Radioulnar synostosis refers to the bony bridge between the radius and the ulna. This can be either congenital or post-traumatic. The precise cause of congenial synostosis is unknown. Embryologically, the elbow forms from the three cartilaginous parts representing the humerus, radius, and ulna. A programmed cavitation process leads to formation of the elbow joint; if this process fails, endochondral ossification results in a bony synostosis. Because the forearm bones differentiate at a time when the fetal forearm is in pronation, almost all forearm synostoses are fixed in this position.