Go to content Go to navigation

Note: This text is a work in progress. If you find any inaccuracies or material that needs to be included, please contact us.

Background

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:

Resources

Glossary of prosthetic terms

Concept development

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:

Illustration of a lattice contracting as it is pulled

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:

Illustration of a braid contracting as it is pulled

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.

Prototypes

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.

Test of braided sleeving gripping a simulated stumpJon testing a braided suspension

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.

All content and designs on this site are in the public domain, and we place no restrictions on their use. We encourage any derivative works, but all designs are registered periodically so that our work cannot be kept from the public by patents.