Introduction

Mallet finger injuries are commonly encountered in everyday clinical practice. Mallet, which means hammer, was the term used to describe the hammer-like deformity that occurred in sports-related injuries in the 1800s. As some individuals do not see the hammer resemblance, some have proposed changing the name to drop the word "finger" due to its appearance. It is also known as "baseball" finger. Although it is the most common closed tendon injury seen in athletes as a result of high velocity and contact sports, it also can be the result of a relatively minor trauma such as doing household chores (tucking in a shirt, tucking in sheets) or work-related activities. Though some athletes and coaches often believe mallet injuries to be minor, each case should have a systematic evaluation performed.[1][2][3]

Etiology

The bones making up the digits are called phalanxes or phalanges. They include two bones in the thumb or three bones in the fingers as well as two to three joints between the phalanges. These joints are the distal interphalangeal joint (DIPJ); the metacarpophalangeal joint (MCPJ), which is the joint that connects the digit to the carpal or hand bones, and the proximal interphalangeal joint (PIPJ), which is the joint between the DIP and MCP joint. The joints sit in volar plates, which are collateral ligaments attached to dense fibrous connective tissue, to increase stability. Tendons are tissues that connect muscles to bone. The muscles that move the digits (fingers and thumbs) are located in the forearm and are connected to the bones of the digits by long tendons. The tendons on the top of the hand are called extensor tendons and extend or straighten the digits, while the flexor tendons on the pals side of the hand flex or bend the fingers. The tendons of the digits extend over three joints. Mallet finger injuries result when the extensor tendon is disrupted.[4][5][6]

Epidemiology

The most commonly affected fingers are as follows: long finger greater than ring finger greater than index finger greater than little finger, thumb. The most common cause of mallet finger is a violent flexion or laceration to the dorsum of the finger at the distal interphalangeal joint  (DIPJ).  The most common mechanism is the finger being struck at the tip or “end on” resulting in tearing of the extensor tendon where it joins at the distal phalanx, distal to the DIPJ, with or without a small fragment of bone from the insertion point.  Extensor tendon disruption causes the distal phalanx to droop or remain in a flexed position without the ability to actively extend the DIPJ, resulting in the deformity. Doyle classified mallet finger into four categories:

• I: Closed with or without small avulsion fracture
• II: Open laceration with tendon discontinuity
• III: Deep abrasion with skin loss and loss of tendon continuity.
• IV: Mallet finger includes 3 subtypes:

• A: Transepiphyseal plate fracture in children
• B: Fracture of articular surface between 20-50%
• C: Fracture of articular surface >50%

History and Physical

Patients usually present with pain and swelling over the end of finger, the DIPJ flexed and the distal phalanx in a drooped or bent position, and inability to hold the finger straight at the DIPJ. The physical exam is pertinent for DIPJ flexion at rest, inability to straighten or extend the DIPJ during active range of motion testing, and tenderness and swelling near DIPJ. It is important to isolate the DIPJ for accurate assessment. All open injuries affecting the dorsum should raise suspicion for this type of injury as well.

Evaluation

Mallet finger is a clinical diagnosis that requires a thorough history and physical exam. Imaging studies are integrated as supportive measures to assess bony injuries.  An anterior-posterior (AP), lateral, and oblique view Xray centered at the DIPJ of the affected finger should be obtained to classify the mallet finger category - to differentiate a bony injury versus tendinous mallet injury. The lateral view is the most useful for assessing for avulsion fractures and volar (palmar) subluxation of the distal phalanx. Some have suggested that ultrasound may also be utilized for diagnosis.[7][8][9]

Differential Diagnosis

• Hand and wrist surgery in rheumatoid arthritis

• Jammed finger

• Metacarpophalangeal injuries

• Open wounds of the extensor tendon

• Osteoarthritis

• Phalangeal fractures

• Seymour lesions

• Swan-neck deformities

Prognosis

Outcomes after treatment can be assessed based on Crawford Classification [23]. 

• Excellent outcome: No pain and a full range of motion at the DIP joint.
• Good outcomes: less than a 10-degree extension deficit.
• Fair outcome:10–25 degrees of extension deficit with no pain.
• Poor outcome: more than 25 degrees of extension deficit or persistent pain.

Missed diagnosis or inappropriate treatment can result in DIPJ dysfunction. Lengthening of the terminal extensor tendon by 1mm results in 25 degrees of extension lag, and a shortening of 1 mm will restrict DIPJ flexion [24].

Complications

Dorsal skin complications: These are the most commonly encountered complications e.g ulceration, maceration, nail deformity [25].

Mild extensor lag < 10° and prominent bump on the dorsal aspect of the DIPJ: may be present following nonoperative or operative treatment [26]. However, no functional deficits or patient dissatisfaction have been reported [27][28]. There are no reported differences in the outcomes in terms of extensor lag or patient satisfaction following nonoperative treatment regardless of nighttime splinting after full time splinting [29]. 

Swan neck deformities: this occurs due to weakness of the volar plate and transverse retinacular ligament at PIPJ level,  and dorsal slippage of the lateral bands with subsequent PIPJ hyperextension. The deformity is maintained by the contracture of the triangular ligament.

Specific operative treatment complications: Superficial and deep infection, deformity of the nail, secondary displacement of the fracture, joint incongruity, avascular necrosis, or extensor tendon rupture have been reported with operative management [30][31].

Pearls and Other Issues

Another possible complication is swan neck deformity (the DIPJ remains abnormally flexed, and the distal interphalangeal joint (PIPJ)) rests in a hyper-extended position). Swann neck deformity results from the extensor tendon disruption interfering with the volar plate’s provided stability. The disrupted extensor tendon leaves the flexor digitorum superficialis unopposed, leading to DIPJ hyperflexion while concentrated extensor on the middle phalanx results in PIPJ hyperextension. A DIPJ extensor lag of 5-10 degrees is not uncommon following treatment, as it occurs in about 40% of cases.  Approximately 70% of cases may have transient skin problems due to the splint placement.  Approximately 60% of mallet fingers have satisfactory results after splintage, followed by 20% improving in due course.

Enhancing Healthcare Team Outcomes

Mallet finger is often encountered by the nurse practitioner, primary care giver, emergency department physician and orthopedic surgeon. It is important to be aware that current evidence suggest that non-surgical treatment be undertaken first. Splinting the finger may lead to recovery in most patients. Only recalcitrant cases should be referred to the hand surgeon.

Physical therapy is vital to help recover joint function and strength. Unfortunately, returning back to sports can lead to recurrence.