New York School Of Regional Anesthesia - Popliteal Block For Foot Surgery: Posterior Or Lateral Approach?

The need to position the patient in the prone position is the main disadvantage of the posterior approach to the sciatic nerve block in the popliteal fossa (popliteal block). This may preclude the use of the popliteal block in patients who could benefit the most from this technique (i.e. advanced pregnancy, morbid obesity, spine and hemodynamic instability, mechanical ventilation). A recent study in a human cadaver model demonstrated that the sciatic nerve in the popliteal fossa can also be reliably accessed in the supine position using the lateral approach, Figure 1 (1).

Figure 1. A cadaveric model for the lateral approach to popliteal block: A cross-sectional anatomy of the popliteal fossa. The sloped lines represent two different angles of needle insertion. Circles represent injectates at 30 degree angle and squares injectates at 60 degree angle.

A needle inserted seven cm above the knee in the groove between the biceps femoris and vastus lateralis muscles, and advanced at a 30 degree downward angle relative to the horizontal plane, resulted in a reliable approximation of the needle tip to the popliteal nerve. While the study in cadavers provided guidance for the design of the lateral approach to the popliteal block, its ease and effectiveness has not yet been examined in clinical practice. The aim of this study was to test the clinical utility of the lateral approach to popliteal block and compare its effectiveness and patient acceptance to that of the posterior approach.

Fifty patients undergoing lower leg surgery were randomized to receive a popliteal block using either the lateral or the posterior approach. All patients received midazolam 10 to 30 µcg/kg and fentanyl 1 to 2 µcg/kg intravenously prior to introduction of the anesthetic. Additional doses of midazolam and fentanyl in increments of 1 mg and 50 µcg, respectively, were administered as necessary during the block performance and surgery, in order 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, additional infiltration with a local anesthetic by the surgeon was implemented or an alternative anesthetic was considered.

Technique Of The Popliteal Block Using The Lateral Approach

The patients were in the supine position with their legs extended in the knee joint.
The long axis of the foot was placed at a 90 degree angle to the table.
A 100 mm long, 21G insulated stimulating needle 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 on right) until the shaft of the femoral bone was intentionally contacted.
When the femur was not contacted within the depth of approximately 50 mm, the needle was reinserted 5 to 10 mm anteriorly (above) to the first insertion.
After the femur was contacted, the needle was withdrawn to the skin, and redirected posteriorly at a 30 degree angle to the horizontal plane, Figure 3.
If the stimulation of the sciatic nerve was not obtained, the needle was withdrawn to the skin and reinserted through the same skin puncture, first 5 to 10 degrees anterior and then 5 to 10 degrees posterior relative to the initial insertion (30 degree) plane.
If these redirections (1st attempt) did not result in nerve localization, the same technique was repeated through new skin punctures (subsequent attempts) in 5 mm increments posterior to the initial insertion plane.


Figure 2. A 100 mm longus is inserted in a horizontal plane between the biceps femoris and the vastus lateralis muscles until the femur is contacted.		Figure 3. The needle is inserted posteriorly at a 30 degree angle to the horizontal plane.

Technique Of The Popliteal Block Using The Posterior Approach

The patients were in the prone position (2,3). With the leg fully extended and the long axis of the foot perpendicular to the horizontal plane a 50 mm insulated block needle attached to a nerve stimulator was inserted perpendicular to the skin at the midpoint between the tendons of biceps femoris and semitendinosus muscles, 7 cm above the popliteal fossa crease (Figure 3), Figure 4.
The innitial stimulating current was 0.8 mA.
The needle was slowly advanced while seeking a plantar or dorsiflexion of the foot or toes.
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 and then at 10 degrees laterally to the initial insertion plane (1st attempt).
Failure to obtain the sciatic nerve stimulation prompted removal of the needle, and repeating the same maneuvers, but through a new puncture site 5 mm laterally to the initial insertion site (2nd attempt).
This technique was repeated through new insertion sites (subsequent attempts) in 5 mm incremental lateral insertions, until the desired response was obtained.


Figure 4. The needle is inserted perpendicularly at the midpoint between the tendons of the biceps femoris and semitendinosus muscles		Figure 5. The stimulating current is controlled using a foot-controlled stimulator. This allows a single anesthesiologist to perform the block without the need for an assistant

While the techniques of the popliteal block differed between the lateral and posterior approach, the preparation for the block, local anesthetic solution used for the blocks, and the technique of nerve stimulation were similar.

