Lumbosacral plexus
Lumbosacral plexus: femoral, obturator and sciatic nerves for FCPS Part 1
Lumbar and sacral plexuses for FCPS Part 1: femoral, obturator, gluteal, sciatic and pudendal nerves, their root values and lesions. Reflexes, dermatomes and the pattern of weakness point straight to the nerve, root or trunk involved.
Lumbar plexus (L1–L4)
Formed from the ventral rami of L1–L4 within psoas major. Like the brachial plexus, it splits into anterior divisions (to the adductor, medial compartment) and posterior divisions (to the extensor, anterior compartment).
| Nerve | Roots | Supplies |
|---|---|---|
| Iliohypogastric | L1 | Lower abdominal muscles; skin above the pubis |
| Ilioinguinal | L1 | Lower abdominal muscles; runs in the inguinal canal and out of the superficial ring to the skin of the upper medial thigh, root of penis or mons and anterior scrotum or labium; can be compressed or cut in hernia repair, giving groin pain or numbness |
| Genitofemoral | L1–2 | Genital branch: cremaster and skin of scrotum or labium (efferent limb of the cremasteric reflex); femoral branch: skin over the femoral triangle |
| Lateral cutaneous nerve of thigh | L2–3 | Skin of lateral thigh; compressed under the inguinal ligament near the ASIS, causing meralgia paraesthetica (burning lateral thigh, no weakness) |
| Femoral | L2–4, posterior divisions | Iliacus, pectineus, sartorius, quadriceps; skin of anterior thigh; saphenous nerve to medial leg and foot |
| Obturator | L2–4, anterior divisions | Adductors (longus, brevis, adductor part of magnus), gracilis, obturator externus; skin of medial thigh; branches to hip and knee joints |
| Lumbosacral trunk | L4–5 | Descends over the pelvic brim to join the sacral plexus |
- The femoral nerve emerges at the lateral border of psoas and enters the thigh outside the femoral sheath, lateral to the femoral artery. The obturator nerve emerges at the medial border of psoas and leaves through the obturator canal.
- Anterior thigh skin is supplied by the intermediate and medial cutaneous nerves of the thigh (femoral), so its loss points to the femoral nerve.
Sacral plexus (L4–S4)
Formed on the front of piriformis from the lumbosacral trunk and S1–S4.
| Division | Nerves |
|---|---|
| Posterior divisions | Superior gluteal (L4–S1), inferior gluteal (L5–S2), common peroneal part of the sciatic, nerve to piriformis, posterior cutaneous nerve of thigh (in part) |
| Anterior divisions | Tibial part of the sciatic, nerve to quadratus femoris, nerve to obturator internus, pudendal (S2–4) |
- Superior gluteal nerve: gluteus medius, gluteus minimus, tensor fasciae latae; injury gives a Trendelenburg sign (pelvis drops on the opposite side when standing on the affected leg).
- Inferior gluteal nerve: gluteus maximus, the powerful hip extensor for rising from sitting and climbing stairs.
- Sciatic nerve (L4–S3): the thickest nerve in the body. It leaves the pelvis through the greater sciatic foramen below piriformis, runs down the back of the thigh on adductor magnus, supplies the hamstrings (and the hamstring part of adductor magnus), and divides at the apex of the popliteal fossa into the tibial and common peroneal (fibular) nerves. The short head of biceps is the only thigh muscle supplied by the common peroneal part.
- Pudendal nerve (S2–4): perineum; leaves through the greater and re-enters through the lesser sciatic foramen.
Greater sciatic foramen
Piriformis is the muscle that passes through it and divides it.
| Above piriformis | Below piriformis |
|---|---|
| Superior gluteal nerve and vessels | Sciatic nerve, inferior gluteal nerve and vessels, posterior cutaneous nerve of thigh, nerve to quadratus femoris, pudendal nerve, internal pudendal vessels, nerve to obturator internus |
Nerve lesions
| Nerve | Typical cause | Deficit |
|---|---|---|
| Femoral | Pelvic or hernia surgery, iliacus haematoma (anticoagulants), femoral catheters | Weak knee extension and hip flexion, lost knee jerk, numb anterior thigh and medial leg |
| Obturator | Pelvic surgery, obturator hernia, pelvic tumour, labour | Weak adduction (leg swings outwards), numb medial thigh; obturator hernia gives pain referred to the medial knee (Howship–Romberg sign) |
| Sciatic | Posterior hip dislocation, misplaced gluteal injection, hip replacement | Weak knee flexion, foot drop, all movement below the knee lost; numb leg and foot except the saphenous strip; lost ankle jerk |
| Lumbosacral trunk | Fetal head at the pelvic brim in a long or forceps delivery | Foot drop plus weak inversion and hip abduction, numb dorsum of foot |
| Common peroneal | Fibular neck fracture, tight plaster, crossed legs | Foot drop, weak eversion, numb dorsum of foot; inversion and hip abduction normal |
| Tibial | Popliteal or tarsal tunnel injury | Weak plantarflexion and toe flexion, numb sole |
- Why the peroneal part suffers most: the common peroneal component of the sciatic nerve has fewer, larger fascicles with less protective connective tissue, and it is tethered at the sciatic notch and the fibular neck, so it cannot slide away from stretch. After a posterior hip dislocation the usual picture is foot drop with numb dorsum but near-normal calf power and sole sensation. Always test dorsiflexion before reducing a hip.
