Abstract
The purpose of this research project is to describe the methods of fabrication of a mounting apparatus for the Ossur Proprio™ battery. While the technology of the foot itself is impressive, a problem remains as to how to attach the rather cumbersome battery unit to the definitive socket. Currently the manufacturer provides a dummy piece which is meant to be laminated into the final lamination of the definitive socket. The Proprio™ battery holster described in this project is cost effective in terms of materials and time efficiency, is lighter weight, and more effective because it fully encompasses the battery unit. The original fabrication process provided by Ossur for the battery retainer within the lamination of the socket will first be described. Finally, the methods and materials for the battery holster fabrication will be described in detail.
Introduction
The technology of the Ossur Proprio Foot™ is one of the most impressive and innovative additions to the prosthetic field. Despite this impressive technology, there is one minor flaw that seems to consistently cause a problem. The rechargeable lithium-ion battery of the foot weighs approximately 0.5 lb, and has dimensions of 2 1/4" wide, 4 3/4" long, and 1 1/4" thick. Furthermore, this bulky power source is external and separate from the foot. The problem presents itself: how to effectively attach this cumbersome battery to the prosthesis? There has been a strong desire by not only the manufacturer, Ossur, to have some sort of simple and timely solution to this problem, but also several practitioners that have had the chance to fit several of their patients with a Proprio Foot™.
From the beginning, Ossur has provided a plastic dummy piece which has the same dimensions of the battery itself with the exception of it being only 1/4" thick. The original idea was to laminate this dummy piece into the final lamination of the socket, such that once the dummy piece was removed from the lamination, a void was created for the battery to click into. At first, the small lip of lamination material that remained to hold the battery in place seemed to work rather well. However, after a couple of days, the lamination seemed to wear out, spread or even delaminate and the battery would slip out. This was combated with the addition of a strip of sticky-back velcro. When the velcro failed, a small strap was added across the battery to help hold it in the designated compartment. The point is, the original battery suspension system is flawed, ineffective, time consuming, and uses a significant amount of materials. Instead of continuing to use band-aid solutions to make the original laminated compartment system work to hold the battery in place, the purpose of this project is to create a new battery suspension system that is more effective, less time consuming, and uses less materials.
The Proprio™ battery holster is a thermoplastic holding compartment that is pulled over a modified version of the provided Proprio™ battery dummy piece. The methods for the fabrication of the holster are provided in the methods section.
There are several benefits to using the battery holster. First, the battery holster is more effective in keeping the battery attached to the prosthesis. The holster completely encompasses the battery, so there is no threat of the battery slipping free and falling out. When compared to the original laminated compartment system which is completely open and relies on the snug fit of the battery against the lip of the lamination, the holster simply secures the battery in place better by design. Second, the fabrication time of the new battery holster is significantly less than the lamination process by which the original system relied on. This is simply due to the fact that draping a piece of plastic under vacuum takes less time than the extra lamination step. Third, the materials needed to create the battery holster are less in volume, cost, and weight. The original lamination system required an extra lamination step at the end of the fabrication of the socket. An extra lamination step means another PVA bag, more lay-up materials (nyglass, carbon, fiberglass), and more resin, all of which add weight and time. The thermoplastic battery holster system requires only the materials necessary to pull thermoplastic (polypro/co-poly, aliplast, nylon stockinette, cut-strip material) and the crepe material used to modify the dummy piece. In comparison, these materials cost far less. In addition, most of the plastic material is cut away leaving only the material needed to secure the battery, thus, being significantly lighter weight. It is for these reasons that the Proprio™ battery holster design has been pursued in this project as a simple and effective solution to the problem of suspending and securing the lithium-ion battery to the definitive socket of a patient fit with the Proprio Foot™.
