TABLE III

Definitions of OPLL Characteristics

CharacteristicDefinition
Occupying ratioThickness of ossification divided by anteroposterior diameter of the osseous spinal canal
Type of OPLL
 ContinuousLong lesion extending over several vertebral bodies
 Segmental1 or several separate lesions behind vertebral bodies
 MixedCombination of the continuous and segmental types
 CircumscribedLocated posterior to a disc space
Shape of OPLL lesion
 Hill-shapedMassive beak-shaped ossification localized to certain levels
 Plateau-shapedRelatively narrow spinal canal without any localized massive ossification
ThicknessAnteroposterior extent of OPLL
LengthLongitudinal extent of OPLL
Configuration of spinal cord
 BoomerangSpinal cord has convex posterior surface and concave anterior surface with smooth round corner
 TeardropSpinal cord has convex posterior surface and concave anterior surface with smooth round corner on only 1 side
 TriangularSpinal cord has an angular lateral surface and a flat anterior surface
Space available for spinal cordDiameter of ossified mass subtracted from the developmental canal diameter
K-line classification (+/−)K-line is a straight line connecting the midpoints of the spinal canal at C2 and C7 on a neutral lateral radiograph. K-line (–) indicates that OPLL extends beyond K-line, whereas K-line (+) indicates that it does not
Dural ossificationOssification of dural membrane
 Type AHypodense mass is crescent-shaped and OPLL is much more extensive than dural ossification
 Type BShort straight hypodense straight line less than or equal to half of the base width of vertebrae
 Type CLong straight hypodense straight line more than half of the base width of vertebrae
No. of levelsNo. of ossified levels