The spread of injectate during saphenous nerve block at the adductor canal: a cadaver study

Background: The spread of injectate during a saphenous nerve block at the adductor canal has not been clearly described.

Methods: We examined the spread of 15 ml dyed injectate during ultrasound-guided saphenous nerve blocks at the adductor canal in 15 unembalmed cadavers' lower limbs followed by comparative dissections of the same limbs.

Results: The spread of the injectates was determined by the fascial limits and the muscles surrounding the adductor canal. The anteromedial limit of the adductor canal (the roof) was found to be a continuous fascia, with a thin proximal part and a thicker distal part (the vastoadductor membrane) covering the canal from the apex of the femoral triangle to the adductor hiatus. The fascial limits of the adductor canal formed a conduit around the femoral neurovascular bundle. The dyed aqueous injectate spread throughout the entire adductor canal to the femoral triangle and reached 1-2 cm into the popliteal fossa. Injections superficial to the adductor canal spread over the femoral artery within the subsartorial fat compartment resembling the injections within the canal but with ultrasonographic distinct features. These injections spread only half the length of the adductor canal. The only nerve observed within the adductor canal was the saphenous nerve.

Conclusions: Injection of 15 ml dye was sufficient to spread throughout the adductor canal and beyond both proximally and distally. Distinct ultrasonographic features could be identified separating a subsartorial injection from an injection within the adductor canal with consequent differences in the spread.