Interspace between Popliteal Artery and posterior Capsule of the Knee (IPACK) Injectate Spread: A Cadaver Study
David J. Harris, MD,* Adam D. Niesen, MD,* Christopher S. Johnson, MD,* David E. Stoike, DO,* Hugh M. Smith, MD, PhD,*
Adam K. Jacob, MD,* Adam W. Amundson, MD,* Wojciech Pawlina, MD,** and David P. Martin, MD, PhD*
*Department of Anesthesiology and Perioperative Medicine and **Department of Anatomy, Mayo Clinic, Rochester, MN
Total knee arthroplasty is associated with significant postoperative pain mediated primarily by branches of the femoral and sciatic nerves.1 Peripheral nerve blockade attenuates pain and shortens hospital length of stay following this procedure.2-7 However, traditional proximal approaches to sciatic nerve blockade result in motor weakness which may prevent early rehabilitation and mask common peroneal nerve stretch or injury.8-9 Therefore, a motor-sparing technique for posterior knee analgesia is desirable to facilitate rehabilitation while maintaining the ability to monitor for neurologic deficits.
Local anesthetic injection into the Interspace between the Popliteal Artery and the posterior Capsule of the Knee (IPACK) has been suggested to anesthetize small, articular sensory nerves while avoiding the tibial and common peroneal nerves.10 However, the ability of the IPACK block to achieve this goal remains speculative as the current literature on this block is limited to a single peer-reviewed publication with limited discussion of the relevant neuroanatomy. Therefore, the objective of this cadaveric study was to evaluate IPACK injectate spread to relevant anatomical structures in order to advance our understanding of the mechanisms, risks, and benefits of this technique. We hypothesized that injectate would spread throughout the popliteal fossa without contacting the common peroneal or tibial nerves.
After receiving Institutional Review Board Biospecimen Subcommittee approval, IPACK injection was performed on fresh frozen cadaver specimens by a regional anesthesiology fellow (DES). A sonographic image (Figure 1) of the posterior surface of the femur just proximal to the termination of the femoral condyles was obtained with a SonoSite X-Porte (Fujifilm SonoSite, Inc. Bothell, WA) and HFL38xp transducer. Colored latex solution was injected through a 20-gauge, 10 cm B.Braun (Bethlehem, PA) Stimuplex® Ultra 360® needle within 1 cm of the posterior aspect of the lateral and medial condyles (10 mL behind each condyle). Following latex polymerization, the popliteal fossa in each specimen was dissected and examined for injectate spread.
Ten fresh frozen cadaver knees were included in this study. Injection was performed on average 3.0 ± 0.5 cm proximal to the popliteal crease. Injectate was observed to spread in the popliteal fossa 6.1 ± 0.7 cm in the medial-lateral dimension, and 10.1 ± 3.1 cm in the proximal-distal dimension. In general, the medial condyle and lateral condyle injections coalesced, creating a continuous mass of latex in the popliteal fossa. Most specimens also exhibited injectate spread distal to the knee, which typically tracked caudally along the gastrocnemius muscle.
No latex was noted to be in contact with the proximal segment of the sciatic nerve, but its major branches illustrated the potential to be reached by injectate. Two specimens exhibited latex contact with the tibial nerve, as well as possible contact with the common peroneal nerve. Additionally, one specimen revealed injectate spread to the tibial nerve, and one specimen exhibited possible spread to the common peroneal nerve (Figure 2). In six specimens, injectate remained distant from both the tibial and common peroneal nerves. All specimens showed injectate coating the middle genicular artery (Figure 3) except for the one specimen with intraarticular latex (Figure 4).
This cadaver study of IPACK injection demonstrated spread throughout the popliteal fossa without proximal sciatic involvement. However, the potential for injectate to spread to the tibial or common peroneal nerve was demonstrated. In vivo studies are necessary to determine whether this finding would be associated with clinically significant weakness. Consistent coating of the middle genicular artery in latex solution suggests a potential mechanism of analgesia for the IPACK block, due to the predictable relationship between the posterior articular nerve and this artery,11 as the posterior articular nerve is a primary contributor to the popliteal plexus innervating the posterior knee.12 Further study is needed to determine the ideal site of IPACK injection that simultaneously optimizes analgesia while mitigating accompanying motor blockade.