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Sciatic Nerve Block

Title
Ultrasound-guided anterior approach to sciatic nerve block: a comparison with the posterior approach
Authors
Ota J, Sakura S, Hara K, Saito Y et al.
Journal
Anesthesia and Analgesia 2009 Feb; 108(2): 660-665.

 

Study Summary

 

Study design
  • Randomized controlled clinical study
Patient population
  • 100 adult patients undergoing minor knee surgery
  • Patients received a femoral + lateral femoral cutaneous nerve block, + sciatic nerve block, and sedation during surgery.
Intervention
  • Ultrasound-guided anterior sciatic nerve block
  • Curved array 2-5MHz transducer
  • In-plane approach with a 100mm 21G block needle
  • End-point for needle insertion was proximity to the nerve as visualized on ultrasound, and a minimum threshold current for motor response of 0.7mA or less.
  • End-point for injection was circumferential spread of local anesthetic solution: 20mL of 1.5% mepivacaine with 1:400,000 epinephrine
Comparator(s)
  • Ultrasound-guided posterior subgluteal sciatic nerve block
  • Curved array 2-5MHz transducer
  • In-plane approach with a 100mm 21G block needle
  • Endpoints for needle insertion and injection as above
Main findings
  • The sciatic nerve could not be identified in 5 patients (2 anterior, 3 posterior approach)
  • Recognition time and block execution time were similar for both techniques (31-36 seconds and 5-6 min respectively)
  • The average depth of the nerve was 5.9 cm and 3.4 cm for the anterior and posterior approach respectively
  • Incidence of complete sensory block was similar at 30 minutes between the 2 groups, except in the distribution of the posterior femoral cutaneous nerve (68.1% vs 14.9% in posterior and anterior groups respectively)
  • Of note, the incidence of complete sensory block of the tibial nerve at 30 minutes was only 51.1% in both groups
  • Onset and duration of block was similar between groups

 

Comments on the study

  • This study nicely illustrates the ultrasonographic anatomy of the anterior and posterior subgluteal approaches to the sciatic nerve
  • It should be noted that this study was performed in Asian patients with an average BMI of 23 kg/m2. Our experience with patients with BMI >30 kg/m2 is that identification of the sciatic nerve in the posterior gluteal region can be difficult. The depth to the nerve is also greater in these patients
  • An out-of-plane approach to the nerve may be preferable in patients with deeper and less-easily visualized nerves. Needle visibility is difficult at steep angles and increased depth. In addition, the sciatic nerve is anisotropic; the manipulation of the transducer required to maintain needle-beam alignment in the in-plane approach can result in decreased visibility of the target nerve.
  • The low rate of complete sensory block of the tibial nerve at 30 minutes is disconcerting. One explanation may be the use of only 20 ml of local anesthetic. Further studies are needed to determine the minimum effective volume for ultrasound-guided sciatic nerve blockade.
  • We agree that the ultrasound-guided anterior sciatic approach is feasible and useful, especially if turning the patient into the lateral position is to be avoided. However the authors’ finding that the posterior femoral cutaneous nerve is usually missed in this approach indicates that this is not a useful block for operations above the knee.