- After a sciatic injury the muscles spared are those supplied by the femoral, obturator and gluteal nerves: quadriceps, adductors and the gluteal muscles. Hip extension for standing up survives partly through gluteus maximus, but the hamstrings are lost.
- Lumbosacral trunk versus common peroneal palsy: both give foot drop, but weak inversion (tibialis posterior, tibial nerve, L4–5) and weak hip abduction (superior gluteal, L4–S1) show the lesion is above the sciatic division, at the L4–5 trunk on the pelvic brim. An epidural would not produce this pattern.
Referred knee pain from the hip
The obturator and femoral nerves supply both the hip and the knee joints (a joint is supplied by the nerves of the muscles that move it). Irritation of hip afferents is felt in the area of skin and joint served by the same nerve, so a child with Perthes disease or slipped upper femoral epiphysis may complain only of knee pain. Always examine the hip in a child with knee pain.
Obturator artery and the corona mortis
In about a quarter to a third of people, an abnormal (accessory) obturator artery arises from the inferior epigastric artery and runs down to the obturator foramen close to the lacunar ligament, on the medial side of the femoral ring. Cutting the lacunar ligament to free a strangulated femoral hernia can divide it and cause severe bleeding.
Myotomes, reflexes and dermatomes
| Root | Movement | Reflex | Dermatome |
|---|---|---|---|
| L2–3 | Hip flexion, adduction | None | Front of thigh |
| L3–4 | Knee extension | Knee jerk (L3–4) | L3 front of knee; L4 medial leg and medial malleolus |
| L4 | Ankle dorsiflexion and inversion (tibialis anterior) | Contributes to knee jerk | Medial leg |
| L5 | Great toe extension (EHL), hip abduction | None reliable | Lateral leg, dorsum of foot, big toe |
| S1 | Plantarflexion, eversion | Ankle jerk (S1) | Lateral foot, sole, heel |
| S2–4 | Bladder, anal sphincter | Anal reflex (S2–4) | S2 back of thigh; S3–5 saddle area |
Disc prolapse
Lumbar roots exit below the pedicle of their own vertebra, but a posterolateral disc usually catches the root passing down to the next foramen.
- L4/5 disc: compresses the L5 root: weak big toe extension and dorsiflexion, numb dorsum of foot and big toe, sciatic pain down the lateral leg, reflexes usually normal.
- L5/S1 disc: compresses S1: weak plantarflexion, lost ankle jerk, numb lateral foot.
- A large central disc compresses the cauda equina: saddle anaesthesia, urinary retention, bilateral leg signs; a surgical emergency.
Remember Knee jerk L3–4, ankle jerk S1; an L4/5 disc catches L5.
Exam trap The saphenous nerve is a branch of the femoral nerve, so medial leg sensation survives a sciatic injury.
Exam trap Foot drop with weak inversion is not a common peroneal palsy; look higher, at L5 or the lumbosacral trunk.
How FCPS asks about this
- Root values of femoral and obturator nerves: both L2–4; femoral from posterior divisions, obturator from anterior divisions.
- Structures through the greater sciatic foramen: piriformis; superior gluteal above it; sciatic, inferior gluteal and pudendal below it.
- Posterior divisions of the sacral plexus: superior and inferior gluteal nerves and the common peroneal part of the sciatic.
- Foot drop after posterior hip dislocation: peroneal fascicles of the sciatic nerve are more vulnerable (fewer, larger fascicles, tethered).
- Foot drop with weak inversion and hip abduction after a forceps delivery: lumbosacral trunk compressed at the pelvic brim.
- Child with hip disease complaining of knee pain: shared obturator and femoral supply of hip and knee.
- Bleeding after cutting the lacunar ligament: abnormal obturator artery from the inferior epigastric.
Test yourself on this topic
5 of the 32 questions in our bank on this topic. Choose an answer to see why every option is right or wrong.
After reduction of a posterior hip dislocation, a 30-year-old man has foot drop and numbness over the dorsum of the foot, but calf power and sole sensation are nearly normal. Why is the peroneal component mainly affected?
Answer: Peroneal fascicles are more vulnerable
In the sciatic nerve, the common peroneal division lies laterally, has fewer and larger fascicles with less protective connective tissue, and is tethered at the sciatic notch and at the neck of the fibula. It is therefore stretched and damaged more easily than the tibial division. Most sciatic injuries from hip dislocation or hip surgery thus present mainly as foot drop with numbness of the dorsum of the foot.
Peroneal division vulnerability
- Lies laterally in the sciatic
- Fewer, larger fascicles
- Tethered at notch and fibular neck
- Injury looks like foot drop
Remember Sciatic injury at the hip often looks like a peroneal palsy.
