For both cosmetics and functionality, the replacement of a missing body part with a prosthesis (Greek for attachment) has always been a necessity. The current technology went beyond the mere mechanical replacement of the limb, elevating it to the biomechanical, thanks to the introduction of myoelectric sensors for activating the prosthesis through the muscle activity. Here, we will focus on the materials constituting the prosthesis, in particular the ones designed for sports, rather than the electronics.
Back in time, amputation was the only treatment for any serious wound in a limb. However, the replacement of the missing limb was rare and reserved for noblemen. The first documented prosthesis was discovered in Egypt. There, a mummy dated around 950 B.C. of a noblewoman was found with a prosthetic toe of wood and leather and a carved toenail as a replica of the missing part (Figure 1) .
During the Middle Age, prosthesis started to become more functional (Figure 2). Men who lost their arms during battles had their limb replaced with a device of iron with elements that allowed them to place a shield during combats. Meanwhile, on ships, sailors had their forearms replaced with the famous hook and their legs with wooden sticks, both materials of easy availability on board.
The first functional prosthesis was ideated by the French surgeon Ambroise Paré in the 16th century. Prosthesis included a bending knee, able to be locked while standing, and hands that permitted the French captains on horses to grip and release their reins . In the 17th century, a Dutch surgeon, Pieter Verduyn, included articulations in his prosthesis as well as better attachment to the leg. Later, the prosthesis included springs to simulate muscles and tendons. In the 1990s, microprocessors were introduced to control the knee movement of the prosthesis, and sensors recorded the electromyographical stimulus that moved the prosthesis (Figure 3) .
Three main components
The prosthesis can replace four different body parts named as a consequence of their locations: trans-humeral, trans-radial, trans-tibial and trans-femoral. Independently from their application and placement, the prostheses need to be light to facilitate their use (it would not be helpful to have an artificial limb weighing as the original one, i.e. 10% and 30-40% of the bodyweight for the two arms and the two legs, respectively).
The prosthesis is composed of three main components (suspension, pylon and socket) that generally remain the same among types of prosthesis (esthetic or functional) and location .
The socket is the part of the prosthesis that attaches to the residual limb. For guaranteeing the comfort of the user and the total efficiency of the prosthesis, it is essential that the socket does not irritate the skin of the residual limb and can transfer the impact or force. The socket is usually made from polypropylene, replacing the previously used wool.
The suspension is the junction between the pylon and the socket. It is vital for keeping the pylon attached to the residual limb, and usually, a suction method is used to create a vacuum and keep the two parts attached.
The pylon is the core of the prosthesis. Typically, it is created from titanium or carbon fibers (more resilient, light and stronger than steel), replacing the wood that was used back in time. The pylon is often covered with soft material that matches the color of the natural skin.
Prosthesis in sports
After World War II, the involvement of amputees in sports activities became an opportunity for them to go back to normality, increasing their well-being and social inclusion feeling. Therefore, after the replacement of the missing limb, a further step went in the direction of the optimization of the prosthesis for the use in sports.
This development is mainly famous for running. In the 1980s, one of the first prostheses created for more strenuous physical activity was the Seattle Foot (Figure 4). An internal flexible Delrin (a crystalline plastic with characteristics between metals and plastics) keel surrounded by a polyurethane shell acted as a spring, returning part of the energy .
Utilizing Flex-Foot (Figure 5) and Re-Flex VSP, lower limb amputees managed to reach more energy-efficient running. The introduction of carbon fibers permitted, in fact, to run more on the toes, a characteristic of the normal runners . In particular, the Flex-Foot showed the highest energy return ratio in comparison with the other prosthesis made with polyurethane or polyacetal .
Lately, the name of the South African Oscar Pistorius hit the headlines being the first double-amputee athlete to compete at the Olympic Games and starting the debate about technological doping (Figure 6). The sprint runner utilized the Cheetahs, invented by the medical engineer Van Phillips. Their shape is designed for going forward, and thus, a heel is not included.
