Mallet Finger

Mallet finger is an injury to the terminal extensor tendon at its insertion into the dorsal base of the distal phalanx, resulting in an inability to actively extend the DIP joint. The injury may be a pure tendinous avulsion (most common), a bony avulsion fracture (mallet fracture), or rarely an open laceration. The result is a characteristic droop of the distal phalanx — the `mallet` or `dropped finger` deformity. Left untreated, it can progress to a swan neck deformity of the PIP joint. The overwhelming majority of mallet fingers are managed non-operatively with splinting.

• Mechanism: forced flexion of the actively extended DIP joint — classically when a ball strikes the tip of the outstretched finger (hence `baseball finger`); also from trivial injuries (tucking in bed sheets, putting on socks); the DIP joint is forced into flexion while the extensor mechanism is under tension, avulsing the terminal extensor tendon from the distal phalanx
• Epidemiology: the most common closed tendon injury of the hand; affects all ages; middle and ring fingers most commonly affected; right (dominant) hand more often; the terminal extensor tendon inserts into the dorsal base of the distal phalanx through a broad, flat insertion; it is particularly vulnerable at the DIP joint because there is minimal overlying soft tissue protection
• Consequences of untreated mallet finger: if the terminal extensor is not treated, the DIP joint adopts a permanent flexed posture; the proximal extensor mechanism (lateral bands and central slip) becomes hyperactive; over time, the lateral bands migrate dorsally and the PIP joint is pulled into hyperextension — producing a swan neck deformity (PIP hyperextension + DIP flexion); this is the most important long-term consequence of neglected mallet finger; swan neck deformity from mallet is preventable with appropriate DIP splinting

• History: mechanism of injury (ball sports, trivial injury); time since injury (late presentation changes management); dominant hand; occupation and sporting activities; assess whether the patient can tolerate 6–8 weeks of splint wear
• Examination: characteristic drooped DIP joint; inability to actively extend the DIP joint; the DIP joint can be passively extended to neutral (no fixed contracture in acute cases); assess the PIP joint for any pre-existing hyperextension tendency (swan neck risk); open wounds must be carefully assessed to exclude open tendon injury
• Plain radiographs (AP and lateral of the affected finger): mandatory in all mallet finger injuries to identify: bony avulsion fragment (mallet fracture Type IV); the size of the fragment relative to the articular surface (determines Type IVB vs IVC); subluxation of the distal phalanx volarly (>50% articular fragment with volar subluxation = surgical indication — the fragment is large enough that without fixation the distal phalanx will sublux volarly as the FDP pulls it); physeal fracture in children (Type IVA)

• DIP extension splinting — the gold standard for Types I, IVA, and IVB: the DIP joint is held in full extension (0°) or slight hyperextension (5°) continuously for 6–8 weeks; the PIP joint is left FREE (very important — immobilising the PIP increases stiffness without improving outcomes); the splint must be worn continuously — even brief periods of DIP flexion restart the healing clock; patient compliance is the biggest predictor of outcome; commercially available mallet splints (Stack splint) or custom thermoplastic splints are used; the skin under the splint must be inspected regularly to prevent maceration and pressure sores
• If the splint is removed (e.g., for washing), the patient must hold the DIP in extension with the other hand — the splint must NEVER be removed without the DIP held extended; patients must understand this instruction explicitly
• After 6–8 weeks of full-time splinting: a further 2–4 weeks of night splinting is recommended; then gradual weaning from the splint; the DIP will often have a small residual extensor lag (5–10°) — this is acceptable and usually does not impair function; most patients return to sport at 10–12 weeks with a protective splint for contact sports
• Late presentation of mallet finger: even late-presenting mallet fingers (up to 3 months after injury) can be treated with splinting with reasonable results; the outcome is less reliable than acute treatment but the extension deficit often improves; it is worth attempting splinting even in patients presenting late (4–6 weeks) before considering surgery; beyond 3 months, established fibrous tissue means splinting is less likely to be effective and DIP arthrodesis may be the most appropriate treatment for a symptomatic or functionally limiting deformity