In both techniques the following procedure was followed:

The skin was prepared with a solution of povidone iodine, and the site of needle insertion was infiltrated with 2 mL of 1% lidocaine using a 25 gauge needle.
The nerve localization was achieved using insulated needles connected to a nerve stimulator. A 100 mm long 21G needle was used for the lateral approach and 50 mm long 22G for the classical approach.
The stimulating needle was connected to the negative lead of the constant voltage nerve stimulator (DualStim" DX, Professional Instruments, Houston, TX), (DualStim™ DX, Professional Instruments, Houston, TX) while the reference electrode was connected to the lateral calf via an EKG electrode ("Red Dot," 3M, London, Ontario).
The initial current of 0.8 mA (1 Hz) was gradually decreased by a single operator using a foot-controlled unit (Hadzic A, Vloka JD.: U.S. Patent Serial No. No. 08/419,419 for "Apparatus for Locating and Anesthetizing Peripheral Nerves and a Method Therefore"), Figure 5.
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 of 1.5% alkalinized mepivacaine (1 mEq of NaHCO3 per 30 mL of mepivacaine) with 1:200,000 epinephrine was injected.
When stimulation of the nerve required a current of greater intensity than 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 to 10 mL of 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 for performance of the block), 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 was ascertained using the pinprick method every 5 minutes following injection of the local anesthetic, for up to 30 minutes. The presence of motor block was assessed by testing the ability of the patient to perform plantar or dorsal flexion of the foot 30 minutes following the injection. A successful block was defined as a complete sensory block affecting both divisions of the sciatic popliteal nerve within 30 minutes and absence of pain on surgical instrumentation. Surgery proceeded once sensory anesthesia in the surgical field was documented by pinching the skin by the surgeon using a hemostat clamp.

Postoperative pain assessments were performed by postanesthesia care unit (PACU) nurses blinded to the purpose of the study. Patients were assessed every 15 minutes from the time of admission to PACU (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, since the data could not be reliably obtained in all patients who were discharged to their homes. All patients were interviewed by an anesthesiologist (blinded to the popliteal block technique) in person (inpatients) or by telephone (outpatients) 24 hours after the operation. During the interview, patients' responses to questions relating to discomfort and pain during block placement, and 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 popliteal block technique to their friends.

Data are presented as mean +/- standard deviations 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 by 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 by P 2, or by Fisher's exact test when numbers were small. Discomfort was assessed by a P 2 for trend. All analyses were performed using the Statistical Package for the Social Sciences (SPSS for Windows), version 5.02 (Chicago, IL, 1993). Differences were considered significant at the 0.05 level.

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

	Posterior Approach (n=25)	Lateral Approach (n=25)
Age (years)	51 +/- 16	49 +/- 14
Height (cm); mean +/- SD Range (cm)	164 +/- 12 147-184	162 +/- 17 125 +/- 194
Weight (kg); mean +/- SD Range (kg)	74 +/- 17 50-109	73 +/- 16 51-104
Sex (M:F)	11:14	16:9
ASA physical status I II-III	11 (44%) 14 (56%)	13 (52%) 12 (48%)
Surgeries: Bunionectomy TMTA I&D foot ORIF ankle Achilles tendon BKA Other	9 (36%) 3 (12%) 2 (8%) 3 (12%) 3 (12%) 1 (4%) 4 (16%)	8 (32%) 5 (20%) 1 (4%) 3 (12%) 1 (4%) 1 (4%) 6 (24%)
Tourniquets used none ankle calf thigh	5 10 9 1	7 11 7 0
Duration of operation (min)	64 +/- 40	60 +/- 37
Surgical setting Inpatients Ambulatory surgery	8 (32%) 17 (68%)	11 (44%) 14 (56%)

Table 1: Patient demographics and surgical procedures.

There were no significant differences in demographics, surgical procedures or the use of tourniquets between patients receiving the popliteal block through the posterior or through the lateral approach. Legend: TMTA-trans-metatarsal amputation, I&D-incision and drainnage, ORIF-open reduction and internal fixation, BKA-below knee amputation.

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 microgram/kg/min) for completion of the procedure. On admission to PACU, however, the patient had a full sensory and motor block, and did not require any analgesics for 630 minutes after surgery. The time to the first analgesic in the posterior PNB group was 568 +/- 159 minutes (range 320-760 min) and 595 +/- 133 minutes (range 365-780 min) in the lateral PNB group; p> 0.05.

Five saphenous nerve blocks (two in the posterior and three in the lateral group) were judged as having failed. Although these patients required supplemental injections of local anesthetics in the distribution of the saphenous nerve, as well as additional doses of intravenous midazo-MTA- trans-metatarsal amputation, I&D-incision and drainnage, ORIF-open reduction and internal fixation, BKA-below knee amputation.

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 microgram/kg/min) for completion of the procedure. On admission to PACU, however, the patient had a full sensory and motor block, and did not require any analgesics for 630 minutes after surgery. The time to the first analgesic in the posterior PNB group was 568 +/- 159 minutes (range 320-760 min) and 595 +/- 133 minutes (range 365-780 min) in the lateral PNB group; p> 0.05.

