Automatic craniotomy and bonework via milling, force sensing, and impedance sensing

A method of performing a craniotomy, comprising:

• connecting an electrical contact to a subject, the electrical contact being electrically coupled to a computer numerical control (CNC) end mill;
• locating an outer surface of a cranium of the subject;
• determining an estimated final depth of a hole to be formed at a target plunge mill location based on a medical image of the cranium of the subject;
• plunge milling into the cranium from the outer surface with the end mill at the target plunge mill location;
• measuring, during the plunge milling, an impedance through the end mill, the subject, and the electrical contact;
• gauging, during the plunge milling, an axial force of the end mill;
• and
• stopping the plunge milling based upon a circuit that monitors changes in the impedance simultaneously with changes in the axial force, wherein the changes in the axial force are determined by the axial force being proportionally less than a maximum axial force by a predicted breakthrough ratio.

A method of performing a craniotomy using a craniotomy system, which includes a computer numerical control (CNC) milling machine including a spindle and an end mill, the spindle being positioned relative to a craniotomy location on a cranium of a patient, and a controller configured to control a feed rate of the end mill, the controller including a processor and a memory, the memory of the controller being programmed with a predicted cortical impedance range indicative of the end mill plunge milling in an inner layer of cortical bone of the cranium and a predicted dura impedance range indicative of the end mill plunge milling in dura of the cranium, the method comprising:

• selecting a plurality of plunge mill locations being circumferentially located about a center of the craniotomy location;
• creating a plurality of holes in the cranium, each hole created by:
• plunge milling into the cranium at a respective plunge mill location of the plurality of plunge mill locations;
• measuring an impedance through the end mill to the patient while plunge milling;
• measuring an axial force applied to the end mill while plunge milling;
• stopping the plunge milling in response to a change in the impedance or in response to a change in the axial force;
• and
• storing a final depth of the respective hole in the memory of the controller;
• and
• face milling, with the end mill, in a path corresponding to the plurality of plunge mill locations to create an outer edge of the craniotomy, wherein a cutting depth at a plurality of points of the path is determined at least in part by the final depths of the plurality of holes.