Isolated Third Finger Extension Weakness: Posterior Interosseous Nerve Syndrome
Your inability to lift (extend) the third finger while maintaining normal flexion, extension, and grip strength strongly suggests posterior interosseous nerve (PIN) compression, not a radial nerve issue, and requires immediate clinical examination to differentiate this from an extensor tendon injury.
Understanding Your Presentation
Your symptom pattern is highly specific:
Preserved functions indicate intact radial nerve main trunk and median/ulnar nerves:
- Normal wrist extension rules out high radial nerve palsy
- Strong grip and pinch demonstrate intact flexor tendons and median/ulnar motor function
- Ability to flex and extend other fingers shows selective involvement
Isolated third finger extension loss points to either:
- Posterior interosseous nerve (PIN) branch injury affecting extensor digitorum communis to the middle finger
- Extensor tendon rupture or laceration specific to the third finger
Immediate Clinical Assessment Required
Test for PIN syndrome versus tendon injury:
Passive extension test: Have someone gently extend your third finger at the MCP joint while you relax completely. If the finger extends fully with passive motion but you cannot actively extend it, this confirms motor nerve or tendon pathology rather than joint contracture 1
Tenodesis effect: Make a fist with your wrist flexed, then extend your wrist while keeping fingers relaxed. If the third finger extends passively with wrist extension, the extensor tendon is intact and PIN palsy is more likely
Resisted extension of other fingers: Extend your index and ring fingers against resistance. If this is strong and painless, isolated third finger involvement becomes more suspicious for focal pathology
Critical Diagnostic Pathway
Obtain plain radiographs immediately to exclude:
- Occult fracture fragments that could entrap nerve branches
- Bony abnormalities causing nerve compression
- Joint subluxation affecting tendon mechanics 2
If radiographs are normal, proceed with:
- Clinical examination by hand surgeon or neurologist within 48-72 hours
- Nerve conduction studies and EMG if PIN syndrome suspected (typically delayed 3-4 weeks to allow Wallerian degeneration for accurate localization)
- MRI of forearm if space-occupying lesion suspected as cause of PIN compression
Management Algorithm
Conservative management (if no urgent surgical indications):
Immediate splinting: Buddy tape the third finger to the fourth finger in slight extension to prevent PIP joint contracture while awaiting definitive diagnosis 1
Active ROM exercises: Begin immediate active flexion exercises of all uninvolved joints (PIP, DIP of third finger if flexion intact, and all other finger joints) to prevent stiffness, which is the most functionally disabling complication 1, 3
Observation period: Most nerve injuries associated with closed injuries recover spontaneously without surgical intervention, but documentation of progression is essential 3
Surgical exploration indicated if:
- Open injury or penetrating trauma history
- Progressive weakness developing after initial injury/reduction
- No clinical or electrodiagnostic evidence of recovery at 6 months 3
- Space-occupying lesion identified on imaging requiring decompression
Critical Pitfalls to Avoid
Do not delay evaluation beyond 7-10 days, as early intervention for nerve compression or tendon repair yields superior outcomes compared to delayed treatment 1
Avoid immobilizing uninvolved joints, as failure to maintain joint mobility during the observation period leads to permanent stiffness that is difficult to reverse 3
Do not assume this will resolve spontaneously without proper diagnosis, as misdiagnosed extensor tendon injuries can result in permanent extension lag if not addressed within 4-6 weeks 2
Monitor for secondary nerve palsy development, which may indicate progressive nerve entrapment requiring urgent surgical intervention 3