Superior Gluteal Nerve Anatomy

Origin

• From the posterior divisions of the ventral rami of L4, L5 and S1 in the sacral plexus.
• Mixed: motor to the abductors, with a sensory branch to the hip joint capsule, especially its superior part. No cutaneous territory.

Exit from the pelvis

• Leaves the pelvis through the greater sciatic foramen, above piriformis - the suprapiriform foramen.
• It is the only nerve to do so. Every other nerve leaving through the greater sciatic foramen - inferior gluteal, sciatic, posterior femoral cutaneous, pudendal, nerve to obturator internus, nerve to quadratus femoris - passes below piriformis.
• It is accompanied by the superior gluteal artery and vein, the artery being the largest branch of the posterior division of the internal iliac artery.
• At the notch the bundle is tethered where it crosses the bone, which is why traction and retractor pressure here injure it.

In the gluteal region

• The nerve immediately enters the plane between gluteus medius and gluteus minimus, and runs anteriorly and laterally with the deep branch of the superior gluteal artery.
• It divides into a superior and an inferior branch, at a point that varies considerably - some divide close to the notch, others much further laterally.

The superior branch

• Runs along the upper border of gluteus minimus, close to the superior gluteal artery.
• Supplies gluteus medius and gluteus minimus.

The inferior branch

• Runs transversely across the hip, between gluteus medius and minimus, from posteromedial to anterolateral.
• Supplies both gluteus medius and minimus along its course, and continues anteriorly to terminate in tensor fasciae latae, entering it on the deep surface.
• This is the branch that defines the safe zone. It crosses the field of a gluteus medius split, and its distance from the tip of the greater trochanter is the number every surgeon should know.

Measured distances

• In 30 dysplastic hips assessed intra-operatively with a nerve stimulator, the inferior branch lay a mean of 37 mm (range 25 to 45) above the trochanteric tip at the anterior third of gluteus medius, 40 mm (30 to 50) at the middle third and 44 mm (35 to 55) at the posterior third. The distance decreased as dysplasia became more severe, and the authors concluded that a 3 cm safe zone is appropriate in most dysplastic hips.
• In a cadaveric study of 33 dissections, a nerve-free area over the greater trochanter measured 7 cm above and behind the trochanter, 5 cm above its posterior angle and only 3 cm above its anterior angle - a more restrictive safe area than earlier descriptions.
• In 40 hips studied for a muscle-sparing anterolateral approach, the inferior branch lay a mean 74 plus or minus 6 mm (60 to 88) from the anterior tubercle of the greater trochanter, and the authors recommended keeping the proximal part of the incision within 6 cm of that tubercle.
• In 23 cadaveric hips, the distance from the point of entry of the nerve into gluteus medius to the greater trochanter correlated significantly with cadaver height, and the nerve lay outside the conventional safe area in 22 per cent of posterior regions - the authors' conclusion being that "the safe area is not always safe."

The abductor tendons it drives are developed in gluteal tendon tears, the regional vasculature in hip blood supply, and the commonest clinical context in total hip arthroplasty indications.

The three muscles

What it does NOT supply

• Gluteus maximus - that is the inferior gluteal nerve (L5, S1, S2), which leaves the pelvis below piriformis. This distinction is the single most common examination question on the gluteal nerves.
• Piriformis and the short external rotators - separate branches from the sacral plexus.
• No skin. There is no cutaneous territory, so an isolated superior gluteal nerve lesion produces no sensory sign whatsoever.

The motor entry points that matter surgically

• The nerve enters gluteus medius on its deep surface, which is why a split of the muscle from its superficial surface does not injure it until the split reaches the transverse course of the inferior branch.
• The anterior third of gluteus medius is the closest to the nerve - a mean 37 mm above the trochanteric tip in dysplastic hips - and this is precisely where the direct lateral approach splits the muscle.
• The branch to tensor fasciae latae is the most anterior and the most distal, which is why the Watson-Jones interval between gluteus medius and tensor fasciae latae is not a true internervous plane and why proximal extension of that interval denervates the tensor.

