Posterior Approach to the Sciatic Nerve in Adults: Is Euclidean Geometry Still Necessary?

A total of 24 sciatic nerves in 12 adult cadavers were dissected. Some of the dissections were performed by medical students and some by the author himself. All cadavers were dissected on prone position with the lower extremities in adduction. The sciatic nerve and neighboring structures were identified. Position, trajectory, and different measurements were recorded. The information obtained was analyzed for repeated patterns and compared to the available literature. 9–13Originating from L4–5, S1–3 the sciatic nerve exits the pelvis through the greater sciatic foramen. At this point it rests on the ventral surface of the piriformis muscle describing a lateral and downward curve. At the inferior border of this muscle the nerve enters the buttocks covered superficially by the gluteus maximus muscle. From this point, and for the rest of its trajectory in the buttocks, the sciatic nerve became the target of our attention.

The analysis of our data as well as the review of the available literature shows that:

1. Caudal to the piriformis muscle, the sciatic nerve adopts a vertical trajectory that is parallel to the midline. In surface anatomy the midline is represented by the intergluteal sulcus. The intergluteal sulcus, also known as natal cleft, is the result of the anchoring of the deep layers of the skin to the anococcygeal raphe and the dorsum of the coccyx up to the tip of the sacrum.

2. The point at which the sciatic nerve emerges in the buttocks caudal to the inferior border of the piriformis can be projected on a horizontal plane that passes through the uppermost aspect of the intergluteal sulcus. This is the same point at which the inferior gluteal vessels branch off.

3. Because the sciatic nerve's trajectory in the buttocks is parallel to the midline, the distance from it is constant. Such linear distance, as measured from the midline of the buttocks to the center of the nerve ranged from 9.7 cm to 10.3 cm in all 24 specimens (mean 10.1 cm ± 0.2) with no specific tendency in either sex.

4. Soon after the sciatic nerve begins its parallel course to the midline of the buttocks, it enters a narrow valley carved between the ischium and the greater trochanter (fig. 1). A horizontal plane crossing this point projects in the surface roughly to the midpoint of the intergluteal sulcus.

5. The posterior femoral cutaneous nerve of the thigh emerges distal to the piriformis alongside the sciatic nerve and runs behind it until both nerves reach the thigh. Concerns about missing this branch if the block is performed low in the buttocks seem unfounded.

6. In as many as 12% of subjects 9,13the sciatic nerve exits the pelvis as two individual nerves, the posterior tibial and the common peroneal. However, because they share a common sheath that keeps them in intimate contact, 9,12,13this situation should not have clinical relevance.

7. Caudal to the piriformis, the gluteus maximus is the only muscle superficial to the sciatic nerve in the buttocks. The gluteus medius and minimus are proximal and lateral to it.

The main difference between male and female bony pelvises is that the minor pelvis is relatively “roomier” in females. 14The female bones, on the other hand, are thinner than those of males. 15The net result is that the overall width of the pelvis is similar in both sexes, measuring, according to one source, 14280 mm in males and 275 mm in females. In fact, the width of the pelvis according to a different source 6is similar in males and females at any given age, and variation in hip width reflects soft tissue, not bony, differences between the sexes.

Armed with this information the study progressed to the clinical stage. Institutional Review Board approval and written informed consents were obtained. All patients ASA I–III that came to the operating room (OR) for a surgical procedure below the knee without the need for a thigh tourniquet were eligible for the study. The blocks, as per our OR policy, were performed in the OR suite. An intravenous line was started and ASA standard monitors applied. Most of the patients were given 1 mg of midazolam plus 50 μg of fentanyl in the OR; some patients received less. Based on associated pathology, body habits, and/or the patient's preference, the block was performed with the patient in either the prone position or in true lateral decubitus (both hips and knees flexed in a position similar to the one used to perform a neuroaxial block).

The blocks were performed with a 21-gauge, 10-cm, short-bevel insulated needle (Stimuplex, B. Braun, Bethlehem, PA) and a nerve stimulator (Stimuplex-DIG, B. Braun, Bethlehem, PA). Most of the blocks were performed by residents (CA-1 to CA-3) under the author's supervision. All residents had discussed the new technique with the author and were aware of its anatomic foundations. After estimating the midpoint of the intergluteal sulcus by simple inspection, a mark on the skin was placed 10 cm lateral to it in a straight line disregarding the buttocks curvature (fig. 2). Care was taken not to stretch the soft tissues during this measurement, because the subsequent recoil would translate into an underestimation of this distance. In a few cases performed on the lateral decubitus, we purposely stretched the redundant soft tissues of the nondependent buttock by taping the skin to the side of the bed (no recoil). This maneuver facilitated the measurement. When the operator was ready a handheld timer (Casio, Japan) was started and left running until incision time. The following times were recorded as actual readings:

1. Time to skin mark on the buttocks (needle's entrance point). This time period includes the identification of the midpoint of the intergluteal sulcus and the 10-cm measurement lateral to it.

2. Time to first sciatic nerve response. Representing the time it takes to apply a disinfectant solution to the skin, raise a skin wheal, and give a subcutaneous injection of 2 to 3 ml of 1% lidocaine plain. Most importantly, it is the time it takes to bring the blocking needle in the vicinity of the sciatic nerve.

3. Time to injection. This time period starts with the first elicited response at 2 mA and finishes with a response still visible at 0.6 mA. It measures the time spent refining the position of the needle close enough to the nerve to elicit a response at low output.

4. Time to incision. This corresponds to the elapsed time between the start of injection and the incision and it is a reflection of onset and development of an adequate sensory block.

5. Total accumulated time. This is measured from the beginning of the procedure to incision, and reflects the overall efficiency of the technique.

The timer was left running continuously. The time periods were later calculated by subtracting the previous elapsed time from the actual reading. When the technique was performed prone the needle was advanced parallel to the midline (perpendicular to the table). When the technique was performed in the lateral position the needle was still advanced parallel to the midline (parallel to the table). Thus in either case the needle was advanced in reference to the midline (or the table or both) but independent of the buttocks’ curvature (fig. 3). The technique was started with an output of 2.0 mA at 1 Hz (1 stimulation per second) and injected at 0.6 mA. If the needle failed to elicit a response, provided that the nerve stimulator was functional, a reposition was performed. As figure 4shows, the nerve could only be located in a parasagittal plane slightly lateral or slightly medial to the needle's original position. It should be kept in mind that at a theoretical depth of 9 cm, a 10° correction in the insertion angle would move the tip of the needle approximately 1.6 cm away, and a 20° correction would move it approximately 3.4 cm away. All blocks were injected slowly and with frequent aspirations, using 30 ml of a solution of 1.5% mepivacaine plain plus 3 ml of 8.4% sodium bicarbonate. If no technical difficulties were encountered the patient was positioned for surgery and scrubbing was allowed to proceed. A saphenous nerve block, when indicated, was performed below the knee or in front of the medial malleolus using 3–5 ml of 1% mepivacaine plain.

The surgical procedures included below the knee amputations and different foot procedures such as bunionectomies, hammertoe corrections, and others. At incision time the surgeon probed both the plantar and dorsal surfaces of the foot with a hemostatic clamp, followed by the incision.

The end of anesthesia was recorded as the time when the patient felt the subjective sensation of local anesthetic dissipation. If that happened after the patient had left the postanesthesia care unit, he or she was instructed to record the time of occurrence. The patient was visited or contacted by telephone 24 h later, and questioned about analgesia, side effects, residual anesthesia, and overall satisfaction.

Eleven males and nine females were recruited for the study (table 1). Out of 20 blocks, 11 were performed with the patient in the prone position, and 9 were performed with the patient positioned laterally. Residents performed 15 blocks and the author performed the remaining 5. Table 2shows that 40% of the blocks were accomplished at the first attempt, 85% needed three attempts or fewer, and all blocks were completed after six attempts or fewer. The height of patients who required only one attempt ranged from 147 cm (female) to 178 cm (male). The patients’ weight in this subgroup ranged from 73 kg (female) to 100 kg (female). No pattern of reposition developed. Out of 20 blocks, 18 were successful and two failed.

