Significance of the double-layer and single-layer signs in the ossification of the posterior longitudinal ligament of the cervical spine.

Object: The purpose of this study was to elucidate the significance of the signs of dural penetration, which were previously described by Hida et al. This goal was accomplished by an analysis of preoperative computed tomography scans and a review of the medical records of patients who underwent removal of the ossification of the posterior longitudinal ligament (OPLL) via the anterior approach.

Methods: Outcomes in 197 patients with cervical OPLL who underwent anterior decompression and fusion were studied retrospectively. The types of OPLL, single- and double-layer signs of dural penetration, diameter of the central hypodense mass of the double-layer sign, and the presence of actual dural penetration were evaluated. Signs of dural penetration were found in 30.5% of patients. These signs were much more prevalent in patients with nonsegmental OPLL. Dural defects were present in 20 (52.6%) of 38 patients with double-layer signs and in three (13.6%) of 22 patients with single-layer signs. Among patients in the double-layer sign group, the mean diameter of the central hypodense masses was thicker in the group with an actual dural defect. Although not to the degree reported by Hida et al., the double-layer sign had a significant association with dural defects. In particular, the thicker the central hypodense mass of the double-layer sign, the greater the possibility of a dural defect. However, a single-layer sign had less significance than a double-layer one.

Conclusions: Surgeons should be alert to the increased possibility of a dural defect when there is a double-layer sign with a thick central hypodense mass in nonsegmental OPLL.