====== Cervical ossification of the posterior longitudinal ligament classification ======

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%
((Tetreault L, Nakashima H, Kato S, et al. A systematic review
of classification systems for cervical ossification of the posterior longitudinal ligament. Global Spine J 2019;9:85-103.)).

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]])