanter, just anterior to the trochanteric fossa and below and slightly anterior to the piriformis insertion on the superomedial trochanter.
• Footprint: the conjoint tendon footprint is roughly 1.5-2 cm in vertical dimension on the medial trochanter, immediately anterior to the trochanteric fossa where obturator externus inserts.
• Variability: superior gemellus is the smaller and more variable twin. It is hypoplastic or absent in a proportion of anatomical specimens, whereas the inferior gemellus is more consistently present and larger. The conjoint tendon is present regardless.

The Trochanteric Insertion Map

Nerve Supply

• Nerve: the nerve to obturator internus.
• Root values: L5, S1 (some descriptions include L4).
• Also supplies: the obturator internus — the two muscles it lies against, hence the shared nerve.
• Entry: the branch to superior gemellus arises as the parent nerve crosses the ischial spine and enters the muscle on its deep (anterior) surface almost immediately, since the muscle lies directly on the spine.

Course of the Nerve — the Double Passage

• Arises from the anterior divisions of the sacral plexus (L5, S1).
• Leaves the pelvis through the greater sciatic foramen, below piriformis.
• Crosses the dorsal surface of the ischial spine, lateral to the internal pudendal vessels and lateral to the pudendal nerve.
• Re-enters the pelvis through the lesser sciatic foramen to reach the pelvic (deep) surface of obturator internus.
• It gives its branch to superior gemellus as it crosses the spine, before re-entering.

Why the Double Passage Matters

• Only three structures leave the pelvis through the greater sciatic foramen and re-enter through the lesser: the pudendal nerve, the internal pudendal vessels, and the nerve to obturator internus. This is a stock viva question.
• Their order at the ischial spine, medial to lateral, is: pudendal nerve, internal pudendal artery and vein, nerve to obturator internus.
• Clinical application: the ischial spine is the landmark for a transvaginal or transperineal pudendal nerve block. It is also the site at which the pudendal nerve is entrapped between the sacrospinous and sacrotuberous ligaments in pudendal neuralgia, and where a sacrospinous ligament fixation suture placed too laterally can injure the pudendal bundle.
• Orthopaedic relevance: dissection medial to the ischial spine in a Kocher-Langenbeck approach, or aggressive medial retraction, threatens the pudendal bundle. Injury produces perineal numbness and sexual and continence dysfunction — a far more distressing complication than a small muscle deficit.

Nerve Supply of the Group — the Exam Table

Supply to the Muscle

• Inferior gluteal artery — the principal supply, reaching the muscle as it emerges below piriformis.
• Internal pudendal artery — small branches as it crosses the ischial spine.
• Medial circumflex femoral artery — branches to the tendinous portion near the trochanteric insertion.
• The muscle is small, segmentally supplied and of no value as a flap donor.

The Vessel That Governs Everything: the MFCA Deep Branch

• The deep branch of the medial circumflex femoral artery is the dominant blood supply to the adult femoral head via the superior retinacular (lateral epiphyseal) arteries.
• Its course relative to the rotators: it runs along the inferior border of obturator externus, crosses posterior to the obturator externus tendon, then passes anterior (deep) to the tendons of the inferior gemellus, obturator internus, superior gemellus and piriformis, before ascending on the posterosuperior femoral neck.
• The consequence, stated precisely: the conjoint tendon — including superior gemellus — lies posterior (superficial) to the artery. Dividing the conjoint tendon therefore does not endanger the femoral head blood supply, provided the obturator externus tendon is preserved.
• The corollary that gets candidates marks: it is not piriformis-sparing that protects the artery. It is obturator externus preservation.

The Trochanteric Anastomosis

• Formed on the posterior greater trochanter by the medial and lateral circumflex femoral arteries and the superior and inferior gluteal arteries.
• Why this governs the release: stripping the short external rotators flush from the femur takes this anastomosis and the terminal MFCA branches with them. In the Ganz surgical dislocation the external rotators are not divided at all, and the artery is shielded by the intact obturator externus.
• Practical translation for the conjoint tendon: divide it a short distance medial to the trochanteric footprint, leaving a cuff on the bone both for the repair and to leave the vascular anastomosis undisturbed.

