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Department of Anesthesia, Orthopedic University Clinic of Zurich/Balgrist, Zurich, Switzerland
To the Editor:
We have some concerns regarding the methodology of the Casati et al. (1) study. At the level of the inguinal ligament, the femoral nerve splits into its three main branches, which then separate at the level of the inguinal crease (2). We agree with the authors that the multiple injection technique at this level permits to localize and successfully block each branch separately with a constant volume of local anesthetics. In our opinion, however, the minimum volume needed to block the femoral nerve and its main branches will depend on the exact positioning of the tip of the needle, which can elicit the same motor response despite being placed either slightly median or lateral of the nerve. Even with a standardized technique as used in this study (localization of the middle branch: vastus intermedius nerve), we believe that the position of the needle may vary from the theoretical midpoint of the three nerves, which may influence the spread of the injected local anesthetics and consequently the quality as well as the onset time of the block.
To minimize this anatomical bias in the single injection group, we believe that instead of blocking the femoral nerve at the level of the inguinal crease, it should be blocked just below the inguinal ligament where the three branches are close to each other. Even at this level there may be a variation of the needle position using the same stimulating technique, but this may not have the same consequences on the quality of the nerve block.
To indisputably demonstrate the advantages of the multiple injection technique, the authors should have included in their study a third group receiving the single shot just below the inguinal ligament.
References
Department of Anesthesiology, Vita-Salute University of Milano - IRCCS San Raffaele Hospital, Milano, Italy
In Response:
We thank Dr. Ekatodramis and colleagues for their comments on our study (1). We also would like to take this opportunity to acknowledge Dr Borgeat’s group for their relevant contribution to the field of interscalene block techniques and welcome their new interest in femoral nerve block techniques.
Although everyone with expertise and interest in neuroanatomy recognizes that anatomic variations are common, according to McMinn and Hutchings (2) the femoral nerve enters the femoral triangle by passing below the inguinal ligament, which itself attaches to the pubis tubercle and the anterior superior iliac spine. The advantage of the cited anatomy reference is that this atlas displays "the parts of the body in their natural size." According to Figure A (page 296) and Figure D (page 297), the femoral nerve provides seven branches. The first three nerves (to the sartorius, rectus femoris and vastus lateralis muscles) originate 4.5 cm below the inguinal ligament and 1 cm above the attachment of the adductor longus muscle on the body of the pubis. The last four branches originate 1 cm below, and include the nerves to the vastus intermedius, vastus medialis, and adductor brevis muscles and the saphenous nerve. Although our edition was reprinted from the original 1977 edition, it is possible that the original edition may have presented an anatomical variation of the one we just described.
The femoral nerve is usually reached 2–3 cm caudal to the inguinal ligament at the level of the inguinal crease (3), and this is undoubtedly the most widely used approach. Femoral nerve blocks performed at this level using a single injection technique and eliciting the contraction of the vastus intermedius muscle with the movement of the patella have been reported to result in a 100% success rate for surgical anesthesia (3). Multiple stimulation techniques use the same anatomic landmarks as the single stimulation technique (3–7) and have been shown to increase the success rate and decrease the total volume of local anesthetic required to produce a successful blockade. Our data suggested that using a multiple stimulation technique for the femoral block produces the same benefits. Dr. Ekatodramis and colleagues need to recognize that the aim of our study was to compare the effects of a single injection technique versus a multiple injection technique on the total volume of local anesthetic required to get a complete block within 20 min with a 95% confidence interval. Such a comparison dictated that the same landmark be used. The authors’ hypothesis that using a more proximal approach to the femoral nerve may reduce the local anesthetic volume required to get a similarly successful femoral block is intriguing but theoretical. This hypothesis also contradicts previous reports demonstrating in anatomical models that needle insertion at the inguinal crease level results in the highest rate of needle-femoral nerve contacts as compared with other insertion sites (3). However, this is a matter of another new study, and until Dr. Ekatodramis and colleagues demonstrate that their new approach indeed reduces the total volume of local anesthetic required to get a complete block within 20 min with a 95% confidence interval, as compared with the approach at the inguinal crease level, the value of their comments is at the least limited.
References
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