Few studies have examined the use of ultrasound for sciatic nerve localization. The authors evaluated the usefulness of low-frequency ultrasound in identifying the sciatic nerve at three locations in the lower extremity and in guiding needle advancement to target before nerve stimulation.
In this prospective observational study, 15 volunteers underwent sciatic nerve examination using a curved ultrasound probe in the range of 2-5 MHz and a Philips-ATL 5000 unit (ATL Ultrasound, Bothell, WA) in the gluteal, infragluteal, and proximal thigh regions. Thereafter, an insulated block needle was advanced inline with the ultrasound beam to reach the nerve target, which was further confirmed by electrical stimulation. The quality of sciatic nerve images, ease of needle to nerve contact, threshold stimulating current, and resultant motor response were recorded.
The sciatic nerve was successfully identified in the transverse view as a solitary predominantly hyperechoic structure on ultrasound in all of the three regions examined. The target nerve was visualized easily in 87% and localized within two needle attempts in all patients. Nerve stimulation was successful in 100% after two attempts with a threshold current of 0.42 +/- 0.12 (mean +/- SD) eliciting foot plantarflexion or dorsiflexion.
These preliminary data show that a curved 2- to 5-MHz ultrasound probe provides good quality sciatic nerve imaging in the gluteal, infragluteal, and proximal thigh locations. Ultrasound-assisted sciatic nerve localization is potentially valuable for clinical sciatic nerve blocks.
SCIATIC nerve block is indicated for foot and ankle surgery and commonly combined with lumbar plexus block for hip and knee surgery. Many techniques are available for sciatic nerve block, but all rely on surface anatomical landmarks for approximating the sciatic nerve location before needle insertion. The classic Labat approach1aims to target the sciatic nerve adjacent to the ischial spine in the gluteal region, whereas the more recently described parabiceps2–4and anterior5,6approaches target the nerve next to the ischial tuberosity in the infragluteal region and the lesser trochanter of the femur in the proximal thigh, respectively.
Ultrasound is a useful tool for localizing the brachial plexus and its branches at different levels along its course.7,8Also useful is real-time imaging guidance at the time of needle advancement during ultrasound-assisted nerve block. In principle, ultrasound imaging can also identify the sciatic nerve, which is more prominent in size but deeper in location than the brachial plexus. Bones are consistent structures easily identifiable on ultrasound. When the ultrasound beam encounters bone, the reflected signal generates a linear predominantly hyperechoic (bright) bony density and a hypoechoic (dark) bony shadow underneath. Therefore, the consistent anatomical relation between the sciatic nerve and its neighboring bony structures (ischial spine, ischial tuberosity, and lesser trochanter) along its course may be valuable anatomical cues for ultrasound localization of the sciatic nerve. If successful, ultrasound-assisted sciatic nerve block may improve the accuracy of needle placement and local anesthetic injection and decrease failure rate.
The purposes of the current study were to evaluate the usefulness of ultrasound imaging in identifying the sciatic nerve and guiding the block needle to reach target by showing the desired site, direction, and depth of needle penetration.
Materials and Methods
After institutional review board approval (University Health Network, Toronto, Ontario, Canada) and written informed consent, 15 healthy male volunteers (American Society of Anesthesiologists physical status I or II, 20–36 yr old, 168–183 cm, 57–100 kg, body mass index 18.3–32.3 kg/m2) participated in this study. The sciatic nerve was scanned using a low-frequency curved 7-cm ultrasound probe in the 2- to 5-MHz range and a Philips ATL HDI 5000 unit (ATL Ultrasound, Bothell, WA) with color flow doppler, compound imaging, and image capturing capabilities.
Each subject was scanned at three anatomical locations—gluteal, infragluteal, and proximal thigh levels—with the goal to localize the sciatic nerve at the level of the ischial spine, ischial tuberosity, and lesser trochanter, respectively. To scan the sciatic nerve in the first two locations, the subject was positioned semiprone (Sim’s position) with the hip flexed at approximately 90°. To scan in the proximal thigh, the probe was positioned approximately 8 cm distal to the inguinal crease, and the subject was supine with the hip and knee flexed and the leg externally rotated at approximately 45°. The ultrasound probe was positioned perpendicular to the skin and oriented at each location to obtain the best possible transverse cross-sectional view of the sciatic nerve (i.e. , the ultrasound beam perpendicular to the nerve) as shown in figures 1A–C.
The following assessments were made at each anatomical location: the quality of ultrasound sciatic nerve images (good image = nerve recognized within 10 s by two independent investigators, one performing ultrasound scanning and one independent observer; poor image = one in which the nerve cannot be identified by one or both investigators with confidence and confirmation required nerve stimulation); nerve diameter (medial to lateral measurement); minimum skin-to-nerve distance (reported as mean ± SD); and identification of neighboring vascular, muscular, and bony structures.
