Home / Conditions / Peroneal nerve palsy
The nerve that lifts your foot runs just under the skin at the outside of the knee, where almost nothing protects it. Compress it and the foot stops lifting — but the same thing can come from the spine, and the difference decides the treatment.
The common peroneal nerve is a branch of the sciatic nerve. It supplies the muscles that lift the foot and turn it outward, and carries sensation from the outer lower leg and the top of the foot.
Its problem is its route. Just below the knee it wraps around the head of the fibula — the bony bump on the outside of the leg — where it sits directly under the skin with no muscle or fat covering it. Almost nowhere else in the body is a major nerve so exposed.
Anything that presses there for long enough damages it, and the foot stops lifting.
The right-hand view shows why the site matters. The common peroneal nerve crosses the outside of the knee as a single trunk, then divides into two branches lower down. Compression at that crossing affects both branches at once — which is why the foot loses both its lift and its sensation together.
The shaded areas on the left are where sensation is typically lost: the outer lower leg and the top of the foot. That distribution is one of the things an examination checks, because it differs from the pattern produced by an L5 root problem in the spine.
The branches also explain why presentations vary. Where only the deeper branch is affected, foot drop dominates and sensory loss is minimal — a small patch between the first two toes.
Foot drop is the headline sign, and it has several possible causes — this page covers the one arising at the knee.
Notably, pain is often absent or mild. People frequently describe the problem as clumsiness rather than as an injury, which is one reason it goes unassessed for weeks.
The causes are unglamorous, and most are avoidable once you know them:
Foot drop is a symptom, not a diagnosis. The nerve that lifts the foot can be interrupted at the knee — peroneal nerve palsy — or much higher, where the L5 nerve root leaves the spine, usually from a slipped disc.
The foot behaves the same way in both. What differs:
This is the single most important distinction on this page. One is treated at the knee, the other in the spine — and an examination plus nerve studies separates them reliably.
The examination does most of the work: which movements are weak, where sensation has changed, and whether tapping over the fibular head reproduces the tingling. From there:
Nerve studies are most informative after two to three weeks. Performed too early they can look normal even when the nerve is significantly affected, which occasionally leads to false reassurance.
Most cases caused by compression recover once the pressure is removed. The job is protecting the foot while that happens, and identifying the minority that will not recover on their own.
Stopping leg crossing, changing how you sit or kneel at work, padding the outside of the knee, adjusting a cast or brace. Simple, and for compression palsies it is the treatment rather than a preliminary to one.
A light brace that holds the foot at the right angle so you can walk normally and stop tripping. It does not treat the nerve, but it prevents falls and stops the ankle stiffening while the nerve recovers — and stiffness is a complication worth avoiding.
Maintaining ankle movement, keeping the calf from tightening, and strengthening as the nerve returns. Gait retraining matters too — people adopt compensations that persist after the foot works again.
Considered where the palsy is not recovering, where nerve studies show the fibres themselves are damaged rather than simply blocked, or where something structural is pressing on the nerve.
Decompression releases the nerve at the fibular head. Where a cyst or mass is responsible, removing it treats the cause directly. Where the problem is an L5 root instead, the surgery is in the spine, not the knee — which is why the distinction above matters.
This condition turns on one question — is the problem at the knee or in the spine — and that question happens to sit exactly where a neurosurgeon works.
A knee surgeon examines the knee. A spine surgeon examines the spine. The trouble with foot drop is that you do not know which one you need until someone has looked at both. Dr Tung assesses the nerve along its whole course, from the L5 root where it leaves the spine to the fibular head where it wraps the knee.
Nerve conduction studies and EMG do two things: locate where conduction is blocked, and show whether the nerve fibres are damaged or merely compressed. The first answers knee versus spine. The second determines whether this recovers on its own or needs operating on — and both readings shape what happens next.
Where an L5 root is the cause, the treatment is spinal — and decompression, foraminotomy and discectomy are among his listed procedures. There is no onward referral and no second opinion to arrange.
Most compression palsies recover, so watching is often right — but only for a while. Judging when a nerve has stopped recovering and intervention is warranted is the call that determines whether the foot ends up working. It is a judgement made better by someone who performs the operation than by someone who refers for it.
It depends on what the nerve has actually suffered, which is what nerve studies establish.
Delay costs more here than in most conditions. A nerve compressed for a short period usually recovers completely; one compressed for months may not. If your foot is not lifting, that is a reason to be assessed now rather than to wait and see.
Most compression cases do, often completely, once the pressure is removed. How long depends on whether the nerve is blocked or the fibres are damaged — nerve studies distinguish these, and that determines the outlook.
Both produce foot drop, and they are treated in entirely different places. Examination plus nerve conduction studies separates them reliably. Back or leg pain points towards the spine; a painless foot drop more often points to the nerve at the knee.
It can, particularly if you sit that way for long periods, have lost weight recently, or have diabetes. It is one of the commonest identifiable causes, and it is straightforward to stop.
It is usually worth it while the nerve recovers. It stops you tripping, which matters, and it prevents the ankle stiffening — stiffness can outlast the nerve problem and cause difficulty of its own.
Do not wait. A nerve compressed briefly usually recovers fully; one compressed for months may not. Early assessment also establishes the cause while it is still reversible.
No. Bring any nerve conduction studies or scans, and note when you first noticed it, what you were doing beforehand, and whether there is any back or leg pain.
Foot drop · Neuropathic pain · Sciatica · Nerve pain in the leg · Slipped disc · Foot pain · Decompression surgery
A foot that will not lift is worth assessing now rather than waiting. One appointment establishes whether the problem is at the knee or in the spine — and early compression usually recovers, while months of it may not.
