PROJECT SUMMARY The ankle and hindfoot are made up of complex interlocking bones and joints that work together to enable movement of the foot. Unfortunately, these bones and joints can become severely damaged as a result of disease or injury, leading to devastating pain and disability. Patients facing severe ankle-hindfoot damage have limited treatment options: they can either amputate the leg below the knee, or pursue a salvage operation called ankle-hindfoot arthrodesis (“fusion”), which involves using orthopaedic nails and screws to connect the bones in the foot so that the joints no longer move. Unfortunately, about 30% of ankle-hindfoot fusions fail, requiring the limb to be amputated anyway. Even for the estimated 70% of ankle-hindfoot fusions that are “successful”, the functional outcomes are so severely restrictive that patients may eventually choose to have an amputation, because a prosthetic leg would provide more function than their fused joints. In short, current treatment options are so limited that patients and clinicians often choose to amputate the injured leg, even when the muscles, skin, and other bones of the foot are still viable. In this work, we aim to fundamentally reinvent the care paradigm for traumatic injury or disease of the ankle and hindfoot by advancing a novel Compliant Ankle Endoprosthesis with the potential to restore function and alleviate pain, without requiring fusion or amputation. By preventing limb loss and eliminating the need for ankle-hindfoot fusion, our goal is to alleviate suffering and restore much more functional capacity than is currently possible. Our product restores joint motion via deformation of flexible elements, rather than rubbing or sliding of bearing surfaces. The device originated in one of the applicants’ engineering lab at UCLA. Feasibility of this product has already been demonstrated in the applicant’s lab in simulation, on the benchtop, and in a large animal model. In this work, we will conduct the late-stage pre-clinical studies required for regulatory approval of this device, prior to clinical testing. Specifically, we intend to show that i) acute failure of the implant happens only under super- physiologic loads and in a way that does not endanger the surrounding tissues, and ii) the implant has the durability necessary to support human gait for decades inside the body. We will conduct this research over 12 months, and the data we obtain will be used in an FDA Investigational Device Exemption application to support an early feasibility study, and in the NIH SBIR Phase II application. Major milestones to be achieved with this grant include direct experimental validation of short- and long-term mechanical survivability of the Compliant Ankle Endoprosthesis, in preparation for clinical testing.