The Utah Terminal Device (UTD) project began in 1978 at a University of Utah lab, the Center for Engineering Design, known for bionic arms and cutting-edge technology in robotics and other medical areas. At the time, the standard for high-functionality and robust durability was the body-powered hook. Remarkably, it still is.
The lab was started a few years earlier by Stephen C. Jacobsen, PhD, who founded nine companies, including Motion Control, now a division of Fillauer/Hanger. I was a graduate student in that lab from 1976 until I was hired by Motion Control to introduce the Utah Arm to the marketplace six years later.
The University of Utah was very active in bioengineering research at the time. Teams were making headlines for artificial hearts, an early myoelectric elbow (the Utah Arm), and others. Even the first graphite prosthetic foot was introduced by a former student.
Jerome Stenehjem, MD, then also a student, had ideas for developing a body-powered hook that could convert easily into a hand. Together with Jacobsen, he built the first-generation terminal device, which featured new shapes, a two-position thumb, and center-pull cable actuation (making a hand/hook combination more practical).1
When Stenehjem left the program for medical school in 1978, he left a working prototype ready for field trials, which I was tapped to perform. My first step was to canvass the local prosthetics providers and recruit a group of willing and active hook wearers. Four subjects’ sockets were modified with the device, and after several weeks of trials, unfortunately, this group found little to like about it.
After some introspection, we realized that a simple but tough question had been overlooked: What were the unmet needs of these users? The design had been driven by some clever ideas, but they did not line up with what these experienced users wanted. Clearly, the first iteration was design-driven when it needed to be consumer-driven.
We needed to reevaluate and create a plan to answer specific questions:
— What was lacking about existing terminal devices?
— What was most useful about existing terminal devices?
— Why didn’t these hook users use hands, and would they ever want to convert a hook to a hand?
— What features of the preliminary model might be worth saving?
Further, the new approach had to minimize any bias inherent with evaluation by the device’s designers. Once we understood these pros and cons, new design goals were needed.
A well-known engineering axiom was a guideline: “If you can’t measure it, you can’t improve it,” and what we wanted to measure was usefulness of the terminal devices. Through detailed interviews with the specifically designed questionnaire, we documented (and counted) exactly what tasks these wearers performed, how the terminal device was used to perform it, what worked well, and what didn’t work well. This process produced a great deal of useful data, and we felt it addressed the problem of bias, because our data would be the same, no matter who asked the questions.
We avoided the variability of new sockets by adapting the new hook to each subject’s existing wrist and socket.
Identification of the tasks and the specific terminal device functions of the early prototype and the subjects’ usual terminal devices helped prevent “throwing the baby out with the bathwater,” as a new design was formulated.
New UTD Design
More secure gripping was a high priority, so in the new design we widened the gripping surfaces for flat and cylindrical gripping and coated the surfaces with tough urethane rubber (Figures 2 and 3). The cylindrical grip shapes were optimized for a typical drinking glass or beverage can. Knife gripping was improved by designing a new way for a knife blade to be securely interlocked (Figures 4 and 5).
Passive function, i.e., push-pull functions, were important to hook function, so rubber coating was installed on the outer surface of hook fingers in the new design.
The center-pull action of the first prototype added complexity, but the benefit it offered was retained by adding an additional bracket to the base of the terminal device where the cable housing was attached. This improved cable wear and reduced the problem of longer cable excursion when the terminal device was rotated at the wrist (Figure 7).
UTD Evaluation Results
The final-stage field trials with UTD prototypes showed positive results, with both the all-purpose and the work hook terminal device groups (Table 3 and Table 4). We used a five-level rating scale, chosen for ease of use in the interviews and the ability to reach a simple numerical result.4
Conclusions based upon the field trial data for the UTD were:
- The majority of both groups favored the UTD over their existing terminal device, both overall and in most functional areas. The study design produced clear data documenting the wide range of functions demonstrated by hook terminal devices, including the usual terminal device types and the UTD, which are still relevant to current needs. (A recent study of bilateral prosthesis use further demonstrates the ongoing importance of hook terminal devices to this population in particular.5)
- Grip security was improved for flat surfaces and cylindrical grips, and those who utilized the knife grip found it secure.
- The outside coating on the hook fingers improved passive functions of pushing and pulling.
- Prototypes showed promising durability, although multiyear experience would be the acid test.
- Urethane materials, in place of nitrile rubber, proved to be very durable (although challenging to fabricate).
