Complete Bilateral Ankle Fusion: A Rare Complication of Ankylosing Spondylitis

Ankylosing spondylitis (AS) is a chronic inflammatory disorder primarily affecting the axial skeleton and is strongly associated with a positive human leukocyte antigen B27 (HLA-B27) genotype. Patients typically present with chronic low back pain that typically starts before the age of 40 years. Common initial clinical features include lower back, hip, and joint pain with stiffness that is worse in the morning and with inactivity. As the disease progresses over a prolonged period, it leads to fusion of sacroiliac joints and ankylosis of the vertebrae with the iconic "bamboo spine" on imaging. Joint fusion or ankylosis is the sequela of either undiagnosed or untreated AS. We report a case of a 69-year-old male with complete fusion of the ankle joint, hindfoot, and midfoot of both feet in the clinical context of an incidental finding of an ankylosed spine on computed tomography (CT) imaging. The ankle joint is a very uncommon site for fusion in ankylosing spondylitis. We would like to suggest the terminology “boot sign” for this rare radiographic finding of complete ankle and subtalar fusion given the appearance of a boot. “Boot sign” is associated with either inability to ambulate or a steppage gait from loss of ankle dorsiflexion as a result of ankle and hindfoot fusion with or without fusion of forefoot and midfoot.