Extensor Tendon Injuries
Summary
Extensor tendon injuries are common hand injuries with important clinical implications. The extensor mechanism is divided into 9 zones (odd numbers = joints, even numbers = between joints). Zone 1 (DIP joint) injuries cause mallet finger — a classic droop of the distal phalanx. Zone 3 (PIP joint) injuries involve the central slip and can lead to boutonniere deformity if missed. Zone 5 (MCP joint) injuries are often caused by "fight bites" (knuckle injury from punching teeth) and carry high infection risk. Management depends on the zone: closed mallet finger is treated with splinting; open injuries require surgical repair; boutonniere deformity requires prolonged splinting or surgery.
Key Facts
- Zones: 9 zones; Odd = joints, Even = between joints
- Zone 1 (DIP): Mallet finger — droop of distal phalanx
- Zone 3 (PIP): Central slip injury — boutonniere deformity
- Zone 5 (MCP): Fight bite risk
- Mallet treatment: Extension splinting (6-8 weeks continuous)
- Open injuries: Surgical repair
- Fight bite: High-risk for infection; Requires washout + antibiotics
Clinical Pearls
"Odd Zones = Joints": Zone 1 = DIP; Zone 3 = PIP; Zone 5 = MCP; Zone 7 = Wrist. This helps localise injuries.
"Mallet Finger = Splint for 6-8 Weeks": Closed mallet finger (Zone 1) is splinted in extension continuously for 6-8 weeks. The DIP must never flex during this period or healing restarts.
"Boutonniere = Central Slip Injury": An untreated central slip injury at PIP (Zone 3) causes the lateral bands to slip volarly, resulting in boutonniere deformity — PIP flexion + DIP hyperextension.
"Fight Bite = Emergency": A laceration over the MCP joint from punching teeth is a human bite injury. High risk of joint sepsis. Requires thorough washout, antibiotics (cover oral flora), and often admission.
Why This Matters Clinically
Missed extensor tendon injuries lead to significant functional disability and deformity. Recognising mallet finger, central slip injuries, and fight bites is essential. Early splinting or surgical repair produces good outcomes; delayed treatment leads to chronic deformity.[1,2]
Incidence
| Parameter | Data |
|---|---|
| Mallet finger | Common; Ball sports, direct trauma |
| Fight bites | Common; Often underreported |
| All extensor injuries | Less common than flexor injuries |
Extensor Tendon Zones
| Zone | Location | Key Structures |
|---|---|---|
| 1 | DIP joint | Terminal tendon |
| 2 | Middle phalanx | Lateral bands |
| 3 | PIP joint | Central slip |
| 4 | Proximal phalanx | |
| 5 | MCP joint | Sagittal bands |
| 6 | Metacarpal | Extensor tendon |
| 7 | Wrist | Extensor retinaculum |
| 8 | Distal forearm | Musculotendinous junction |
| 9 | Proximal forearm | Muscle bellies |
The Extensor Mechanism (It's a Web, not a Rope)
Unlike flexors (ropes in tubes), the extensors are a complex interconnected web (aponeurosis).
- Sagittal Bands (Zone 5): At the MCPJ, these lasso the ED tendon to the volar plate. If they rupture (usually radial sagittal band), the tendon subluxes into the ulnar gutter ("Boxer's Knuckle").
- Juncturae Tendinum: Fibrous connections between the EDC tendons in Zone 6 (Hand dorsum).
- Significance: If you cut the EDC to the middle finger proximal to the juncturae, the middle finger can STILL extend (via the ring/index pulling on the juncturae). You can miss a 100% laceration if you are not careful.
- Test: You must test extension with other fingers flexed (isolating the ED).
Zone 1: Mallet Finger (DIPJ)
- Terminal Tendon: Very thin (1mm).
- Pathology: Rupture -> Unopposed Flexion by FDP.
- Swan Neck Effect: The extensor force that should go to the DIP retracts proximally to the PIP (Central Slip). This hyperextends the PIP. So a Mallet Finger -> Swan Neck.
Zone 3: Central Slip (PIP)
- The Extensor Digitorum trifurcates over the P1.
- Central Slip: Inserts into Middle Phalanx Base. (Extends PIP).
- Lateral Bands: Go around the sides to form the terminal tendon.
- Boutonniere Deformity:
- If Central Slip is cut, the PIP cannot extend.
- The Lateral Bands fall "volarly" (below the axis of rotation).
- They become flexors of the PIP.
- Result: PIP Flexion + DIP Hyperextension.
Zone 5: "Fight Bite"
- The Clenched Fist: When you punch, the MCP joint is flexed. The tooth penetrates skin, tendon, and capsule.
