Two Nerves That Only Carry Movement. When They Fail, Nothing Feels Wrong.
Most nerve problems announce themselves with pins and needles.
But these two nerves…don’t.
The anterior and posterior interosseous nerves are branches that run deep into the forearm carrying almost nothing but instructions to muscles. They supply no skin worth speaking of.
So when one stops working, sensation stays completely normal and something in the hand simply stops doing its job. People notice they can’t do a task, not that anything hurts or tingles, and that’s exactly why these get missed or mistaken for a torn tendon.
The anterior interosseous nerve: the OK sign test
This one is a branch of the median nerve. It runs the muscle that bends the tip of your thumb and the muscle that bends the tip of your index finger.
Try to make an OK sign: thumb tip to index tip, a proper round circle – if the nerve isn’t working you just can’t. The tips won’t bend, so instead of a circle you get a flat triangle, with both digits pinching straight and meeting pad to pad.
Compare with your other hand. It’s one of the cleanest bedside tests in the whole upper limb, and it takes two seconds.
A circle means the nerve is working. A triangle means it isn’t.
The story that changes everything
Here’s the part I most want people to know, because it changes the treatment completely.
A large share of these are not a nerve being squeezed at all.
They’re neuralgic amyotrophy, sometimes called Parsonage-Turner syndrome: an inflammatory condition, thought to be immune-driven, that attacks nerves in the arm.
It has a distinctive story. Severe, deep, relentless pain in the shoulder or upper arm, often waking you, lasting days to a couple of weeks. Then the pain fades, and as it fades weakness appears.
That sequence, bad pain first and weakness afterwards, is the fingerprint. It often follows a viral illness, a vaccination, or surgery elsewhere.
It matters because there’s nothing to release.
Operating on a compression that isn’t there helps nobody. Most cases recover over many months to a couple of years, and the job in the meantime is keeping the joints moving and the hand working.
The posterior interosseous nerve: fingers that won’t lift
This one is a branch of the radial nerve, running through the same radial tunnel that causes forearm pain when it’s merely irritated. When it stops working altogether, you get weakness instead.
Lay your forearm on a table, palm down, hand off the edge. Try to lift your fingers at the knuckles.
In a posterior interosseous palsy they just cant come up. The thumb won’t lift out either.
The wrist is the giveaway. Unlike a radial nerve injury higher in the arm, the wrist still extends here, because the muscle that does most of that work is supplied above the branch point. But it drifts off to the thumb side as it lifts, because the muscle on the other side has gone quiet. A wrist that lifts but wanders is a very specific finding.
And again: no numbness anywhere.
Nerve or tendon? One test separates them
A finger that won’t straighten could be a dead nerve or a snapped tendon, and they need completely different surgeons on completely different timelines.
Let your wrist flop fully forwards, palm-side down, and relax the hand entirely. Healthy tendons are pulled tight by that position and the fingers will straighten out on their own, without you doing anything.
- Fingers straighten passively: the tendons are intact, so the problem is the nerve.
- One or two fingers stay curled: those tendons are ruptured.
Worth knowing that in someone with rheumatoid arthritis, tendon rupture is the far more likely answer, and it tends to arrive one finger at a time starting with the little finger.
Finding the cause
For both nerves, the question is always the same: is something pressing on it, or is it inflamed?
Nerve conduction studies confirm which nerve and how badly. Imaging, usually ultrasound or MRI, looks for a cause: a lipoma or ganglion sitting on the nerve, joint synovitis in inflammatory arthritis, a fracture or a plate, or scarring after a previous injury.
A found lesion gets removed. A compression that fits gets decompressed. An inflammatory picture with the classic painful prodrome gets watched, supported and rehabilitated.
Recovery, and the hand therapy in the middle of it
Recovery is slow because nerve recovery is slow: roughly a millimetre a day of regrowth, so several months before muscles that are some distance away start responding.
The whole risk during that wait is that your hand seizes up while you’re waiting for it to come back, and then the nerve recovers into a stiff hand. That’s a genuinely avoidable bad outcome and it’s the main reason to be in therapy through this.
- Splinting to hold the affected joints in a working position and stop them settling into deformity. For a posterior interosseous palsy, a splint that lifts the knuckles lets you use the hand while you wait.
- Full passive range daily, every joint, every day, whether or not you can move it yourself.
- Task adaptation, so the hand stays in use rather than being carried around.
- Re-education the moment flickers of movement return, which is when progress accelerates.
Expect months for recovery. It wont be fast.
If there’s genuinely no recovery by around 12 to 18 months, tendon transfers can restore the lost movement by rerouting a working muscle to do the missing job, and those work well.
The short version
Weakness with completely normal sensation is the signature. Make an OK sign, and try lifting your fingers at the knuckles with your forearm flat.
If either fails, that needs assessing properly, and mention any episode of severe shoulder or arm pain in the weeks beforehand, because that single detail can change the entire plan.
Written by Nigel Chua, hand occupational therapist in practice since 2005. Tens of thousands of hands, wrists and elbows assessed and treated. My credentials are here. This article explains; it doesn’t examine. New weakness in a hand needs assessing now, not in a few weeks.