Residency · Residency · Orthopedic Surgery
Extensor Tendon Injuries and Mallet Finger
Overview
Extensor tendon injuries are more common than flexor injuries but historically received less surgical attention. The extensor mechanism is a complex, interlinked system where injury at one level affects the entire cascade. Understanding the zone system and the intricate anatomy of the extensor hood is essential for proper management.
Anatomy
Extrinsic Extensors
The extensor digitorum communis (EDC) extends the MCP joints of digits 2-5. The extensor indicis proprius (EIP) provides independent index extension (ulnar to the EDC of the index). The extensor digiti minimi (EDM) provides independent small finger extension (ulnar to the EDC of the small finger). The extensor pollicis longus (EPL) extends the thumb IP joint and hooks around the Lister tubercle. The extensor pollicis brevis (EPB) extends the thumb MCP joint, and the abductor pollicis longus (APL) abducts the thumb CMC joint.
Extensor Hood Mechanism
At the MCP level, the sagittal bands stabilize the EDC tendon over the metacarpal head. Distal to the MCP, the EDC trifurcates into the central slip (inserting on the dorsal base of the middle phalanx to extend the PIP) and two lateral bands that continue distally with contributions from the intrinsics (lumbricals, interossei) to form the terminal tendon, inserting on the dorsal base of the distal phalanx to extend the DIP. The triangular ligament holds the lateral bands dorsal to the PIP axis. The transverse retinacular ligament holds the lateral bands volar to the PIP axis, allowing flexion. The oblique retinacular ligament (Landsmeer) links PIP extension to DIP extension through a tenodesis effect.
Extensor Compartments of the Wrist (Dorsal Retinaculum)
The six compartments contain: I (APL, EPB), II (ECRL, ECRB), III (EPL), IV (EDC, EIP), V (EDM), and VI (ECU).
<image>Anatomy of the extensor mechanism over the finger showing central slip, lateral bands, and terminal tendon insertion</image>
Zone Classification (Kleinert and Verdan)
The zones follow the rule that odd zones are over joints and even zones are between joints. Zone I is at the DIP joint (terminal tendon, mallet finger). Zone II is over the middle phalanx. Zone III is at the PIP joint (central slip, boutonniere). Zone IV is over the proximal phalanx. Zone V is at the MCP joint (sagittal bands, "fight bite"). Zone VI is the dorsum of the hand at the metacarpal level. Zone VII is at the wrist under the extensor retinaculum. Zone VIII is the distal forearm. Thumb zones T1-T5 are analogous.
| Zone | Location | Key Structure | Classic Injury | Primary Treatment |
|---|---|---|---|---|
| I | DIP joint | Terminal tendon | Mallet finger | Extension splinting 6-8 weeks |
| II | Middle phalanx | Lateral bands | Laceration | Splint (<50%) or repair (>50%) |
| III | PIP joint | Central slip | Boutonniere deformity | PIP extension splinting 6 weeks |
| IV | Proximal phalanx | Broad extensor | Laceration | Repair + extension splinting |
| V | MCP joint | Sagittal bands | Fight bite / snapping knuckle | Splint (acute) or surgical repair |
| VI | Dorsum of hand | EDC tendons | Laceration | Core suture repair; early motion |
| VII | Wrist | Tendons in retinaculum | Laceration | Repair with controlled motion |
| VIII | Distal forearm | Muscle-tendon junction | Laceration | Direct repair |
Zone-Specific Injuries and Management
Zone I -- Mallet Finger
Definition
Mallet finger involves disruption of the terminal tendon at or near its insertion on the distal phalanx, resulting in an extensor lag at the DIP joint. It is the most common closed tendon injury in athletes, typically occurring when a ball strikes an extended fingertip.
Classification
A tendinous mallet involves tendon rupture without fracture. A bony mallet involves an avulsion fracture of the dorsal base of the distal phalanx. The Wehbe and Schneider classification subdivides bony mallets into Type I (no DIP subluxation) and Type II (DIP subluxation), with subtypes A (less than 1/3 articular surface), B (1/3-2/3), and C (greater than 2/3).
Treatment
For tendinous mallet, nonoperative treatment is the standard. Continuous DIP extension splinting (Stack splint or custom thermoplastic) is maintained for 6-8 weeks, followed by nighttime splinting for an additional 4-6 weeks. The DIP must not flex at all during treatment, as any flexion resets the clock. The success rate is 80-90% with compliant splinting.
For bony mallet, treatment depends on fracture characteristics. Small fragments (less than 1/3 articular surface) without subluxation are treated with extension splinting identical to the tendinous protocol. Large fragments (greater than 1/3 articular surface) with volar subluxation of the distal phalanx require surgical fixation, typically extension block pinning (Ishiguro technique) or open reduction with screw or hook plate fixation.
Zone II -- Middle Phalanx
Isolated Zone II injuries are rare. Partial lacerations involving less than 50% of tendon width are splinted; those involving greater than 50% are repaired. Complete lacerations are treated with direct repair using a running epitendinous suture.
Zone III -- Central Slip (Boutonniere Deformity)
Pathoanatomy
Disruption of the central slip at the PIP joint allows the lateral bands to migrate volar to the PIP axis of rotation, resulting in the characteristic PIP flexion combined with DIP hyperextension (boutonniere deformity). The Elson test (flexing the PIP over the edge of a table and attempting extension against resistance) reveals weak PIP extension with the DIP rigid in extension, indicating a central slip injury.
