Total knee replacement (TKR) is a procedure associated with severe pain.1 Interest is growing in motor sparing regional anesthetic techniques to enhance early rehabilitation.2 The recently described iPACK block for posterior knee pain relief involves ultrasound guided local anesthetic infiltration of the interspace between the popliteal artery and the posterior knee capsule.3 It is believed that this block targets only the terminal nerve branches involved in posterior knee pain, without causing motor block.4 This cadaveric study aims to evaluate dye spread distribution in the popliteal fossa and the nerves affected following an iPACK block injection.
Materials and methods
After IRB approval, 6 lightly-embalmed cadaveric specimens were injected with 10 mL of a methylene blue solution, using the ultrasound-guided iPACK block technique originally described by Dr. Sanjay Sinha. With the thigh in neutral position, a 22-G 8-cm echogenic block needle was inserted into the medial aspect of the knee approximately 1 finger breadth above the patella under ultrasound guidance using a curved 2-5 MHz probe (Figure 1). A fine guide wire was passed into the popliteal artery, the movement of which helped identify the artery’s location in the ultrasound image. The needle was advanced in plane to the level of the artery. Dye was injected in 1 mL increments while the needle was gradually withdrawn. During dissection, the sciatic, common peroneal (fibular) and tibial nerves and their articular branches, posterior branch of the obturator nerve, and superior medial and superior lateral genicular nerves were identified. In each specimen, nerves stained with methylene blue and staining frequency were recorded (Figure 2).
In all specimens, posterior branch of the obturator nerve and superior medial genicular nerve were stained. However, superior lateral genicular nerve was stained in 67% of specimens, the main common fibular nerve in 67%, and the main tibial nerve in 33%. Articular branches of the tibial nerve were found in all specimens with a 50% (n=3) stain rate. Articular branches of the common fibular nerve were found in 4 of 6 specimens with staining occurring only in 2. In all specimens, dye was found anterior to the popliteal vessels with spreading through the adductor hiatus into the anterior compartment of the thigh, between the vastus medialis muscle and the surface of the femur. Lateral dye spread did not extend into the anterior compartment.
Our preliminary data suggest that an iPACK block injection can reach both the articular branches of the sciatic nerve and posterior branch of the obturator nerve, as well as the superior medial genicular nerve. It also reaches the adductor canal through back diffusion through the adductor hiatus. This block potentially provides relief for both posterior and anterior knee pain but the clinical relevance requires evaluation in future studies.
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