Orthopaedic Surgery - Posterior Interosseous Nerve Entrapment

Basics

Posterior interosseous nerve syndrome (PINS) is a compressive neuropathy involving the:

Posterior interosseous nerve (PIN)

which is the predominantly motor branch of the radial nerve supplying many of the finger and thumb extensor muscles.

The syndrome produces:

Motor weakness without sensory loss.

The related condition known as:

Radial tunnel syndrome (RTS)

is considered separately.

RTS is characterized primarily by:

Pain over the proximal lateral forearm

without the motor weakness typical of PINS.

The diagnosis of radial tunnel syndrome remains somewhat controversial because objective electrodiagnostic abnormalities are often absent.

Anatomy

The posterior interosseous nerve arises from the:

Deep branch of the radial nerve.

It passes through the proximal forearm and enters the:

Supinator muscle.

During this course, it may be compressed at several potential anatomical sites.

Arcade of Frohse

The most common site of compression is the:

Fibrous proximal edge of the superficial head of the supinator

known as the:

Arcade of Frohse.

Other Potential Compression Sites

Additional sites include:

Fibrous bands near the radiocapitellar joint

Fibrous tissue over the radial head

Radial recurrent vessels, known as the leash of Henry

Medial or fibrous edge of the extensor carpi radialis brevis

Distal margin of the supinator

Compression may also result from:

Ganglion cyst

Lipoma

Tumor

or another space-occupying lesion.

Posterior Interosseous Nerve Syndrome Versus Radial Tunnel Syndrome

The distinction is important.

Posterior interosseous nerve syndrome primarily produces:

Motor weakness.

Radial tunnel syndrome primarily produces:

Pain without objective motor weakness.

Neither usually produces cutaneous sensory loss because the superficial sensory branch of the radial nerve travels separately.

Epidemiology

PINS is uncommon.

It most frequently occurs in individuals between approximately:

20 and 50 years of age.

The true incidence is uncertain and may be underestimated because mild cases can be missed or attributed to other causes.

Risk Factors

Risk may increase with repetitive activities involving:

Forearm supination

Powerful gripping

Repetitive wrist or finger extension

Occupational and athletic repetitive-loading activities may therefore contribute.

Genetics

There is no known Mendelian inheritance pattern.

Etiology

The most common mechanism is:

Mechanical compression of the posterior interosseous nerve.

Compression may occur spontaneously or secondary to:

Repetitive mechanical stress

Inflammatory tissue

Fibrous bands

Vascular structures

Mass lesions

Associated Conditions

PINS or radial tunnel symptoms may coexist with:

Lateral epicondylitis

Radial tunnel syndrome

Rheumatoid arthritis

Systemic lupus erythematosus

Inflammatory synovitis around the proximal radioulnar region may increase local nerve compression.

Diagnosis

Diagnosis is based primarily on:

History

Motor examination

Anatomic localization

and, when needed,

Electrodiagnostic testing or imaging.

Signs and Symptoms

The hallmark of PINS is:

Weakness of finger and thumb extension without sensory loss.

Onset

Weakness may develop:

Spontaneously

without an obvious traumatic event.

Finger Extension Weakness

Patients may have difficulty extending the:

MCP joints of the fingers.

Because the interphalangeal joints can still extend through intrinsic muscle function, the deficit may be most apparent at the:

MCP joints.

Thumb Extension Weakness

Weakness may affect:

Thumb extension

and sometimes:

Thumb abduction through the extensor mechanism.

Wrist Extension

Wrist extension is usually preserved because:

Extensor carpi radialis longus

is innervated proximal to the PIN.

However, the wrist may extend with:

Radial deviation

because the ulnar-sided wrist extensor, extensor carpi ulnaris, is PIN-innervated and may be weak.

Pain

Prominent aching or radiating pain is more suggestive of:

Radial tunnel syndrome

than pure PINS.

Sensory Symptoms

Numbness and cutaneous sensory loss are usually:

Absent

in both PINS and radial tunnel syndrome.

The superficial radial sensory nerve does not travel through the same deep motor tunnel.

Physical Examination

A complete neurovascular examination of the upper limb should be performed.

Motor Examination

Assess PIN-innervated muscle function, particularly:

Finger MCP extension

Thumb extension

Thumb retropulsion

Wrist extension pattern

Weakness in these functions supports:

Posterior interosseous neuropathy.

Wrist Extension Pattern

Preserved wrist extension with:

Radial deviation

is characteristic because extensor carpi radialis longus remains functional while more distal PIN-innervated extensors are impaired.

