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Prosthetic Design and Engineering

Medical device engineers face tough challenges when designing products, especially in the world of prosthetics. As engineers, we have common goals to solve problems and create solutions, but sometimes extreme performance characteristics overshadow what is truly valued by the end-user. For example, prosthetic feet become part of the body, and our goal as product designers is to make the amputee forget they are wearing a prosthesis. To meet this goal, we’ve found a recipe that has worked for us, focusing on customer needs to overcome design challenges while creating innovative solutions.

Voice of Customer

Product engineers or designers are faced with difficult problems that often have competing, underdefined, or missing requirements. How successful the product will be is typically determined by meeting these requirements. If one requirement is missed, we risk a general lack of product acceptance, which is one of our biggest fears. So, the question becomes—how do we ensure that we’re creating effective solutions to meet everyone’s needs?

In 2018, College Park celebrated 30 years of providing high-end prosthetic solutions for amputees. Over these years, we sought out and received feedback from prosthetists, end-users, surveys, market research, and by analyzing returns. This is what we call the Voice of Customer (VOC). By collecting and analyzing data, we have gained great knowledge with respect to design criteria that sometimes gets overlooked. Aside from the look, feel, and raw performance stats, we put a big emphasis on designing products that are durable, noise-free, lightweight, and compact. We often go to extreme measures to save a few grams in weight and design features/surface finishes that reduce noise or increase durability. These measures often include extra machining steps to remove unnecessary material or the use of expensive coatings and materials, such as some higher-grade aluminum alloys or titanium. When we use the VOC to guide design criteria, we can achieve goals like the creation of the Odyssey K2/K3 hydraulic feet.

Applying Principles to Designs

In 2012, we embarked on a project to develop College Park's next prosthetic foot, the Odyssey. The hydraulically dampened prosthetic foot market was growing, and existing products had proven patient benefits. The idea of using hydraulics was nothing new to the industry. The benefits of utilizing hydraulics to control prosthetic joint motion has been much publicized. The problem with applying hydraulic control to a prosthetic ankle joint is doing so while still meeting the needs of the end-user. Whether hydraulic cylinders are used to dampen motion in a motorcycle’s suspension or to create digging power in the bucket of an excavator, they all must be connected through a series of linkage points. These linkage points and complexity not only add weight but also wear over time, which may introduce noise or added play. Although these design challenges can be easily overcome in the case of the motorcycle or the excavator, it’s not so easy in prosthetics. Prosthetics have to be safe and reliable, but there comes a point where the benefits of a certain design may start to be overshadowed by its weight or durability. When looking at applying a hydraulically dampened cylinder to a prosthetic foot, we had packaging concerns and saw the linkage points as an issue for both weight and durability.

Innovative Solutions

When we started conceptualizing the Odyssey foot, some key design requirements stood out for look and function. These included durability, simplicity, anatomical motion, a low build height, and a lightweight design. We try to design prosthetic feet that mimic the motion of anatomical feet. This motion tends to pivot about an ankle center that we generally try to keep located as anatomically correct as possible. It’s this relatively low, circular pivot motion that gave birth to the curved cylinder concept. Rather than trying to adapt a linear cylinder with multiple links and pivots to dampen rotary motion, which doesn’t package very well within the envelope of the anatomical space, we thought—what if the cylinder was curved and swept the same arc that extended some distance radially from the ankle center?

If this was possible, it could potentially package very well, be lighter in weight, less complicated, and more reliable. At this point, a quick hand sketch made the design look slightly possible. Design challenges quickly began popping up, the biggest being manufacturability. How can we make one? This concept featuring a curved cylinder bore and curved rod portions was unlike anything we had ever designed or made before.

After a quick meeting with our machinist over the napkin sketch, he said he would attempt it. We transformed the napkin sketch into cad files, spent a couple months proving out a machining approach, then we had our very first prototype that we put on an amputee. For the first time, we could clearly start to see we had something that would meet all our key VOC requirements. It was off to the races to bring it to market!

How to Repeat Innovation

When solving a complex problem, it is key to have a complete understanding of it. The innovative aspects of the design would not have been possible if we had never gathered any VOC data or ignored our mistakes over the years. Innovation is not something that any one person is good at. It requires the input from many different perspectives. These ideas often come about as a result of many years of experience. It’s when these learning experiences are understood and applied to problems that innovation occurs.

Aaron Taszreak

Aaron Taszreak is College Park’s Engineering Manager. Since he joined our team in 2001, Aaron has played an integral role in our growing product offerings. He led our team’s efforts in bringing to market successful products like the Odyssey K2 and Espire Elbow series. His experience includes design and development of Class I/II products in accordance to ISO 13485, risk management, program management, strategic planning, computer-aided design, product testing, process validation, mold design, machining, prototyping, and composites manufacturing. Aaron is an innovative, hands-on leader with a Bachelor of Science degree in Mechanical Engineering (BSME) from Lawrence Technological University. We are lucky to have him on our team!

“The most rewarding part of my job is helping people, solving complex problems, and making lives better through innovative products. I love seeing ideas turn into reality.”