The Pathophysiology of Migrated Lumbar Intervertebral Disc Herniations.

Introduction: Herniated lumbar intervertebral discs migrate into the rostral or caudal anterior epidural space (AES). Previous studies have reported varying frequencies of migration direction, and the factors influencing the migration direction include patient age, affected disc level, and AES structural differences. However, the relationship between AES volume and migration direction remains unclarified. The purpose of this study was to measure the AES volume using computed tomography (CT) imaging and investigate the factors affecting herniated nucleus pulposus (HNP) migration in the sagittal direction. Methods: We reviewed 42 patients who were surgically treated for migrated lumbar intervertebral disc herniation between 2014 and 2023. The primary endpoint was the ratio of the AES volume between vertebrae adjacent to the herniated disc. The secondary endpoints were patient demographics, disc level, clinical symptoms, disc degeneration, and lumbar instability. AES volume was measured by calculating the area between the posterior vertebral wall concavity and a line connecting the posterior walls on CT images, with the total volume determined as the sum of the slice areas multiplied by the slice width. Results: A total of 14 patients exhibited rostral HNP migration, while 28 exhibited caudal HNP migration. Rostral HNP migration was associated with a higher prevalence of double-root involvement (p<0.05) and a greater superior/inferior ratio of the AES volume (p<0.01). Multivariate analysis identified the AES volume superior/inferior ratio (odds ratio: 9.551) as a factor significantly associated with the direction of HNP migration. Conclusions: The HNP tends to migrate toward the direction with a larger AES volume because the herniated material follows the path of least resistance. Clinical presentation of double-root symptoms was strongly indicative of rostral HNP migration.