Superficial (Posterior)

• Gluteus maximus and its overlying fascia.
• The sciatic nerve, which descends over the posterior surface of the short external rotators. Below piriformis it lies on the conjoint tendon and then on quadratus femoris.
• The posterior femoral cutaneous nerve, medial and superficial to the sciatic nerve.
• The inferior gluteal artery and nerve, emerging below piriformis at the superomedial corner of the field.

Deep (Anterior)

• The posterior hip capsule — the conjoint tendon lies directly on it, separated by a thin layer of areolar tissue. The tendon and capsule are often adherent and are frequently taken as one layer in a posterior approach.
• The deep branch of the medial circumflex femoral artery, running anterior to the tendon toward the posterosuperior neck.
• The nerve to quadratus femoris, which lies deep to the conjoint tendon on the capsule.
• The obturator externus tendon, deeper and more inferior, crossing the femoral neck to the trochanteric fossa.

Superior

• Piriformis, and above it the greater sciatic foramen with the superior gluteal neurovascular bundle emerging above piriformis.
• The piriformis interval — the gap between the inferior border of piriformis and the superior border of superior gemellus — is a working plane in the piriformis-sparing posterior approach.

Inferior

• Obturator internus tendon, then inferior gemellus, then quadratus femoris.

Medial

• The ischial spine and the sacrospinous ligament, and just medial to those, the pudendal nerve and internal pudendal vessels — the medial limit of safe dissection.

Structures at Risk

• Sciatic nerve — superficial to the muscle; the commonest neurological complication of the posterior approach.
• Pudendal nerve and internal pudendal vessels — medial to the nerve to obturator internus at the ischial spine.
• Deep branch of the MFCA — deep to the conjoint tendon; safe if obturator externus is intact and the release leaves a cuff rather than stripping flush.
• Inferior gluteal artery — superomedially, retracts into the pelvis if avulsed.
• Nerve to quadratus femoris — deep to the conjoint tendon on the capsule.

The Conjoint Tendon in Surgery

• In the posterior (Moore or Southern) approach to the hip, the conjoint tendon — superior gemellus, obturator internus and inferior gemellus — is the principal structure released, along with piriformis.
• Sequence:
  1. Split the iliotibial band and gluteus maximus fascia; split gluteus maximus bluntly along its fibres.
  2. Internally rotate the hip to bring the rotators into view and to tension their tendons.
  3. Identify the sciatic nerve — it lies superficial to the rotators and deep to gluteus maximus. Palpate rather than dissect it out.
  4. Tag piriformis and the conjoint tendon with heavy non-absorbable sutures before dividing.
  5. Divide them a short distance medial to the trochanteric insertion, leaving a cuff on bone.
  6. Reflect the tagged sleeve posteriorly — it now protects the sciatic nerve for the remainder of the case.
  7. Perform the capsulotomy, tagging its corners.
• Quadratus femoris is usually left intact. Obturator externus should be preserved.

Piriformis-Sparing Modification

• Works through the interval between piriformis above and the conjoint tendon below, dividing only the conjoint tendon (or in some descriptions, only a portion of it).
• Superior gemellus is therefore the first structure divided in a piriformis-sparing approach — it is the upper border of the released sleeve.
• Justification: soft tissue preservation, better repair quality and possibly faster early recovery. Not a vascular justification, since piriformis lies superficial to the MFCA deep branch.

The Repair

• Transosseous drill holes in the posterior greater trochanter, or suture anchors.
• Repair the capsule first, then the tendon sleeve.
• Position: hip in extension, neutral or slight external rotation.
• Check stability through flexion, adduction and internal rotation before closing, and document the safe arc.
• Evidence: pooled data show the dislocation rate falls from roughly 4 per cent without repair to under 1 per cent with it.

