Optimal location of local anesthetic injection in the interspace between the popliteal artery and posterior capsule of the knee (iPACK) for posterior knee pain after total knee arthroplasty: an anatomical and clinical study.

Background: This study aimed to determine the optimal location of local anesthetic injection in the interspace between the popliteal artery and posterior capsule of the knee (iPACK), using the anatomical pattern of the articular branch of tibial nerve (ABTN). We hypothesized that injection at the level of ABTN forming a popliteal plexus would mainly spread throughout the popliteal fossa without contacting the tibial or peroneal nerves.

Methods: The anatomical study included 30 soft cadavers. Ultrasound-guided dye injection was performed in legs of 10 cadavers after identifying the position of the ABTN and surrounding structures, followed by dissection to assess its spread. Clinical study was conducted in 15 patients undergoing total knee arthroplasty (TKA) with ultrasound-guided injection in the iPACK. All patients also received continuous adductor canal block. Sensorimotor function of the tibial and common peroneal nerves was determined.

Results: In the distal portion of the popliteal fossa, the tibial nerve and popliteal vessels ran superficially and closely together. The trajectory of ABTN ran lateral to the popliteal vasculature, forming a plexus towards the posterior capsule of the knee below the medial side of the upper edge of lateral femoral condyle. In cadavers, the ABTN and surrounding area of the popliteal plexus were stained with dye after injection. In the clinical study, no patients experienced complete motor or sensory blocks.

Conclusions: We described a modified iPACK technique injection at the level of the ABTN forming the popliteal plexus, and it may constitute an optional anesthetic regimen to promote early ambulation following TKA.