The Investigation Committee for Ossification of the Spinal Ligaments (part of the Japanese Ministry of Health, Labor and Welfare) developed a classification system for cervical OPLL comprising 4 types:
(1) continuous: a long lesion extending over several vertebral bodies
(2) segmental: one or several separate lesions behind the vertebral bodies
(3) mixed: a combination of the continuous and segmental types
(4) localized (focal or circumscribed): located mainly just behind a disc space, ossification surrounding intervertebral disc herniation.
The frequency of each type has been reported as continuous, 25.97%; segmental, 34.26%; mixed, 32.47%; and localized, 7.30% 1).
other variants: includes a rare type of OPLL that is contiguous with the endplates and is confined to the disc space (involves focal hypertrophy of the PLL with punctate calcification)