Ultrasound Guided Sub-Sartorial Saphenous Nerve Block
Overview of the Block
The saphenous nerve is a sensory only branch of the femoral nerve which covers the medial portion of the lower leg to the medial malleolus. It is most commonly used in combination with a popliteal sciatic or lower anterior sciatic block for procedures and injuries of the mid-lower leg and ankle.
The saphenous nerve branches from the femoral nerve fairly high in the thigh and then travels with the superficial femoral artery in the “adductor tunnel” between the medial border of the vastus medialis muscle and the medial border of the adductor muscles. This pathway runs from lateral to medial beneath the course of the sartorius muscle.
At the point in the distal thigh where the superficial femoral artery changes course to a deeper plane to become the popliteal artery, the saphenous nerve follows the small descending genicular artery to emerge from between the “adductor tunnel” to eventually diverge from the artery and become subcutaneous. Once the saphenous nerve becomes subcutaneous it follows the saphenous vein along the medial side of the knee to the lower leg.
In this version of the saphenous nerve block, we will locate the nerve before it becomes subcutaneous, while it is still partnered with the easy-to-find descending genicular artery, and block it there.
Patient Position & Technique
Place the patient in the supine position with the knee slightly flexed and the leg rotated externally enough to expose the distal inner thigh.
Prep the inner thigh from just above mid-thigh to just below the knee, and from lateral of the midline to the mattress.
The target structures can usually be seen with a high frequency, linear array (straight face) ultrasound probe, like that used for the femoral nerve or the brachial plexus, but on patients with larger legs it may be necessary to use a lower frequency, curved array probe to see deeper structures.
Follow the superficial femoral vessels on ultrasound as they course along the medial portion of the thigh, from proximal to distal watching the large artery. As the artery changes course to become deeper and go posterior to the distal femur, a small artery will branch off and take a more superficial course. This is the descending genicular artery and with it will travel the saphenous nerve.
The needle approach can be either In-Plane or Out-Of-Plane for this block. After satisfactory needle placement and and aspiration, 8 – 15 ml of local anesthetic will create an acceptable block.