A research team designed a compact AFO joint based on a worm gear mechanism with steel and brass counterparts and found that stress levels during normal walking did not affect the performance of the device. The joint is also self-locking to prevent backdrive, which is beneficial for foot drop recovery. The researchers recommended high-grade structural steel for the ankle shaft as the highest stresses in the AFO were located there.
The design was subjected to a nonlinear finite element analysis based on the maximum torque generated during the heel-strike and toe-off phases of a gait during normal walking. The ankle torques generated during normal walking were calculated to be 40 Nm clockwise and 120 Nm counterclockwise during the heel-strike and toe-off phases, respectively. The structures were also subjected to safe stress levels indicating a structurally feasible design, wherein the maximum stress values of 75 MPa and 130 MPa were observed at the ankle joint shaft for the heel-strike and toe-off phases, respectively, according to the study’s authors.
Future design modifications will include a serial spring element to further minimize the stress on the joint and improve the stability.
The open-access study, “Effect of Ankle Torque on the Ankle–Foot Orthosis Joint Design Sustainability,” was published in Materials.