If a person cannot fully extend their finger after a pop, which structure is likely torn?

Extensor Tendon Rupture (Likely Central Slip)

When someone cannot fully extend their finger after hearing or feeling a "pop," the most likely structure torn is the extensor tendon, specifically the central slip of the extensor mechanism at the proximal interphalangeal (PIP) joint. This injury, if missed or untreated, will progress to a boutonnière deformity with PIP joint flexion contracture and compensatory DIP hyperextension 1.

Clinical Presentation and Mechanism

The "pop" followed by inability to extend suggests an acute disruption of the extensor apparatus. The injury typically occurs from:

• Forced flexion of an extended finger
• Direct trauma to the dorsum of the PIP joint
• Tangential forces at the extensor hood with forced ulnar deviation 2

Key diagnostic finding: Loss of active extension at the PIP joint with the tenodesis effect absent—when you passively flex the wrist, the injured finger fails to extend at the PIP joint if the extensor tendon is completely disrupted 3.

Differential Diagnosis by Location

Most Common: Central Slip Rupture (Zone III)

• Location: Over the PIP joint
• Presentation: Immediate or delayed loss of PIP extension
• Mechanism: The central slip tears, allowing lateral bands to slip volarly, creating the boutonnière deformity 4
• Critical timing: Early recognition prevents progression to fixed deformity

Alternative: Mallet Finger (Zone I)

• Location: DIP joint
• Presentation: Inability to extend DIP joint, finger droops at tip
• May include: Bony avulsion visible on radiograph 1, 5

Less Common: Extensor Hood Rupture

• Location: Metacarpophalangeal (MCP) joint
• Presentation: Jerky dislocation of extensor tendon to ulnar side during MCP flexion
• Often misdiagnosed as trigger finger or recurrent MCP dislocation 2

Immediate Diagnostic Approach

Start with plain radiographs (3-view hand series) to exclude:

• Bony avulsion fragments (particularly at DIP or PIP joint base)
• Fracture-dislocations
• Large fragments requiring operative fixation 1

Perform the Elson test under digital block: With PIP joint flexed over table edge, patient attempts extension. If central slip is ruptured, DIP joint becomes rigid in extension due to unopposed lateral band tension 3.

When Radiographs Are Negative

If clinical suspicion remains high despite negative radiographs:

• MRI without contrast is the gold standard for soft tissue evaluation
• Sensitivity 89-92% for extensor hood injuries (though variable 28-85% for some extensor injuries) 1
• Can identify central slip tears, lateral band displacement, and associated injuries
• Do NOT delay treatment waiting for advanced imaging if clinical diagnosis is clear 1

Management Algorithm

Acute Supple Deformity (within days to weeks, PIP still passively extends fully)

Surgical reconstruction is the definitive treatment for complete ruptures:

• Resection-suture of central slip with redorsalization of lateral bands
• 90% excellent/good results when PIP joint is normal 4
• Timing matters: Direct repair within days provides excellent functional outcomes 6

Post-operative protocol:

• PIP joint immobilized in extension for 4 weeks 2
• Early motion protocols can be employed with proper surgical technique 3
• Additional 2 weeks of nighttime splinting 5

Chronic Stiff Deformity (fixed PIP flexion contracture)

If PIP joint remains flexed despite passive stretching:

• Consider two-stage approach: tenoarthrolysis first, then reconstruction after achieving suppleness 4
• If PIP joint shows osteoarthritis: silicone spacer arthroplasty may be required
• Poor results (6/8 cases) when reconstruction attempted on arthritic joints 4

Critical Pitfalls to Avoid

1. Misdiagnosis as "sprain": Any loss of active extension after trauma warrants careful extensor mechanism evaluation
2. Delayed diagnosis: Missed central slip injuries progress to fixed boutonnière deformity requiring more complex reconstruction 4
3. Inadequate immobilization: Less than 4 weeks extension splinting leads to re-rupture
4. Assuming distal injury: Zone III ruptures can masquerade as distal avulsions—extend exploration proximally if distal insertion is intact 6
5. Non-compliance with therapy: Two of four poor surgical results were due to absent post-operative physical therapy 4

Bottom Line

The combination of audible "pop" plus inability to extend strongly indicates extensor tendon disruption. Obtain radiographs immediately to exclude bony injury, then refer urgently to hand surgery for clinical examination and likely operative repair within the first week 1, 3. Conservative treatment cannot heal complete extensor hood tears 2, and delayed treatment significantly worsens outcomes by allowing progression to fixed deformity requiring more extensive reconstruction or even joint replacement 4.