Prosthetic Legs PDF

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Luke Donforth, Paper on Prosthetic Legs...

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Physics Behind Prosthetics Prosthetics are artificial extensions to facilitate functional and aesthetic rehabilitation of a missing limb. They were designed to allow amputees to carry out their everyday tasks like walking and holding objects. The presence of prosthetics has allowed people with limbs absent from birth and impacted by trauma to continue pursuing their aspirations. When engineering a prosthetic appendage, compression, flexibility, stress resistance and durability are just a few of the factors taken into account. These factors are influenced by the physical forces the prosthetic will be subjected to in day-to-day use. Ultimately, it is the patient’s needs and status that determine the goal of the prosthetic treatment. Figure 1 Phases of the Gait Cycle from ProtoKinetics When you are walking, there are multiple forces acting on your lower extremities. The force of gravity is the force being applied on your entire body while walking. The normal force is the force perpendicular to the surface you are walking on. The frictional force from the ground on to your feet is what drives you forward. To rehabilitate function, lower extremities prosthetics are engineered to act like springs. When in contact with the ground, the prosthetic compresses and stores elastic energy until midstance (Fig. 1). From mid stance, there is mechanical energy returned as the elastic energy is converted to gravitational potential energy and kinetic energy. Due to the different forces that are involved in walking, the material used in production is largely based on the spring constant, k. The spring constant measures how stiff and strong the material is. Running specific prosthetics (RSP) have been designed to mimic the spring like mechanics of walking and running. This was done by incorporating a physical spring in the prosthetic and altering the leg stiffness. With the incorporation of a physical spring, the leg compresses and from mid stance the spring recoils. RSP are categorized by predetermined stiffness categories (17), the lower the number, the lower the stiffness(Beck, Taboga, & Grabowski, 2016). Patients with a greater body mass are typically prescribed prosthetics with greater stiffness. Patients who intend to sprint are also prescribed prosthetics with greater stiffness. Figure 2. Relationship of Running Speed and Leg Stiffness The average stiffness of the leg including the residual limb and prosthetic from 3 m/s - 7m/s. In a published study investigating the relationship between the stiffness of a material and velocity it was found that as you increase the stiffness of the prosthetic leg, the step frequency would increase. The step frequency had increased 0.32 Hz resulting in a final running speed of 7 m/s ((Beck, Taboga, & Grabowski, 2017)Fig. 2). Every 1 m/s increase, the overall leg stiffness decreased 0.58 kN/m and the residual limb stiffness decreased kN/m ((Beck et al., 2017) Fig. 2). Most prosthetics are made using fiberglass and carbon fiber due to their cost effectiveness. In a study published by Au, the effects of spring stiffness and torque were explored to engineer a prosthetic that mimics normal gait. The prosthetic was designed to have a unilateral spring and force-controlled actuator. The result was a prosthetic that improved the amputee metabolic economy from 7% to 20% although the system was two times heavier compared to other

devices(Au, Herr, Weber, & Martinez-Villalpando, 2007). Figure 3. The Vertical Force of Walking Study conducted by the Department of Veteran Affairs that addresses the ground reaction force of a 89-kg male walking with a Flex-Foot prosthesis. In a study published by the Department of Veterans Affairs, the absence of stump pain and fatigue while walking were the most important factors taken into consideration for veterans with amputated limbs(Klute, Kallfelz, & Czerniecki, 2001). Stump pain and fatigue can be greatly reduced by taking into consideration pressure and friction. While walking is an unconscious action, our legs are subjected to high frequency repetitive loading. The study conducted by Veterans Affairs analyzed the ground reaction forces of an 89 kg man walking with a Flex Foot and observed a spike of 1000N at 0.2 seconds when walking ((Klute et al., 2001)Fig. 3). Over time, this repetitive loading can lead to tissue damage, osteoarthritis, prosthetic joint loosening and a myriad of other health complications. Vertical shock absorbing pylons have been engineered to relieve some of the forces endured by the residual limb while walking to reduce fatigue and increase comfort. Pressures can be adjusted by increasing the area the force is applied over in the prosthetic socket. (P=F/A). Additionally, the repetitive rubbing of the residual limb and socket can result in pain and irritation. As a result, most prosthetic limbs are attached through seals in suction. Suction is made possible when the pressure surrounding the stump and socket is less than the pressure surrounding the limb. Friction can be reduced by designing a prosthetic using a material with a lower coefficient of friction (Ff=u*Fn). To reduce gravity, the prosthetic foot is also designed to be shorter so there is a higher center of gravity. This improves the prosthetics ability to control acceleration and reduce friction.

References Au, S. K., Herr, H., Weber, J., & Martinez-Villalpando, E. C. (2007). Powered ankle-foot prosthesis for the improvement of amputee ambulation. Conf Proc IEEE Eng Med Biol Soc, 2007, 3020-3026. doi:10.1109/iembs.2007.4352965 Beck, O. N., Taboga, P., & Grabowski, A. M. (2016). Characterizing the Mechanical Properties of Running-Specific Prostheses. PLoS One, 11(12), e0168298. doi:10.1371/journal.pone.0168298 Beck, O. N., Taboga, P., & Grabowski, A. M. (2017). How do prosthetic stiffness, height and running speed affect the biomechanics of athletes with bilateral transtibial amputations? Journal of The Royal Society Interface, 14(131), 20170230. doi:doi:10.1098/rsif.2017.0230 Klute, G. K., Kallfelz, C. F., & Czerniecki, J. M. (2001). Mechanical properties of prosthetic limbs: adapting to the patient. J Rehabil Res Dev, 38(3), 299-307....