A mallet finger (Figure 1) results from a separation of the extensor digitorum from its insertion on the distal phalanx. This separation by tearing away (technically, an "avulsion") results from hyperflexion of the distal interphalangeal (DIP) joint. A commonly seen mechanism of injury is a blow against the tip of the finger by a ball being caught. The injury can either cause the avulsion of the tendon off the bone or the avulsion of the tendon with a piece of the bone attached. When there is bony involvement of the distal phalanx, the injury is referred to as a "bony mallet." An injury to the extensor digitorum more proximally, at the level of the middle phalanx, damages the so-called “central slip” and can lead to a boutonnière (meaning "button hole" in French) deformity. In a boutonnière deformity (Figure 2), the head of the proximal phalanx pops through the central slip (like a button through a button hole). This causes the characteristic deformity of flexion at the proximal interphalangeal (PIP) joint and hyperextension at the DIP joint.

Figure 1: Mallet finger. A patient with a mallet finger cannot extend the DIP joint.

Structure and Function

The extensor digitorum communis extends the four lesser digits of the hand (excluding the thumb) via four tendons emanating from a single muscle (Figure 3). At the level of the metacarpophalangeal (MCP) joint, there is a common extensor tendon, which is maintained in its central position by “sagittal bands” on both the radial and ulnar sides. These sagittal bands attach to the proximal phalanx and allow the common extensor tendon to extend the metacarpophalangeal (MCP) joint.

Just distal to the MCP joint, the extensor tendon trifurcates: a “central slip” and two “lateral bands.”

The central slip crosses the PIP joint and attaches to the base of the middle phalanx. The central slip is therefore responsible for extension of the PIP joint.

The lateral bands project in the radial and ulnar directions, where they are joined by the tendons of the interosseous and lumbrical muscles. The lateral bands then move more centrally and cross the dorsal surface of the DIP joint where then insert commonly on the distal phalanx and therefore extend the DIP joint.

An injured sagittal band, it follows, will fail to keep the extensor tendons centralized. Such an injury can cause subluxation of the extensor tendons over the MCP joint when a fist is made, causing a snapping sensation in both flexion and extension at the joint.

An injury to the central slip (Figure 4) can lead to an inability to extend the PIP joint. Also, such an injury may allow the lateral bands to fall in the volar direction (that is, toward the palm). If the lateral bands end up on the volar side of the PIP joint, they will in turn flex that joint, which accentuates the boutonniere deformity.

Patient Presentation

Patients with mallet finger will present after a traumatic injury to the tip of the finger. The dorsal DIP may be ecchymotic and usually painless. There is an obvious droop of the tip of the finger and the patient will be unable to fully actively extend the finger at the DIP joint.

A chronic mallet finger can present as a so-called "swan neck deformity" characterized by DIP hyperflexion with PIP hyperextension. (This the "opposite" of a boutonnière deformity.) The failure of the extensor tendon at the distal phalanx coupled with unopposed action of the flexor digitorum superficialis (FDS) leads to the DIP hyperflexion. Concentration of the extensor on the middle phalanx alone causes the PIP hyperextension.

Patients with central slip injuries may present with pain and swelling along the dorsal PIP joint. The patient may initially have mild weakness in extending the finger at the PIP joint. If the patient presents weeks to months after the injury a progressive boutonniere deformity may develop.

On physical examination a central slip injury can be diagnosed with the Elson test. In order to perform the Elson test, the PIP joint is held fully flexed. If the central slip is injured the patient will be able to extend the DIP joint despite maintaining the PIP in flexion.

Patients with sagittal band injuries will present with a traumatic injury over their knuckle. Sagittal band injuries are commonly called “boxer’s knuckles.” The extensor tendon may slip back and forth over the MCP joint. Patients may report a snapping sensation while flexing the digit with a visible volar subluxation or dislocation of the extensor tendon when making a fist. With progression of the injury, patients may develop difficulty extending the finger at the MCP joint.

Objective Evidence

The best objective evidence for an extensor tendon injury is the physical exam. All finger injuries should have an x-ray on initial evaluation. An x-ray of the finger (A/P, lateral, and oblique) should be ordered. A dedicated finger x-ray should be reviewed, as a hand x-ray will often have overlap of the digits leading to difficultly in diagnosing fractures and dislocations. A finger x-ray may show a bony avulsion at the proximal aspect of the distal phalanx (bony mallet). An x-ray can also assess the presence or absence of a dislocation.

