Background: The spring ligament is an important medial arch stabilizer. However, when disrupted, it does not cause planovalgus deformity until the foot is cyclically loaded. We propose that the tibionavicular (TN) ligament plays an important role. However, this ligament is not imaged in routine magnetic resonance imaging (MRI) sequences. Methods: A prospective case-control study using a novel MRI sequence to image the TN ligament in 20 normal feet creating a baseline appearance of the ligament. We then scanned 20 patients with adult acquired flatfoot deformity (AAFD). All patients had weightbearing anteroposterior and lateral radiographs. We followed up patients, the end point being surgery or 18 months’ follow-up. Results: The normal ligament was reliably identified on the novel sequences. It had a reproducible appearance in 2 views, and consistent length and width. Two groups of patients were identified in the AAFD cohort: Normal TN (11/20) (The mean Meary angle was 6.8 degrees) and Abnormal TN (9/20). The ligament was thickened proximally, with distal attenuation and intrasubstance edema. On sagittal sequence, it had dorsal bulging and high signal. The mean Meary angle was 13.2 degrees (P =.013). All patients had posterior tibial tendon dysfunction and 8 had spring ligament complex attenuation. Five patients have undergone corrective surgery compared to none in the other group. Conclusion: This study adds to the evidence that AAFD is multifactorial. With this imaging technique, we were able to reliably image the TN ligament. We hope that including this sequence into routine scanning will help us understand its role in flatfoot deformity. This poses the question of whether this structure will play a role in reconstructive surgery in future. Level of Evidence: Level II, prospective comparative study.