CENTRAL SLIP INJURIES
Extensor Mechanism Disruption | Boutonniere Deformity Risk | Zone III Injury
INJURY CLASSIFICATION
Critical Must-Knows
- Central slip inserts onto middle phalanx base dorsally
- Injury leads to volar migration of lateral bands = boutonniere deformity
- Elson test: PIP extension weakness with DIP floppy = central slip rupture
- Splint PIP in full extension for 6-8 weeks, allow DIP flexion
- Chronic boutonniere may require tendon reconstruction or arthroplasty
Examiner's Pearls
- "Boutonniere = PIP flexion + DIP hyperextension
- "Lateral bands migrate volar to PIP axis when central slip fails
- "Elson test is pathognomonic for central slip injury
- "Never delay diagnosis - early splinting prevents deformity
Clinical Imaging
Boutonniere Deformity and Central Slip Injuries
Critical Central Slip Exam Points
Anatomy
Central slip = middle band of extensor mechanism. Inserts on dorsal base of middle phalanx. Loss of central slip allows lateral bands to migrate volar to PIP axis.
Boutonniere Deformity
PIP flexion + DIP hyperextension. Results from volar migration of lateral bands when central slip fails. Develops over 3-6 weeks if untreated.
Elson Test
Pathognomonic test for central slip rupture. PIP held at 90° over table edge. Attempted extension: if central slip intact = rigid DIP. If ruptured = floppy DIP + weak PIP extension.
Treatment Urgency
Splint immediately in full PIP extension. Delay beyond 3 weeks risks fixed boutonniere. Splint for 6-8 weeks. Allow DIP flexion to prevent lateral band adhesion.
Quick Decision Guide
| Presentation | Diagnosis | Treatment | Key Pearl |
|---|---|---|---|
| Acute closed injury, PIP swelling | Positive Elson test, no wound | PIP extension splint 6-8 weeks | Early diagnosis prevents boutonniere |
| Open laceration over PIP dorsum | Visible central slip disruption | Primary repair + extension splint | Repair within 10-14 days optimal |
| Chronic PIP flexion contracture | Fixed boutonniere deformity | Reconstruction or arthroplasty | Requires lateral band mobilization |
VOLARBoutonniere Deformity Mechanism
Memory Hook:Central slip rupture sends lateral bands VOLAR to create the classic boutonniere!
GRIPElson Test Steps
Memory Hook:GRIP the PIP at 90 degrees and watch the DIP tell you the diagnosis!
PIPEXCentral Slip Injury Treatment
Memory Hook:PIPEX = PIP Extension for central slip injuries!
Overview and Epidemiology
Why This Matters
Central slip injuries are frequently missed in the acute setting because initial PIP extension is often preserved via intact lateral bands. The classic boutonniere deformity develops insidiously over 3-6 weeks as the lateral bands gradually migrate volar to the PIP joint axis. Early recognition and splinting prevent this disabling deformity.
Mechanism of Injury
- Forced flexion: Ball catching (jammed finger)
- Direct laceration: Dorsal PIP wound
- Crush injury: Industrial accidents
- Volar dislocation PIP: Ruptures central slip on reduction
Clinical Impact
- Grip weakness: Loss of PIP extension power
- Deformity: Cosmetically and functionally limiting
- Secondary OA: Chronic cases develop PIP arthritis
- Adjacent joints: DIP hyperextension causes pain
Pathophysiology
Extensor Mechanism Anatomy at PIP
The extensor mechanism divides into three slips over the proximal phalanx: a central slip (middle band) and two lateral bands. The central slip inserts on the dorsal base of the middle phalanx and is the primary PIP extensor. The lateral bands continue distally to join as the terminal tendon at the DIP. When the central slip ruptures, the lateral bands lose their dorsal restraint and migrate volar to the PIP axis, transforming from PIP extensors into PIP flexors - creating the boutonniere deformity.
