Background The innervation of the sacroiliac joint (SIJ) is complex, with a dual innervation originating from the lumbosacral plexus anteriorly as well as the sacral lateral branches posteriorly. Nociceptors are found in intra-articular structures as well as periarticular structures. In patients with SIJ pain, a fluoroscopy-guided SIJ injection is usually performed posteriorly into the bottom one-third of the joint with local anesthetic and corticosteroids, but this does not always reach all intra-articular structures. The correlation between a cranial contrast spread and clinical success is undetermined in patients with SIJ pain.Methods In a tertiary referral pain center, electronic medical records of patients who underwent an SIJ injection were retrospectively analyzed. Only patients with at least three positive provocation maneuvers for SIJ pain were selected. Contrast images of the SIJ were classified as with or without cranial spread on fluoroscopy as a marker of intra-articular injection. Clinical success was defined as >= 50% improvement in the patient's global perceived effect after 3-4 weeks. The primary outcome was defined as the correlation between cranial contrast spread and clinical success after an SIJ injection.Results 128 patients in total were included. In 68 patients (53.1%) fluoroscopy showed cranial contrast spread. Clinical success was higher in patients with cranial spread of contrast (55 of 68, 81%) versus those without (35 of 60, 58%) (p=0.0067). In a multivariable analysis with age, gender, presence of rheumatoid arthritis, side, and number of positive provocation maneuvers, the cranial spread of contrast remained the only independent factor of clinical success (p=0.006; OR 3.2, 95% CI 1.4 to 7.7).Conclusion In patients with SIJ pain, identified by positive pain provocation maneuvers, cranial contrast spread as a marker of intra-articular injection, with subsequent injection of 3 mL of local anesthetic and methylprednisolone 40 mg, was significantly correlated with clinical success up to 4 weeks. Therefore, attempts should be made to reach this final needle position before injecting local anesthetic and corticosteroids. This result needs to be confirmed in a high-quality prospective trial.