Recent advances in the field of prosthesis have led to the widespread use of advanced plastics and carbon fiber composites. These materials lead to the production of a lighter, stronger and a more suitable prosthetic limb compared to the rudimentary artificial attachments in the less advanced times.
The incorporation electronics in prosthesis provides more flexibility and functionality of the limb. It is much more realistic and has the feature of adapting their function during under certain conditions, such as gripping or walking.
Although newer materials and technology have revolutionized prosthesis, the basic components remain the same.
Components of a prosthesis
Pylon
It is the framework upon which the prosthetic limb is built. It is used to provide structural support, like the skeleton in human body, and conventionally metal rods were used as pylons. In the modern age, lighter carbon-fiber is used to form pylons. Pylons are sometimes covered by foam like material which is colored to match the skin of the recipient giving the prosthetic a more life-like appearance.Socket
This is the interface between the stump (remaining part of the human limb) and the prosthetic. The socket must be fitted accurately as it forms the part of region from where mechanical forces are transferred to the patient’s body. This careful fitting ensures that no damage is done to the skin or the tissues. One or more layers of socks are worn to facilitate better fitting.The suspension system
This is responsible for keeping the prosthetic attached to the body. There can be several kinds of suspension systems. There’s the harness system, where straps or belts or sleeves are used to attach the prosthetic. In some cases prosthetic remains attached to the body by just fitting around the shape of the limb. A common mechanism is suction whereby attachment is maintained by snugly fitting the prosthetic to the residual limb and using an airtight seal to keep it together.Although most prosthesis retain these components in some form, each device is unique. Amputation above or below a joint makes a huge difference in the type of prosthetic required, for example, amputation above knee will require a device having artificial knee while amputation below knee means the patient could use his or her own knee.
Design and fabrication
This process consists of various steps starting with the precise measurements which include detailed measurements of the patient’s body such as height, weight etc.Following this, amputation surgery and time for the wound to heal is given, after which a plaster mold of the residual limb is taken. A duplicate is constructed and is used to test the fit of prosthetic limb.
The interface between the residual limb and the prosthetic socket needs to be meticulously monitored as the residual limb will shrink due to muscle atrophy i.e. shrinkage due to lack of use. Children amputees have to visit the doctor regularly because their prosthesis need to be resized and reshaped to cope with their natural growth.
Thermosets vs. thermoplastics as the polymer matrix
A prosthetic nowadays is made using plastic polymer laminates. Thermosets such as acrylic, polyester or usually epoxy is used. The advantage these thermosets offer is that they allow greater degree of control over controlling variables such as strength, stiffness and thickness of the final product. This control allows the prosthetic to be strong and stiff in certain areas and thin and light in others.For instance, during above knee prosthesis, it is essential that the weight bearing area must be stiff so as to not bend under the weight of the body. To achieve this, extra reinforcement is applied in this area. In the rest of the socket, these requirements can be sacrificed so the laminates are thin and lighter prosthesis can be made.Similarly, rigidity or flexibility can be achieved by adjusting the resin and fiber content.
However, once fabrication is complete the laminate cannot be remolded and it is difficult to make adjustments in case of thermosetting resins compared to thermoplastics on the account of the cross-linking in them.
Why use Epoxy?
Acrylics and epoxies are popular thermosetting resin systems employed in production of laminates for prosthesis purposes providing different advantages. Epoxy results in a strong, stiff structure with very good adhesion to carbon fibers. It has a longer setting time so it is preferred in situations where bigger devices are to be produced.Acrylics produce a colorless transparent structure which is skin friendly and has antibacterial properties. It has a shorter setting time but its adherence to carbon fiber is not as good as epoxy resin.
Epoxy, polyesters or vinyl esters?
Epoxies are the most expensive of these three resins but they justify their cost. They are almost three times stronger than the next strongest resin. They adhere to the fibers and older epoxies much better than the other two and form a virtually leak proof barrier.Vinyl esters are not very strong (about one-third the strength of epoxy) and are usually only used for aesthetic purposes. Due to their decreased strength and adherence, they are not effective materials for prosthetics.
Polyesters are the cheapest and have poor bonding capabilities especially with carbon fibers so they cannot be used in load bearing circumstances.
Why use carbon fibers as reinforcements?
Reinforcement fabrics for prosthesis include fiberglass, carbon and Kevlar. Each comes with its own pros and cons. Fiberglass for instance is the most economical and easily saturates with resin and can be found in different forms. Fiberglass is durable and flexible because glass fibers are twice as strong under compression as in tension. But, fiberglass is heavier than the other two and is not as strong or stiff as the carbon fiber.Kevlar is the lightest material and provides excellent fracture toughness under impact loading. It can also absorb high levels of torque and stress. Unfortunately, Kevlar is not very good at maintaining form or structure under load and it is very difficult to saturate it with resin.
Carbon fiber is the most suitable alternative. It is almost as light as Kevlar and is able to hold its shape under load due to excellent stiffness and strength in both compression and tension. In addition, carbon fiber has excellent fatigue resistance and is bio-compatible. Carbon fibers are very versatile and innovative designs can enhance the effect of their mechanical properties. Laminates can be made to be either stiff or strong. Fiber orientation can improve properties in more than one direction.
Structural compromises can overcome the undesirable properties of carbon fiber composites such as brittleness. All three fibers can be used in unison to obtain a hybrid which can exhibit the properties of all three fibers.
Manufacturing
As mentioned earlier, the process of prosthetic production starts with taking careful measurements of the residual limb and other key factors such as the weight of the patient, length of the other whole limb etc. Once the wound is healed, a plaster mold of the residual is created. Using this mold a duplicate residual limb is produced. This duplicate is used for testing with the prosthetic limb.The prosthetic limbs are created by using prepreg carbon fiber. This is basically carbon fiber that has been pre-impregnated with epoxy resin and the epoxy is allowed to be partially cured. These prepregs come in sheets or rolls with optimum balance between the fiber and the resin.
A plaster model of the prosthetic is dried in an over to remove moisture from the mold. Prepregs are meanwhile stored in a freezer to prevent further curing. Patterns are traced on a plastic sheet such as PVC foils and then these patterns are cut out of a single sheet of prepreg. Prepregs come in different orientations (bi-directional, unidirectional) and depending on the area of the prosthetic and the amount of load that particular area is going to bear, several layers of prepregs are applied with varying orientations to accommodate torsion and flexion.
This assembly is sealed in a vacuum bag and it goes into an oven for 4-5 hours of curing. High vacuum environment is maintained to remove excess resin. Finally, buffing is done to remove sharp edges.
good initiative (y)
ReplyDeletemuch appreciated
Delete