Posterior interosseous nerve test: Anatomy, technique, and interpretation

Key takeaways

The posterior interosseous nerve test assesses the deep motor branch of the radial nerve, checking for finger and thumb extension weakness without wrist drop.

The arcade of Frohse, the fibrous proximal edge of the supinator, is the most common site of PIN entrapment. It produces motor deficits with no sensory loss.

A positive test shows weak finger and thumb extension at the MCP joints, with wrist extension that deviates radially. Wrist drop is absent because ECRL is spared.

Practice management software like Pabau keeps MRC grades, Tinel’s findings, and investigation results in one patient timeline, so follow-up visits compare like with like.

What is the posterior interosseous nerve test?

The posterior interosseous nerve test is a clinical examination sequence that detects motor dysfunction of the posterior interosseous nerve (PIN).

The PIN is the deep motor branch of the radial nerve. A positive test reveals finger and thumb extension weakness at the metacarpophalangeal (MCP) joints, with preserved but radially deviated wrist extension. Because the PIN carries no sensory fibers, sensory function stays intact throughout.

That combination is what makes the test worth doing properly. PIN palsy looks like a hand problem, and a vague note makes the deficit disappear. The sequence below is written to be repeatable and gradable. It covers the anatomy that explains the findings and the four examination steps in order. It then sets out how to read the result against its differentials, and which investigations confirm it.

Anatomy: Nerve course and the arcade of Frohse

The radial nerve divides at the lateral epicondyle into a superficial sensory branch and the posterior interosseous nerve. The PIN descends into the radial tunnel, passing through or beneath the arcade of Frohse, the fibrous proximal arch of the supinator muscle. From there it enters the posterior forearm compartment.

One anatomical detail explains the whole clinical picture. The extensor carpi radialis longus (ECRL) receives its innervation proximal to the arcade of Frohse, before the PIN enters the supinator. When the PIN is compressed at or below the arcade, ECRL remains intact.

So wrist extension is still possible in PIN palsy. The wrist deviates radially because extensor carpi ulnaris (ECU) is weak while ECRL pulls unopposed. The map below shows where that dividing line falls.

According to Orthobullets’ anatomical reference, the arcade of Frohse is the most commonly cited site of PIN entrapment. Repetitive pronation and supination loads that arch, which is why the presentation clusters in manual workers and racquet sports.

Muscles innervated by the posterior interosseous nerve

Knowing which muscles the PIN supplies is the foundation of every posterior interosseous nerve test. ECRL is innervated above the arcade and is therefore always spared in PIN palsy. All other posterior forearm extensors are PIN territory.

ECRB is the row worth reading twice. Its innervation varies between individuals, so it may sit above or below the arcade in any given patient. That variation is one reason wrist extension power grades inconsistently across cases that are otherwise identical.

Causes of posterior interosseous nerve entrapment

PIN entrapment can be traumatic, compressive, or inflammatory. Knowing the cause guides both the examination focus and the investigation choice.

• Arcade of Frohse compression: the most common cause. Fibrous tissue at the proximal supinator edge compresses the PIN, often in repetitive pronation-supination activities.
• Space-occupying lesions: lipomas and ganglia at the radial tunnel are well-recognized causes. Ultrasound and MRI are the investigations of choice for these.
• Radial head fracture or dislocation: direct trauma disrupts the PIN at or near its entry into the supinator. This is common in Monteggia fracture-dislocation patterns.
• Rheumatoid synovitis: inflammatory synovium at the radial head can progressively compress the PIN.
• Iatrogenic injury: surgical approaches to the radial head or proximal radius carry PIN risk.
• Idiopathic: some cases, particularly in manual workers, have no identifiable structural cause.

Clinical presentation of PIN syndrome

PIN syndrome presents as a pure motor neuropathy. That alone separates it from most other peripheral nerve lesions at the bedside.

• Finger extension weakness: reduced power or inability to extend the fingers at the MCP joints. The patient may hold the fingers in slight flexion at rest.
• Thumb extension weakness: weakness of EPL at the IP joint and EPB at the MCP joint, so the thumb drops.
• Radially deviated wrist extension: ECRL is intact and ECU is weak, so the wrist extends but deviates radially. This is pathognomonic of PIN palsy.
• No wrist drop: the hallmark of PIN syndrome. Complete wrist drop indicates a higher radial nerve lesion, not PIN palsy.
• No sensory deficit: the PIN is purely motor. Sensory loss points to a more proximal radial nerve injury or a different diagnosis.
• Lateral forearm pain: present in some cases, particularly at the radial tunnel. Pain without motor deficit suggests radial tunnel syndrome instead.

