The ultrasound-guided infiltration between the posterior of the knee capsule (PCK) and the popliteal artery (PA), commonly known as IPACK, plays a pivotal role in total knee arthroplasty when performed alongside an ultrasound-guided adductor canal block.[1,2] The precise positioning of the needle between the PKC and the PA at the level of the femoral condyles, followed by an adequate diffusion of injectate, is of utmost importance. Traditionally, the needle is inserted from medial to lateral as it courses between the PKC and the PA (Video 1).
Following institutional ethics committee approval, we investigated the distance between the PKC and the PA at the level of the femoral condyles in a sample of 20 patients, and found a mean distance of 1.2 cm. However, in eight patients (40%), the distance was less than 0.7 cm (Figure 1). This finding is crucial as a distance of less than 0.7 cm may hinder the administration of IPACK, leading to potential complications. In these eight patients, the needle could not be negotiated between the PKC and the PA, and the IPACK was deferred.
Distance between the PKC and the PA would be critical to determine the possibility of administrating an IPACK. Thus, a pre-scout scan and evaluation of the distance are of utmost importance. With a shorter distance, the diffusion of local anesthetic toward the sciatic nerve and its branches in the popliteal fossa is imminent, resulting in a possible foot drop. This small sample study stems from the two cases of foot drop that lasted for two to three weeks, following an IPACK with 30 mL 0.1% ropivacaine performed at the level of femoral condyles. In both these cases, we faced difficulty in negotiating the needle toward the target site due to the short distance between PKC and PA (0.62 cm in one patient).
In conclusion, we suggest a pre-scout scan for evaluating the distance between PKC and the PA and assessing needle track and tip positioning. If the distance is less than 0.7 cm, as in our experience, the IPACK can be abandoned at this level and can be administered at the level of the lower femoral shaft (LFS). However, it should be remembered that a cadaveric study demonstrated an anteromedial to posterolateral insertion of the needle at LFS in close approximation to the nerve to vastus medialis and the saphenous nerve.[3] An alternative technique, such as local infiltration anesthesia and injection into the posterior knee capsule by the surgeon, can be considered. Moreover, the close approximation of the vascular structures should alert the surgeon to the posterior compartment, if necessary. Besides, it may be unnecessary to inject more than 10 mL drug since a cadaveric study revealed staining of the tibial (43%, both proximal and distal approaches) and common peroneal nerves (57% proximal and 71% distal) using 10 mL dye, respectively.[4] Conversely, following an IPACK, it may be unnecessary for the surgeon to inject into the posterior capsule of the knee capsule. We reckon additional research would be needed regarding the site of needle tip positioning, concentration and volume of local anesthetic to achieve adequate block efficacy without local systemic and neurotoxicity.