Lower limb
Introduced| English | Meaning |
|---|---|
| femoral nerve | The nerve to the front thigh muscles, including quadriceps, which extend the knee. |
| obturator nerve | The nerve to the medial thigh muscles that pull the leg inward (adductors). |
| sciatic nerve | The largest nerve, supplying the back of the thigh and everything below the knee. |
| common peroneal nerve | common peroneal nerve |
| compartment syndrome | Dangerous pressure rise inside a closed fascial compartment that cuts off blood supply. |
| foot drop | foot drop |
A lifetime of carrying
- The leg carries the body for a lifetime. Its muscles sit in closed sleeves, its nerves pass landmarks you can name, and its arteries run from groin to toe.
- Each sleeve has its own nerve, its own job and its own emergency.
The three thigh nerves
- The femoral nerve 股神经 runs in front and straightens the knee. The obturator nerve 闭孔神经 supplies the inner thigh, which pulls the legs together.
- The sciatic nerve 坐骨神经 runs behind and splits above the knee into the tibial nerve and the common peroneal nerve 腓总神经.
Match each nerve to the movement it powers.
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Front, inner and back of the thigh each have their own nerve, so the movement that fails names the nerve.
Four sleeves in the leg
- The leg below the knee has four fascial compartments. Anterior lifts the foot (dorsiflexion); lateral turns the sole outwards (eversion).
- Superficial posterior points the foot down (plantarflexion); deep posterior turns the sole inwards (inversion) and bends the toes.
Which leg compartment turns the sole of the foot outwards (eversion)?
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The lateral compartment (peroneal muscles, superficial peroneal nerve) everts the foot.
When a sleeve becomes a trap
- Fascia does not stretch. If muscle swells inside it, the pressure rises, and above about 30 mmHg the small vessels stop flowing: compartment syndrome 骨筋膜室综合征.
- The sign is pain far worse than expected, especially when the muscle is passively stretched. Pulses usually remain, because arterial pressure is much higher. Treatment is to open the sleeve.
A patient's diastolic blood pressure is 70 mmHg and the anterior compartment pressure is 45 mmHg. What is the difference (ΔP) in mmHg?
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70 − 45 = 25 mmHg. A ΔP below about 30 mmHg means the muscle is not being perfused well: a sign that the compartment needs opening.
If the foot pulses can be felt, compartment syndrome is ruled out.
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Arterial pressure is far above the pressure that stops capillary flow, so pulses usually stay until very late.
Worked example: the nerve at the fibular neck
- The common peroneal nerve wraps around the neck of the fibula, just under the skin. Crossed legs or a tight cast can press it there.
- Result: foot drop 足下垂 and weak eversion, but inversion is kept, because the tibial nerve still works. That pattern places the lesion at the fibular neck.
A patient has foot drop after a tight cast below the knee, but can still turn the sole inwards. Where is the lesion?
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Foot drop and weak eversion with inversion kept (tibial nerve intact) point to the common peroneal nerve where it wraps the fibular neck.
Worked example: the narrowed artery
- (Fictional patient) Calf pain after 200 m of walking, relieved by standing still for a few minutes. An artery higher up is narrowed, so the calf runs out of blood when it works.
- Examine four pulses from top to bottom: femoral, popliteal, posterior tibial, dorsalis pedis. Where they disappear tells you where the narrowing is.
Put the leg pulses in order from the groin to the foot.
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Checking them in order finds the level where blood flow is lost.
Three common slips
- Dermatome map for nerve map. In the limbs, the skin patch of a root and of a nerve differ.
- Mixing up eversion and inversion. Eversion is superficial peroneal; inversion is mainly tibial. The pair locates the lesion.
- "A pulse rules out compartment syndrome." It does not; the key sign is pain on passive stretch.
Recap
- Femoral (front), obturator (inner), sciatic (back, splitting into tibial and common peroneal).
- Four leg compartments, each with a job and a nerve; tight fascia makes swelling dangerous.
- Common peroneal palsy: foot drop with inversion kept. Four named pulses map the arteries.