High-Fi Flies High in New Prosthetic Interface: Tissue Compression/Release Concept Gains Control
"I've been an amputee for 35 years, and I am very, very particular about the fit and finish of the socket. In fact, when I was asked to try Randy's new interface socket, I thought, 'Okay, I'll agree to have him mold his new and improved, whiz-bang socket-and then I will show him why it won't work on a VERY active amputee.' Now when I see Randy, I can hardly talk to him because my mouth is so full of black feathers from eating crow."
|The radial High-Fidelity Interface. Images courtesy of biodesigns inc., unless otherwise noted.|
Ron Currier, a retired chief of prosthetics at the Manchester (New Hampshire) Department of Veterans Affairs (VA) Medical Center, made these comments while discussing a new prosthetic-interface design, the patent-pending High-Fidelity Interface with Vector-Enhanced Compression and Soft Tissue Release (VECTR), the brainchild of Randall "Randy" Alley, CP, LP, FAAOP, CEO of biodesigns inc., Thousand Oaks, California. Currier, a bilateral transradial amputee, was a confirmed skeptic. Not only is he extremely active-having worked as a well driller, homebuilder, and maintenance contractor, plus enjoying such sports as hunting, fishing, scuba diving, kayaking, and motorcycle riding-he also has experienced severe socket-fitting problems. At times, he says, "I would take off my prosthesis and pour blood out of my socket because my skin would break down. I've tried just about every kind of socket there is-self suspending, Münster, sleeve suspension, all with the same results. There was always a severe loss in range of motion, skin breakdown with repetitive motion, loss of suspension due to lifting requirements, pulling, hammering, or heavy sweating, which due to burns, is severe and uncontrollable."
He continues, "I honestly figured I would never find a self-suspension socket that would work on me. But I was wrong. It's light, it's comfortable, and my range of motion is 100 percent. One of the things I require from my socket is durability and reliability. With the interface, I was picking up 55-pound water jugs and doing arm curls. I'm completely blown away."
|Bone Motion in a Socket Interface|
Full tissue compression is shown by the dashed lines. In a conventional socket, tissue is displaced internally until full compression transfers force. When depressions in the socket pre-compress tissue, force transfer occurs much sooner and more force is transferred away from the bone ends. Images courtesy of biodesigns inc and T. Walley Williams III.
Chuck Hildreth, Gifford, New Hampshire, who has a left short humeral and right interscapulothoracic amputation, is equally enthusiastic. Before trying the High-Fidelity Interface, wearing prostheses was so uncomfortable that he preferred doing most of his daily tasks with his feet or his teeth. "Now I'm able to concentrate on what I'm doing rather than fighting with the arm to [keep it] in place. Previously, all of the soft tissue in the residual limb would move in the socket. In this new technology, there's no more slushy skin. I have more freedom of movement and more stability and better control of the arm."
Both Currier and Hildreth are participating in a clinical study involving the high-tech Luke Arm developed by famed inventor Dean Kamen's DEKA Research and Development, Manchester, New Hampshire, as part of the Defense Advanced Research Projects Agency (DARPA) Revolutionizing Prosthetics program.*
The goal of the DEKA/DARPA project is to create a production version of the DEKA Luke Arm, Alley explains. "The High-Fidelity Interface design and its VECTR concept form the foundation for the socket interface utilized in this project."
Alley is working with Next Step Orthotics and Prosthetics, Manchester, New Hampshire, on the DEKA/DARPA project. Alley says that he and the Next Step clinical team are fitting people for home clinical study and also advising DEKA on the arm's optimization for future use. Matthew Albuquerque, CPO, Next Step vice president, says that he is enthusiastic about the High-Fidelity's benefits, adding that the only contraindication he sees would be for persons with hypersensitive skin. Alley notes that for these cases, the design can be adjusted so that it is less aggressive.
How It Works
Every so often a new idea comes along that might be described as "revolutionary." The High-Fidelity Interface design could well fit this description. Alley explains that in traditional socket designs, a patient's soft tissue is encapsulated by the socket, restricting the amount of control a socket can impart upon the bone buried beneath the soft tissue. Skeletal motion should be the focus, not the soft tissue surrounding it, Alley says. "Soft tissue that hasn't been optimally preloaded allows significant skeletal motion within the interface prior to the interface responding.... This internal movement is extremely inefficient, resulting in a tremendous loss of energy that could be otherwise used to propel or stabilize the prosthesis."
|Two views of a CAD/CAM-rendered High-Fidelity positive image, followed by a lateral view of the resulting test frame.|
Noted prosthetic engineer T. Walley Williams III, director of product development, Liberating Technologies, Holliston, Massachusetts, refers to this concept as "compression stabilization" in a poster he co-authored with Alley and presented during the recent 13th International Society for Prosthetics and Orthotics (ISPO) World Congress in Leipzig, Germany. "Bone is centered in tissue, which must move before pushing the socket wall," Williams explains. "In a typical compression-stabilized [CS] socket, four longitudinal depressions compress tissue between the socket wall and the bone to reduce lost motion as the bone moves toward the socket wall."
In contrast, the High-Fidelity Interface is stabilized by longitudinal struts creating depressed areas alternating with relief areas. "In between these longitudinal areas of compression that travel nearly the entire length of the bone are areas or windows-depending on whether the interface is a solid body or an open, cage-style interface-where soft tissue can flow out of the way or out of the interface entirely. This allows for increased compression on the intrinsic bone, far greater than can be achieved in a hydrostatic or other traditional socket," Alley says.
