Numbness, Zaps and Nerve Pain After a Knee Replacement

Almost everybody who has a knee replacement experiences something odd in the way the leg feels afterwards, and almost nobody is warned about it in enough detail beforehand. The sensations are strange, they arrive without explanation, and on a bad night at three in the morning they are easy to interpret as something having gone wrong.

Most of them are ordinary. A few are not. This page covers which is which.

The numb patch

If you run a hand down the outer side of your scar, the skin will probably feel dulled, distant, or as though there is a layer between your hand and the leg. That area may extend a hand’s width outwards and some distance below the knee.

This happens in essentially every knee replacement performed through a front incision, and it is unavoidable rather than a complication. Small nerves supplying sensation to the skin cross the front of the knee from the inner side towards the outer. The incision runs straight down through their path. They are divided, and the skin they supplied goes numb.

The nerve involved is usually a branch of the saphenous nerve — the infrapatellar branch. It carries sensation only. It does not supply any muscle, which is why the numbness has no effect at all on how the knee bends, straightens or takes your weight.

What happens over time: the patch usually shrinks over the first one to two years, as neighbouring nerves gradually extend their territory into the gap. A smaller area often remains permanently numb. Most people stop noticing it within the first year.

The two practical consequences:

• Be careful with heat. Hot water bottles, heat packs and hot showers on skin that cannot feel properly are a genuine burn risk. Check the temperature with your other hand or your other leg
• Kneeling feels strange on that patch — not painful so much as unpleasant and oddly disconnected. This is part of why kneeling is often uncomfortable after a knee replacement even when nothing is wrong

The zaps, zings and electric shocks

This is the one patients most often describe with a slightly apologetic laugh, as though it is too odd to be a real symptom. It is entirely real and extremely common.

Divided nerves do not simply stop. The cut ends attempt to regrow, and regenerating nerve fibres fire spontaneously — without anything touching the leg, without movement, without warning. What you feel is a sharp electrical jolt, a zing, a shooting sensation, or a sudden stab lasting a fraction of a second.

They tend to be:

• Around or just below the scar, occasionally travelling down the shin
• Worse at rest than during activity, and often worst at night, when there is nothing else competing for your attention
• Unpredictable, which is much of what makes them unsettling
• Accompanied by other odd sensations — pins and needles, crawling, itching deep under the skin, a burning line along the scar, or the scar feeling hypersensitive to a bedsheet

They are a sign of nerves recovering, not of nerves being injured. They generally become most noticeable somewhere in the first few months and then fade, though occasional zaps a year on are not unusual.

Ordinary pain relief works poorly on them, because they are not that kind of pain. If they are genuinely interfering with sleep rather than simply being irritating, it is worth raising — nerve-type pain responds to different medication, and that is a conversation to have with your GP or at a review rather than something to endure silently for months.

Pain behind the knee at night

A distinct and very common complaint, and usually not a nerve problem at all despite feeling like one.

Two things account for most of it. If the knee had a fixed bend before surgery, the hamstrings, capsule and ligaments behind it have been short for years. The joint now straightens; those tissues are being stretched back out, and that is uncomfortable — particularly at night, when the leg has been still and then moves.

The second is swelling inside the joint pressing on the capsule at the back, which is why it is often worse on the evening of a day you did more than usual.

What tends to help: ice before bed rather than after the pain starts, a pillow under the calf rather than under the knee, keeping up the regular pain relief through the evening rather than letting it lapse overnight, and shifting position before the ache builds rather than after.

It is worth mentioning if it is persisting well beyond the early months or getting worse rather than better.

When it is coming from your back

This one causes real confusion, and it is worth knowing about.

Having had a knee replacement does not protect you from the ordinary problems of a lower back, and a period of reduced activity, altered walking and sitting awkwardly is exactly the kind of thing that stirs one up. Nerve irritation in the lumbar spine refers pain, pins and needles or numbness down the leg — and understandably, a patient who has just had knee surgery attributes anything happening in that leg to the knee.

Clues that point towards the back rather than the knee:

• Symptoms below the knee — the calf, the shin, the ankle, and particularly the sole of the foot, which the knee incision has nothing to do with
• A pattern that runs in a stripe or band down the leg rather than spreading around the knee
• Changes with spinal position — worse sitting, worse bending forward, easier lying flat, worse after a long car trip
• Back or buttock pain alongside it
• Symptoms that were there before the operation, even mildly

None of this is something to work out alone, but noticing the pattern and describing it accurately makes the assessment much faster. “It goes down the back of my calf into the sole of my foot and it is worse when I sit” is a far more useful sentence than “my leg hurts”.

The symptoms that need to be seen

Most of what is on this page settles on its own. These do not belong on that list.

On foot drop specifically. Weakness lifting the front of the foot comes from the peroneal nerve, which wraps around the outer side of the knee just below the joint and is the most exposed nerve in the leg. It is an uncommon problem after knee replacement, and it is somewhat more likely where a significantly bowed or long-standing stiff knee has been straightened at surgery — correcting the alignment stretches a nerve that has been sitting short for years. It usually recovers, and recovery is helped considerably by recognising it early.

Pain that simply does not settle

It should be said plainly, because a page like this that only lists reassuring explanations is not an honest one.

A minority of people have ongoing pain after a technically sound knee replacement, and nerve-type pain is part of that picture. Sometimes it is a nerve that has not recovered as expected, sometimes it is the pain system itself becoming over-sensitised after months or years of an arthritic knee, and sometimes no single cause is identifiable.

If you are several months out and the pain is not following the downward trend described on the recovery timeline, that is worth a proper conversation rather than waiting to see. There are approaches that help — medication aimed specifically at nerve pain, targeted rehabilitation, and referral onwards where appropriate — and they work better when started earlier.

Being told a knee looks fine on an X-ray does not mean the pain is not real. It means the answer is somewhere other than the X-ray.

If something about the way your leg feels is worrying you, phone the rooms on (08) 6332 6365. Phoning about something that turns out to be ordinary is always the right call. Appointments can also be made through the contact page.