• Benjamin J Walker, Sean H Flack, and Adrian T Bosenberg.
• Department of Anesthesiology and Pain Medicine, University of Washington School of Medicine, Seattle Children's Hospital, 4800 Sandpoint Way NE, W-9824, Seattle, WA 98105, USA.
• Anesth. Analg. 2011 Mar 1; 112 (3): 661-5.

BackgroundLumbar plexus block provides effective analgesia for hip, thigh, and knee surgery. A simple measurement that accurately predicts lumbar plexus depth would be invaluable for clinicians performing this block in children, in whom plexus depth varies with age.MethodsSurface anatomy measurements were taken on children having lower extremity surgery and lumbar plexus block. All blocks were placed under general anesthesia using peripheral nerve stimulation. The distance from the posterior superior iliac spine (PSIS) to the intercristal line (ICL), i.e., PSIS-ICL distance, and from midline to the intersection of a perpendicular line drawn from the PSIS and the ICL were measured. Lumbar plexus depth was recorded at the point at which maximum quadriceps stimulation was elicited using the lowest current output. Linear regression was used to explore the least squares line of best fit for each measure.ResultsMeasurements were made on 350 consecutive patients aged 1 month to 24 years. A very strong linear relationship between lumbar plexus depth and PSIS-ICL distance was noted. The median (interquartile range) absolute difference between observed lumbar plexus depth and that predicted by PSIS-ICL distance was 2 mm (1-5 mm), 95% CI for median = 1.36 to 2.64. Ninety-two percent of patients (95% CI, 88.7%-94.6%) had lumbar plexus depths within ±10 mm of the predicted depth. The strongest correlation to lumbar plexus depth was found with PSIS-ICL distance (R(2) = 0.89, P < 0.0001). Weaker correlations were found for weight, height, body mass index, midline-PSIS line distance, and age.ConclusionPSIS-ICL distance provides an accurate, patient-specific predictor for lumbar plexus depth in children over a wide range of age and body habitus. The strong linear relationship obviates the need for complex calculations. This measurement can be used as a guide for ultrasound location, to choose an appropriate needle length, and may reduce complications associated with this block.

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