Leonard V. Bunting, M.D.
Saphenous Nerve Block -
The saphenous nerve is the largest cutaneous branch of the femoral nerve. Physicians wishing to achieve anesthesia of the medial lower leg can block the femoral nerve proximally or the saphenous nerve along its course. This section covers blockade of the saphenous nerve near the ankle.
Innervation areas of the 5 nerve blocks are shown in illustration 1-3.
The three illustrations below may also help you choose which nerves to block based on the location of the injury. As a simplified rule, two nerves innervate the volar aspect of the foot, three nerves innervate the dorsal aspect of the foot.
- Posterior tibial nerve
- Sural nerve
- Saphenous nerve
- Anterior tibial nerve (also called the deep peroneal nerve)
- Superficial peroneal nerve (also called the dorsal cutaneous or musculocutaneous nerve)
|Illustration 1: Sensory innervation of the foot, dorsal view.||Illustration 2: Sensory innervation of the foot, plantar view.||Illustration 3: Sensory innervation of the lateral view.|
The saphenous nerve divides off the femoral nerve in the proximal thigh. It follows the superficial femoral vessels in the medial thigh, deep to the sartorius muscle. Proximal to the knee the nerve emerges superficially to associate with the greater saphenous vein. It continues down the medial lower leg with the greater saphenous vein to lie anterior to the medial malleolus at the ankle.
The saphenous nerve is a sensory nerve. Blockade of the nerve at the ankle provides anesthesia to a small medial portion of the ankle and foot.
The patient is placed supine on the stretcher with their leg overhanging the bed or eleveated off the stretcher by towels. The leg is externally rotated to give access to the anterior medial ankle. A high-frequency (12 – 18 MHz) linear array probe is used with appropriate depth adjustment. Under sonographic guidance, the greater saphenous vein is identified anterior and superficial to the medial malleolus. The vein will collapse easily in most patients. A tourniquet placed around the calf and light probe pressure can aid in detection. The saphenous nerve may be identified adjacent to the vein, but it is commonly very difficult to see. If the nerve is not apparent, tracing the vein proximally may highlight the hyperechoic nerve following the vein. If the nerve is still not visualized, the saphenous vein is used as the target for injection.
Figure 1: Saphenous Nerve (yellow) and greater saphenous vein (blue)
are found close to the distal tibia.
Given the superficial lie of the nerve and shallow needle angle, a smaller gauge needle (25 gauge, 2.5cm long) is generally used for this block. The needle is inserted in an in-plane orientation from the side of the probe. Once the tip is identified the needle is advanced to the deep border of the nerve, or the greater saphenous vein if the nerve is not apparent. Once movement of the needle causes movement of the nerve, a small amount of local anesthetic is injected. The anesthetic should spread evenly around the nerve and separate it from the vein. If anesthetic spread is inadequate, the needle may need to be directed towards the anterior surface of the nerve. If the nerve remains obscured, the vein is surrounded in local anesthetic instead. A total of 3-5 mls is injected for complete anesthesia.
Figure 2: Saphenous nerve block with hands in-plane technique.
Figure 3: Saphenous nerve block with hands out-of-plane technique.
Video 1: Saphenous nerve in-plane (under review)
Pearls and Pitfalls
Clear all needles and syringes of air prior to injecting. Injected air will obscure images.
The saphenous nerve is commonly not seen at this level. Aim for the saphenous vein to start.
Ensure spread of local anesthetic between the vein and nerve.
Placing a rolled towel or pillow underneath the calf may aid in positioning for the procedure.
Smaller needles are more difficult to visualize using ultrasound. Novices should consider using 23 gauge needles to start.
Avoid vascular injection and injury by frequently aspirating. Any injected anesthetic should result in a bolus of fluid on the image. If no bolus is seen after injection, this may indicate intravascular injection.
For patients with little subcutaneous tissue, the probe may not sit well near the medial malleolus. It is sometimes necessary to perform the procedure more proximal to avoid boney projections.