Fractures of the metacarpals generally occur with a straightforward history of trauma. The exception is an open fracture over the dorsal aspect of the metacarpal phalangeal (MCP) joint - particularly over the fourth or fifth digit. In this case, clinical suspicion for "fight bites" must be raised as the appropriate treatment is specialized for this type of injury. Pertinent history in hand injuries includes mechanism of injury, handedness and vocation. The patient may hold the hand in the typical pseudoclawing position with the MCP joint hyperextended and the proximal interphalangeal (PIP) joint flexed.
The metacarpals are tubular bones. The four bones of the second to fifth digit are connected by two sets of ligaments - the interosseous ligaments at the base proximallyand the deep transverse ligaments distally around the MCP joint. These ligaments act a checkrein and minimize shortening with fractures. Intrinsic muscles of the hand have attachments to the metacarpals and play a role in the deforming forces on the metacarpals.
Metacarpals can be grouped on locations as fractures of the head, neck, shaft or base.
The Orthopaedic Trauma Association classification uses an alpha-numeric classification scheme. The schematic for metacarpal fractures is 25(modifier) - _ _ . _.
The modifier specifies which digit is involved:
T - Thumb
I - Index
M - Middle
R - Ring
L - Little
The first blank after the modifier is a letter specifying the articular nature if the fracture:
A - Extra-articular
B - Articular
C - Artcular / extraarticular
The second blank is a number specifying the bony location of the fracture:
1 - Head (includes the neck as extra-articular fractures - 25-A1)
2 - Shaft
3 - Base
The decimal point after the first two modifiers further characterizes the fracture pattern - simple, oblique, degree of comminution, etc.
A patient with a metacarpal fracture may hold the hand in a pseudoclaw position as described above. Dorsal swelling and ecchymosis may be present. The skin should be inspected for open fractures. Malrotation of the digits also needs to be evaluated on inspection. Physical examination of the hand should include 2 point discrimination with >5mm considered abnormal.
AP and lateral x-rays of the hand should be preformed. Brewton and skyline views may be needed to minimize bone overlap. The Brewton view evaluates the metacarpal base, and the skyline view is for vertical impaction of the metacarpal head.
The typical presentation of a metacarpal shaft fracture is one with an apex dorsal angulation from the pull of the volar, proximal intrinsic muscles.
Standard goals of treatment are restoration of length, alignment and rotation.
Closed treatment in isolated, stable fracture can be treated with closed methods and immobilization. Splints should have the wrist in 20-30 degrees of extension, MCP in 90 degrees of flexion and the interphalangeal (IP) joints in extension, though some advocate leaving the proximal IP free to allow finger flexion. Once swelling subsides, cast application may be instituted for 4-5 weeks.
Open reduction is indicated with unstable patterns (spiral, oblique, comminuted), inadequate reductions, or multiple metacarpals are fractured. Open reduction occurs through an extensile incision (longitudinal) on the dorsal aspect of the hand often between two metacarpals. The extensor tendons overly the dorsal surface of the bones and are connected by juncturae tendinum. The juncturae may be incised, split and repaired after reduction to allow access to the fracture site.
Extra-articular fractures of the thumb may tolerate up to 30 degrees of angulation and up to 4mm of shortening given the mobility of the thumb. Greater than 30 degrees of angulation starts to interfere with the ability to pinch. The majority of these fractures may be treated closed in a thumb spica.
Intra-articular fractures of the thumb represent a fracture-dislocation of the carpal metacarpal (CMC) joint. There are two main patterns - Bennett's and Rolando's fractures.
Bennett's - This pattern is with a small base fragment on the ulnar side of the metacarpal. This fragment is attached to the trapezium by beak ligament (primary stabilizer of the thumb CMC) and the remainder of the metacarpal is displaced by the pull of the abductor pollicis longus with some contribution of the adductor pollicis. Treatment usually requires percutaneous pinning or screw fixation to achieve a stable reduction.
Rolando's - The Y fracture pattern includes a fragment on the trapezium similar to a Bennett's but also a radial fragment as well. Treatment is ORIF though external fixation may be used also.
Reductions are required with severe dorsal angulation (10-20 degrees for the index and long finger and 30 degrees for the ring and small finger), shortening of more than 5mm, or malrotation. The upper limit of rotation accepted is 10 degrees, though 5 degrees of malrotation can lead to 1.5cm of overlap on flexion of the digits.
Fixation of unstable fractures - spiral, oblique or comminuted fractures - can be achieved with:
Interfragmentary screw fixation
Mini- fragment plate fixation (acts as a dorsal tension bend) with four cortices of fixation above and below the fracture
External fixation or
Occur most frequently on the 4th and 5th digit when an axial load is applied through the metacarpal. The so-called Boxer's fracture occurs with a closed fist injury. The increasing degree of mobility of the metacarpals from radial to ulnar allows for increasing angulation acceptance. The dorsal apex displacement tolerated is 10 degrees for the index finger, 20 degrees for the long finger, 30 degrees for the ring finger and 40 degrees for the small finger. These fractures can generally be treated closed with cast immobilization, though physical exam must evaluate the rotation of the digits in flexion.
Possible complications of injury and treatment include malunion, nonunion, arthritic degeneration, compartment syndrome, reflex sympathetic dystrophy. In particular dorsal plates may result in attritional rupture of the extensor tendon.