Paramedian approach is more useful than midline approach when degenerative changes are encountered in the interspinous structure and when ideal positioning of the patient can not be achieved. The purpose of this study was to determine the guideline of depth from skin to subarachnoid space by midline or paramedian approach in Korean adult.

The distance from skin to subarachnoid space was measured in 47 patients recieving spinal anesthesia by midline approach and 30 patients by paramedian approach at L3-4 level.

The mean depth to subarachnoid space was 5.44±0.47 cm in midline approach. In paramedian approach by 18.0degrees of caudal and 18.5degrees of lateral angle, mean depth was 6.24±0.60 cm. There were significant correlation between depth and weight, and depth and ponderal index. In ages over 50 years old, difficult cases were significantly more in midline approach than in paramedian approach.

Measurement of factors as weight or ponderal index can be used to predict the depth. Paramedian approach is more useful than midline approach in ages over 50 years old.