The Finer Points of Ply Reduction With Vacuum

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Jason Kimmel

In the April 2011 issue of The O&P EDGE, I wrote an article describing the technical process of using socks and vacuum to perform ply reductions (“How to Do Ply Reductions Using Vacuum,”). I still receive questions on the topic, so I thought it would be helpful to revisit it and share some additional tips.

Let’s start with a recap of the process for a ply reduction using vacuum:

1. Thread a barbed fixture into the socket.

2. Place the required amount of ply socks into the socket, and tape them in place over the brim.

3. Insert an isolation rubber into the socket, over the ply socks, and then fold it over the brim.

4. Install a vacuum hose onto the barbed fitting and apply vacuum.

5. Fill with plaster, and then insert a pipe.

6. Once the plaster has cured, remove the socket, ply socks, and isolation rubber.

The process is not always as easy as these steps make it sound. Here is my advice for the most common challenges:

What if wrinkles appear in the ply sock and/or the isolation rubber while applying vacuum?

Figure A

Figure A: Some socket designs, like a transtibial socket with high medial-lateral trim lines, make it difficult to evenly fold the ply sock over the brim.

This usually happens because either the ply sock or the isolation rubber is too big for the socket. When it comes to the isolation rubber, use one that is considerably smaller than the circumference of the socket because it is capable of stretching a great deal. A good way to test for the correct size of ply sock is to insert the sock into the socket until it just touches the distal end. Fold the proximal portion over the brim, and lightly tape it in place. Now, place your hand inside the sock, and firmly hold it to the distal end. Using your free hand, press the sock to the inside walls of the socket. Do you want a good indicator that the ply sock will not wrinkle? You’ll find it in a sock that evenly conforms to the socket, without a lot of excess material.

How do you get the ply sock to conform evenly and smoothly to the brim of the socket?

Not all socket designs allow the ply sock to be evenly folded over the brim. One example is a transtibial socket with high medial-lateral trim lines (Figure A). A lot of sock material tends to be left over the posterior and anterior trim lines. When this happens, I find it helpful to make a vertical cut in the sock above the portion of the brim that the sock needs to fold over (Figure B). Occasionally, more than one cut needs to be made before the sock can be evenly folded over the brim.

Another method to get ply socks to conform to the socket is to apply spray glue to the proximal portion of the sock and then adhere the sock to the socket (Figure C). Using scissors, cut the excess sock even with the brim of the socket (Figure D).

Figure B

Figure B: Make a vertical cut in the sock where it must fold over the brim.

Figure C

Figure C: Apply spray glue to the proximal portion of the sock, and adhere the sock to the socket.

Figure D

Figure D: Cut the excess sock even with the brim of the socket.

Why don’t the sock and isolation rubber fully conform to the distal end of the socket?

Any of the following factors could prevent vacuum and prevent the sock and isolation rubber from fully conforming.

  • The barb fitting is not completely sealed.
  • The isolation rubber is not sealed off around the socket.
  • There is a hole in the isolation rubber.

If none of these is the issue, remove the isolation rubber and verify that the ply sock reaches the bottom of the socket. If not, the sock is too short for the socket. Insert a longer sock, verify that it reaches the bottom, and begin the process again.

Can the ply-vacuum method be used on a plaster wrap?

Yes, but first you’ll need to do some prep work. Be sure the wrap is sturdy. If it needs additional strength and stiffness, you may need to apply more plaster bandage around the brim. Then, because the plaster wrap is not airtight, you’ll have to seal it. Using stretch tape, completely cover the exterior surface of the wrap (Figure E). Next, install the barb fitting and seal it off. Then insert your ply socks and isolation rubber, and apply vacuum. You may find that the wrap wants to collapse under vacuum. If this happens, reduce the vacuum until it no longer collapses. Then you’re ready to fill the wrap with plaster.

Figure E

Figure E: Before using the vacuum method with a plaster wrap, seal the wrap by covering the entire exterior surface with stretch tape.

I hope the vacuum process and these suggestions are useful the next time you need to make a ply reduction.

Jason Kimmel is a founding partner of Motion Unlimited, Minneapolis. He has specialized in prosthetic and orthotic fabrication since 1999. He can be reached at .

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