Student Mercer University and Brookwood High School Lilburn, Georgia, United States
Introduction: There are approximately more than 50 million amputees worldwide, with about 40 million living in developing countries. An average rudimentary prosthetic leg costs between $4,000 and $13,000 with additional costs for adjustments and replacement of the devices. These devices are usually unaffordable for amputees living in the poorest of the third world countries who make on average $150 to $200 monthly. Non-profit and volunteer organizations such as Mercer University in Macon, Georgia, have been providing prostheses in developing countries at no cost to the patients. These organizations set up short-term mobile clinics to fit as many patients possible. Therefore, time and cost are major considerations.
A prosthetic leg with modular components is ideal because it allows the patient to replace a section instead of the entire device. A leg prosthesis requires a foot which serves as the balancing unit and supports all the body weight. The foot should be strong, durable, lightweight, and function similarly to a natural foot.
Prosthetic devices have developed rapidly due to the advances in additive manufacturing (commonly known as 3D printing) over the past forty years. Computer Aided Design (CAD) software allows the parts to be customized, designed with precision, scaled to different sizes, and manufactured through 3D printing. 3D printing technology includes a wide selection of materials and the production process can be automated, reducing time and costs. This project explores which technologies and materials are optimal to manufacture a prosthetic foot.
Materials and
Methods: The criteria of the prosthesis are low cost, lightweight, efficiently manufactured, strength, and durability. Three common 3D printing techniques and four materials were studied.
For Fused Deposition Modeling (FDM) technology, filament is melted and extruded through a hot nozzle and printed onto a heated bed layer by layer. The printer used in this study costs approximately $2,000 and does not require any post-printing machines or tools. Polylactic acid (PLA) filament was chosen because it was the least expensive and most common material.
For Stereolithography (SLA) technology, an ultraviolet light solidifies photosensitive resin poured into a tank layer by layer. The printer used in this study costs about $3,750 and requires post-printing machines, including a washer and curing device which together cost about $1,400. Two resins were chosen, clear resin and tough resin due to availability in the lab.
For Selective Laser Sintering (SLS) technology, a laser melts and fuses polymer powder layer by layer. This method is advantageous because it does not require supports and separate pieces may be linked together during printing. However, SLS is the most expensive machine, costing almost $20,000 for the printer and post-printing sander and powder recycler. Nylon (PA) powder was the standard material used here.
Cost and time were assessed by calculating how much material was used through the specific printer software. Mechanical testing of the foot samples were conducted on a Materials Testing System (MTS) to assess strength and durability . Weight was measured with a digital scale.
Results, Conclusions, and Discussions: Two designs of a pediatric foot were studied, one with springs and one without springs. Each foot was printed with the four materials (PLA, clear and tough resin, and PA). See Figure 1. Each sample was placed in the MTS machine and an increased force was applied onto the foot until the software detected a fracture in the structure. Three trials were performed for each sample. The ultimate failure point was measured to assess strength. The samples were further placed in the MTS for a cyclic test mimicking walking to test for deflection; the less the deflection, the more durable the model.
The spring foot design manufactured with PLA through FDM technology is the optimal method. The device costs about $10, the FDM printer costs $2,000 which is the least expensive technology, print time is less than 10 hours with only three simple steps, and PLA is the second strongest and most ductile material, weighing less than ½ a pound. See Figures 2, 3, and 4.
The spring foot design is stronger than the design without springs. The spring design reduces deflection, distributes the stress concentrated on the c-shape curve, returns energy, and allows for a more natural gait. See Figures 2, 3, and 4.
While clear standard resin is the strongest, it is too stiff and brittle for the intended application because the foot repeatedly receives impact and may chip. The PLA spring foot can support about 18,000 N (4,000 pound-force), satisfying the safety factor of five times the body weight of an active child 8 to 14 years old – a minimum of 238 kg. See Figures 2, 3, and 4.
While the SLS method using powder produced the lightest weighing sample, the prints did not come out with precision and felt flimsy. Also, the SLS method took almost twice as long as the other technologies and was not efficient. See Figures 2 and 3.
This design will undergo additional testing to ensure safety and durability, including additional failure and cyclic testing. Human testing to assess gait and comfort would be the next step to assess the device.
Acknowledgements (Optional): I would like to thank my mentors, Dr. Ha Van Vo, Trung Le, and Bich Nguyen, and Mercer University for their time and guidance with this two year project, and for the opportunity to travel to Vietnam and Cambodia for the past three years to fit amputees with prosthetic legs to enhance my understanding of the biomedical engineering field.
References (Optional): Arora, A.A., Nguyen, B., Le, T., Lian, B., Webb, L.W., Vo, H.V. (2018). Using 2D Gait Motion Analysis to Evaluate the Mercer Universal Prosthetic Device in a Vietnamese Population. Harvard Medical Student Review, 4, 27 – 34. https://static1.squarespace.com/static/57e561738419c29418bd7e3b/t/5bb574df4192025957c71968/1538618611517/Mercer+Universal+Prosthetic.pdf
Arora, A.A., Nguyen, B., Le, T., Vo, H.H. (2018, October 25 - 28). Using 2D Gait Motion Analysis to Test The Functional Efficacy of The Mercer Universal Prosthetic Device in a Vietnamese Sample Population [Poster presentation]. American Academy of Physical Medicine and Rehabilitation Annual Assembly 2018, Orlando, Florida. DOI: 10.13140/RG.2.2.29363.89123
“Biomechanics of Amputees” (H. Vo, personal communication, May 26, 2022)
“Clinical Fitting Below the Knee (BK) & Above the Knee (AK) Prostheses” (H. Vo, personal communication, May 26, 2022)
James, Beth. “Enabling the Disabled with Affordable Prosthetics.” Brown School of Engineering, 18 Feb. 2018, https://engineering.brown.edu/news/2018-02-20/matthew-lo-18.
Le, T., Nguyen, B., Vo, H., Webb, L., O’Brien, E. Kunz, R., McMahan, C. (2019, October 16 – 19). A Study Of Muscle Activities of Below Knee Amputee Fit With Mercer® Universal Prosthesis. Biomedical Engineering Society Annual National Conference 2019, Philadelphia, Pennsylvania. DOI: 10.13140/RG.2.2.25012.88965
“Sinterit Sp. Z O. O. Safety Data Sheet.” Sinterit.com, 13 Apr. 2022, https://sinterit.com/wp-content/uploads/2022/02/PA12-SMOOTH_CDN-en_10.pdf
Vo, H.V. (2014). U.S. Patent No. 8,870,968 B2. Washington, DC. Patent and Trademark Office.https://image-ppubs.uspto.gov/dirsearch-public/print/downloadPdf/8870968
Vo, H.V., Nguyen, B.N., Le, T.T., McMahan, C.T., O’Brien, E.M., Kunz, R.K. (2018). The Novel Design of Mercer Universal Prosthesis. International Federation for Medical and Biomedical Engineering, 63, 197 - 204. http://doi.org/10.1007/978-981-10-4361-1_33
Weaver, Marcia, and Juanita Haagsma. “Global Prevalence of Traumatic Non-Fatal Limb Amputation.” Institute for Health Metrics and Evaluation, 4 Dec. 2020, https://www.healthdata.org/research-article/global-prevalence-traumatic-non-fatal-limb-amputation.