There is none

The superior gluteal nerve has no cutaneous sensory territory. It supplies only muscle, with small articular branches to the hip joint.

Why that matters clinically

• An isolated superior gluteal nerve lesion produces a pure motor deficit. There is no numb patch to find and no sensory sign to document.
• The diagnosis therefore rests entirely on gait, the Trendelenburg test, abduction power and electromyography.
• Buttock numbness after hip surgery is not this nerve. It is a cluneal nerve, most often the superior cluneal nerves from the dorsal rami of L1 to L3 crossing the posterior iliac crest, or the inferior cluneal branches of the posterior femoral cutaneous nerve. Attributing buttock numbness to the superior gluteal nerve is a classic error.
• Lateral thigh numbness after an anterior approach is not this nerve either. That is the lateral femoral cutaneous nerve, and it is the characteristic sensory complication of the direct anterior approach.

Articular supply

• Small articular branches reach the hip capsule, contributing to deep hip pain and to proprioception. They are not clinically testable and are of anatomical interest only.

Testing the neighbours to localise a lesion

Points of vulnerability, proximal to distal

1. The greater sciatic notch The bundle is tethered as it crosses the bone above piriformis. It is compressed by retractors placed into the notch during acetabular fracture surgery and revision arthroplasty, and it is stretched in pelvic ring and acetabular fracture displacement. The superior gluteal artery lies with it, and in posterior wall and transverse acetabular fractures the artery can be lacerated, producing torrential retroperitoneal or gluteal bleeding.

2. The suprapiriform foramen itself A narrow outlet. Piriformis hypertrophy, an anomalous piriformis, or a space-occupying lesion in the notch can compress the nerve, though a genuine entrapment syndrome here is rare and should be a diagnosis of exclusion.

3. The plane between gluteus medius and minimus Where the branches run. Blunt dissection in this plane is generally safe if it follows the direction of the nerve; sharp transverse division is not.

4. The transverse course of the inferior branch, 3 to 5 cm above the trochanteric tip The single most important point. Any longitudinal split of gluteus medius that reaches this line divides the branch.

5. Intramuscular injection The classical hazard of gluteal intramuscular injection is the sciatic nerve, which is why the upper outer quadrant or the ventrogluteal site is used. But repeated injection into the gluteal muscles also causes gluteal fibrosis and can injure the superior gluteal branches, particularly in populations where repeated intramuscular injection is common in childhood.

Relations to know for the operating theatre

• Piriformis is the key landmark at the notch. Structures above it are superior gluteal; everything below is sciatic and its neighbours.
• The superior gluteal artery is with the nerve. Brisk arterial bleeding from the depths of a notch dissection means the nerve is immediately adjacent. Blind packing and blind diathermy at the notch both injure the nerve; pack, obtain proximal control or embolise, but do not cauterise blindly.
• The plane between medius and minimus contains the nerve and the deep branch of the artery. Both are preserved by mobilising the muscles as a unit rather than separating them.
• The abductor sleeve in a direct lateral approach - the anterior third of gluteus medius in continuity with vastus lateralis - carries the nerve branches that reach it from above and behind. Both the split and the repair matter.

The mechanism

Failure of the abductor mechanism to balance the pelvis in single-leg stance. The patient either allows the contralateral hemipelvis to drop (the classic sign) or shifts the trunk over the stance hip to reduce the required abductor moment (the compensated lurch).

Five causes, and they are not all neurological

The examination sequence that sorts them

1. Watch the gait from behind and from the side, before touching the patient.
2. Trendelenburg test with fingertip support for 30 seconds.
3. Grade abduction power in side-lying with the hip slightly extended.
4. Palpate the greater trochanter for tendinopathy or a defect.
5. Measure leg lengths and check for a fixed adduction contracture, which mimics abductor weakness.
6. Test tibialis posterior and tibialis anterior, and examine the back - if these are abnormal the problem is an L5 root, not the superior gluteal nerve.
7. Radiographs to measure offset, hip centre and trochanteric position.
8. Electromyography and ultrasound or MRI only after the clinical and radiographic assessment has narrowed the possibilities.

