Mallet finger is an injury to the terminal extensor mechanism at the level of the distal interphalangeal (DIP) joint. This can be caused by either tendon rupture in zone 1 or an avulsion fracture of the distal phalanx with a fragment of bone that remains attached to the tendon.

There are two proposed mechanisms of injury.

Mallet fingers comprise approximately 9% of all tendinous/ligamentous lesions with an incidence estimated at 5.6% of all tendinous lesions in the hand. The literature does not show any gender difference; however, high-energy injuries are seen in younger, male patients and low-energy mechanisms are seen in the elderly.

The ulnar three fingers are the most commonly affected digits, and tendinous injuries are more common than bony avulsion injuries.

With the loss of the terminal extensor tendon insertion, the central slip receives all of the tension; the volar plate and transverse retinacular ligament attenuate; the lateral bands sublux dorsally; and the proximal interphalangeal (PIP) joint may be forced into extension in chronic injures. The inability to extend the DIP joint and the PIP joint extension is referred to as a swan neck deformity of the finger (Figure 32.2).

Patient’s history usually includes mechanism of injury, and the patient will usually present in the acute phase.

Patients will most commonly endorse painful and/or swollen DIP joint as the primary complaint. In addition, patients will complain of an inability to extend the DIP joint.

On examination, the patient will have a painful and swollen DIP joint with the joint held in flexion (Figure 32.3). The patient will lack the ability to actively extend the tip of the finger.

Plain radiographs can reveal a bony avulsion of the dorsal lip of the distal phalanx articular surface (Figure 32.4). If the injury is purely tendinous, the DIP joint will appear to rest in flexion without a bony avulsion (Figure 32.5).

There are two classification systems that are used most commonly: