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March 2006 • Vol. 2, No. 1
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Advancing Orthotic and Prosthetic
Care Through Knowledge
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Jeff Denune, CP/LP
Ask yourself this question: why are there so many unique socket designs to choose from today? Are we looking for better suspension? Are we looking for better control of the prosthesis for our patients during ambulation? Experience and testing tells us that what we are ultimately looking for as trained practitioners is a socket that will provide the maximum amount of comfort for each individual patient.
Time and time again, patients have said, “By the end of the day, I take my prosthesis off sooner than I would like.” Do comments like this honestly show that we are helping amputees to have the quality of life that we as clinicians would like them to have? It all comes down to amputees being comfortable in their prosthesis.
Just as no two patients are alike, no two residual limbs are alike. With so many unique limb shapes, many of which are less than optimal, our job as prosthetists to provide a comfortable prosthesis can be challenging at best. As a result, the field of prosthetics has searched for solutions to the comfort question since the industry’s inception. Various approaches for increasing the amount of time that amputees are able to spend in their sockets have included innovative socket design, socks, pelite inserts, flexible sockets and, finally, gel liners. Substantial progress has been made in enhancing comfort for standard-shaped limbs, but the more unique the shape of the amputee’s limb, the more challenging it is to create a comfortable fit in the socket.
There are new, unique systems that allow us to design a custom liner—in other words, a liner that is “made to order”—so that patients with prominent bony limbs, uneven shapes, and deep invaginations can finally be fit with a more uniform—and comfortable—socket design.
The choices in the custom liner market are abundant, but without a full understanding of the materials, choosing a liner can be confusing. In the custom liner market, several material types are available such as silicone, urethane, and thermoplastic elastomer with mineral oil. All three of these materials react very differently to load force, shear force, and have different stiffness. It is our responsibility as professionals to choose the material that would work best for each individual patient’s needs.
What is happening to the patient’s limb inside the liner at full weight bearing? How is the material reacting to these loads when applied? Basically, there are two different types of materials that are used to create liner interfaces, thermoplastic and thermoset materials. Liners made with thermoplastic materials change shape under conditions of elevated temperature and/or loading, but these changes in shape do not ultimately change the gel’s properties. Thermoplastic materials also have more of a flow characteristic which better accommodates geometric changes to an amputee’s limb shape. Urethane and silicone materials are generally considered to be thermoset. Thermoset liners can also be changed by elevated loading, but any changes to these types of materials are typically temporary and prolonged shape change can ultimately lead to the deterioration of the material properties.
Some manufacturers of thermoset liners recommend a lubricant or powder to reduce shear force, while others recommend an interface material between the patient’s residual limb and the liner. However, with thermoplastic liners, no additional lubricants are required due to the gel’s ability to move coincident with the limb tissue. The movement of the gel allows it to absorb the shear force and reduce the shear transferred to an amputee’s limb in a socket.
Another factor to consider when choosing a custom liner is the stiffness of the gel. The stiffer the material, the less ability it has to absorb the loads applied to the amputee’s limb. The greater the stiffness, the more critical the socket design and liner fit. A stiffer liner material such as urethane or silicone, when poorly fit, will have a greater tendency to cause discomfort and/or damage to the liner. The increased material stiffness localizes the forces applied during use and can increase
the stress level in both liner and tissue. The ideal prosthetic liner should provide good response (stiffness) and protection from impact. This does not mean, however, that a stiffer liner is always better, since many times increased stiffness comes at the very high price of increased forces transferred to the patient’s limb.1
Based on these characteristics, it appears that a fabric-covered liner made of thermoplastic elastomer with mineral oil—specifically, the Alpha® Liner—would be preferred by amputees and prosthetists. In actual clinical use, the ability for a thermoplastic liner to adapt to the shape of the amputee’s limb has proven to increase the comfort that the amputee will have within the socket. Additionally, Alpha Liner’s level of stiffness has shown to be extremely comfortable in daily use
by amputees.
These clinical conclusions are further supported by a survey conducted on 30 patients who had been fitted with the Alpha Liner.2
83% of the patients reported that they were "very satisfied" or "satisfied" with the Alpha Liner 67% had no difficulties donning the liner 63% felt that they were able to walk for longer periods 14% were mildly satisfied, but were not interested in returning to their previous sockets 3% were dissatisfied and chose to return to their previous sockets 97% of the survey participants wanted to continue their use of the Alpha Liner
This survey applied to the off-the-shelf Alpha Liner, which until recently was the only type of Alpha Liner available. Patients with uniquely shaped limbs were unable to experience the characteristics that led the survey participants to prefer the Alpha. What if this level of comfort could be applied to patients who don't fit the profile for an off-the-shelf liner?
