Implications of different patterns of "double-layer sign" in cervical ossification of the posterior longitudinal...

To make a preliminary classification of double-layer sign according to the morphological characteristics of the ossified and central hypodense mass and clarify implications of different patterns of "double-layer sign".

The 268 patients of cervical ossification of the posterior longitudinal ligament (OPLL) who underwent anterior corpectomy were retrospectively analyzed from January 2009 to January 2014. All these patients were performed cervical plain X-rays, CT and MRI. The double-layer sign was observed on axial bone window of CT images. According to the morphological characteristics of the ossified and central hypodense mass, this sign was classified into three types: type A was crescent shape, type B was short-straight shape and type C was long-straight shape. Type A was named when the central hypodense mass traced an arc and the OPLL is much more extensive than the dural ossification (DO). It belonged to type B when the central hypodense mass traced a short-straight line, less than or equal to half of the base width of the vertebrae. The OPLL may be extensive or equal to the DO. Type C was defined when it was more than half of the base width, presenting with a long-straight line and DO is much more extensive than OPLL. Intraoperative findings including dural mater ossification and adhesion, postoperative CSF leakage and outcome were all studied. Two spinal surgeons with rich experiences read the CT images according to this classification method and verified its consistency.

Ninety-two patients were found in association with DO during the anterior decompression procedure, meanwhile the double-layer sign could be seen on axial bone window of CT imaging. In these 92 patients, there were 51 patients presenting with type A, and the ossified mass was completely resected with ossified dura mater reserved. Only two patients presented with dural defect and postoperative CSF leakage. Thirty-five patients was classified as type B, of which six accompanied by CSF leakage. All the other six patients of type C presented with CSF leakage after operation. There was a significant correlation between the occurence of CSF leakage and pattern of double-layer sign, but not gender, age, duration of symptoms, extent of OPLL and occupying rate. The Kappa value between the two surgeons was 0.82, showing a good consistency of the method.

OPLL patients with double-layer sign of type C is almost inevitably followed by CSF leakage after anterior decompression. For type A and B, occurrence of CSF leakage is not as high as we thought before. Pattern of double-layer sign should be a considered factor when anterior or posterior approach is chosen.