"Findings: There is ossification of the posterior longitudinal ligament over 4 contiguous levels and to a lesser extent of the anterior longitudinal and transverse ligaments, best seen on the CT exam. Ossification is difficult to confirm on MRI, but is suggested by the presence of fatty marrow elements on the T1-weighted image. The T2-weighted sagittal MRI best demonstrates narrowing of the spinal canal and cord compression.
A. A sequestered disc herniation would involve a single disc level. The abnormality shown extends over multiple contiguous vertebral levels.
B. There is ossification of the posterior longitudinal ligament (OPLL) over 4 contiguous levels, best seen on the CT exam. Ossification is difficult to confirm on MRI in the absence of fatty bone marrow elements. However, MRI best demonstrates narrowing of the spinal canal and cord compression.
C. The density of the abnormality on CT is not characteristic of a hematoma. It is characteristic of dense calcification or ossification and is confined to the posterior longitudinal ligament.
D. Diffuse idiopathic skeletal hyperostosis (DISH) is characterized by flowing paravertebral ossification over at least 4 contiguous levels and primarily involving the anterior longitudinal ligament. This case demonstrates involvement of 4 contiguous levels but primarily involves the posterior longitudinal ligament.


Imaging Findings
Best diagnostic clue: Flowing multilevel ossification posterior to vertebral bodies, with relatively minimal degenerative disc disease, absent facet ankylosis
Location: Midcervical (C3-C5) > mid-thoracic (T4-T7)
Characteristic ""upside down T"" or ""bowtie"" configuration on axial images
Pathology
Epidemiology: 2-4% prevalence in Japan; much less common elsewhere
Bulky posterior ligamentous ossification with normal cortical bone, marrow
Clinical Issues
Symptomatic myelopathy almost universal if canal diameter < 6 mm; rare if > 14 mm
↑ Risk for developing progressive myelopathy if > 60% canal stenosis, ↑ cervical range of motion
Diagnostic Checklist
Overly ""thick"" hypointense PLL on MRI strongly suggestive of OPLL
Requires high index of suspicion, careful observation to diagnose on plain radiographs