A Comparison of the Posterior versus Lateral Approaches to the Block of the Sciatic Nerve in the Popliteal Fossa 

After institutional review board approval was granted and informed consent obtained, 50 patients undergoing lower leg surgery were randomized to receive a PB using either the lateral or the posterior approach. Random assignment to the lateral or posterior PB groups was done using sealed envelopes immediately before surgery. Exclusion criteria included patient refusal, neurologic or neuromuscular disease, or skin infection at the site of needle insertion. All patients received 10 - 30 [micro sign]g/kg midazolam and 1 - 2 [micro sign]g/kg fentanyl intravenously before introduction of the anesthetic. Additional doses of midazolam and fentanyl in increments of 1 mg and 50 [micro sign]g, respectively, were administered as necessary while the block was administered and during surgery to allay any accompanying anxiety and discomfort. The nature and exact origin of the discomfort were elicited from the patients before any additional sedatives or analgesics were given. When it was determined that patients experienced pain on surgical instrumentation, the surgeon infiltrated additional local anesthetic or considered an alternative anesthetic.

The lateral approach to PB was performed with patients in the supine position and with their legs extended at the knee joint. The long axis of the foot was positioned at a 90 [degree sign] angle to the table. A 100-mm, 21-gauge insulated stimulating needle (Stimuplex B; Braun Medical, Bethlehem, PA), attached to a nerve stimulator was inserted in a horizontal plane 7 cm cephalad to the most prominent point of the lateral femoral epicondyle, in the groove between the biceps femoris and the vastus lateralis muscles (Figure 1) until the shaft of the femoral bone was intentionally contacted. If the femur was not contacted within the depth of approximately 50 mm, the needle was inserted against 5 - 10 mm anterior to the first insertion. After the femur was contacted, the needle was withdrawn to the skin and redirected posteriorly at a 30 [degree sign] angle to the horizontal plane (Figure 2). If the sciatic nerve was not stimulated, the needle was withdrawn to the skin and reinserted through the same skin puncture, first 5 - 10 [degree sign] anterior and then 5 - 10 [degree sign] posterior relative to the initial insertion (30 [degree sign]) plane. If this redirection (first attempt) did not result in nerve localization, the same technique was repeated through new skin puncture (subsequent attempts) in 5-mm increments posterior to the initial insertion plane.

The posterior approach to the PB was performed with patients in the prone position. [2,3]With the leg fully extended, a 50-mm insulated block needle (Stimuplex) attached to a nerve stimulator was inserted pependicular at the midpoint between the tendons of the biceps femoris and semitendinosus muscles, 7 cm above the popliteal fossa crease (Figure 3). The needle was advanced slowly while a plantar or dorsiflexion of the foot or toes was sought. When the nerve was not localized on the first needle insertion, the needle was slowly withdrawn to the skin and reinserted through the same skin puncture. The first reinsertion was at an angle of 5 [degree sign] and then at 10 [degree sign] lateral to the initial insertion plane (first attempt). Failure to stimulate the sciatic nerve prompted removal of the needle and repetition of the same maneuvers through a new puncture site 5-mm lateral to the initial insertion site (second attempt). This technique was repeated through new insertion sites (subsequent attempts) in 5-mm incremental lateral insertions until the desired response was obtained.

Although the techniques of the PB differed between the lateral and posterior approach, preparation for the block, local anesthetic solution used for the blocks, and the nerve stimulation technique were similar. In both techniques, the skin was prepared with a solution of povidone iodine, and the site of needle insertion was infiltrated with 2 ml 1% lidocaine using a 25-gauge needle. The stimulating needle was connected to the negative lead of the constant voltage nerve stimulator (DualStim DX; Professional Instruments, Houston, TX), whereas the reference electrode was connected to the lateral calf via an electrocardiogram electrode (Red Dot; 3M, London, Ontario, Canada). The initial current of 0.8 mA (1 Hz) was gradually decreased in both approaches after an initial response to nerve stimulation was obtained. The localization of the nerve was considered successful when either tibial nerve response (plantar flexion) or common peroneal response (lateral inversion - dorsal flexion) was obtained. The output current of the nerve stimulator was adjusted to the lowest current at which these responses were still observed. At this point, after inadvertent intravascular placement of the needle was ruled out by gentle aspiration, 40 ml 1.5% alkalinized mepivacaine (1 mEq of NaHCO³per 30 ml mepivacaine) with 1:200,000 epinephrine was injected. When stimulation of the nerve required a current >0.4 mA, an attempt was made to stimulate the division of the popliteal nerve that predominantly innervated the surgical area. When necessary, a supplementary block of the saphenous nerve was performed at the level of the tibial tuberosity using 8 - 10 ml pH-adjusted 1.5% mepivacaine with 1:200,000 epinephrine. [4] 

