Anterior interosseous nerve test: How to perform and interpret it

Found our content helpful?

Key takeaways

The anterior interosseous nerve test, also called the OK sign or pinch grip test, checks three forearm muscles for motor weakness.

A positive result is a triangular pinch instead of a round circle, with no numbness or tingling anywhere in the hand.

EMG and nerve conduction studies confirm the diagnosis, and they are ordered no earlier than three weeks after symptoms begin.

Conservative care comes first, most patients recover within 3 to 12 months, and surgery is considered only after six months without improvement.

Structured clinical forms and outcome tracking keep findings comparable between visits, so a recovery plateau is easy to spot.

The anterior interosseous nerve test is a bedside exam for motor weakness in the anterior interosseous nerve (AIN).

The AIN is a purely motor branch of the median nerve, so weakness shows up with no numbness at all. You ask the patient to form an OK sign with the thumb and index finger.

A triangular or lateral pinch instead of a round circle is a positive result. The full sequence runs to six steps, needs no equipment, and takes about a minute. A positive test tells you which muscles are weak, not why. Electrodiagnostic studies answer that second question.

What the anterior interosseous nerve test assesses

The test evaluates motor integrity of the AIN. Failure to form a perfect circle points to weakness in the three muscles the AIN exclusively innervates. Because the AIN carries no sensory fibers, patients present with isolated motor weakness and nothing else.

That absence of sensory involvement is the fastest way to separate AIN palsy from carpal tunnel syndrome and pronator syndrome. Both of those compress the median nerve higher up, and both produce numbness the patient will mention unprompted.

The test goes by several names in practice, including the OK sign test, the pinch grip test, and the circle sign. The syndrome itself is sometimes called Kiloh-Nevin syndrome. Knowing which muscles are at stake makes the deformity easier to read when the findings are subtle.

Anatomy of the anterior interosseous nerve

The AIN arises from the median nerve roughly 5 to 8 cm distal to the lateral epicondyle, in the proximal forearm. From there it runs along the interosseous membrane between the radius and ulna. It travels deep to the flexor digitorum superficialis and alongside the flexor digitorum profundus.

The nerve terminates in the pronator quadratus, which it innervates distally. Three muscles receive their sole motor supply from the AIN. The table below maps each one to the joint it controls and to the finding you expect when that muscle is paralyzed.

How to perform the anterior interosseous nerve test (pinch grip or OK sign)

The pinch grip test requires no equipment and takes about 60 seconds. Follow these six steps in sequence for a reliable result.

1. Position the patient. Seat the patient with the elbow flexed to about 90 degrees. The forearm rests in a neutral, mid-prone position, supported on the exam table or an armrest.
2. Demonstrate the target shape. Show the patient a perfect OK circle. Press the pad of your own thumb against the pad of your index finger to form a round loop.
3. Instruct the patient. Ask them to copy the circle as precisely as they can, matching pad to pad.
4. Observe the shape formed. A healthy AIN produces a round circle. AIN weakness produces a triangular or lateral pinch instead. The thumb IP joint stays extended and the index DIP fails to flex, so the tips meet rather than the pads.
5. Apply gentle distraction force. Try to pull the finger and thumb apart. A patient compensating with the FDP of the ring or middle finger can pass the visual test but give way under load.
6. Record the finding. Note whether the result is normal or positive, and whether the weakness is partial or complete. Describe the contact point as pad to pad, tip to tip, or tip to nail.

How to perform the pronator quadratus isolation test

This test targets the third AIN-innervated muscle directly. Flex the elbow fully, to roughly 130 degrees, which mechanically shortens the pronator teres so it cannot contribute to pronation. Then resist the patient’s attempt to pronate from that position.

Compare that result against pronation strength with the elbow closer to extension. Weakness only in the flexed position isolates the pronator quadratus from the pronator teres. It also confirms AIN involvement at the distal forearm level.

How to interpret the results

A positive anterior interosseous nerve test means the patient cannot form a round circle. Instead they produce a pinch deformity, contacting tip to tip or tip to nail rather than pad to pad. That pattern indicates weakness of the flexor pollicis longus and the FDP to the index finger.

Partial and complete palsy produce different pictures, and the distinction is worth making. In complete palsy all three AIN-innervated muscles are affected. The patient fails the pinch test, cannot flex the index DIP against resistance, and shows pronator quadratus weakness on the isolation test.

In partial palsy only one or two muscles are weak. The most common partial pattern spares the pronator quadratus and affects only the FPL and FDP. That produces a positive pinch test alongside normal pronation strength.

