CT Vacuum Disk Sign: A Highly Specific Predictor of Lumbar Nonunion

Summary: 61 patients with a prior lumbar fusion were evaluated with a CT scan prior to undergoing surgical fusion exploration. Of the 17 previously fused levels with a vacuum disk sign on preoperative CT, all 17 were subsequently found to have a surgical nonunion (100 % specificity).

Purpose: To determine the validity of the vacuum disk sign on CT scan as an indicator of lumbar nonunion. Methods: 61 patients at a single institution with prior lumbar fusion underwent CT imaging. A single spine surgeon assessed the presence or absence of a vacuum disk sign at previously fused levels in a blinded fashion. The presence of a vacuum disk sign on CT images was compared to the presence of surgically confirmed union status (n=143 levels).

Results: 61 patients (mean age: 58.0 years) had a CT scan of the lumbar spine an average of 4.7 years after their index lumbar spine fusion procedure. Of the previously 'fused' levels, vacuum disk signs were detected at 17 levels in 15 patients. When present, the vacuum disk sign was consistently visualized in each imaging plane (axial, sagittal, coronal). Of the 17 levels with vacuum disk signs, 14 had prior posterior only fusion, and 3 circumferential fusion. Surgical exploration confirmed the presence of a nonunion at 51 of 143 levels in 33 patients. A vacuum disk sign was present at 17 of 51 surgically confirmed nonunion levels (specificity: 100%, sensitivity: 33.3%, positive predictive value: 100%).

Conclusion: A vacuum disk sign on CT, when present, is an extremely strong predictor for the presence of a lumbar nonunion.