Background: Lumbar vacuum phenomenon (VP) within the intervertebral disc has been classified based on CT imaging. We compared same-patient sagittal CT images and dynamic flexion-extension x-rays to determine if there is a difference in the amount of vertebral instability present between three VP morphologies on CT.Methods: Anterior subluxation measurements on x-ray were compared with same-segment VP on CT images from the same patient when both findings were present. VP were classified as spot, island, or linear. It was determined if there was a difference in the amount of anterior subluxation between the three morphologies. Secondary analysis looked at whether there was a difference in anterior subluxation between the three groups if patients had undergone a prior lumbar fusion surgery or not. Results: There was no difference in anterior subluxation between the three groups on dynamic flexion-extension x-rays. There was also no difference between the three groups on flexion-extension x-rays when patients were separated based on if they had received or not received a previous lumbar fusion surgery. Conclusion: IVD VP morphology is not a useful indicator in determining vertebral instability pre-operatively according to CT scan. Further fine-tuning of an IVD VP CT classification is needed to help radiologists and spine surgeons know when IVD VP presence is important.