Sciatic Nerve & Gluteal Region Anatomy

The gluteal region is a high-frequency NEET PG anatomy zone because it ties together muscle actions, nerve supply, the two sciatic foramina, the clinically vital intramuscular injection site, and the classic foot-drop / Trendelenburg lesions. Master the layered topography here and a cluster of lower-limb MCQs falls into place.

Boundaries & Surface Orientation

The gluteal region lies posterior to the pelvis, bounded above by the iliac crest and below by the gluteal fold (which does not correspond to the lower border of gluteus maximus — the muscle's lower border crosses the fold obliquely). The bony landmarks tested repeatedly are the posterior superior iliac spine (PSIS) (dimple of Venus, level of S2 and the middle of the sacroiliac joint), the ischial tuberosity, and the greater trochanter.

High-yield: A line from PSIS to the ischial tuberosity marks the surface line of the sciatic nerve in the upper part; the nerve then runs midway between the ischial tuberosity and the greater trochanter. This is the safe vs. unsafe quadrant logic for injections.

The Gluteal Muscles — Classification, Action, Nerve Supply

The three glutei plus the deep lateral rotators form the muscular bulk. Gluteus maximus is the most superficial; medius and minimus lie deeper and more laterally, fanning from the ilium to the greater trochanter.

Muscle	Origin → Insertion	Nerve	Main Action
Gluteus maximus	Ilium behind posterior gluteal line, sacrum, sacrotuberous ligament → iliotibial tract + gluteal tuberosity	Inferior gluteal nerve (L5, S1, S2)	Powerful extensor of flexed hip (climbing, rising from sitting); lateral rotator
Gluteus medius	Ilium between anterior & posterior gluteal lines → lateral greater trochanter	Superior gluteal nerve (L4, L5, S1)	Abduction; anterior fibres medially rotate; stabilises pelvis in single-leg stance
Gluteus minimus	Ilium between anterior & inferior gluteal lines → anterior greater trochanter	Superior gluteal nerve (L4, L5, S1)	Abduction & medial rotation; pelvic stabiliser
Tensor fasciae latae	ASIS / iliac crest → iliotibial tract	Superior gluteal nerve	Flexes, abducts, medially rotates; tenses IT band

High-yield: The superior gluteal nerve supplies gluteus medius, gluteus minimus, and tensor fasciae latae — and supplies NO gluteus maximus. Gluteus maximus is the only large muscle served by the inferior gluteal nerve. This single fact resolves a huge proportion of gluteal MCQs.

Mnemonic for superior gluteal nerve targets — "Medi-Mini-Tensor" (Medius, Minimus, Tensor fasciae latae).

Deep lateral (short) rotators — superior to inferior

Piriformis → superior gemellus → obturator internus → inferior gemellus → quadratus femoris (mnemonic: "Play Golf Or Go Quietly"). All are lateral rotators of the extended hip. Quadratus femoris is supplied by the nerve to quadratus femoris (L4, L5, S1); obturator internus and superior gemellus by the nerve to obturator internus.

Greater & Lesser Sciatic Foramina — Contents

The sacrospinous and sacrotuberous ligaments convert the sciatic notches into foramina. Piriformis is the key landmark that divides the greater sciatic foramen into suprapiriform and infrapiriform compartments.

Foramen	Above piriformis (suprapiriform)	Below piriformis (infrapiriform)
Greater sciatic foramen	Superior gluteal vessels & nerve	Inferior gluteal vessels & nerve; sciatic nerve; posterior femoral cutaneous nerve; nerve to quadratus femoris; pudendal nerve & internal pudendal vessels; nerve to obturator internus

Lesser sciatic foramen contents: tendon of obturator internus, nerve to obturator internus, and the pudendal nerve with internal pudendal vessels (these re-enter the perineum here).

High-yield: The pudendal nerve and internal pudendal vessels leave the pelvis via the greater sciatic foramen (below piriformis), hook around the ischial spine/sacrospinous ligament, and re-enter through the lesser sciatic foramen. This "exit-and-re-enter" pattern is a perennial favourite. The ischial spine is the landmark for the pudendal nerve block.

