PROJECT SUMMARY Tibiotalar arthrodesis, or ‘ankle fusion’, is the gold standard surgical treatment for posttraumatic ankle osteoarthritis (OA). Ankle fusion brings pain relief, at least in the short term; however, elimination of motion at the tibiotalar joint imposes new demands at the subtalar joint, which may exceed the limit of cartilage and result in secondary OA. Most patients undergoing ankle fusion surgery have preoperative deformities in the alignment of the tibiotalar and subtalar joints. Notably, 60% of all patients with ankle OA have varus malalignment of the tibiotalar joint. Recently, it was discovered that the ipsilateral subtalar joint adopts ‘compensatory’ valgus alignment in approximately half of those patients with varus-type ankle OA. Opposing varus-valgus deformities tend to neutralize the overall alignment of the leg and hindfoot, making it appear normal on gross clinical inspection. Unfortunately, while current surgical recommendations for ankle fusion provide clear guidance for how to correct varus malalignment of the tibiotalar joint, they make no stipulation for compensatory valgus alignment of subtalar joint. Results from our recently completed R21 project illustrate the potential for surgeons to strategically align the ankle fusion to control for postoperative motion, joint space distance, and congruency of the subtalar joint, which could in turn mediate secondary OA. Still, the specific effects of ankle fusion surgery on the form and function of the subtalar joint have not been evaluated longitudinally in-vivo. Our broad goal is to identify mechanical and structural factors that lead to inferior patient outcomes after ankle fusion. The R01 proposed herein addresses this goal through quantitative, longitudinal characterization of the effects of ankle fusion surgery on the form and function of the subtalar joint in-vivo. Here, we take a focused, mechanistic look at patients with varus ankle OA that are subdivided into two equal groups (with and without compensatory valgus alignment of the subtalar joint; n = 24 per group). Guided by our preliminary data, we hypothesize that patients with preoperative compensatory alignment of the subtalar joint are at greater risk of experiencing altered form- function relationships and inferior outcomes at 24 months post ankle fusion surgery. Aim 1 will quantify how ankle fusion changes the alignment and morphology of the subtalar joint from longitudinal statistical shape models constructed from weight-bearing computed tomography scans. Aim 2 will quantify how ankle fusion changes the in vivo kinematics of the subtalar joint during walking, stair ascent, and stair descent using dual fluoroscopy. Aim 3 will synthesize these data to identify and isolate the role preoperative alignment of the subtalar joint has on subtalar joint morphology, in vivo motion, and patient outcomes. If successful, this project will provide the first and only mechanistic understanding of the effects of ankle fusion. We anticipate that completion of these aims will also provide prerequisite information for us to pursue a future interventional study, where we would evaluate whether accounting for valgus compensation at the time of surgery yields better outcomes.

PROJECT NARRATIVE The results of this research will provide quantitative characterization of anatomical form and its relationship with function in patients that undergo ankle fusion surgery to treat ankle osteoarthritis. These data should help surgeons to better address deformities at the time of ankle fusion surgery, which could in turn improve clinical outcomes and reduce the likelihood that patients will develop secondary subtalar arthritis later in life.