Mallet Thumb: Pathophysiology, Diagnosis, and Management

✋ What is Mallet Thumb?

Mallet thumb is the terminal extensor tendon injury of the thumb at the interphalangeal (IP) joint. It’s analogous to the more common mallet finger, but involves the thumb, a digit with unique biomechanics and functional importance.

🧠 Pathophysiology

Injury occurs from:

Axial load or hyperflexion to the distal thumb (e.g., striking the ground while gripping an object)
Results in:
- Tendinous rupture of the extensor pollicis longus (EPL)
- Or an avulsion fracture of the dorsal base of the distal phalanx

🔍 Clinical Presentation

📋 Symptoms:

Inability to actively extend the IP joint of the thumb
Drooping posture of the distal phalanx
Swelling, pain, and tenderness at the dorsal aspect of the thumb

📏 Physical Exam:

IP joint flexed at rest
No active extension
Check for instability or subluxation

🧪 Imaging

X-ray (Lateral View):
- Look for small avulsion fragment or large fracture
- Assess for volar subluxation of the distal phalanx
MRI may be helpful if X-ray is inconclusive but rarely required acutely

🩺 Management Strategy

🟢 Non-Surgical (First-line)

Indication: No fracture or small avulsion fracture with stable joint

Splinting:
- Use thumb spica splint or modified stack splint
- Position: Full extension of IP joint
- Duration:
  - 6–8 weeks full-time wear (no flexion allowed)
  - 2–4 weeks night splint thereafter
Key Rule: Any flexion restarts the clock on immobilization

🔴 Surgical (Selective)

Indications:

Fracture >30–40% of joint surface
Volar subluxation or joint instability
Failed conservative treatment

Surgical Options:

Open reduction and internal fixation (ORIF)
Suture repair of tendon with anchor
Temporary K-wire fixation across joint

📚 Prognosis

Excellent outcomes with early diagnosis and adherence to splint protocol
Delayed or incorrect treatment ➜ chronic deformity, extension lag, or arthritis

🔑 Clinical Pearls

Don’t confuse with mallet finger—IP joint in thumb is unique and demands different splinting.
Always check for joint stability—an unstable joint needs surgical consult.
Reassess at 1–2 weeks to ensure no secondary displacement if fracture is present.