According to Josh McHugh , “The Cheetahs seem to bounce of their own accord. It’s impossible to stand still on them, and difficult to move slowly. Once they get going, Cheetahs are extremely hard to control.” The reason lies in the fact that the Cheetahs are made in carbon-fiber-reinforced polymer (as polyester, epoxy or nylon that is binding the carbon fiber). Depending on the direction and density of the fibers, different stiffness levels can be given.
The carbon fiber works as a spring, storing and releasing the kinetic energy of the athlete during each step. In particular, the ratio of the mechanical work at the ankle joint between the negative and positive phase is 0.907 for the Cheetahs as compared to 0.401 for healthy athletes .
The mechanical work in the knee was 11 and eight times higher in the negative and positive phase, respectively, in the Cheetahs than in the able athletes . Due to the enhanced elastic properties of the artificial limb, the Paralympian long jumpers use the prosthetic leg to take off during the jump.
While running, trans-femoral amputee athletes have more disadvantages than the trans-tibial ones. The main reason lies in the fact that the knee joint is difficult to mechanically reproduce due to its high complexity. The trans-femoral athlete’s running is characterized by an asymmetry up to 36% in the swing phase between the able and unable side . Therefore, different solutions have been proposed to remedy the problem of inertia influencing the acceleration of the prosthesis during the recovery phase.
Not only running
The evolution of prosthesis for sports is not restricted to the world of running. Normally, lower limbs prostheses are used in sports that need an upright position like skiing, while upper limb prostheses are used for sports such as rowing or cycling. In the latter, the propulsion and stability given by the arms are essential. For many sports, an adaptation of the prosthetics is not necessary for playing it, but in the majority of cases, it is .
In cycling, the prosthesis of the upper limb needs to be permitted to break and change the gears. A standard opening/closing mechanism should be sufficient. However, for competitive cycling, the prosthesis should guarantee the athlete’s ability to change his/her position on the handle, too . For mountain biking, a shock absorber can reduce the vibrations of the ride that are transmitted to the handle (Figure 7).
For the lower limb, the energy-storing feet that are helpful for walking and running constitute a disadvantage in cycling, not permitting proper propulsion due to their elasticity . Generally, a normal lower body prosthesis is sufficient for guaranteeing the thrust. Still, some adaptations need to be considered, such as a wider pedal and bends to fix the prosthesis to the pedal.
Upper and lower limb unilateral amputees can usually swim without problems, as long as they are waterproof prostheses. However, in order to improve their efficiency, a flipper is frequently directly attached to the socket of the sound limb (Figure 8) and at the same length of the sound limb .
Furthermore, the Bartlett Tendon Universal Knee and the XT9 are prosthetics utilized in extreme sports, from skiing to snowboarding and motorbiking. Both prostheses have been invented by sportsmen who lost their limbs in accidents.
Nike, Adidas, and other companies alike have been developing their prosthesis for sports. Adidas created the Symbiosis prosthetic line utilizing material as carbon fiber, sorbothane (a polyurethane) and aluminum . Nike moved instead in the creation of prosthetics that could interface with the carbon fiber blade of Ossur, providing benefits such as stability, energy return and recovery (Figure 9).
In order to reduce the price of the sport and normal use prosthesis, 3D printing has been employed in their production. Just as normal prosthetics, 3D printed prosthetics are made of plastics like polypropylene, polyethylene, acrylics and polyurethane, with an inner pylon of titanium, aluminum or carbon fiber.
The future of sports prosthetics and normal use seems to lie in osteointegration, i.e. the attachment of the prosthesis directly in the bone of the amputee using titanium. However, osteointegration presents pros and cons. The absence of the socket will permit to reduce the discomfort and pressure on the skin. On the other side, the risk of infection is high and the patient should take care of the abutment skin area daily, with the possibility of not being able to do activities as jumping or running.
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