Table 3shows the different time periods recorded. All incisions were performed in less than 29 min from injection. Anesthesia dissipated 2–4 h after the injection in all patients, but a strong analgesia persisted in some patients up to 24 h later. During the 24-h evaluation all patients expressed satisfaction with the technique and reported no residual numbness, side effects, or complications. We could not reach one Hispanic male patient at the telephone number he provided as a contact, but his block had worn off at discharge from the hospital and he had no complaints at the time.

Anatomy is the foundation on which the success of regional anesthesia rests. It contributes to the safety and consistency of a technique, but most importantly, it simplifies it. It does not seem reasonable that blocking a nerve averaging 2 cm in width, 9indeed the largest peripheral nerve in the body, would require the accurate palpation of pelvic bone structures that are sometimes hard to find. Several authors agree that determining the bony landmarks and/or performing a classic technique is usually complicated. 6–8,16–19Several attempts at simplification have been made. Rucci et al. , in 1989, 19proposed an approach in which the ischial tuberosity also needs to be identified. Chang et al.  18on the other hand, proposed in 1993 a rectal exam to find the ischial spine and indirectly the sciatic nerve. It is hard to believe that these modifications can qualify as “simplified” techniques, but perhaps they help to illustrate the need for a truly simplified approach.

The main goal of this study was to clinically prove what was learned in the anatomy laboratory. Therefore the stress was on feasibility and not necessarily on success rate. We chose for this study a volume of 30 ml of local anesthetic without epinephrine. The short duration of the anesthesia itself (2–4 h) may be related to this. Because the nerve is covered superficially by the gluteus maximus muscle, an argument could be made for the use of epinephrine, whenever not contraindicated, as the absorption from this site could potentially be significant.

While the population of this study is small, the technique was successfully used in adult patients of different gender, height, body habits, and ethnic backgrounds, including a 147-cm tall Hispanic woman and a 196-cm tall Caucasian male, all at approximately 10 cm. This seems to be proof of its versatility. The technique was also easy to teach; residents of all levels performed it successfully. In one case the skin mark was mistakenly placed at 8.5 cm from the midline (possibly because of stretching of the skin during measurement). This was in fact the only case where more than just a slight correction in the insertion angle was necessary.

It is not surprising that the time to place a mark on the skin was measured in seconds, as there are no buried landmarks to find. It is significant however, that the time to first sciatic response was less than 5 min in all cases, more so since this time also includes the infiltration of the skin. All incisions were performed in fewer than 29 min from the time of the injection. One of the reasons for this relatively short time to incision is that, having to perform the blocks in the OR, we allowed the scrubbing to proceed before ascertaining the quality of the sensory blockade. All patients were supine for surgery; the prone position would have required a longer waiting period. The mean accumulated time from the beginning of the procedure to incision was approximately 25 min (range 18–31 min) attesting to the efficiency and speed of the new technique.

One of the two blocks that failed was performed on an obese female (BMI 31) scheduled for a below the knee amputation. The 10-cm needle was found to be too short to reach her sciatic nerve (at the time we did not have a 15-cm insulated needle). After six attempts and with the needle hub pushed against the skin, an intermittent sciatic response was obtained, as the needle kept recoiling back. The surgeon probed the surgical site at about 16 min after the injection and the patient complained of pain. The other failed block occurred in a young female scheduled for removal of a bony mass over the lateral side of her foot. The block was performed at first attempt without any technical difficulties. More than 10 min after the incision was made she referred to an ill defined “discomfort” at the surgical site and requested to be asleep. At the end of a 55-min surgery the patient awoke pain free. Among the 18 patients that received a successful block nobody received more than 2 mg of midazolam and 100 μg of fentanyl total for the entire case and many patients received less. Only the two patients with failed blocks received more sedation and/or propofol.

In this technique there are no bony landmarks to determine, and the needle is advanced in straight angles to the patient's midline. This takes away the difficulty of determining what is “perpendicular to all planes” as needed with the classic and other approaches. 1,3,7,20–22A perpendicular to all planes approach in a round structure, such as the buttocks, means aiming the needle for the center of a sphere. There is a good chance of missing that center around a 360° radius.

