Category: Hindfoot Introduction/Purpose: Tibiotalocalcaneal (TTC) arthrodesis using a hindfoot intramedullary nail has been shown to be a safe and reliable technique in patients with severe ankle and hindfoot disease. There is debate about the use of straight nails versus curved nails. Proponents of the curved nail argue that straight nails predispose to greater risk of lateral plantar nerve injury, poor calcaneal bone purchase and inability to maintain satisfactory hindfoot valgus alignment. However, the subtalar joint is a condyloid joint enabling rotation of the talus on the calcaneum to create varus/valgus position of the calcaneal tuberosity. We present a clinical series of patients undergoing TTC fusion using a straight nail assessed by clinical and radiological outcomes to establish whether theoretical risks of straight nails are reflected in clinical practice. Methods: This single centre, retrospective study of prospectively collected data on a sequential series of patients to undergo TTC fusion, with pre and post-operative weight bearing CT imaging. Data was collected on indication for procedure, co-morbidities, post-operative complications, union rate as well as clinical scores, EQ5D and MOXFQ. Weight bearing CT imaging was analysed using the validated TALUS™ (torque ankle lever arm system) method on weight bearing CT, providing calcaneal offset, hindfoot angle and hindfoot alignment. These were used as markers of hindfoot alignment. Results: 65 patients (37 males, 28 females) were included in the study, with an average age of 57 years. Average follow-up was 20 months. Indications for TTC fusion included Charcot arthropathy, talar avascular necrosis, post traumatic arthritis and non-union of previous arthrodeses. Union rate was 91% (59/65). Overall complication rate was 11% (7/65) and most were minor wound complications. There were no plantar nerve injuries noted. There were no nail cut outs from the calcaneum. On all markers of hindfoot alignment, cases showed a more physiological degree of hindfoot valgus compared to pre-operative measures. There were significant improvement in both EQ5D and MOXFQ scores (p<0.05). Conclusion: We present the largest series of TTC fusion using a straight intramedullary nail and the first series to analyse hindfoot alignment using weight bearing 3D CT imaging. Our data regarding correction of hindfoot alignment is supported by biomechanical theories of subtalar varus/valgus, being related to rotatory changes at the subtalar joint, questioning the perceived need for a curved nail. This is particularly at the level of the entry point of the nail. The principles of valgus hindfoot nails are based on biomechanical laboratory studies and cadaveric studies, neither of which reflect our findings in clinical practice.