Safety margin for needle placement during lumbar plexus block: An anatomical study using magnetic resonance imaging

We aimed to provide anatomic characteristics of the lumbar plexus and a safety margin for needle placement during lumbar plexus block in adults.

Lumbar spine magnetic resonance images from 377 adults were reviewed. We determined the depth of the lumbar plexus from the needle insertion point for the modified traditional and Capdevila's approaches at the L4 and L5 levels. The relationship of age, height, and body weight with lumbar plexus depth, and the presence of the kidney on the transverse plane at the L4 level were assessed.

The lumbar plexus was deeper at the L5 level than at the L4 level for both approaches (mean difference 3.2 mm [95% confidence interval (CI), 2.4 to 4.0] for the modified traditional approach; mean difference 4.4 mm [95% CI, 3.7 to 5.2] for Capdevila's approach). Eighty-six (22.8%) patients had an L4 transverse process shorter than 40 mm, which implies that the needle may not contact the L4 transverse process with the modified traditional approach. The mean (standard deviation) of safety margins for needle insertion, defined as the distance from the posterior aspect of the L4 transverse process to the anterior margin of the psoas muscle, were 45 (8) and 44 (6) mm, respectively for the modified traditional and Capdevila's approach (mean difference, 0.5 mm; 95% CI, -0.1 to 1.1]). The lumbar plexus depth for each approach at the L4 level was predicted using the following equations: Depth (mm) = 87.24 - 0.36 × height (cm) + 0.69 × weight (kg) for the modified traditional approach (r² = 0.37) and Depth (mm) = 86.51 - 0.35 × height (cm) + 0.61 × weight (kg) for Capdevila's approach (r² = 0.33). The kidney was observed at the L4 level in 60 (15.9%) patients, with higher incidence in patients over 70 yr and those shorter than 150 cm.

The surface anatomic relations of the lumbar plexus found in this study may be useful for performing successful and safe lumbar plexus block.