Peter Jepsen and Dr. Anton Bowden, Mechanical Engineering
Amputees in developing countries struggle to find employment because of their disabilities, but also struggle to afford the artificial limbs that are available to them. Further compounding the problem is the lack of comfortable fitting prosthetics, which are hard to find and design even in advanced nations. I researched, designed and tested suspension systems that could be made for a transtibial (below the knee) prosthetic leg that would be both comfortable and inexpensive.
The suspension part of a prosthetic leg consists of attaching the prosthetic leg to the stump in a way that offers functionality, comfort, and reattachment ease. A prosthetic leg needs to stay firmly attached to the leg when desired but also needs to be removed and reattached daily. In addition to these characteristics the improper fitting or reattachment can lead to skin irritation, rashes and eventually infection.
The socket is a hard shell that houses the stump. Professor William Pitt suggested that we try using two-liter soda bottles to create our socket. In order to add strength to a single Polyethylene terephthalate (PET) two-liter bottle we attempted to combine and melt two units together. Unfortunately, PET does not reflow easily with the temperature and atmospheric pressure we used and it have failed. We still believe using a two-liter bottle may be the most practical solution. Much of the weight will be absorbed directly into the main shaft of the prosthetic leg through the mounting at the bottom of the socket and through fasteners (all throughout the socket) to shaft extensions. We found that High Density Polyethelyene (HDPE) also had thermal and reflow problems when melting multiple layers together to create a stronger structure. The current socket design will use a PET socket with strong connections to the shaft for handling the pressure put on it.
To make the prosthetic leg beneficial to more amputees, the socket must be able to interface with various sizes of stumps. From the initial brainstorming of our design, we have planned too many socks (cloth put around the stump), which are inexpensive and will be needed for practically every prosthetic design. The number of socks, sock thickness, and sock material can change to better fit the patient. Foam was selected to augment the socks to provide comfort to the sensitive and different sized stump. Foam is rather inexpensive and as we found out is already used for bedding by most Africans. In an attempt to make a more comfortable and better fitting the prosthetic, we experimented with a polyurethane foam mix that begins as a two liquids but combined fill a space and solidifies into the consistency of foam. Creating this foam is difficult because it will fill as far as there is open space and will spray through unsealed gaps. We plan to use Plaster of Paris to make a mold of the patient’s stump and then make a positive mold of the stump so the foam from the original mold. The positive mold will allow us to make the foam to form in a shape matching the curvatures of the stump. Previous journals have shown that uniform pressure around the stump will be just as good as designing the stump to put less pressure on sensitive parts of the stump (where the bone is) . Although we are still experimenting with this material, we have found that properly creating the foam will required some short special training for the local medical technician.
Our research on existing designs revealed that many prosthetic users complain about irritation and skin decay due to resins in neoprene sleeves. We considered the use of spandex as an alternative in order to use suction to attach the prosthetic, but found that the specific blend material that would be strong enough would be impractical due to its cost. Our backup option was to use leather, which is commonly available, to make a strap holding the prosthetic leg onto the stump. We originally envisioned a cross-strap configuration that would wrap around the leg behind the knee, but we found that a simple above the knee strap coming down along the sides of the leg would do just fine. Placement just above the knee will allow the knee to bend and keep the strap from moving. The side straps will also provide the prosthetic leg with the ability to move with the knee and allow for some angling movement. This strap design should provide movement similar to a natural leg without requiring expensive materials or specialized training. Making this strap wide (approximately 2 inches) will provide the stability and strength for the attachment while spreading the pressure on larger parts of the leg so that the strap won’t pinch the leg. The side straps will be bolted on to the rest of the prosthetic device or possibly onto the socket.
This design allows for a custom fitting to maximize comfort but will do so in a way to keep costs low. The molds will be made first so the foam fits the shape of the stump. The number of socks put around the stump will vary depending on the swelling and shrinking of the stump with small size changes being taken care of by the foam. A strap, customized to the person, will then be attached to the prosthetic and the amputee’s leg. The only tools needed to make and fit this prosthetic are a sharp knife and a hammer (or something that performs the same function as these things).
A provisional patent has been issued for the initial design of the inexpensive prosthetic leg. Further work on this project will include several stress tests on the leather strap and of the connection to the prosthetic leg. A volunteer will wear the prosthetic leg for a few days to see if the design has sufficient comfort and if the fitting process is inadequate in preventing skin problems. While much work still needs to be done in order to finalize and validate the design, we have found many valuable solutions to the challenging problems of designing a capable yet inexpensive prosthetic leg. This project has taken longer than anticipated to produce a feasible design, but we are still confidently pursuing the goal to provide an excellent design that will reach those in need.