Materials
- 1/8" thermoplastic (either polypro or co-poly)
- Ossur provided plastic dummy piece
- 1/2" crepe material
- Master's glue
- Aliplast
- Dacron cut strip material
- >Nylon stockinette
- Vacuum system set up for drape forming
- Cast saw
- Troutman or shoe-machine
- Four speedy rivets
Discussion
Although the Proprio™ battery holster has proven benefits such as more effectively securing the battery to the socket, being lighter weight, and more cost and time efficient in its fabrication process, there are still several conditions that have given rise for the need for future development of the system. Originally, the battery holster was first designed for a below knee socket with the Ossur Iceross® Seal-In® liner suspension with a flexible inner liner and rigid frame. The flexible inner liner allowed for the suction suspension to be maintained while the speedy rivets holes went through the rigid frame. Obviously, this is an ideal situation and not everyone is an ideal candidate for the seal-in suspension system. It is likely, however, that if the patient is a candidate for the Proprio Foot™ that they must have the best possible suspension due to the weight of the foot distally. Ideally, the patient is either using suction suspension with the seal-in liner, a closed suction suspension system with a one-way valve and a sealing sleeve, or possibly an elevated vacuum suspension system such as Ohio Willow Wood's Limb Logic™ or Otto Bock's Harmony ® e-Pulse (also needing a closed system with a sealing sleeve). The later two suspension systems pose a problem because of the need to speedy rivet the battery holster to the socket, and the holes would cause a leak in the closed system. Some discussion of sealing the speedy rivets in with epoxy after attaching the holster to the socket in order to maintain the closed suction environment has been presented, but not tested.
Another problem that has been presented is the sealing sleeve itself. If the residual limb is fairly short, there needs to be sufficient room for the sealing sleeve to roll down past the trim lines of the socket. The placement of the battery holster, or even the placement of the laminated compartment system, takes away from that room needed for the sealing sleeve.
Continued discussions have taken place about alternate ways to connect the holster to the socket without the use of speedy rivets, thus taking away the problem of the compromised closed suction environment. A strong epoxy manufactured by the company 3M called Dura-mix™ appears to have potential in terms of reliability, durability, and strength, however it has not been tested in this project for any significant length of time. In addition, a fully encapsulating holster that is pulled separately from the socket over a plaster mold of the battery so that the thermoplastic is along the back of the battery has been discussed. This would make it possible to perhaps attach the battery holster to a pylon or any other surface using two slots cut out of the back of the holster and a strap. These discussions have not been pursued as of yet, and maybe cause for further research in the near future, or until the power source of the Proprio Foot™ changes to be either smaller and perhaps internal to the foot unit.
Methods
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Match the dummy piece that Ossur provides for the lamination to the actual battery. Remove the antennae extension piece on the top using a band-saw or hack saw, then grind and smooth.
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The top of the dummy piece does not need to be modified.
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Attach the modified dummy to the final lamination of the socket.
Trace the placement of the dummy to the laminated socket. Place sticky back Velcro hook at the corners of where the dummy was traced, and a stripe down the middle. This will help the plaster adhere to the lamination.
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Using plaster, attach the dummy piece to the lamination. The plaster on the sides of the dummy piece should be smooth and straight down to the socket.
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Prepare and pull plastic. Remember to put a cut strip on the anterior of the socket to help protect the lamination when cutting off the plastic. Pull a thin stocking over the cut strip and plastered dummy piece for an air wick. Make a seal from the vacuum pipe to the socket using aliplast. Pull plastic over the dummy with the seem off-set from the cut strip so that you can access and use the cut strip.
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Trim and attach the holster. Once the plastic is cooled, it can be cut off. Separate the dummy piece from the plastic and cut the plastic down to the desired holster design. Be sure to leave approximately 1" of plastic "wings" on either side of the battery in order attach the holster to the socket.
Once the socket is separated from the cast, remove the flexible inner liner, cut the desired socket trim lines, and then attach the holster to the rigid frame of the socket using speedy rivets.
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Also, make sure that the sides of the holster are cut low enough proximally so that the wire from the foot to the battery is easily attached and unobstructed. The battery charger adapter input should also be easily accessible as the top of the holster is completely open.
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