Medical Research: Machining Prostheses

The Pocket NC is used in a variety of medical applications both in research at universities as well as in the medical industry. One medical application area is the automation of machining a prosthesis for a skull during surgery. This research is discussed in a recent publication, “Development of an Automatic Robotic Procedure for Machining of Skull Prosthesis” at the University of Pavia in Italy. 

Additive manufacturing is widely used in surgical applications. In the case of a craniectomy the planned cut versus the actual cut can be off which would require the prior generated 3D prosthesis or the opening on the patient to be altered. The researchers propose scanning the skull opening, comparing it to prior scans, then modeling, cutting and installing the compatible prosthesis on a CNC during the course of the surgery. In this instance milling is more appealing than 3D printing because of the time to produce the part. This leads to a reduction in the time it takes to perform the operation as well as reduces cost. 

stock for prosthesis oriented on CNC machine

Figure 1: How stock is oriented on the machine*

Several aspects of the process of generating the prosthesis are automated. The digitized STL file is uploaded and based on the size of the prosthesis part a starting stock size is chosen from the stock available within the hospital inventory. Stock is selected to minimize the machining time to produce the prosthesis part. The prosthesis is then oriented within the stock available and tool paths are automatically generated cutting each side of the part individually using proprietary software, leaving a holding strip of a certain thickness which will also serve to ensure that the part was made correctly. A holding strip that is too thick or thin indicates an issue with the part. The strip is removed in the parting off process. Once the prosthesis is machined it will be deburred and sterilized.

final orientation of prosthesis part
Figure 2: Examples of how the prostheses parts are oriented*

The prosthesis parts are made from two varieties of polymer, a soft and hard plastic. Only one tool is used during machining to minimize machining time and required technician involvement in creating the part. As many parameters as possible are kept the same from part to part such as tool diameter and material left for removal, but parameters that affect finishing of the part like feeds, speeds and depth of cut are varied for the material machined.

steps in machining strategy
Figure 3: Steps in machining strategy*

By simplifying, optimizing and automating much of the process and machining strategy the group at the University of Pavia demonstrates that it is possible to use a CNC machine to cut a skull prosthesis during the time it takes to operate on a patient. This approach leads to a better fit prosthesis, decreased operating time and cost, and minimal involvement from medical personnel to generate the prosthesis. 

A Pocket NC V2-10 machine was used to make these sample prostheses. To find out more about the process find the publication available for download here. *All images are from the publication referenced.

prosthesis once it is machined and deburred

Figure 4: Prosthesis once it is machined and deburred*