Metacarpal Neck Fracture

Anatomy

In human anatomy, the metacarpal bones, form the intermediate part of the skeletal hand located between the phalanges of the fingers and the carpal bones of the wrist.

The metacarpals form a transverse arch to which the rigid row of distal carpal bones are fixed. The border metacarpals (those of the thumb and little finger) form the sides of the cup of the palm, and as they are brought together they deepen this arch or concavity. The index metacarpal is the most firmly fixed to the carpal bones, while the thumb metacarpal articulates with the trapezium, has the most degrees of freedom, and acts independently from the others. The index and middle metacarpals are tightly held to the carpus by intrinsic interlocking bone elements at their bases. The ring metacarpal is somewhat more mobile while the small metacarpal is semi-independent.^{[1, 2]}

Each metacarpal bone consists of a shaft, and two extremities: the head at the distal end, and the base at the proximal end.

Body

The shaft is prismoid in form, and curved, so as to be convex in the longitudinal direction dorsally, concave volarly. It presents three surfaces: radial, ulnar, and dorsal.

The radial and ulnar surfaces are concave, for the attachment of the interosseus muscles, and separated from one another by a prominent volar ridge.
The dorsal surface in its distal two-thirds has a smooth, triangular, flattened area which is covered by the tendons of the extrinsic extensor muscles. More proximally, there is a ridge which runs along the rest of the dorsal surface to the base. This ridge separates two sloping surfaces for the attachment of the dorsal interossei muscles.

Base

The base is of a cuboidal form, and broader dorsally than volarly: it articulates with the carpal bones and with the adjoining metacarpal bones.

Head

The head presents an oblong surface markedly convex from volar to dorsal, less so transversely, and flattened from side to side; it articulates with the proximal phalanx. It is broader and more prominent on the volar compared to the dorsal aspect, and is longer in the volar-dorsal than in the transverse diameter. On either side of the head is a tubercle for the attachment of the collateral ligament of the metacarpophalangeal joint.

The dorsal surface, broad and flat, supports the tendons of the extensor muscles.

The volar surface is grooved in the midline, and marked on either side by an articular eminence or condyle.

Neck

The neck is the transition zone between the body and the head.

Articulations

Besides the metacarpophalangeal joints, the metacarpal bones articulate by carpometacarpal joints as follows:

the thumb with the trapezium and index metacarpal;
the index with the trapezium, trapezoid, capitate, and middle metacarpal;
the middle with the capitate, and index and ring metacarpals;
the ring with the capitate, hamate, and middle and small metacarpals;
the small with the hamate and ring metacarpal.

Mechanism of Injury

Most metacarpal neck fractures are the result of striking a hard object with a fist, and the most commonly fractured digit is the small finger.[3] Often this occurs due to a physical altercation between two individuals, and the fist strikes the other’s head. Walls or other solid structures are also struck, usually in anger or frustration. This has led to popularly calling these fractures “boxer’s fractures”, but some have made the distinction that true boxers are more likely to strike with the center of their fist, thus fracturing the middle or ring finger metacarpal neck, and that a small finger metacarpal neck fractures should more appropriately be called a “brawler’s fracture”. It should be noted that a metacarpal neck fracture is not solely caused by aggressive or violent activity – a slip and fall on a closed fist may cause the same fracture pattern – and also that many misinformed individuals will call any fracture of the small finger metacarpal a “boxer’s fracture”, even though the term only applies to the neck. Because of these reasons, this and other eponyms should be abandoned in favor of a descriptive terminology that more accurately communicates information.

Symptoms

Patients will complain of pain and swelling in the area of the MP joint of the digit that is involved, usually on the ulnar side of the hand.

Physical Examination

The findings include swelling, tenderness, and deformity. There may be a palpable prominence on the dorsum of the hand, and the metacarpal head may be flexed (less prominent dorsally) into the palm. The digital cascade should be assessed for rotational malalignment. If there is a laceration, then some effort should be made by the examiner to determine if the traumatic event included a punch to the jaw or mouth. In this case, a tooth may have penetrated through the skin. If this is a possibility, the wound must be irrigated, and if there is suspicion for inoculation of the joint with oral flora, operative irrigation should be considered. The sequellae of delayed treatment of a joint infection can be serious, and should be avoided.

Radiology

Three views of the hand (PA, lateral, oblique) should be obtained. The fracture pattern can be transverse, oblique, or comminuted.[4] More complex fractures may involve the shaft or head. A transverse fracture of the neck is inherently unstable, and the forces acting on the head (intrinsic muscles) usually cause apex dorsal angulation. Rotation is poorly assessed on radiographs, and this should be evaluated clinically.

