Direct Empowerment for Joints Weakened By Limb-Loss

The medical community classifies persons with amputations into “K-levels” according to their athleticism and confidence in maneuvering with their prostheses. While the most capable K4 ambulators dominate the subject pools for engineering experiments in powered prosthesis, the K2 “community ambulators” may not be able to fully benefit from the powered-knee and powered-ankle configurations of modern devices such as the Open Source Leg. The reason is that many K2 ambulators have a shorter residual limb, and therefore have lost significant muscle mass in the hip-actuating muscles that attach to the thigh. But these hip muscles are critical to actuating the biological hip and effecting the motion of passive prostheses and especially the inherently heavier powered prostheses. This presents an obstacle to effective robotic intervention and impedes K2 ambulators from activities of daily living, negatively impacting their quality of life. Recently, Ref. [IshmaelTranLenzi2019ICORR] proposed to address this problem mechanically, by fitting patients with an under-actuated orthoprosthesis—a powered hip orthosis connected to a passive prosthesis—and applying assistive torques to the weakened hip.  However, the control strategy remains task-dependent, since it assumes that the person is moving periodically, and is thus unable to assist in the full range of non-periodic behaviors included within activities of daily life. These include sit-to-stand maneuvers, transitions between tasks, and recovering from unexpected disturbances in order to avoid falling. The overall goal of my research thrust in this area is to extend this mechanical assistance strategy to a general class of under-actuated orthoprosthesis feedback systems for task-invariant assistance of joints weakened by amputation surgery, with initial emphasis on K2 transfemoral amputees. I hypothesize that by implanting a six axis force/torque sensor in the passive prosthetic, just below the socket, the biological hip torque can be both estimated to a high degree of accuracy and fed back via the powered orthosis to increase the wearer’s perception of physical strength at the hip, thus facilitating their use of the prosthesis. More specifically, I aim to (1) design and assemble a specialized under-actuated orthoprosthesis that features an instrumented passive prosthesis, (2) understand the dynamics of the residual limb of K2 ambulators for the purpose of guaranteeing stability under feedback with a hip exoskeleton, and (3) determine the extent to which hip strength amplification changes user perception of the mass and inertia of the passive prosthesis while walking. As this research addresses a key technological barrier to biomedical adoption of an assistive exoskeleton device, it would align with the mission of the NIH NIBIB. It also aligns with the NIH NCMRR’s Devices and Technology Department and research priority in prevention and treatment of secondary conditions. Follow up investigations could introduce hardware for amplifying other affected joints and body-powered hand prostheses.