Large portions of amputees in smaller, developing countries are victims of landmines (Strait, 2006). In Angola alone, there are approximately 15 million landmines, which have created nearly 70,000 amputees. Eight thousand of these amputees are children (Oppong 2005). As a result of these injuries, people who need prosthetics in these countries aren’t able to afford high tech bionic limbs that are affordable to some citizens of Western countries. Patients have yet to see a prosthetic designed specifically for their needs with the strength and durability to last longer than a few years. As a result of the quick rate at which children grow, pediatric prosthetics must be replaced every six to twelve months (Strait, 2006). The International Council for the Red Cross (ICRC) currently provides low-cost solutions, but not long-term solutions, but the adjustable prosthetic limb system we designed provides a long-term solution, while still fitting the needs to be durable, low-cost, high strength, easily available, and also easy for technicians to learn use and repair (“Trans-Tibial” 2006).
Introduction and Background
Current solutions provided by the ICRC include a pylon system enclosed in a cosmetic shell with a rubber foot attached and cost about $250. This is made as one full system, therefore when a child grows, the entire system must be replaced. Replacing the entire system is a process that takes nearly a month to complete, only to complete again within 6 months of additional growth. A system was designed using the standard adaptors used in the industry today, which is low-cost, adjustable, detachable, and durable as needed.
Our device was created to provide a low-cost, adjustable, and durable transtibial prosthetic limb for children in developing countries. Our system is an adjustable pylon system modeled to be easily implemented with existing socket and pyramid adaptors, which will allow certain parts to be replaced on an as needed basis instead of replacing the entire system. This device will enable a child to keep and use a prosthetic leg for years at a time and adjust the length of their new leg in the comfort of their own home, eliminating the need to travel to clinics or overseas for fittings of new prosthetic legs. Carbon fiber resin composite (CFRC) combinations are researched, tested, and analyzed to determine the best combination to create a foot that is low-cost, easy to manufacture in developing countries, will hold a maximum weight, and still remain as flexible as current solutions.
Problem Statement
The ICRC currently provides low-cost solutions, but not long-term solutions in regards to a pediatric transtibial prosthetic. Our goal is to design an adjustable prosthetic limb system that can be used by children between the ages of 7-17 years old, which will replace the current short-term prosthetic limb. The system will also include a low-cost and easily manufactured CFRC foot that can support a maximum weight of 300lb and include an angle of flexion of 25°, which will allow it to be competitive in the market for current low-cost prosthetic limbs for children in developing countries.
Design and Development
Adjustable pylon system
Two Aluminum 6061 pylons were obtained and machined to fit into standard 30mm and 22mm socket and pyramid adaptors. Proper lengths were chosen to provide a minimum tibial length of 9.5inches and a maximum tibial length of 16.5inches. According to CDC growth charts, this range in length supports children between the ages of 7 and 17 years old (CDC). We used Al 6061 because it is the standard used in the industry.
Figure 1: The 2 pylon models we used to fit the standard adaptors
Clamping mechanism
In order for the 2 pylons to be adjusted then hold in place until the next adjustment, a clamping system was needed. We decided to use a bolted clamp system that is typically used to hold 30mm bike posts together. This clamp only costs $6 and was purchased from the local bike store. The quick release clamp has the potential to be adjusted by a child, which is a safety hazard. Using a bolted clamp instead of a quick release clamp allows only the caregiver the ability to adjust the length of the rods by using an Allen wrench, which eliminated the safety hazard.
Figure 2: The bolted clamp design chosen to hold the 2 pylons in place
CFRC foot
The foot was designed based off of current feet in the market today made by Ottobock and Ossur. The C-shaped design adds flexibility in the heel, while the flat bottom adds stability to the foot. Different combinations of CFRCs were tested on top of a medium density foam core. The final CFRC foot created is made of 19.7 osy 2×2 twill weave fiber double layered to the foam core.
Figure 3: The CFRC foot model
Evaluation
The CFRC foot was created to withstand a weight of 300lb, while still allowing for plantar flexion and have the ability to be reproduced on site in a developing country. Along with compression testing of the pylon system at the maximum and minimum lengths and torque testing of the clamp system to provide the maximum torque needed to support the maximum weight, we also tested the maximum support weight of the foot, while testing the angle of flexion. We performed an analysis of the proper process for laying CFRC, while also analyzing the different effects of using a low density weave versus a high density weave.
