Inferior Gluteal Nerve Anatomy

One nerve, one muscle. The inferior gluteal nerve arises from the sacral plexus from the roots L5, S1 and S2 and supplies exactly one muscle: it is the sole motor supply to gluteus maximus. That single fact organises everything else. It is purely motor, with no cutaneous branch, so an injury is silent on sensory testing and shows up only as weak hip extension.

The course. The nerve leaves the pelvis through the greater sciatic foramen below piriformis, as does the sciatic nerve, and immediately breaks up into branches that enter the deep (anterior) surface of gluteus maximus. The superior gluteal nerve leaves above piriformis, which makes piriformis the key landmark between the two.

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Its neighbour. Learn the nerve as a pair with the superior gluteal nerve: below versus above piriformis, extension versus abduction, lurch versus Trendelenburg.

Because the nerve is the sole supply of gluteus maximus, its functional importance follows from the muscle's attachments. Gluteus maximus is the largest and most superficial gluteal muscle, and it arises broadly from:

• The posterior gluteal line of the ilium and the bone behind it
• The posterior aspect of the lower sacrum and coccyx
• The sacrotuberous ligament
• The thoracolumbar fascia

A dual insertion. The upper roughly three-quarters of the muscle inserts into the iliotibial tract, and the deep lower quarter into the gluteal tuberosity of the femur. Through the iliotibial tract it tenses the fascia lata and helps stabilise the extended knee and the lateral hip, so its weakness affects more than straight-line extension.

Its actions. It is the body's most powerful hip extensor and the only powerful extensor of the flexed hip, which is why it matters most for rising from sitting, climbing stairs and running or jumping, exactly the activities lost when the nerve is injured. The whole muscle is a lateral rotator. The upper fibres, above the axis of the hip joint, assist abduction through the iliotibial band, and the lower fibres assist adduction.

The deficit. Injury to the nerve weakens hip extension: the patient has difficulty rising from a low chair, climbing stairs and running, and the buttock atrophies and flattens.

The lurch. The gait sign is the gluteus maximus lurch, a backward thrust of the trunk at heel strike. It keeps the hip extended over the stance limb by bringing the line of gravity, and the ground reaction force, behind the hip, so the weak extensor is not needed.

Testing the muscle. Examine the patient prone with the knee flexed to 90 degrees, which slackens the hamstrings so they cannot substitute as hip extensors, and ask them to lift the thigh off the couch against resistance. What extension power remains is the maximus. Inspect and palpate the buttock for flattening and atrophy, comparing sides.

The differential of weak hip extension. If weak extension comes with sensory loss, a reduced ankle reflex or distal weakness, look beyond the nerve to an S1 root or a proximal sciatic or sacral-plexus lesion.

• Isolated inferior gluteal nerve palsy - pure motor maximus weakness, no sensory loss, normal hamstrings and ankle
• S1 radiculopathy - adds sensory loss (posterolateral leg and sole), a reduced ankle jerk and plantarflexion weakness, with back or leg pain (developed in the lumbar radiculopathy topic)
• Proximal sciatic nerve or sacral plexus lesion - a wider deficit, with the hamstrings and the sciatic distribution (foot drop, sensory loss) involved alongside the maximus
• Hamstring injury - knee-flexion weakness, with hip extension largely preserved by the maximus (developed in the hamstring injury topics)

The 5 cm rule. The nerve enters the deep surface of gluteus maximus about 5 cm from the tip of the greater trochanter. In the posterior approach to the hip the muscle split should not extend more than about 5 cm proximal to the trochanter, because beyond that the nerve can be damaged before it is seen. Hence the recommendation of a modified, "safe-split" posterior approach that limits the proximal extent of the split.

Where the danger lies. The skin incision is not the hazard. The danger is how far proximally the muscle is split and how hard it is retracted apart, since the nerve lies on the undersurface and is injured before anyone sees it. Keep the split short and distal, and part the fibres rather than levering them.

How often it matters. Partial denervation of gluteus maximus is most associated with the posterior approach: EMG finds it in about seven of ten posterior-approach patients, whereas the anterolateral approach mainly threatens the superior gluteal nerve branch to tensor fasciae latae. Abductor power is not measurably reduced, and in primary THA the clinical sequelae of these partial lesions are usually not significant. Respect the anatomy because a complete lesion is disabling, but do not tell a patient that a posterior approach will weaken their hip.

Beyond the hip. The nerve is also at risk in sacral and pelvic surgery and in deep gluteal procedures around the greater sciatic foramen.