Kocher-Langenbeck Approach

• The same rotator release is used to expose the posterior column and posterior wall of the acetabulum.
• Additional considerations:
  • Keep the hip extended and the knee flexed throughout to protect the sciatic nerve.
  • The ischial spine is a medial landmark for the quadrilateral surface; palpate it, do not dissect 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 superior superior-acetabular access.

The Concept

• Deep gluteal syndrome is extrapelvic entrapment of the sciatic nerve in the space bounded by the 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.
• 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.

Obturator Internus and Gemelli as a Cause

• The obturator internus and gemellus complex is a recognised cause of deep gluteal sciatic entrapment, distinct from piriformis. The nerve passes over the conjoint tendon, and fibrous adhesions or a hypertrophied complex can tether it.
• Presentation: buttock pain worse on sitting, radiating down the posterior thigh, without a clear dermatomal pattern and with a normal lumbar spine.
• Distinguishing feature from piriformis: pain reproduced by resisted external rotation with the hip flexed and by direct palpation lower and more lateral than the piriformis point, closer to the lesser sciatic notch.

Investigation

• 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 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 — a positive response is the most useful single test.

Management

• Non-operative first: activity and sitting modification, neural mobilisation, hip external rotator and abductor rehabilitation, and image-guided injection.
• Endoscopic sciatic nerve decompression for refractory cases: 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.
• 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

• Inflammation of the bursa between the obturator internus tendon and the lesser sciatic notch.
• Presentation: deep buttock pain, worse on resisted external rotation and on sitting; occasionally associated with a snapping sensation as the tendon moves over the notch.
• Imaging: fluid distending the bursa on MRI, best seen on axial fluid-sensitive sequences.
• Management: image-guided corticosteroid injection into the bursa is usually effective; surgery is rarely needed.
• Important 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; missing it delays treatment of a septic collection.

Denervation Patterns

• Nerve to obturator internus lesion: selective atrophy of obturator internus and superior gemellus.
• Nerve to quadratus femoris lesion: selective atrophy of quadratus femoris and inferior gemellus.
• Nerve to piriformis lesion: isolated piriformis atrophy.
• A pattern involving all of them plus the hamstrings indicates a sacral plexus or proximal sciatic lesion, not an individual nerve.
• Recognising these patterns 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.

Iatrogenic Injury to the Nerve to Obturator Internus

• Occurs during deep dissection at the ischial spine, posterior column acetabular fixation, pelvic and gynaecological surgery, and sacrospinous ligament fixation.
• Isolated injury is functionally silent — the muscles it supplies are expendable — but its proximity to the pudendal nerve means that an injury to it should prompt a careful assessment of pudendal function.

Pudendal Nerve Injury at the Ischial Spine

• The complication that matters at this site. Perineal numbness, faecal or urinary incontinence, and sexual dysfunction.
• Mechanisms in orthopaedic practice:
  • Traction against a perineal post during hip arthroscopy or fracture table use — the commonest cause. Mitigated by a well-padded, large-diameter post, minimising traction force and time, and using a post-free or lateral traction system where available.
  • Medial dissection beyond the ischial spine in a Kocher-Langenbeck approach.
  • Retractor placement on the quadrilateral surface or over the ischial spine.
• Prevention is the answer: limit traction time to the minimum, document it, use adequate padding, and stop dissecting when you reach the spine.

Anatomical Variants

• Absent or hypoplastic superior gemellus in a proportion of specimens — of no functional consequence, but it means the conjoint tendon may appear to have only two contributors at surgery.
• Accessory slips between superior gemellus and piriformis or the capsule.
• Sciatic nerve variants relative to piriformis matter far more at this site. A systematic review and meta-analysis of 44 cadaveric studies found the typical pattern — the undivided nerve passing below piriformis — in a pooled 90 per cent, meaning that in greater than 10 per cent of people the nerve courses through or above the muscle. These variants are implicated in deep gluteal syndrome and place the common peroneal division at particular risk.