Letters to the Editor

fossa. The obturator nerve pierces the oblique posterior ligament of the knee and supplies the posteromedial capsule of the knee joint and contributes to the innervation of the cruciate ligaments. As Jæger et al1 correctly point out, local anesthetic injected in the AC can be speculated to spread distally inside the canal and block the saphenous nerve as well as the posterior branch of the obturator nerve (if present in the canal). Local anesthetic theoretically might spread as far as the distal part of the AC with the approach described by the authors— reportedly demonstrated with magnetic resonance imaging in only 1 patient.4 Unfortunately, the referred solitary T2-weighted magnetic resonance image does not verify spread of liquid in the AC. Both water and fat are bright on T2weighted images without fat suppression, and the bright signal present in the AC has the same intensity as the fat signal seen around the other vessels and nerves in the image. Consistent distal local anesthetic spread in the AC will most probably require an injection within the AC and not in the femoral triangle as performed by the authors. The injection technique described in the present and the previous ACB studies is more correctly termed a “subsartorial volume block in the femoral triangle” because the position halfway between the base of the patella and the ASIS is well inside the femoral triangle and certainly proximal to the entrance of the AC. To use the term “adductor canal block” with this needle entry point is a misnomer. To locate the AC correctly, we suggest identification of sonographic internal landmarks (VAM, sartorius muscle, adductor longus muscle, vastus medialis muscle, and the femoral vessels) and not based on external surface landmarks and measurements. If the goal of the ACB volume block is to anesthetize the saphenous nerve and the posterior branch of the obturator nerve, then the preferred needle entry point is the distal thigh within the AC where the femoral artery is sonographically seen to descend posteriorly toward the adductor hiatus, as described by Manickam et al5 in 2009. In our opinion, rather than using a large volume of local anesthetic to fill the entire AC, it is more logical to use the subsartorial approach to selectively anesthetize the saphenous nerve in the femoral triangle with a small local anesthetic volume (∼5 mL) and to selectively block the posterior branch of the obturator nerve with an equally small local anesthetic volume in the interfascial plane between the adductor brevis and magnus muscles in the

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subinguinal region.6 Further studies are warranted to compare the usefulness of combined selective blockade of the saphenous nerve subsartorially and the posterior branch of the obturator nerve with the ACB for total knee arthroplasty.

Thomas Fichtner Bendtsen, MD, PhD Department of Anesthesia and Intensive Care Medicine Aarhus University Hospital Aarhus, Denmark

Bernhard Moriggl, MD, PhD Division of Clinical and Functional Anatomy, Department of Anatomy Histology and Embryology Innsbruck Medical University Innsbruck, Austria

Vincent Chan, MD Department of Anesthesia Toronto Western Hospital University of Toronto Toronto, Ontario, Canada

Erik Morre Pedersen, MD, DMSc Department of Radiology Aarhus University Hospital Aarhus, Denmark

Jens Børglum, MD, PhD Department of Anesthesia and Intensive Care Medicine Copenhagen University Hospital Bispebjerg, Denmark

The authors declare no conflict of interest. REFERENCES 1. Jæger P, Zaric D, Fomsgaard JS, et al. Adductor canal block versus femoral nerve block for analgesia after total knee arthroplasty. A randomized, double-blind study. Reg Anesth Pain Med. 2013;38: 526–532. 2. Jæger P, Grevstad U, Henningsen MH, Gottschau B, Mathiesen O, Dahl JB. Effect of adductor canal blockade on established, severe post-operative pain after total knee arthroplasty: a randomised study. Acta Anaesthesiol Scand. 2012;56: 1013–1019. 3. Jenstrup MT, Jæger P, Lund J, et al. Effects of adductor canal blockade on pain and ambulation after total knee arthroplasty: a randomized study. Acta Anaesthesiol Scand. 2012;56:357–364. 4. Lund J, Jenstrup MT, Jæger P, Sorensen AM, Dahl JB. Continuous adductor canal blockade for adjuvant post-operative analgesia after major knee surgery: preliminary results. Acta Anaesthesiol Scand. 2011;55:14–19.

5. Manickam B, Perlas A, Duggan E, Brull R, Chan VW, Ramlogan R. Feasibility and efficacy of ultrasound-guided block of the saphenous nerve in the adductor canal. Reg Anesth Pain Med. 2009;34:578–580. 6. Soong J, Schafhalter-Zoppoth I, Gray AT. Sonographic imaging of the obturator nerve for regional block. Reg Anesth Pain Med. 2007;32:146–151.

Reply to Dr Bendtsen Accepted for Publication: February 3, 2014. To the Editor: e would like to thank our colleagues1 for their interest in our randomized trial2 and especially in our preliminary report of the adductor canal block published in 2011,3 and we appreciate the important questions they raise regarding the anatomical basis of the adductor canal block. Although the issues our colleagues address certainly have an academic interest, it is our belief that they may have limited clinical relevance. Whether the technique is most correctly named an adductor canal block has no impact on the result of our randomized study—that by depositing local anesthetics in the proximity of the femoral artery at the midthigh level with a subsartorial approach—we demonstrated preserved quadriceps strength with an analgesic effect similar to that of the femoral nerve block.2 To address their first set of concerns, we respectfully disagree with their evaluation that “…it is an anatomical matter of fact that the needle insertion point defined above is within the femoral triangle proximal to the adductor canal.” Our colleagues continue with a thorough, but not referenced, anatomical description of the adductor canal and femoral triangle. However, there is nothing in that description contradicting that our needle tip is within the adductor canal. The adductor canal is an aponeurotic tunnel containing the femoral vessels in the middle third of the thigh.4 It runs from the apex of the femoral triangle, which is situated in the proximal third of the thigh.4 A needle insertion in the midpoint of the thigh would per definition be within the adductor canal, rather than in the femoral triangle. This is supported by the boundaries of the 2 regions; the adductor canal is roofed by the sartorius muscle and the underlying fascia, whereas the femoral triangle has no muscular roof.4 Furthermore, the lateral boundary of the femoral triangle is the medial margin of the sartorius muscle.4 This means that any block being

