Prosthetic Foot and Ankle Design

Background Information

Prosthetic limbs, or artificial limbs, are fabricated devices that provide amputees with a replacement for their missing limb, restoring some motion and function. Lower limb prosthetic devices must restore the capacity for locomotion if they are to be effective. Lower limb prosthetics have been made since the time of the ancient Egyptians, but just in the past 20 years with the advent of the microprocessor, great strides have been made in the realism and capabilities of lower limb prosthetics. Prosthetic limbs now utilize hydraulic and pneumatic controls that, along with microprocessors, allow for fine control and natural motion. However, these devices are very expensive. A lower limb prosthetic can cost anywhere from $10,000 to $35,000 and must be replaced every three to four years due to wear. In many situations, it is not feasible to use advanced prosthetics due to their high cost and increased complexity.

The Clinic

The Centro de Miembros Artificiales (CMA), a clinic in La Paz, Bolivia that opened in 2011, is dedicated to providing low-income Bolivian amputees with low cost prosthetics to improve their quality of life. The CMA manufactures all devices in the clinic in La Paz out of locally sourced materials, and the custom fit devices are provided to amputees for little or no cost. The current prosthetic design operates on passive locomotion, a knee joint is attached for above the knee amputees. The Polycentric knee is fabricated from Delrin and designed by LIMBS International. Below the knee a metal pylon replaces the lower limb, and a rigid ankle coupling connects the pylon to the foot prosthetic.

Prosthetic Leg

Our design project will focus on improving the design of the foot and ankle. The current ankle coupler is rigid, and connects the lower limb pylon to the upper face of the foot prosthetic. The clinic uses one of three foot designs, two of which are heat formed out of a copolymer plastic; these two designs depend on the flexibility of the copolymer to alter their shape during each step. The third foot design in use is carved out of wood and uses foam to provide some articulation at the toe. All foot designs are susceptible to permanent deformation after being used to walk for some time. They suffer additional damage from rotation and off-axis motion that also occurs as the patient is walking. Catastrophic failures of the feet of several prosthetics have occurred, endangering the patients.