Targeting Safe Analgesia of the Posterior Knee Capsule.

To the Editor, We read the article "Integrating IPACK (Interspace between the Popliteal Artery and Capsule of the Posterior Knee) Block in an Enhanced Recovery after Surgery Pathway for Total Knee Arthroplasty-A Prospective Triple-Blinded Randomized Controlled Trial" by Pai et al in the Journal for Knee Surgery (2022) with great interest1. We applaud the authors in finding that when added to an enhanced recovery after surgery pathway consisting of a spinal anesthetic, adductor canal nerve catheter, and surgeon-administered periarticular joint infiltration (PAI), the IPACK block did not significantly impact postoperative opioid consumption or measured functional outcomes on postoperative day 1. Interestingly, the authors found that addition of the IPACK block did significantly decrease posterior knee pain. We found this notable as both the IPACK block and PAI target the posterior knee capsule. Innervation of the posterior knee capsule is complex and includes branches of the obturator, sciatic, common fibular, and tibial nerves2. PAI has variable effectiveness in this anatomical area3. Prior studies have shown that addition of the IPACK and adductor canal blocks to PAI provides superior analgesia and decreases opioid consumption in TKA4. This study found that the IPACK block resulted in improved posterior knee analgesia compared to PAI. This could signify that an ultrasound-guided approach may be more consistent in targeting the posterior knee capsule. The authors also astutely mention concern for local anesthetic toxicity (LAST). Peripheral nerve blocks, especially in the lower extremity, have a decreased risk of LAST compared to PAI5. Further study of the effectiveness of the IPACK block compared to PAI may show that the IPACK is a reasonable replacement of PAI, while also potentially decreasing the risk of LAST. We greatly appreciate the work Dr. Pai and team performed to further our understanding of posterior knee analgesia and its contribution toward the creation of patient care pathways that target analgesia and prioritize patient safety.