The Devil is in the Details

Wednesday, 06 July 2011 14:26

as published in Product Design & Development, February 11, 2011

With a growing amputee population, two firms partner to design artificial limbs that use vacuum pumps to increase circulation, comfort, and quality of life.

Until recently, artificial limbs were held on via archaic methods, such as straps, sleeves, or pins. All of the former methods allowed for certain types of motion, motion that creates sores and pressure. Ray McKinney, certified prosthetist orthotist (CPO) of McKinney Prosthetics, and his partner in design, William Fleming, president of Dynaflo, have developed a vacuum suspension system to create a seal that securely holds the prosthesis to the limb.

The new system eliminates excess motion and, since the vacuum pressure is high enough, it converts perspiration from the socket to a vapor that seeps out through the pump back into the atmosphere—curing another ailment of the below-knee amputee community.

The vacuum not only holds the leg on, but it also makes the prosthetic feel lighter and stabilizes the bind between the limb and prosthesis.

“Normally, as amputees will tell you, when they put the leg on in the morning it will usually fit fine, but it shrinks and continues to shrink all day as a result of the positive pressure of the prosthesis,” says McKinney. “But with elevated vacuum, if it’s higher than 15 in-Hg, which these pumps are, it keeps the limbs from shrinking.”

According to McKinney, limbs normally shrink anywhere from seven to 12 percent of their limb volume in the course of the day. In vacuum, they shrink less than one percent, and problems typically don’t occur until four percent. The seal between the socket and the limb and liner is not perfect. Over the course of several hours, the vacuum will diminish and the pump has to be able to reestablish the vacuum.

The pump was Fleming’s brainchild and was designed specifically for this application.

“What’s unique about the pump itself is the fact that it’s very small, but it’s capable of generating a relatively high vacuum level [up to 25 in-Hg] and that compares to an absolute vacuum level of about 29 in-Hg,” says Fleming. “In most actual installations, nowhere near 25 in-Hg is required. They typically run up to a maximum of about 20 in-Hg.”

The Devil is in the Details continue...