Courtesy: Prof Nabil Ebraheim, University of Toledo, Ohio, USA

 

Common Conditions Affecting Thumb Function

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Overview

The thumb plays a critical role in:

• Pinch
• Grip
• Fine motor function

Various conditions can impair thumb function, leading to:

• Pain
• Weakness
• Deformity
• Loss of dexterity

Early recognition is essential for accurate diagnosis and optimal management.

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1. Trigger Thumb (Stenosing Tenosynovitis)

Definition

• A form of stenosing tenosynovitis involving the:
  • Flexor pollicis longus (FPL) tendon
  • A1 pulley

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Pathophysiology

• Thickening of:
  • Flexor tendon
  • A1 pulley

Leads to:

• Impaired tendon gliding
• Formation of a tendon nodule

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Clinical Features

• Pain at the base of the thumb
• Clicking or triggering
• Locking of the thumb in flexion

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Key Mechanism

• Tendon nodule gets caught beneath the A1 pulley during extension

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Examination Finding

• Tender palpable nodule over:
  • A1 pulley at MCP joint

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2. Extensor Pollicis Longus (EPL) Tendon Rupture

Function of EPL

• Extends:
  • Interphalangeal (IP) joint of thumb
• Assists in:
  • Thumb extension
  • Retropulsion

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Common Causes

• Distal radius fractures:
  • Especially minimally displaced fractures
• Delayed rupture due to:
  • Tendon attrition
  • Ischemia

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Clinical Features

• Inability to extend thumb IP joint
• Loss of thumb retropulsion
• Often presents weeks after fracture

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3. Posterior Interosseous Nerve (PIN) Entrapment

Anatomy

• Motor branch of radial nerve
• Passes through:
  • Supinator muscle
  • Arcade of Frohse

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Causes

• Compression at supinator
• Trauma:
  • Radial head fractures
  • Elbow fracture-dislocations

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Clinical Features

• Pure motor deficit (no sensory loss)
• Inability to extend:
  • Fingers
  • Thumb

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Important Feature

• Wrist extension preserved
• May show radial deviation

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4. Proximal Radial Nerve Injury

Cause

• Injury proximal to elbow

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Clinical Features

• Loss of:
  • Wrist extension
  • Finger extension
  • Thumb extension

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Characteristic Finding

• Wrist drop

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5. Anterior Interosseous Nerve (AIN) Syndrome

Anatomy

• Motor branch of median nerve
• Arises distal to elbow

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Muscles Supplied

• Flexor pollicis longus
• Flexor digitorum profundus (index ± middle finger)
• Pronator quadratus

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Clinical Features

• Weakness of:
  • Thumb flexion
  • DIP flexion of index finger

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Characteristic Sign

“OK Sign” Test

• Patient unable to form a round “OK”
• Produces a pinch deformity

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6. Ulnar Nerve Compression at Elbow (Cubital Tunnel Syndrome)

Definition

• Compression of ulnar nerve at elbow

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Muscles Affected

• Intrinsic hand muscles:
  • Interossei
  • Adductor pollicis

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Clinical Features

• Weakness of:
  • Finger abduction
  • Finger adduction
• Inability to cross fingers

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Froment’s Sign

Mechanism

• Weak adductor pollicis (ulnar nerve)

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Test

• Patient holds paper between thumb and index finger

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Positive Sign

• Thumb IP flexion due to compensation by:
  • Flexor pollicis longus (median nerve)

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7. Rheumatoid Arthritis of the Thumb

Pathology

• Chronic inflammatory synovitis leading to:
  • Pain
  • Swelling
  • Joint instability
  • Progressive deformity

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Functional Impact

• Weakness of:
  • Grip
  • Pinch
  • Grasp

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Common Deformity

Thumb Boutonnière Deformity

• MCP joint flexion
• IP joint hyperextension

Leads to significant functional impairment

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Key Summary Points

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Importance of Thumb

• Essential for:
  • Pinch
  • Grip
  • Hand dexterity

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Common Conditions

• Trigger thumb
• EPL tendon rupture
• PIN entrapment
• AIN syndrome
• Cubital tunnel syndrome
• Rheumatoid arthritis

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Clinical Insight

• Early diagnosis is crucial to:
  • Prevent deformity
  • Preserve hand function
  • Improve outcomes