Three views of the hand, demonstrating the PA, oblique, and lateral orientations. There is a fracture through the neck of the small finger metacarpal. There is minimal comminution, and there is apex dorsal angulation. Rotational malalignment should be evaluated clinically.
Image licensed under CC BY-NC-SA 4.0

The oblique view may be use to measure the degrees of angulation, as this provides the clearest view of the small finger metacarpal. Some have stated that only the lateral view should be used for this measurement, but it could be argued that one should use the view that demonstrates the most angulation (which is usually the oblique view). In reality, the exact measured angle is not that important. The decision for treatment will be based on the evaluation of multiple factors, including the history, physical examination, and radiographic images, as well as considerations of the patient’s hand usage, work activities, preferences, and social factors.
Image licensed under CC BY-NC-SA 4.0

This lateral view demonstrates that it is more difficult to measure the angulation, but also provides some data about the fracture pattern. For example, evaluation of the base of the metacarpal is important on this view, as an injury to the CMC joint could also be seen, in general, with the same mechanism of injury.
Image licensed under CC BY-NC-SA 4.0

Treatment

Treatment should begin with a discussion with the patient regarding the risks, benefits, and expected outcomes. Because of the proximity of the injury to the MP joint, stiffness can be a complication, especially with manipulation, fixation, or open reduction.

Indications for reduction

Because of the significant laxity of the volar plate of the MP joint of the small finger, there is normally at least 60 degrees of hyperextension present, and therefore, considerable flexion deformity of the distal fragment can be accepted without significant loss of function. Some guidelines recommend treatment only if more than 45 degrees of apex dorsal angulation is present, and some authors even accept up to 90 degrees.[5] The concern for a prominent metacarpal head causing pain in the palm with grip seems to be unfounded, as patient complaints after healed displaced metacarpal neck fractures are exceedingly rare. It seems that the degrees of angulation is much less important than the presence of rotational malalignment. It is generally agreed that malrotation should be corrected.

Closed reduction

Intrinsic muscle pull at baseline is volar to the axis of the metacarpal, and this is the major factor responsible for deforming forces on the distal fragment. The fracture pattern is usually unstable, and reduction is often not maintained on follow-up radiographs. There is little harm in performing a closed reduction; it may be successful in correcting malrotation, and it may be stable, especially in children. Local anesthesia should begin with an ulnar nerve block performed at the wrist, and additional local anesthesia infiltrated into the hematoma/fracture site after the block has set. The reduction maneuver involves some traction to loosen the muscles, followed by pressure on the volar side of the metacarpal head with counterpressure on the dorsal metacarpal shaft. Mobility of the fracture should be felt, but can be difficult to appreciate if swelling is present. The amount of force to apply is probably more than the junior trainee expects – it is a good idea to imagine the force that was required to displace the fragment in the first place. In some cases, because of swelling and/or hand morphology (fat, short hands and fingers), it may be difficult to apply pressure directly to the distal fragment during reduction. The Jahs maneuver has been described. This maneuver relies on the collateral ligaments of the MP joint to transmit force to the metacarpal head during reduction. Care should be taken during reduction to not injure these structures. Once the reduction has been performed, a splint should be applied (usually an ulnar gutter splint with the ring and small fingers included in the safe position), and a post-reduction x-ray should be performed (even if improvement in alignment was not felt). The post-reduction x-ray should be evaluated for fracture alignment as well as looking for other injuries not previously seen, or caused by the reduction maneuver. It is reasonable to perform another reduction attempt (followed by another x-ray) if the first was inadequate.

Operative reduction and fixation

If there are continued indications for treatment after attempted closed reduction, then reduction in the operating room should be followed by fixation, either with k-wires, or more stable internal fixation. Each manipulation and each invasive maneuver can be associated with additional soft tissue trauma, with resultant increased swelling, scarring, and stiffness.

Consult Summary/Pearls

The history should include details about the mechanism, and combined with the physical examination, should help determine the risks of joint inoculation from “fight bite” and other expectations about outcome after treatment. X-ray evaluation should include three views of the hand, and the CMC joints should be evaluated for associated injuries. Metacarpal neck fractures are distinct from head or shaft fractures, and may require different treatment. The fracture pattern should be described specifically, rather than using eponyms that can be misleading or misunderstood. The most important indication for reduction is the presence of malrotation, evaluated by physical examination. Even severe angulation deformity can be well-tolerated with little functional deficit. Considerable force is required for reduction, but care should be taken to avoid damage to associated structures. Splinting should be in the safe position. The more invasive the reduction and fixation, the more stiffness can occur.