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Two Nerves That Only Carry Movement. When They Fail, Nothing Feels Wrong.
If you suddenly can’t make a round OK sign, or can’t lift your fingers at the knuckles even though your wrist still lifts, and nothing is numb, think AIN or PIN palsy. These are movement-only nerve branches in the forearm. Get assessed within days. Many are caused by nerve inflammation rather than squeezing, and recover over 6 to 18 months, sometimes longer. Your job meanwhile: keep every joint moving so the hand is ready when the nerve comes back.
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.
And a miss costs you twice. Treated as a tendon, you can end up with the wrong operation. Left alone, the joints quietly stiffen during the months the nerve needs, and the nerve recovers into a hand that can’t use it.
What are the AIN and PIN?
The anterior interosseous nerve (AIN) branches off the median nerve just below the elbow, on the palm side of the forearm. The posterior interosseous nerve (PIN) branches off the radial nerve around the same level, on the back of the forearm. Both run deep, close to the bone, and both are almost pure motor nerves. See the radial tunnel article for where the PIN gets irritated when it’s merely grumbling rather than failing.
AIN palsy: can’t make an OK sign
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.
What you’ll notice day to day: writing gets awkward, buttons and coins slip, and picking up small things feels oddly clumsy even though the fingers feel perfectly normal.
In children, the AIN is the nerve most often affected by a supracondylar elbow fracture, and most of those recover on their own over weeks to months.
A circle means the nerve is working. A triangle means it isn’t.
Parsonage-Turner syndrome: 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.
PIN palsy: 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 can’t 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.
Finger drop, not wrist drop. 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 forward, 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.
How is AIN or PIN palsy diagnosed?
For both nerves, the question is always the same: is something pressing on it, or is it inflamed?
- Examination first. The OK sign, the finger lift, the wrist drift, the tendon test above, and a normal sensory check. That pattern alone narrows it right down.
- Nerve studies. Nerve conduction and EMG confirm which nerve and how badly. They’re often done around 3 weeks after onset, because the changes take that long to show, and repeated later to look for early recovery.
- Imaging. Usually ultrasound or MRI, looking 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.
Where are you in the window?
Two clocks run at once here, and the Closing Window Method names both: the nerve’s, and your joints’.
- OPEN (first few months): the nerve has time. The thing at risk right now is joint stiffness, and that part is fully in your hands.
- CLOSING (around 3 to 6 months with no sign of recovery): if there’s nothing on examination or repeat nerve studies, this is when surgeons start discussing whether to look at the nerve directly. Muscle waiting without a nerve doesn’t wait forever.
- SHUT for the nerve, not the hand (around 12 to 18 months with no recovery): the goal changes from waiting for the nerve to rerouting a working muscle. Tendon transfers restore the lost movement, and those work well.
How long does AIN or PIN palsy take to recover?
Recovery is slow because nerve recovery is slow: roughly a millimeter a day of regrowth, so several months before muscles that are some distance away start responding. Typical shape, not a promise:
- Weeks 0 to 3 (OPEN): assessment, the hunt for a cause, splint if needed, full passive range every day.
- Months 1 to 3 (OPEN): little visible change. Normal, and the most frustrating stretch. Keep the joints moving.
- Months 3 to 6 (OPEN to CLOSING): first flickers often appear. If none, and studies show none, talk to your surgeon about next steps.
- Months 6 to 18 and beyond: strength builds. Inflammatory cases can keep improving for up to two years or more.
- 12 to 18 months with nothing: tendon transfer territory.
Expect months for recovery. It won’t be fast. (Think less Fast and Furious, more slow-cooker.)
Hand therapy while you wait: splints and exercises
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, worn during the day for activity and off for exercises and skin checks. See the wrist drop splint for the radial nerve family of splints.
- 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. Thicker pens, key turners, larger buttons.
- Re-education the moment flickers of movement return, which is when progress accelerates.
The exercises, specifically:
- AIN, passive tip bends. Support the thumb just below the tip joint and bend the tip fully with your other hand. Hold 5 seconds. 10 reps, 3 times a day. Same for the index fingertip, supporting the middle joint.
- PIN, passive finger straightening. Lay the hand flat on a table and gently press the fingers and thumb straight. Hold 5 seconds. 10 reps, 3 times a day.
- Once a flicker appears, place and hold. Put the joint into position with the other hand, then try to hold it there yourself for a few seconds. 10 reps, several times a day. Stop before the muscle tires out, cos a recovering muscle fatigues fast.
Can I work and drive with AIN or PIN palsy?
- Desk work: usually yes. AIN makes writing and fine pinch awkward, so thicker pens help. PIN makes typing harder because the fingers can’t lift off the keys, and the knuckle splint helps.
- Manual work: depends on what the hand has to do. Keep it in use. A therapist can work out what’s safe and what needs adapting.
- Driving: you need to grip and release the wheel quickly and safely. Many people with an isolated AIN palsy can. With a PIN palsy, check with your doctor, and your insurer, before driving.
- Gym: keep training. Avoid loading the weak muscles to fatigue while they’re recovering.
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.
Pain somewhere else in the forearm? Forearm pain by location sorts it by spot. Tingling in the thumb side of the hand with a forearm ache is a different median nerve problem: pronator teres syndrome.
Get it checked today if
- You have sudden weakness in the hand or arm with a drooping face, slurred speech or a weak leg. Call emergency services. That’s a stroke until proven otherwise.
- The weakness started after an injury, fracture, elbow dislocation, or surgery near the elbow. In a child after an elbow fracture, a cold, pale or very painful hand means the emergency department now.
- Severe arm pain comes with fever, or the weakness is spreading to more muscles.
- You have rheumatoid arthritis and a finger has suddenly dropped. See a hand surgeon within days, because the next tendon tends to follow.
Get it checked if
- You have new weakness with normal feeling, even if nothing hurts. Within days, not weeks.
- There’s no sign of recovery by around 3 to 6 months.
- Joints are stiffening while you wait.
- You can feel a lump in the forearm.
Nothing tingles, so nobody hurries. Hurry anyway, then keep the joints moving while the nerve takes its time.
Written by Nigel Chua, hand occupational therapist in practice since 2005. 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.