Unintentional Injection Into the Retrodural Space of Okada During Transforaminal Epidural Steroid Injection

Background: Transforaminal epidural steroid injection (TFESI) is commonly used for radicular pain, but can lead to an unintentional injection into the retrodural Space of Okada (RSO), an extradural space located dorsal to the ligamentum flavum, instead of the epidural space. Objectives: To determine the prevalence and describe the fluoroscopic imaging features of an unintentional injection into the RSO during a TFESI and to review the history of injections into the RSO. Study Design: Observational study and original research. Setting: This work was conducted at Jeju National University School of Medicine, Jeju, Republic of Korea. Methods: A total of 5,429 lumbar TFESIs performed from the September 1, 2018 through October 31, 2021 were analyzed for unintentional RSO injections using fluoroscopic -guided contrast medium patterns. Results: The rate of unintentional injection into the RSO was 0.20% (11 incidents). Contrast medium patterns in the RSO had a sigmoid or ovoid shape confined to the affected facet joint, or a butterfly -shaped pattern extending into the contralateral facet joint, but rarely extending beyond the upper or lower level. Limitation: The rarity of unintentional injection into the RSO prevented a randomized controlled study design. Conclusions: Careful fluoroscopic examination of contrast medium patterns during lumbar TFESI is crucial to identify needle placement in the RSO. If detected, the procedure can be corrected by slightly advancing the needle into the foramen.