Magnetic Lower-limb Prosthetic Design

Currently, there are an estimated 2.1 million amputee patients in the US, and this number is
expected to double by the year 2050 from increasing rates of cancer and diabetes. Within this
group, 80-90% of the amputee patients undergo transtibial (lower-leg) amputation surgeries with
185,000 lower-leg amputation surgeries performed annually within the US. One of the key areas
that researchers and clinicians have identified as a problem in current prosthetics lies in the
socket, where it is incorrectly fit to the amputee stump. On one hand, a loose fit results in vertical
and horizontal movements of the stump in relation to the socket through phenomenon such as
pistoning and bell clapping, and this results in repetitive skin irritation due to friction forces. On
the other hand, a tight fit results in blood constriction and a lack of breathability for the stump.
57% of prosthesis users report pain and general discomfort due these factors and the repetitive
skin irritation of the prosthetic. This paper outlines a solution to this problem by describing a
system consisting of a metal implant inside an amputee patient’s limb, and a magnetically
engineered socket to attract the implant and create a secure and comfortable fit for the user. A
series of four neodymium magnets are secured to the sides of the socket to create moment forces
that prevent lateral displacements of the stump in the socket which is called bell clapping while a
circular axial magnet is placed on the bottom of the socket to prevent vertical displacement
which is called pistoning. The design works effectively to address three main technical
requirements related to pistoning, bell clapping, and pressure applied on the user to prevent pinch
points. The socket was tested to withstand 18.2 lbs of force before separating from the implant
(passing the minimum requirement of 1.6 lbs) which demonstrates the strength that the device
has before pistoning occurs. The socket also was tested for bell clapping by effectively being
able to prevent displacements between gyroscopic sensors on the socket and the stump up until
an angular velocity of 300 °/s. Finally, the socket places a max pressure of 4.4 psi on the user,
well within the 40 psi maximum requirement. Overall, the prototype is able to function properly
as a prosthetic with the potential to optimize comfort for the user.