Metacarpophalangeal joint (MCPJ) dislocations are uncommon dislocations of the hand.
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Epidemiology
Metacarpophalangeal joint dislocations account for ~3-5% of all dislocations 1,2.
The thumb is the most commonly affect digit, followed by the little finger 2.
Clinical presentation
The initial presentation of metacarpophalangeal joint dislocation may differ depending on whether there is a simple or complex dislocation. Simple dislocations usually present with hyperextension, while in a complex dislocation it is only slightly extended and flexion is blocked due to the interposition of soft tissue structures, commonly the volar plate or the ulnar collateral ligament of the thumb in the context of a Stener lesion 3,4.
Puckering of the palmar skin next to the metacarpal head may be present 3,5.
Pathology
The usual mechanism of dorsal dislocations – which are much more common than volar ones – is a forced hyperextension of the digit, such as might happen due to a fall onto the outstretched hand or when forced against the handlebars of a motorcycle 3.
Radiographic features
While radiographs are frequently enough on their own to make a diagnosis of an metacarpophalangeal joint dislocation, it is important to differentiate between a simple and a complex dislocation, which may require other imaging modalities.
Plain radiograph
Simple or incomplete dislocations of the metacarpophalangeal joint dislocation are actually subluxations. Complex dislocations are complete luxations 3,4. Either one is better visualized on oblique or lateral projections of the hand, in which the proximal phalanx will be dislocated, almost always dorsally.
Small bony fragments may be visualized near the dorsal aspect of the metacarpal head or the collateral ligaments, these associated findings are often related to complex dislocations with volar plate and ligaments injuries 4.
An interposed sesamoid into the metacarpophalangeal joint space is pathognomonic of a complex dislocation 4.
Treatment and prognosis
It is of great importance to differentiate between a simple and a complex dislocation to guide adequate management.
Simple dislocations are reducible by closed methods, while complex dislocations are not. Reduction attempts must be performed with caution and proper technique in order to prevent converting a simple dislocation into a complex one due to inadequate attempts to reduce the metacarpophalangeal joint dislocation 3-5.
Complex dislocations are irreducible and require surgical management 3-5.
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