The data collected included patient demographics, time from the first needle insertion to successful nerve localization (time required to perform the block), and the number of attempts to localize the popliteal nerve (defined as a number of skin punctures). The presence of sensory block in the distribution of the peroneal and tibial nerves (and saphenous nerve, when indicated) was ascertained using the pinprick method every 5 min after injection of the local anesthetic, for as long as 30 min. The presence of motor block was assessed by testing the ability of the patient to perform plantar or dorsal flexion of the foot 30 min after the injection. A successful block was defined as a complete sensory block affecting both divisions of the sciatic popliteal nerve within 30 min and absence of pain on surgical instrumentation. Surgery proceeded once sensory anesthesia in the surgical field was documented with a pinch to the skin by the surgeon using a hemostat clamp.

Postoperative pain assessments were performed by postanesthesia care unit nurses blinded to the purpose of the study. Patients were assessed every 15 min from the time of admission to the postanesthesia care unit (phase 1 recovery) until their discharge home or transfer to the floor. The time to the first administration of analgesics was evaluated only among inpatients, because the data could not be obtained reliably in all patients who were discharged to their homes. All patients were interviewed by an anesthesiologist (blinded to the PB technique) in person (inpatients) or by telephone (outpatients) 24 h after the operation. During the interview, patients' responses to questions relating to discomfort and pain during block placement and to satisfaction with anesthesia technique were recorded. Pain and discomfort during block placement were rated as no pain, slightly painful, moderately painful, painful, and extremely painful. Patient satisfaction with the anesthetic technique was assessed by questioning as to whether they would choose the same anesthesia technique for any eventual subsequent surgery or if they would recommend the PB technique to their friends.

Data are presented as means +/- SD for continuous variables and as counts and percentages for nominal data. Differences between patients receiving the posterior or the lateral approach with respect to continuous variables (age, height, weight, intensity of the stimulating current, time to complete the block, time to surgical incision, duration of operation) were tested using a Student's t test for independent samples. Differences in proportions of patients in the two groups with respect to nominal data (sex, type of surgery, inpatient vs. ambulatory surgery, number of attempts, number of successes) were tested using the chi-square or Fisher's exact tests when numbers were small. Discomfort was assessed using a chi-square test for trend. All analyses were performed using the Statistical Package for the Social Sciences (SPSS for Windows, version 5.02, Chicago, IL). Differences were considered significant at the 0.05 level.

Fifty patients were enrolled in the study. There were no significant differences in demographics (age, weight, height), American Society of Anesthesiologists status, or type of surgical procedure between the groups (Table 1).

Both techniques resulted in successful blockade in all but one patient in the lateral approach group, who underwent transmetatarsal amputation of the foot. Despite adequate anesthesia on assessment, bone incision proved to be painful, which required infusion of propofol (100 - 130 [micro sign]g [middle dot] kg^-1[middle dot] min^-1) for completion of the procedure. On admission to the postanesthesia care unit, however, the patient had a full sensory and motor block and did not require any analgesics for 630 min after surgery. The time to the first analgesic in the posterior PB group was 568 +/- 159 min (range, 320 - 760 min) and 595 +/- 133 min (range, 365 - 780 min) in the lateral PB group (P > 0.05).

Five saphenous nerve blocks (two in the posterior and three in the lateral group) were judged to have failed. Although these patients required supplemental injections of local anesthetics in the distribution of the saphenous nerve, as well as additional doses of intravenous midazolam, propofol, or fentanyl, none of them required induction of an alternative anesthetic.

There was no difference in the PB success rate between the two techniques. However, although most of the patients in the posterior group required only one or two attempts, most in the lateral group required a third or fourth attempt to localize the nerve (Table 2). This difference was statistically significant (chi-square trend, P < 0.001). The initial response to nerve stimulation also varied significantly between the two techniques, with stimulation of the common peroneal nerve being the most common first response (72%) in the lateral approach group, and the tibial nerve response being the most common (76%) in the posterior approach group (Table 2).

The stimulus intensity required for nerve localization did not show any significant difference between the groups, with most patients requiring a current <or= to 0.4 mA (Table 2). Most patients reported only minimal or no pain during needle insertion (Table 3).

