By Zach Harvey, CPO
Perhaps more fundamentally important than how prostheses are made is the why those of us in the field are driven to do this work. Getting someone mobile again is one of the greatest feelings in the world, and it’s why I do what I do. However, as prosthetists, we don’t just do what we do to feel good about ourselves. In addition to the internal feeling of satisfaction I get when I fit someone with a new prosthesis, there is certainly also economic value to what we do. Restoring function improves the health and quality of life for the individual and this translates into saving health care dollars. Getting patients back to work means generating tax dollars and may reduce the reliance on public aid. A question I often get is why prosthetic limbs are so expensive. The way our industry works is that we bill by device, not by time. Each device comes with a warranty period. All the work leading up to the delivery device and follow-up afterward, inside the warranty period, is not billable separate from the amount we billed for the device. The components such as feet, hands, elbows, and knees are expensive for us to purchase. The costs of these items are expensive because the cost to develop is high compared to the number of units sold. The socket is custom-made to the individual, and since each person is different, this involves a time consuming step-by-step process each time a socket is made.
Making a Socket
The socket is important to get right in order to fit the person properly. The skill and experience of the prosthetist determines the end result. This end result is perhaps more important than the method used to get there. In order to test the fit of a socket before fabricating out of definitive materials such as carbon fiber, a test socket made of clear thermoplastic is usually needed. The process starts with an assessment of the residual limb and often fitting with a properly sized gel liner. Measurements are taken over the skin and/or liner. Capturing the shape of the residual limb can be done using traditional methods, such as hand casting, or can be done using a scanner, most commonly an optical scanner.
The Traditional Method
The traditional method of socket fabrication is the most common method of creating a socket in our industry. For the traditional method, plaster bandage is wetted and rolled around the residual limb and liner (if present). Anatomical landmarks can be marked using an indelible pencil prior to casting and will transfer to the inside of the cast. The prosthetist may push areas of the cast in while it is setting up to create a shape which places more pressure on soft tissue and less on boney prominences. This ability to create a shape on the patient is why many prosthetists, including myself, prefer this method. Once hardened, the cast is removed from the limb and is poured with liquid molding plaster. A pipe is placed in the mold while still liquid. Once hardened, the plaster bandage is removed, revealing a positive model with marked anatomical landmarks. The prosthetist then modifies this model using surform rasps until arriving at a shape and size s/he thinks will fit. Measurements can help with this process. Different theories in modification technique are also learned in school and continuing education classes. As much as prosthetists may differ in their preferred techniques, each patient may require different theoretical approaches as well. The most advanced prosthetists vary their approach based on the presentation of the patient and may combine techniques in modification. That is when the use of biomechanical principles culminate into an art form. Once the positive model is modified and smoothed, a sheet of clear plastic is heated and vacuum formed over the model. This plastic is cut off the model and the edges are smoothed, forming the check socket. This is then attached to the rest of the prosthesis and is ready for fitting. For more about traditional methods of manufacturing, you can check out: https://www.youtube.com/watch?v=V5Vtgjy00Bg&feature=share
Another method of creating a socket is computer aided design and manufacturing (CAD-CAM), and it has been around since the 1980's. For this method, the shape of the residual limb is captured using a scanner, The scanner is moved around the limb and a digital image immediately pulls up on a computer screen. This shape can be modified by the prosthetist, then it is sent to a central fabrication facility to manufacture a plastic test socket. For facilities with in-house carvers, the file can be sent to the carver. A carver uses a bit which moves in and out as well as up and down. The bit carves out the desired shape into a spinning blank piece of foam. A check socket is then fabricated over the modified foam model in the same way as a plaster model, or it can be created with an automated thermoformer machine. The advantage of CAD-CAM is that the digital file take up virtually no space and can be pulled up again later if changes are made. Advocates for CAD-CAM also state that it is faster than the traditional method, although I think this all depends on the comfort level of the prosthetist with the technology.
There's a lot of talk about 3D printing and we are actively looking at ways to implement this technology into our practice. 3D printing is an additive manufacturing process, the opposite of CAD-CAM, and it saves steps because it avoids the mold-making process associated both with the traditional method and CAD-CAM. It will be a time saver and could save money after recouping the initial costs of the printer. However, the print size can currently be limiting, material strength is generally poor, and print times can be somewhat long. The future does hold promise though, and it won't be long before these technicalities are worked out. The really exciting thing about 3D printing is what happens after verifying a good check socket fit. Instead of the time-consuming process of laminating a socket using carbon fiber, the file could be printed in a definitive plastic or carbon material with virtually no effort. That will certainly be a time and money saver and will allow prosthetist to help more people get mobile in less amount of time, reducing waste and feeling even better about our jobs.