Josh Caputo and HuMoTech: Creating a Way to Test-drive Prosthetic Feet

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By Laura Fonda Hochnadel

While there are a wide variety of prosthetic foot options available, a 2005 survey shows that prosthetists choose from about five when fitting patients, and only consider others when none of those are appropriate.1 Despite the age of that survey, with well over 100 different types of prosthetic feet on the market, it can be difficult for a prosthetist to learn all the unique characteristics, including cost, design and materials, and pros and cons. Choosing from a limited selection may decrease reimbursement headaches, but is this in the best interest of the patient? It's a conundrum that Josh Caputo, PhD, is seeking to address.

A close-up view of the prosthetic foot emulator. Photographs courtesy of Caputo.

During graduate school at Carnegie Mellon University (CMU), Caputo became interested in prosthetic feet when he began working with his advisor, Steve Collins, PhD, who runs CMU's Experimental Biomechatronics Laboratory. Collins had developed a controlled energy-storage-and-return (CESR) prosthetic foot, and it was in Collins' lab that Caputo began applying a passion he had developed for robotics as an undergraduate to improve quality of life for individuals with gait impairments.2

Early in my PhD studies, I focused on how to design more effective prosthetic feet...," Caputo explains. However, as he began researching the role of magnitude and timing of active push-off from the prosthetic foot and its effect on amputee gait, his interests took a turn.3-5 "I started to get interested in the fact that we've got these exciting advanced robotic devices… but few patients are using them because reimbursement is such a challenge," he explains. "So what do we do about that? Why are the payers skeptical? Why is there not more evidence out there that demonstrates the advantage of these technologies?"

At the same time, Caputo says he began contemplating the question many doctoral students do: "What am I going to do with my degree?" He realized he might make a greater positive impact on the world by trying to solve some of these underlying problems rather than contributing to the growing "catalog of new tech out there."

Toward this aim, Caputo is developing a prosthetic foot emulator that would enable clinicians to measure which prosthetic foot characteristics will work best for each patient— essentially providing a patient with the opportunity to test-drive different prosthetic feet prior to prescription so his or her prosthetist can select the most appropriate device. The prosthesis emulator consists of an off-board motor and real-time control system, a flexible tether that transmits sensor signals and mechanical power, and a lightweight robotic ankle-foot prosthesis. Caputo founded Human Motion Technologies (HuMoTech), Pittsburgh, in 2015 to build the technology, among other things.

Data is the key that could unlock a new world of possibilities, Caputo says. "Patients will be able to trial different devices and generate individualized data about what they prefer and how they perform with different products, opening the door to evidence-based decision making," he says. "This would empower patients, elevate the quality of care, provide an opportunity for manufacturers of new devices to test efficacy much earlier, and generate data to meet the rising expectations for proof of medical necessity."

The prosthetic foot emulator can be programmed with torque-angle data measured from the mechanical testing of an off-the-shelf prosthetic foot. "We can take the [prosthetic] device and put it in a mechanical testing rig, which pushes on it in various ways, measures how the device responds, and then we turn that data into a format that our emulator understands…. In theory, if the robotic device deflects in the same way that the off-the-shelf device does, its effect on the patient and their gait will be the same."

The goal, Caputo says, is for a user to wear the emulator and interface with it while walking on a treadmill. Simultaneously, the prosthetist will change the behavior of the device, even when the user is midstride, so that direct comparisons can be obtained without having to change an actual prosthetic foot. "What we've done to date has all been treadmill walking and a bit of running," he says. "We are starting a project now that involves other ambulation conditions. We think it will be straightforward to do slow over-ground walking, sitto- stand activities, etc., but it is all in the conceptual phase."

Caputo examines the prosthetic foot emulator.

Currently, the emulator offers one degree of freedom and is programmed with the characteristics of several conventional devices, with more to be added as the work evolves. During initial pilot studies, patients trialed four foot behaviors with the emulator—SACH, dynamic elastic response, an active robotic boot, and a conceptual high-powered robotic foot design—and Caputo and his team could tune what the device was doing, using ankle torque versus angle relationships, to maximize the subjects'  satisfaction, energy expenditure, or walking speed.6 A follow-up study, which is not yet published, focused on a narrower range of behaviors (all dynamic elastic response–like) and on patient preference and/ or satisfaction. "We will use the results of this study to motivate future work," Caputo says. "We will need inversion/eversion control if we want to match the characteristics of multiaxial feet, and we will want to add a controllable heel."7,8

The team is collaborating with scientists from the U.S. Department of Veterans Affairs (VA) and U.S. Department of Defense (DoD) on a project funded by the DoD Congressionally Directed Medical Research Programs with David Morgenroth, MD, from the Seattle VA Center for Limb Loss and Mobility (CLIMB) as the principal investigator. "This project is focused on studying the predictive validity of a patient-centered test-drive strategy for the prescription of prosthetic feet," Caputo says. "This is an exciting, multisite project with a stellar team of investigators with expertise in clinical amputee rehabilitation, robotics, prosthetic foot properties, and amputee outcome measures," he emphasizes.

