Locked thumb metacarpophalangeal joint

Locked thumb metacarpophalangeal (MCP) joints are rare hyperextension injuries caused by the radial sesamoid preventing flexion ^1,2.

Epidemiology

These injuries have been reported in all age groups but are most common in patients in their 20s ². They can be due to ball sports, in particular dodgeball and basketball ².

Clinical presentation

Patients present with reduced of active and passive flexion of the thumb after a hyperextension injury ².

Pathology

Etiology

Various theories as to the cause of a locked thumb MCP joint have been proposed ^2,4:

• entrapment of the radial sesamoid (may be due to accessory collateral ligament rupture)

• entrapment of the volar plate

• intra-articular body or fracture fragment

Thumb metacarpal head morphology may be a risk factor, with thumb MCP locking joint associated with a flat metacarpal head with a prominent radial condyle ^2-4.

Radiographic features

Plain radiograph / CT

A lateral radiograph or CT may show dorsal subluxation of the thumb MCP joint with intra-articular displacement of the radial sesamoid ⁴.

Ultrasound

Longitudinal scanning of the volar surface of the thumb MCP joint shows distal displacement of the radial sesamoid into the joint space ³.

Treatment and prognosis

If closed reduction fails, open reduction with metacarpal head shaving, soft tissue release, and/or excision of the radial sesamoid may be required ^2-4.