Two limitations apply. Patients with generalized joint hypermobility can appear to form a normal circle through hyperextension rather than through FPL and FDP strength. The test also cannot separate compressive AIN neuropathy, Parsonage-Turner syndrome involving the AIN fascicle, and incomplete median nerve injury.

Clinical presentation of AIN syndrome

AIN syndrome presents as painless weakness of pinch grip, usually developing over days to weeks. The classic complaint is difficulty pinching small objects, opening jars, or writing. Some patients report a prodromal ache in the proximal forearm, particularly in cases linked to Parsonage-Turner syndrome, but frank pain is unusual.

There is no numbness, no tingling, and no sensory change in the hand, forearm, or fingers. That is the feature separating AIN syndrome from pronator syndrome and carpal tunnel syndrome. Clinicians in sports medicine practices will recognize it in throwing athletes, gymnasts, and manual workers doing repetitive forearm pronation.

Common causes and etiology

AIN compression occurs at the proximal forearm, where several structures can impinge the nerve. The most frequently implicated are:

• Fibrous bands from the pronator teres or flexor digitorum superficialis
• Aberrant muscle slips of the flexor digitorum superficialis or palmaris longus
• Pronator teres hypertrophy in repetitive forearm use
• Supracondylar fractures of the humerus, a recognized cause in pediatric patients, with incidence figures that vary by study
• Parsonage-Turner syndrome (neuralgic amyotrophy), which can selectively involve the AIN fascicle without external compression
• Penetrating trauma or forearm hematoma from venipuncture or catheter placement

Idiopathic AIN syndrome, where no compressing structure is identified, remains the most common presentation in practice.

Differential diagnosis: Distinguishing AIN syndrome from similar conditions

A positive test narrows the differential considerably, but the picture still has to be weighed against the conditions below. The table compares AIN syndrome with its most common alternatives, using sensory findings and the OK sign as the discriminating criteria.

Confirming the diagnosis with electrodiagnostic studies and imaging

Electromyography (EMG) and nerve conduction studies (NCS) are the gold standard for confirming AIN syndrome. Both StatPearls and the American Association of Neuromuscular and Electrodiagnostic Medicine treat electrodiagnosis as the confirmatory step. The workup serves three functions.

• It confirms denervation in the AIN-innervated muscles
• It rules out a more proximal lesion of the median nerve
• It establishes a baseline for monitoring recovery

Key findings include fibrillation potentials and positive sharp waves in the FPL and the FDP to the index finger on needle EMG. Normal sensory nerve action potentials confirm the purely motor distribution. Motor conduction to the pronator quadratus may show reduced amplitude or slowing.

MRI is not first line, but it earns its place when the clinical picture is atypical. Denervation edema on STIR sequences in the FPL, FDP and pronator quadratus correlates with acute or subacute AIN injury. It also helps separate Parsonage-Turner syndrome from compressive neuropathy, since the former shows a patchy, multifocal pattern.

Treatment and management of AIN syndrome

Management depends on the likely cause, the severity of the weakness, and how long symptoms have been present. Conservative care suits most cases and should be the default first approach.

• Activity modification: reduce or stop activities needing sustained forearm pronation or repetitive elbow flexion and extension under load
• Splinting: a volar forearm splint in slight supination reduces tension on the AIN, worn at night or during aggravating activities
• Anti-inflammatory medication: NSAIDs, or a short course of oral corticosteroids in the acute phase if Parsonage-Turner is suspected
• Watchful waiting: in idiopathic cases, six months of observation before surgical intervention is considered
• Rehabilitation: structured nerve gliding exercises and progressive strengthening, as detailed below

Most patients are referred on to physical therapy, so the practice taking that referral needs a record it can compare visit to visit. Purpose-built physiotherapy clinic management software keeps those measures in one place rather than scattered across visit notes.

Baseline outcome measures belong in the record before any exercise program starts, so progress is tracked against numbers rather than recall. State licensing rules shape what else has to be documented, and they vary widely. Our guide to physical therapy clinic requirements covers Arizona’s version in full.

Surgical decompression (neurolysis) is considered when there is no clinical or electrodiagnostic improvement after six months of conservative management. It is also considered when imaging identifies a discrete compressing structure. Outcomes are generally good for compressive causes and less predictable for Parsonage-Turner cases, which is why separating the two matters before operating.

Laid out on a single timeline, the decision points sit further apart than they read on the page.

