Delayed Presentation of a Locked Metacarpophalangeal Joint Secondary to Incarcerated Sesamoid with Associated Proximal Ulnar Collateral Tear in a 15-Year-Old Boy.

A 15-year-old right-handed boy presented 7 weeks after a hyperextension injury while playing basketball with a suspected metacarpophalangeal (MCP) joint dislocation to his right thumb and subsequent reduction by his mother at the time of injury. Initial radiographs were negative for acute fracture or dislocation. However, pain and stiffness persisted in the MCP joint, and subsequent magnetic resonance imaging demonstrated an incarcerated radial sesamoid and a proximal ulnar collateral ligament (UCL) injury with migration of the ulnar sesamoid under the proximal UCL, a finding not previously documented in the literature. He underwent surgical excision of the sesamoid bone and repair of the UCL injury with successful improvement in symptoms and return to all activities, including basketball, by his 4-month postoperative visit. This case demonstrates that it is imperative to scrutinize radiographs and consider advanced imaging when evaluating a locked MCP joint following a hyperextension injury.