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AN16.1-6 | Gluteal region & back of thigh — Self-Directed Learning
CLINICAL SCENARIO
A 4-year-old boy is brought to the outpatient department of a government hospital in Chennai. His mother says he has been limping since he started walking. On examination, when the child stands on his right leg and lifts the left, his right hip drops — the opposite side dips instead of rising.
What is this sign called? Which muscles are responsible? Why does it happen?
By the end of this module, you will understand the exact anatomy behind this presentation — and recognise it in every child who walks through your door.
WHY THIS MATTERS
The gluteal region and back of thigh are sites of some of the most preventable iatrogenic injuries in Indian medicine. Every intern gives intramuscular injections — and incorrect gluteal injections are responsible for dozens of sciatic nerve palsies every year in our country.
Beyond injections, you will encounter:
- Sciatica — the most common neurological complaint in adults, caused by disc prolapse compressing the sciatic nerve
- Hamstring tears — common in athletes and students who sprint without warm-up (think inter-college sports day)
- Popliteal artery injury — a limb-threatening emergency after posterior knee dislocations
- Baker's cyst — a bulge in the popliteal fossa that patients often mistake for a tumour
Understanding this anatomy will guide your examination, protect your patients from harm, and help you localise the level of a nerve lesion precisely.
RECALL
Before we start, recall from your earlier Lower Limb sessions:
- The hip joint is a ball-and-socket joint. The femoral head sits in the acetabulum.
- The femur has a greater and lesser trochanter. The gluteal tuberosity is on the posterior surface.
- The lumbosacral plexus (L1–S3) forms the nerves of the lower limb. The sciatic nerve is the largest — L4, L5, S1, S2, S3.
- Fascia lata is the deep fascia of the thigh; the iliotibial tract is its thickened lateral band.
Keep these in mind as we build on them.
The Gluteal Muscles — Layers and Actions
The gluteal region contains three main muscles and a group of six deep lateral rotators.
Gluteus Maximus
- Origin: Ilium (posterior gluteal line), sacrum, coccyx, sacrotuberous ligament
- Insertion: Iliotibial tract (upper 3/4) + gluteal tuberosity of femur (lower 1/4)
- Nerve: Inferior gluteal nerve (L5, S1, S2)
- Actions: Powerful extension of the hip (e.g., rising from a chair, climbing stairs); lateral rotation
- Clinical note: The largest muscle in the body. Used as the donor site for flap surgery. The safe zone for IM injection is its upper outer quadrant — but gluteus medius is preferred.
Gluteus Medius and Minimus
- Both originate from the ilium (between anterior and posterior gluteal lines for medius; anterior and inferior gluteal lines for minimus)
- Both insert onto the greater trochanter
- Both are supplied by the superior gluteal nerve (L4, L5, S1)
- Both abduct the hip; anterior fibres medially rotate
- Critical function: When you stand on one leg, these muscles CONTRACT on the weight-bearing side to prevent the pelvis from tilting to the other side. Weakness → Trendelenburg sign.
The Six Deep Lateral Rotators (mnemonic: Piece Of Outgoing Internal Girls Quietly)
| Muscle | Mnemonic |
|---|---|
| Piriformis | P |
| Obturator internus | O |
| Gemellus superior + inferior | G (x2) |
| Quadratus femoris | Q |
All insert on the greater trochanter or intertrochanteric crest. All laterally rotate the extended hip. All supplied by direct branches from sacral plexus.
Piriformis is the key landmark: the sciatic nerve usually exits below piriformis (through the greater sciatic foramen).
CLINICAL PEARL
The sign: When the patient stands on the affected leg, the pelvis drops on the opposite (healthy) side.
Why it happens: Gluteus medius and minimus (superior gluteal nerve, L4, L5, S1) should contract on the standing leg side to hold the pelvis level. If they are weak — due to superior gluteal nerve palsy, hip joint disease, or femoral neck fracture — they fail to do this, and the opposite pelvis sags.
Remember: The sign is positive on the standing (affected) side. The drop is on the opposite side.
Causes in India: Developmental dysplasia of the hip, old polio (L4/L5 level), superior gluteal nerve injury during posterior hip approaches.