Exam trap Check and record foot dorsiflexion before reducing a dislocated hip.
Why the other options are wrong
- The tibial nerve leaves the sciatic in the pelvis Both divisions leave the pelvis together in one sheath.
- Peroneal fibres run separately through piriformis The peroneal division sometimes pierces piriformis, but this is not the reason.
- The tibial fibres lie lateral in the nerve The tibial division lies medially.
- Only the peroneal fibres cross the hip joint Both divisions are equally close to the joint.
Which nerve roots form the femoral nerve?
Answer: L2, L3, L4
The femoral nerve arises from the posterior divisions of the ventral rami of L2–L4 in the lumbar plexus. It passes lateral to the femoral artery under the inguinal ligament, supplies the anterior thigh muscles (quadriceps, sartorius, pectineus), the knee jerk (L3–L4) and, via the saphenous nerve, the skin of the medial leg. The obturator nerve comes from the anterior divisions of the same roots.
Femoral nerve
- L2–L4, posterior divisions
- Lateral to femoral artery
- Quadriceps; knee jerk
- Saphenous: medial leg skin
Remember Femoral and obturator share L2–L4: back and front.
Exam trap The saphenous nerve is the only branch of the femoral nerve reaching the foot.
Why the other options are wrong
- L1, L2 Ilioinguinal and genitofemoral region.
- L4, L5, S1 Part of the sciatic nerve.
- S1, S2, S3 Sacral plexus.
- L5, S1, S2 Part of the sciatic nerve.
After a complete obturator nerve block, a patient can still weakly adduct the thigh. Which muscle explains this?
Answer: Adductor magnus
Adductor magnus has two parts with different nerve supplies. The adductor part, arising from the ischiopubic ramus, is supplied by the obturator nerve. The hamstring part, arising from the ischial tuberosity and inserting on the adductor tubercle, is supplied by the tibial division of the sciatic nerve. After obturator block or injury, this hamstring part still produces some adduction, along with pectineus, which often has femoral nerve supply.
Adductor magnus
- Adductor part: obturator nerve
- Hamstring part: tibial (sciatic)
- Adduction survives obturator loss
Remember Adductor magnus has two nerves, so it is a hybrid muscle.
Exam trap Pectineus may also have a femoral supply, so adduction is rarely lost completely.
Why the other options are wrong
- Adductor longus Supplied only by the obturator nerve.
- Gracilis Supplied only by the obturator nerve.
- Adductor brevis Supplied only by the obturator nerve.
- Obturator externus Supplied by the obturator nerve; a lateral rotator.
A 45-year-old man has foot drop. Ankle eversion and dorsiflexion are weak, but so are ankle inversion and hip abduction. He has back pain radiating down the lateral leg. Where is the lesion?
Answer: L5 nerve root
Dorsiflexion and eversion are weak in both L5 root and common peroneal lesions. Inversion is produced mainly by tibialis posterior, supplied by the tibial nerve, and hip abduction by gluteus medius and minimus, supplied by the superior gluteal nerve. Both are L5-dependent but outside the peroneal territory, so their weakness places the lesion at the L5 root, usually from a disc prolapse. A common peroneal palsy spares inversion and hip abduction.
Foot drop: root or nerve?
- Peroneal: inversion spared
- L5: inversion and hip abduction weak
- Look for back pain
Remember Test inversion and hip abduction in every foot drop.
Exam trap Ankle jerk is normal in both an L5 root and a peroneal lesion.
Why the other options are wrong
- Common peroneal nerve at the fibular neck Spares inversion and hip abduction.
- Deep peroneal nerve Causes dorsiflexion weakness only.
- Superficial peroneal nerve Weakens eversion only, with no foot drop.
- S1 nerve root Weakens plantarflexion and loses the ankle jerk.
Which spinal segments mediate the knee (patellar) jerk?
Answer: L3, L4
Reflex roots: biceps C5–C6, brachioradialis C6, triceps C7, knee L3–L4 (femoral nerve) and ankle S1 (tibial nerve). An absent knee jerk suggests an L3–L4 root lesion (for example an L3/4 disc) or femoral neuropathy; brisk jerks suggest an upper motor neurone lesion.
Reflex roots
- Biceps: C5–C6
- Triceps: C7
- Knee: L3–L4
- Ankle: S1
Remember One-two buckle my shoe (S1–2), three-four kick the door (L3–4).
Exam trap Diabetic neuropathy loses the ankle jerks first.
Why the other options are wrong
- L1, L2 Cremasteric reflex.
- L5, S1 Ankle jerk mainly S1.
- S1, S2 Ankle jerk.
- C5, C6 Biceps jerk.
Where this sits in the CPSP syllabus
- Paper 1: Lower limb, Lumbosacral plexus; femoral, obturator and sciatic nerves
- Surgery & Allied, Paper 2: Lower limb, Lumbosacral plexus; femoral, obturator and sciatic nerves