Continuous Sciatic Blocks

A. Parasacral Approach

Elizabeth Gaertner

Patient Position: Sims position.

Indications: Anesthesia and postoperative analgesia for total knee arthroplasty; above and below the knee amputation; popliteal fossa surgery (e.g., tumors, cysts); cancer surgery of the lower limb; and major leg, ankle, and foot surgery with a thigh tourniquet.

Volume and Infusion Rate: A 15 to 20 mL ropivacaine 0.75% bolus followed by an infusion of 7 mL/hour or a patient-controlled analgesia (PCA) ropivacaine 0.2% with a basal rate of 5 mL/hour, bolus 5 mL, lockout time 45 minutes.

Needle Size and Catheter: An 18-gauge, 100-mm plastic introducing cannula with a stimulating short beveled stylet or insulated Tuohy needle and a 20- or 21-gauge catheter.

Anatomic Landmarks: The posterior superior iliac spine (PSIS) and the ischial tuberosity (IT). With this approach, the sciatic nerve is approached at the level of the greater sciatic foramen, where it leaves the pelvis with the pelvic fascia medial, piriformis muscle posterior, and obturator internus muscle lateral.

Approach and Technique: A line is drawn between the PSIS and the lowest point of the IT. The site of needle insertion is three fingerbreadths caudal to the PSIS on the PSIS–IT line. The insulated Tuohy needle with the bevel oriented laterally, connected to a nerve stimulator (2.0 mA, 2 Hz, 0.1 ms) is introduced perpendicularly at a 10° caudal and lateral angle (Fig. 29-1). The position of the needle is adjusted to produce a motor response corresponding to a stimulation of the sciatic nerve, either the common peroneal nerve (inversion of the foot or dorsiflexion of the foot and toes) or the tibial nerve (eversion of the foot or plantar flexion of the foot and toes) with a current less than 0.5 mA. After negative aspiration for blood, local anesthetic is slowly injected with repeated aspiration for
blood. The catheter is introduced 3 to 5 cm beyond the needle tip. The introducer needle is removed, and the catheter is secured in place with Steri-Strip (3M, St. Paul, MN) and covered with a transparent dressing.

Figure 29-1. The insulated Tuohy needle connected to a nerve stimulator is introduced perpendicularly.

Tips

The site of the introduction of the needle is just below the PSIS.
A motor response at the level of the hip or thigh should not be considered sufficient because it can be produced by direct muscle stimulation or can correspond to a nerve stimulation of the piriformis or the obturator internus muscle (needle too lateral). Conversely, a stimulation of the obturator nerve (adduction of the thigh) indicates that the needle is too anterior and medial. Finally, a gluteal muscle contraction indicates that the position of the needle is too superficial.
In case of bony contact (sacral or iliac bone, near the sacroiliac joint, at the top of the greater sciatic notch), the needle needs to be withdrawn and reintroduced more caudally on the PSIS–IT line. The depth at which the bone was contacted is an important indicator. Since the nerve is approached on the top of the greater sciatic foramen, where the sciatic nerve leaves the pelvis, the needle tip should be introduced no more than an additional 20 mm.
This block is often associated with a block of the obturator nerve (>90%) because of the anatomic proximity of the sacral plexus and the obturator nerve. An extension of the block to the pudendal plexus is also possible, especially to the pudendal nerve (up to 80%). This explains the associated unilateral anesthesia of the perineum. Finally, the proximity of the pelvic splanchnic nerves and the terminal branches of the sympathetic trunks and the inferior hypogastric plexus may explain associated urinary retention.
Although no specific serious complications have been reported with this technique, caution should be exercised to avoid damaging the pelvic vessels and organs because of the proximity of the sacral plexus and the pelvic vessels and organs.
In the recovery room, a radiograph may be performed after injection of a contrast agent to verify the position of the parasacral catheter.
This is the only approach to the sciatic nerve that allows consistent blocking of the tibial, common peroneal, and posterior cutaneous nerves of the thigh and also the superior and inferior gluteal nerves and the quadratus femoris nerve.
Compared with a single parasacral block, the Tuohy needle is oriented caudally and laterally to facilitate the placement of the catheter.

Suggested Readings

Birnbaum K, Preschera A, Hessler S, et al. The sensory innervation of the hip joint—an anatomical study. Surg Radiol Anat 1997;19:371–375.

Bruell P. Sciatic nerve block: parasacral approach. Reg Anesth Pain Med 1998;23:78.

Cuvillon P, Ripart J, Jeannes P, et al. Comparison of the parasacral approach and the posterior approach, with single- and double-injection techniques, to block the sciatic nerve. Anesthesiology 2003;98(6):1436–1441.

Jochum D, Iohom G, Choquet O, et al. Adding a selective obturator nerve block to the parasacral sciatic block: an evaluation. Anesth Analg 2004;99:1544–1549.

Mansour NY, Bennetts FE. An observational study of combined continuous lumbar plexus and single shot sciatic nerve blocks for post-knee surgery analgesia. Reg Anesth 1996;21:287–291.

Morris GF, Lang SA, Dust WN, et al. The parasacral sciatic nerve block. Reg Anesth 1997;22: 223–228.

Ripart J, Cuvillon P, Nouvellon E, et al. Parasacral approach to block the sciatic nerve: a 400-case survey. Reg Anesth Pain Med 2005;30:193–197.

B. Gluteal Approaches

Rama Joshi

1. Gluteal Approach

Patient Position: Lateral (Sims position).

Indications: Anesthesia and postoperative analgesia for major surgery at the level of and below the knee, including the foot and ankle.

Needle Size and Catheter: 18-gauge, 100-mm insulated Tuohy needle and a 20-gauge catheter.

Volume and Infusion Rate: 15 to 20 mL of 0.5% ropivacaine followed by either a continuous infusion of 6 to 10 mL/hour or a PCA with a basal infusion rate of 5 mL/hour, bolus of 5 mL, and a 30-minute lockout time 0.2% ropivacaine.

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