Patient reference device

A method of tracking patient anatomy during a surgical operation using an optical navigation system and a patient reference device, the patient reference device being attached to an anatomical feature of the patient, the patient reference device including an attachment base having an attachment mechanism to secure the attachment base to the anatomical feature and a optically-trackable array including a plurality of fiducials in a fixed geometric pattern to be detected by the optical navigation system and having a longitudinally-extending arm to space apart the fixed geometric pattern from the anatomical feature, the arm including a connector to be detachably secured to the attachment base, the method comprising:

• determining, based on comparing a threshold level to a motion signal from an inertial measurement unit within the attachment base, that the attachment base has changed position, wherein the motion signal represents the change in position and its magnitude;
• based on determining that the attachment base has changed position, generating an alarm signal;
• and
• outputting, via an output device in the attachment base, an alarm in response to the alarm signal.