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Creating a well-made prosthesis always starts with a shape capture of a residual limb. Traditionally this was taking a cast. However, in the 1990s, some digital options for shape capture became available. For traditional shape capture, the cast is poured with plaster of paris and then modified with various sculpting tools to create a final shape for either thermoforming or wet lamination. (Plaster of paris is a quick-setting gypsum plaster consisting of a fine white powder (calcium sulfate hemihydrate), which hardens when moistened and allowed to dry.) With digital fabrication, the shape capture is acquired digitally and then modified on a computer. Digital fabrication can include CNC carving a foam, plaster mold, or 3D printing. Whether traditionally fabricated or digitally fabricated the socket is then skillfully fit by a Certified Prosthetist to evaluate and align the prosthesis. Is one process better than the other? The answer can be yes or no, but the real answer lies in how a prosthetist applies the technology and the patient outcome.
Five years ago, I would have balked at the notion that a 3D scanner would ever be a part of my practice. I would think to myself, “Surely, I would not be able to get the fit I needed or the modification appropriate if I went digital.”
One day, however, a colleague challenged me by saying, “Brent, just trying to scan a cast of yours and have it carved and fabricated and let me know how the test socket fits.” Scan the cast? Outsource fabrication? I was in doubt but did it anyway. The socket was traditionally fabricated over a CNC carved foam mold. As I was on the way to fit the socket, my mind was already made up, “it was never going to work, such a dumb idea, and at least I have a cast, and I can do it the way I always do it.” The patient was waiting for me to put the test socket on, and I did. I couldn’t believe it; the test socket was so close! I thought, maybe this scanning idea IS a viable option. Perhaps this process could save me some of my most valuable asset, time.
I started researching and exploring how I could consistently scan and get the results I needed. Should I direct scan and rectify, or should I create a cast with my modifications already in it and then scan that? I settled on digitizing the inside of my cast. However, I also found that scanning the outside of the cast got me very close to the shapes I needed for a good fit. I manipulate the cast in its green state through a variety of customized shapes that are 3D printed. These shapes allow me to design the shapes in the cast rather than rectify digitally after the fact.
You may ask why not a direct scan of a patient’s limb? While scanners are very good at capturing shape, they do not capture shape with weight bearing considerations. If you digitally modify a direct scan, you are guessing what the underlying anatomy and tissue are going to do. If you cast and then scan that you have the volume needed plus what the tissue is going to do under load. It may be an extra step, but I would rather be close than a guess.
Well, how do we cast?
For transtibial patients (with below the knee amputation), we are firm believers in a wide AP socket style similar to what Coyote Design does with Stabilflex. The basic idea is we use a fiberglass cast as tight as the fiberglass can handle (one step under ripping) and cast a patient with the knee fully extended. This casting method provides a baseline and can be customized with extra care on shaping the cast anteriorly, adding pads under the cast to get reliefs, and thus holding tight in medial-lateral direction but not touching in the anterior-posterior direction. The purpose of this cast is to capture what the tissue does under load and achieve an appropriate volume. The reason the knee is extended for the cast is to have the best fit when the weight is entirely on the prosthesis which is during heel strike and single limb support. We know that a cast or a scan is only a snapshot of the patient’s limb in motion, and we choose that snapshot to be when they are fully extended and weight-bearing. At that point, we scan the inside or outside of the cast.
For transfemoral (above the knee amputation) patients, we still use fiberglass, but we do not pull very tight. While the cast is getting set up, we use plastic shapes from our library to push into the cast similar to what the ISPO casting jig and the Berkley Cast jig historically did. Once you take the cast off and it is modified, then we scan the inside or outside of the cast.
What do we use to scan?
We have two scanners that we use consistently. Daily, we use an app called Digiscan3D on an iPad with a Structure Sensor. LifeNabled initially commissioned the development of this app to allow non-profits specifically in the developing world; however, many private clinics asked for access, and they added it to the app store. Digiscan3D is a paid app with all proceeds going to helping those in the developing world gain access to a prosthesis. The unique aspect of this app is the only app that exports natively as a .STL file with an appropriate scale in millimeters. The scans are clean, and it can be used without a connection to the internet as all data is located locally on the iPad or iPhone.
The other scanner we use is the Artec Eva. Eva allows us to split casts and scan both sides and put them together for a positive mold. Between both scanners, we can cover all shapes that we would encounter in the orthotic and prosthetic realm.
About the Author:
W. Brent Wright is an ABC-Certified Prosthetist and BOC-Certified Orthotist with Eastpoint Prosthetics and Orthotics in Raleigh, NC. He has been in the field since he started as a technician at age 16 and has seen first hand how additive technology is evolving and impacts patients not only in the US but also in the developing world.
Brent is pioneering techniques to effectively use additive technology such as MultiJet Fusion and Selective Laser Sintering to create dynamic prostheses that are light as well as flexible. He is excited that prosthetists have a chance to be a part of the industrial revolution where mass customization and patient outcomes collide.
Brent is the Clinical Director of LifeNabled, a non-profit that specializes in manufacturing all new low-cost and high quality prostheses. LifeNabled has developed a scanning app for the iPad and iPhone called Digiscan3D and also has developed a suitcase 3D printer that can be deployed anywhere in the world.