Case Study: Using PCTG for 3D Printed Prosthetic Sockets
Amputees often use prosthetic limbs to re-gain mobility and independence. A patient usually has a plaster mold created of their existing leg which is then used to create clear check sockets and then modified to create a definitive socket.
The challenge in the past is that the layup process for creating a new socket is labor intensive and expensive, usually on the order of a new car. Enter 3D printing.
Using a 3D scan of a person’s geometry, a 3D printer can create a socket for the patient overnight. This gives the clinician a great boost in time available for seeing more patients or taking on more tasks. Careful selection of materials and thoughtful print parameters can make all the difference in the world. In this case the material chosen was a PCTG plastic. It is transparent as a filament with a silvery shine which transfers well to printed parts. Additionally, it has incredible impact strength, which means it absorbs energy well, and is still stiff enough to hold its shape during patient fitting.
Late one night we took one of the extra printed sockets that had a blemish and decided to do qualitative human stress testing to determine strength and fatigue performance. In short this means we put it sideways on the ground and started bouncing on it. We were astonished at what we found once it finally broke.
1. It took a lot of weight and a lot of bouncing before it finally broke.
The person standing on the socket was about 150 lbs and could bounce forty-two times before it failed. This socket was not designed to support weight in this fashion and was probably the worst way anyone could stress it.
2. It didn’t break along layer lines!!
In fact, just the opposite, it broke along the length of the socket. One of the well-known weaknesses of FDM printing is its inner layer bonding strength and so a logical failure point would be along layer lines. The fact that it didn’t speaks to the idea that, with well specified printing parameters, a 3D printed part can exhibit excellent strength and inner layer bonding that isn’t necessarily the weak point in all prints.
3. It didn’t break along the seam.
During the printing process the nozzle will start at a particular point on a looped path, travel and extrude all the way around the perimeter, and eventually end where it started. Then it will move over and do this again to make up the correct thickness of the socket. There is a fair amount of time for the plastic to cool in between these start and stop points and because of that it was thought that the seam would be an inherently weak point. Again, even though it is placing hot plastic on cool plastic it can still bond to itself well enough to make a strong joint.
4. The fracture lines are smooth and there is no visible layering in the cross section.
Since inner layer adhesion is the weakest point of a 3D printed object, it was expected that the weak bond would be apparent in the layer lines of the cross section. It is easy to see layer lines on the outside and surely it must be easy to see them on the inside as well. However, the reality was the exact opposite, the cross-section looked very smooth and uniform. Only in a few places were the layer lines even slightly visible which lends to the idea that the material is fully filling in between layers and leaving no voids or visible borders where the plastic meets. Since there is full material contact any detriment to strength must come from how the material is bonded together. This certainly opens up clues into how to address this typical strength issue.
This test leads to some interesting conclusions. The material is relatively flexible and tough. It’s ability to deform and spring back shows its elastic properties in that it will elongate but not permanently deform. The fact that it could be loaded in this way speaks to the strength of the material itself as well as the print settings, poor print settings can quickly and severely undercut the strength of a part. Looking at the break and where it happened it was notable that it did not happen along the seam and that the cross section looked uniform. This shows that the process is laying down plastic in a way where it can bond with itself so that the layers and lines don’t separate during a fracture.
Overall this lends strength to the idea that 3D printed parts and prosthetics have a place in the world where they can serve as functional parts that are manufactured quickly and in an automated fashion. Improvements in the technology are constantly being explored with industry innovators like Standard Cyborg and TriFusion Devices currently 3D printing prosthetic devices. In the future, look to 3D printed prosthetics becoming the go to solution for doctors around the world.