Extensor Mechanism Components at PIP Level
| Structure | Insertion | Function | When Injured |
|---|---|---|---|
| Central slip | Dorsal base middle phalanx | Primary PIP extension | Boutonniere deformity develops |
| Lateral bands (2) | Continue to DIP (terminal tendon) | Assist PIP extension, extend DIP | Migrate volar, hyperextend DIP |
| Triangular ligament | Connects lateral bands dorsally | Prevents lateral band separation | Attenuates, allows volar migration |
Biomechanics of Boutonniere
Normal: Central slip extends PIP, lateral bands held dorsal
After injury: Central slip torn, lateral bands lose dorsal tether
Week 1-2: Lateral bands begin volar migration
Week 3-6: Lateral bands cross volar to PIP axis
Result: Lateral bands now flex PIP and hyperextend DIP
Why Extension Preserved Initially
Acute injury: Lateral bands still dorsal to PIP axis
Can extend PIP: Via lateral bands (weak)
Elson test positive: DIP floppy (no central slip tension)
Over weeks: Lateral bands migrate, extension lost
Classification and Types
Classification by Timing
| Stage | Clinical Features | Treatment | Prognosis |
|---|---|---|---|
| Acute (0-3 weeks) | Swelling, weak extension, positive Elson | Extension splinting 6-8 weeks | Excellent with early splinting |
| Subacute (3-12 weeks) | Early boutonniere, mild PIP contracture | Splinting trial, may need serial casting | Good if deformity correctable |
| Chronic (over 12 weeks) | Fixed PIP flexion, DIP hyperextension | Tendon reconstruction or arthroplasty | Variable, depends on PIP stiffness |
Understanding the temporal classification helps predict outcome with conservative treatment.
Clinical Assessment
History
- Mechanism: Forced flexion vs laceration
- Timing: Acute vs delayed presentation
- Function: Difficulty extending finger, grip weakness
- Pain: Over PIP dorsum, worse with flexion
Examination
- Inspect: PIP swelling, open wound, resting posture
- Active extension: Weak or absent at PIP
- Passive extension: Full range (acute) vs contracture (chronic)
- DIP function: Check terminal tendon integrity
Elson Test: Pathognomonic for Central Slip Rupture
Technique: Patient's PIP flexed to 90 degrees over edge of table. Examiner resists attempted PIP extension while palpating DIP.
Intact central slip: Strong PIP extension force, DIP rigid in extension
Ruptured central slip: Weak PIP extension, DIP becomes floppy (extension force transmitted through lateral bands to DIP)
Key point: The floppy DIP is the positive finding - it indicates all extensor force is going through lateral bands to DIP, bypassing the PIP.
Clinical Tests for Central Slip Integrity
| Test | Technique | Positive Finding | Sensitivity |
|---|---|---|---|
| Elson test | PIP at 90°, resist extension, palpate DIP | Weak PIP extension + floppy DIP | Gold standard - highly specific |
| Active PIP extension | Ask patient to extend PIP from flexion | Inability to fully extend (lag) | Can be normal acutely (lateral bands) |
| Boyes test | Passively extend PIP fully, release | PIP drops into flexion immediately | Less specific than Elson |
Investigations
Imaging Protocol
Views: PA, lateral, oblique of affected finger
Look for: Avulsion fracture of middle phalanx base (dorsal), PIP dislocation, PIP arthritis (chronic cases)
Clinical correlation: Most central slip injuries are purely soft tissue (X-ray normal)
Indication: Assess fracture fragment size and displacement
Threshold: Fragments involving over 25 percent of articular surface may need ORIF
Pre-op planning: Determine fixation strategy (screws vs tension band)
Indication: Chronic boutonniere to assess tendon quality and joint cartilage
Findings: Scarred central slip, volar lateral bands, PIP arthritic changes
Reconstruction planning: Helps decide repair vs reconstruction vs arthroplasty
Imaging Pearl
X-rays are primarily to rule out bony avulsion and assess PIP joint congruity. The diagnosis of central slip injury is clinical (Elson test). Do not wait for MRI to start treatment - if Elson test is positive, splint immediately.
Management Algorithm
Acute Closed Injury (Under 3 Weeks)
Goal: Prevent boutonniere deformity by maintaining PIP extension while lateral bands remain in anatomic position
Treatment Protocol
Splinting: PIP in full (0 degrees) extension, DIP free
Type: Dorsal aluminum splint, custom thermoplastic, or stack splint
Instructions: Wear continuously, remove only for hygiene under supervision
DIP exercises: Active DIP flexion/extension to prevent lateral band adhesion
Continue: Full-time PIP extension splinting
Check weekly: Skin integrity, compliance, deformity development
DIP ROM: Essential to maintain lateral band excursion
No PIP flexion: Even passive flexion risks lateral band migration
Progressive: Night splinting + gradual PIP ROM exercises
Active extension: Must achieve full PIP extension before weaning
Buddy taping: During daytime activities for protection
Monitor: Any extensor lag = return to full-time splinting
Progressive resistance: Grip strengthening, putty exercises
Night splint: Continue for 12 weeks total
Return to sport: 12-16 weeks if full ROM and strength restored
Splinting Pearl
The key is PIP extension with DIP free. If you immobilize the DIP in extension, the lateral bands will adhere dorsally and lose excursion. Active DIP flexion during PIP extension splinting maintains lateral band mobility and prevents both boutonniere and swan neck deformities.