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Regional Anesthesia and Pain Medicine • Volume 39, Number 3, May-June 2014

placed subsartorially is per definition not in the femoral triangle, but must be lateral or distal to it. We do not claim to be experts in anatomy, but injecting local anesthetics subsartorially, within the aponeurotic space containing the femoral artery in the middle part of the thigh, is both at a level as well as within the anatomic boundaries of the adductor canal, but not within those of the femoral triangle. Of course, these are only theoretical arguments; more convincing may be the clinical implication: By placing the block at the midthigh level with a subsartorial approach, there is no clinically relevant muscle weakness.5 On the contrary, a femoral nerve block applied with a subinguinal approach is definitely in the femoral triangle, and as we know, leads to quadriceps weakness. Considering the referred “solitary T2 weighted MRI image” presented in the preliminary report,3 it is true that both fat and water are bright on T2-weighted images, and we concur that a T2-weighted sequence should be combined with other sequences when evaluating spread of liquid. Unfortunately, only the T2-weighted sequence (referring to the published figure) is described in our article,3 but 3 different sequences were actually performed, namely, T1 weighted, T2 weighted, and turbo inversion recovery magnitude. We admit that this information should have been included in the article. In Figure 1, we present another MR image, from another subject, where we have included

the contralateral leg as a reference. This image demonstrates that even in a T2weighted sequence, where both water and fat seem bright, the injectate can easily be localized. Of note, we do agree with our colleagues that 1 solitary MRI scan does not verify consistent spread of liquid in the adductor canal; this, of course, needs to be verified in a larger cohort. Regarding our colleagues’ opinion that a combined selective blockade of the saphenous nerve and the posterior branch of the obturator nerve would be “more logical,” we believe that the advantages of the ACB are that it only involves 1 injection and is fairly simple to perform. After any subsequent injection, there is an increased risk of infection, nerve injury, and penetration of blood vessels. Furthermore, the posterior obturator block is technically more complicated. Although Bendtsen and the coauthors master these blocks to perfection, a posterior obturator block may be a bigger challenge for less experienced colleagues, with the inherent risk of lowering the success rate. In our opinion, for the proposed combined selective blockade technique to be relevant, these disadvantages would need to be outweighed by clinically relevant and superior analgesia compared with the adductor canal block. We will welcome a randomized study investigating this, but the primary aim of our research has been to investigate a motor-sparing alternative to the femoral block.

Letters to the Editor

Finally, we would like to congratulate Dr Bendtsen and coauthors on their recently published article in Regional Anesthesia and Pain Medicine,6 and we appreciate their description of a saphenous nerve block performed “at the level of the adductor canal in the midthigh.” Pia Jæger, MD Department of Anaesthesia Centre of Head and Orthopaedics Copenhagen University Hospital Rigshospitalet, Copenhagen Denmark

Jørgen Lund, MD Morten T. Jenstrup, MD Department of Anaesthesia Aleris-Hamlet Hospitals Copenhagen Søborg, Denmark

Vibeke Brøndum, MD Department of Radiology Aleris-Hamlet Hospitals Copenhagen Søborg, Denmark

Jørgen B. Dahl, MD, DMSc, MBAex Department of Anaesthesia Centre of Head and Orthopaedics Copenhagen University Hospital Rigshospitalet, Copenhagen Denmark

The authors declare no conflict of interest. REFERENCES 1. Bendtsen TF, Moriggl B, Chan V, et al. Defining Adductor Canal Block. Reg Anesth Pain Med. 2014;39:253–254. 2. Jæger P, Zaric D, Fomsgaard JS, et al. Adductor canal block versus femoral nerve block for analgesia after total knee arthroplasty: a randomized, double-blind study. Reg Anesth Pain Med. 2013;38:526–532. 3. Lund J, Jenstrup MT, Jæger P, Sorensen AM, Dahl JB. Continuous adductor-canal-blockade for adjuvant post-operative analgesia after major knee surgery: preliminary results. Acta Anaesthesiol Scand. 2011;55:14–19. 4. Standring S. Gray’s Anatomy: The Anatomical Basis of Clinical Practice. Edinburgh, Scotland: Elsevier Churchill Livingstone; 2005.

FIGURE 1. T2-weighted cross-sectional MR image of the adductor canal. Arrow points to the adductor canal; bounded anterolaterally by vastus medialis, posteriorly by adductor longus, and anteriorly by sartorius. After injection of 30 mL of local anesthetic subsartorially at the midthigh, the adductor canal is seen to be filled with fluid. Localization of injectate was verified by a T1-weighted and a turbo inversion recovery magnitude sequence. ALM indicates adductor longus muscle; SM, sartorius muscle; VMM, vastus medialis muscle. © 2014 American Society of Regional Anesthesia and Pain Medicine

5. Jaeger P, Nielsen ZJ, Henningsen MH, Hilsted KL, Mathiesen O, Dahl JB. Adductor canal block versus femoral nerve block and quadriceps strength: a randomized, double-blind, placebo-controlled, crossover study in healthy volunteers. Anesthesiology. 2013;118:409–415. 6. Borglum J, Johansen K, Christensen MD, et al. Ultrasound-guided single-penetration dual-injection block for leg and foot surgery: a prospective, randomized, double-blind study. Reg Anesth Pain Med. 2014;39:18–25.

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