Background and Objectives: Posterior lumbar plexus block is a well established anesthetic technique for lower limb surgeries. Both magnetic resonance imaging and clinical prospective studies were performed to verify if a new palpable landmark could be used for lumbar plexus block in pediatric patients.

Methods: Twenty-five magnetic resonance imaging exams were analyzed to verify the relationship between the ipsilateral lumbar plexus and the sulcus formed by the division between the longissimus and the iliocostal muscles. The mean angle between the point of needle introduction and the lumbar plexus was measured. In a prospective series, 100 consecutive patients (1 to 15 years old) were given posterior lumbar plexus block. Under general anesthesia, the first palpable depression found by sliding the finger laterally at the L4 spine level was used as a landmark. A needle was introduced perpendicularly at the point where this sulcus crossed the line connecting the iliac crests. The ease of sulcus palpation, time spent in the block procedure, and postoperative pain score at 3 time points (M1, awake; M2, discharge from the postanesthesia care unit; M3, the fifth postoperative hour) were registered.

Results: The mean angle found in 25 magnetic resonance images was 5.6 degrees. Clinical data from 99 patients were analyzed. The intramuscular groove was easily palpable in 93% of the patients. More than 95% of the blocks were completed in less than 5 minutes. The median pain scores were 0 for M1/M2 and 1 for M3. The block was effective in 97% of patients (95% confidence interval, 93.6%-100%). There was 1 case of subarachnoid puncture (95% confidence interval, 0%-3%).

Conclusions: The sulcus formed by the division between the longissimus and the iliocostal muscles is a simple, direct, and easy-to-identify landmark for posterior lumbar plexus block in children.