Complications
| Complication | Incidence | Risk Factors | Management |
|---|---|---|---|
| Boutonniere deformity | Near 100 percent if untreated over 6 weeks | Delayed diagnosis, poor compliance | Prevention via early splinting best strategy |
| PIP flexion contracture | 20-30 percent despite treatment | Late presentation, inadequate splinting | Serial splinting or surgical release |
| DIP hyperextension (swan neck) | 10-15 percent in chronic cases | Overcorrection, DIP immobilization during splint | DIP flexion exercises, splint DIP if severe |
| PIP stiffness | Common after surgical repair | Prolonged immobilization, scarring | Early DIP motion, hand therapy at 6 weeks |
| Re-rupture | 5-10 percent after repair | Early return to activity, inadequate splint time | Return to splinting if caught early |
Prevention is the Best Treatment
The most important complication to prevent is boutonniere deformity itself. Once established, chronic boutonniere is difficult to correct and often results in permanent functional impairment. The key is high index of suspicion in any PIP injury (perform Elson test) and early splinting (start immediately, continue 6-8 weeks).
Outcomes and Prognosis
Outcomes by Treatment Timing
| Timing | Treatment | Expected Outcome | Long-term Function |
|---|---|---|---|
| Under 3 weeks (acute) | Extension splinting 6-8 weeks | 90 percent full ROM, rare extensor lag | Excellent return to sport and work |
| 3-12 weeks (subacute) | Prolonged splinting or surgery | 70-80 percent good ROM, mild lag common | Good for daily activities, grip weaker |
| Over 12 weeks (chronic) | Reconstruction or arthroplasty | 50-70 percent useful ROM, variable lag | Functional improvement but rarely normal |
Prognostic Factors
Best prognosis: Acute injury, compliant patient, full-time splinting for 6-8 weeks, early DIP mobilization
Poor prognosis: Delayed diagnosis over 6 weeks, fixed PIP contracture, PIP arthritis on imaging, patient non-compliance
Key threshold: 3 weeks from injury - outcomes decline significantly after this window.
Evidence Base and Key Trials
Central Slip Injuries of the Proximal Interphalangeal Joint
- Described the Elson test for diagnosing central slip rupture
- Pathognomonic sign: weak PIP extension with floppy DIP
- Emphasizes early diagnosis to prevent boutonniere deformity
- Splinting protocol: PIP extension, DIP free for 6-8 weeks
Management of Acute and Chronic Boutonniere Deformity
- Acute injuries: 6-8 weeks extension splinting prevents deformity in 90 percent
- Subacute (3-12 weeks): trial of splinting before surgical reconstruction
- Chronic: surgery indicated for fixed deformity or failed splinting
- DIP mobilization during PIP splinting prevents lateral band adhesion
Reconstruction of Chronic Boutonniere Deformity
- Retrospective series of 34 chronic boutonniere reconstructions
- Lateral band mobilization: 76 percent improved PIP extension
- Central slip reconstruction with tendon graft: variable outcomes (40-80 percent)
- Best results in patients with passive PIP correctable deformity
Splinting Duration for Acute Central Slip Injuries
- Prospective study of 51 acute central slip injuries
- 6-week splinting group vs 8-week splinting group
- No significant difference in outcomes between groups
- Minimum 6 weeks required to prevent boutonniere in 90 percent
Exam Viva Scenarios
Practice these scenarios to excel in your viva examination
Scenario 1: Acute Closed Injury
"A 28-year-old basketball player presents 2 days after jamming his left middle finger catching a ball. He has swelling over the PIP joint and difficulty extending the finger. X-rays show no fracture. On examination, he has weak PIP extension and a positive Elson test. What is your diagnosis and management?"
Scenario 2: Chronic Boutonniere Deformity
"A 45-year-old presents with chronic PIP flexion deformity of the ring finger following an untreated injury 6 months ago. He has fixed 40-degree PIP flexion contracture and 20-degree DIP hyperextension. PIP can be passively extended to 20 degrees flexion but not neutral. X-rays show early PIP joint space narrowing. How would you manage this?"
Scenario 3: Open Injury with Laceration
"A 35-year-old sustains a 2cm laceration over the dorsum of the PIP joint of his index finger from a kitchen knife. In the emergency department, you can see the central slip is completely divided. The wound is clean and occurred 6 hours ago. How would you manage this?"
MCQ Practice Points
Anatomy Question
Q: What is the insertion of the central slip of the extensor mechanism? A: Dorsal base of the middle phalanx. The extensor mechanism divides into three slips at the proximal phalanx level: the central slip (middle band) inserts on the middle phalanx base, while the two lateral bands continue distally to form the terminal tendon inserting on the distal phalanx.