Acute Management
Closed injuries are treated with PIP extension splinting for 6 weeks with the DIP left free for lateral band gliding. Open injuries require direct surgical repair of the central slip plus splinting. Critically, the DIP must not be splinted; active DIP flexion exercises prevent lateral band adherence.
Chronic Boutonniere
Chronic boutonniere deformity is a difficult reconstruction problem. Options include Fowler central slip tenotomy, lateral band rerouting (Matev), terminal tendon tenotomy, or extensor tendon reconstruction. Results are generally inferior to acute management.
Zone IV -- Proximal Phalanx
At this level the extensor mechanism is broad and thin. Partial injuries are splinted. Complete injuries require repair with MCP and PIP splinting in extension.
Zone V -- MCP Joint / Sagittal Band
Sagittal Band Tears
Acute traumatic rupture of the sagittal band causes subluxation or dislocation of the EDC tendon, usually in the ulnar direction, producing a "snapping knuckle" where the tendon subluxates with flexion. This is most common in the long finger. Acute tears are splinted with the MCP in extension for 4-6 weeks. Chronic or complete tears with persistent subluxation require surgical repair or reconstruction using juncturae tendinum.
"Fight Bite" (Clenched Fist Injury)
A human bite wound over the MCP joint occurs when a tooth penetrates the joint capsule and extensor tendon. Eikenella corrodens is a characteristic pathogen. Management requires surgical irrigation and debridement, IV antibiotics (ampicillin-sulbactam or equivalent), and leaving the wound open. A fight bite wound should never be primarily closed.
<image>Boutonniere deformity showing PIP flexion and DIP hyperextension with diagram of lateral band migration</image>
Zone VI -- Dorsum of Hand
Tendons are superficial and easily injured at this level. Juncturae tendinum may mask a complete EDC laceration because the patient can still partially extend the MCP. Repair uses a core suture technique, and early active motion is possible due to the broad tendon bed.
Zone VII -- Wrist
At the wrist, tendons lie within tight extensor retinaculum compartments, and the risk of adhesion formation post-repair is high. Repair with early controlled motion protocols is recommended. Partial retinaculum release may be needed for access, but complete release must be avoided to prevent bowstringing.
Thumb Extensor Injuries
Mallet Thumb (Zone T1)
EPL avulsion at the thumb IP joint is treated with extension splinting similar to finger mallet. Surgical repair or reconstruction is reserved for failed conservative management.
EPL Rupture
EPL rupture can occur as a closed injury weeks after distal radius fracture due to attritional rupture at the Lister tubercle. Treatment is EIP to EPL tendon transfer rather than direct repair, as the tendon quality is poor at the rupture site.
Swan Neck Deformity
Swan neck deformity presents with PIP hyperextension combined with DIP flexion (the opposite of boutonniere). Causes include volar plate laxity, FDS rupture or insufficiency, intrinsic tightness, and rheumatoid arthritis. While not a primary extensor tendon injury per se, it is an important differential diagnosis. Treatment depends on etiology and includes silver ring splinting, FDS tenodesis, lateral band rerouting, or dermadesis.
<image>Comparison of mallet finger, boutonniere deformity, and swan neck deformity patterns</image>
Rehabilitation Principles
Zones I-IV are managed primarily with splinting protocols and controlled motion programs similar in concept to flexor tendons but generally more forgiving. Zones V-VIII allow early active motion within a relative motion splint (Merritt/yoke splint) where adjacent fingers share the load. Relative motion extension splinting holds the injured digit in slightly more extension than adjacent digits, protecting the repair while permitting composite motion.
Clinical Pearls
Mallet finger splinting must maintain continuous DIP extension; if the splint is removed and the DIP flexes, the 6-8 week clock must be restarted. The Elson test should always be performed in PIP injuries because a missed central slip injury leads to chronic boutonniere, which is far harder to treat. Juncturae tendinum can mask a complete EDC laceration, so each tendon must be tested independently. The "fight bite" is a surgical emergency, and a small wound over the MCP joint after an altercation should never be underestimated. EPL rupture after distal radius fracture is a known late complication, and patients should be counseled to present if they lose thumb extension weeks after cast removal. In rheumatoid patients, extensor tendon ruptures are common (Vaughan-Jackson syndrome with attritional rupture starting with EDM and progressing radially), and prophylactic tenosynovectomy with Darrach procedure may prevent progression.
References
- Doyle JR. Extensor tendons — acute injuries. In: Green's Operative Hand Surgery. 7th ed.
- Kleinert HE, Verdan C. Report of the Committee on Tendon Injuries. J Hand Surg Am. 1983;8(5):794-798.
- Wehbe MA, Schneider LH. Mallet fractures. J Bone Joint Surg Am. 1984;66(5):658-669.
- Elson RA. Rupture of the central slip of the extensor hood of the finger. J Bone Joint Surg Br. 1986;68(2):229-231.
- Merritt WH. Relative motion splint: active motion after extensor tendon injury and repair. J Hand Surg Am. 2014;39(6):1187-1194.
