Abstract
We investigated the mechanism of the hanging-drop sign used for identification of the epidural space in the lateral position. We found that the movement of a drop in the hub is momentory and not continuous.
Method: In 29 patiens, we advanced 17G Tuohy needle to the interspinous ligament in the second or third lumbar interspace in lateral position. Then we connected the electric pressure transducer to the needle, and measured the pressure changes through epidural puncture. We also measured pressure changes using an epidural space model made from two thin rubber sheets representing the ligamentum flavum and dura mater.
Results: There were three pressure patterns in clinical cases. In the first 19 cases, the pressure went down to the negative range momentary with the feeling of penetration of the ligamentum flavum and continued negative pressure. In the second 8 cases, at first the negative pressure was same to the first cases but there was no negative pressure after penetration of the ligamentum flavum. In the third 2 cases, there was no negative pressure. In epidural space model, there was the same pressure patterns to clinical investigations according to the distance between two rubber sheets. Momentaly negative pressure was recorded when the rubber sheet which was regarded as the ligamentum flavum was penetrated.
Discussion: We thought that the changes of epidural puncture pressure were synthesis of penetration of the ligamentum flavum and compression of the dura mater. The ligamentum flavum could be the cause of the hunging drop sign.
