Popliteal Nerve Block

Popliteal Sciatic Nerve Block: Clarifying Terminology and Technique

The term “popliteal nerve” is often used to describe the continuation of the sciatic nerve within the popliteal fossa; however, this term is somewhat of a misnomer. While the name reflects the nerve’s location, anatomically, the sciatic nerve has typically already bifurcated at this level into its two terminal branches: the common peroneal nerve (laterally) and the tibial nerve (medially), relative to the popliteal artery and vein. As noted by Frank Netter, MD, the sciatic nerve retains its name deeper in the posterior thigh and gives rise to these two branches well before it reaches the crease of the popliteal fossa.

In adults, this bifurcation usually occurs approximately 7 cm proximal to the popliteal crease. Thus, one could argue that there is, technically, no distinct “popliteal nerve.” Nevertheless, a popliteal block performed at this level will anesthetize the same sensory and motor distributions as a more proximal sciatic block, with the exception of a small area of skin innervated by the saphenous nerve, which should be considered when planning blocks for procedures involving a tourniquet.

Technique Overview: Nerve Stimulator-Guided Posterior Popliteal Block

The posterior approach is the most commonly described and widely practiced technique for popliteal sciatic nerve blocks using a nerve stimulator. While relatively simple to perform, a key limitation is that nerve stimulation may activate only one branch—either the tibial or the common peroneal nerve—which may result in an incomplete block if only one distribution is targeted.

Key Anatomical Landmarks:

• The popliteal fossa is bordered laterally by the biceps femoris tendon and medially by the semitendinosus tendon.

• The popliteal artery typically lies deep to the sciatic nerve bifurcation, with the popliteal vein located adjacent to the artery.

• The sciatic nerve bifurcates into the tibial and common peroneal nerves approximately 5–7 cm proximal to the popliteal crease.

Procedure:

1. Positioning: Place the patient in the prone position or lateral decubitus as appropriate.

2. Identify the Pulse: Palpate the popliteal artery about 5–7 cm above the popliteal crease.

3. Skin Preparation: Cleanse the area and infiltrate ~5 mL of local anesthetic for skin anesthesia, anticipating potential needle redirection.

4. Needle Insertion: Insert the insulated block needle perpendicular to the skin, adjacent to the artery.

5. Nerve Identification:

   • Dorsiflexion of the foot indicates stimulation of the common peroneal nerve.

   • Plantarflexion indicates stimulation of the tibial nerve.

6. Confirmation: Once the desired twitch is obtained, decrease the current to 0.2–0.5 mA. Maintain stimulation at this level. If a strong twitch persists below 0.2 mA, adjust needle position slightly to avoid intraneural placement.

7. Injection: Inject the local anesthetic after negative aspiration. Repositioning is recommended to ensure coverage of both branches, as relying on spread from a single injection may not reliably anesthetize both nerve distributions.

Clinical Consideration

Due to variability in bifurcation level and nerve trajectory, a thorough understanding of anatomical relationships and precise nerve localization is critical for successful block performance. Although commonly referred to as a "popliteal block," practitioners should remain aware that the sciatic nerve may have already divided proximal to the injection site. Ensuring adequate blockade of both distal branches—either through ultrasound visualization or careful nerve stimulation—is essential for comprehensive anesthesia of the lower leg.

Ultrasound guidance for the popliteal block

Ultrasound-Guided Popliteal Sciatic Nerve Block (Posterior Approach)

While the posterior approach is the most commonly described technique for popliteal sciatic nerve blocks, several ultrasound-guided approaches have been developed, allowing for flexibility based on patient anatomy and clinical circumstances. One of the advantages of the ultrasound-guided technique is that it can be performed with the patient in a lateral, supine, or prone position, making it well-suited for diverse patient presentations.

Scanning Technique

Begin by preparing all necessary equipment, including a high-frequency linear transducer, typically set to a depth of 3–5 cm. Position the probe transversely across the popliteal crease to obtain a short-axis view of the popliteal artery. Once the artery is visualized, identify the adjacent sciatic nerve—most often appearing as the tibial nerve, located superficial and lateral to the artery.

With the nerve centered on the screen, scan proximally (cephalad) until the bifurcation of the tibial and common peroneal nerves is clearly visualized. Due to the tension of the surrounding hamstring tendons—the biceps femorislaterally and semitendinosus medially—maintaining stable probe contact may be challenging. Adequate pressure and probe angling will help optimize image quality.

Needle Insertion and Injection

Once the bifurcation is visualized, anesthetize the skin and subcutaneous tissue with local anesthetic. Using an in-plane approach, advance a long block needle toward the sciatic nerve bifurcation, guiding the tip between the tibial and common peroneal nerves. Administer small aliquots of local anesthetic to achieve circumferential spread, ensuring complete perineural coverage of both branches.

Patients may report sensory changes during this process, which is expected.

Catheter Placement (Optional)

If a continuous block is desired, a perineural catheter may be inserted following the initial bolus. The catheter tip should be advanced just deep to the bifurcation of the nerve, allowing gravity-assisted spread and optimal alignment with the nerve fascicles. Secure the catheter using skin adhesive, looping or tunneling as needed, and cover with a clear occlusive dressing for protection.

Occasionally, a large, fluid-filled structure may be visualized in the popliteal region during routine ultrasound scanning for a popliteal block. When this occurs, activate Doppler mode to assess for vascularity. If no flow is detected, the structure is likely a Baker’s cyst—a benign synovial fluid collection that can become quite enlarged.

While the presence of a Baker’s cyst is typically incidental and does not preclude the performance of a popliteal block, it is advisable to notify the orthopedic team of the finding. The block can usually be performed safely by adjusting needle trajectory to avoid the cyst and preserve its integrity. If the cyst significantly impedes access or visualization, consider aborting the procedure.

The initial ultrasound scan should be used to determine whether the cyst’s location will interfere with safe and effective block placement.

The image below demonstrates a large Baker’s cyst identified incidentally during scanning for a popliteal block.