Table 2.

The type of direct causes.

Direct cause	Description	Main forms
Operator-device	A safety accident that may be caused by a problem in the interaction between the operator and the device	Usability problems: display interface; control interface; HMIa matching (space, seat, and workspace); label or specification; other
Patient-device (P-D)	A safety accident that may be caused by a problem in the interaction between the patient and the device	P-D-1 usability problems: display interface; control interface; HMI matching (space, seat, and workspace); label or specification; other
		P-D-2 internal risk: biocompatibility (blood, tissue, and immunoreaction); tissue or organ infection; tissue or organ damage; other
Environment-device	A safety accident that may be caused by a problem in the interaction between the environment and the device	Effect on the environment or disturbance by other devices: pollution (eg, air pollution); disturbance (eg, electromagnetic interference; other
Device (D)	A safety accident that may be caused by component failure of the device	D-1 (hardware failure); D-2 (software failure)
Unknown	Unknown causes or unexpected injuries	Unknown scientific principle involves multiple chaotic factors and unexpected events

^aHMI: human-machine interface.