- •
A high-frequency transducer is preferred for this block.
- •
This approach is the most effective and easiest one for saphenous nerve block.
- •
It can be used in lieu of femoral nerve block after total knee arthroplasty to avoid quadriceps muscle weakness.
- •
It can be used as a block for the saphenous nerve after surgeries in the medial side of the foot and the ankle.
Sonoanatomy
The saphenous nerve, a terminal branch of the posterior division of the femoral nerve, provides sensory innervation to the medial, anteromedial, and posteromedial aspects of the lower extremity from the distal thigh to the medial malleolus. It travels along the lateral aspect of the superficial femoral artery in the proximal artery within the adductor canal (Hunter’s canal). It then crosses over the superficial femoral artery anteriorly just proximal of the lower end of the adductor magnus muscle and runs medially alongside the superficial femoral artery until emerging from the canal with the saphenous branch of the descending genicular artery. After leaving the adductor canal, the saphenous nerve divides into the infrapatellar branch, which provides a sensory branch to the peripatellar plexus of the knee, and the sartorial branch, which perforates the superficial fascia between the gracilis and sartorius muscles and emerges to lie in the subcutaneous tissue below the knee fold. It then descends along the medial tibial border with the saphenous vein giving cutaneous branches to the medial aspect of the leg, ankle, and the forefoot. The nerve to the vastus medialis is also a branch of the posterior division of the femoral nerve. It travels lateral to the superficial femoral artery within the adductor canal sending multiple branches to the vastus medialis, and supplies the anteromedial portion of the knee capsule.
The adductor canal is an aponeurotic tunnel in the middle third of the thigh. It courses between the anterior–medial compartment of the thigh and is covered by strong aponeurosis, the vastoadductor membrane. The canal contains the superficial femoral artery, vein, saphenous nerve, nerve to the vastus medialis, and the terminal nerve endings of the posterior branch of the obturator nerve.
The short-axis ultrasound image of the adductor canal at the midthigh usually shows the sartorius muscle and the saphenous nerve as a hyperechoic structure, which lies lateral to the artery and anterior to the vein. The vastus medialis muscle lies laterally to the saphenous nerve, and the adductor longus and adductor magnus muscles are on its medial side ( Figs. 20.1 and 20.2 ).
