This study was conducted to investigate the optimal angle of needle insertion during caudal epidural injection in chronic low back pain patients using ultrasound imaging.

One hundred eight patients (40 male and 68 female patients) with low back pain and sciatica were studied. Soft tissue ultrasonography was performed to identify the sacral hiatus. The optimal angle of the needle to the skin was measured with an imaginary line drawn parallel to the sacral base using a protractor on a longitudinal plane. A 22-gauge caudal epidural needle was inserted and was guided by ultrasound to the sacral hiatus and into the caudal epidural space.

The mean ± SD for the intercornual distance, depth of the caudal space and the thickness of the sacrococcygeal membrane were 19.0 ± 3.2 mm, 3.6 ± 0.9 mm and 1.8 ± 0.8 mm, respectively. The optimal angle showed a significant correlation with the depth of the caudal space and the thickness of the sacrococcygeal membrane. The mean ± SD for the optimal angle of the needle insertion was 23.5 ± 6.9 degrees.

We conclude that the needle should be inserted at an angle of approximately 23.5 degrees to the skin in order to avoid injury to the periosteum and an inadvertent intra-osseous injection.