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Regional Anesthesia: Research Report

Does Sciatic Parasacral Injection Spread to the Obturator Nerve? An Anatomic Study

Valade, Nathalie MD; Ripart, Jacques MD, PhD; Nouvellon, Emmanuel MD, MSc; Cuvillon, Philippe MD, MSc; Prat-Pradal, Dominique MD, PhD*; Lefrant, Jean-Yves MD, PhD; de La Coussaye, Jean-Emmanuel MD, PhD

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doi: 10.1213/ane.0b013e3181607205
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To provide complete anesthesia for lower limb surgery, peripheral nerve block requires blockade of both lumbar and sacral plexuses, thus blocking the five major nerves of the limb: the sciatic nerve, the posterior cutaneous nerve of the thigh, the femoral nerve, the lateral cutaneous nerve of the thigh and the obturator nerve. This is usually done by performing several injections on the sciatic nerve and various approaches to the lumbar plexus.1 It is clear now that only the posterior approach to lumbar plexus (psoas compartment block) provides a reliable blockade of the femoral, lateral cutaneous of the thigh and obturator nerves.1 The classical “three-in-one” in fact, unreliabily provides obturator blockade1–7; therefore, an additional obturator block is frequently required, which requires an additional puncture.8

The parasacral approach is widely used for sciatic nerve block.9–12 It has been reported to provide reproducible obturator nerve blockade.9,13 If accurate, parasacral nerve block would avoid the need for a systematic selective obturator nerve block, but this point remains controversial.14 Using an adductor strength reduction as a confirmation of obturator blockade, Jochum et al.14 concluded that parasacral block does not provide adequate obturator blockade, thus challenging the opinion that parasacral block can provide obturator blockade. These clinical findings are opposed to previous results.9,13 The controversy involves lack of knowledge about the spreading of local anesthetic injected through the parasacral route. Finally, if local anesthetic spreads into the pelvis to block the pelvic portion of the obturator nerve, it could also spread to each sacral plexus root, thus providing a complete perineal block.

The aim of this anatomical study was to describe the spread of colored latex in human cadavers to mimic parasacral block, with special attention to the spread of dye toward the pelvis including roots of the sacral plexus and the intrapelvic portion of the obturator nerve.



Seven preserved male human cadavers were used (deceased for less than 7 days). Needle positioning was performed as described by Mansour, by a physician experienced in this block (J.R.).9,10 Cadavers were positioned prone in order to allow bilateral injection. A line was drawn from the posterior superior iliac spine to the lower point of the ischial tuberosity. A 17-gauge Tuohy needle was introduced 6 cm from the iliac spine on this line, and advanced perpendicularly to the skin. When the bony margin of the sciatic notch was encountered, the needle was withdrawn and reintroduced 1 cm caudad on the same line, thus avoiding the bone and penetrating the greater sciatic foramen to a depth of 2 cm farther than the previous depth of bone contact. If bone was encountered a second time, the needle was withdrawn and reintroduced 1 cm lower. Specially prepared latex was used, as described previously15,16: latex was diluted to 50% in water with natural blue or green pigment added. This technique allows an injection on each side and prevents confusion between them. Fifteen to 20 mL was injected, depending on the estimated weight of the cadavers, at a standardized speed of injection (1 mL/10 s).


Dissections were undertaken after 12 to 48 h (time required to allow the latex to polymerize) and consisted of two steps for each injection.

The first dissection sought to confirm the location of the initial injection and to confirm the spread of the latex near the sciatic nerve, mimicking a theoretically successful parasacral nerve block. It consisted of a posterior approach to the gluteal area as far as the piriformis muscle and sciatic nerve at the greater sciatic foramen level.

The second dissection consisted of an anterior approach inside the pelvis in order to observe the spread of the latex to the obturator nerve and sacral roots.

Measured Variables

The distance between the pelvic portion of the obturator nerve and sacral plexus (at the approximate location of the injection) was measured. Pictures were taken at each step of each dissection. The pictures were later scored by two independent investigators (N.V. and J.R.). In case of divergence between the two scores, the pictures were reevaluated by the two investigators together.

Evidence of injection around the considered nerves (both sciatic, obturator and sacral roots) was described from 0 to 3 as follows:

  • 0 = Totally failed injection, no latex reaching the nerve, thus suggesting a missed block.
  • 1 = Poor injection: very little latex reaching the nerve.
  • 2 = Fair injection: latex clearly reaching the nerve but only partially surrounding it.
  • 3 = Totally successful injection: latex completely surrounding the nerve, thus mimicking a totally successful blockade.

Only cadavers corresponding to a successful parasacral (sciatic) injection (score 2 and 3) were analyzed for obturator and sacral roots spread.


The individual results are detailed in Table 1. Figures 1 and 2 show the typical spread of the colored latex on the posterior and anterior dissections, respectively. The mean ± sd distance between the sacral plexus and the obturator nerve was 2.9 ± 0.7 cm.