During telephone interviews, all patients expressed high satisfaction with both techniques and stated that they would choose it for any eventual, subsequent surgery, and they would recommend it to their friends.

The lateral approach to the PB consistently provided reliable surgical anesthesia in the distribution of both divisions of the sciatic nerve. The lateral technique allowed performance of the PB block with patients in the supine position, with the onset and quality of anesthesia and patient comfort during block placement comparable to that of the posterior approach.

Consistent with our previous report in human cadavers, [1]the lateral approach to PB in patients resulted in a predictable approximation of the needle tip to the popliteral nerve. Although the lateral approach differed from the posterior in regard to patient positioning, anatomic landmarks, and the technique of block performance, the current requirements and principles of nerve localization were similar. Of note, the common peroneal nerve was stimulated more frequently through the lateral than through the posterior approach. This is not surprising, because the peroneal nerve is located laterally (Figure 4). In addition, the lateral approach required more attempts and slightly longer time to complete the block. One possible contributing factor was the lack of experience with the new technique. A 100-mm, 21-gauge needle used in the lateral technique tended to bend on insertion more than did the 50-mm, 22-gauge needle used in the posterior approach, which possibly contributed to the longer time to nerve localization. A shorter needle (70 mm) or a needle with a larger diameter (20 gauge) could result in a more consistent needle path and therefore might be preferable for the lateral approach. Although it is possible that the lateral approach might be associated with somewhat greater patient discomfort because of the needle passage through the biceps or vastus lateralis muscles (tendons), as well as a larger diameter of the needle, we did not observe this under the conditions of our study. Premedication with midazolam and local infiltration of the skin with local anesthetic may have altered the patients' perceptions of pain, making it difficult to discern any differences in discomfort during block performance.

Local contractions of the biceps femoris muscle, frequently observed during needle advancement, provided a useful estimate of the required depth of needle insertion in the lateral approach to the PB. Because at this level the sciatic nerve is located close to the medial aspect of the biceps femoris muscle (Figure 4), stimulation of the nerve was usually obtained with further advancement of the needle 3 - 5 mm after the local twitches of the biceps femoris muscle disappeared. Failure to localize the nerve with further needle advancement after cessation of the twitches of the biceps femoris muscle indicates that the plane of needle insertion is either anterior or posterior to the nerve. In this case, the needle should be withdrawn to the skin and reinserted with a slight anterior or posterior angulation. The use of a higher initial stimulating current (about 0.8 mA) was beneficial during nerve localization through the lateral approach by accentuating local contractions of the biceps femoris muscle, which in return served as an additional guide during block placement.

Although clinical use of the PB has many potential advantages compared with neuraxial techniques in patients having lower extremity surgery, [5]the technique is used infrequently by anesthesiologists in the United States. [6]In addition to unfavorable local practice settings, [7]possible explanations for infrequent clinical use of the PB include inadequate exposure to this technique during residency training [7]and the inability of patients to lie in the prone position, which is necessary with the posterior approach. [2,8] 

Another report of the lateral approach to the PB [9,10]described use the procedure for postoperative pain management. The technique involved inserting a needle horizontally, anterior to the tendon of the biceps femoris muscle, with the leg slightly flexed at the knee joint and placed on a pillow. Although this technique provided good postoperative pain relief in patients after foot surgery, its frequent failure to provide adequate analgesia [9,10]makes it unsuitable for surgical anesthesia. The high success rate of the PB in our study may be the result of precise nerve localization using low current intensity nerve stimulation (<0.4 mA). This is in contrast to the common belief that confirmed successful localization of the nerve is not essential for block effectiveness. [11]Injections of larger volumes of local anesthetic (40 ml) in the immediate vicinity of the nerve allow substantial spread of local anesthetic solution within the epineural sheath. This results in exposure of both divisions of the sciatic nerve to the local anesthetic, ensuring consistent and dense blockade of the entire popliteal nerve. [12] 

In conclusion, the lateral approach to PB for ankle and foot surgery resulted in reliable anesthesia, comparable to that of the PB using the posterior approach. Performance of the block using the lateral approach appeared straightforward when the described technique was followed, although it took more attempts at nerve localization. In addition to using the PB in patients who cannot assume the prone position, this technique provides the option of performing supplementary blocks (i.e., saphenous or femoral nerve blocks) and surgery without the need for patient repositioning.

The authors thank Gorica Hadzic for editorial assistance.