The Trendelenburg gait (waddling gait): The patient compensates by swinging the trunk to the affected side with each step — the classic duck gait seen in bilateral hip disease.
SELF-CHECK — Self-Check 1
A patient with left superior gluteal nerve palsy stands on the left leg. What do you observe?
A. Left pelvis drops
B. Right pelvis drops
C. Both sides drop equally
D. No visible change
Reveal Answer
Answer: B. Right pelvis drops
Which muscle of the gluteal region has the MOST important role in walking on level ground?
A. Gluteus maximus
B. Gluteus medius
C. Piriformis
D. Quadratus femoris
Reveal Answer
Answer: B. Gluteus medius
Structures Deep to Gluteus Maximus & Safe IM Injection
Structures under gluteus maximus (from above downward in the greater sciatic foramen):
1. Superior gluteal nerve and vessels (above piriformis)
2. Piriformis (landmark)
3. Inferior gluteal nerve and vessels
4. Sciatic nerve
5. Pudendal nerve and internal pudendal vessels
6. Nerve to obturator internus
7. Posterior cutaneous nerve of thigh
The Sciatic Nerve
- Formation: L4, L5, S1, S2, S3 — from the lumbosacral plexus
- Exit: Through the greater sciatic foramen, below piriformis (in most people)
- Course: Runs midway between the ischial tuberosity and greater trochanter, then descends the back of the thigh, covered by biceps femoris
- Division: Usually at the apex of the popliteal fossa into common peroneal (fibular) and tibial nerves
- Branches in thigh: None (the nerve is silent in the thigh — branches emerge in the popliteal fossa)
Safe Zone for Gluteal IM Injection
- The upper outer quadrant of the gluteus maximus is the traditional zone
- Better: Ventrogluteal site (Hochstetter's method) — the gluteus medius and minimus over the greater trochanter; no major nerves or vessels at this site
- Danger: Injecting into the lower inner quadrant risks hitting the sciatic nerve — causing foot drop, sensory loss on the entire sole and dorsum of foot, and paralysis of all muscles below the knee
Piriformis syndrome: In a small percentage of people, the sciatic nerve passes through (not below) piriformis. These individuals are more prone to sciatic irritation from piriformis spasm — common in people who sit for prolonged periods (long-distance drivers, cyclists).
The Hamstring Group — Back of Thigh
The hamstrings are three muscles occupying the posterior compartment of the thigh.
| Muscle | Origin | Insertion | Action | Nerve |
|---|---|---|---|---|
| Biceps femoris (long head) | Ischial tuberosity | Head of fibula | Flex knee, extend hip, laterally rotate leg | Tibial part of sciatic (L5, S1, S2) |
| Biceps femoris (short head) | Lateral lip of linea aspera | Head of fibula | Flex knee, laterally rotate leg | Common peroneal part of sciatic (L5, S1) |
| Semitendinosus | Ischial tuberosity | Medial surface of tibia (pes anserinus) | Flex knee, extend hip, medially rotate leg | Tibial part of sciatic (L5, S1, S2) |
| Semimembranosus | Ischial tuberosity | Posterior medial condyle of tibia | Flex knee, extend hip, medially rotate leg | Tibial part of sciatic (L5, S1, S2) |
Pes anserinus ("goose foot") is where sartorius, gracilis, and semitendinosus all insert on the medial tibia — a common site of bursitis.
Hamstring strain (a very common sports injury):
- Occurs at the musculotendinous junction, most often in the long head of biceps femoris
- Mechanism: Sudden sprinting or kicking — the muscle is overloaded eccentrically (contracting while being stretched)
- In India: common in cricket (fast bowlers), kabaddi players, athletic events
- Predisposing factors: Poor warm-up, fatigue, previous injury
Nerve supply note: The short head of biceps is the ONLY hamstring supplied by the common peroneal division of the sciatic nerve. All others are from the tibial division. This is tested in viva examinations.
