Designing Prosthetics: A Minimalist Approach With Major Benefits

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It’s a beautiful fall day in Chicago. The weather is cool, the breeze is refreshing, and the sky is a clear blue. It’s a perfect day to go for a run, so you set off at a nice, easy jog along the lakefront. You can smell the crisp autumn air, and hear the fallen leaves crunch beneath your feet. Another runner approaches, and you nod in greeting.  But something seems a little strange. Wait a minute – that runner isn’t wearing any shoes! Most likely, it’s because he is one of the thousands of barefoot runners that eschew traditional running shoes in the hopes of avoiding the all-too-common joint injuries that plague recreational runners.

Many recreational runners are reconsidering the traditional cushioned running shoe approach in favor of a barefoot or minimalist running style. Some experts argue that our feet have evolved to handle the impacts of everyday life, and that cushioned running shoes result in reliance on flawed human engineering. Proponents of the minimalist approach claim that the “springy” properties of the arch of the foot can provide equivalent or even superior shock absorption to anything manmade.

No consensus has yet been reached on whether the minimalist running style results in fewer running injuries than a more traditional running approach. Nevertheless, some of the key principles may be applied to a quite different scenario: artificial limbs.

Persons with a lower-limb amputation are often prescribed artificial limbs, or prostheses, to help them perform daily activities. Leg prostheses are fitted around what remains of the person’s amputated limb, known as the residual limb. An amputation can occur anywhere within the lower limb, and therefore the residual limb consists of varying quantities of bone and soft tissue. Unlike an intact foot, these structures are not intended to bear substantial weight. Tissue breakdown and discomfort may occur from high-impact forces, leading to disuse of the prosthesis and lack of activity or independence.

To prevent this, prosthetists must create a prosthesis that protects the residual limb from the impact forces of walking and running. However, unlike running shoe companies, which have a single interface (the shoe) at which to influence shock absorption, a prosthetist has the ability to affect shock absorption at many locations in a prosthetic limb: at the foot, at the shank (between the knee and ankle), at the knee, and at the socket.

Typical prosthetic components including a prosthetic socket with a gel liner, a shock-absorbing pylon with springs, and two different types of prosthetic feet. Erin Boutwell/NORTHWESTERN

The socket refers to the part of the prosthesis that surrounds the residual limb, and is comparable to a shoe in many ways. The socket must fit well, without causing blisters or uncomfortable pressure, and is the location where manmade material comes in contact with the biological limb. Therefore, shock absorption in a prosthesis must occur at the level of the socket, or below, to protect the residual limb.

This shock absorption may be provided in very different ways, including springs that compress under force, foam or gel materials that deform under force, and hydraulic prosthetic joints that resist joint motions due to an applied force. The possible combinations are endless, and are further complicated by the many factors a prosthetist must consider when selecting components for an artificial leg, including cost, residual limb health, activity level, and age.

A major goal of many clinical practitioners is to create a comfortable prosthesis, and shock absorption is a key factor that plays into the perception of comfort. Comfort is a difficult quantity to define, however, and may be influenced by other factors (e.g. residual limb health). To achieve a comfortable prosthesis, prosthetists may – incidentally or deliberately – include multiple components that provide shock absorption in a prosthesis.

For example, a typical below-knee prosthesis may include a gel liner that is placed over the residual limb as well as a prosthetic foot with a cushioned or flexible heel. Both the liner and foot will provide shock absorption independently, and their combined effect on the prosthesis may result in “too much” shock absorption. But, what is the downside to excessive shock absorption?

Counterinutitively, at least one study (Boutwell et al., 2012) demonstrated larger impact forces when excessive shock absorption was added to a prosthesis. The study authors hypothesized that the additional shock absorption made it more difficult for prosthesis users to sense contact with the ground, a scenario that may be analogous to the traditional, cushioned running shoe paradigm.

Excessive shock absorption may also result in higher energy costs to the prosthesis user because more work is required to support body weight in the presence of shock-absorbing prosthetic components, a result that has been demonstrated with prosthetic feet (Fey et al., 2011). This preliminary evidence indicates that redundant shock absorption could disadvantage prosthesis users, and therefore a minimalist approach may be warranted.

A minimalist approach would aim to provide the minimum amount of shock absorption necessary to allow safe and comfortable prosthetic use. Rather than incorporate shock absorption throughout an artificial limb, the minimalist approach would involve modifying a single interface (e.g., the prosthetic foot, or the residual limb-socket interface) and implementing just enough shock absorption to make the prosthesis comfortable for the user.

Suppose that a young, fit athlete comes to the clinic for a prosthetic limb. He has a high activity level and good residual limb health, so having a dynamic, carbon fiber foot will be important for carrying out his daily activities. Such carbon fiber feet are effectively comprised of a number of springs that allow shock absorption and forward progression during walking. By selecting the right spring stiffness of the foot, adequate shock absorption could be provided without any further inclusion of specialized components.

By contrast, an older diabetic person may have very different needs. She may need her artificial limb mainly to move around her home, but pressure sores on her residual limb make prosthetic use difficult. Rather than provide her with a state-of-the-art foot, a prosthetist may give her a thick gel liner that protects her residual limb from pressure transmitted through the socket. With comfort as the primary concern, it may also be appropriate to consolidate shock-absorbing components at the limb-socket interface. Furthermore, the diminished sensation associated with diabetes would make it even more important that this individual does not have excessive shock absorption in her prosthesis, assuming that it contributes to lack of stability, as hypothesized by Boutwell et al. (2012). 

These examples represent simplifications of the prosthetic fitting process. However, they are intended to highlight the advantages and disadvantages of shock-absorbing components when incorporated into prostheses. The ideal amount of shock absorption is as yet unknown, and further mechanical studies of prosthetic components – as well as their function when worn by a human – are critical to understanding the complexities of prosthetic shock absorption. Research studies are currently underway to provide answers to some of these unknowns. But, in the meantime, a minimalist prosthesis could yield substantial benefits for some users. While the jury is still out on the long-term effects of running barefoot, there may be some practical advantages to applying minimalist principles to artificial limbs.

Header image courtesy of Wikimedia Commons

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My son, 32, lost his right

My son, 32, lost his right leg above the knee two years ago. He is very over weight and has trouble with the fit of his prostheses. He frequently gets blisters on the top inner part of his leg almost into the groin area. Do you have any suggestions? He is working on losing weight.

Hi Mary, You might be able to

Hi Mary, You might be able to find more information/advice through The Northwestern University Prosthetics-Orthotics Center (http://www.nupoc.northwestern.edu/) or the Rehabilitation Institute of Chicago (http://www.ric.org/conditions/prosthetics-orthotics/). Hope that helps! Sincerely, Bethany Hubbard (HELIX Editor-in-Chief)

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