The 10-cm distance from the intergluteal sulcus proposed here might well become 9.5 cm or 10.5 cm in different hands. Some others may even learn to use one measurement in some people and a slightly different one in others, and that is perfectly fine. The anatomic and clinical evidence suggest that the 10-cm mark can accommodate a large segment of the adult population, provided that the lower extremities are in adduction.

Because the nerve position in the bony pelvis is dictated by the location of the narrow passage between the ischium and the greater trochanter, the actual size of the buttocks does not play a role. Thus, if a particular adult has a sciatic nerve located 10 cm from the midline when he weighs 70 kg, it should still be the case when his weight has reached 100 kg or more. What it does change is the relative position of the nerve in the buttocks. The adult with small buttocks has a nerve that seems “eccentric,” being much closer to the side than to the midline. In a “medium” sized patient the nerve seems to be very much centrally located, equidistant from the midline and the lateral side, while a large patient seems to have a nerve located closer to the midline. The fact that both male and female bony pelvises are similar in width 6makes these observations applicable to either sex.

The author has proposed to perform this block at the level of the midpoint of the intergluteal sulcus. This point is totally arbitrary but easy to visualize. Because the nerve in the buttocks runs parallel to the midline, the block can be attempted at different levels up and down the buttocks. Thus theoretically other techniques, such as supine 8and subgluteal, 23could also benefit from this 10-cm measurement, avoiding the identification of bone structures. However, the block should not be attempted lateral to the top part of the intergluteal sulcus. The nerve at this point is describing a curve, so the distance to the midline is variable. Also, this is the area where the large inferior gluteal vessels are branching off. A block at this level could potentially carry a higher risk for intravascular injection.

During the performance of this technique with a nerve stimulator, a local twitch confined to the buttocks is obtained when the needle reaches the gluteus maximus muscle. As the needle penetrates the small amount of connective tissue that separates this muscle from the sciatic nerve there is a short period of “silence.” If this silence continues for more than a few centimeters beyond the gluteus maximus the operator should consider repositioning the needle. Any block performed in rational steps is usually described as a “technique.” The repositioning of the needle should be no different. Frequently a failure to elicit a response turns into an almost random search for a “lucky strike.” This new technique offers only two rational alternatives to repositioning, slightly lateral or slightly medial. If bone is contacted it usually indicates that the needle should be repositioned laterally, for the 10-cm measurement from the midline is more likely to fall short of the nerve, hitting the ischium, than being long enough to strike the greater trochanter at the outer side of the nerve.

Because the point of needle insertion falls close to the classic approach, the success rate with either technique should be similar, provided that the operator elicits similar endpoints (twitch or paresthesia). It is tempting to argue that because the new technique seems fast and easy to perform, more blocks could be accomplished successfully in less time. However this remains to be determined.

This newly described technique is a rational effort to simplify both the posterior approach to the sciatic nerve in adults and the repositioning of the needle when that is necessary. It is grounded on anatomic facts and some common sense. The author understands that many practitioners may be reluctant to try a method advocating a “one size fits all” approach, instead of other methods that seem tailored to each particular patient. However, the pitfalls of the latter approaches have been outlined, while the simplicity and rationality of this new approach seem appealing. This new technique has nothing of the geometric beauty of the classic approaches. Instead, it is a technique based on averages and common occurrences. No doubt there will be patients to whom this does not apply; it remains to be seen how often that nonconformity will occur, and whether it will become relevant in clinical practice. The test of time should tell whether this attempt at simplification is instead an oversimplification.

The author thanks the Department of Anatomy, Rush Medical College, Chicago, IL, especially Thomas E. Durica, Ph.D. (Assistant Professor of Anatomy, Director of the Gross Anatomy Laboratory), and Robert J. Beck, Ph.D. (Anatomy Instructor), for their invaluable help. He also thanks Amir B. Rafizad, M.D. (Chief Anesthesiology Resident, Cook County Hospital, Chicago, IL) for his help in gathering the clinical data. The author dedicates this work to the memory of his father.