Potential S1 Nerve Root Blocks Associated with Sacroiliac Joint Injections.

Background: Sacroiliac (SI) joint dysfunction is a common cause of lower back pain. The diagnosis of SI joint pain remains challenging. Sacroiliac joint injection remains the gold standard of diagnosis of SI joint pain as well as providing therapeutic effect. One complication related to SI joint injection is temporary numbness and weakness of the leg. Objectives: To evaluate the anatomy of the SI joint and the flow of the contrast in the sacroiliac joint and to understand how local anesthetic can affect the nerve roots and cause temporary weakness and numbness of the leg. Methods: Patients who underwent SI joint injection with three-dimensional cone beam computed tomography with fluoroscopy (3D-CBCT) imaging were identified through retrospective review of two providers' case log from the electronic medical record. The cone beam CT images were reviewed to study the contrast spread and flow in the SI joint. Results: 27/32 patients with the mean age of 56 years (range 39-87 years), 20 females, and 7 males were included in this study. After reviewing cone beam CT images, 4/27 (14.8%) patients showed contrast spread in the SI joint and spread into the S1 posterior neuroforamen. The remainder 23/27 (85.2%) patients had contrast localized in the SI joint. Conclusion: Our results indicate that the injection of lower concentration of local anesthetic with less volume may be necessary to decrease the risk of S1 nerve root block and epidural block. Furthermore, to improve the specificity of a diagnostic SI injection, an appropriate evaluation should be considered to rule out any S1 nerve pathology as a significant pain generator.