Admir Hadzic, Ana M. Lopez, Catherine Vandepitte, and Xavier Sala-Blanch
POPLITEAL SCIATIC BLOCK AT A GLANCE
- Indications: foot, ankle and Achilles tendon surgery
- Transducer position: transverse over the popliteal fossa
- Goal: local anesthetic spread surrounding the sciatic nerve within the epineural sheath
- Local anesthetic: 15–20 mL
The anatomy of the sciatic nerve in the popliteal fossa is variable, and the division into the tibial nerve (TN) and common peroneal nerve (CPN) occurs at an inconstant distance from the popliteal crease (Figure 33G–1). With nerve stimulator–based techniques, larger volumes (eg, > 40 mL) of local anesthetic have
been used to increase the chance of block success. However, US guidance reduces the volume required for reliable block because the injection can be halted once adequate spread is observed. The most common approaches to the popliteal sciatic block are the lateral approach, with the patient in the supine or lateral position, and the posterior approach in the prone or lateral position (Figure 33G–2). While the patient position and needle path differ between the two approaches, the rest of the technique details are similar.
The injection of local anesthetic must occur within the sciatic nerve sheath that contains both components of the nerve.1–14 The injection is ideally accomplished at the position where both components of the nerve are within the sheath but slightly
separated by adipose tissue, allowing for safe placement of the needle between them.15 Although the sciatic nerve block can be
accomplished with an injection around either nerve component, injecting into the space between both is more common in clinical practice.16,17
Beginning with the transducer in the transverse position at the popliteal crease, the popliteal artery is identified, aided with color Doppler US when necessary, at a depth of approximately 3–4 cm. The popliteal vein accompanies the artery at it is positioned just
superficial (posterior) to it. On either side of the artery are the biceps femoris muscles (laterally) and the semimembranosus and semitendinosus muscles (medially). The tibial nerve is positioned superficial and lateral to the vein and is seen as a hyperechoic, oval or round structure with a honeycomb pattern (Figure 33G–3). Asking the patient to dorsiflex and plantar flex the ankle makes the two sciatic nerve branches twist or move in relation to each other. Usually, tilting the transducer caudally is necessary to bring out the nerve from the neighboring adipose tissue.
Once the tibial nerve has been identified, the CPN is visualized slightly more superficial and lateral to the tibial nerve. The transducer should be slid proximally until the tibial and peroneal nerves are visualized coming together to form the sciatic nerve before its division (Figure 33G-4). This junction usually occurs at
a distance 5–10 cm from the popliteal crease but may occur very close to the crease or, less commonly, more proximally in the thigh. As the transducer is moved proximally, the popliteal vessels move deeper and become more challenging to image. Adjustments in depth, gain, focus, and direction of the US beam should be made to keep the nerve visible at all times. At the popliteal fossa, the sciatic nerve typically is visualized at a depth of 2–4 cm.
• Ultrasound imaging should specifically focus on identifying the sciatic nerve sheath (Vloka’s sheath) containing both components of the sciatic nerve (tibial and common peroneal nerves). Successful injection will deposit local anesthetic within the Vloka’s sheath
Sciatic nerve block results in anesthesia of the lower limb below the knee, both motor and sensory, with the exception of the medial leg and foot, which is the territory of the saphenous nerve, a branch of the femoral nerve. The motor fibers to the hamstring muscles are spared; however, fibers to the posterior aspect of the knee joint are blocked (Figure 33G-5). For a more comprehensive review of the sciatic nerve distribution, see Chapter 3.
The equipment recommended for a popliteal sciatic block includes the following:
• Ultrasound machine with linear transducer (8–12 MHz), sterile sleeve, and gel
• Standard nerve block tray
• A 20-mL syringe containing local anesthetic
• 50- to 100-mm, 21- to 22-gauge, short-bevel, insulated stimulating needle
• Peripheral nerve stimulator
• Injection pressure monitor
• Sterile gloves
LANDMARKS AND PATIENT POSITIONING:
This block is performed with the patient in the supine or lateral position. This can be accomplished either by resting the foot on an elevated footrest or flexing the knee while an assistant stabilizes the foot and ankle on the bed (Figure 33G–6). If nerve stimulation is used, exposure of the calf and foot is required to observe motor responses.
