Seeking the Perfect Marriage in Prosthetic Liners

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Photograph courtesy of Ohio Willow Wood.
Photograph courtesy of Ohio Willow Wood.

Otto Bock liner inspector Rocio Holguin. Photograph courtesy of Otto Bock HealthCare.
Otto Bock liner inspector Rocio Holguin. Photograph courtesy of Otto Bock HealthCare.

Suppose you want a car that can reach 200 miles per hour—and you also want 60 miles to the gallon fuel economy. Given the current state of automotive technology, consumers can't have both. As one prosthetist mused, "That's an analogy for liners today. For more durability, you're looking at harder, firmer materials, but that could mean less comfort."

"What everybody clamors for is more durable liners," says noted orthotic and prosthetic consultant John Michael, MEd, CPO, FISPO, FAAOP. "This has been improved in recent years, but further improvement would be very welcome." Michael notes the usual trade-off: the lower the durometer, the greater the cushion but with lower durability; the higher the durometer, the greater the durability but at the cost of cushioning. The other trade-off, he adds, is thickness: the thinner the better if the liner will do the job clinically, Michael believes, but currently prosthetists must use thicker liners for more cushioning, adding weight and difficulty in limb control.

Michael
Michael

The comfort-versus-durability conundrum is just one challenge involved in the continuing quest for better liners. Other design and materials challenges include maintaining an intimate fit, reducing movement within the socket, reducing perspiration buildup, preventing skin irritation and breakdown, and maintaining secure suction suspension.

The liner is a vital part of socket fit and comfort. "You can build the prosthesis with the most expensive, technologically advanced componentry, but if socket fit and design are inferior and not comfortable, the amputee will not wear it," says Jeff Denune, CP, Ohio Willow Wood, Mount Sterling, Ohio.

According to one study, "The single most critical aspect of any prosthesis is the quality of the interface between the residual limb and the prosthesis." (Linda J. Marks & John W. Michael, 2001, www.bmj.com/cgi/content/full/323/7315/732).

Choices: Factors to Consider

Jeff Denune, CP
Jeff Denune, CP

No one liner design or material works for everyone, and all those interviewed for this article agreed about the importance of evaluating each patient as a unique individual. Factors to consider include amputation level; limb shape and condition; socket design; activity level; the patient's functional needs and goals; and, most importantly, the type of suspension system to be used.

"It's almost impossible to separate a discussion about liners from a discussion about suspension since the suspension mode can affect liner performance, especially with thicker liners," says Michael.

"Prosthetists need to first consider the suspension mode they will be using and then consider which material will work best in combination with that," says Scott Weber, MS, the U.S. marketing manager for prosthetic feet and socket technologies, Otto Bock HealthCare, Minneapolis, Minnesota. For instance with pin suspension, in which there is a constant pull on the end of the liner, a more durable material such as silicone would be a logical choice, he notes.

With so many variables in the mix, choosing the best liner for a particular patient can be a challenge, especially since there has not been much research in the correlation of materials with liner performance in patient use. Manufacturers including Ossur, Ohio Willow Wood, and Otto Bock have provided guidelines that offer valuable insight into matching liner choices with suspension systems, patient activity levels, amputation levels, and residual limb shapes. Of course, these guidelines have the limitation of including only that particular company's products and materials. "There have been very few unbiased cross-material comparisons," observes Michael.

Materials Properties

Photograph courtesy of Ossur Americas.
Photograph courtesy of Ossur Americas.

Commonly used liner materials include silicone, urethane, gels (which can be made of some of these other materials), and thermoplastic elastomers. Although each material has certain general characteristics, many manufacturers have their own proprietary formulas; materials may be combined with others, changing their characteristics, so no materials description is absolute.

A quick chemistry review: elastomers are polymers that resist and recover from deformation produced by force, similar in behavior to natural rubber. Polymers are natural or synthetic compounds of usually high molecular weight consisting of up to millions of repeated linked units, each a relatively light and simple molecule.

There are two general classes of polymers based on their behavior when exposed to heat.

Thermoplastic polymers are normally produced in one step and then made into products in a subsequent process. They become soft and formable when heated. The polymer melt can be formed or shaped when in this softened state. When cooled significantly below their softening point, they again become rigid and usable as a formed article.

Fothergill
Fothergill

Thermosetting polymers are normally produced and formed in the same step. Upon heating, thermosetting polymers will become soft but cannot be shaped or formed to any great extent and will definitely not flow.

