Anatomical Coordinates for Adductor Canal Block: Targeting Saphenous Nerve and Nerve to Vastus Medialis.

Abstract

The adductor canal block effectively targets the saphenous nerve and nerve to vastus medialis, providing pain relief after total knee arthroplasty while preserving quadriceps strength. Despite its widespread use, the ideal needle insertion point remains uncertain. This study aims to determine the optimal segment for adductor canal block using anatomical dissections and morphometric analysis. Cadaveric dissections of 30 limbs measured the morphometric data including the thigh length, adductor canal length, and distances from bony landmarks. The Saphenous nerve and nerve to vastus medialis entry at the beginning of the adductor canal, their lengths and course within it, in relation to the femoral artery and vastoadductor membrane, were measured. A single dye injection was performed in one cadaver to analyze its spread within the adductor canal. From the beginning of the adductor canal, both the saphenous nerve and nerve to vastus medialis were found together for a distance of 3.5 cm. This region is identified as the critical zone. Key findings in the critical zone include: (1) The saphenous nerve and nerve to vastus medialis were closely positioned (3.5-6.9 mm apart). (2) Absence of the vasto-adductor membrane in ~50% of specimens enhancing nerve access. (3) Both nerves were lateral to the femoral artery, possibly minimizing vascular injury risk. In relation to the bony landmarks, the adductor canal begins ~7 cm from intersection of the line across anterior superior iliac spine to adductor tubercle and the line across the mid-inguinal point to superior border of patella (28 ± 3 cm from anterior superior iliac spine). Administering anesthesia here with a spread of 3 cm proximally and distally might ensure coverage of the critical zone. The nerve was 3.5-5 cm deep, with dye injection at this site staining both nerves effectively with a spread of 5 cm. The study identifies the proximal 3.5 cm from the beginning of the adductor canal as the critical zone to target both the saphenous nerve and nerve to vastus medialis, ensuring effective adductor canal block. The aim of the cadaver study is to identify the optimal segment of the adductor canal to facilitate and improve the success of adductor canal blocks. Injecting local anaesthetic 3.5 cm from the beginning of the adductor canal is an effective way of targeting both the saphenous nerve and the nerve to the vastus medialis. Superficial landmarks and ultrasound guidance can be used to identify the injection point.