Soft Tissue Depression at the iliac crest prominence: a new landmark for identifying L4-L5 Interspace.

BACKGROUND: One of the most common approaches to identifying the L4-L5 interspace is using the iliac crest as a landmark. We propose a new landmark to identify the L4-L5 interspace based on the soft tissue depression palpable at the iliac crest prominence. The aim of this study was to assess the reliability and time saving when using this new landmark compared to using the iliac crest to perform a lumbar plexus block. METHODS: Fifty-four patients scheduled for lower limb surgery were randomly allocated to have a lumbar plexus block performed using the iliac crest (Chayen's approach) or the soft tissue depression (Borghi's approach). The landmarks for both approaches were drawn on each patient prior to randomization (N.=27 per group). All the blocks were performed by an anesthesiologist familiar with both techniques using a nerve stimulator and 30 mL of 0.5% levobupivacaine. The time to achieve successful needle placement and the number of needle re-directions, as well as the onset time for the sensory and motor blockade, were recorded. RESULTS: All the blocks using Borghi's approach were performed successfully. With the Chayen's approach, there were 5 needle placement failures. The mean times to onset of a successful block after injection of the local anesthetic did not differ between the two groups: 17.8±3.9 min for the Chayen vs. 15.9±2.4 min for the Borghi's approach (P=0.14). However, the mean time to achieve correct needle placement was 7.6±3.2 min with the Chayen's approach compared to 5.1 (±2.6 SD) min with the Borghi's approach (P<0.01). The Chayen's approach also required a significantly higher median number of needle redirections (2 [inter-quartile range (IQR): 0-4] vs. 0 [IQR: 0-4], P<0.01). In obese patients (BMI ≥30 kg/m(2)), the mean placement time was 10.5±1.7 min vs. 4.8±2.1 min (P