Sensory Examination

Sensation over the:

Forearm

Dorsal radial hand

and

Digits

should remain normal in isolated PINS.

Sensory loss suggests another diagnosis or a more proximal radial nerve lesion.

Tenodesis Effect

Passive wrist flexion normally causes finger extension through the:

Tenodesis effect.

Preservation of passive tenodesis despite inability to actively extend the fingers supports a:

Neurologic deficit

rather than rupture of the extensor tendons.

Radial Tunnel Tenderness

In radial tunnel syndrome, maximal tenderness is usually located approximately:

3–5 cm distal to the lateral epicondyle.

This is more distal than the tenderness typically seen with:

Lateral epicondylitis.

Resisted Supination

Pain reproduced by:

Resisted forearm supination

may support radial tunnel syndrome.

Resisted Middle-Finger Extension

Pain produced by resisting extension of the:

Middle finger

has historically been described as a provocative maneuver for radial tunnel syndrome.

However, this test is not highly specific and should not be interpreted in isolation.

Lateral Epicondylitis Comparison

Lateral epicondylitis typically produces maximal tenderness:

Directly over or just distal to the lateral epicondyle

rather than farther down the radial tunnel.

Diagnostic Injection

A local anesthetic injection into the radial tunnel may occasionally help distinguish:

Radial tunnel pain

from other sources such as:

Lateral epicondylitis.

Interpretation should be cautious because local anesthetic can spread to adjacent structures.

Laboratory Tests

Routine laboratory studies are not helpful for isolated PINS.

Tests may be obtained only when an associated inflammatory or systemic disorder is suspected.

Imaging

Plain Radiographs

Radiographs are not routinely required.

They may be useful when there is concern for:

Fracture

Arthritis

Bony deformity

or another osseous source of compression.

MRI

MRI may be useful when there is suspicion of:

Ganglion cyst

Tumor

Lipoma

Synovitis

or another space-occupying lesion.

It may also demonstrate denervation changes in affected muscles.

Ultrasound

High-resolution ultrasound may also identify:

Nerve enlargement

Compression

Ganglion cyst

or other superficial masses.

Dynamic examination can occasionally assist with localization.

Electrodiagnostic Studies

Electromyography and nerve-conduction studies are useful for:

Confirming PINS

Localizing the lesion

Estimating severity

Assessing denervation

They can also help exclude:

Cervical radiculopathy

Brachial plexopathy

More proximal radial neuropathy

Radial Tunnel Syndrome and EMG

Electrodiagnostic studies are often:

Normal or nonspecific

in radial tunnel syndrome because the condition is primarily a pain syndrome without major axonal injury.

Pathological Findings

At surgical exploration, the PIN may show compression most commonly at:

The arcade of Frohse.

Nerve Appearance

Chronic compression may produce:

Flattening

Focal constriction

Swelling proximal to the compression

or a:

Pseudoneuroma.

Differential Diagnosis

Important alternatives include:

Lateral epicondylitis

Cervical radiculopathy

Neuralgic amyotrophy

Extensor tendon rupture

Superficial radial sensory nerve entrapment

More proximal radial nerve palsy

Lateral Epicondylitis

Lateral epicondylitis typically causes:

Pain directly over the lateral epicondyle

with pain during resisted:

Wrist extension

but does not cause PIN-pattern motor weakness.

Cervical Radiculopathy

Cervical radiculopathy may cause:

Neck pain

Radiating arm pain

Sensory abnormalities

Weakness involving several muscle groups

The distribution depends on the involved cervical nerve root.

Neuralgic Amyotrophy

Neuralgic amyotrophy, or:

Parsonage–Turner syndrome

may mimic a focal motor neuropathy.

It often begins with:

Severe acute shoulder or arm pain

followed days or weeks later by:

Weakness and muscle wasting.

Superficial Radial Nerve Entrapment

Entrapment of the superficial radial sensory nerve is known as:

Wartenberg syndrome

or

Cheiralgia paresthetica.

It produces:

Pain

Burning

Numbness

over the dorsoradial aspect of the hand without motor weakness.

Treatment

General Principles

Initial management is usually:

Nonoperative

unless there is:

Progressive motor weakness

A compressive mass

or another structural lesion requiring treatment.

Activity Modification

Repetitive activities involving:

Supination

Forceful gripping

Repetitive forearm rotation

should be reduced when they reproduce symptoms.

Rest

Temporary avoidance of the suspected causative activity may permit recovery in mild compressive neuropathy.