Alley emphasizes that the compression must be precise; he often uses a blood-volume perfusion sensor to ensure a safe compression level. "Above a certain level, you could lose adequate blood flow; below a certain level, you minimize the benefits."
"The soft tissue is displaced and the connective tissue to the bone is pressed and thus controls the motion of the bone better," Williams says. "The idea is to provide just enough pressure so that the bone can no longer move front and back and sideways." For example, Williams notes that the socket design stabilizes the end of the humerus so that it can't move. Forces controlling the humerus are not applied at the ends of the bone but rather along its length. Thus, humeral motion within the socket is greatly reduced or eliminated, providing more comfort and control.
Discussing the transradial application, Williams says, "Many other designs put a lot of pressure at the end of the residual limb within the prosthesis. The force is mainly distributed at the ends of the bone, but with this design, which distributes force all along the bone, the prosthetic arm does not seem to weigh as much."
Although the upper-limb applications of the High-Fidelity design have captured more spotlight, with ongoing clinical studies at DEKA producing "extremely positive results," the transfemoral application has so far been the most widely used, Alley says. Williams designed a specialized femoral-level casting jig, which Alley is currently modifying. They have applied for a patent on the High-Fidelity Interface design and the jig.
Pending Institutional Review Board (IRB) approval, Alley says, a comprehensive study of the transfemoral application is expected to begin soon at the University of California, San Francisco (UCSF). Heading the study will be Matthew Garibaldi, CPO, clinical manager of UCSF's Orthopaedic Institute. Garibaldi, who formerly worked at biodesigns, was the first person Alley trained in the lower-limb application of the High-Fidelity design. "Randy had asked me to apply the High-Fidelity concept to the lower-extremity arena, specifically for transfemoral amputees in an attempt to maximize femoral stabilization," Garibaldi says, adding that the socket underwent various modifications to maximize comfort and function. Garibaldi says his main focus was to maximize motion capture and stabilization of the femur while concurrently eliminating zero-level socket complications that are characteristic of traditional transfemoral designs. "By stepping outside the confines of hydrostatic and other traditional fitting principles to allow for greater compression depth of the stabilizing struts, we found that it was quite possible to eliminate concerns of ischial containment, skeletal/soft tissue MLs, as well as trochanteric encapsulation by simply focusing on the primary mover of the transfemoral amputee, the femur," he says.
|A carbon-fiber frame interface on a finished prosthesis.|
Alley believes the lower-limb trials will show the limitations of traditional hydrostatic theory in interface design and will demonstrate how much of the energy lost during ambulation is lost within the interface itself. "I am very excited as to what we will ultimately discover."
Discussing another aspect of the issue, Alley adds that he believes the prosthetic community has missed an opportunity in dealing with window edema, generally choosing to completely avoid the issue rather than quantifying its proper management and utilization to gain a functional advantage while avoiding its negative consequences. Although some prosthetists have used windows for suspension, heat dissipation, and muscle hypertrophy, Alley points out that the High-Fidelity design not only can incorporate these advantages but it can also utilize enhanced compression via soft-tissue release to increase intrinsic bone stability and minimize energy loss due to wasted motion to improve performance.
With clinical studies, Alley intends to discover the relationship between aperture size, force, pressure, and duration under weight bearing so that safe levels of window edema are optimized for the individual's needs. Post-delivery assessment will focus on skin issues and limb-shape changes for future cosmetic, comfort, and donning/doffing issues.
"Because the preferred methods utilized to date for High-Fidelity lower-limb applications have employed either solid suction designs with tissue-relief areas or cage-style designs utilizing silicone liners and a strut system, adequate tissue compression to ensure proper blood flow has always been present," Alley says. "Thus, we haven't tested the upper limits of what can be achieved. This will be one of the goals of the university study." Alley is also looking at more effective suspension techniques. "The current methods of suction, elevated-vacuum, pin, and auxiliary suspension all work fine, but they aren't optimal. I am looking at really pushing the envelope in this area."
Alley is currently considering training courses for both upper- and lower-limb applications, with a potential roll-out schedule in 2011 based on the completion of the casting jig. "I also will be working with select CAD [computer-aided design] software companies capable of reproducing the intricacies of the design, both in terms of software modifications as well as on the fabrication side, particularly with the precision required for small upper-limb applications," he says. "The most exciting aspect will be the capability to merge MRI and other detailed imaging with precisely targeted compression and release elements."
Alley is also working on a high-performance tibial design. "Suffice it to say we are losing far too much energy with traditional approaches at this level, and we need to rethink what we've been doing with respect to tibial capture."
Putting Energy to Use
|Michael Hart wearing the High-Fidelity radial sports prosthesis.|
The High-Fidelity design has already helped persons with amputation to do what they love. For instance, Michael Hart, 17, a member of the Litchfield Hills Rowing Club, Litchfield, Connecticut, previously experienced difficulties when he participated in races. "Before, when Michael got hot and sweaty, the prosthesis would slip off and he would lose his grip," his father, Michael Hart Sr., says. "This new interface works really great. It grips his arm aggressively, but comfortably. Michael says it feels more like part of his body."
Alley points out, "When it comes to high-energy activities such as Michael's rowing, traditional socket designs fall significantly short of what is required to maximize performance.
"What an athlete needs most is to maximize use of the tremendous amount of energy he/she expends during the event," Alley continues. "Rather than allow the interface to absorb and waste it, why not capture this energy and put it to greatest use?"
Miki Fairley is a freelance writer based in southwest Colorado. She can be reached at
*This project is sponsored by the Defense Advanced Research Projects Agency and the U.S. Army Research Office. This information does not necessarily reflect the position or policy of the government; no official endorsement should be inferred.