The Composition of the L5-S1 Neural Foramen on MRI—A Retrospective Cohort Study Examining the Anatomy Relevant to Transforaminal Epidural Steroid Injections

Abstract

Transforaminal epidural steroid injections are commonly used for the treatment of radicular pain. Some providers opt for an antero-superior approach and others a postero-inferior approach. In this retrospective cohort study, we evaluated MRI evident anatomic differences between the antero-superior and postero-inferior neural foramen at L5-S1 that may be relevant when choosing an approach for injections. A total of 29 L5-S1 neural foramina that were targeted for transforaminal epidural steroid injections were included. Pre-procedure MRIs were assessed for the distribution of the fat within the foramen. Additionally, the presence of foraminal vessels and foraminal stenosis and the presence/absence of anterolisthesis was also observed. Final imaging data were obtained by majority opinion of three or four radiologists. There was a statistically significant difference in the distribution of foraminal fat between the postero-inferior foramen and the antero-superior foramen (p < 0.001), with more fat generally in the postero-inferior foramen. Foraminal vessels were not consistently visualized. There was weak inter-reader reliability for the presence of vessels. In conclusion, this study suggests that there is a difference in the distribution of foraminal epidural fat between the postero-inferior and antero-superior foramen at L5-S1. Through MRI, vessels are inconsistently visualized and cannot be reliably detected on conventional MRI between readers.