A Bifid Eleventh Rib Presenting as a Dorsal Mass in an Infant

Infantile dorsal masses arise from a wide range of congenital, inflammatory, and neoplastic conditions, and accurate diagnosis often depends on imaging. We report a 10-month-old boy who presented with a palpable left dorsal mass that was ultimately diagnosed as an isolated bifid 11th rib. Routine chest radiographs were inconclusive, whereas ultrasonography suggested an osseous lesion, an oblique rib radiograph localized the abnormality to the 11th rib, and three-dimensional computed tomography clearly demonstrated the bifid morphology. Magnetic resonance imaging (MRI) excluded an associated soft tissue lesion. The patient was managed conservatively. This case illustrates that a bifid 11th rib may present as a dorsal mass because of the posterior orientation of a floating rib and highlights the value of stepwise imaging, particularly three-dimensional computed tomography, for establishing the diagnosis when conventional imaging is inconclusive.