Structures passing through greater sciatic foramen — flow: Above piriformis → superior gluteal nerve + vessels. Below piriformis (lateral to medial roughly) → sciatic nerve → inferior gluteal vessels/nerve → posterior femoral cutaneous nerve → internal pudendal vessels → pudendal nerve → nerve to obturator internus.

The Sciatic Nerve — Roots, Course, Divisions

The sciatic nerve (L4, L5, S1, S2, S3) is the thickest nerve in the body, arising from the sacral plexus. It is really two nerves bound in a common sheath: the tibial component (anterior divisions of the ventral rami) and the common peroneal/fibular component (posterior divisions).

Course: sacral plexus → exits pelvis through greater sciatic foramen below piriformis → descends midway between the ischial tuberosity and greater trochanter → crosses the posterior surface of the short rotators (gemelli, obturator internus, quadratus femoris) → enters the back of the thigh deep to the long head of biceps femoris → divides into tibial and common peroneal nerves usually at the superior angle of the popliteal fossa (apex).

High-yield: In the gluteal region the sciatic nerve lies on the posterior surface of (superficial to) the obturator internus, gemelli, and quadratus femoris, and is crossed superficially by gluteus maximus. The nerve gives no branches in the gluteal region.

Beaton & Anson classification of sciatic nerve–piriformis relationship

A high-yield named classification describing anatomical variation:

Type	Description	Approx. frequency
Type A (Type I)	Undivided nerve passes below piriformis (normal)	~85–90%
Type B (Type II)	Common peroneal passes through piriformis; tibial below	~10%
Type C	Common peroneal above, tibial below piriformis	~2–3%
Type D	Undivided nerve passes through piriformis	rare
Type E/F	Variants passing above piriformis	very rare

High-yield: The most common variant after the normal pattern is the common peroneal (fibular) division piercing the piriformis while the tibial division passes below. This anatomical variant predisposes to piriformis syndrome and explains why the peroneal component is preferentially affected.

Trochanteric / Gluteal Anastomosis & Blood Supply

The cruciate and trochanteric anastomoses connect the internal iliac (gluteal arteries) with the femoral system (medial & lateral circumflex femoral, first perforating). This collateral circulation maintains lower limb perfusion if the external iliac/femoral artery is occluded. The superior and inferior gluteal arteries are branches of the internal iliac artery.

Clinical Anatomy & Lesions (the MCQ core)

1. Intramuscular injection — the safe quadrant

Divide the buttock into four quadrants by a vertical and a horizontal line through the midpoint. The upper outer (supero-lateral) quadrant is safe because the sciatic nerve and superior/inferior gluteal vessels lie in the lower medial and lower quadrants.

High-yield: Injection in the wrong (lower medial) quadrant risks sciatic nerve injury → foot drop. The safe site is the upper outer quadrant (or the ventrogluteal/von Hochstetter site over gluteus medius — increasingly preferred as it avoids the sciatic nerve entirely).

2. Superior gluteal nerve injury → Trendelenburg gait/sign

The superior gluteal nerve powers the hip abductors (medius/minimus) that stabilise the pelvis during single-leg stance.

Mechanism flow: Stand on the affected leg → paralysed abductors cannot hold the pelvis → pelvis drops on the unsupported (swinging, contralateral/sound) side → positive Trendelenburg sign → "lurching/waddling gait" (trunk leans toward the affected side to compensate).

High-yield: A positive Trendelenburg sign on the side of the lesion is shown by the pelvis sagging on the opposite (sound, lifted) side. Causes: superior gluteal nerve injury, gluteus medius weakness, congenital hip dislocation, coxa vara, fracture neck of femur. The superior gluteal nerve is at risk during the posterior approach for total hip replacement and in deep gluteal IM injections.

3. Sciatic nerve injury → foot drop pattern

A complete proximal sciatic lesion affects the hamstrings, all muscles below the knee, and all sensation below the knee except the medial leg (saphenous nerve, from femoral). Because the common peroneal component is more vulnerable (lateral position, tethered at the fibular neck), the foot-drop pattern often predominates.

Feature	Tibial component	Common peroneal component
Movements lost	Plantarflexion, inversion, toe flexion	Dorsiflexion, eversion (→ foot drop)
Gait	—	High-stepping (steppage) gait
Sensory loss	Sole of foot	Dorsum of foot, lateral leg
Reflex	Ankle jerk (S1) lost	—

High-yield: Foot drop = loss of dorsiflexion = common peroneal (fibular) nerve / L4–L5. Sciatic nerve injury characteristically spares the medial leg sensation (saphenous from femoral nerve) — a classic distractor.

