Pediatric patients undergoing caudal block for urologic surgery were included in this study.

After induction of general anesthesia, the patients were placed in the lateral position. The ultrasound evaluation include transverse and longitudinal view.

First, investigators place the transducer at the sacral cornua to obtain a transverse view.

In the transverse view, the distance between two cornua and the depth of caudal space at the sacral hiatus were measured. In the longitudinal view, the distance from the skin to the sacrococcygeal membrane and the optimal angles and the safety distance for needle insertion depending on the points of insertions were measured. From the end of dura sac to the sacral hiatus were regarded as safety distance for the needle insertion during caudal block.

First, investigators place the transducer at the sacral cornua to obtain a transverse view.

In the transverse view, the distance between two cornua and the depth of caudal space at the sacral hiatus were measured. In the longitudinal view, the distance from the skin to the sacrococcygeal membrane and the optimal angles and the safety distance for needle insertion depending on the points of insertions were measured. From the end of dura sac to the sacral hiatus were regarded as safety distance for the needle insertion during caudal block.