Dislocation of the thumb MP joint most commonly occurs dor-sally. The injury is secondary to a sudden hyperextension load. On-field examination of these injuries may demonstrate a mild to moderate hyperextension deformity of the proximal phalanx and dimpling within the palm (Fig. 41-18). The injuries can be classified into simple and complex. Complex dislocations are irreducible by closed methods. Radiographs demonstrating interposition of the sesamoid bones between the metacarpal head and proximal phalanx is suggestive of a complex dislocation. Simple MP joint dislocations result in proximal disruption of the volar plate. Concomitant injuries include sesamoid fracture and varying degrees of collateral ligament injuries. In reducing the joint, it is important to avoid longitudinal traction with the possibility of converting a simple dislocation into an irreducible one. After regional anesthesia is administered, closed reduction is attempted by flexion of the wrist and axial loading of the proximal phalanx onto the metacarpal head with gentle MP flexion. The majority of MP dislocations are easily reducible
and stable after reduction. The collateral ligaments should be assessed after reduction. If postreduction radiographs demonstrate concentric reduction and the joint is stable, then the thumb is immobilized for 3 weeks. If the joint is found to be clearly stable, then a more abbreviated period of immobilization could be considered depending on the specific requirements of the sport.
Complex dislocations are irreducible and require open reduction. The volar plate, flexor pollicis longus, and sesamoids have all been found to block reduction at the time of surgical exploration. The joint is approached through a volar, dorsal, or lateral approach. Once the interposed tissue has been removed, the joint is usually found to be stable. The MP joint may be pinned in 20 to 30 degrees of flexion in cases of instability after congruent reduction. Four weeks of immobilization is recommended following open reduction. Volar MP dislocations of the thumb are much less common. They are sometimes associated with concomitant injury to the extensor mechanism and the dorsal capsule requiring open repair.
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