4. Piriformis syndrome

Entrapment/irritation of the sciatic nerve by a hypertrophied or spasming piriformis (favoured by the Beaton Type B variant). Presents with deep buttock pain radiating down the leg, worse on sitting, and pain on resisted external rotation. FAIR test (Flexion, Adduction, Internal Rotation) reproduces symptoms. It is a key mimic of lumbar disc sciatica but without true radiculopathy / back pain; straight-leg-raise may be positive but neuro exam and imaging of the spine are normal.

High-yield: Piriformis syndrome is a diagnosis of exclusion; investigation of choice to exclude disc prolapse is MRI of the lumbosacral spine (normal), and MR neurography may show piriformis–sciatic abnormality. Management: NSAIDs, physiotherapy/stretching, local anaesthetic + steroid or botulinum toxin injection; surgical release if refractory.

5. Posterior approach hip surgery & nerve risk

• Superior gluteal nerve — at risk if the gluteus medius split extends >5 cm above the greater trochanter (Hardinge/lateral approach).
• Sciatic nerve — most commonly injured nerve in total hip replacement and posterior hip dislocation; the peroneal division is preferentially damaged → foot drop.

Differentials / Distinguishing Concepts

Condition	Key discriminator
Lumbar disc prolapse (L5/S1)	Back pain + dermatomal radiculopathy; MRI shows disc; positive SLR with neuro signs
Piriformis syndrome	Buttock-centred pain, normal spine MRI, positive FAIR test
Trochanteric bursitis	Point tenderness over greater trochanter, no neuro deficit
Superior gluteal nerve palsy	Trendelenburg, no sensory loss (it is purely motor)
Common peroneal palsy at fibular neck	Isolated foot drop, eversion loss, dorsum sensory loss, hamstrings intact

Recently asked / exam angle

• "Superior gluteal nerve supplies all EXCEPT gluteus maximus" — recurring single-best-answer.
• Trendelenburg sign: pelvis dips on the contralateral (sound, unsupported) side when standing on the affected leg.
• Foot drop localisation to common peroneal division and L4–L5 root.
• Contents below piriformis in the greater sciatic foramen (sciatic, pudendal, inferior gluteal, PFCN).
• Pudendal nerve leaves via greater and re-enters via lesser sciatic foramen; ischial spine = pudendal block landmark.
• Safe IM injection site = upper outer quadrant / ventrogluteal site; lower medial quadrant injures sciatic nerve.
• Beaton & Anson Type B (common peroneal through piriformis) as commonest variant.
• Sciatic nerve gives no branch in the gluteal region and divides at the apex of the popliteal fossa.
• Saphenous nerve sparing of medial leg in sciatic injury.

Rapid revision

1. Sciatic nerve roots = L4, L5, S1, S2, S3; thickest nerve in the body; exits below piriformis.
2. Superior gluteal nerve (L4,L5,S1) → medius, minimus, TFL; never gluteus maximus.
3. Inferior gluteal nerve (L5,S1,S2) → gluteus maximus only (chief extensor of flexed hip).
4. Deep rotators top-to-bottom: Piriformis, sup. gemellus, obturator internus, inf. gemellus, quadratus femoris.
5. Above piriformis → superior gluteal nerve & vessels; everything else passes below.
6. Pudendal nerve: greater sciatic foramen out → around ischial spine → lesser sciatic foramen in.
7. Trendelenburg sign = pelvis drops on the opposite/sound side; lesion = superior gluteal nerve / gluteus medius.
8. Foot drop = common peroneal (fibular) division; steppage gait; loss of dorsiflexion & eversion.
9. Sciatic injury spares medial leg sensation (saphenous = femoral nerve).
10. Upper outer quadrant = safe IM injection; ventrogluteal site avoids sciatic nerve.
11. Beaton Type B (common peroneal pierces piriformis) → predisposes to piriformis syndrome.
12. Sciatic nerve = most commonly injured nerve in THR / posterior hip dislocation; peroneal part worst hit.