Isolated dorsoradial capsule injuries of the thumb metacarpophalangeal joint are different from those associated with collateral ligament disruption. Early suspicion of this rare injury is important because, if overlooked, ulnarward subluxation of extensor pollicis longus tendon can develop. Functionally, active thumb extension becomes impaired, and over the long term, a thumb Boutonniere's deformity becomes established. Joint hypermobility/instability may predispose to this injury. The 2 cases presented illustrate this through anatomic differences. At the time of acute injury, 3 presenting clinical features should raise suspicion of dorsoradial capsular rupture: a history of isolated hyperflexion injury to the thumb, stable collateral ligaments on examination, and x-ray evidence of palmar subluxation of the proximal phalanx on the metacarpal. Ulnarward subluxation of the extensor pollicis longus is a delayed sign. Diagnostic imaging, beyond x-ray studies, may not be helpful in defining the injury. Early exploration and repair of this injury give the best long-term outcome. Postrepair, metacarpophalangeal joint range of motion may not be fully restored, but stability and a preinjury level of hand function can usually be reestablished.