Study design: To establish the diagnosis of dural penetration on preoperative computed tomographic studies of the cervical spine in patients with ossification of the posterior longitudinal ligament (OPLL).
Objectives: To define before surgery the pathognomonic computed tomographic findings of OPLL extending to and through the dura.
Summary of background data: On preoperative computed tomographic studies, Hida et al have described the single-layer sign characterized by a solid mass of hyperdense OPLL and the double-layer sign defined by two (anterior and posterior) ossified rims surrounding a central nonossified but hypertrophied posterior longitudinal ligament. Only 1 of the 9 patients exhibiting the single-layer sign but 10 of 12 patients showing the double-layer sign had no separate dural plane identified at surgery.
Methods: Only 2 of 54 patients undergoing multilevel cervical circumferential OPLL procedures had absent dura at surgery. Computed tomographic examinations for all patients were retrospectively reviewed to determine unique signs of dural penetration.
Results: Dura was absent in 1 of 12 patients who had the single-layer CT sign that was additionally characterized by an irregular C angular configuration. Only 1 of 4 patients exhibiting the double-layer computed tomographic sign had absent dura at surgery. The remaining 38 patients had the smooth-layer sign, characterized by more regular margins of classic (22 patients) or early OPLL (16 patients).
Conclusions: The double-layer computed tomographic sign is more pathognomonic for dural penetration than the single-layer sign. The smooth-layer sign, indicating a clean dural plane, is more typical in North American patients.