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Neuroradiology Case of the Week

Case 54

Ravinder Sidhu MD, Ramon de Guzman MD, Leena Ketonen MD, PhD,
and P-L Westesson MD, PhD, DDS

Clinical Presentation: A 55-year-old male presented with a five year history of cervical and lumbar radiculopathy. On examination, there was no neurological deficit.

Radiological Findings: Myelogram of the cervical and lumbar spine followed by CT-myelo was performed. There were prominent extradural indentations at anterior thecal column seen from C 3/4 until C7/T1 levels, suggestive of disc bulge. Ossified posterior longitudinal ligament was seen at C 4/5 level (Fig. 1). There was a lucent line between the ossified ligament and posterior margin of the vertebral body (Fig. 2). Axial T2-weighted MR image revealed a hypointense signal posterior to vertebral body margin, suggestive of ossified posterior longitudinal ligament (Fig. 3). Both these factors attributed to spinal canal stenosis. Lumbar spine also showed disc bulges.

Diagnosis: Ossification of posterior longitudinal ligament

Discussion:  

Clinical Discussion: Ossification of posterior longitudinal ligament is more common in the cervical (70%), compared to either thoracic (15%) or lumbar (15%) spine. In cervical spine, middle and upper cervical vertebrae are commonly involved. The disease is most prevalent in Japanese population and has a genetic linkage. Most individuals with this condition are asymptomatic and only a minority of them develops radiculopathy [1].
    Posterior longitudinal ligament runs from the second cervical vertebra to sacrum along the posterior surface of vertebrae and discs. The posterior longitudinal ligament is closely adherent to and blends with annulus fibrosus of the discs and adjacent margins of the vertebral bodies. At the mid-vertebral levels, the ligament is separated from the vertebral margins by 1-2 mm gap that is filled with connective tissue and venous plexus.
   The posterior longitudinal ligament has two strata of fibres-superficial and deep part. The ossification of posterior longitudinal ligament initially occurs in its superficial layer. The ossified mass consists mainly of lamellar bone with areas of calcified cartilage in between. It expands in thickness and width beyond its anatomical boundaries and is firmly attached to the posterior aspect of the vertebral body and disc. In approximately 50% of instances, the mass is separated from the vertebral margin by intervening connective tissue, venous plexus and the emulsified but thickened deep component of posterior longitudinal ligament. These findings attribute for the typical radiographic appearance of a sharp and thin radiolucent line separating the ossified posterior longitudinal ligament from the posterior vertebral margin [2].

Imaging Discussion: Morphologically, four forms of ossification of posterior longitudinal ligament have been described. The continuous and segmental forms account for up to 95% of all cases. The continuous form extends over several contiguous vertebral bodies while the segmental form is situated fragmentally behind each vertebra. The mixed form includes the combination of continuous and segmental form. The fourth or retrodiscal form, is situated exclusively behind the intervertebral disc and radiologically may be indistinguishable from a posterior osteophyte [3].
   CT scan is the method of choice for detecting the presence and extent of ossified mass. Sagittal reformations are invaluable in showing the exact extent of the disease. On axial sections, the ossified posterior longitudinal ligament is seen as a ’mushroom’, a ‘hill’, and a ‘square’ or a mixture of these shapes. And a characteristic sharp radiolucent line separates the ossified posterior longitudinal ligament from the posterior vertebral margin representing the unossified deep component of posterior longitudinal ligament [4].
   MR imaging is helpful in depicting the nature of cord compression such as myelomalacia, edema, demyelination or cyst formation and root sleeve involvement. The extent of myelomalacia and edema of the cervical cord is best seen in sagittal and axial T-2 weighted sequences. The ossified posterior longitudinal ligament is seen as a constant signal void in both T1-and T2- weighted images.
    The segmental type of ossified posterior longitudinal ligament needs to be differentiated from calcified discs and large posterior osteophytes. Osteophytic growth, however, is along a horizontal axis whereas ossified posterior longitudinal ligament extends vertically. Neither of these two conditions show the characteristic sharp radiolucent line.

References:

1. Terayama K. Genetic studies on ossification of the posterior ligament of the spine. Spine 1989; 14:1184-1191.
2. Enzman D, De La Paz R, Ruben J. Magnetic Resonance Imaging of the Spine. CV Mosby Co., St Louis, 1994:129.
3. Epstein N. The surgical management of ossification of the posterior longitudinal ligament in 51 patients. J Spinal Disord 1993; 6:432-455.
4. Soo MY, Rajaratnam S. Symptomatic ossification of the posterior longitudinal ligament of the cervical spine: pictorial essay. Australasian Radiology 2000; 44: 14-18.