Being a thermoset material, silicone is not affected by changes in climate temperatures encountered by amputees, observes Ian Fothergill, BSc (Hons), MBAPO, clinical marketing manager at Ossur Americas, Aliso Viejo, California. Silicone is also practically inert, meaning it does not interact chemically with the body, making it a very safe material.

Silicone and urethane don't "pack out" or thin out over bony prominences and other pressure points as do some copolymer materials, notes Weber. Harder durometer materials pack out less, but currently all gel interfaces are subject to packing over time, observes Denune.

Unlike silicone and copolymers, urethane has a "flow" characteristic, explains Weber. "Urethane will flow and distribute pressure more evenly, so the patient doesn't feel high-pressure points."

Socket Design and Liner Performance

Socket design plays a part in liner life and performance. For gel liners, some manufacturers urge the use of a total surface weight bearing (TSB) socket rather than specific weight bearing sockets; i.e., patellar tendon bearing (PTB) and variations.

"It's very important to utilize a total surface weight bearing socket when using a TPE [thermoplastic elastomer] gel liner," says Denune.

MacKenzie
MacKenzie

"A true patellar bearing socket is disastrous for all gel liners although silicone can withstand the high- and low-pressure areas better than thermoplastic elastomer or urethane," says Craig MacKenzie, CP, CEO of Evolution Liners and Velocity Labs, Orlando, Florida. He recommends a hybrid design with very mild medial, lateral, and posterior pressure, and says that this design allows for better rotation control.

Denune offers suggestions for addressing pressure discomfort from bony prominences without compromising the TSB design. Having a void, buildup, or relief in the socket can cause gel liners to fail prematurely, according to Denune. He suggests placing gel pads in the socket interface at the bony prominences to add softness and relieve pressure, rather than grinding the socket to provide relief. Grinding relief in the socket can result in potential skin breakdown as the TSB design of the socket is lost, he notes.

Other strategies to promote better liner performance have included adding textile covers to gel liners to increase durability and ease of donning and doffing, along with various ways of maintaining an intimate fit and secure suction. Several gel liner manufacturers offer external fabrics or an internal matrix to diminish pistoning and increase control, regardless of the liner material used, Denune points out.

Otto Bock urethane liners. Photograph courtesy of Otto Bock HealthCare.
Otto Bock urethane liners. Photograph courtesy of Otto Bock HealthCare.

"The inclusion of anisotropic reinforcement fabrics and rigid reinforcement elements are as significant as gel material, durometer, and thickness in overall liner performance—or more so," says Fothergill.

Promoting Healthy Skin

Several manufacturers have taken advantage of the absorbent properties of silicone and other liner materials to infuse them with emollients, antioxidants, and other skin-friendly substances that diffuse into residual limb skin to promote health and help prevent skin irritation and breakdown. For instance, Ossur's Dermo line includes DermoGel® with aloe vera, Vaseline®, and menthol. Ohio Willow Wood's Alpha® liners use a thermoplastic elastomer gel infused with mineral oil and vitamin E.

Alps Corporation, St. Petersburg, Florida, includes vitamin E and another antioxidant in its line of thermoplastic elastomer gel liners. Antioxidants scavenge free radicals such as hydroxls that can inflict cell damage, thus preventing damage and promoting healing. "This approach of promoting healthy residual skin has proved its efficacy," says Kevin McLoone, Alps' vice president, marketing. A success story is pictured on Alps' website, www.easyliner.com, which illustrates a case history in which an EasyLiner played a vital role in healing a residual limb with severe skin breakdown. Jim McElhiney, CPO, Nashville Orthotic & Prosthetic Services Inc., Tennessee (owned by Hanger Prosthetics & Orthotics), was called in to evaluate a patient with diabetes and compromised vascularization. Three weeks post-fitting, the limb was healed—apparently due largely to the hydroxyl-scavenger appetite of the EasyLiner. "We have clinical trials under way at leading hospitals across the United States to better understand the outcomes we are seeing with our products," says McLoone.

McLoone
McLoone

Alps also now offers an antimicrobial liner which incorporates the antibacterial properties of silver salts.

Several O&P manufacturers offer sprays, creams, and oils to promote skin health and lubrication between the socket and a liner or sock.

