In this Innovation article, The O&P EDGE presents problem-solving techniques developed by an upper-limb prosthesis user to address durability issues.
Background
I sustained bilateral hand amputations in an accident in 1954, at age 15. During my teen years, I spent many Saturdays in the company of Leonard Madison, my first prosthetist. I took an immediate interest in my prosthetic equipment as breakdowns were a weekly event. I have always been a physically active person, involved in sports and outdoor recreation. I was sanctioned by the National Collegiate Athletic Association to compete as an epee fencer and was on The Ohio State University fencing team from 1958-60.
During my time with Madison, I watched him make cables and I tried different terminal devices-anything to regain my independence. Madison was happy to teach me and I assembled a tool kit. With supplies bought from him, I began making up my own cables. I still do this and have continued my quest for better equipment. Over the years, I have experimented with a variety of types and generations of upper-limb prosthetic systems and devices.
In my experience, one of the weakest links in body-powered upper-limb prosthetic systems is the wrist unit. The idea of a locking/ quick-change wrist unit is excellent. The problem is they suffer from a relatively weak, easily broken or worn-out lockup. After trying the available locking units, I selected the Pope (originally made by U.S. Manufacturing) as being the least worst. The 3/32-inch dowel pin in the Pope wrist unit locks in two places in the circle of a dozen teeth in the adapter. The other units I tested feature a robust cog-tooth or grooved locking surface in one part while the adjustable locking element features a few shallow teeth or two thin prongs to maintain the locked position. Wear, breakage, and slippage occur within a couple weeks.
Figure 1. The Dorrance terminal devices used in this project. The Pope adapter is on the right hook.
Figure 2. The first prototype was a good fit but proved impractical.
Figure 3. Left, the original Pope socket; right, the new socket.
Figure 4. Left, the original locking lever was subject to wear and bending.
Figure 5. A heavier laminated-in socket replaced the original. Left, the original socket; center, the locking socket for the Pope adapter; right, the heavy-duty laminated-in socket.
Figure 6. Left, the original Pope laminated-in socket; right, the current commercial model, top view.
Figure 7. As seen from the bottom, the commercial model relies on a friction-fit insert to anchor the locking socket.
Figure 8. The commercial model after breakage at the edge of a screw hole.
Figure 9. Left, the original Pope faceplate. The current model (right) can be modified to function.
With a terminal device, used mainly in a single position, wear on the adjoining teeth becomes greater at this point in the unit, resulting in a poor fit and an undesirable rotary motion of the terminal device. This problem is exacerbated in the Pope unit when the pin wears, bends, or breaks, which necessitates frequent replacement. The mounting hole for the pin soon becomes enlarged in the aluminum socket, causing the pin to slip out. Additional problems occur with wear and bending of the locking lever, causing the unit to fail to lock or remain locked. Continued pin replacement, with the mounting screws sufficiently tightened for the unit to function properly, leads to thread failure in the laminated socket, necessitating replacement (i.e. tearing down the socket for relamination or total replacement).
The Project
My goal was to replace the pin lock on the Pope wrist unit with a full-circular system based on a spline linkage akin to that used in a tractor's power-takeoff. Secondarily, the plan was to use as many prefabricated parts as possible. The spline idea was dropped to retain the Pope adapter (Figure 1).
The first effort involved horizontal drilling to duplicate the shape of the teeth in the socket adapter. This was in the form of an insert to be set in the bottom of a deeper, longer socket. The problem was that a dowel pin was still used to secure the insert. While this dowel would not be subjected to wear and bending, it would receive considerable torque. Manufacturing the prototype was time consuming. The deeper base adapter also required a deeper socket, which created issues with the inner-arm socket (Figure 2).
The most economical approach was to create a socket with teeth cut into the bottom. This was fabricated at Precision Machine, Edison, Ohio. The initial prototype was made from 4140 carbon steel. The teeth were cut using a four-axis CNC bed mill with a hand-ground carbide, ball-end mill (Figure 3).
