Diagnosis

Complex Dorsal Metacarpalphalangeal Dislocation

Widening of the MP joint and mild hyperextension favor a complex dorsal dislocation of the MP joint. The differential diagnosis includes subluxation of the MP joint, but the joint is usually hyperextended at 60 to 80 degrees with subluxation.

Dorsal dislocation of the MP joint usually results from a hyperextension injury. The index and small fingers are the most commonly involved digits. It is important

Dislocated Index FingerDorsal Sublustion
Figure 49—1. Posteroanterior radiograph of a complex dorsal dislocation of the index metacarpophalangeal joint. Note the ulnar deviation.

Figure 49—2. Lateral radiograph of a complex dorsal dislocation of the index metacarpophalangeal joint. Note the widened joint space, dorsal displacement, and hyperextension.

to differentiate between subluxation and simple dislocation, which can be reduced by closed manipulation, and complex dislocation, which requires open reduction. Subluxation produces a greater dorsal angulation and should be reduced by applying pressure to the dorsal base of the proximal phalanx without longitudinal traction or forced hyperextension to re-create the mechanism of injury. A subluxation can be converted to a complex dislocation if improperly manipulated.

A complex dislocation typically presents with dorsal displacement and slight hyperextension of the proximal phalanx relative to the metacarpal. The interphalangeal joints are slightly flexed. In the index finger, the proximal phalanx is usually deviated toward the long finger. The metacarpal head is prominent in the palm, and careful inspection may reveal the pathognomonic skin pucker at the proximal palmar crease. However, swelling may obscure these findings; therefore, radiographs are essential for confirming the diagnosis and excluding associated fractures.

The volar plate is the key structure blocking reduction in complex dislocations. Hyperextension of the MP joint avulses the volar plate from the weaker proximal attachment to the metacarpal neck and displaces it over the dorsum of the metacarpal head. The deep transverse metacarpal ligament is partially torn at its attachment to the side of the volar plate. In the case of the index finger, the metacarpal head becomes trapped by the volar plate dorsally, the lumbrical radially, and the flexor tendons ulnarly. Longitudinal traction tightens the flexor tendon and lumbrical, creating a "noose" around the metacarpal head, thus the recommendation that closed reduction be performed with the wrist flexed to relax the flexor tendon and lumbrical. Even with the wrist flexed, the collateral ligaments may prevent adequate distraction of the joint to allow for escape of the volar plate and reduction of the joint. Therefore, the key to open reduction is division of the proximal portion of the volar plate to allow the metacarpal head to slide through.

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