LANDMARKS AND PATIENT POSITIONING:
This block is performed with the patient in the prone or lateral position (Figure 33G-2). A small footrest is useful to facilitate identification of a motor response if nerve stimulation is used. A footrest also relaxes the hamstring tendons, making transducer placement and manipulation easier.
The goal is to inject the local anesthetic within the common connective tissue (Vloka’s) sheath that envelops the TN and CPN. Alternatively, separate blocks of TN and CPN can be performed.
The skin is disinfected and the transducer positioned to identify the sciatic nerve. If the nerve is not immediately apparent, tilting the transducer toward the feet can help improve the contrast and bring the nerve “out of the background”. Sliding the transducer slightly proximal or distal may improve the quality of the image and allow for better visualization.
It is recommended to perform the block at the level where TN and CPN start diverging but are still in the common sciatic nerve (Vloka’s) sheath. For the lateral approach, a skin wheal is made on the lateral aspect of the thigh 2–3 cm above the lateral edge of the transducer, and the needle is inserted in plane in a horizontal orientation from the lateral aspect of the thigh and
sPATNCPNFemurBicepsfemorisBicepsfemorisadvanced toward the sciatic nerve (Figures 33G–7 and 33G-8). For the posterior approach, the needle is inserted in plane from lateral to medial (Figure 33G-9) or out of plane (Figure 33G–10). If nerve stimulation is used (0.5 mA, 0.1 msec), the contact of the needle tip with either branch of the nerve usually is associated with a motor response of the calf or foot. Once the needle tip is placed within the common sciatic nerve sheath, 1–2 mL of local anesthetic is injected to confirm the proper injection site. Such injection should result in a distribution of local anesthetic within the sheath, and separation of the TN and CPN within Vloka’s sheath (Figure 33G-11).
When local anesthetic injection does not appear to result in a spread inside the sciatic nerve sheath and around the sciatic nerve components, additional needle repositions and injections may be necessary. Correct injection is recognized when local anesthetic spreads proximally and distally to the site of the injection around both divisions of the nerve, which can be documented by observing the spread of the local anesthetic within Vloka’s sheath proximal to the site of injection. A single injection of local anesthetic typically suffices.
• To improve needle visualization, a skin puncture site 2–3 cm lateral to the transducer will reduce the angle between the needle and the footprint of the transducer (see Figure 33G–6).
• The presence of a motor response to nerve stimulation is useful but not necessary if the nerve, needle, and local anesthetic spread are well visualized.
• Never inject against high resistance because this may signal an intraneural injection (injection pressure must be < 15 psi).
• In the posterior approach to the popliteal block, either an in-plane (lateral or medial) or out-of-plane technique can be used (Figures 33G-9 through 33G-12). While the in-plane lateral approach is commonly used, the advantage of the out-of-plane approach is that the needle path is through skin and adipose tissue rather than the muscles and thus, less painful.
CONTINUOUS ULTRASOUND-GUIDED POPLITEAL SCIATIC BLOCK
The goal of the continuous popliteal sciatic block is to place the catheter within the sciatic nerve sheath in the popliteal fossa (Figure 33G-13).
The catheter is inserted 4-5 cm beyond needle tip and its correct placement is documented by observing injection of LA within the sciatic nerve sheath. The catheter is secured either by taping it to the skin or tunneling.
The lateral approach may have some advantage over the prone approach with regard to catheter placement. First, the biceps femoris muscle tends to stabilize the catheter and decrease the chance of dislodgement, compared with the subcutaneous tissue of the popliteal fossa in the prone approach. Second, if the knee is to be flexed and extended, the side of the thigh is less mobile than the back of the knee. Finally, access to the catheter site is more convenient with the lateral approach compared with the prone approach.
A common starting infusion regimen is ropivacaine 0.2% at 5 mL/h with a patient-administered bolus of 5 mL every 60 minutes.
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