Mallet Finger 

Loss of extensor tendon continuity at the distal interphalangeal joint (DIPJ) causes the joint to rest in an abnormally flexed position, as shown below. This occurs with a laceration to the dorsum of the digit near the DIPJ. Mallet finger describes the condition in which the skin remains closed and the extensor tendon is either forcibly stretched or avulsed from the distal phalanx.

The terminal portion of the extensor mechanism that crosses the distal interphalangeal joint (DIPJ) in the midline dorsally is responsible for active extension of the distal joint. A flexion force on the tip of the extended finger jolts the DIPJ into flexion. This may result in a stretching or tearing of the tendon substance or an avulsion of the tendon's insertion on the dorsal lip of the distal phalanx base. In either instance, active extension power of the DIPJ is lost, and the joint rests in an abnormally flexed position (mallet finger), as shown below.

Frequency

Several different athletic injuries can occur at the interphalangeal joints. The most common injury is a sprain of the proximal interphalangeal joint (PIPJ), the so-called jammed finger. Mallet fingers are less common than PIPJ sprains, but they are more common than PIP fractures or fracture dislocations. Similar injuries to the extensor mechanism at the interphalangeal joint of the thumb occur, although infrequently.

Any forced flexion of the finger while it is held in an extended position risks the integrity of the extensor mechanism at the distal interphalangeal joint (DIPJ). The classic mechanism of injury is a finger held rigidly in extension or nearly full extension when the finger is struck on the tip by a softball, volleyball, or basketball. Other common mechanisms of injury include forcefully tucking in a bedspread or slipcover or pushing off a sock with extended fingers.

From experimental studies, the rate of loading determines whether a tendon (or ligament) ruptures in mid substance or is avulsed from its bony attachment. Rapid loading rates are more likely to cause a tear in the tendon itself. Lower loading rates are more likely to cause a bony avulsion. This occurs because the bone is relatively more viscoelastic than the tendon.

Following a forced distal interphalangeal joint (DIPJ) flexion injury, the patient notices the inability to actively extend the distal joint, although full passive extension remains intact. The dorsum of the joint may be slightly tender and swollen, but often the injury is painless or nearly painless. Patients may think that the joint is only sprained. They continue playing sports and notice loss of active extension after 1 or more days.

Patients may not present to the orthopedist with mallet finger for weeks or even months, perhaps having received no treatment or ineffective treatment. Bony injuries heal within weeks; thus, an old bony injury without functional deficit is best left untreated. A tendinous injury generally can be improved by extension splinting up to 6 months from the time of injury. The period of splinting for such an old injury is extended because the area becomes less inflamed as time passes. Therefore, fibroplasia and wound contraction occur more slowly and less completely.

Attempted open reduction and internal fixation (ORIF) of a mallet injury (mallet finger), either tendinous or bony, often results in a stiff, infected, or painful finger. When a large, bony fragment is observed, the surgeon instinctively wants to anatomically reconstruct the articular surface. Remember, however, this is a non-weight-bearing joint, and articular incongruity, which would not be tolerated in the ankle or knee, is well tolerated in the distal interphalangeal joint (DIPJ). This joint has been demonstrated to remodel beautifully over time, even in the presence of volar subluxation of the distal phalanx. Late osteoarthritis at the DIPJ after an untreated mallet finger or a mallet finger that is treated without anatomic reduction of the fracture is rare, if not nonexistent. The risk of poor outcome from ill-advised open treatment far outweighs any risk of early dysfunction or late arthritis from splint treatment.

The terminal extensor tendon is a thin, flat structure measuring approximately 1 mm thick and 4-5 mm wide. This tendon occupies the sparse space between the bone and dorsal skin and inserts onto the dorsal lip of the distal phalanx, well proximal to the germinal nail matrix. At the distal interphalangeal joint (DIPJ), the tendon's excursion is only several millimeters from full joint extension to 80° of flexion. The tendon is the terminal extension of the dorsal mechanism, which is a complex crossing of fibers at the proximal interphalangeal joint (PIPJ) and at the metacarpophalangeal joint (MCPJ).

Powered by the lumbrical and interosseous muscles, the dorsal mechanism flexes the MCPJs and extends both the PIPJs and DIPJs. With a disruption of the dorsal mechanism at the DIPJ, the entire power of extension is directed to the PIPJ. Over time, and especially if the volar plate is lax, this concentrated extension force results in PIPJ hyperextension and a swan-neck deformity (ie, the DIPJ rests in an abnormally flexed position and the PIPJ rests in a hyperextended position). This deformity frequently causes a functional deficit.^[1] Therefore, even if a mallet finger is not particularly symptomatic from a functional or cosmetic perspective, treatment of the mallet injury may preclude development of this swan-neck deformity.

In most instances, the surgeon should resist any urge to treat these injuries surgically. (See the reasons listed in Indications.)