DESCRIPTION (provided by applicant): PROJECT SUMMARY Our aim with this proposal is to better understand how in-shoe foot orthoses achieve improvements in foot and ankle function for people with ankle osteoarthritis (OA) and/or adult acquired flatfoot resulting from posterior tibial tendon dysfunction (PTTD). We also aim to be able to predict what the optimal, personalized orthotic device is for each patient is. These are common, painful, and often highly debilitating conditions, with ankle OA estimated to affect around 6% of the adult population and adult acquired flat foot around 3.3% percent of females. It has been shown that foot orthoses can be an effective conservative intervention for these conditions, and can help to postpone or negate surgery. However, for a significant proportion of patients foot orthoses are unsuccessful, and there is evidence that this may be a result of significant inter-individual variability in joint movement and loading response to the intervention. This may be due to a number of factors, including foot bone shape, muscle strength, and/or joint range of motion. In addition, the design of foot orthoses is often inconsistent between suppliers, largely because of the manual approach that is used to design and manufacture them. A further complicating factor is that prescriptions for foot orthoses are often vaguely written. Improving our understanding of different foot and ankle responses to variation in foot orthotic design is essential if we are to improve how these devices function at the level of the individual patient. To measure how the individual bones of the foot move using traditional techniques is, however, very difficult. Such methods rely on skin-mounted markers that are tracked in space to determine foot and ankle kinematics. However the size and position of the foot and ankle bones means that it is not possible to measure them all of them individually. Moreover, the movement between the skin and the underlying bones, known as soft tissue artifact, introduces significant errors into the measurements. This is further complicated by the need to wear shoes for orthoses to function properly. Our group has developed a biplane fluoroscopy system that is tailored to address the unique issues of measuring foot kinematics. This system has the additional advantage of being able to measure the effects of foot orthotics in unmodified shoes. To achieve our objective of understanding and being able to predict the effects of orthoses, our specific aims are: [1]: To collect, via biplane fluoroscopy, kinematic dat describing the effect of varying the angle of hindfoot posting in foot orthotics. These data will b obtained from 90 participants: 30 with ankle OA; 30 with symptomatic PTTD; and 30 healthy controls. [2]: Using the data from SA1, carry out a regression analysis to identify factors obtained from biplane fluoroscopy and clinical exam that significantly influence an individual's response (i.e., hindfoot kine...