- The Relaxed Hand: When you relax, the tendon slides back proximally. It drags the bacteria under the skin, away from the visible wound.
- Management: You must extend the wound proximally to find the "inoculum" inside the tendon/joint.
Zone 7: Extensor Retinaculum
- 6 Compartments.
- APL + EPB (De Quervains).
- ECRL + ECRB.
- EPL (Lister's Tubercle pivot).
- EDC + EIP.
- EDM (Little finger).
- ECU.
- Repair here usually requires dividing the retinaculum to prevent bowstringing/adhesions.
Deformities
| Deformity | Mechanism |
|---|---|
| Mallet finger | Zone 1 — Terminal tendon rupture/avulsion; DIP droop |
| Boutonniere | Zone 3 — Central slip injury; PIP flexion + DIP hyperextension |
| Swan neck | Opposite: PIP hyperextension + DIP flexion (often from FDS injury) |
Mallet Finger (Zone 1)
| Feature | Description |
|---|---|
| Mechanism | Forced flexion of extended DIP (ball striking finger) |
| Appearance | DIP droop; Unable to actively extend DIP |
| Types | Tendinous (soft tissue) or Bony (avulsion fracture) |
Central Slip Injury (Zone 3)
| Feature | Description |
|---|---|
| Mechanism | Forced flexion of extended PIP |
| Acute | Tender over PIP; Weak extension |
| Delayed | Boutonniere deformity develops over weeks |
Fight Bite (Zone 5)
| Feature | Description |
|---|---|
| Mechanism | Punch to mouth; Tooth lacerates MCP |
| High risk | Joint penetration; Oral bacterial contamination |
| Examination | Laceration over MCP; Check with fist clenched (wound position changes) |
Assessment
| Test | Purpose |
|---|---|
| Active DIP extension | Mallet — cannot extend |
| Elson test | Central slip injury — Tests extension at PIP |
| Wound exploration | Open injuries — Assess depth and tendon status |
Elson Test (Central Slip)
| Step | Finding |
|---|---|
| Flex PIP to 90° over table edge | |
| Ask patient to extend PIP | |
| Intact central slip | PIP extends; DIP stays floppy |
| Ruptured central slip | DIP extends forcefully; PIP weak |
| Investigation | Purpose |
|---|---|
| X-ray | Bony mallet (avulsion fragment); Joint congruity |
| Wound swab | Open/fight bite — If infection suspected |
Bony Mallet Classification
| Type | Description |
|---|---|
| Type I | Tendon rupture or small avulsion (<30% articular surface) |
| Type II | Avulsion involving >30% articular surface |
| Type III | Subluxation of DIP joint |
Management Algorithm
EXTENSOR TENDON INJURY MANAGEMENT
↓
┌───────────────────────────────────────────────────────────┐
│ ZONE 1 (MALLET FINGER) │
├───────────────────────────────────────────────────────────┤
│ CLOSED / SMALL BONY FRAGMENT (<30%): │
│ ➤ Stack splint (Zimmer) in extension │
│ ➤ Wear CONTINUOUSLY for 6-8 weeks (never flex!) │
│ ➤ Then night splinting for 2-4 weeks │
│ │
│ BONY MALLET (>30% articular or subluxation): │
│ ➤ Surgical fixation (K-wire; Ishiguro technique) │
│ │
│ OPEN MALLET: │
│ ➤ Surgical repair │
└───────────────────────────────────────────────────────────┘
↓
┌───────────────────────────────────────────────────────────┐
│ ZONE 3 (CENTRAL SLIP) │
├───────────────────────────────────────────────────────────┤
│ CLOSED: │
│ ➤ Splint PIP in extension; Allow DIP flexion │
│ ➤ 6 weeks │
│ ➤ Hand therapy for DIP flexion exercises │
│ │
│ ESTABLISHED BOUTONNIERE: │
│ ➤ Prolonged splinting; May require surgery │
│ │
│ OPEN: │
│ ➤ Surgical repair of central slip │
└───────────────────────────────────────────────────────────┘
↓
┌───────────────────────────────────────────────────────────┐
│ ZONE 5 (FIGHT BITE / MCP) │
├───────────────────────────────────────────────────────────┤
│ ⚠️ TREAT AS HUMAN BITE UNTIL PROVEN OTHERWISE │
│ │
│ ➤ Thorough wound washout / exploration │
│ ➤ Assess joint penetration (with fist clenched) │
│ ➤ X-ray: Tooth fragment? Fracture? │
│ ➤ Antibiotics: Co-amoxiclav (oral flora + Eikenella) │
│ ➤ If joint penetrated: Washout in theatre; IV antibiotics│
│ ➤ Leave wound open; Delayed closure │
│ ➤ Tetanus prophylaxis │
│ │
│ ➤ Surgical tendon repair if lacerated │
└───────────────────────────────────────────────────────────┘
↓
┌───────────────────────────────────────────────────────────┐
│ OPEN INJURIES (OTHER ZONES) │
├───────────────────────────────────────────────────────────┤
│ ➤ Surgical repair │
│ ➤ Core suture (horizontal mattress or figure-of-8) │
│ ➤ Post-op splinting │
│ ➤ Hand therapy — Static or dynamic splinting │
└───────────────────────────────────────────────────────────┘
Extensor tendons are weaker than flexors but have "extensor habitus" (they heal better but tend to stick).
