Introduction:: Despite improvements in prosthetic limb devices, most patient-reported problems relate to how these devices are attached to the body [1]. These problems, such as heat/sweating, skin irritation, and tissue damage, lead to reduced prosthesis use and high abandonment rates. Maintaining residual limb health is also costly: patients average 7 prosthetist visits per year, and annual prosthesis-related care for a patient with above-knee amputation costs between $5K and $10K. This is compounded by the fact that 77% of major limb amputations are due to dysvascular causes, such that patients must already manage conditions like diabetes or peripheral artery disease. Importantly, these issues disproportionately affect people from underprivileged socioeconomic communities and racial minorities, who are much more likely to undergo preventable amputations due to disparities in access to healthcare and other resources [2].
In current socket-based systems, loads required to suspend a prosthesis are transferred to soft tissues via friction or suction between the socket (or liner) and the skin. Because soft tissues deform under load, these tissues are repeatedly deformed during use, causing damage and breakdown over time. We present a new attachment paradigm with the potential to greatly reduce soft tissue deformation. Our system has two core components: a subcutaneous ferromagnetic implant in the residual bone, and an external electromagnet housed at the bottom of the socket. Magnetic attraction between the electromagnet and implant allows transfer of prosthesis suspension loads directly to the residual bone across a closed skin envelope.
Materials and Methods:: To develop an electromagnetic attachment system (Fig. 1a) for a transfemoral amputation level, first the loads required to suspend a knee-ankle prosthesis during the swing phase of gait were calculated. Using a biomechanical model built to simulate a prosthesis user (Fig. 1b), along with published gait data from persons with transfemoral amputation, inverse dynamics analyses were performed in the software OpenSim to determine the forces required at the limb-socket interface along the long axis of the femur. A preliminary implant was designed through cadaveric dissections to fit within a limb and the thickness of tissue covering the implant was measured. Based on the required loading profile, implant design, and tissue thickness, an electromagnet was designed in the electromechanical simulation software JMAG.
The feasibility of electromagnetic attachment was tested based on power requirements during gait. Simplifying the electromagnet to a resistive load (due to the coil’s low inductance), the instantaneous power at during gait was calculated from the current required to track the socket force profile using the relationship between electromagnet current and attractive force. Because the system only handles suspension, the electromagnet was powered off during the stance phase.
Results, Conclusions, and Discussions:: The force required to attach a knee-ankle prosthesis during gait is shown in Fig. 2a. During stance phase, compressive loads for weight bearing vary greatly between subjects, however during swing the tensile pulloff force on the socket is consistent in timing and magnitude between subjects. The force profile during swing has two main components, a static baseline force roughly corresponding to the mass of the prosthesis, and dynamic force peaks related to the limb’s kinematics. The designed electromagnet has a mass of around 1 kg and is composed of a permanent magnet core surrounded by the coil, all enclosed by a ferromagnetic shell. The strength of the permanent magnet core was designed to match the baseline force such that the mass of the prosthesis was suspended passively, while the dynamic forces are handled by actively modulating the current through the coil. This passive force is seen in the zero-current force on the electromagnet current and force relationship (Fig. 2b). Because the baseline force during swing is covered passively in addition to the magnet being powered off during stance, the electromagnet power during gait is isolated to three distinct peaks (Fig. 2c). This results in an average power during gait for of 32 W, well within the capabilities of prosthesis power electronics. These power requirements show that electromagnetic attachment would be theoretically feasible. While the working principle of this system should result in decreased soft tissue deformation, future work will focus on creating the physical system and comparing deformation performance to that of conventional sockets.
Acknowledgements (Optional): :
References (Optional): : [1] K. Hagberg and R. Brånemark, “Consequences of non-vascular trans-femoral amputation: A survey of quality of life, prosthetic use and problems,” Prosthet. Orthot. Int., vol. 25, no. 3, pp. 186–194, 2001
[2] K. Newhall, E. Spangler, N. Dzebisashvili, D. C. Goodman, and P. Goodney, “Amputation Rates for Patients with Diabetes and Peripheral Arterial Disease: The Effects of Race and Region,” Ann. Vasc. Surg., vol. 30, pp. 292-298.e1, 2016
