Saphenous Nerve Block

Saphenous Nerve Block Overview

The saphenous nerve block serves as a valuable adjunct to the various sciatic nerve block techniques, particularly for procedures requiring anesthesia along the medial aspect of the lower leg. As a terminal sensory branch of the femoral nerve, the saphenous nerve provides cutaneous innervation to the medial calf, with no motor component.

Motor function of the lower leg is primarily mediated by the distal branches of the sciatic nerve. The saphenous nerve branches from the femoral nerve approximately midway along the femur, then courses medially, traveling through the fascial plane deep to the junction of the vastus medialis and sartorius muscles. Anatomical variations are common—the nerve may pass beneath or through the sartorius muscle.

As a purely sensory nerve, motor responses cannot be elicited using a nerve stimulator. However, at low current intensities, patients may perceive a pulsing or tingling sensation in the saphenous distribution. Higher outputs may produce discomfort without aiding in localization.

Several techniques exist for saphenous nerve blockade, both with and without ultrasound guidance. The ultrasound-guided approach, preferred by the author, is typically performed at the level of the medial thigh, offering reliable visualization and localization of the target fascial plane.

Ultrasound-Guided Saphenous Nerve Block Technique

Begin the procedure by selecting a high-frequency linear transducer set to a depth of 3–5 cm. Position the probe on the anterior aspect of the mid-thigh, obtaining a short-axis view of the thigh musculature. In the initial image, identify the rectus femoris and vastus medialis muscles, which appear as large, marbled ovoid structures near the superficial portion of the screen. The femur may be seen as a hyperechoic, crescent-shaped structure at the bottom of the image.

Gradually slide the probe medially until the sartorius muscle—a smaller, oval structure—comes into view. The saphenous nerve typically lies between the sartorius and vastus medialis muscles or deep to the sartorius within the adductor canal. Due to its small size and variable course, the nerve may not always be visualized. High-resolution imaging may occasionally reveal it adjacent to the femoral artery or a secondary branch on the contralateral side of the artery. At the mid-thigh level, the nerve is generally located lateral and anterior to the artery.

Needle Insertion and Local Anesthetic Administration

Once optimal visualization is achieved and the skin has been prepped, infiltrate the anticipated needle path with local anesthetic to raise a skin wheal. Advance a long block needle in-plane, under continuous ultrasound guidance, to a position just deep to the sartorius muscle, within the adductor canal.

After confirming negative aspiration, deposit local anesthetic within the canal. Reposition the needle between the vastus medialis and sartorius muscles, and administer an additional 5–10 mL of local anesthetic after aspirating again. Take care to avoid vascular structures—most notably the great saphenous vein and inferior genicular artery—which course in close proximity to the nerve. Careful aspiration and slow injection during the first few milliliters are critical for patient safety.

THE EVOLUTION OF THE ADDUCTOR CANAL BLOCK: 

The emerging technique for motor sparing analgesia to the knee.

Adductor Canal Block: A Focused Approach to Knee Analgesia with Motor Preservation

The adductor canal block has recently garnered increased interest from both anesthesia and orthopedic communities due to its ability to provide effective analgesia while preserving motor function of the anterior thigh. This selective sensory blockade is associated with shorter hospital stays, earlier rehabilitation, improved pain control, and the preservation of neurologic feedback in adjacent distributions, which can be compromised by more proximal femoral nerve blocks.

In line with modern regional anesthesia principles, the adductor canal block exemplifies a targeted approach—blocking only the nerves responsible for surgical pain, minimizing unnecessary motor impairment.

Literature Review

To better understand this relatively novel technique, a literature review was conducted using the keywords "adductor canal block" and, subsequently, "saphenous nerve block" due to limited initial results. Only articles published within the last five years were included, with the exception of one foundational study to provide historical context.

Historical Background

One of the earliest contributions was by Van Der Wal et al. (1993) in Canada, who described a “subsartorial approach” to the saphenous nerve in cadaveric studies. Although originally intended for foot and ankle procedures, their work laid the foundation for modern access techniques used today to provide selective knee analgesia.

Relevant Anatomy

The adductor canal block specifically targets the saphenous nerve, a purely sensory branch of the femoral nerve. The femoral nerve originates from L2–L4 spinal nerves, forming part of the lumbar plexus. After passing beneath the inguinal ligament, it gives rise to multiple motor branches in the anterior thigh and the saphenous nerve, which accompanies the femoral artery and vein through the adductor canal.

The adductor canal is bordered:

• Anteriorly by the sartorius muscle

• Posteriorly by the vastus medialis

• Medially by the adductor longus

Within this canal, the saphenous nerve provides sensory innervation to the anterior and medial aspects of the knee, including the infrapatellar region via its infrapatellar branch.

Technique

While early methods used landmark-guided, blind approaches, ultrasound guidance is now considered standard due to superior accuracy and safety.

Step-by-Step Approach:

1. Positioning: Slightly externally rotate the patient’s thigh.

2. Ultrasound Setup: Use a high-frequency linear probe set to a depth of ~4 cm.

3. Probe Placement: Begin at the midpoint between the inguinal crease and patella on the medial thigh.

4. Identify Structures: Locate the femoral artery in short axis. If uncertain, trace it proximally from the inguinal crease. Use Doppler to confirm pulsatile flow.

5. Anatomic Landmarks: Identify the sartorius and adductor longus muscles—visualized as a characteristic triangular canal (see Figure 1).

6. Needle Insertion: Prep the skin, then infiltrate with local anesthetic (e.g., lidocaine) using a small gauge needle. Insert a blunt-tipped block needle in-plane, guiding it beneath the sartorius and adjacent to the artery.

7. Needle Redirection: The saphenous nerve is often not visible, but probe tilting (20–30° caudal) may help localize it.

8. Injection: After negative aspiration, inject local anesthetic and observe for spread within the canal. Volume may vary, but up to 30 mL has been shown to effectively fill the space.

Clinical Considerations

The adductor canal block is still evolving in clinical practice. It differs from traditional techniques by sparing motor function while providing robust analgesia for knee procedures.

Nerve Stimulator Use

While the saphenous nerve is purely sensory and does not produce a motor twitch, nerve stimulation can still be of limited utility. At higher current settings (>1.0 mA), sensory nerve activation may elicit a subjective pulsing or tingling sensation in awake patients. However, most studies on this technique have relied exclusively on ultrasound guidance, and nerve stimulator use is not routinely recommended.

Current Evidence and Future Directions

Most recent studies (2–15, 17) support the efficacy of the adductor canal block in providing targeted analgesia with preservation of quadriceps strength. While more research is needed to establish standardized dosing and long-term outcomes, the current literature is overwhelmingly positive. Only one study reported no significant benefit over IV analgesics following knee arthroscopy.

Given its favorable profile and increasing clinical adoption, the adductor canal block appears poised to play a significant role in modern multimodal analgesia protocols—particularly for knee surgery and outpatient orthopedic procedures.