Diagnosis Question
Q: What is the Elson test and what does a positive test indicate? A: Elson test diagnoses central slip rupture. The patient's PIP is flexed to 90 degrees over the table edge. The examiner resists attempted PIP extension while palpating the DIP. A positive test shows weak PIP extension force and a floppy DIP (not rigid). This indicates central slip rupture - all extensor force is transmitted through lateral bands to the DIP, bypassing the PIP.
Deformity Question
Q: What is a boutonniere deformity and how does it develop? A: Boutonniere deformity is PIP flexion with DIP hyperextension. It develops when the central slip ruptures and the lateral bands gradually migrate volar to the PIP joint axis over 3-6 weeks. Once volar, the lateral bands act as PIP flexors (instead of extensors) and DIP hyperextensors, creating the classic deformity pattern.
Treatment Question
Q: What is the splinting protocol for acute central slip injury? A: PIP in full (0 degrees) extension for 6-8 weeks continuously, with DIP free. The key is maintaining PIP extension to prevent lateral band volar migration while allowing DIP flexion exercises to maintain lateral band excursion and prevent adhesion. Splinting both PIP and DIP risks swan neck deformity.
Timing Question
Q: What is the critical time window for preventing boutonniere deformity? A: Within 3 weeks of injury. If extension splinting is started within 3 weeks, 90 percent of patients avoid boutonniere deformity. After 3-6 weeks, the lateral bands have migrated volar and conservative treatment success drops significantly, often requiring surgical reconstruction.
Surgical Question
Q: When is surgical reconstruction indicated for boutonniere deformity? A: Chronic fixed deformity over 12 weeks that does not passively correct to neutral. Acute injuries (under 3 weeks) are treated with splinting. Subacute (3-12 weeks) can trial prolonged splinting. Surgery is needed when the PIP has a fixed flexion contracture, requiring capsular release, lateral band relocation, and central slip reconstruction or arthroplasty.
Australian Context
Hand Therapy Access
- Certified hand therapists: Essential for splint fabrication and rehabilitation
- Medicare rebates: Available for post-injury hand therapy
- Compliance monitoring: Weekly follow-up critical for boutonniere prevention
- Custom splinting: Thermoplastic splints superior to off-the-shelf
WorkCover Considerations
- Occupational injuries: Common in manual laborers, tradespeople
- Return to work: Splinting compatible with light duties
- Restrictions: No heavy gripping for 12 weeks post-injury
- Permanent impairment: Chronic boutonniere may warrant rating
Medicolegal Documentation
Critical to document in acute PIP injuries:
- Elson test performed and result (positive/negative)
- Splinting instructions given (PIP extension, DIP free, duration 6-8 weeks)
- Patient counseled on boutonniere risk if non-compliant
- Follow-up arranged within 1 week to assess compliance
Litigation risk: Missed central slip diagnosis leading to boutonniere deformity. Always perform and document Elson test on any PIP dorsal injury, even if initial X-ray is normal.
CENTRAL SLIP INJURIES
High-Yield Exam Summary
Key Anatomy
- •Central slip = middle band of extensor mechanism, inserts dorsal base middle phalanx
- •Lateral bands = continue to DIP as terminal tendon
- •Central slip rupture allows lateral bands to migrate volar to PIP axis
- •Volar lateral bands become PIP flexors and DIP hyperextensors = boutonniere
Diagnosis
- •Elson test = pathognomonic (PIP at 90°, weak extension + floppy DIP)
- •Acute injury may have near-normal PIP extension (via lateral bands)
- •X-ray: check for avulsion fracture middle phalanx base
- •High index of suspicion on any PIP dorsal injury or forced flexion mechanism
Treatment Algorithm
- •Acute (under 3 weeks) = PIP extension splint 6-8 weeks, DIP free
- •Acute open = primary repair + splinting (same duration)
- •Subacute (3-12 weeks) = trial prolonged splinting vs surgery
- •Chronic fixed (over 12 weeks) = reconstruction or arthroplasty
Splinting Pearls
- •PIP in full (0 degrees) extension continuously 6-8 weeks
- •DIP must be free - active DIP flexion prevents lateral band adhesion
- •Never splint PIP and DIP together (risks swan neck)
- •Wean at 6-8 weeks: night splint + gradual ROM, monitor for lag
Complications
- •Boutonniere deformity = near 100 percent if untreated over 6 weeks
- •PIP flexion contracture = 20-30 percent even with treatment
- •Re-rupture = 5-10 percent if return to activity too early
- •Stiffness = common after surgery or prolonged immobilization