A rare bone disease characterized by spontaneous adult-onset tarsal navicular osteonecrosis. Patients present with chronic mid- and hindfoot pain, swelling and tenderness over the dorsomedial aspect of the midfoot, flattening of the medial longitudinal arch, and pes planovarus. Radiographic findings include comma-shaped deformity due to collapse of the lateral part of the navicular bone and medial or dorsal protrusion of a portion or the entire bone. The condition may be bilateral or asymmetric and associated with pathological fractures.