Rehabilitation exercises for AIN syndrome

Peripheral motor nerve recovery follows reinnervation rather than the calendar, so the weeks below are a guide and not a schedule. Deliver the program under qualified physical therapist supervision and adapt it to the patient in front of you. A staged return to running protocol applies the same logic to a different injury.

1. FPL tendon gliding (weeks 2 to 6): gentle active-assisted flexion of the thumb IP joint through full range. Guide the thumb with the other hand to avoid substitution. Ten repetitions, three to four times a day.
2. FDP tendon gliding for the index finger (weeks 2 to 6): isolated DIP flexion with the PIP held in extension. Progress from passive assist to active movement as reinnervation occurs. Ten repetitions, three times a day.
3. Pronation re-education (weeks 4 to 8): resisted pronation with the elbow moving gradually from 130 degrees toward neutral. This recruits the pronator teres as pronator quadratus strength returns. Use a forearm weight or a resistance band.
4. Pinch strengthening (weeks 6 to 12): tip pinch and lateral pinch against a foam ball or putty, adding resistance as strength recovers. Watch for compensation, and use pad-to-pad pinch quality as the marker.
5. Functional reintegration (weeks 10 to 16): task-specific work matching the patient’s job or sport, such as racquet grip, instrument handling, or precision object manipulation.

Prognosis and recovery expectations

Spontaneous recovery occurs in a meaningful proportion of cases, particularly those attributed to Parsonage-Turner syndrome or idiopathic compression. Recovery timelines run from 3 to 12 months with conservative management. Individual variation is considerable, and some patients take longer than a year.

Published series report outcomes that vary by cause. Compressive cases treated surgically show good to excellent results in the majority. Parsonage-Turner cases recover less predictably, and often incompletely, whichever treatment is chosen.

Several factors shape recovery speed. Younger patients, a shorter delay before treatment, and partial rather than complete palsy all correlate with faster and more complete recovery. Repeating the same measures at set intervals is what shows a plateau early enough to act on.

Clinical pearls and common pitfalls at the bedside

The maneuver is quick, but it has several interpretive traps that a hurried exam walks straight into. The table below collects the ones worth checking every time.

How practice management software supports AIN assessment documentation

Recording this exam the same way at every visit is harder than it sounds in a busy physical therapy or sports medicine practice. The deformity pattern, the PQ isolation result, grip strength, and the EMG referral date tend to land in four different places in a free-text note. Six months later, nobody can say whether the pinch has improved.

Practice management software like Pabau replaces that with a structured nerve assessment template. Each field is captured the same way at every appointment, so the second visit is directly comparable with the first. Test results, imaging referrals and treatment notes sit in one longitudinal record, which is what physical therapy EMR software is for.

The practical payoff is timing. When the numbers stop moving, the team can see it in the record and action a surgical referral at month six instead of month nine. Date-stamped outcome records at each visit also stand up to medicolegal scrutiny for patients with peripheral nerve conditions. You can book a demo to see how that workflow fits a nerve assessment practice.

Keep every nerve assessment in one record

Pabau lets physical therapy and sports medicine practices build structured clinical templates. Every outcome measure and nerve assessment finding stays in one patient record.

Book a demo

Conclusion

AIN syndrome is underdiagnosed because it does not look like median nerve pathology. There is no numbness, no night pain, and no tingling to anchor the diagnosis. What the patient has is a hand that will not pinch, and a test that takes a minute to confirm it.

So run the OK sign on any painless pinch weakness, and add the pronator quadratus isolation test before you call the exam normal. Then commit to the timeline: EMG after three weeks, conservative care first, and a surgical conversation at month six if nothing has moved. The trade-off is patience against nerve recovery, and the record is what tells you which one is winning.

That record only works if every visit is written down the same way. Book a demo to see how Pabau keeps nerve assessment findings comparable from the first visit to discharge.

Continue your research

Building out your upper limb screening? The Wright test covers the neurovascular exam for thoracic outlet compression, a differential worth ruling out alongside AIN palsy.

Testing cranial nerves too? The hypoglossal nerve test walks through another purely motor nerve exam, including what a positive finding rules in and out.

Assessing the shoulder in the same visit? The infraspinatus test explains how to isolate external rotation weakness and grade the result.

Running a sports medicine practice? Sports medicine software compared reviews the systems that handle assessment workflows, recalls, and outcome tracking.

Keeping on top of compliance? Compliance for physical therapy practices sets out the documentation standards that apply to multi-visit musculoskeletal care.

Frequently asked questions