The PIP joint should be assessed for an avulsion fracture of the dorsal base of the middle phalanx indicating a possible central slip injury. A central slip disruption is diagnosed when a volar dislocation of the PIP joint is noted on lateral x-ray.

Patients with a sagittal band injury will usually have normal x-rays. Radiographs are obtained to rule out an associated fracture or subluxation of the MCP joint.

If the diagnosis is still in question, an MRI or ultrasound should be considered to further assess the integrity of the extensor mechanism.

Epidemiology

Mallet finger is most commonly seen in the small, ring, and middle fingers in the dominant hand. Mallet finger more commonly affects men, usually during work or sports related activities. With appropriate splinting most patients can return to work.

Injuries to the sagittal bands or the central slip are less common. Central slip dysfunction may be related to direct trauma or to a volar dislocation of the PIP joint. The sagittal bands may also be damaged from a direct injury to the MCP joint, as occurs in boxing. The radial sagittal band is most commonly injured leading to ulnar subluxation of the extensor mechanism. In addition, patients with rheumatoid arthritis can develop attritional ruptures of the sagittal band.

Differential Diagnosis

Patients who present with trauma to the distal phalanx and DIP joint may have other associated injuries including nail bed injuries, tuft fractures, nail plate avulsion, and distal phalanx fracture dislocations without tendon injury to name a few.

Patients with a flexion contracture of the PIP joint often appear as if they have a boutonnière deformity. Patients with this pseudo-boutonnière will have full flexion of the DIP joint whereas a true boutonnière deformity leads to hyperextension and limited flexion of the DIP joint.

Sagittal band injuries are commonly misdiagnosed as a trigger finger due to the snapping sensation with finger flexion and extension.

Red Flags

Mallet finger leads to the inability to actively extend the DIP joint. It is important to obtain an x-ray to rule out a fracture dislocation of the DIP joint which can present in a similar fashion.

An injury to the central slip may initially lead to mild tenderness about the dorsal PIP. One must have a high index of suspicion for this injury and splint early to prevent a boutonnière deformity. If left untreated, a central slip injury may lead to a stiff deformed finger as the ligaments and volar plate contract. It is very important to refer these patients to a hand specialist. Beware of the volar PIP joint dislocation since the central slip is torn during this injury and treatment is different from the more common dorsal PIP joint dislocation.

It is critical to observe the location of the extensor mechanism as the patient makes a fist to best assess for subluxation.

Treatment Options and Outcomes

In the acute setting, a mallet finger is treated with splinting (Figure 5) of the DIP joint in full extension for six weeks and occasionally up to three months. Even in the chronic setting extension splinting can be tried. If non-operative treatment is unsuccessful, a primary repair or reconstruction may need to be performed by a hand specialist. If there is subluxation of the DIP joint due to a large bony articular fragment avulsion, primary repair may be the first line of treatment.

Central slip injuries are initially treated by extension splinting of the PIP joint. DIP joint flexion exercises are started to prevent stiffness and tendon migration. If non-operative treatment with splinting fails, occasionally operative repair or reconstruction is considered.

Sagittal band injuries are initially treated with extension splinting of the MCP joint for four to six weeks. Sagittal band injuries are one of the rare instances where the MCP joints are immobilized in extension. The decision whether repair or reconstruction of the sagittal band is needed is one best made in consultation with a hand specialist.

The goal in treating a patient with an extensor mechanism injury is to regain full motion of the digit without pain. Despite non-operative or operative treatment of a mallet finger, patients often develop a mild extensor lag (droop). The most important aspect of treating a patient with an extensor tendon injury is coordinating care with a skilled hand occupational therapist; this allows the patient to regain motion while preventing contracture.

A central slip injury treated acutely often leads to full range of motion. Once a boutonnière deformity develops, operative or non-operative treatment typically results in a persistent mild deformity.

Sagittal band injuries treated with extension splinting or by operative repair or reconstruction can be expected to have good results with near full range of motion.

Risk Factors and Prevention

In order to prevent injuries to extensor tendons of the hand it is important to wear appropriate protective devices during high risk activities.

Unfortunately, injuries to the hand occur despite use of these devices, and it is important to seek treatment early to prevent further deformity and disability.

Miscellany

The word "boutonnière" comes from the French word for "buttonhole." In boutonnière deformities, the head of the proximal phalanx “buttonholes” through the defect in the extensor mechanism from the central slip injury leading to the deformity.

Key Terms

mallet finger, swan neck deformity, boutonnière deformity, extensor tendon, central slip, extensor digitorum communis, lateral bands, sagittal bands, boxer’s knuckles.

Skills

Examination of extensor mechanism of the finger.