Bipartite navicular bone: spectrum of imaging findings of uncommon anatomic variant frequently inducing mid-tarsal osteoarthrosis

Abstract Background Bipartite navicular bone is a relatively uncommon pathological condition, characterized by splitting of the navicular bone into two parts. Patients with this anomaly typically present with chronic pain at the dorsomedial aspect of their foot and a flatfoot deformity. This study aimed to describe the imaging findings related to the presence of this uncommon anatomical variant. Results Magnetic resonance imaging was done for all 24 patients with bilateral involvement in four patients, yielding 28 feet. On MRI, separation of the navicular bone with unequal size of the two fragments was observed in all examined feet and secondary talonavicular degenerative osteoarthrosis identified in 16 feet. Bone marrow edema was seen at the parent navicular bone in 10 feet, while the lateral fragment showed bone marrow edema in 18 feet. Ten patients, with bilateral involvement in four of them, yielding a total of 14 feet did MDCT studies which were reviewed. Separation of the navicular bone into two parts by oblique cleft with unequal size of the two fragments in all examined feet. Comma-shaped appearance of the main navicular bone with medial translation relative to the talar head was identified in 8 feet. Secondary talonavicular degenerative osteoarthrosis was observed in 10 feet. Plain X-ray of the symptomatic foot was reviewed in eight patients yielding a total number of 12 examined feet due to bilateral involvement in four patients. On AP view, the lateral bone fragment was obscured in all examined feet. On lateral view, an irregular bone separated from the dorsal aspect of the navicular bone was identified in 10 feet, while failure of identification of the cleft between the two fragments on the lateral view in 2 feet but was nicely demonstrated on oblique view. Irregularity in the talonavicular joint space was observed in 8 feet. Conclusions Bipartite navicular bone is rare anatomic variant that causes mid-foot pain and osteoarthritic changes around the navicular bone. In cases where pathology in relation of the presence of this anatomical variant is suspected, detailed clinical correlation and careful assessment with MRI and MDCT will play an important role.