After ultrasound scanning, a block needle was inserted for nerve localization and electrical stimulation in 15 subjects. That is, each subject received two needle punctures (first, in the ischial spine location in all subjects, and second, in the ischial tuberosity location [8 subjects] or the lesser trochanter location [7 subjects] according to a computer-generated randomization table). Subsequent electrical stimulation confirmed target sciatic nerve at both locations. After sterile skin preparation with povidone-iodine and local infiltration with 1% lidocaine, an 8-cm, 22-gauge insulated block needle (Pajunk, Geisingen, Germany) was inserted parallel and inline with the ultrasound probe covered with a sterile plastic cover and gel (fig. 1D). The needle was advanced under real-time ultrasound imaging guidance until it made contact with the target nerve. When the needle was judged to be in satisfactory position, a nerve stimulator with a 100-μs pulse duration (Stimuplex; Braun Medical, Bethlehem, PA) was turned on to elicit foot plantarflexion or dorsiflexion using a maximum of 1.5 mA. After electrical stimulation, 10–20 ml dextrose 5% solution was injected incrementally in 2- to 3-ml aliquots under ultrasound observation to mimic a local anesthetic injection. The pattern of 5% dextrose solution spread, and the threshold stimulating current (mean ± SD) was noted.
All subjects successfully completed the study. In the transverse view, the sciatic nerve appeared predominantly hyperechoic on ultrasound as a flat and thin elliptical structure at the gluteal region (ischial spine level, fig. 2A), lip- to oval-shaped at the infragluteal region (ischial tuberosity level, fig. 2B), and round or oval-shaped at the proximal thigh (lesser trochanter level, fig. 2C). Good quality images of the sciatic nerve were obtained in 13 of 15 subjects (87%) at the gluteal region (ischial spine level) and 100% in the other two locations.
When scanned obliquely in the buttock at the ischial spine level (fig. 1A), the hyperechoic sciatic nerve was consistently located deep to the gluteal maximus muscle, lateral to the ischial spine, and above the hyperechoic ischial bone (fig. 2A). The sciatic nerve was 1.67 ± 0.17 cm (mean ± SD) in width (medial to lateral measurement) and 3.48 ± 0.91 cm from the skin surface on ultrasound. Often, the pulsatile inferior gluteal artery pudendal artery, or both could be found medial to the sciatic nerve (fig. 3A).
At the ischial tuberosity level, ultrasound scanning in the transverse plane (fig. 1B) consistently showed the ischial tuberosity medially and greater trochanter laterally, both hyperechoic in appearance (fig. 2B). In between was the hyperechoic oval- or lip-shaped sciatic nerve deep to the gluteal maximus muscle, most commonly 1.57 ± 0.17 cm (mean ± SD) in width and 3.34 ± 0.85 from the skin surface. No nearby vessel was identified.
At the lesser trochanter level, when scanned approximately 8 cm distal to the inguinal crease and transverse on the medial side of the thigh that was semiflexed and externally rotated (fig. 1C), the hyperechoic sciatic nerve was found posterior and medial to the lesser trochanter (fig. 2C). The nerve appeared most commonly oval or round, 1.66 ± 0.21 cm in width and 6.21 ± 0.68 cm (mean ± SD) from the skin surface. The sciatic nerve was deep to the adductor magnus muscle, and the femoral neurovascular bundle was noted far lateral to the sciatic nerve in this projection (fig. 3B).
Under real-time ultrasound guidance, all 15 subjects underwent nerve localization and stimulation successfully in two anatomical locations, ischial spine (n = 15) plus ischial tuberosity (n = 8) or lesser trochanter (n = 7). When the block needle was passed inline with the ultrasound beam (fig. 1D), needle advancement was most commonly observed as needle and tissue (muscle) movement without clear view of the needle shaft due to the steep angle of needle penetration. We observed needle movement in real time until the needle made contact with the hyperechoic sciatic nerve as indicated by nerve movement. The needle-to-nerve distance was further minimized by adjusting the needle position based on nerve stimulation response.
When in contact with the sciatic nerve, electrical stimulation through the needle evoked muscle contraction in all cases as a confirmatory signal. The minimum threshold currents required for nerve stimulation were 0.46 ± 0.14, 0.42 ± 0.08, and 0.39 ± 0.16 mA at the level of the ischial spine, ischial tuberosity, and lesser trochanter, respectively, to elicit plantarflexion or dorsiflexion of the foot. Injection of 1–2 ml of the 5% dextrose solution intensified the motor response, and solution spreading around the nerve was observed after 10–20 ml of injection (fig. 4). There was no complication except mild buttock and posterior thigh paresthesia in one subject that lasted for 3 days after the study. Paresthesia was not reported during needle advancement in any subject.