A further improvement was made by replacing the release lever with a 0.53-inch, high-strength, 17-4 PH stainless steel unit, which is thicker, harder, and less prone to wear and warping. This part was made using a water-jet cutter. The modified units were installed in a new set of prosthetic arms at Mountain State Prosthetics, Charleston, West Virginia, in consultation with the late David Hansford, CPO. We had Precision Machine fabricate custom sockets from aircraft aluminum for a more durable design, with deeper faceplate screw holes, to be laminated into this set of prosthetic arms (Figures 4 and 5).
The construction of these arms presented the additional problem that the current manufacturing of the Pope wrist unit has been altered from the original design (Figure 6). The current design uses smaller-diameter, longer screws and a circular recess in the faceplate to retain the release-lever operating spring. These features increase the difficulty of inserting the spring without crushing it. Furthermore, the laminated-in portion, which was originally a single, solid unit, currently features a friction-fitted ring subject to slippage and breakage where the screw holes are near the edge. Some years ago, in a discussion with an experienced prosthetist about the original Pope wrist unit, we agreed that the Pope wrist could be improved by using fewer, larger-diameter screws. Therefore, I used the original parts, although the current faceplate design can be drilled and otherwise slightly modified to fit (Figures 7, 8, and 9).
Performance
As previously mentioned, the first version of the toothed sockets was constructed of 4140 carbon steel. These sockets lasted about 18 months, during which time they were used for heavy lifting and hard work-specifically, cutting, loading, splitting, and stacking more than five cords of firewood; helping my brother lift 150- pound sections of dock; and constructing a seawall using 20 to 50-pound boulders. By the end of this time, wear was visible and lock-point slippage reached a point of uselessness.
The second version was the same system, however, this time the 4140 steel was casehardened in the grooved portion in the socket bottom. Unfortunately, this did not produce a noticeably longer lifespan.
In a third version, the sockets were made of hardened 4140 steel. With this version, vibration/tool wear during the milling process resulted in a slight offset of the grooves between the teeth. This required additional hand-fitting to allow the adapter teeth to mesh properly with those in the socket. After a year, these sockets function well, though they are exhibiting visible wear. However, the machinist pointed out that the additional machining time and degree of hand-fitting required would raise costs to a level disqualifying the device as a viable commercial product using the current technology.
In a potential third design iteration, we gave up the idea of using the prefabricated, toothed adapter in favor of an adapter with an octagonal base that would fit in a socket with an octagonal bottom. This system could lower the manufacturing cost of the socket and adapter to create a unit cost on par or less than the current prefabricated models. Beyond this, such a system appears to be potentially more durable and have the added feature of giving the user eight locking points as opposed to the current six points.
While this idea seemed promising, achieving a feasible, easy-to-use lockup system did not work, and I have been financially unable to pursue further experimentation. Nevertheless, I consider this effort a qualified success. The toothed socket did not last for the hoped-for several years, but it held up for 18 months of punishing activity. The annoyance of pin replacement every ten days to two weeks has been eliminated, as well as the wear and damage to the laminated-in socket from the constant removal and tightening of the mounting screws that eventually leads to failure of the threads, which necessitates replacement through a tear-down and relamination or a new socket (Figure 10).
Figure 10. The final version with original faceplate, replacement locking lever, and locking socket.
By my assessment, it appears that many upper-limb prosthetic components are built to the stress/durability level of use by the average office worker, making them unsuitable for those who pursue such occupations as military service, mining, farming, lumbering, factory work, and millwork. Those who wish to continue in these occupations require equipment built to the demand specifications of their work. Further information on costs and details of manufacturing of the units I designed and tested may be obtained from Jeff Taylor. He can be contacted at .
C. Rodney James, PhD, earned his doctoral degree in mass-media studies from The Ohio State University, and is a freelance writer. He can be contacted at .