CLINICAL PEARL
The cruciate anastomosis is the main arterial anastomosis at the back of the thigh. It connects:
- Descending branch of inferior gluteal artery (from internal iliac)
- Ascending branch of medial circumflex femoral artery
- Ascending branch of lateral circumflex femoral artery
- 1st perforating branch of profunda femoris
Why it matters: This anastomosis provides collateral circulation when the femoral artery is ligated above profunda femoris. The limb can still be perfused via internal iliac → inferior gluteal → cruciate anastomosis → profunda perforators → popliteal artery.
Compare this to the trochanteric anastomosis at the hip joint level (formed by the circumflex femoral arteries and gluteal vessels).
Popliteal Fossa — The Diamond at the Back of the Knee
The popliteal fossa is a diamond-shaped space behind the knee joint.
Boundaries:
- Supero-lateral: Biceps femoris
- Supero-medial: Semitendinosus and semimembranosus
- Infero-lateral: Lateral head of gastrocnemius
- Infero-medial: Medial head of gastrocnemius (and plantaris)
Roof: Deep fascia of the leg (popliteal fascia) — pierced by the small saphenous vein and posterior cutaneous nerve of thigh
Floor (from above downward):
1. Popliteal surface of femur
2. Posterior capsule of knee joint (with oblique popliteal ligament)
3. Popliteus muscle and its fascia
Contents (superficial to deep, medial to lateral):
- Tibial nerve (most superficial and lateral of neural structures)
- Popliteal vein (middle)
- Popliteal artery (deepest — most protected but also most vulnerable in dislocation)
- Common peroneal nerve along medial border of biceps femoris
- Genicular branches of the popliteal artery (anastomosis around knee)
- Popliteal lymph nodes (deep)
Clinical anatomy:
- Popliteal aneurysm: Most common peripheral arterial aneurysm. Pulsatile swelling in popliteal fossa. Can cause distal embolism or acute limb ischaemia.
- Baker's cyst (popliteal cyst): Herniation of synovial membrane through posterior capsule, usually communicates with the knee joint. Common in rheumatoid arthritis and osteoarthritis. Feels like a soft fluctuant swelling, best palpated in extension.
- Popliteal artery injury: Occurs in posterior knee dislocation (hyperextension injury). The popliteal artery is tethered at both ends — it tears easily. Compartment syndrome and limb loss can follow within 6 hours if not repaired.
SELF-CHECK — Self-Check 2
A patient presents with inability to flex the knee and loss of sensation on the back of the leg and sole of the foot. The most likely injured structure is:
A. Common peroneal nerve
B. Tibial nerve
C. Superficial peroneal nerve
D. Saphenous nerve
Reveal Answer
Answer: B. Tibial nerve
Which structure in the popliteal fossa is most deeply placed (closest to the posterior capsule of the knee)?
A. Tibial nerve
B. Popliteal vein
C. Popliteal artery
D. Common peroneal nerve
Reveal Answer
Answer: C. Popliteal artery
REFLECT
A junior nurse gives a gluteal injection to a 6-month-old infant and the child develops foot drop the next day. Trace the anatomical pathway: which nerve was injured, at what point, and what functional losses would you expect? How would you counsel the mother and what is the prognosis?
KEY TAKEAWAYS
Gluteal Muscles
- Gluteus maximus: extension + lateral rotation; inferior gluteal nerve (L5, S1, S2)
- Gluteus medius/minimus: abduction + medial rotation; superior gluteal nerve (L4, L5, S1)
- Six deep rotators: all laterally rotate; piriformis is the key landmark
Trendelenburg Sign
- Pelvis drops on OPPOSITE side to the weak gluteus medius
- Caused by superior gluteal nerve palsy, hip joint disease, or neck of femur fracture
IM Injection Safety
- Safe zone: upper outer quadrant of gluteus maximus, or ventrogluteal (preferred)
- Danger zone: lower medial quadrant → sciatic nerve palsy
Hamstrings
- Ischial tuberosity origin; knee flexion + hip extension
- Short head of biceps femoris = only hamstring from common peroneal division
Sciatic Nerve
- L4-S3; exits below piriformis; divides at popliteal fossa apex
- Silent in thigh; supplies all muscles below the knee
Popliteal Fossa
- Diamond shaped; popliteal artery is deepest (vulnerable in dislocation)
- Tibial nerve → posterior leg + sole; common peroneal → anterior + lateral leg