1. Static Splinting (Immobilisation)
- Zone 1/2 (Mallet): 6-8 weeks continuous. Stax or Zimmer splint.
- Zone 3/4 (Boutonniere): Splint PIP straight. Leave DIP free (DIP exercises pull the lateral bands dorsal, helping correction).
- Zone 5-7: 4 weeks wrist extension splint.
2. Early Active Motion (Short Arc Motion) - Evans Protocol
- For Zone 3-6 repairs.
- Concept: A specific splint (ICAM) allows 30 degrees of flexion but blocks full flexion.
- Allows the tendon to glide 3-4mm (preventing adhesions) without gapping the repair.
- Better than static splinting for grip strength.
3. Dynamic Splinting
- "Outrigger" splints. Rubber bands hold fingers in extension, patient flexes against resistance. Less common now (bulky).
1. Extensor Lag
- Definition: Active extension is less than Passive extension. (e.g., finger droops 20 degrees).
- Cause: The repair has stretched ("gapped").
- Management: Re-splinting or Tenolysis (if adherent).
2. Adhesions
- Less common than flexors but catastrophic over the proximal phalanx (Zone 4) where tendon slides directly on bone.
- Tenolysis: Often required.
3. Chronic Mallet (Swan Neck)
- If mallet is ignored, the PIP hyperextends.
- Management:
- Soft Tissue: Spiral Oblique Retinacular Ligament (SORL) reconstruction.
- Bony: Fusion of DIPJ.
4. Chronic Boutonniere
- Very difficult to fix.
- The lateral bands are stuck volarly.
- Surgery: Central slip reconstruction (using lateral band weave).
5. Juncturae Syndrome
- If you repair the EDC to the middle finger too tight, it pulls the other fingers (via juncturae) and they cannot flex.
The Fowler Tenotomy (for Boutonniere)
- If the deformity is passive correctable but the DIP is stiff in extension.
- We cut the terminal tendon (Zone 1) distally.
- This releases the tight lateral bands. They slide back proximally to help Extend the PIP.
- Essentially, creating a Mallet finger to cure a Boutonniere!
Key Studies
- Doyle (1993): Classification of zones.
- Evans (1989): Short Arc Motion protocol reduced adhesions compared to static splinting.
- Newport: Investigated the excursion of extensor tendons. Zone 5-7 require more excursion than Zone 3.
What is a mallet finger?
A mallet finger is when the tip of your finger droops and you can't straighten it. It happens when the tendon that straightens the end of your finger is damaged.
What causes it?
- A ball hitting the end of your finger
- Jamming your finger against something
How is it treated?
- A special splint keeps the finger straight for 6-8 weeks
- You must wear the splint all the time — never let the finger bend
- If it's more serious, surgery may be needed
What is a fight bite?
A fight bite is a cut on your knuckle from punching someone's teeth. It's very high risk for infection because the wound can go into the joint. You need antibiotics and may need surgery to wash out the wound.
- Doyle JR. Extensor tendons – acute injuries. In: Green's Operative Hand Surgery. 2011.
High-Yield Exam Topics
| Topic | Key Points |
|---|---|
| Zone 1 | Mallet finger; DIP droop; Stack splint 6-8 weeks |
| Zone 3 | Central slip; Boutonniere if missed |
| Zone 5 | Fight bite; Joint sepsis risk |
| Elson test | Detects central slip injury |
| Fight bite antibiotics | Co-amoxiclav (cover oral flora + Eikenella) |
Sample Viva Question
Q: How do you manage a closed mallet finger injury?
Model Answer: Closed mallet finger (Zone 1 injury) is managed with extension splinting. Apply a stack (Zimmer) splint to hold the DIP in slight hyperextension. The splint must be worn continuously for 6-8 weeks — the DIP must never flex during this period or healing is disrupted. After 6-8 weeks, wean with night splinting for a further 2-4 weeks. Indications for surgery: Bony mallet with >30% articular surface or subluxation of DIP joint. Patient compliance is critical for success.
Last Reviewed: 2025-12-24 | MedVellum Editorial Team