The results of the interim analysis showed significant results.

Caudal epidural injections have been commonly performed in patients with low back pain and radiculopathy. Conventional caudal epidural injections, which the needle is advanced into the sacral canal, present a potential risk of penetration of the epidural venous plexus or dura. The investigators hypothesized that a new caudal injection technique, which the needle only penetrates the sacrococcygeal ligament without being inserted into the sacral canal, might represent a safe alternative, with a lower incidence of intravascular injections and patient's discomfort during the procedure than the conventional technique. The study is designed to investigate the influence of the depth of the inserted needle on successful epidurogram and clinical outcome in caudal epidural injections under the ultrasound and digital subtraction angiography.

Inclusion Criteria:

• Adult patients (20-80 years of age) who were scheduled to receive caudal epidural injection for lumbosacral radicular pain at our pain management clinic

Exclusion Criteria:

• pregnancy
• coagulopathy
• systemic infection
• any active infection at the injection site
• history of allergy to contrast media, local anesthetics, corticosteroid
• patients unable to communicate or patients with cognitive dysfunction