Management principles

• Treat the cause, not the sign. A denervated abductor will not be helped by a tendon repair, and a failed tendon repair will not be helped by waiting for nerve recovery.
• Physiotherapy targeting the abductors and the trunk is worthwhile in all groups, and a walking stick in the contralateral hand reduces the abductor moment required and is a genuinely effective intervention.
• Restore offset surgically where it has been lost - this is often the most effective single intervention after arthroplasty.
• Abductor reconstruction by gluteus maximus transfer or vastus lateralis transfer is a salvage for a chronically deficient abductor mechanism, and it depends on the nerve to the transferred muscle being intact.

The Hardinge approach and the safe zone

• Technique: split gluteus medius in line with its fibres at the junction of the anterior third and posterior two thirds, carry the split distally through the trochanteric insertion and into vastus lateralis, and elevate the anterior sleeve of gluteus medius, gluteus minimus and vastus lateralis in continuity to expose the anterior capsule.
• The safe zone: conventional teaching limits the proximal extent of the split to 5 cm above the tip of the greater trochanter, because the inferior branch of the superior gluteal nerve runs transversely across the muscle at approximately that level.
• Tighten the limit in dysplasia and in short patients. Intra-operative nerve stimulator measurement in 30 dysplastic hips found the inferior branch a mean of 37 mm above the trochanteric tip anteriorly, as little as 25 mm, with the distance shortening as dysplasia became more severe - and the authors recommended a 3 cm safe zone with the option of using a nerve stimulator in severe dysplasia. Separately, the distance correlates with body height, so the same absolute figure is not equally safe in every patient.
• Practical rules:
  • Place a stay suture at the apex of the split before retraction so it cannot propagate proximally.
  • Retract with blunt, broad retractors and release them intermittently. Sustained retraction on the anterior sleeve is a genuine mechanism of injury independent of the split.
  • Do not extend the split proximally to improve exposure - improve the exposure by other means, such as capsular release or femoral positioning.
  • Repair the abductor sleeve securely to bone or through drill holes. The single greatest determinant of abductor function after this approach is the quality of the repair.

What the outcome data show

• In 40 patients undergoing a Hardinge approach, electromyographic damage was present in 42.5 per cent at four weeks, in 7.5 per cent at six months, and only one of those patients had a positive Trendelenburg test at one year.
• The Trendelenburg test was positive in 50 per cent pre-operatively - a reminder that hip arthritis itself produces abductor dysfunction - and in 25 per cent at one year, so the approach improved abductor function overall.
• Critically, nine of the 37 patients with normal electromyography had a positive Trendelenburg test, confirming that most persistent abductor insufficiency after this approach is mechanical rather than neurological.

Anterolateral (Watson-Jones)

• Uses the interval between gluteus medius and tensor fasciae latae - which is not a true internervous plane, because both muscles are supplied by the superior gluteal nerve. Proximal extension of the interval denervates tensor fasciae latae.
• In a study of 40 hips developed for a muscle-sparing anterolateral approach, the inferior branch lay a mean 74 plus or minus 6 mm from the anterior tubercle of the greater trochanter, and the recommendation was to keep the proximal part of the incision within 6 cm of that tubercle, directing the incision from the anterior tubercle toward a point 6 cm posterior to the anterior superior iliac spine, with distal extension being safe.
• The same study found that excessive distraction was the main cause of injury to the anterior border of gluteus medius, and recommended protecting the muscle from retractor distraction before the femoral neck cut and during femoral preparation.

