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

1 . 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 method comprising:

selecting a plurality of plunge mill locations being circumferentially located about a center of the craniotomy location;

determining, by the craniotomy system, an estimated final depth of a hole to be formed at each of the plurality of plunge mill locations based on a medical image of the cranium of the patient;

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;

calculating a predicted breakthrough ratio;

stopping the plunge milling in response to a change in the impedance or in response to a change in the axial force, wherein the stopping the plunge milling in response to the change in the axial force is due to the axial force being proportionally less than a maximum axial force by the predicted breakthrough ratio; and

storing the estimated 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 estimated final depths of the plurality of holes.

2 . The method of claim 1 , further comprising:

determining a starting location of a top surface of the cranium at the plurality of plunge mill locations by positioning the end mill concentric with the respective plunge mill location and touching, without cutting, the end mill against the cranium at the respective plunge mill location.

3 . The method of claim 2 , further comprising:

storing the starting location of the top surface of the cranium at the plurality of plunge mill locations in the memory of the controller.

4 . The method of claim 1 , wherein the memory of the controller is 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.

5 . The method of claim 4 , wherein the memory of the controller is programmed with a predicted cancellous impedance range.

6 . The method of claim 4 , wherein stopping the plunge milling in response to the change in the measured impedance is due to the measured impedance being within the predicted dura impedance range.

7 . The method of claim 6 , wherein the predicted dura impedance range is lower than the predicted cortical impedance range.

8 . The method of claim 1 , wherein the memory of the controller is programmed with a predicted minimum axial force, wherein stopping the plunge milling in response to the change in the measured axial force is due to the measured axial force being less than the predicted minimum axial force.

9 . The method of claim 1 , further comprising:

determining a maximum axial force from the axial force;

storing the maximum axial force in the memory of the controller; and

storing the predicted breakthrough ratio in the memory of the controller, the predicted breakthrough ratio measuring a proportional difference between the maximum axial force and the axial force.

10 . The method of claim 9 , wherein the predicted breakthrough ratio is between 80% and 90%.

11 . The method of claim 1 , wherein face milling with the end mill creates an interior section of cranium material located at the center of the craniotomy location, wherein the interior section of cranium material is removable such that an interior portion of the craniotomy is created.

12 . The method of claim 1 , further comprising:

obtaining a computed tomography (CT) scan of the cranium of the patient as the medical image.

13 . The method of claim 1 , further comprising:

storing the estimated final depth of the hole to be formed at each of the plurality of plunge mill locations in the memory of the controller.

14 . The method of claim 13 , wherein

the creating the plurality of holes in the cranium by plunge milling into the cranium further comprises recalling the estimated final depth of the hole to be formed, and

the stopping the plunge milling in response to the change in the measured impedance or the change in the measured axial force further comprises stopping the plunge milling in response to plunge milling the estimated final depth into the cranium.

15 . The method of claim 1 , wherein the impedance is measured via an impedance measurement system electrically coupled to the CNC milling machine and the controller.

16 . The method of claim 15 , wherein

the impedance measurement system includes a first electrical contact in the end mill and a second electrical contact configured to be removably coupled to the patient, and

the impedance is measured between the first electrical contact and the second electrical contact.

17 . The method of claim 1 , wherein the axial force is measured via an axial force sensor electrically coupled to the CNC milling machine and the controller.

18 . The method of claim 1 , further comprising:

determining, optically via an optical assembly, the center of the craniotomy location through an optical window of the optical assembly, the optical assembly being attached to the CNC milling machine.

19 . 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.

20 . 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.