Metacarpal neck fractures account for 40% of all hand fractures. A fracture of the neck of the fifth metacarpal, or Boxer’s fracture, named for the classic mechanism of injury in which direct trauma is applied to a clenched fist, is the most common, representing 10% of all hand fractures. Treatment for a Boxer’s fracture varies based on whether the fracture is open or closed, the degree of angulation, rotation, and other concomitant injuries. Immobilization with an ulnar gutter splint may be the definitive treatment for closed, non-displaced fractures without angulation or rotation, while open fractures, significantly angulated or malrotated fractures or those involving injury to neurovascular structures require referral to a hand surgeon.
The most common mechanism of injury for Boxer’s fracture is punching, e.g., the axial pressure applied to the metacarpal bone when the fist is in a clenched position. Direct trauma to the dorsum of the hand may also cause a fracture of the fifth metacarpal neck. The Boxer’s fracture typically does not occur with a fall onto an outstretched hand, unlike many other hand and wrist fractures.
The incidence of metacarpal neck fractures presenting for hospital care in the United States is 13.6 per 100,000 person-years. Metacarpal neck fractures account for 40% of all hand fractures. Fractures of the fifth metacarpal neck account for 10% of all hand fractures. For this injury males, incidence in males is five times higher than in females. Males aged ten to 19 have the highest incidence, followed by males aged 20 to 29. The most common setting for a Boxer’s fracture is in the home, followed by recreational settings.
The fifth metacarpal bone is one of the five metacarpal bones of the hand. The fifth metacarpal is associated with the fifth digit. The metacarpal bone consists of a head (distally located), neck, body, and base (proximally located). Direct trauma to a clenched fist transfers energy to the metacarpal bone axially, causing fractures most commonly at the neck, and typically resulting in apex dorsal angulation due in part to the forces exerted by the pull of the interosseous muscles. The interosseous muscles, responsible for adduction and abduction of the fingers, originate from the metacarpal shafts and insert onto proximal phalanges. The collateral ligaments also join the metacarpal bones to the proximal phalanges and must be taken into consideration during splinting to minimize the risk of loss of motion due to shortening of the ligaments. The ligaments are taut in flexion, and more slack in extension, therefore the MCP joints should be splinted in flexion to prevent shortening. The arteries and nerves supplying the fingers are adjacent to the metacarpal bones and can be injured in severely displaced Boxer’s fractures, requiring surgical intervention.
Patients with Boxer’s fractures present with complaints of dorsal hand pain, edema, and possible deformity in the setting of one of the mechanisms typically associated with this injury involving direct trauma to the hand.
Complete physical exam of a potential Boxer’s fracture should include an examination of the entire hand, comparison to the contralateral (presumably uninjured) hand, with special attention to the following:
Plain radiographs are the standard of care to diagnose Boxer’s fractures and determine a degree of angulation. Anteroposterior, lateral, and oblique views should be obtained. The lateral view should be used to measure the degree of angulation of the shaft of the metacarpal as compared to the mid-point of the fracture fragment. Normal angulation of the metacarpal head to the neck is 15 degrees, so the angulation of the fracture should be measured as that more than the baseline of 15 degrees.
Recent literature suggests that bedside ultrasound may also be used to make an initial diagnosis of a Boxer’s fracture.
CT is generally not used for the diagnosis of metacarpal fractures; however occult fractures may be detected via CT in patients for whom there is a high degree of clinical suspicion for fracture and negative plain radiographs.
The appropriate treatment for a Boxer’s fracture on initial presentation varies based on whether the fracture is open or closed, the degree of angulation, rotation, and other concomitant injuries. Due to the risk of infection from "fight bite," even very small wounds should be thoroughly irrigated, and there should be a low threshold for antibiotic treatment.
For a Boxer’s fracture that is closed, not angulated, and not malrotated or otherwise displaced, splinting is used for initial immobilization. A Boxer’s fracture should be immobilized with an ulnar gutter splint. Alternatively, a pre-made Galveston splint or a custom orthosis may be used.
The hand should be positioned for splinting as follows: mild wrist extension, 70 to 90 degrees of flexion at MCP joint, and slight flexion at the DIP and PIP joints. Flexion of these joints is important to prevent shortening of the collateral ligaments and subsequent loss of range of motion and functional impairment.
Closed reduction is required for a Boxer’s fracture with significant angulation. Fifteen degrees of angulation is the normal metacarpal head to neck angle, and the fifth metacarpal can tolerate angulation of up to 30 degrees. Closed reduction and immobilization is indicated for fractures with angulation greater than 30 degrees. Fractures with pseudo-clawing should also undergo closed reduction.
Analgesia options for the procedure include a hematoma block or an ulnar nerve block. Younger children or very anxious patients may require procedural sedation, but this procedure typically is tolerated well without sedation.
Closed reduction of a Boxer’s fracture is accomplished by using the “90-90 method.” The MCP, DIP, and PIP joints should all be flexed to 90 degrees. The clinician should then apply volar pressure over the dorsal aspect of the fracture site while applying pressure axially to the flexed PIP joint. This axial pressure to the PIP applies dorsal force to the distal fracture fragment. The clinician should be able to feel the reduction when it has been achieved. The injury should be immobilized with an ulnar gutter splint, and post-reduction films should be taken to assess for adequate reduction.
Surgical referral is indicated for fractures that are open, severely comminuted, associated with neurovascular injury, or for fractures with any malrotation. Surgical referral is also appropriate for fractures with significant angulation if the initial provider is unsuccessful in achieving adequate reduction and alignment.
It is reasonable to have a lower threshold for referral to a specialist for patients whose occupation requires a high level of manual dexterity, especially if the patient has injured the dominant hand.
Boxer’s fractures should be sent for repeat radiographs within one week to assess alignment. Radiographs should be obtained every two weeks following, until clinical and radiographic healing are present, typically between four to six weeks. Even with the adequate reduction, some cosmetic deformity may persist, with loss of the normal knuckle contour. After a period of immobilization, the passive and active range of motion exercises should be performed to alleviate stiffness of the MCP and PIP joints. If any loss of function persists after several weeks of these exercises, referral to occupational therapy is warranted.
The delayed presentation is not uncommon with Boxer’s fracture, possibly due to some hesitation to present for care given the classic mechanism of the injury. When patients present 2 to 3 weeks after sustaining this injury, as with initial presentation, assess function, angulation, and pain. If patients have minimal pain and no impairment of function, conservative treatment may be sufficient, even for angulated fractures. However, if there is any evidence of severe angulation, comminuted fractures, neurovascular injury, functional impairment, or unmanageable pain, patients should be referred to a hand surgeon.