Ossification of the Posterior Longitudinal Ligament

Pathogenesis, Management, and Current Surgical Approaches: A Review

Zachary A. Smith, M.D.; Colin C. Buchanan, M.D.; Dan Raphael, P.A.-C.; and Larry T. Khoo, M.D.

Disclosures

Neurosurg Focus. 2011;30(3):e10 

In This Article

Neuroimaging of OPLL

Given that the cervical dura is often involved with the ossification of the ligaments, the ability of the surgeon to anticipate the degree of dural ossification and erosion prior to going into the operating room is critical. Once the dura is ossified, it becomes intimately associated with the OPLL. This makes it difficult to cleanly separate the ossified ligament from the dura. As a result, one of the most common complications that results from an anterior approach to decompress OPLL is a CSF leak.[5] In addition, in cases in which there is significant OPLL, the risk of injury to the spinal cord or nerve roots may also be increased as the white matter and vessels of the pial layer become intimately associated with areas of ossification.[39,47] For these reasons, preoperative CT identification of either an ossified ligament or ossification of the dura is critical.

Computed tomography scanning often shows early signs of OPLL, including multiple small areas of bone contained within an enlarged ligament. In patients with progressive disease, these areas form a large, bony plaque within the ligament and ventral to the cord. Hida et al.[24] reported on 2 CT findings that were associated with dural ossification. A "single-layer sign," as described in this report, described dense ossification within the ligament that extended to the periphery. In 9 patients with this single-layer sign, only 1 patient experienced a CSF leak. A double-layer sign was also described, in which there is ossification of the ligament directly behind the vertebral body as well as the hypodense mass of the PLL (Fig. 3). Penetration of the dura (and an associated CSF leak) were significantly more common when this CT finding was present.[16,24]

Figure 3.

A: Preoperative midline sagittal CT scan obtained in a 53-year-old man, demonstrating segmental and continuous regions of ossification starting at the C3–4 level and extending to C6–7. The patient's relative kyphosis was believed to be a contraindication to an anterior approach. B–D: Axial CT images obtained in the same patient showing an ossified bar (asterisk) with an associated pearl of calcification (B), "double-layer" sign (arrow), consistent with dural ossification (C), and a lateral bar of ossification leading to right-sided compression (D).

On MR imaging, early OPLL appears dorsal to the interspaces and can be seen on axial and sagittal views. As the disease progresses, the dense signal behind the vertebral bodies and interbody spaces becomes hypointense on all MR imaging sequences. However, in the progressed disease, there are smaller areas of increased signal. These areas are indicative of new bone formation within the ligament. In addition, OPLL does not enhance with Gd. Thus, on enhanced MR images, it is possible to differentiate between a hypertrophied ligament and postoperative scarring. Associated changes in the spinal cord may be seen on T2-weighted imaging in association with OPLL. This includes areas of increased T2-signal associated with cord edema.

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