Direct anterior (Smith-Petersen and modifications)

• Uses the interval between sartorius and rectus femoris (femoral nerve) and tensor fasciae latae (superior gluteal nerve) - a genuine internervous plane.
• The characteristic nerve complication is to the lateral femoral cutaneous nerve, not the superior gluteal nerve.
• The superior gluteal nerve is nevertheless at risk from vigorous proximal retraction against tensor fasciae latae, which can produce a denervation of that muscle, and from retractors placed over the superior acetabular rim.

Posterior (Southern, Moore, Kocher-Langenbeck)

• The superior gluteal nerve is furthest from the field in a posterior approach, which is the single most consistent conclusion of the comparative anatomical work: the nerve is nearer and more exposed in anterior transgluteal approaches, and neither dislocation nor acetabular exposure threatens it as much from behind.
• The risk here is to the sciatic nerve and, at the greater sciatic notch, to the superior gluteal bundle when retractors are placed into the notch.
• An extended posterior approach has been described specifically to preserve the glutei and their neurovascular supply: mobilising gluteus medius from the posterior gluteal line proximal to distal exposes and mobilises the superior gluteal neurovascular bundle between the sciatic notch and its entry into the muscle, allowing wide supra-acetabular exposure. Intra-operative electromyography in that series confirmed innervation was not affected and all patients had good abduction function.

Acetabular fracture surgery and pelvic osteotomy

• Retractors in the greater sciatic notch are the principal mechanism of injury. Use a blunt notch retractor placed on bone, release it intermittently, and never lever against the soft tissues of the notch.
• The superior gluteal artery is the vessel most often injured in acetabular and pelvic fractures, and its embolisation combined with an extensive exposure risks gluteal necrosis.
• The extended iliofemoral approach strips the abductors from the ilium and puts both the nerve and the artery at risk; it is reserved for cases where nothing less will do.
• Periacetabular osteotomy requires careful protection of the bundle during the posterior column and retroacetabular cuts.

Femoral nailing and proximal femoral fixation

• A piriformis or trochanteric entry point requires a split of the abductor mechanism. Keep the split within the safe zone and repair it.
• Proximal extension of a lateral femoral plate under the abductors risks the branches entering the muscle from above.
• Trochanteric osteotomy and its non-union produce abductor failure by shortening or losing the lever arm, independently of the nerve.

A structured differential

A patient with a limp and weak abduction after total hip arthroplasty needs all five causes considered, and the order matters.

1. Pain and infection. Always excluded first. Inflammatory markers, aspiration where indicated. Pain inhibition produces the same gait as denervation.
2. Lever arm. Measure femoral offset, hip centre position, leg length and trochanteric position against the contralateral side on a standardised radiograph. Lost offset is common, is mechanical, and is correctable.
3. Tendon. Ultrasound or MRI with metal artefact reduction for abductor tendon avulsion or a failed repair. Trochanteric tenderness and a palpable defect support it.
4. Nerve. Electromyography of gluteus medius and tensor fasciae latae. Remember that early denervation is common and usually transient, and that a normal study does not exclude a mechanical abductor problem.
5. Heterotopic ossification and adhesions. Radiographs and, occasionally, computed tomography.

Why the order matters

Because the data show that persistent abductor insufficiency after a direct lateral approach is more often mechanical than neurological. Jumping to electromyography and attributing a limp to nerve injury misses a correctable lost offset or a repairable tendon avulsion.

Treatment by cause

• Lost offset: revision to a higher offset stem, or lateralisation of the femoral component, restores the lever arm and is often dramatically effective.
• Tendon avulsion: direct repair to bone with suture anchors where the tissue is adequate; augmentation with allograft or synthetic ligament for chronic retracted tears; the repair depends on an innervated muscle, so document nerve function first.
• Denervation: no direct treatment. Physiotherapy, a contralateral walking stick, and, in a severe permanent case, an abductor reconstruction using an innervated donor - gluteus maximus transfer, with or without vastus lateralis - to restore an abduction moment.
• Heterotopic ossification: excision once mature, with prophylaxis.

