Note: This text is a work in progress. If you find any inaccuracies or material that needs to be included, please contact us.
The most essential part of any prosthesis is its attachment to the body. The fit must be comfortable enough to wear the prosthesis all day with minimum discomfort, and for a functional prosthesis it must also be rigid enough to transfer force from the body to an object that is being grasped, lifted, or leaned upon. Often these two requirements compete with one another, because a prosthesis that fits more tightly for a strong connection is also likely to be less comfortable. The consequences of a bad fit can be skin irritation, pain, and even tissue breakdown.
The system used to attach a prosthesis is called a suspension system. There are several types of suspension systems currently in use, and the choice of which to use is often based on the specifics of the user’s residual limb and lifestyle. The following general methods are common:
- Adjustable sleeve: This type of suspension achieves proper fit through user adjustment of a strap or buckle. It is often used temporarily while the user’s stump is changing shape or volume or if other suspensions are unsuitable. The silesian belt for artificial legs is an example of this method.
- Elastic sleeve: An elastic sleeve is an stretchy tube lined with frictionate material that grips the user’s stump with a combination of friction and vacuum. These are usually made of silicone with a fabric layer on the outside for easier handling. Silicone gel is often incorporated for a softer and more comfortable interface.
- Suction socket: A suction socket attaches by creating a vacuum between the stump and the prosthesis. As the patient dons the prosthesis, air is expelled from the socket through a one-way valve. The negative pressure around the stump holds the prosthesis in place until the user releases it by opening the valve. Here is an example of a suction socket that uses an impermeable mebrane between stump and vacuum.
Our original idea for a new suspension is based on the principle of a Chinese finger trap, which is easy to insert your fingers into but tightens as you try to pull your fingers out of it. This effect happens because the trap is made from a lattice structure which contracts on one axis as it expands on the other. Here is a two-dimensional illustration of the effect:
Fabric used in this way is said to be cut on the bias in the textiles industry. When fibers are woven on the bias into a continuous tube, this arrangement is called a braid. There are many types of braid produced commercially, and they are typically used as the basis of composite tube structures. Braided structures are very efficient at absorbing torsional and tension loads and impacts. Braided composite material is already used for the structural part of many prostheses because it is strong and can be easily shaped.
The aspect of braid that makes it potentially useful for a suspension is that it constricts under tension, as illustrated by the following diagram:
This means that the grip force of the suspension sleeve is roughly proportional to the tension load on it. There is precedent for using braid this way in the medical industry, which uses braided sleeving for orthopedic traction.
Our first prototype of this idea was simply testing the mechanism with a commercially available product called FLEXO Expandable Sleeving from TechFlex, Inc. This material is basically biaxial braided sleeving in a variety of materials. We tried the 2.5” PET material and the 4” Heavy Wall material by sliding them over Jon’s stump and pulling on them. The 2.5” material gripped well, and would comfortably support loads without slipping. Here’s a picture of the sleeving on a simulated stump (rubber wrapped on a cylindrical carton) and a picture of Jon testing it.
The material needed at least 8-10 inches of engagement with the stump before it gripped properly. It would be more widely useful if it could grip a shorter residual limb, and it seems we might be able to do this by changing the material and/or geometry of the braid. Clearly this material is not as frictionate as it could be, since the monofiliments it is made of are very smooth and slick. Also, it would be desirable to be able to wick moisture away from the skin instead of retaining it on the surface of the fibers. The pitch of the braid may also be important, since it controls to some extent the proportion between pulling force and contracting force. A better fit would help as well, since the diameter of the stump at any given point affects the effective pitch of the braid at that point. An additional issue is dealing with the movement of the elbow or knee.
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