Sheet thermoplastics are widely used in prosthetics for prosthetic interfaces as well as structural components. These materials are available in sheet form in various thickness and colors. The most basic types of sheet thermoplastics are polypropylene and polyethylene. Polypropylene (PP) is a very rigid plastic that has found several uses in prosthetics. The supporting structure of the prosthetic interface is often fabricated of polypropylene. Low density polyethylene (PE) is a soft and flexible thermoplastic that may be used for prosthetic interfaces. A benefit of these and other thermoplastics is that they can be reshaped after initial fabrication. In order to reshape the material a heat gun is used to heat the desired area to its thermoformable temperature, and then the heated area can be recontoured. This can be a great advantage when a pressure area occurs in the socket.
In addition to PP and PE , there are a multitude of "blended" plastics available to the prosthetist. Characteristics of these materials vary from very stiff to very flexible. A good example of this is Co-Polymer, which is a blend of polypropylene and ethylene creating a material that is fairly rigid yet more flexible and crack resistant than straight polypropylene. Stiff materials are often used for support so that the forces associated with walking can be transmitted from the amputee to the floor. Flexible materials are advantageous when used as interface materials, i.e., those in contact with the residual limb. These materials provide a more comfortable and adjustable socket. Recently, blends of flexible plastic have included silicone providing a very soft feel to these flexible interfaces. When choosing the appropriate plastic for a particular application the prosthetist must walk the fine line between allowing the material to yield, which should enhance comfort, yet offering some resistance to bending in order to support the limb in a comfortable fashion. Sometimes a clear material may be used to evaluate the fit of the interface prior to final fabrication. These "test sockets" have become a valuable and indispensable tool for the prosthetist. The most common test socket material today is Clear Co-Polyester thermoplastic.
The material of choice is formed over a plaster cast of the residual limb after being heated in a high temperature oven until the thermoformable temperature is reached, usually around 300-400 degrees. The hot plastic is molded to the plaster model using vacuum pressure to ensure an exact fit over the cast.
Silicone and similar materials
Silicone has found increasing use in limb prosthetics over the years. It is now used as a padding material in sockets, as a means of suspension in the silicone suction socket (3S Iceross type) and is the material of choice when making high quality cosmetic hand restorations, to name a few. This material comes in many forms and can be made in a firm durometer or a very soft durometer. When made in a very soft durometer, silicone not only provides excellent padding but also protects the skin from friction (shear). This can be very important because friction often is the cause of skin breakdowns. The silicone suction socket uses what could be called a silicone "sock" worn directly against the skin that incorporates an attachment pin at the bottom, locking the sock, and the amputee, into the prosthesis.
This means of suspension is so successful that it is widely used for suspending lower-limb and upper-limb prostheses. 3S "socks" are now available in varying thickness allowing for additional padding in critical areas. Other materials that behave similar to silicone are urethane as used in the TEC liner and the mineral oil based gel used in the Alpha liner.
In prosthetics, metal components are generally those that are manufactured by one of the many prosthetic component suppliers. These include knees, pylons, ankles and rotators to name a few. Examples of these components can be found made of aluminum, stainless steel and titanium.
Aluminum, in general, is considered as a lightweight alternative to steel. It's not as strong but depending on the particular application it is often strong enough to meet the design criteria and pass the necessary testing procedures. Certain knees are fabricated of aluminum, taking advantage of its light weight. Some of these knees are very strong and durable owing much of their strength to the geometry of the knee as well as the material used.
Steel is certainly strong but it is relatively heavy. Because steel is strong, it can be used to create small components that may rely more on the strength of the material than the geometry of the design. Small knee units used for endoskeletal prostheses were originally made of steel. The material is fairly heavy however very little material is needed to construct these knees.
Titanium is a strong, lightweight alternative. The penalty for using titanium is higher cost. Many of the endoskeletal components originally designed of steel are now available in titanium. As with all of the trade-offs that must be considered in choosing prosthetic components, one must carefully weigh the advantages and disadvantages and choose wisely. The most expensive, strong and lightweight materials may not provide any discernible advantage over less exotic options if chosen inappropriately.
Liners and Sleeves
Liners are the materials that are either made during the fabrication process to fit inside the socket or those materials added after the leg is in use in order to accommodate for shrinkage of the residuum. Pelite is probably the most commonly used "soft socket" liner material. It is a closed-cell polyethylene foam material that is available in various durometers. This material is thermoformable, meaning that it can be heated and formed over the plaster cast. Pelite and materials like it have the advantage that they are easily adjusted by adding additional material when the residual limb shrinks. It is also convenient to use these materials if suspension of a below-knee prosthesis is to be provided by means of a supracondylar wedge. In this case the liner is built up above the wide, bony prominence of the knee in order to suspend the prosthesis.
Sleeves are worn over the outside of the prosthesis and are used to provide suspension. Some common materials used for sleeves are: neoprene, silicone, latex, and urethane. Sleeves are worn over the prosthesis and extend onto the thigh of the below-knee amputee. If the sleeve is non-porous, suspension will be achieved by a combination of suction (no air can get into the socket) and mechanical friction.
Endoskeletal prostheses are generally covered with a foam material in order to match the shape of the sound leg. Foam covers are available made of polyurethane and polyethylene foams. These foams range from the very soft, usually used to finish an above-knee prosthesis with a one-piece cover, to fairly firm used for below-knee and discontinuous above-knee cosmetic covers. These materials come in oversized "blanks" that are custom shaped in order to produce the optimal cosmetic appearance.
The traditional, and still the most common, finishing material for the cosmetic cover is nylon hose. These are similar to heavy duty women's thigh high (or knee high) stockings. More recently custom and semi-custom prosthetic skins have been introduced which offer a more natural finish to the prosthesis. Prosthetic skins are color matched to the amputees' skin tone and may have detail down to matching the appearance of hair on the leg. These prosthetic skins are generally made of a latex like material.
The advantage of a custom skin is obviously the improved, more "life-like," appearance it provides. The disadvantages are: increased cost, increased difficulty in making adjustments after the prosthesis is finished, and in the case of moving joints such as the prosthetic knee on an above knee prosthesis or a heel height adjustable ankle, the skin may wrinkle or impede optimal function of the underlying componentry.
Due to the influence that the cosmetic cover has over the prosthetic componentry, some amputees have elected to leave the prosthesis uncovered in order to allow the components to function optimally. It is common for amputee athletes to wear their limb without the cover, and in recent years this has become more popular with other amputees as well.
Whatever your personal preferences and needs regarding your prosthesis, I hope the information presented in this series has been helpful in improving your understanding of your prosthesis and how it works. Advances in materials have improved prostheses incredibly over the years, this has meant an improved quality of life for countless individuals. And I am certain that new developments are right around the corner. In order to take full advantage of the latest developments I recommend that you discuss your prosthetic options with your ABC Certified Prosthetist, and together decide on the prosthetic components and materials that will optimize your function and comfort.