Table 1
Table 1:
Individual Results of 14 Injections
Figure 1.
Figure 1.:
Typical successful parasacral injection (Posterior view, left side). 1—Piriformis muscle (inferior part left in place, superior part reclined upward to show the sciatic nerve emergence through the muscle). 2—Sciatic nerve. Note the presence of blue latex in the muscle sheath and spreading along the sciatic nerve.
Figure 2.
Figure 2.:
Typical successful obturator nerve spreading of the latex (Endopelvic view, right side). 1—psoas muscle, 2—obturator nerve, and 3—sacral nerve roots. Note the presence of green latex near the obturator nerve and on the sacral nerve roots (part of the latex spreading frontward to the obturator nerve had to be removed during dissection in order to see it).

Spreading to the Sciatic Nerve

Of the 14 injections, 11 (78%) were scored as 2 or 3, meaning a success rate of 78%. In the remaining three failed injections, 2 (N° 11 and 12) were performed in the gluteal muscles, suggesting the needle was not introduced deep enough. In the third failed injection (N°7), the majority of latex was found intravascularly. Failed sciatic injections were excluded from further analysis.

Spread to the Obturator Nerve and Sacral Roots

Nine of 11 (82%) injections were scored 2 or 3 for both obturator and sacral roots spread. The two remaining failed cases were associated with partial intravascular placement that might have prevented spread toward the obturator nerve.


The primary finding from this anatomic work is the ability of a successful parasacral nerve block to frequently (82%) provide an obturator nerve and perineal blockade. Figure 3 shows the proximity between obturator and sciatic nerves at the level of the greater sciatic foramen.

Figure 3.
Figure 3.:
Transverse semischematic view of a human pelvis, passing through the greater ischiatic foramen (where the parasacral block is performed). IP = iliopsoas muscle surrounding the femoral nerve; IB = Iliac bone; ON = obturator nerve; GM = greater gluteal muscle; SN = sciatic nerve; PM = piriformis muscle; S = Sacrum. Dotted line: fascia of pelvic muscles. Note the proximity between sciatic and obturator nerve. One can see that local anesthetic injected around the sciatic nerve may easily spread under the fascia of pelvic muscles toward the obturator nerve anteriorly and the sacral roots posteriorly. Redrawn from anatomical slice.

Clinical studies are controversial on this point. Bouaziz et al. have confirmed that there is no sensory skin dermatome in 57% of cases.17 Therefore, obturator nerve blockade can only be tested by evaluation of the decrease in adductor muscle strength and not by a pinprick or ice test. Injecting 30 mL of 1.5% lidocaine, Morris et al.13 observed that adductor muscle strength was decreased after parasacral nerve block in 100% of the injected blocks and concluded that parasacral nerve block reliably provides obturator nerve block. However, the measurement of this decrease was only semiquantitative. Finally, an ipsilateral perineal blockade was observed, thus confirming a complete sacral plexus block provided by parasacral block. Helayel et al.18 reported two cases of transient urinary incontinence after a bilateral parasacral block that was similarly attributed to a complete perineal block. By contrast, after a parasacral injection of 0.75% ropivacaine 30 mL, Jochum et al.14 found an adduction strength decrease of only 11%. A more complete (69%) adduction strength decrease was observed when a selective obturator block was added. In an anatomic portion of the same investigation, the authors evaluated latex spread after four parasacral injections in two cadavers. No spread to the obturator nerve was noted in any of the four injections and it was concluded that the parasacral nerve block cannot provide obturator blockade.14 The discrepancy between the results of this study and ours is somewhat disturbing. The latex volume injected by Jochum et al. (30 mL) was larger than ours (17–20 mL), which has been demonstrated to be clinically sufficient for parasacral injection.11 With a larger volume, they should have observed a greater spread of latex than in the current study. We speculate that the improved spread in our study occurred because our latex was diluted with water resulting in a less viscous mixture that was able to spread before polymerizing.

As with any anatomical work, our description has some limitations. First, the needle was positioned only using external landmarks and contacting bone. The correct location of the needle was not confirmed, i.e., electric nerve stimulation for obvious reasons, ultrasonography, or fluoroscopy. Second, spread of latex in cadavers may be different from the spread of local anesthetic in living subjects. However, as latex is more viscous than local anesthetics, one would expect latex to have a more limited spread. Therefore, if latex can reach the obturator nerve and sacral roots, it is conceivable that local anesthetic should at least do the same. However, an imaging (ultrasonography, magnetic resonance imaging, or computed tomography scan) study with associated clinical assessment of obturator and sacral root block in living subjects is required to confirm our results.

In conclusion, this anatomic work demonstrated that a successful parasacral nerve block can theoretically provide obturator and complete sacral plexus blockade. When a combined sciatic and obturator block are required, the use of a parasacral block may avoid the need to perform a separate obturator block.


Thank you to Lana Zoric for English editing.


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