Kinematics of Scaphoid Nonunion

Scaphoid nonunion causes abnormal wrist kinematics and typically leads to carpal collapse and a subsequent degenerative arthritis of the wrist. The natural history, including carpal collapse and degenerative arthritis, in scaphoid nonunions is different at different fracture locations. This lecture was to review recent studies related to natural history of the scaphoid nonunion based on the biomechanical analysis. The key is the fracture location relative to the scaphoid apex, which is the ulnar apex of the dorsal scaphoid ridge where the dorsal scapholunate interosseous ligament (DSLIL) and proximal fiber of the dorsal intercarpal ligament (DIC) attach. In type B2 of Herbert classification where the fracture located distal to the scaphoid apex, the proximal scaphoid fragment and lunate extend together and the distal fragment of the scaphoid individually flexes. Therefore, untreated type B2 fractures result in humpback and dorsal intercalated segment instability (DISI) deformity relatively early after injury. In type B1 fracture where the fracture located proximal to the scaphoid apex, the connection between the distal fragment and lunate is preserved via DSLIL and DIC so that the scaphoid–lunate complex remains stable, and the carpal collapse is less severe than that in B2 type fractures. The fracture location relative to the scaphoid apex is a reliable landmark in the determination of the natural history of scaphoid nonunions.