Materials Science: Producing Lighter, More Cosmetically Appealing O&P Devices

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By Sherry Metzger, MS

The history of orthotics and prosthetics has been marked by spurts of technology and advances in materials. Prosthetists, sensitive to their patients' needs, are driving technological improvements in materials manufacturing to better suit those needs. Reacting to the biggest complaint of orthotic and prosthetic users that their prostheses are heavy and unattractive, efforts to create lightweight and lifelike prostheses from new materials abound. A comfortable fit, which reduces rubbing and the formation of sores on the residual limb, also is important, in addition to providing improved mobility and function.

Progress in Lightweight Materials

One main goal in orthotic and prosthetic technology is to make devices lighter. Although metal is used for rotating components such as knees and ankles, lighter-weight aluminum is an often-used alternative to heavier steel. Steel now is more conservatively used in such components as small knee units for endoskeletal prostheses. Titanium is an attractive, though pricey, option for adapters, connectors, and rotating bases. The non-magnetic, nontoxic metal weighs 40 percent less than carbon steel and is totally immune to corrosion caused by human body fluids, making it ideal for prosthetic devices.

The advent of thermoplastic decades ago led to a breakthrough in fabrication, replacing heavy metals for structural components and increasing patient comfort. Thermoplastics are available in sheets of varying thickness and color. Polypropylene (PP) is a rigid form used in supporting structures and polyethylene (PE) is more flexible. Because PE is a softer thermoplastic, it often is used in the prosthetic interface with the residual limb for a more comfortable and adjustable socket. The two forms often are combined in various ratios to create thermoplastics called copolymers with greater flexibility and ductile strength, resulting in less cracking. The greatest advantage to thermoplastic is that it can be heated and reshaped after initial fabrication to accommodate changes in the residual limb or pressure areas that can occur in the socket.

Friddle
Friddle

"I have been witness to many great advances in O&P fabrication over the years," says Frank Friddle Jr., CO, FAAOP, owner of Friddle's Orthopedic, Honea Path, South Carolina. "The development and usage of thermoplastics was the first major development that I can remember. Being so involved with spinal orthotics and seeing the migration from leather and metal pelvic interfaces to thermoplastic was astonishing."

Plastic polymer laminates are used to bond layers of reinforcement textiles, such as fiberglass, nylon, and carbon. Textiles vary in strength, malleability, and brittleness. They are molded onto models of the residual limb, then laminates are poured over the fabrics under vacuum pressure. In wet laminations, resins and hardeners are mixed together, poured on the textile, and then left to set. Thermosets harden by applying high heat. Often used in sockets and made from acrylic, epoxy, and polyester, laminates can vary in thickness as needed. However, a disadvantage as compared to thermoplastics is that thermoset resins are difficult to reshape after initial fabrication. Varying the type, quantity, and combination of textiles and resins creates a range of "composites" particularly suited for a patient's weight and activity level.

"New laminates and resins are quickly replacing the fiberglass and polyester resins that were so common just a few years ago," Friddle says. "Weight reduction and strength seem to be the goals for prosthetic fabrication today. The development of carbon fibers and other exotic textiles is important. Ultralight prostheses using strong composite materials in conjunction with titanium components and carbon energy-storing feet represent a quantum leap forward in the profession of prosthetics."

Improved Socket Materials Advance Patient Comfort

The most important contribution to patient comfort has been advances in socket material technology. Silicone and urethane are materials used to create padding in sockets to protect skin from friction. Gel liners conform to the bony prominence of the residual limb to provide comfort and protection. The Double Socket Gel Liner™, for example, consists of two layers of prosthetic socks with Silipos® proprietary polymer gel in between. Liners can be made during fabrication or after the prosthesis has been in use to accommodate for residual limb shrinkage. One of the most commonly used materials for a socket is Pelite®, "an expanded, cross-linked, closed-cell, white sponge foam material [that] is lightweight, soft to the touch, moisture proof, and provides comfortable cushioning," according to JMS Plastics Supply Inc., Neptune, New Jersey( www.jmsplastics.com ). Pelite comes in a variety of density and durometers and can be adjusted easily after initial fabrication.

