PROJECT SUMMARY/ABSTRACT The first metatarsophalangeal joint (MTPJ1) is one of the sites most often affected by osteoarthritis, leading to a condition called hallux rigidus (HR). This is very common, estimated to affect 25% of the adult population and increasing in prevalence with age. The number of patients seen by the Veterans Health Administration (VHA) for HR has more than doubled over the last decade. In contrast to degenerative osteoarthritis at the hip and knee, which is commonly treated with joint replacement arthroplasties, the most common surgical treatment for severe HR is arthrodesis, which eliminates joint function. This approach does not allow modification of footwear, interferes with some activities (e.g., yoga, Pilates) and may lead to secondary complications such as metatarsalgia and mobility restrictions. To date, various designs for MTPJ1 arthroplasties have been proposed, but none have been particularly successful, with high failure rates due to loosening and regular reports of migration of the implant. This may be in part because of the relatively small amount of cortical bone in the metatarsal head and proximal phalanx regions, making it difficult to achieve adequate fixation of the prosthetic components. Development of new implants aimed at addressing these problems has been limited by the sparseness of the literature regarding the mechanical environment of the MTPJ1. Similarly, there is very little detailed information on the typical 3D movement of the MTPJ1 required during activities of daily living. Our recent work has established the groundwork for a computational-based modeling workflow that is intended to optimize the design of a novel MTPJ1 implant, and we have had some initial success in generating new, evidence-based implant concepts. In this project, we intend to advance this work, increasing our ability to further MTPJ1 implant technology through computational and robotic gait simulation-based testing. This will ultimately lead to improved patient outcomes. We intend to achieve these aims by: 1) characterizing pathological and healthy MTPJ1 function during different activities of daily living; 2) using a robotic gait simulator to measure the effectiveness of existing and novel MTPJ1 implants at restoring joint function; and 3) further refining our musculoskeletal and finite element models of MTPJ1 to improve their accuracy and provide further validation of their ability to generate useful results. We believe this work has the potential to reinvigorate the study of MTPJ1 arthroplasty, which at present is primarily driven by ideas and not data. The knowledge disseminated from this research will allow surgeons and patients to make better decisions regarding surgical treatments for HR. Specifically, these data will help better understand the disease process of HR and lead to more physiologic MTPJ1 replacements, that will ultimately result in an improvement in mobility and quality of life for Americans with HR. Upon c...