Sciatic, Tibial and Fibular: The Largest Nerve in the Body and Its Two Destinies
Somewhere deep in your buttock, hidden under a slab of muscle, runs a cord as thick as your thumb. It is the widest nerve any human body builds, and it has to be — it carries the instructions for standing, walking, running and every step you will ever take, all the way from the lower back to the tips of your toes. But the sciatic nerve keeps a secret that explains almost every one of its injuries: it was never really one nerve. It is two nerves travelling together in a shared sheath, like two cables taped side by side, and they separate behind the knee to live two completely different lives. One goes down the back to the sole of the foot; the other swings round the outside of the leg, hugs a bone so closely and so shallowly that a tight plaster cast can silence it, and gives us the most famous drop in medicine.
A young man is brought into the emergency department after a car crash; his knee struck the dashboard, the hip is flexed, adducted and internally rotated, and the leg looks short. The orthopaedic registrar reduces the posterior hip dislocation — and then, before anything else, asks him to pull his foot up. He cannot. Down the corridor, a woman who spent six hours in a coach seat with her legs crossed has woken with a foot that flops when she walks; she lifts the knee unnaturally high so the toes clear the floor, and the foot slaps down. In the plaster room, a teenager's cast is being split open because his toes stopped moving overnight. Three stories, three mechanisms — a dislocation, a crossed leg, a tight cast — and every one of them is the same nerve, hurt in the same place, producing the same sign. Anatomy rarely repeats itself this loudly.
A thumb-thick cord out of the pelvis
The sciatic nerve is the collected output of half the sacral plexus. The sciatic nerve carries the root value L4, L5, S1, S2 and S3 — five spinal levels gathered by the lumbosacral plexus into a single flat band about as wide as a finger and as thick as a thumb. It leaves the pelvis through the greater sciatic foramen, and the single most useful fact about that exit is that it emerges BELOW the piriformis muscle. Piriformis is the great signpost of the gluteal region described in the muscles of the gluteal region: everything above it and everything below it can be named by its relationship to that one muscle belly, and the sciatic nerve is the largest structure on the lower side. From there it runs downwards midway between the ischial tuberosity and the greater trochanter of the femur — a line you can draw on the skin — then descends through the back of the thigh, deep to the hamstrings, resting on the flat posterior surface of adductor magnus like a cable laid on a wall.
Two nerves in one sheath, from the very beginning
The sciatic nerve usually divides at the apex of the popliteal fossa, a hand's breadth or so above the knee crease, into the tibial nerve and the common fibular nerve. But that division is only the moment the wrapping comes off. From the instant the nerve is formed, it contains two anatomically distinct divisions running side by side, each with its own fibres, its own root pattern and its own destiny. You can prove this from its branches in the thigh. The TIBIAL division supplies the hamstrings described in the muscles of the thigh — semitendinosus, semimembranosus and the long head of biceps femoris — plus the hamstring (ischiocondylar) part of adductor magnus. The COMMON FIBULAR division supplies exactly one muscle in the whole thigh: the short head of biceps femoris. That lopsided arrangement is not a curiosity; it is the reason a high sciatic lesion so often looks like a pure common fibular lesion. The fibular fibres are more lateral, more superficial, contain fewer and larger fascicles with less protective connective tissue, and are tethered at the fibular neck — so when the trunk is stretched or compressed, they are damaged first and recover last.
Think of the sciatic nerve as a single armoured motorway cable running from the small of the back to the knee, carrying two separate bundles of wiring inside one black sleeve. The bundle destined for the back of the leg sits deep and cushioned, padded in plenty of connective tissue. The bundle destined for the front sits at the outer edge of the sleeve, packed into fewer, fatter strands with less padding, and further down it is stapled to a bare bone at the surface. Cut the sleeve behind the knee and you finally see them as two cables. But damage the cable anywhere along its shared course — stretch it, squeeze it, stab it — and it is almost always the outer, thinly padded, stapled bundle that fails.
The tibial nerve: down the back, into the sole
The larger of the two branches, and the one that carries you up onto your toes. The tibial nerve (L4–S3) continues the line of the parent trunk straight down through the popliteal fossa, where it is the most superficial of the three main structures — nerve, then vein, then artery, from skin to bone. It then passes under the tendinous arch of soleus into the deep posterior compartment of the leg and runs down on tibialis posterior with the posterior tibial vessels. It supplies the ENTIRE posterior compartment described in the muscles of the leg: the superficial group — gastrocnemius, soleus and plantaris, the great plantarflexors that fire every time you push off — and the deep group — popliteus, flexor digitorum longus, flexor hallucis longus and tibialis posterior, the last of these the muscle that inverts the foot and holds up the medial longitudinal arch. Then it makes its famous turn: it passes BEHIND the medial malleolus, deep to the flexor retinaculum, in the tarsal tunnel, and divides into the medial and lateral plantar nerves, which supply all the intrinsic muscles of the sole and the skin that reads the ground beneath you.
