Lumbosacral plexus
32 named structures.
Draft — not yet clinically reviewed. The structure of this map is checked automatically, but its wording has not been fact-checked against a textbook. Do not rely on it for an exam answer yet.
Hover or tab a structure to trace what it connects to. Some structures reveal further branches.
Clinical detail
- L4 anterior ramusL4L3/4 disc prolapse or foraminal stenosis compressing the L4 root1 question
- L5 anterior ramusL5L4/5 posterolateral disc prolapse compressing the traversing L5 root (with L5/S1, the commonest level)1 question
- S1 anterior ramusS1L5/S1 disc prolapse compressing the S1 root1 question
- S4 anterior ramusS4Central lumbar disc prolapse or tumour compressing the sacral roots of the cauda equina1 question
- Lumbosacral trunk (L4–L5)L4 L5Fetal head compressing the trunk against the pelvic brim in a long or obstructed labour (short mother, cephalopelvic disproportion)
- Sacral plexus anterior divisionsS1 S2 S3 S4Locally invasive pelvic malignancy (cervix, rectum, prostate) or radiation plexopathy infiltrating the sacral plexus
- Iliohypogastric nerve (L1)L1Cut in a low gridiron appendicectomy incision, a Pfannenstiel incision or anterior iliac crest bone harvest2 questions
- Ilioinguinal nerve (L1)L1Caught in suture or mesh at inguinal hernia repair, or cut at a Pfannenstiel incision1 question
- Genitofemoral nerve (L1–L2)L1 L2Genital branch damaged at inguinal hernia repair or varicocele surgery, or the whole nerve stretched by a psoas abscess1 question
- Lateral femoral cutaneous nerve (L2–L3)L2 L3Meralgia paraesthetica: compression at the inguinal ligament by obesity, pregnancy, tight belts or trousers, prolonged prone or hip-extended positioning, or iliac crest bone harvest1 question
- Femoral nerve (L2–L4)L2 L3 L4Iliacus or psoas haematoma (anticoagulation, haemophilia), retractor pressure in pelvic surgery, prolonged lithotomy, or diabetic amyotrophy1 question
- Obturator nerve (L2–L4)L2 L3 L4Obturator hernia in a thin, elderly, multiparous woman; pelvic malignancy; damage at pelvic lymphadenectomy or hip surgery1 question
- Superior gluteal nerve (L4–S1)L4 L5 S1Injury in lateral or posterior hip arthroplasty, an intramuscular injection placed in the upper inner buttock, or an L5 root lesion1 question
- Inferior gluteal nerve (L5–S2)L5 S1 S2Damage in posterior-approach hip surgery or a deep buttock injection3 questions
- Sciatic nerve (L4–S3)L4 L5 S1 S2 S3Posterior hip dislocation, acetabular fracture, hip arthroplasty, or a buttock injection placed too medial or inferior2 questions
- Pudendal nerve (S2–S4)S2 S3 S4Stretch in a prolonged second stage of labour or forceps delivery; chronic compression in Alcock's canal (cyclists)
- Tibial nerve (L4–S3)L4 L5 S1 S2 S3Tarsal tunnel syndrome: compression under the flexor retinaculum by a ganglion, tenosynovitis, pes planus or old fracture1 question
- Common fibular (peroneal) nerve (L4–S2)L4 L5 S1 S2Fibular neck fracture, tight plaster cast or knee brace, prolonged leg crossing or squatting, rapid weight loss, lateral knee surgery, pressure during anaesthesia or in ICU
- Saphenous nerve (L3–L4)L3 L4Great saphenous vein harvest or stripping, knee arthroscopy through a medial portal, or a saphenous cut-down at the ankle1 question
- Deep fibular (peroneal) nerve (L4–S1)L4 L5 S1Anterior compartment syndrome after a tibial fracture or unaccustomed exertion1 question
- Superficial fibular (peroneal) nerve (L5–S2)L5 S1 S2Lateral compartment syndrome, traction from a severe inversion ankle sprain, or entrapment where it pierces the deep fascia1 question
- Sural nerve (S1–S2)S1 S2Injury during small saphenous vein surgery, Achilles tendon repair or lateral ankle surgery; or deliberately excised for biopsy1 question
- Medial and lateral plantar nervesMorton's neuroma: a common plantar digital nerve compressed between the 3rd and 4th metatarsal heads by tight shoes or high heels3 questions
Common questions
How do I separate a common fibular palsy from an L5 radiculopathy in a patient with foot drop?
Both weaken dorsiflexion and numb the dorsum of the foot. An L5 root lesion also weakens inversion (tibialis posterior, tibial nerve), hip abduction (gluteus medius, superior gluteal nerve) and often gives back pain. A common fibular palsy at the fibular neck spares inversion and hip abduction and weakens eversion. The ankle jerk (S1) is normal in both. A complete sciatic lesion adds plantarflexion loss, a flail foot and an absent ankle jerk.
Which nerve leaves the greater sciatic foramen above piriformis?
Only the superior gluteal nerve (with its vessels). The sciatic, inferior gluteal, pudendal, posterior femoral cutaneous nerves and the nerves to quadratus femoris and obturator internus all pass below piriformis. The pudendal nerve then re-enters through the lesser sciatic foramen.
Why is the medial leg spared after a complete sciatic nerve injury?
The medial leg and medial foot are supplied by the saphenous nerve, a branch of the femoral nerve (L3–L4). Every other patch of skin below the knee belongs to sciatic branches: superficial fibular (dorsum), deep fibular (first web space), sural (lateral foot) and the tibial plantar nerves (sole).
Which side does the pelvis drop in a positive Trendelenburg sign?
On the side opposite the weak hip. Standing on the affected leg, the paralysed gluteus medius and minimus (superior gluteal nerve) cannot hold the pelvis level, so the unsupported contralateral hemipelvis sags. The trunk lurches over the weak side during walking to compensate.
Where is a gluteal intramuscular injection safe?
The upper outer quadrant of the buttock or the ventrogluteal site. The sciatic nerve emerges midway between the ischial tuberosity and greater trochanter in the lower inner region, and the superior gluteal nerve runs across the upper inner quadrant. Injury to the sciatic gives foot drop plus a flail foot; injury to the superior gluteal gives a Trendelenburg gait.