• Surgery is required in a minority of mallet finger cases — approximately 5–10%
• Indications for surgery: Type IVC mallet fracture (bony fragment >50% articular surface + volar subluxation of the distal phalanx — the FDP is pulling the distal phalanx volarly and the fragment is too large to leave unfixed); open mallet with significant tendon loss; failed non-operative treatment with significant functional deficit; symptomatic swan neck deformity from chronic untreated mallet (DIP arthrodesis)
• Extension block pinning (Ishiguro technique): for Type IVC bony mallet; a K-wire is placed through the dorsal skin into the base of the middle phalanx, acting as a physical `block` that prevents the fracture fragment from displacing when the DIP is extended; a second K-wire is then placed across the DIP joint to hold it in extension; the fragment is not directly fixed — the reduction is maintained by the extension block principle; the procedure is performed closed (no open incision); the K-wires are removed at 6–8 weeks
• Open ORIF: for displaced bony mallet fractures; miniature screws or K-wires used to fix the fragment; technically demanding due to the small size of the fragment; complication rate is significant (nail bed injury, osteonecrosis, infection, DIP stiffness); open surgery for mallet fractures has a higher complication rate than non-operative or extension block techniques
• DIP arthrodesis: for chronic symptomatic mallet with established deformity that has failed splinting or presents late; fused in 0–5° of flexion; reliable pain relief and deformity correction; loss of DIP motion (which is relatively small and less functional than PIP motion) is generally well-tolerated

• The `50% rule` for bony mallet fractures: fragments involving >50% of the articular surface are associated with volar subluxation of the distal phalanx (the FDP pulls the remaining articular segment volarly); this subluxation is the indication for surgical intervention — not the fragment size alone; a fragment >50% without subluxation can be treated non-operatively; it is the subluxation that renders non-operative treatment unreliable; always assess the lateral X-ray carefully for volar subluxation of the distal phalanx when a mallet fracture is present
• Skin complications with DIP extension splinting: continuous splinting at 0° extension over 6–8 weeks carries a risk of dorsal skin maceration, blistering, and pressure necrosis over the DIP joint; the skin over the dorsal DIP must be monitored closely; the patient should be instructed to air the skin briefly (holding the DIP in extension) and rotate between splints if possible; a blistered or broken skin over the DIP in a continuously splinted mallet is a real complication requiring splint modification and wound care — if the skin breaks down and the wound becomes infected, the treatment must pause (an infected open wound cannot be splinted)
• Mallet thumb (extensor pollicis longus avulsion at the IP joint of the thumb): the same mechanism can occur at the interphalangeal joint of the thumb, avulsing the EPL tendon; the thumb IP joint droops into flexion; treated identically with IP extension splinting for 6–8 weeks; bony avulsion with fragment >50% articular surface requires ORIF; outcomes are equivalent to finger mallet injuries

• Mallet finger: terminal extensor avulsion at DIP; forced flexion of extended DIP; drooped DIP; inability to actively extend DIP; PIP free; X-ray mandatory to assess for bony avulsion
• Type I (pure tendinous): most common; X-ray normal; DIP splint 6–8 weeks; then 2–4 weeks night splinting; PIP left FREE; excellent results
• Splinting rules: DIP in 0° (full extension) or 5° hyperextension; continuous — NEVER removed without DIP held extended by the other hand; restart the clock if removed; 6–8 weeks minimum
• Untreated mallet → swan neck deformity: DIP flexion → PIP hyperextension develops over time as lateral bands migrate dorsally; prevent by treating mallet appropriately
• Type IVC: bony fragment >50% articular surface + volar subluxation of distal phalanx = surgical indication; extension block pinning (Ishiguro technique) or ORIF
• Extension block pinning (Ishiguro): K-wire dorsal block + DIP K-wire; closed technique; for Type IVC; removes K-wires at 6–8 weeks
• 50% rule: fragment >50% + subluxation = surgery; fragment >50% WITHOUT subluxation = splinting still appropriate; the subluxation is the surgical trigger
• Late presentation (<3 months): splinting still worth attempting; >3 months + symptomatic: DIP arthrodesis in 0–5° flexion; loss of DIP motion generally well tolerated
• Open ORIF for mallet fracture: higher complication rate than conservative or extension block; only for cases where other approaches are not feasible
• Skin maceration: dorsal skin complication from prolonged splinting; monitor closely; rotate splints; if skin breaks down — pause splinting + wound care