Introduction
Ossification of the posterior longitudinal ligament (OPLL) is a slowly progressive disorder that can result in spinal stenosis and myelopathy. Myelopathy in patients with OPLL is related to both static and dynamic factors. The most common anatomic location is the cervical spine, although the process may also involve the thoracic and lumbar regions. The prevalence of OPLL increases with age and is most common in the elderly population along with a slight male predominance. There is also increased incidence in Asian populations, specifically the Japanese population.
Pathophysiology and Natural History: Overview
Evolution
The evolution of changes in OPLL begin with hypervascular fibrosis of the posterior longitudinal ligament (PLL), which is followed by focal calcification that eventually leads to proliferation of periosteal cartilaginous cells and ultimately ossification. Therefore, the natural history of OPLL consists of fibrosis followed by calcification and ultimately ossification. Importantly, the process often extends into the adjacent dura (a finding discussed in greater detail later). It is the hypertrophy and ossification that may cause myelopathy secondary to direct spinal cord compression or ischemia.
Distribution
The PLL extends along the posterior surface of the vertebral bodies from the clivus to the sacrum. Although OPLL may involve any portion of the PLL, by far the most common anatomic location is the cervical spine—accounting for approximately 75% of cases. The process typically involves 2.5 to 4 levels beginning at approximately C3/C4 and progressing distally to involve C4/C5 and C5/C6, although generally sparing C6/C7. Thoracolumbar involvement is less common, accounting for roughly 25% of cases, usually involving the upper thoracic spine rather than the lumbar segments.
Pathologic Classification and Imaging Features
The initial neuroimaging features can be variable. The most widely used classification system distinguishes between four types of OPLL based on sagittal CT images. The four types are continuous , segmental , mixed , and circumscribed ( Fig. 23.1 ).
Continuous OPLL, located posterior to several contiguous vertebral bodies and intervertebral disks, with relatively minimal degenerative disc disease and absent facet ankylosis, defines the continuous type. The segmental type of OPLL demonstrates discontinuous ossification posterior to one or multiple vertebral bodies. The mixed type is a combination of continuous and segmental types. The fourth category, termed circumscribed, occurs with ossification posterior to an intervertebral disc and must be distinguished from disc calcification. The posterior margin of ossified regions may appear relatively smooth or irregular and jagged adjacent to the thecal sac. It should be noted that OPLL more frequently involves the adjacent dura than do other discogenic lesions. Accompanying adjacent dural ossification occurs more commonly with the nonsegmental type in the cervical spine but can also be seen with both segmental and nonsegmental types when encountered in the thoracic spine.
Rate of Progression
OPLL is a slowly progressive disorder that evolves over a period of years ( Fig. 23.2 ). It may progress in a continuous or discontinuous fashion in relation to the initial areas of ossification and may involve segments of the spine that are widely separated from previously involved regions. Separate foci of thoracic involvement may occur in a patient originally noted to have ligamentous ossification in the cervical spine, for instance. Although the rate of progression is variable among patients, a slow, gradual rate of progression is the norm. The millimetric average annual rate of progression in the anterior-posterior dimension is 0.67 mm and longitudinally 4.1 mm.
Pathophysiology
The pathogenesis of OPLL is incompletely understood. However, OPLL is most prevalent in the Japanese population, suggesting a partial genetic component among many other factors. OPLL can be associated with diffuse idiopathic skeletal hyperostosis (DISH), ankylosing spondylitis, and other spondyloarthropathies ( Fig. 23.3 ). Although OPLL may be an incidental finding, it can present with pain, neurological deficit, or acute neurological injury.
Although the morphologic appearance of OPLL on axial imaging is quite variable, “square”, “hill”, and “mushroom”-like configurations have been described ( Fig. 23.4 ). “Upside down T” and “bow-tie” configuration descriptions have also been used. Mushroom or hill shape on axial images is typical of PLL ossification contiguous with the vertebral body. A sharp line of hypodensity between the posterior vertebral body and the ossified ligament is characteristic of OPLL at sites where the hypertrophic but partially unossified PLL is not closely apposed to the vertebral body ( Fig. 23.5 ).
Lateral cervical radiographs are the simplest method to detect ligamentous ossification; however, x-rays often fail to demonstrate OPLL. Computed tomography (CT) including CT myelography is the ideal modality for accurate diagnosis and characterization of OPLL, best depicted on sagittal images. The thickened ossified ligament may have a protuberant appearance extending posteriorly towards the thecal sac.
On magnetic resonance imaging (MRI), the appearance of OPLL is varied but typically demonstrates T1 and T2 hypointensity of the thickened, ossified segments, with findings best identified on sagittal T2 images. Thickening of the PLL is often not appreciated until approximately 5 mm of thickness. Of crucial importance, MRI most accurately characterizes the extent of spinal stenosis, cord impingement, and edema. Some studies, for instance, have reported a correlation between a spinal canal occupying ratio of greater than 60% with increased risk of myelopathy and, in surgical cases, increased intraoperative blood loss. MRI is also better suited for detection of early stages of OPLL, termed EOPLL or HPLL (early or hypertrophic OPLL), in which the ligament appears thickened and hypointense in the absence of compact bone lamellar formation on corresponding CT or plain radiograph ( Figs. 23.6–23.10 ).