Solutions for Special Needs

Liners have been developed for highly specialized needs. For instance, the Vacuum Hip Liner developed by Evolution Liners for hip disarticulation amputees combines sleeve and liner in one. The liner is worn like a pair of shorts, which utilizes the compression and tackiness of the liner to adhere to the skin, explains MacKenzie. A carbon fiber plate is inserted into the vacuum circle in the liner. Vacuum is applied between the liner and the carbon plate, locking them together. The plate is then connected to the socket with an anterior and posterior lanyard. A contralateral belt controls lateral movement at mid-stance.

What Is the Best Liner?

The following research article, published by the Department of Veterans Affairs (VA) Rehabilitation Research and Development Service, compares 15 specific liners by product name and translates some of the results into clinical application suggestions: "Testing of Elastomeric Liners Used in Limb Prosthetics: Classification of 15 Products by Mechanical Performance," by Joan E. Sanders, PhD; Brian S. Nicholson, BS; Santosh G. Zachariah, PhD; Damon V. Cassisi, BSME; Ari Karchin, MSE; and John R. Fergason, CPO, published in the March/April 2004 issue of the Journal of Rehabilitation and Research & Development (JRRD). Read it online at www.rehab.research.va.gov/jour/04/41/2/pdf/Sanders.pdf

The ingenuity of Ossur's engineers solved a problem last summer for Gunnery Sgt. Angel Barcenas, an amputee with heterotopic ossification (HO) due to blast injuries. Barcenas was using an Ossur suction system with a seal on the silicone liner, which put pressure directly on a sensitive area, according to a story by Timothy Jacobsen, Associated Press ("Wounds Lead to Better Tech," USA Today, July 15, 2007). To alleviate the pressure, Ossur, which was already developing a liner for military amputees suffering from HO, devised a liner with five to eight seals extending to about half the length of the liner, reducing the pressure from any one seal.

Innovative Materials, Designs

The ideal solution has yet to be found, but the search has led to a number of material and design innovations.

Evolution Liners/Velocity Labs developed a liner using a new silicone formulation that allows for easy donning and doffing, plus providing a good vacuum seal without requiring a fabric cover or coating, explains MacKenzie. He notes that coatings can peel or crack, and that fabric can delaminate, harbor bacteria, and have an odor.

Alps is introducing a new transfemoral liner at the upcoming Hanger Education Fair and the Annual Meeting of the American Academy of Orthotists and Prosthetists (the Academy). Also making its debut is another Alps innovation: the PSI (Proximal Seal Interface) Liner. The distal one-third portion of the gel liner is covered with a fabric that allows air to wick out of the prosthesis, leaving the proximal portion of the liner as a means for sealing against either the socket wall or a suspension sleeve.

This design also has been successfully used for transfemoral amputees utilizing a suction socket with the addition of a comfortable interface, McLoone adds.

A number of other manufacturers also have new developments under way; however, they were unable to discuss them since the patent process had not yet been completed.

With so many choices and features available in the market, how can a prosthetist decide which liner is best for a specific patient? Due to the scarcity of independent, objective research, prosthetists currently "have to make the best decisions they can based on their clinical experience and the collective experience of their peers," says Michael. "Each manufacturer believes their approach is the best, but when they groan that they can't get prosthetists to change because they're already successfully using someone else's product, that just reflects the lack of solid scientific evidence that would lead clinicians to go against their previous experience."

However, product-specific or materials-specific studies can provide insight into performance characteristics to aid in liner choices since many of them report improved clinical outcomes, notes one manufacturer, "but available research is rarely referenced."

Where should liner developments go in the future? "If there is a way to make a liner that is thinner, lighter, more durable, and more protective, that would obviously be a winner in the marketplace," says Michael. "It might be possible in the future to have liners with both high durability and a high degree of cushioning."

"Can you have both comfort and durability?" asks Weber. "I think urethane comes close to that, but I think we can do better than what is currently available in the marketplace." He notes that Otto Bock chemists in Germany and the United States are working on better materials.

Fothergill envisions a future generation of liners that can actually change properties as requirements in the socket change with the amputee's activities—softer when needed, then stiffening and controlling movement in the socket better with gait acceleration.

As in other areas of prosthetics, new developments in liner materials and design will be exciting to see as they provide more benefits for amputees.

Miki Fairley is a contributing editor for The O&P EDGE and a freelance writer based in southwest Colorado. She can be contacted via e-mail at  miki.fairley@gmail.com

Editor's note: The O&P EDGE does not endorse any company or product. Companies and products mentioned in this article are for reader information only as a representative sample of the range of liner designs and materials currently available. This article does not provide complete coverage of manufacturers providing prosthetic liners.

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