Given the high detail, low volume benefits of additive manufacturing (3D printing), the manufacturing process is particularly attractive in the medical space. 3DP can take the unique body geometry of a patient into consideration for the creation of a single, personalize device. One application this benefit of 3DP can be capitalized upon is in the creation of prosthetic sockets.
A prosthetic socket is the interface between a residual limb and a prosthetic device; it is how the prosthetic device is non-permanently attached to the remaining limb. Essentially, a socket is analogous to a shoe. Sockets remain attached in a non-permanent way through either a vacuum pump or through an elastic band that covers both the socket and remaining limb.
Figure  A tradition test socket
A socket is currently created uniquely for each individual by their prosthetist. Traditionally, a plaster cast of the residual limb is made, from which the physician will sculpt in order to add or take away material to keep pressure off of sensitive areas. This piece is then the basis for which a “test socket” is created. The physician will then work with the amputee, trying on the socket, to see if it fits and is comfortable.
Fig.  Revisions to be made to a traditional test socket
Fit and comfort are the primary factors of a good socket. An individual can have an amazing prosthetic device, but if they lack a comfortable socket, they most likely will not use it. Creating a comfortable socket is a difficult task, relying on a test-check method in order to find what works for the patient.
3D printing offers an enhancement to this process. Let alone the ability of 3DP to create a socket unique to a patient’s geometry, a baseline requirement, it can do so in such a way that the socket can be printed with multiple materials, each with different elastic properties. Even more so, the distribution of these different elastic materials can be determined through a data-driven method. By analyzing medical images of the amputee’s residual limb, the areas most likely to be sensitive or not sensitive can be mapped. The end result is a unique, data-driven multi-material prosthetic socket.
Fig.  Segmented topographical model made from patient CT scan
I created such a socket in the summer of 2014. By taking a publicly available set of CT scan medical images, I turned a set of 2D images into a 3D model. I then shaped the individual’s leg (below the knee) to resemble that of an amputee. I was able to take the shape of this residual limb as the basis for creating a socket. I created and ran an algorithm to map areas of sensitivity based on the proximity of bone to the skin. From this segmentation, I created a printable model of a socket. The socket was printed as a single piece, with different portions having different materials. The areas with bone close to the surface of the skin was made with soft, elastic material while the areas with the most muscle and fat in between the bone and skin had hard, supportive material. I had this model printed using a Connex multi-material printer.
Fig.  Segmented volumetric model ready for printing
This first socket served as a proof of concept, and can be enhanced in a number of ways. More data than bone location can be accounted for to map sensitivity. Material can be moved farther towards or away from the limb based on this data, in addition to just changing the material. Load analysis would be tested with completed sockets to ensure their strength during use. Longevity of the material would be tested given exposure to the elements.
Fig.  3D printed data-driven multi-material prosthetic socket
Ultimately, while this process may still require the test-check method, being data driven and originating from a CAD model can lead to more informed decisions as to the changes, aimed towards the creation of a more comfortable socket in less time. The digital models can also easily be stored to show changes in the limb over time and make the process of mapping sensitivity better as more of the sockets are made.
There are potential hurdles this process faces, however. The process of printing a multi-material socket is one of the more expensive printing processes for the time being. However, if the process does result in less cycles with the physician, this could offset the material costs. Reducing the number of complications down the road due to a better fit, such as rashes and swelling of the limb, could also offset the cost. The material is also relatively fragile, but could perhaps be strengthened with a durable shell. The material is also light sensitive, given the printing process, and degrades overtime with exposure to light.
There is another means of overcoming the problem of ill-fitting, uncomfortable interfaces between a residual limb and the prosthetic device. This process does not use a socket at all, but is instead a permanent implant into the patient’s skeletal system. Osseointegration is the surgical procedure of inserting a metal rod into the remaining bone of an amputated limb. This rod extrudes outside of the skin, offering a point at which to attach a prosthetic device. While the rod is permanently attached, the interface outside of skin is not. The primary benefit osseointegration aims to offer to transfer the forces from the prosthetic device to the skeletal system, versus into soft tissue as a socket does. It can also offer ease of use to the patient, as attaching and detaching devices to the implanted bar is much faster than taking on and off a socket. This process is not yet FDA approved in the United States, but is being made available by a number of countries (most notably Germany).
In closing, 3DP offers many benefits to the creation of data-driven prosthetic sockets to offer a more comfortable interface faster, as so too may osseointegration.
Please join 3DHEALS for a stimulating conversation on 3D Printing in Orthopedics in their upcoming June event.
About the Author
Alex Madinger – bachelors of science in mechanical engineering
Award-winning mechanical engineer with proven success developing and executing mechanical designs, analysis, and tests to achieve effective solutions to complex problems. Proven expertise in designing for additive manufacturing, especially in the focus of healthcare.