Landmark-based approach

As for all regional anesthetic procedures, after checking that emergency equipment is complete and in working order, intravenous access, ECG, pulse oximetry, and blood pressure monitoring are established. Asepsis is observed.

The patient is placed in the lateral decubitus (Sims) position (Fig. 24.6). A 100-mm insulated needle is used. The stimulating current is set at 0.75 mA, 2 Hz, and 0.1 ms. The needle is oriented 90° to all planes (Fig. 24.7) and advanced slowly until the appropriate muscle response is obtained: tibial nerve component produces plantar flexion and inversion of the foot, while common peroneal stimulation produces dorsiflexion and eversion of the foot. Hamstring contraction should not be accepted as a suitable response because it can be due to direct gluteal muscle stimulation.

Figure 24.6 Patient position for the posterior landmark-based approach to sciatic nerve block. The patient is placed in the lateral position with a slight forward tilt. The upper hip is flexed, with the foot resting on the extended knee of the lowermost limb.

Figure 24.7 Posterior landmark-based sciatic block technique. The needle is inserted in a 90° orientation to all planes.

Figure 24.8 Axial T1-weighted MR image after injection of 20 mL of contrast, showing spread of contrast. Note spread toward the sacral roots and lateral wall of the pelvis. 1: ischium; 2: sciatic nerve roots; 3: gluteus maximus muscle; 4: sacrum.

The nerve is usually 7–9 cm deep to the skin. If bone is contacted, the needle is redirected medially or laterally. The initial stimulating current should be kept at 0.75 mA or less because direct stimulation of the gluteus maximus muscle (Fig. 24.2) with higher currents can mimic sciatic nerve stimulation. The needle may stimulate the inferior gluteal nerve (a branch from the sacral plexus). This results in rhythmic contractions of gluteus maximus; in this circumstance, the sciatic nerve is a few centimeters deeper and possibly 1 cm laterally. It is best to reattempt the block with needle insertion 1 cm lateral to the original site. The needle position is adjusted while reducing the current to 0.35 mA with maintenance of the muscle response.

Incremental injections of local anesthetic are made with repeated aspiration. For this block, 20 mL of local anesthetic is adequate.

Ultrasound-guided approach

Intravenous access, electrocardiogram (ECG), pulse oximetry and blood pressure monitoring are established. Maximized comfort for the operator and patient is an important step in pre-procedure preparation. For the ultrasound-guided sub-gluteal sciatic block, the patient is placed in the prone or lateral position. The operator stands on the side to be blocked with the ultrasound screen on the opposite side (Fig. 24.9). Room lights may be turned down to enhance image viewing. The operating lights can be used to maintain some working lighting in the background.

Figure 24.9 Global view of the block field for the ultrasound-guided posterior sciatic nerve block.

The skin is disinfected with antiseptic solution and draped. A sterile sheath (CIVCO Medical Instruments, Kalona, IA, USA) is applied over the ultrasound transducer with sterile ultrasound gel (Aquasonic, Parker Laboratories, Fairfield, NJ, USA). Another layer of sterile gel is placed between the sterile sheath and the skin. The sub-gluteal region is scanned in the transverse plain using a linear or curvilinear transducer. The ultrasound screen should be made to look like the scanning field. That is, the right side of the screen represents the right side of the field. Adjustable ultrasound variables such as scanning mode, depth of field, and gain are optimized.

A skin wheal of local anesthetic is raised at the medial aspect of the ultrasound transducer. The needle bevel should face the active face of the transducer to improve visibility of the needle tip. A free hand technique rather than the use of a needle guide is preferred. A 21-GA × 120-mm insulated regional block needle (Pajunk, Geisingen, Germany; or B.Braun, Bethlehem PA) is inserted within the plane of imaging to visualize the entire shaft and bevel along the path of the ultrasonic beam (Fig. 24.10). The needle is attached to sterile extension tubing, which is connected to a 20-mL syringe and flushed with local anesthetic solution to remove all air from the system. The operator can slide and tilt the transducer to maintain the needle tip within the plane of imaging as much as possible. The needle tip should be clearly identified within the plane of imaging before advancing the needle. The needle is advanced until it reaches the side of the target sciatic nerve (Fig 24.11).

Figure 24.10 Ultrasound transducer and needle positioning during ultrasound-guided posterior sciatic block. Note the needle orientation in the same plane as the ultrasound beam.

Figure 24.11 Ultrasound image of the ultrasound-guided posterior sciatic block after administration of 20 mL of local anesthetic solution. N: needle; LA: local anesthetic; SN: sciatic nerve.

Once the needle tip has been confirmed by ultrasonography to lie adjacent to the sciatic nerve, this can be confirmed by using a nerve stimulator (Stimplex; B. Braun, Bethlehem PA). If an electrical nerve stimulation technique is used, characteristic motor activity of either dorsi- or plantar-flexion is elicited in the foot. Test injections for assessment of local anesthetic spread should be small (0.5–2 mL). If local anesthetic spread is not seen on the ultrasound screen, injection should be stopped. Local anesthetic is deposited on four sides by manipulating the needle tip position. Local anesthetic: 20 mL of lidocaine 2% with epinephrine 1 : 200 000. If the block is used for postoperative analgesia bupivacaine 0.5% with epinephrine (1 : 200 000) or ropivacaine 1% with epinephrine (1 : 200 000) can be used, which give nearly 24 hours of analgesia.

Continuous technique

Continuous posterior sciatic block is similar to the single-shot technique, although it can be difficult to thread the catheter due to the perpendicular approach to the nerve. An alternate technique is to use the same needle insertion site but to aim the needle caudally toward the midpoint between the greater trochanter and the ischial tuberosity. An insulated Tuohy needle, oriented appropriately, may be useful to assist catheter placement with the classic posterior landmark-based approach.

As with other continuous nerve block techniques, the initial dose of local anesthetic is usually injected and only then is the infusion of a more dilute local anesthetic initiated. Once the local anesthetic is injected, the catheter is carefully inserted some 5 cm beyond the tip of the needle while keeping the needle immobile. Other approaches to the sciatic nerve (including parasacral, subgluteal, and popliteal) are favored for continuous sciatic block. The catheter should be accessible with the patient supine and thus secured in a lateral direction. Administer a continuous infusion of ropivacaine 0.2% at 5–12 mL per hour, achieving postoperative analgesia without significant motor weakness.