What is the likely diagnosis for a fifth digit with a puncture injury at the distal interphalangeal joint five months ago that now cannot be extended?

Chronic Mallet Finger (Extensor Tendon Rupture)

The most likely diagnosis is a chronic mallet finger deformity resulting from extensor tendon disruption at the distal interphalangeal (DIP) joint, which occurred at the time of the penetrating wood injury 5 months ago and was left untreated.

Clinical Reasoning

The presentation is classic for mallet finger: inability to actively extend the DIP joint following trauma to the fingertip. The mechanism—a pointed wood puncture at the DIP joint—likely caused either:

• Direct laceration/rupture of the terminal extensor tendon, or
• Bony avulsion fracture at the extensor tendon insertion on the distal phalanx

The 5-month delay means this is now a chronic mallet finger, which has significantly worse outcomes than acute treatment 1, 2.

Immediate Diagnostic Steps

Obtain 3-view radiographs of the affected finger immediately 3. The imaging must include:

• Posteroanterior (PA) view
• True lateral view
• Oblique view

These radiographs will differentiate between:

• Tendinous mallet (no bony fragment visible)
• Bony mallet (avulsion fracture at the base of the distal phalanx) 3, 2

The radiographs are also critical to assess for:

• Retained foreign body (wood fragment)
• Joint subluxation or volar displacement of the distal phalanx
• Size of any bony fragment (if >1/3 of articular surface, surgery is typically required) 3
• DIP joint arthritis (common complication of chronic untreated mallet finger) 2

Treatment Implications Based on Chronicity

At 5 months post-injury, conservative splinting alone will likely fail 2. The literature indicates:

• Acute mallet fingers (treated within 2-4 weeks) have good outcomes with 6-8 weeks of continuous DIP joint splinting in slight hyperextension 4, 5, 1
• Chronic mallet fingers often require surgical intervention due to tendon contracture, joint stiffness, and potential swan-neck deformity development 1, 2

Referral Decision

Refer to a hand surgeon urgently for the following reasons:

1. Chronicity: 5 months is well beyond the window for simple conservative management
2. Penetrating mechanism: Risk of retained foreign body, infection, or joint involvement 3
3. Potential complications: Chronic mallet finger leads to swan-neck deformity and DIP joint osteoarthritis if left untreated 2
4. Surgical options needed: May require tendon reconstruction, pinning, or correction of secondary deformities 1, 2

Critical Pitfalls to Avoid

• Do not attempt splinting alone at this stage—the tendon has been contracted for 5 months and simple immobilization will not restore function 2
• Do not miss a retained wood foreign body—if radiographs are negative but clinical suspicion remains high, obtain CT without contrast (wood can be difficult to visualize on plain films) 3
• Do not delay referral—further delay worsens the prognosis for functional recovery and increases the likelihood of permanent disability 2

Expected Surgical Approach

The hand surgeon will likely consider 1, 2:

• Open reduction and tendon reconstruction if tendinous injury
• Kirschner wire fixation if bony avulsion with joint subluxation
• Possible correction of any developing swan-neck deformity
• Arthrodesis (joint fusion) if severe arthritis has developed

The key message: This is a surgical problem now, not a splinting problem, due to the 5-month delay in diagnosis.