It's important to remember that no one material or type of component is the best for all individuals. Each individual needs to be evaluated with careful consideration given to their lifestyle, expectations and physical characteristics. The prosthetist must be knowledgeable about working with such traditional materials as wood, steel and leather as well as the space age materials like titanium, carbon fiber, and plastics.
Plastic Polymer Laminates (Thermosets)
Plastic polymer laminates are widely used for the fabrication of prosthetic sockets. The plastic starts as a liquid, which is mixed with a catalyst and used to saturate reinforcement textiles that have been applied over a model of the residual limb. The plastic polymer bonds the fabric layers together creating a lamination. This process is performed under vacuum pressure in order to create a product that is lightweight and strong.
Common types of plastic polymer laminates used in prosthetics are acrylic, epoxy and polyester. The advantage of plastic laminates is that the prosthetist has a great deal of control over the strength, stiffness and thickness of the finished product. These variables can be controlled such that the finished product may be strong and thick in certain areas and thin and relatively light in others. For example, on the above knee prosthesis, it is important that the weight-bearing area be stiff enough to prevent bending under the load of body weight; therefore, extra reinforcement material may be applied in this area. Over the majority of the socket, however, the strength requirements are not as great and the laminate can be thin with little reinforcement providing a lighter weight prosthesis. Also, by adjusting the resin and fabric, the construction can be made fairly rigid or flexible.
Reinforcement textiles
Reinforcement textiles are the fabrics used in a laminate to provide strength. These include fiberglass, nylon, Dacron, carbon, and Kevlar.
These materials all have their advantages and disadvantages. For example, carbon fiber is used to create thin, lightweight and strong prosthetic sockets. The disadvantage of carbon fiber is that it is very difficult to reshape when an adjustment is required and because these laminations are often very thin it may also be impossible to grind a relief area into the material. Carbon fiber is also fairly brittle, meaning that it will break if bent too much, whereas Kevlar and fiberglass will be more likely to bend but not break. This is one reason why a prosthetist might use several reinforcement materials in combination, a composite, to design a prosthetic socket. Carbon fiber is also used in several of the dynamic response (energy storing) feet and to create strong lightweight pylons.
Lamination Process
As mentioned in the previous sections, the lamination process involves the saturation of reinforcement textiles with plastic polymer resin. This process is generally performed under vacuum pressure. To illustrate the process, I will use the example of a below elbow myoelectric prosthesis. The socket, the portion that is custom designed and fabricated to fit over the residual limb, may be constructed of plastic laminate. In addition to serving as the receptacle for the residual limb, the socket must have provision for the electrodes and a means of attaching the forearm section; therefore, as part of the lamination process, electrode dummies and screw anchors will be incorporated into the socket.
First the prosthetist prepares the plaster cast, the model of the residual limb, in a manner that will provide a well-fitted and comfortable socket. A thin film of Polyvinyl Alcohol (PVA) is then applied over the plaster to act as a separator for the plastic resin.
Vacuum is applied under this film so that it is drawn against the cast. Next the various reinforcement textiles are applied (the lay-up). The hardware needed to connect the socket to the prosthetic components may be incorporated into the lay-up.
The prosthetist will use his knowledge of materials science to determine the type and quantity of materials used. Key information in making this decision is weight and activity of the user. Over the lay-up another PVA bag is applied. This PVA bag is open at the top in order to accept the liquid plastic as it is introduced into the fabrics.
Plastic Polymer Laminates (Thermosets)
Plastic polymer laminates are widely used for the fabrication of prosthetic sockets. The plastic starts as a liquid, which is mixed with a catalyst and used to saturate reinforcement textiles that have been applied over a model of the residual limb. The plastic polymer bonds the fabric layers together creating a lamination. This process is performed under vacuum pressure in order to create a product that is lightweight and strong.
Common types of plastic polymer laminates used in prosthetics are acrylic, epoxy and polyester. The advantage of plastic laminates is that the prosthetist has a great deal of control over the strength, stiffness and thickness of the finished product. These variables can be controlled such that the finished product may be strong and thick in certain areas and thin and relatively light in others. For example, on the above knee prosthesis, it is important that the weight-bearing area be stiff enough to prevent bending under the load of body weight; therefore, extra reinforcement material may be applied in this area. Over the majority of the socket, however, the strength requirements are not as great and the laminate can be thin with little reinforcement providing a lighter weight prosthesis. Also, by adjusting the resin and fabric, the construction can be made fairly rigid or flexible.
Reinforcement textiles
Reinforcement textiles are the fabrics used in a laminate to provide strength. These include fiberglass, nylon, Dacron, carbon, and Kevlar.
These materials all have their advantages and disadvantages. For example, carbon fiber is used to create thin, lightweight and strong prosthetic sockets. The disadvantage of carbon fiber is that it is very difficult to reshape when an adjustment is required and because these laminations are often very thin it may also be impossible to grind a relief area into the material. Carbon fiber is also fairly brittle, meaning that it will break if bent too much, whereas Kevlar and fiberglass will be more likely to bend but not break. This is one reason why a prosthetist might use several reinforcement materials in combination, a composite, to design a prosthetic socket. Carbon fiber is also used in several of the dynamic response (energy storing) feet and to create strong lightweight pylons.
Lamination Process
As mentioned in the previous sections, the lamination process involves the saturation of reinforcement textiles with plastic polymer resin. This process is generally performed under vacuum pressure. To illustrate the process, I will use the example of a below elbow myoelectric prosthesis. The socket, the portion that is custom designed and fabricated to fit over the residual limb, may be constructed of plastic laminate. In addition to serving as the receptacle for the residual limb, the socket must have provision for the electrodes and a means of attaching the forearm section; therefore, as part of the lamination process, electrode dummies and screw anchors will be incorporated into the socket.
First the prosthetist prepares the plaster cast, the model of the residual limb, in a manner that will provide a well-fitted and comfortable socket. A thin film of Polyvinyl Alcohol (PVA) is then applied over the plaster to act as a separator for the plastic resin.
Vacuum is applied under this film so that it is drawn against the cast. Next the various reinforcement textiles are applied (the lay-up). The hardware needed to connect the socket to the prosthetic components may be incorporated into the lay-up.
The prosthetist will use his knowledge of materials science to determine the type and quantity of materials used. Key information in making this decision is weight and activity of the user. Over the lay-up another PVA bag is applied. This PVA bag is open at the top in order to accept the liquid plastic as it is introduced into the fabrics.