Currently, high-resolution sonography is valuable in the diagnosis of peripheral nerve entrapment syndromes,9neuropathies,10and nerve tumors.11,12Previous human and cadaver studies have examined anatomical–sonographic correlation of the sciatic nerve, but they are limited to the mid thigh and regions below.10,13–17In this study, we have generated unique ultrasound images of the sciatic nerve in deeper locations, i.e. , the gluteal, infragluteal, and proximal thigh regions. We have demonstrated that a low-frequency ultrasound probe in the range of 2–5 MHz is valuable in localizing the sciatic nerve in these locations and can visually guide needle advancement to target with precision. Ultrasound identification of deep bony landmarks—ischial spine, ischial tuberosity, and lesser trochanter—provides a consistent guide to the sciatic nerve at these locations. Ultrasound-guided sciatic nerve block is potentially valuable for achieving clinical anesthesia with fewer attempts and higher success, but this needs confirmation in future studies.
Labat’s approach to the sciatic nerve relies on intersection of lines adjoining palpable surface bony landmarks (posterior superior iliac spine and the sacral hiatus medially and greater trochanter laterally).1The point of intersection indicates the approximate sciatic nerve location and the gluteal site of needle insertion. Anatomically, the ischial spine is a more reliable bony landmark in proximity to the sciatic nerve, but it is not palpable on the skin surface. Chang et al. 18described a technique of sciatic nerve block by palpating and identifying the ischial spine directly upon rectal examination. Although this technique achieved block success in 76% of the cases, this method of sciatic nerve localization has not gained popularity for obvious reasons.
Our preliminary experience with ultrasound identification of the ischial spine is extremely encouraging. The imaging technique we use closely resembles that reported by Kovacs et al. ,19who developed an ultrasonographic technique for pudendal nerve infiltration for perineal pain. A 3.5-MHz curved-array probe was used to first identify the ischial spine before localizing the pudendal nerve and internal pudendal artery located more medially. The internal pudendal artery (a branch of the internal iliac artery) was found in 98% of the cases and consistently within 1 cm from the ischial spine. Our experience compares favorably with the findings by Kovacs et al. Both the ischial spine and the inferior gluteal or internal pudendal artery (both branches of the internal iliac artery) generate easily identifiable bony and vascular signals on ultrasound and are reliable cues medial to the sciatic nerve location (figs. 2A and 3A).
Our finding is also consistent with previous attempts by Hullander et al. ,20who used Doppler ultrasound to detect superior gluteal arterial pulsation to aid sciatic nerve localization with the Labat approach. Both gluteal arteries are medial to the sciatic nerve with the superior gluteal artery passing between the L5 and S1 nerve roots and emerging from the upper border of the piriformis muscle while the inferior gluteal artery passes between S1 and S2 nerve roots and emerges below the piriformis muscle.
The infragluteal parabiceps technique2–4is a simple approach to the sciatic nerve where the nerve lies halfway between two easily identifiable bony landmarks, the greater trochanter laterally and the ischial tuberosity medially. A case of ultrasound-guided infragluteal sciatic nerve block was reported by Gray et al. 21in a child undergoing Achilles tendon surgery. The block was successful, confirmed with nerve stimulation, and local anesthetic spread was observed surrounding the hyperechoic sciatic nerve deep to the gluteus maximus and biceps femoris muscles. In the current study, we consistently observed enlargement of the presumably “sheath” compartment and extensive spreading of the injected 5% dextrose solution completely or partially encircling the sciatic nerve within this compartment. We chose to pass the needle inline with the ultrasound beam as opposed to the perpendicular direction21so that needle advancement could be observed in real time.
The anterior approach as described by Beck5and Chelly and Delaunay6is to block the sciatic nerve at the lesser trochanter level of the femur. Direct needle passage to the nerve is often blocked at this level when the leg is positioned supine and neutral. In a cadaver study, Vloka et al. 22found that internal rotation of the leg by 45° greatly facilitated but external rotation consistently blocked needle passage at the lesser trochanter level. Alternatively, it is advantageous to block the sciatic nerve below the lesser trochanter because it is more accessible to needle passage when the leg is positioned supine and neutral.23,24
We took an entirely different approach in this study. When the thigh and knee are flexed and the leg is externally rotated (fig. 1C), we find the sciatic nerve easily visible and accessible, posterior and deep to the femur at the lesser trochanter level and below. Furthermore, the femoral neurovascular bundle is far lateral to the site of needle insertion; therefore any unintentional puncture is unlikely (figs. 2C and 3B). In contrast, ultrasound scanning directly over the femur with the leg in the conventional supine and neutral position showed the quadriceps muscles and the femur but not the sciatic nerve at the lesser trochanter level (fig. 5), and the femoral neurovascular bundle is located medially.
In summary, ultrasound technology can provide high-quality images of the sciatic nerve and guide nerve localization and needle placement in the gluteal, infragluteal, and thigh regions. Future studies are required to determine the clinical utility of ultrasound-assisted sciatic nerve blocks in these locations.