Attractive Devices are in High Demand

To meet the demand for more lifelike and well-blended devices, a variety of cosmetic finishes have been developed. Some are made from polyurethane and PE foams that cover the endoskeletal prostheses and are shaped to match the sound-side limb. Nylon stockinettes are most commonly used to enhance the appearance of a prosthesis. Prosthetic skins, often made from latex, allow skin color matches and can even include painted fingernails, freckles, and the appearance of hair. Silicone and polyvinyl chloride (PVC) polymer materials are used to create highly life-like "gloves." These materials are stain-resistant, washable, and impermeable to water. RSLSteeper, UK, is one manufacturer of these "custom-fabricated high-definition silicone cosmeses...sculpted to meet individual requirements and [have a] realistic appearance and feel. Skin pigmentation, freckles, veins, and hair all can be incorporated to ensure an authentic finish. Even tattoos can be copied," according to the company website: www.RSLSteeper.com . "Unlike many similar products, our fabrication techniques involve coloring the silicone material itself prior to sculpting, as opposed to surface painting. This process allows the layering of different shades of silicone to give a translucent finish to the cosmesis, similar to real human tissue."

Hill
Hill

Some silicone and PVC gloves are available in an extra-long style to produce a smooth transition over the proximal socket brim. The most noteworthy feature of the latest silicone is its ability to resist stains, a common problem with cosmetic gloves. Additionally, the increased elasticity allows the hands to operate more efficiently, consuming less power.

An attractive orthotic device often aids in patient compliance; therefore pigmented or patterned orthoses are increasingly popular. "Making a patient more likely to wear the device in turn makes the device more effective," says Steve Hill, CO, technical consultant in Orthotics at Delphi Ortho, Asheville, North Carolina. "However, pigmenting copolymers often weakens the properties of the plastic. [In addition], because they come in sheets, our inventory would have to contain several sheets of each color and each of three thicknesses." Add the endless possible color and pattern combinations, and a stocked inventory can be difficult to maintain.

Pre-Impregnated Carbon Fiber (Pre-preg and ReadyPreg)

Some of the newest materials currently used in the O&P field are pre-impregnated (pre-preg) carbon fibers and ReadyPreg (patent pending) from Kinetic Research Inc., Tampa, Florida. Pre-pregs are carbon textile fabrics pre-impregnated with curable resins by the manufacturer. The amount and type of resin are precisely controlled by the manufacturer to create consistent ratios of fiber to resin for maximum strength and quality. With pre-pregs and ReadyPreg, an extensive inventory of colors and patterns is not needed. "You can add a layer of fabric or ReadyPreg to change the color or add a pattern," says Hill. "Pre-pregs and ReadyPreg makes fabrication of laminated orthoses much easier. With any new technology it takes practice to perfect your technique, but the result is a high-quality, good-looking final product."

LISA Leg Combines Desired
Prosthetic Features

A new leg design has emerged in response to a demand for more attractive prosthetics. The LISA leg (an acronym for Lightweight, Inconspicuous, Shapely, and Active) is designed for transfemoral amputees. Developed by the FDR Center for Prosthetics and Orthotics Inc., Nashua, New Hampshire, and introduced in late 2005, the leg allows heel height adjustment to allow the user to wear a variety of heeled shoes.

"If you push a button on the side, it releases the lock in the ankle so you can manipulate the angle of the foot for an appropriate shoe," Stuart Furusho of FDR Center for Prosthetics said in an interview with WCVB-TV Channel 5 in Boston, Massachusetts.

The leg even allows women to wear up to a two-inch heel! Men likewise are finding that the leg allows them to fit into shoes with heels, such as cowboy boots, platform shoes, or work boots.

The FDR socket, featuring flexible, more comfortable materials, increases the range of motion at the hip. This allows patients greater mobility to enjoy activities such as running and dancing or simply to bend forward to tie shoes or cross their legs while seated. The "inconspicuous," aesthetic shape of the leg gives patients greater confidence about their appearance, and the "lightweight" design makes wearing the prosthesis more comfortable and functional.

The LISA leg includes the FDR socket, knee, and adjustable, energy-storing foot. The leg is shaped with foam and cosmetically finished. Medicare and major insurance plans cover the cost of this leg, according to the company.