Its sensory territory is the whole back of the leg and the whole sole. In the popliteal fossa it gives the sural nerve, which is joined by a contribution from the common fibular side and runs down beside the small saphenous vein to supply the lower posterolateral leg, the lateral border of the foot and the little toe. Near the ankle its medial calcaneal branches pierce the flexor retinaculum to take the skin of the heel, and the plantar nerves take the rest of the sole. Divide the tibial nerve and the consequences are severe. Plantarflexion is lost, so the patient cannot rise onto the toes or push off in a stride; inversion is lost with tibialis posterior; and the toes claw, because the long extensors are unopposed while the intrinsic muscles of the sole are paralysed. The foot is held dorsiflexed and everted — the calcaneovalgus foot. Worst of all is the sensory loss: an anaesthetic sole cannot report a stone in the shoe or a blister forming, and painless plantar ulceration follows. Numb feet are dangerous feet.
The common fibular nerve: the most injured nerve in the leg
The common fibular nerve (L4–S2) is the smaller branch and by far the more fragile. It follows the medial border of biceps femoris to the head of the fibula, then does the thing that defines it: it winds forwards round the NECK OF THE FIBULA, lying almost subcutaneously against bare bone with nothing over it but skin and fascia — a landmark of the tibia and fibula you can roll under your fingertip on your own leg right now. There, in a space of two or three centimetres, it divides into its two terminal branches. The DEEP FIBULAR nerve turns into the anterior compartment and supplies tibialis anterior, extensor digitorum longus, extensor hallucis longus and fibularis tertius — the dorsiflexors — and continues onto the dorsum of the foot to supply extensor digitorum brevis; its sensory share is famously tiny, a patch of skin in the FIRST WEB SPACE between the great and second toes. The SUPERFICIAL FIBULAR nerve enters the lateral compartment and supplies fibularis longus and brevis — the everters — then pierces the deep fascia in the lower leg to become cutaneous and take most of the dorsum of the foot.
Foot drop: the lower limb's twin of wrist drop. Injure the common fibular nerve at the fibular neck and the anterior and lateral compartments die together. Dorsiflexion is lost, so the foot hangs; eversion is lost, so the foot also inverts at rest. The patient walks with a high-stepping, or steppage, gait — lifting the knee and hip exaggeratedly so the drooping toes clear the ground, then letting the foot slap down — and the sensory loss covers the dorsum of the foot and the lateral leg. The list of causes is a list of everyday pressures: a tight plaster cast or a badly padded splint, prolonged leg crossing, deep squatting (the classic "strawberry-picker's palsy"), a fibular neck fracture or a lateral knee ligament injury, and simply lying unconscious or immobile on a hard surface. It is worth learning to distinguish a deep fibular lesion alone — foot drop with eversion PRESERVED and only that tiny first-web-space patch numb — from a full common fibular lesion, in which eversion is lost as well and the numbness covers the whole dorsum. Higher still, a sciatic or L5 root lesion adds weakness that the fibular nerve cannot explain, such as loss of hip extension or of toe flexion.
Hold the two limbs up against each other and the symmetry is beautiful. In the arm, the radial nerve spirals round the shaft of the humerus, supplies every extensor, and when it fails the wrist drops — the picture set out in the radial, axillary and musculocutaneous nerves. In the leg, the common fibular nerve winds round the neck of the fibula, supplies every dorsiflexor and everter, and when it fails the foot drops. Both nerves are the ones that lift the far end of the limb away from resistance; both are the ones that lie naked against a bone; both are the ones a cast, a crutch, a bad night's sleep or an operating table can silence. Learn one and you have learned the other, and you have learned the surgical rule that follows from both: never pad a limb carelessly over a bare bone.
Sciatica, injections and the clinic
"Sciatica" is the pain that radiates from the buttock down the back of the thigh and leg, and the commonest cause is not damage to the nerve trunk at all — it is compression of a lumbosacral nerve root, usually L5 or S1, by a prolapsed intervertebral disc long before those fibres ever unite into a sciatic nerve. That distinction matters clinically, because a root lesion follows a dermatome and a trunk lesion does not. The straight-leg raise (Lasègue's test) exploits the geometry: lifting the extended leg stretches the nerve and its roots over the head of the femur and reproduces the radiating pain, typically between 30 and 70 degrees. Reflexes localise the level — the ankle jerk is S1, the knee jerk L3–L4. Then there are the injuries we cause ourselves. An intramuscular injection given carelessly into the buttock can spear the nerve; the rule is to inject only into the UPPER OUTER QUADRANT, well away from the nerve's course. Posterior dislocation of the hip drives the femoral head straight back onto the nerve, which is why foot drop must be tested before and after every reduction. And the popliteal fossa is a favourite site for regional blocks, where local anaesthetics can be placed around the tibial and common fibular nerves just before they separate to anaesthetise the whole foot for surgery.
