A cadaveric study investigating the spread of injectate following an interspace between the popliteal artery and the capsule of the posterior knee block in a neonatal sample: a pilot study

The infiltration of the interspace between the popliteal artery and the capsule of the posterior knee (IPACK) block, is a novel ultrasound-guided technique used for postoperative pain management of the knee. The success of the block is attributed to the spread of injectate between the capsule of the knee and the popliteal artery. This novel technique is believed to target the articular branches of the tibial, common fibular (peroneal) and obturator nerves. However, the extent of the spread in a paediatric population is unknown. Therefore, this study aims to evaluate the spread of the IPACK block. Using ultrasound guidance, the IPACK block was replicated bilaterally in neonatal cadavers. Methylene blue dye (0.3 ml/kg) was injected proximally and distally. Subsequently, cadavers were dissected to determine the injectate spread. Proximal injections resulted in staining of some of the articular branches, while the distal injections resulted in staining of all four articular branches. Additional staining of the superior lateral and medial genicular nerves was noted irrespective of the technique. Overall, both injections resulted in posterior, anterolateral spread with limited medial spread. Results from this study reveal preservation of the main nerve trunks following the proximal technique, while the distal technique displayed greater staining of the articular branches. We believe that the block can be seen as a more holistic and viable alternative to lower limb blocks for the paediatric population, as it allows for a wider spread in the posterior and medial-lateral compartments of the knee.