Finger Extensor Tendon Injuries
Extensor tendon tears typically occur in two locations in the finger.
- A mallet finger (figure 1), also known as baseball finger or cricket finger, 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 deformity (meaning "button hole" in French). 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.
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.
Figure 3: Extensor mechanism of the finger. In the drawing on the left the common extensor tendon is shown in red. It attaches to the proximal phalanx via bands and extends the MCP joint. In the center figure, the trifurcation is shown. the central slip, shown in sky blue, inserts on the middle phalanx and extends the PIP joint. The lateral bands are shown in blue. In the right-most drawing, the lateral bands are shown joined by the tendons of the interosseous and lumbrical muscles and inserting on the distal phalanx, thereby extending the DIP.
Figure 4: Central slip injury. An injury to the central slip (sky blue) can cause the lateral bands (dark blue) to fall toward the volar side of the finger and make them flexors of the PIP joint.
Patient Presentation
Mallet Finger
There are two types of mallet finger
- Soft tissue : tendon substance disruption
- Bony: Avulsion of extensor tendon from distal phalanx with a small fragment of bone attached to the avulsed tendon.
A bony mallet occurs at the distal phalangeal base, which is the insertion site of the extensor tendon, also known as the terminal tendon. This injury occurs when the distal interphalangeal (DIP) joint is abruptly forced into extreme flexion (i.e. traumatic injury to the tip of the finger), at which time the tendon may pull off a piece of bone.
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.
Boutonnière Deformity
The second most frequent site of injuries involving the extensor tendon is at or near the central slip insertion site of the middle phalangeal base. This injury is known as Boutonnière deformity. The mechanisms underlying Boutonnière deformity include acute violent flexion of the PIP joint, a blow to the dorsum of the middle phalanx, or volar dislocation of the PIP joint.
In the acute stage, Boutonnière deformity presents with pain, swelling of the dorsal PIP joint, mild extension lag, and reduced extension strength against resistance at the PIP joint.
After the acute stage, the intact lateral slips move the volar aspect, inducing flexion of the PIP joint, an increase in the force exerted on the intact terminal tendon insertion, and subsequent extension of the DIP joint. And so 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.
Ultrasound
For mallet finger, instead of tendon echoes at the tendon insertion site of the distal phalanx, an irregular hypoechoic soft tissue lesion is usually present over the distal shaft of the middle phalanx, indicative of the retracted tendon end. In the case of an avulsion fracture, the detached bone fragment at the retracted tendon end and loss of substance in the distal phalangeal base can also be detected on US.
For Boutonnière deformity the central slip of the injured extensor tendon demonstrates a lack of tendon echoes that would indicate insertion into the middle phalangeal base, whereas the lateral slips are intact on both sides of the middle phalanx.
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
Most mallet fingers can be managed non-surgically through splinting (figure 5) of the DIP joint in full extension.
For all soft tissue mallets, splinting is first line. This is usually done with extension splinting for 8-12 weeks 24 hours a day. Splints are either designed to maintain full extension or slight hyperextension. Splinting can be done even with delayed presentation with a successful outcome.
For bony mallets the absolute indication for surgery is volar subluxation or >50% joint involvement or soft tissue laceration. Surgery is controversial for closed acute bony mallet fingers. Some surgeons will offer surgery if there is subluxation of the DIP joint due to a large bony articular fragment avulsion.
Surgical reconstruction can be considered in chronic cases for either type, but complication rates are extremely high (approximately 50%)
Regardless of the treatment option it is common to have a slight extensor lag and prominent bump dorsally.
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.
Resources
References
Part or all of this article or section is derived from Current status of ultrasonography of the finger by Seun Ah Lee et al, used under CC-BY-NC
Part or all of this article or section is derived from Mallet finger and other finger extensor tendon injuries by Orthopaedia.com, used under CC-BY-NC-SA