Prevention at the index operation

• Choose an approach appropriate to the anatomy - the posterior approach places the nerve furthest from the field.
• Respect the safe zone, and tighten it in dysplastic and short patients.
• Use blunt retractors, release them intermittently, and avoid distraction of the anterior border of gluteus medius.
• Restore offset and leg length, and check them intra-operatively.
• Repair the abductor sleeve meticulously and protect the repair postoperatively.

At the gluteus medius split

1. Mark the tip of the greater trochanter and measure the split limit from it, in this patient, not from memory.
2. Place a stay suture at the apex of the split before any retraction.
3. Split in line with the fibres, at the junction of the anterior third and posterior two thirds.
4. Use blunt, broad retractors and release them every few minutes. Sustained distraction is an independent mechanism of injury.
5. Do not extend the split to gain exposure. Improve exposure with capsular release, femoral positioning, or a different approach.
6. Repair the abductor sleeve to bone, through drill holes or with anchors, with the hip abducted, and protect the repair postoperatively.

At the greater sciatic notch

1. Place notch retractors on bone, subperiosteally, and bluntly.
2. Release the retractor intermittently during long acetabular exposures.
3. Never lever against the soft tissue of the notch, and never diathermy blindly in it.
4. If the superior gluteal artery bleeds, pack and obtain control; recognise that embolisation of this vessel devascularises the abductors and, combined with an extensive exposure, risks gluteal necrosis.
5. Where a wide supra-acetabular exposure is genuinely needed, an extended posterior approach that mobilises the superior gluteal bundle under direct vision preserves the glutei and their innervation, and has been shown by intra-operative electromyography to leave innervation unaffected.

Positioning and general measures

• Document abductor function pre-operatively. Half of patients coming to hip arthroplasty already have a positive Trendelenburg test, and a deficit first recorded post-operatively will be assumed to be iatrogenic.
• Restore offset and leg length - protecting the nerve is only half of protecting the abductor mechanism.
• Consider intra-operative nerve stimulation in a severely dysplastic hip where the safe zone may be as little as 2.5 cm.

If the nerve is injured

• Direct repair or grafting of the superior gluteal nerve is not routine practice. The nerve is short, deep and awkward to expose, the muscles are large, and by the time a deficit is confirmed the window has usually passed. Management is therefore functional rather than reconstructive.
• Splint the gait, not the limb: a walking stick in the contralateral hand substantially reduces the abductor moment required and is often the single most effective intervention.
• Targeted physiotherapy for the residual abductors, the trunk and the contralateral side.
• Most iatrogenic lesions from splitting or retraction are neuropraxias and recover over weeks to months, which is why observation with serial electromyography is reasonable in the first 3 to 6 months.

Abductor reconstruction for a permanently deficient mechanism

• Gluteus maximus transfer - the anterior portion of gluteus maximus, innervated by the inferior gluteal nerve and therefore unaffected by a superior gluteal lesion, is mobilised and transferred to the greater trochanter to restore an abduction moment. It is the workhorse salvage.
• Vastus lateralis transfer, alone or combined with the gluteus maximus transfer, adds bulk and a second vector.
• Allograft or synthetic augmentation is used to bridge a chronic retracted abductor tendon defect where the muscle is innervated but the tendon is irreparable.
• Prerequisite: an innervated donor and a stable, well-positioned hip with restored offset. Transferring muscle onto a hip with a short lever arm reproduces the original problem.

The nerve as a consideration in flaps

• Tensor fasciae latae is a workhorse pedicled and free flap, based on the ascending branch of the lateral femoral circumflex artery, and it is innervated by the terminal branch of the superior gluteal nerve. Preserving that branch gives a sensate, tonic flap; sacrificing it produces a passive one. The flap is used for trochanteric pressure sore coverage and abdominal wall reconstruction.
• Gluteus maximus flaps for sacral pressure sores are based on the inferior gluteal system and do not involve this nerve.
• Gluteus medius and minimus are occasionally used as local muscle transfers for trochanteric and hip coverage, and preserving the superior gluteal pedicle and nerve is essential to their function.