"They are defined as heterotopic ossifications involving the anterior longitudinal ligament, paraspinal connective tissues and annulus fibrosus of at least four contiguous vertebral bodies and were originally described on lateral radiographs of the spine. Typically, there are radiolucencies between the anterior aspects of the vertebral bodies and the overlying ossifications of the anterior longitudinal ligament. The radiolucencies again classically end at the level of the vertebral endplates."