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Mallet finger and other extensor mechanism injuries

Description

Mallet finger is a very common injury to the terminal extensor tendon of the finger.  The extensor digitorum communis (EDC) tendon is injured at its insertion site on the distal phalanx.  The injury can involve avulsion of the tendon off the bone as well as an avulsion of the tendon with a piece of the distal phalanx bone attached (bony mallet).  The injury occurs when an extended finger is forcibly flexed at the distal phalanx. This injury is commonly seen in ball sports such as baseball and football.  The patient is unable to actively extend the digit at the distal interphalangeal joint (DIP joint) and there may be a noticeable droop at the DIP joint. 

The central slip is a very specialized portion of the extensor tendon, which inserts on the dorsum at the base of the middle phalanx and receives contributions from the EDC and intrinsic muscles of the hand (interossei and lumbricals).  An injury to the extensor tendon at the level of the central slip can lead to a boutonniere deformity if not addressed.  A boutonniere deformity manifests as flexion at the proximal interphalangeal joint (PIP joint) and hyperextension at the DIP joint. A boutonniere deformity is much more difficult to treat if it is chronic.

The sagittal bands are bands of tissue that originate at the base of metacarpal phalangeal joint (MCP) of the fingers, which keep the extensor tendons centralized and help extend the MCP joint.  An injury to a sagittal band can cause subluxation of the extensor tendons when a fist is made.  A chronically subluxated extensor tendon can lead to difficulty in extending  the MCP joint.

Structure and function

 The extensor mechanism of the finger includes contributions from both extrinsic and intrinsic musculature of the hand.  The EDC innervated by the posterior interosseous nerve, originates from lateral epicondyle of the humerus and inserts on the dorsal expansion of the 2nd through 5th digits. Any injury along the tendon can lead to difficulty with extending the finger.  In a mallet finger the tendon is injured at the level of the DIP joint leading to extensor lag and a droop of the finger at the DIP joint.

The EDC trifurcates proximal to the PIP joint with the central component becoming the central slip and the lateral components joining up with the lateral bands. The intrinsic muscles of the hand and their tendons form the lateral bands. The central slip inserts at the base of the middle phalanx. Any injury to the central slip can lead to a flexion deformity at the PIP joint. As the lateral bands tend to lie volar to the axis of rotation of the PIP joint and dorsal to the DIP joint, they become deforming forces leading to the boutonniere deformity.

The sagittal bands insert on the volar plate at the level of the MCP joint.  These bands keep the EDC tendons centralized over the MCP joints.  There are both radial and ulnar sided sagittal bands that centralize the extensor tendon.  Injury to either radial or ulnar sided sagittal band can lead to subluxation or dislocation of the EDC tendon and if chronic can lead to an inability to extend the finger at the MCP joint. 

Epidemiology

 Mallet finger, central slip and sagittal band injuries are common injuries and are the result of injury to the extensor mechanism of the finger. 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 patient’s can return to work.

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

Clinical presentation

Patients with mallet finger will present after a traumatic injury to the tip of the finger. The injury is often innocuous.  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. 

Patient’s 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. 

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 while making a fist. With progression of the process, patients may develop difficulty extending the finger at the MCP joint.  

Red flags

Mallet finger leads to 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 boutonniere deformity.  If 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.  

Differential diagnosis

Patient’s who present with trauma to the distal phalanx and DIP joint may have associated injuries including nail bed injuries, tuft fractures, nail plate avulsion, distal phalanx fracture dislocations without tendon injury and many other associated injuries. 

Patient’s with a flexion contracture of the PIP joint often appear as if they have a boutonniere deformity. Patients with this pseudo-boutonniere will have full flexion of the DIP joint whereas a true boutonniere deformity leads to hypertension 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.

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, oblique) should  be ordered not a hand x-ray.  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) and rule out a dislocation. 

The PIP joint shoulde 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 x-ray.  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 a sagittal band injury will usually have normal x-rays. The radiographs are obtained to rule out an associated fracture or subluxation of the MP joint.

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

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 these devices and it’s important to seek treatment early to prevent further deformity and disability.

Treatment options

In the acute setting, a mallet finger is treated with splinting 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.  If non-operative treatment with splinting fails occaisionally operative repair or reconstruction is considered.

Sagittal band injuries are initially treated with extension splinting of the MCP joint for 4-6 weeks. It is one of the few instances where the MCP joints are immobilized in extension. If the patient fails non-operative treatment then repair or reconstruction of the sagittal band may need to be performed by a hand specialist.

Outcomes

The goal in treating  a patient with an extensor mechanism injury is to regain full motion of the digit that is painless.  Despite non-operative or operative treatment of a mallet finger  patient’s 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 therapy allows the patient to regain motion while preventing a contracture.

A central slip injury treated acutely often lead to full range of motion.  Once a boutonierre 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 a good result with near full range of motion.  

Miscellany

The word Boutonniere comes from the French term buttonhole.  In the boutonniere deformity the head of the proximal phalanx “buttonholes” through the defect in the extensor mechanism from the central slip injury leading to the deformity. 

Why does a Boutonniere deformity occur with a central slip injury?  If you understand the anatomy of the tendons it will be obvious.

How can a chronic mallet finger lead to a swan neck deformity?

Key terms

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

Skills

Examination of extensor mechanism of finger.

The Elson’s test is the best provocative manuever to diagnose a central slip injury.  In examining a normal digit, the DIP will not be able to be extended actively while holding the PIP fully flexed.  A patient with a central slip injury will be able to extend the DIP joint despite holding the PIP in a flexed position. 

 

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