Pathogenesis
An ulnar claw may follow an ulnar nerve lesion which results in the partial or complete denervation of the ulnar (medial) two lumbricals of the hand. Since the ulnar nerve also supplies the interossei, which flex the MCP joints (aka the knuckles), their denervation causes these joints to become extended by the now unopposed action of the long finger extensors (namely the extensor digitorum and the extensor digiti minimi). The lumbricals and interossei also extend the IP (interphalangeal) joints of the fingers by insertion into the extensor hood; their paralysis results in weakened extension. The combination of hyperextension at the MCP and flexion at the IP joints gives the hand its claw like appearance.
Read more about this topic: Ulnar Claw