All compression testing was performed using a Tinius Olsen tester (ISO 10328). The pylon system held the maximum weight expected and more at both our minimum and maximum lengths when the bolted clamp was at 70in-lbf of torque. Compression testing was performed on the pylon system with the bolted clamp torqued at 35in-lbf to 70in-lbf and the force was recorded. A simple statics analysis showed that the clamp system, after applying a torque of 70in-lbf to the bolt, was capable of holding the 300lb maximum weight.
A medium density foam core was printed using a single axis routing machine. We tested various combinations of resin and hardener mixtures, low density and high-density fiber weaves, as well as double and single layer weaves. One foot was made by laying a single layer of 5.7 ounce per square yard (osy) fiber and 60-minute hardener and resin combination onto the foam core printed. The foot dried for a 24-hour period, then a second layer of fiber was laid on top of the first layer. This foot was compression tested and was shown to hold a maximum weight of 257lb and had a 13.1° of flexion. A second foot was created by soaking the foam core in a 20-minute hardener and resin combination. After the foam core became adhesive, a double layer of 19.7 osy fiber and 60-minute hardener and resin combination was applied and molded to the foam core. There was no waiting period to lay the second layer of fiber. It was laid immediately following the molding of the first. The second foot held a maximum weight of 330lb and had 10.7° of flexion. The third foot was created the same way as the second foot. The only difference between the two was that extra resin was added on top of the layered CFRC. This added extra strength, but took away flexion. The third foot held a maximum weight of over 1000lb, but only had 8.1° of flexion. The process used to create the second foot optimized the use of the 20 and 60-minute resin and hardener combinations, as well as the process of double layering the higher density carbon weaves. This process enabled us to create a foot that supports the maximum weight while still allowing flexion of the foot.
Discussion and Conclusions
In conclusion, while trying to follow ISO standards to perform proper compression testing of our prosthetic limb system, we were able to prove that our system meets our product specifications. The pylon system with the clamp torqued at 70in-lb supports the maximum weight of 300lb at both the minimum and maximum lengths. The entire system is lightweight at only 0.81lb. The CFRC foot created supports the maximum weight while still having nearly 10.7° of flexion. We decided the design of our foot could be improved to add flexion if the C-shape was significantly larger than our current design. With the larger C-shape and the current process for laying the CFRCs, we think we could produce a foot that would support the maximum weight, while having an angle of flexion closer to that of a normal foot. The foot we designed has the ability to be made on site in developing countries. The process can be standardized and utilized in these areas where these prosthetics are needed. The materials can be shipped to the site, where people have been trained to manufacture these CFRC feet according to the proper guidelines set out by the design. However, it should be noted that the procedure must be completed in a clean room where humidity and wind must be controlled to prevent anything from moving into the fabric. Also, the strength of the material is dependent upon the ability of the person laying the fabric; therefore the manufacturer would need to be properly changed. With integrity of our device so dependent upon the way in which the fabric is layered onto the core, further research would be done in the next phase of the project to help find a more suitable combination of epoxy resin and different hardeners.
We tested this system using a torque wrench, which told us exactly how much torque was applied on the bolt. After doing this it was determined that the amount of the torque that needed to be applied for the system to be able to withstand the 300lb load needs an Allen wrench to be able to reach this amount. Also, it was necessary for a bolt clamp system to prevent any accidental manipulation of the fast release. Therefore, our final decision was to go with the bolted clamp system. The total cost of our system is approximately $162US. This includes the adaptors that are provided by the Red Cross in their prosthetics which they provide in other countries. Future work of this device will entail research into possible improvements in the foot design. We would like to increase the size of the C-Shape design of the foot, to increase the overall flexibility of this device.
Acknowledgements
We would like to acknowledge our mentor, Mr. Mike Shipp, the Director of CREST, as well as Ms. Justina Shipley from Shriner’s Children’s Hospital in Shreveport, LA. Without assistance from the Louisiana Tech Eco Car Team, the CFRC foot may not have been as innovative as it is today. We would also like to thank Dr. CLiff Frilot and Dr. Henry Bonin for assistance early on in the project. Funding for this project was made possibl by an NSF Grant.
References
Strait, Erin. “Prosthetics in Developing Countries.” Diss. Iowa State University, 2006. Web. 3 Oct. 2011
Oppong, Joseph, and Ezekiel Kalipeni. “The Geography of Landmines and Implications for Health and Disease in Africa: A Political Ecology Approach.” Africa Today 52.1 (2005) Print.
“Trans-Tibial Prosthesis: Manufacturing Guidelines.” ICRC. Sept. 2006.
Centers for Disease Control and Prevention. CDC. http://cdc.gov/. Web.10 Oct. 2011.
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