# The Transverse Process as a Landmark for Estimating Dural Sac Depth and Feasible Planes for Optimized Paramedian Needle Insertions

## TL;DR

This study shows how the transverse process can help estimate spinal needle insertion depth and angle for successful anesthesia when midline access is not possible.

## Contribution

The study introduces the transverse process as a reliable anatomical landmark for paramedian spinal anesthesia in elderly patients.

## Key findings

- The transverse process aligns with specific zones of the dural sac at different lumbar levels.
- Midline needle insertion was not viable in 20–30% of cases, but paramedian access was possible.
- The vertical range of viable paramedian planes was 8.7 ± 2.9 mm at L4–L5 and 7.9 ± 1.9 mm at L3–L4.

## Abstract

Background: The skin-to-transverse process distance (st) correlates with the skin-to-dural sac depth (d) and may be used to estimate optimal angles for perpendicular needle insertion using the formula inverse cosine d/√(1 + d2), as outlined in free visual guides. Objective: We aimed to analyze the relationship between the transverse process and dural sac depth at lumbar levels relevant to spinal anesthesia and to determine the range of planes where perpendicular paramedian needle insertion is feasible when midline access is not viable. Methods: Ten ex vivo trunks were flexed using an abdominal support, and CT scans were performed. Correlations between the transverse process and dural sac depth were evaluated from L3 to S1. Perpendicular planes at the level of needle paths were examined at L3–L4 and L4–L5. Median path viability was assessed. Results: The transverse process aligned with the dorsal dural sac at L3, the posterior third at L4, and the middle zone at L5 or S1. Median needle insertion was not viable in 20–30% of L4–L5 and L3–L4 levels, respectively. However, paramedian access was possible. The vertical range of viable paramedian planes was 8.7 ± 2.9 mm (L4–L5) and 7.9 ± 1.9 mm (L3–L4). Coronal reconstructions showed that the upper level of the transverse process correlates with the skin-perpendicular planes where insertion is likely to succeed. Conclusion: Many elderly spines lack viable midline paths. The superior aspect of the transverse process serves as a useful landmark for estimating dural sac depth, calculating paramedian angles, and identifying the plane for successful perpendicular needle insertion.