How to perform the posterior interosseous nerve test: Step-by-step

The test runs as a four-part examination sequence rather than a single maneuver. Each step isolates a different component of PIN function, so perform them in order and grade using the MRC scale (0-5) per muscle. Our printable MRC scale template gives you a one-page grading sheet to keep beside the plinth.

Step 1: Resisted finger extension at the MCP joints

Position the patient with the elbow slightly flexed, the forearm pronated, and the wrist in neutral. Stabilize the wrist to remove its contribution. Apply downward pressure over the proximal phalanges of fingers 2-5 while asking the patient to extend at the MCP joints.

Positive finding: weakness or inability to hold MCP extension against minimal resistance. Document each finger individually where possible, since partial PIN lesions may spare some fascicles. Extensor digitorum communis (EDC) and extensor indicis proprius (EIP) are the primary targets here.

Step 2: Resisted thumb extension

Ask the patient to extend the thumb at both the IP joint (EPL) and the MCP joint (EPB) against resistance. Stabilize the first metacarpal to isolate the movement.

Positive finding: weakness at the IP joint in particular indicates EPL involvement. Consider de Quervain tenosynovitis as a differential for localized first-ray symptoms. The Finkelstein test separates the two, because de Quervain produces pain rather than a true extension deficit.

Step 3: Wrist extension deviation assessment

Ask the patient to extend the wrist actively without stabilization. Observe the direction of deviation before applying any resistance. In PIN palsy the wrist extends but deviates radially. Then apply resistance and grade wrist extension power overall.

Positive finding: radial deviation on active extension, with ECU weak and ECRL intact. If the wrist drops entirely on attempted extension, the lesion sits at or above the spiral groove of the humerus rather than at the PIN.

Radial deviation is easy to miss by eye. A described bedside refinement is the window test. The examiner rests a hand along the ulnar border of the patient’s pronated forearm during active wrist extension. A visible window opening between the examiner’s forearm and the patient’s hand marks the radial drift.

Step 4: Tinel’s sign and resisted supination provocation

Percuss over the radial tunnel, roughly 3-4 cm distal to the lateral epicondyle, along the course of the PIN through the supinator. A positive Tinel’s sign produces tingling or shooting pain into the dorsal forearm. Tinel’s sensitivity for PIN entrapment is limited, so a negative sign does not exclude the diagnosis.

Follow with resisted supination. Ask the patient to supinate the forearm against resistance with the elbow flexed to 90 degrees. Reproduction of lateral forearm pain or weakness on this maneuver supports radial tunnel involvement.

Interpreting results and differential diagnosis

A clear positive test shows four findings together.

• Weak finger extension at the MCP joints
• Weak thumb extension at the IP and MCP joints
• Wrist extension that deviates radially rather than dropping
• No sensory loss in the radial nerve territory

The most important differential to exclude is radial tunnel syndrome. The table below separates the three patterns that present around the radial tunnel.

Two further differentials sit outside the forearm. C7 radiculopathy produces sensory symptoms and may involve triceps weakness, and Spurling’s test helps separate a cervical root cause from a local one. Lateral epicondylitis is pain-dominant and produces no motor deficit at all.

Anterior interosseous nerve syndrome is the closest mimic in name only. It is a purely motor palsy of the volar forearm compartment rather than the extensor compartment. The pinch grip test is the bedside screen for it.

Investigations: Electrodiagnostics and imaging

Clinical examination narrows the diagnosis substantially. Electrodiagnostic testing and imaging then confirm it and localize the lesion.

Electromyography (EMG): the gold standard for confirming PIN palsy and excluding radial tunnel syndrome. According to a peer-reviewed electrodiagnostic evaluation of posterior interosseous neuropathy, the key muscles sampled are EDC, EIP, ECU, and APL.

Denervation potentials and reduced MUAP recruitment in those muscles confirm a PIN lesion below the arcade of Frohse, provided ECRL and brachioradialis are normal. Radial tunnel syndrome by definition produces a normal EMG, which is what makes the study decisive between the two.

Nerve conduction studies (NCS): motor latency to EDC via the PIN can be prolonged or absent. Standard superficial radial sensory studies stay normal throughout, consistent with a pure motor neuropathy.

Ultrasound: dynamic imaging at the arcade of Frohse can show nerve enlargement and hypoechogenicity. It is particularly useful for detecting ganglia and lipomas as compressive causes.

