◊ BACKGROUND:
Although the anterior approach to the sciatic nerve block has rarely been performed due to lack of reliable surface anatomical landmarks and technical difficulty, ultrasound guidance may make performance of this approach easier. In this study, we evaluated the clinical use of the ultrasound-guided anterior approach to sciatic nerve block and compared this approach with the posterior approach in adults.
◊ METHODS:
One hundred patients undergoing minor knee surgery were randomly divided into two groups to receive anterior and posterior (subgluteal) approaches to sciatic nerve block, using 1.5% mepivacaine 20 mL with epinephrine combined with femoral and lateral femoral cutaneous nerve blocks. Both approaches to sciatic nerve block were performed using a low-frequency, 5 to 2 MHz, curved array transducer. Measurements included block execution time, depth and size of the nerve, needle depth, onset time of sensory and motor blockade, and duration of the block.
◊ RESULTS:
The anterior approach to sciatic nerve block is performed as easily and successfully as the posterior approach using ultrasound guidance.
◊ CONCLUSIONS:
Our new ultrasound-guided technique has the potential to improve safety and efficacy of the procedure by direct visualization of the needle placement and the distribution of the injected fluid. Furthermore, the precise injection near the optic nerve could lead to a reduction of the amount of the local anaesthetic needed with fewer related complications.
◊ Reviewed by P. Marhofer, MD
Professor, Dept. of Anaesthesia and Intensive Care Medicine,
Medical University of Vienna, Vienna, Austria
This study compared two techniques of sciatic nerve blockade. Both the anterior and the proximal posterior approaches were performed under ultrasonographic guidance and nerve stimulation with similar block qualities and success rates. The descriptions of ultrasonographic identification of the sciatic nerve from anterior and posterior are comprehensible. Ultrasonographic identification of the anterior approach to the sciatic nerve was performed with a 5-2 MHz sector ultrasound probe, which is correct due to the expected depth of the nerve. The choice of a sector probe for identification of the sciatic nerve from posterior and below the gluteal fold can be discussed because the nerve structure is more superficial (3.4 cm median depth) and therefore a high-frequent linear probe would also be appropriate. Beside this slight concern the methods section is appropriate.
I appreciate the honesty of the authors in mentioning that not all ultrasound scans resulted in a straight-forward identification of the sciatic nerve. A percentage of 95 is reliable and reflects clinical practice, even of those who are most experienced with ultrasonographic guided regional anaesthetic techniques. Nevertheless, I would expect that patients with “invisible” sciatic nerves show an increased body mass index. Contrary to this assumption, those patients in this study showed body mass indices from 20.0 – 26.7 kg/m2. One possible explanation for problems to visualize the sciatic nerve might be the high anisotropy of this nerve, which requires an exact angulation of the ultrasound probe. Another explanation could be the low frequent sector probe which was used in the current publication. Once again, in particular for the posterior approach, a higher frequent linear probe should be preferred.
An important note should be made regarding the anterior approach to the sciatic nerve. When the in-plane needle guidance technique is used we should always be aware that the femoral artery can be in-between the puncture site and the nerve and the needle path should be adapted accordingly in order to avoid damage of the artery.
Overall, the paper by Ota et al. is well written, however, there are some questionable aspects. The authors mention that they used a sterile cover for the ultrasound probe, but figures show ultrasound probes without cover. Sterile working is an absolute prerequisite for ultrasonographic guided block techniques. I do not understand the routine use of a nerve stimulator either. The authors call themselves experienced with ultrasonography and a nerve stimulator should only be used as a “trial-and-error” technique in cases of difficult nerve visualization. There are no controlled trials showing the additional use of a nerve stimulator to be superior in terms of block qualities and success rates.
Apart from using epinephrine (which may cause nerve ischemia), my main point of criticism is that the authors used a single-injection technique with a fixed volume of local anaesthetic. The sciatic nerve is such a huge structure that due to my clinical experience a multi-injection technique is superior in terms of block success and reduction of the volume of local anaesthetic.
In summary, the ultrasonographic guided anterior approach to the sciatic nerve is an interesting option. Due to the depth of the puncture it should be limited to a small spectrum of indications (e.g. when supine positioning is required in cases of severe spine injury). |