Splinting

If no clear cause is identified or symptoms are aggravated by motion, temporary immobilization using an:

Elbow or forearm splint

may be considered.

Prolonged immobilization should be avoided because it can lead to stiffness.

Physical and Occupational Therapy

Therapy should begin as soon as it is safe after the acute symptoms settle.

An experienced:

Hand therapist

can assist with:

Range of motion

Nerve protection

Gradual strengthening

Activity modification

Ergonomic changes

Strengthening

Progressive strengthening should begin only after:

Pain is controlled

and

Nerve recovery is evident.

Medication

NSAIDs may be used for:

Short-term pain and inflammation control, particularly when radial tunnel pain or surrounding inflammation is present.

They do not reverse established motor denervation.

Surgical Indications

Surgical decompression should be considered when there is:

Progressive motor weakness

A definite structural compressive lesion

Failure of appropriate conservative treatment

or

No evidence of spontaneous neurologic recovery.

Timing of Surgery

Older protocols suggested considering surgery when there was no spontaneous recovery by approximately:

3 months

after symptom onset despite at least:

6 weeks of conservative management.

Modern timing is individualized according to:

Severity

Electrodiagnostic findings

Rate of progression

Presence of a mass lesion

Surgical Decompression

Surgery consists of exploration and decompression of the:

Posterior interosseous nerve

along its course through the proximal forearm.

Structures Released

Potential constricting structures include:

Arcade of Frohse

Fibrous bands around the radial head

Leash of Henry

Extensor carpi radialis brevis edge

Supinator muscle

Arcade of Frohse Release

The most important component is often release of the:

Fibrous proximal supinator edge.

Vascular Compression

Radial recurrent vessels of the:

Leash of Henry

may be ligated or mobilized if they are clearly compressing the nerve.

Severe Nerve Injury

Rarely, severe focal nerve damage may require:

Excision of a nonviable segment

followed by:

Direct repair

or

Nerve grafting.

Tendon Transfer

If nerve recovery fails and useful motor function cannot be restored, late reconstruction may require:

Tendon transfer procedures

to restore:

Finger extension

Thumb extension

and other lost functions.

Radial Tunnel Syndrome Surgery

Surgical decompression for radial tunnel syndrome remains more controversial than surgery for objective PINS.

Published success rates have varied widely, historically from approximately:

40–95%.

Patient selection is therefore important.

Follow-Up

After conservative or operative treatment, activity is resumed:

Gradually.

Repetitive gripping and supination should be limited initially and increased only as symptoms permit.

Prognosis

The prognosis after decompression for true PINS is generally:

Good.

Motor recovery can be slow because axonal regeneration occurs gradually.

Time to Recovery

Recovery may continue for:

Many months

and in severe cases may take:

Several years.

Older series reported continued improvement for approximately:

2–4 years.

Nonoperative Prognosis

Patients with mild compressive neuropathy may recover with:

Activity modification

Splinting

Therapy

and observation.

Radial Tunnel Syndrome Prognosis

The outcome of radial tunnel syndrome is less predictable because:

Diagnostic criteria remain controversial

and objective nerve dysfunction is usually absent.

There are no strong comparative data establishing superiority of surgery over nonoperative treatment in all cases.

Complications

Recurrence

Symptoms may recur after either:

Conservative treatment

or

Surgical decompression.

Iatrogenic Nerve Injury

Direct injury to the PIN during decompression is:

Rare

but potentially serious.

It may worsen:

Finger and thumb extension weakness.

Persistent Weakness

Incomplete nerve recovery may result in:

Residual motor deficit

Muscle atrophy

Functional impairment

Scar and Adhesion

Postoperative scar formation may occasionally contribute to:

Recurrent compression

or tethering of the nerve.

Patient Monitoring

Patients should be followed periodically, often at approximately:

3-month intervals

until neurologic recovery stabilizes.

Monitoring should include:

Finger extension strength

Thumb extension strength

Wrist extension pattern

Muscle bulk

Pain

Electrodiagnostic recovery when indicated

Key Principle

Posterior interosseous nerve syndrome is a motor neuropathy caused by compression of the deep radial nerve, most commonly at the arcade of Frohse.

The characteristic presentation is:

Weakness of finger and thumb extension with preserved sensation and often preserved wrist extension in radial deviation.

This should be distinguished from:

Radial tunnel syndrome, which primarily causes lateral proximal forearm pain without objective motor weakness.

Most mild cases begin with:

Activity modification and rehabilitation, whereas progressive motor deficit, structural compression, or failure of recovery may require:

Surgical decompression of the posterior interosseous nerve.