Mallet Finger

Mallet finger is disruption of the terminal extensor tendon at the distal interphalangeal joint, with an extensor lag of the distal phalanx. It occurs either as a tendon rupture or as an avulsion fracture of the dorsal base of the distal phalanx. The usual mechanism is sudden forced flexion of an extended fingertip, such as a ball striking the tip, or tucking in a bedsheet.

It is one of the most common closed tendon injuries of the hand, with a peak in young active people and in middle aged women. The long and ring fingers of the dominant hand are often involved.

Classification: type I closed injury (with or without a small avulsion), type II open laceration, type III open abrasion with tendon loss, type IV fracture (A transepiphyseal in children, B hyperflexion fracture with 20 to 50% of the articular surface, C hyperextension fracture with over 50% and volar subluxation).

Untreated injury can progress to swan neck deformity, in which the lateral bands migrate dorsally and the proximal interphalangeal joint hyperextends. Complications are persistent extensor lag, swan neck deformity, nail deformity, skin necrosis from splint pressure and distal joint arthritis.

• A drooping fingertip, inability to actively extend the distal joint, with full passive extension
• Pain, swelling and bruising over the dorsal distal joint, which may be mild
• Extensor lag of 10 to 40 degrees
• Red flags: open wound or nail bed injury, volar subluxation of the distal phalanx, fixed flexion that cannot be passively corrected, fever, and a fracture involving over half of the joint surface

• Clinical diagnosis from the lag and the preserved passive extension.
• True lateral radiograph of the finger (plus anteroposterior): size of the avulsed fragment as a percentage of the articular surface, volar subluxation and joint congruity.
• Ultrasound for tendon integrity when the radiograph is normal.

Differentials: boutonniere deformity, extensor tendon laceration, distal phalanx fracture, distal interphalangeal dislocation, Heberden nodes with osteoarthritis, jersey finger (flexion deficit instead).

Closed injuries (with or without a small avulsion):

• Continuous splinting of the distal interphalangeal joint in neutral to slight hyperextension (a stack splint or a thermoplastic splint), with the proximal interphalangeal joint left free. Wear full time for 6 to 8 weeks, then at night for 4 to 6 weeks.
• The finger must never flex during skin checks. If the joint droops even once during the splinting period, the clock restarts.
• Monitor skin under the splint for pressure necrosis. Chronic cases up to 3 months old can still be splinted for 8 to 12 weeks.
• Paracetamol 1 g orally every 6 hours and ibuprofen 400 mg orally every 8 hours with food for up to 5 days.

Surgery:

• Fracture with volar subluxation, or involving over 30 to 50% of the joint surface with a step over 2 mm, or an unstable joint: extension block pinning (a 1.1 mm Kirschner wire in the middle phalanx head dorsal to the fragment), then reduction of the joint and a transarticular pin for 6 weeks. Alternatively open reduction and fixation with a mini screw or a suture anchor.
• Open injuries: wound irrigation, skin and tendon repair with 4-0 non-absorbable suture, and a transarticular pin for 6 weeks.
• Cefazolin 2 g IV before incision for surgery, and cefalexin 500 mg orally every 6 hours for 5 days for open injuries. Give tetanus toxoid 0.5 mL IM if immunity is not current.
• Children with a physeal injury (Seymour fracture) need irrigation and antibiotics, since the nail bed is often interposed.
• Chronic swan neck deformity: spiral oblique retinacular ligament reconstruction or lateral band tenodesis. Painful arthritis or a fixed deformity: distal interphalangeal arthrodesis.

Outcomes: most patients regain extension, with a residual lag of under 10 degrees being functionally acceptable.

Complications: extensor lag, pin tract infection, nail ridging, joint stiffness and skin ulceration.

Follow up weekly for 2 weeks, then at 6 and 12 weeks. Counsel on strict splint adherence.