The term 'limbus' denotes a small, unusual 'fracture' located on the inferior-posterior corner of a vertebral body (inferior vertebral endplate), usually in the lower lumbar spine. According to Schmorl and Kohler's study, lumbar vertebral ring apophyses, located at the margin of the superior and inferior vertebral endplates, first appear as a cartilaginous rim in childhood and tend to ossify completely with the vertebral body by the age of 18-25 years. Whether this separated triangular bone fragment is a true fracture of the posterior ring apophysis or a separation of the posterior vertebral rim, a number of different names describe this entity as a posterior marginal node, posterior bony avulsion, apophyseal ring fracture or epiphyseal dislocation.
Posterior vertebral rim fractures usually present with symptoms of low back pain and/or radiculopathy, most often in male adolescents (ratio 2:1) or young adults but they can also be incidentally found. Plain radiographs, especially lateral projections, can demostrate this abnormality in 40% of cases. However, CT gives a more complete picture of the morphological changes of the displaced bone fragment, and shows any associated disc herniation and spinal canal compromise. Extruded, degenerated disc material between the fragment and the inferior corner of the vertebra can only be verified by discography, an invasive diagnostic method. An extradural defect at the site of a limbus vertebra can also be demonstrared by myelography, where encroachment on the spinal canal may vary from mild to complete blockage. A fracture of the posterior cortex of the vertebral body has to be excluded in considering a diagnosis of posterior limbus vertebra.