The sural nerve has an unusual second career. Because it is purely sensory, easy to find beside the small saphenous vein behind the lateral malleolus, and supplies a strip of skin whose loss the patient can live with, it is the standard nerve for BIOPSY when a peripheral neuropathy needs a tissue diagnosis — and the standard donor for a nerve GRAFT. A surgeon repairing a severed median nerve in the hand, or a facial nerve after tumour resection, will often harvest 20 or 30 centimetres of sural nerve from the back of the leg and use it to bridge the gap. The patient trades a numb patch on the outer edge of one foot for the return of feeling in a hand or movement in a face. It is one of the quiet bargains of surgery, and it works only because someone first mapped exactly what that nerve does and does not supply.
- Sciatic nerve: L4–S3, the thickest nerve in the body; exits the pelvis through the greater sciatic foramen BELOW piriformis.
- Course: midway between the ischial tuberosity and the greater trochanter, then down the thigh deep to the hamstrings on adductor magnus.
- It is TWO nerves in one sheath from the start; it usually divides at the apex of the popliteal fossa into tibial and common fibular.
- Tibial division: all the hamstrings + the hamstring part of adductor magnus. Common fibular division: ONLY the short head of biceps femoris.
- Sciatica is usually a nerve ROOT problem (prolapsed disc, L5/S1), not damage to the sciatic trunk; the straight-leg raise stretches nerve and roots.
- The nerve is at risk in posterior hip dislocation and in careless gluteal injection — inject only in the upper outer quadrant. Ankle jerk = S1.
- Tibial nerve (L4–S3): most superficial structure in the popliteal fossa, then under the tendinous arch of soleus into the deep posterior compartment.
- It supplies the WHOLE posterior compartment of the leg, then passes behind the medial malleolus in the tarsal tunnel to become the medial and lateral plantar nerves.
- Tibial sensory: sural nerve (lateral foot and little toe), medial calcaneal branches (heel) and the whole sole. Injury → loss of plantarflexion and inversion, clawed toes, calcaneovalgus foot and dangerous plantar ulceration.
- Common fibular nerve (L4–S2): the most commonly injured nerve of the lower limb, because it winds subcutaneously round the NECK OF THE FIBULA.
- Deep fibular = anterior compartment (dorsiflexors) + extensor digitorum brevis; sensory = the first web space ONLY. Superficial fibular = lateral compartment (everters); sensory = most of the dorsum of the foot.
- Common fibular injury → FOOT DROP with steppage gait and loss of eversion — the lower-limb twin of wrist drop. Causes: tight cast, crossed legs, squatting, fibular neck fracture, lying on a hard surface.
- Saying the sciatic nerve divides "at the knee" and leaving it there. It divides at the apex of the popliteal fossa, but functionally it is two separate divisions from the moment it forms — which is why a high lesion can produce a purely fibular picture.
- Treating every foot drop as a common fibular lesion. A deep fibular lesion alone spares eversion and numbs only the first web space; an L5 root lesion adds weakness outside the fibular territory altogether.
- Assuming "sciatica" means the sciatic nerve trunk is damaged. In most patients the lesion is a prolapsed disc compressing an L5 or S1 root in the spine, far above where the sciatic nerve even exists.
A man wakes after sleeping heavily on a hard floor with his leg pressed against a bed frame. He cannot dorsiflex or evert the right foot and walks with a high-stepping gait; sensation is reduced over the dorsum of the foot but the sole is normal. Where is the lesion?
- The sciatic nerve (L4–S3) is the thickest nerve in the body: it leaves the pelvis below piriformis, runs midway between the ischial tuberosity and the greater trochanter, descends deep to the hamstrings on adductor magnus, and divides at the apex of the popliteal fossa.
- It is two nerves in one sheath: the tibial division supplies the hamstrings and the hamstring part of adductor magnus; the common fibular division supplies only the short head of biceps femoris.
- The tibial nerve supplies the entire posterior compartment of the leg and, through the tarsal tunnel and the plantar nerves, the whole sole — injury gives loss of plantarflexion and inversion, clawed toes, a calcaneovalgus foot and a dangerously numb sole.
- The common fibular nerve winds subcutaneously round the neck of the fibula and is the most commonly injured nerve in the lower limb: deep fibular = dorsiflexors + first web space, superficial fibular = everters + dorsum of the foot, and a full lesion gives foot drop with steppage gait.
- Drake RL, Vogl AW, Mitchell AWM. Gray's Anatomy for Students — Lower limb: the sciatic, tibial and fibular nerves.
- Moore KL, Dalley AF, Agur AMR. Clinically Oriented Anatomy — Nerves of the lower limb; sciatic nerve injury and gluteal injections.
- Netter FH. Atlas of Human Anatomy — Nerves of the thigh, leg and foot.
- Last RJ. Last's Anatomy: Regional and Applied — The gluteal region, popliteal fossa and leg.
- Snell RS. Clinical Anatomy by Regions — Common fibular nerve injury and foot drop.
- TeachMeAnatomy — The Sciatic Nerve; The Tibial Nerve; The Common Fibular Nerve.