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Residual limbs that are invaginated, bony or short are all candidates for Alpha DESIGN Liners.
For the above transtibial limb, extra gel can be added over the scarring and on the sides to make the liner more uniform in shape and to provide the most protection from outside shear forces. Practitioners determine the best gel pattern, thickness and placement for their patients, allowing for the most comfort, but also for the most functionality.
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You know your patients best, and now you—the prosthetist—for the first time are given the control to add or subtract gel on the liner as needed for your patient's unique limb shape. By manipulating the placement of the comfortable Alpha gel to follow the patient's anatomy, you can accomplish the goal of providing comfort for patients with very unique limb shapes. Never again will you have to accept a less than optimal fit and try to "make it work" for your patient. With ever-growing advancements in technology, the clinician of today doesn't have to settle for second best for his or her patient.
Consider the following common patient scenarios seen in an average prosthetic practice:
Bony, prominent limbs are common among transtibial patients. With this new custom liner technology, you have the ability to place Alpha gel as needed to provide additional cushioning in sensitive areas. Buildups as thick as 15mm are possible. Patients with deep scars or invaginations could often wear an off-the-shelf liner that appeared to be a good fit from the outside, when actually there was a gap on the inside between the skin and the liner. This could lead to moisture build-up and skin breakdown along the edge of the invagination. Now, you can apply additional gel on the inside of the liner to fill in these gaps to prevent your patient's skin from becoming moist and macerated during daily activities. Patients with irregular limb shapes can often have increased difficulty in donning a well fitting socket. By adding or subtracting gel to accommodate a patient's anatomy, you can create a more uniform limb shape, making it much easier for the patient to don and doff the prosthesis.
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Transfemoral amputees who have severe invaginations on their residual limbs can now have gel placement customized in order to provide maximum support and comfort. Gel can be up to 15mm thick over invaginations.
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With knee disarticulation amputations and Symes patients, the comfort and suspension options are especially advantageous.
Typically in these cases, you would attempt to accommodate the shape of the limb through time-consuming processes such as designing sockets with bladders or doors, or building up pelite liners to create socket shapes that are easier to don and doff.
Now, by adding gel to the liner in the thinner areas of the limb, you can actually use the liner to create a cylindrical limb shape that lends itself to a more user-friendly socket. This approach can be used to promote enhanced suspension as well. When you combine the unique gel placement of a DESIGN™ Liner with a well-fitting socket and a one-way expulsion valve, these
patients will have an amazingly secure suspension system. Patients with shorter limbs who wear standard liners often have difficulty with initiating knee flexion or with bunching of the liner behind the knee when they sit, because the thinned area on an off-the-shelf liner does not extend low enough to be of any benefit to them. Because you can now decide exactly where the thinner areas in each liner need to be, these patients can enjoy the increased comfort of having thinner gel—as thin as 2mm—behind the knee.
Over the past two years, dating back to the initial testing of this unique interface system, the increased comfort and suspension possibilities that practitioners have at their disposal are proving to benefit an extremely wide range of lower limb amputees.
The effort to enhance the comfort that amputees experience in their sockets on a daily basis is a primary responsibility of every prosthetist. Whether your patients are transfemoral or transtibial, a custom designed thermoplastic liner can open the door for unparalleled comfort for a wide range of amputees. Don't all of our patients deserve the comfort and freedom that a custom designed liner can give them?
For more information on the Alpha DESIGN Liner, contact Ohio Willow Wood at 800-848-4930.
“Flow Constraints and Loading Rate Effects on Prosthetic Liner Material and Human Tissue Mechanical Response” S. Covey, J Muonio, G. Street, Journal of Prosthetics & Orthotics 2000, Vol. 12, Num 1, p. 15 “The Fitting of Alpha liners to trans-tibial Amputees: the study of 30 cases”, J. Corti,
Centre Prothetique Houradou; I. Corti, Clinique Chantecler, October 1999.
Jeff Denune, CP/LP has worked in the O&P field since 1986, with an interest in CAD/CAM since 1996. He is an Educational Prosthetist for Ohio Willow Wood.
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