"In parallel, we are working hard to improve the quality of the emulator and providing research-grade emulators to academic labs around the country…but the clinical application is still very much under development," Caputo says

 He acknowledges the problem he is trying to solve is complex, and the challenges are many. Among the challenges are figuring out the level of detail necessary to accurately re-create the patient experience of wearing the real prosthetic foot, determining the evaluation procedure and outcomes to measure, and programming the emulator with the many commercially available prosthetic feet.

Caputo says the initial emphasis is on patient input because that is consistent with current clinical practice, "plus, many of the more objective quantitative metrics are not always consistent with what patients prefer or with what works best for them." He is also seeking input about the types of data practitioners and payers would like to have access to.

A subject trials the prosthetic foot emulator.

"I think that's critical," he says. "We, as scientists, have our ideas about what sort of outcomes we should be paying attention to, but ultimately I think those making decisions in the field should decide."

He continues, "The traditional experimental design studies a proposed intervention and tries to generalize the results across a population. I think we're finding that doesn't work so well in orthotics and prosthetics. Different patients respond differently, and we often don't understand why. Even when you try to pick a homogenous patient population, there is still a big variation—not a great practice if you wish for your results to be clinically relevant. This is one of the reasons why we believe what we are doing is important: the process is individualized. It figures out for this specific patient what works best without any preconceived notions about what should be best for them. We just measure it."

The current focus is on users of transtibial prostheses, regardless of the type of prosthetic foot; however, some devices are more challenging to emulate than others. "For instance, a microprocessor-controlled foot has intelligence programmed into it that cannot simply be measured on a mechanical testing setup. We need to gain insight, presumably from the manufacturer of the device, into how it is programmed to really do a good job of emulation," Caputo explains.

Regardless of this challenge, he envisions not just including all available devices, but also using the emulator to implement ideas for prosthetic feet that do not exist yet, thus allowing manufacturers to research, develop, and test a device before it is even fabricated. "There are so many applications for this technology," he says, and continued engagement with the clinical and research communities is going to be central to HuMoTech's success. "We need feedback from all stakeholders in O&P to identify the key roadblocks holding back patients from achieving their full potential."

Laura Fonda Hochnadel can be contacted at laura@opedge.com. 

References  

1.       Stark, Gerald. 2012. How do clinicians select prosthetic feet? The O&P EDGE, May.

2.       Collins, S. H. and A. D. Kuo. 2010. Recycling energy to restore impaired ankle function during human walking. PLoS ONE (5) 2:e9307. DOI:10.1371/journal.pone.0009307.

3.       Quesada, R. E., J. M. Caputo, and S. H. Collins. 2016. Increasing ankle push-off work with a powered prosthesis does not necessarily reduce metabolic rate for transtibial amputees. Journal of Biomechanics 49 (14):3452-9. DOI: http://dx.doi.org/10.1016/j.jbiomech.2016.09.015.

4.       Malcolm, P., R. E. Quesada, J. M. Caputo, and S. H. Collins. 2015. The influence of push-off timing in a robotic ankle-foot prosthesis on the energetics and mechanics of walking. Journal of NeuroEngineering and Rehabilitation 12:21. DOI 10.1186/s12984-015-0014-8.

5.       Caputo, J. M. and S. H. Collins. 2014. Prosthetic ankle push-off work reduces metabolic rate but not collision work in non-amputee walking. Scientific Reports 4:7213. DOI: 10.1038/srep07213.

6.       Caputo, J. M., P. G. Adamczyk, and S. H. Collins. 2015. Informing ankle-foot prosthesis prescription through haptic emulation of candidate devices. IEEE International Conference on Robotics and Automation (ICRA), Washington State Convention Center, Seattle, Washington.

7.       Collins, S. H., M. Kim, T. Chen, and Tianjian Chen, and Tianyao Chen. 2015. An ankle-foot prosthesis emulator with control of plantarflexion and inversion-eversion torque. IEEE International Conference on Robotics and Automation (ICRA), Washington State Convention Center, Seattle, Washington.

8.       Chiu, V. and S. H. Collins. 2016. Tripod: An ankle-foot prosthesis emulator with force control at three contact points. http://biomechatronics.cit.cmu.edu/publications/Chiu_2016_DW.pdf.