Bone fractures happen to 9.6 million Americans each year. A quarter of all lower-limb fractures are delayed healers and about 1 in 10 are nonunions—fractures that do not heal without another major surgery. The long-term goal of this US NSF-Swiss NSF (NSF-SNSF) project is to improve health outcomes for people with poorly healing fractures by developing computer simulation tools that can predict how bones heal over time. The simulations will use medical imaging to create digital twins – virtual models of real bones – that give insight into how an individual person’s healing may progress with time. Better predictions for healing outcomes will help doctors and patients make more informed decisions about what to do when bone healing goes poorly. The project will also provide a unique training opportunity for a mechanical engineering graduate student. The student will have the opportunity to do an international research exchange in Switzerland at the AO Research Institute, a global leader in biomechanics and bone fracture care. This research will address fundamental knowledge gaps in biomechanics and mechanobiology that are of broad interest to engineers, biologists, and clinicians working in the field of fracture healing. Objective 1 will define the limiting conditions for callus formation by performing spatial colocalization of strain from subject-specific finite element models and mineralization from longitudinal imaging. Objective 2 will measure rate constants for bone remod