For more information visit www.fdrcenter.com

-Sherry Metzger

Pre-pregs are easier to work with than current "wet-lamination" techniques in which the resin must be mixed and poured under vacuum-assisted conditions. "With pre-pregs you don't have to move the resin from one end of a long leg to another which often results in resin-rich areas at one end or the other. Therefore, you get consistent quality every time," explains Hill. "You get the strongest possible outcome, but the technique can be more time-consuming and labor-intensive, and the material is more expensive than thermoplastics. We've waited for this technology for a long time. A KAFO is very difficult to do in one piece using wet-lam techniques, but easier with pre-pregs. It is very strong, but not very adjustable, so it's not for every patient. You gain strength and lose weight, but it costs time, money, and adjustability."

According to one manufacturer, "Composite materials are extremely strong and lightweight. They offer, in many applications, simpler, less expensive alternatives to steel, high-grade aluminum, titanium, and magnesium, especially for industrial applications that require less weight without compromising strength. They exhibit greater resistance to corrosion-especially water and environmental damage-but they retain their mechanical properties longer...[have] great flexibility, impact resistance, vibration damping, are extremely resilient, and maintain superior performance in a wide temperature range." ( www.CarboComp.com )

"ReadyPreg is more like a wet-lam," explains Hill, "The carbon fiber and resin are still separate. But the resin comes in thin layers, like tape that you can cut, lay down on the carbon fiber, and bake." ReadyPreg allows dry layup fabrication in which the impregnation of fibers takes place on the mold during the oven cure cycle.

Wickman
Wickman

"New materials complicate my job," jokes Tony Wickman, RTPO, owner and CEO of Freedom Fabrication Inc., Havana, Florida. "The new materials are wonderful, but they're more complex. The end product ultimately increases the patient's quality of life." Though materials have improved over the years, Wickman says that tool design and technology have not kept up. "Even if we get advanced materials, you still have to manipulate them the way we've been doing it for the last 100 years." Therefore, his company focuses on tool design, such as the newly released Pneumatic Body File, a powered tool to modify molds and rapidly file down plaster. "Though pre-preg carbon composites have been around for several decades, they've just recently become stable enough for us to use in our processes," Wickman says. "The problem with thermoplastics is that they are all one density. The human body is multi-density. Integrated structures are needed; the hard material keeps everything in place, the soft material keeps it comfortable. Composites allow us to create significantly stronger devices that are lighter."

"The direct results are savings in energy expenditure for these patients as well as improved fit." Friddle says. "In terms of fabrication, these new materials and techniques have greatly reduced the fabrication time. However, safety issues have been questioned with many of these new products. The inhalation of vapors from resins, dust from carbon fibers, as well as flammability of titanium dust from grinding-all have to be addressed."

Technicians Look to Future Improvements

Both Hill and Wickman are proud of the accomplishment of founding the Orthotic Prosthetic Technological Association (OPTA) three years ago. "Technicians are a valuable part of this field, and they didn't have a voice before [OPTA]," says Hill. OPTA is a tool for education and communication for technicians in the O&P industry. "The biggest thing we have to overcome in the future is to fill the void of not having enough technicians being trained well enough or fast enough to serve our customer base," adds Wickman.

"With reimbursement levels the way they are today, many facilities need to reduce overhead while seeing as many patients as possible in order to maintain the bottom line of net profit. Central fabrication services are one of the easiest ways to accomplish this act," Friddle agrees. "However, central fabrication companies need to have proper training as well as manufacturing practices in order to keep quality at a level that is needed in their area of expertise. One of the most frustrating aspects of our profession is the fact that technology has advanced the fit, function, and durability of the devices that patients demand. However, all of this advancement comes at a cost, and payers are not willing to increase reimbursement in order for patients to receive these new high-tech products."

Innovations and technological advances in materials such as the pre-pregs and the LISA leg (see sidebar on page 21) offer patients greater comfort, confidence, and mobility. New lightweight and attractive materials greatly increase compliance in orthotic patients.

"It seems many people don't want to use orthoses like they do with prostheses," says Hill. "The perceived benefit has to be a significant increase in function or reduction of pain as an end result in order to get them to comply. And in orthotics, compliance is king."

Wickman adds, "What we do is the final link in helping our patients return to normalcy. That is our ultimate goal and that really is the prize. It's the most important thing that I'll ever do because these things have the opportunity to change people's lives."

Sherry Metzger, MS, is a freelance writer with degrees in anatomy and neurobiology. She is based in Westminster, Colorado, and may be reached at