Aftermath
Eventually, some aspects of the UTD design were used in a successful product, the externally powered ETD2 (Motion Control, 2018) that incorporated most of the gripping shapes of the earlier body-powered UTD, as well as other features.
The positive results from the body-powered UTD field trial still beg the question: When 70 percent of these active wearers of the current alternatives favor the prototype, why doesn’t a body-powered product result? While there’s not one single reason, there are several key factors that influenced this outcome:
Marketing Challenges
— The challenge of educating an uninformed public that considers a hook terminal uncool and simply too conspicuous is daunting. Neither domestic nor foreign markets are receptive to hook-type terminal devices.
— Payers seldom appreciate that people with upper-limb loss may need both a hook and a hand to perform outdoor activities, as well as their indoor work. Often the more expensive electric hand is purchased, but not a second body-powered workhorse.
— A manufacturer already making body-powered terminal devices must consider that a new product may cannibalize their existing market, perhaps resulting in a net-zero increase in total sales.
Reimbursement Limits
A major obstacle to bringing new body-powered terminal devices to market is that reimbursement for body-powered terminal devices is essentially capped by existing L-Codes, which are set by the Centers for Medicare & Medicaid (CMS) but also used as the standard for other third-party payers. Sales data for the entire market is not available, but Table 6 shows the most recently published CMS data. While CMS claims accounted for only about 19 percent of the total US market, they provide a good snapshot of the distribution of device delivery.6
Note that the electric hook code (L-7009) paid nearly $4,000. The aluminum body-powered hook code was reimbursed for only $413, and there were 91 units funded by CMS. A heavy-duty (steel) terminal device was reimbursed for $2,382 on average, and 125 were purchased by CMS, for a total of 216 CMS-funded body-powered hook terminal devices. Obviously, the steel hook has much higher reimbursement, but some wearers cannot tolerate the higher weight of a steel terminal device—the lighter weight product version is still essential for many.
Investments Required
Another important, and discouraging, consideration is the initial investment required to introduce a new terminal device product. New tooling would require as much as $100,000 (and rising), realizing that there are four individual parts to be molded (fixed and moving fingers, right and left). If 3D-printed titanium were pursued, which requires a lower fixed cost, reaching economies of scale could require a commitment to order in the range of 1,000 units.
Summary
These difficult trade-offs exist, and the losers are the consumers and the O&P facilities that provide lightweight body-powered prostheses at a fraction of the margin required for a sustainable business.
But body-powered terminal devices are still chosen by individuals with upper-limb loss who have high functional and rugged needs, especially people with bilateral arm loss, hobbyists, homemakers, and those who make their living with manual tools.5 This project demonstrates that improvements in design could benefit them. Certainly, electric hooks are practical and a growing sector since they eliminate control cables and increase pinch force, but for economy, simplicity, and durability, many still choose body-power. And, for many understandable reasons, the dominant body-powered products are essentially the same as they have been for 75 years.
Harold H. Sears, PhD, retired in 2017 as president of Motion Control, a Fillauer Company, Salt Lake City. Sears earned a bachelor’s degree in mechanical engineering, a Master of Engineering Science from Stanford University, and a doctoral degree in bioengineering from the University of Utah, and defended freedom as a teacher in the Peace Corps in Nepal, 1970-71.
References
- Stenehjem, J. 1980. The Utah Terminal Device [MS thesis]; Salt Lake City: University of Utah Department of Bioengineering.
- Resnik, L., S. Ekerholm, M. Borgia, and M. A. Clark. 2019. A national study of veterans with major upper-limb amputation: Survey methods, participants, and summary findings. PloS One 14(3):e0213578.
- https://www.cms.gov/data-research/statistics-trends-and-reports/part-b-national-summary-data-file; link leads to Download Zip File: National_2022_; then open: Y2022_L5000.
- Sears, H. H. 1983. Evaluation and Development of a New Hook-Type Terminal Device [PhD dissertation]; Salt Lake City: University of Utah Department of Bioengineering. [PDF copy available on request from author]
- Sears, H., K. Doolan, and D. Keenan. 2022. Small-scale study of bilateral UL prosthesis use. Journal of Prosthetics and Orthotics 34(2):95-107.
- https://www.census.gov/library/visualizations/2023/demo/p60-281.html.
Featured photograph: michaelcourtney/stock.adobe.com