MRI: reveals denervation edema and fatty infiltration in PIN-innervated extensor muscles. MRI is the most sensitive modality for identifying space-occupying lesions and quantifying muscle involvement, as confirmed by AJR sonography and MRI research on PIN syndrome.

Documenting PIN test findings in clinical practice

Reference sources describe the examination well and stop at the finding. The note is where a PIN assessment either supports continuity of care or leaves a medico-legal hole, so it is worth specifying. A structured PIN test note should capture four elements.

1. MRC grading per muscle: record a 0-5 MRC score for EDC, EIP, ECU, APL, EPL, and EPB individually. Do not write “finger extension: weak.” Grade and name each muscle, because that is the only format that allows a meaningful comparison at follow-up.
2. Wrist deviation: note the direction and degree of deviation on active extension. “Radial deviation, approximately 15 degrees on active wrist extension” is a usable measurement. “Wrist deviates” is not.
3. Tinel’s response: document the percussion site as a distance from the lateral epicondyle, then the response elicited. Record tingling into the dorsal forearm, pain only, or no response, and name the finger territory involved.
4. Investigation plan: note whether EMG or NCS has been requested, by whom, and the clinical question posed. For imaging, document the modality and the specific question, such as excluding a mass lesion at the radial tunnel.

Recording those four elements the same way at every visit is what makes a follow-up comparison meaningful. Software for physical therapists that carries a reusable assessment template removes the variation between clinicians and between appointments.

Treatment and management of PIN entrapment

Management depends on the cause, the severity, and the duration of symptoms. The evidence base favors a stepped approach, as set out in clinical consensus sources including Physiopedia’s PIN syndrome reference.

Conservative management (first-line, 3-6 months):

• Activity modification to reduce repetitive pronation and supination
• Resting splint holding the wrist in neutral and the MCP joints in slight extension
• Corticosteroid injection at the radial tunnel, particularly where inflammatory synovitis is suspected
• Nerve mobilization techniques and progressive strengthening of the extensor compartment as recovery allows

Surgical decompression: indicated when an adequate conservative trial has failed, when imaging identifies a space-occupying lesion, or when neurology is worsening quickly. Decompression at the arcade of Frohse is the standard approach.

Prognosis is generally good when surgery happens within 12 months of symptom onset. Recovery timelines vary by case, so frame them for the patient as reported ranges rather than as a guarantee.

How Pabau keeps PIN examination findings comparable over time

In most practices the PIN test lands in a free-text note. One clinician writes “finger extension weak”, the next writes MRC grades for three of the six muscles, and the third records only the Tinel’s response. Six weeks later nobody can say whether the patient improved.

Practice management software like Pabau replaces that free text with a form you design once. Build the four-step sequence as fixed fields, with a 0-5 grade per muscle, a deviation measurement, a Tinel’s site and response, and the investigation requested. The form attaches to the appointment and stays on the patient timeline.

The outcome is a set of visits you can read side by side. Grades line up against each other, and the imaging request sits next to the finding that prompted it. Any clinician picking up the case sees the same structure. Every Pabau subscription includes the full clinical records toolset, so this is a setup task rather than an upgrade.

Structured clinical documentation for nerve examinations

Pabau’s digital forms and client records let musculoskeletal and physiotherapy practices capture MRC grading, Tinel’s findings, and investigation results in one structured patient timeline. See it in action.

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Conclusion

PIN palsy is quiet. There is no wrist drop and no numbness to prompt a referral. The deficit sits at the fingers and thumb until someone tests for it deliberately. The four-step sequence takes under five minutes and needs no equipment.

The trade-off worth remembering is that this examination is only as useful as the record it leaves. A grade per muscle turns a subjective impression into a number you can retest in six weeks. A vague note means starting the assessment again from scratch.

Build the sequence into your assessment template once and it runs the same way for every clinician who uses it. Book a demo to see how Pabau keeps MRC grades, Tinel’s responses, and investigation requests together on one patient timeline.

Continue your research

Screening a hand for more than one nerve? Hand nerve tests walks through the median, ulnar, and radial screens you can run in the same appointment.

Need to grade strength consistently? Manual muscle testing covers positioning, resistance, and how to apply the MRC scale without drifting between clinicians.

Suspect a neural rather than muscular cause? Upper limb tension tests gives you a printable sequence for testing neural mechanosensitivity in the arm.

Ruling out a compression at the wrist? Phalen’s test explains the median nerve screen for carpal tunnel syndrome and how to interpret a positive result.

Reviewing the software behind your notes? Physiotherapy clinic management software compares the platforms that handle clinical records and scheduling together.

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