Reply to Drs Johnston and Sondekoppam

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To the Editor:
We thank Drs Johnston and Sondekoppam for their reply1 to our article about the optimal block after total knee arthroplasty (TKA).2 They suggest a technique that aims to block the intermediate and medial femoral cutaneous nerves, the medial vastus nerve, the subsartorial plexus, and the saphenous nerve using 5 injections and 4 needle redirections. They start scanning from a position in the femoral triangle. They move the probe distally until the descending geniculate artery branches off the femoral artery inside the adductor canal (AC) and penetrates the vastoadductor membrane (VAM) together with the saphenous nerve. The needle insertion point is 2 cm proximal to the branching point of descending geniculate artery, which is typically near the proximal end of the VAM. The assessment of the technique cannot be evidence based because of the lack of real data.
Evidence suggests that a femoral triangle block combined with a subpectineal obturator nerve block is very efficient for analgesia after TKA.3 That requires only 2 needle insertions and no redirections of the needle.
The femoral triangle block should be performed a few centimeters proximal to the apex of the femoral triangle, which is easy to identify with ultrasound.2,4 This needle insertion point is approximately at the midpoint between the anterior superior iliac spine and the base of patella, which is practically always inside the femoral triangle.4 The femoral triangle block consistently blocks the saphenous and medial vastus nerves anterolateral to the femoral artery, as well as the medial femoral cutaneous nerve posteromedial to the femoral artery with 1 injection of 10 mL of local anesthetic.4
The femoral triangle block anesthetizes all the branches from the femoral nerve to the anteromedial knee region, where the medial parapatellar arthrotomy is performed. However, the popliteal plexus is equally important for analgesia after TKA as the femoral triangle block. The popliteal plexus is not anesthetized with the technique suggested by Drs Johnston and Sondekoppam, unless local anesthetic spreads distal from the AC into the popliteal fossa to reach the posterior branch of the obturator nerve and the popliteal plexus. That would probably require injection of local anesthetic into the distal end rather than the proximal end of the AC. The popliteal plexus entwines the popliteal vessels inside the popliteal fossa in the vicinity of the adductor hiatus. It is made by branches from the tibial nerve and the posterior branch of the obturator nerve and innervates the posterior knee capsule and intra-articular structures.2 Subpectineal blockade of the obturator nerve provides efficient analgesia after TKA when combined with a femoral triangle block, and analgesia of the contribution from the tibial nerve seems redundant.4
Drs Johnston and Sondekoppam suggest that duration of analgesia is prolonged with a catheter inserted into the AC. However, that would anesthetize only the saphenous nerve, because that is the only nerve contained inside the AC.2 In addition, perineural catheters often become displaced.5,6 Finally, the tunneling of the catheter requires extra uncomfortable penetrations of skin and subcutaneous tissue with a large-bore catheter needle. A combination of bupivacaine, epinephrine, and dexamethasone can prolong the duration of efficient analgesia of all relevant nerves after TKA and makes the use of catheter infusion redundant.4
The technique proposed by Drs Johnston and Sondekoppam does anesthetize the subsartorial plexus. However, the subsartorial plexus is made of branches from the saphenous nerve and the medial femoral cutaneous nerve, which are both anesthetized with a femoral triangle block, as well as branches from the anterior branch of the obturator nerve, which is anesthetized by a subpectineal obturator nerve block.

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