Five saphenous nerve blocks (two in the posterior and three in the lateral group) were judged as having 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 popliteal block success rate between the two techniques. However, while the majority of patients in the posterior group required only one or two attempts, the majority of patients in the lateral group required a third or fourth attempt in order to localize the nerve (Table 2). This difference was statistically significant (p < 0.001). The initial response to nerve stimulation also significantly varied 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).

	Posterior Approach (n=25)	Lateral Approach (n=25)	p-value
Number of attempts¹: One Two Three Four	13 (52%) 8 (32%) 4(16%) 0	2 (8%) 6 (24%) 10 (40%) 7 (28%)	<0.001
Initial response to stimulation Tibial nerve Peroneal nerve	19 (76%) 6 (24%)	7 (28%) 18 (72%)	<0.05
Nerve stimulating current (mA)	0.37 +/- 0.11	0.42 +/- 0.20	ns
Time to complete block (min)² Time - Block to surgical incision (min)³	mean 6 range: 1-16 min mean 19 range: 10-30	mean 8 range: 1-17 min mean 91 range: 10-29	<0.05 ns
Current	0.37 mA	0.43 mA	ns
Success rate: Saphenous nerve	23 (92%)	22 (88%)	ns
Popliteal block	25 (100%)	24 (96%)	ns

Table 2. Ease of nerve localization and success rate.

Legend:
¹Attempt was defined as insertion of the needle through the skin with one or more needle redirection through the same insertion site.
¹Time from the first needle insertion to the successful nerve localization.
¹Time from injection of local anesthetic to surgical incision. Cases in which surgery was not delayed for reasons other than anesthesia (n=19 in each group) are included in this analysis.

Only two patients required pain medication prior to discharge home (104 and 140 minutes after block placement). Both patients, ametatarsal amputation and a procedure on the medial ankle respectively, complained of pain in the medial aspect of the foot with persistant motor blockade of the foot.

The stimulus intensity required for nerve localization, did not show any signifficant difference between the groups, with the majority of patients requiring a current of 0.4 mA, (Table 2).While the majority of 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 would recommend it to their friends.

	Posterior Approach (n=25)	Lateral Approach (n=25)
No pain	19 (76%)	23 (92%)
Slightly painful	3 (12%)	2 (8%)
Moderately painful	3 (12%)	0
Painful	0	0
Extremely painful	0	0

Table 3. Discomfort¹ during popliteal block performance
Legend: Legend: Pttrend=1.01; p>0.05.
¹Discomfort was assessed by interviewing patients 24 hours after surgery.

The lateral approach to the popliteal block consistently provided reliable surgical anesthesia in the distribution of both divisions of the sciatic nerve. The lateral technique allowed performance of the popliteal block block with patients in the supine position, with the onset and quality of anesthesia, as well as 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 popliteal block in patients resulted in a predictable approximation of the needle tip to the popliteal nerve. While the lateral approach differed from the posterior in regard to patient positioning, anatomical 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, since the peroneal nerve is located laterally, Figure 4. Additionally, the lateral approach required a greater number of attempts and slightly longer time for block completion. One possible contributing factor was the lack of experience with the new technique. A 100 mm long 21G needle used in the lateral technique tended to bend on insertion more than the 50 mm long 22G needle used in the posterior approach, which possibly contributed to the longer time to nerve localization. A shorter needle (e.g., 70 mm), or a needle of a larger diameter (e.g., 20G) could result in a more consistent needle path, and therefore might be preferable for the lateral approach. While it is possible that the lateral approach might be associated with a somewhat greater patient discomfort due to the needle passage through the biceps and/or vastus lateralis muscles (tendons), as well as a larger diameter of the needle, this was not observed under conditions of this study. The premedication with midazolam, and local infiltration of the skin with local anesthetic may have altered the patients perception 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 popliteal block. Since 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 to 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 the local contractions of the biceps femoris muscle, which served as an additional guide during block placement.

Although use of the popliteal block in clinical practice has many potential advantages over neuraxial techniques in patients undergoing lower extremity surgery (5), the technique is infrequently utilized by anesthesiologists in the United States (6). In addition to unfavorable local practice settings (7), possible explanations for infrequent clinical use of the popliteal block include inadequate exposure to this technique during residency training (7), as well as the inability of patients to lie in the prone position, mandated in the popliteal block using a posterior approach (2,8).

Another technique of the lateral approach to the popliteal block (9,10) described the use of 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 in the knee joint, 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 popliteal block in our study is likely the result of precise nerve localization using low intensity nerve stimulation (<0.4 mA). This is in contrast to the common belief that confirmation of the successful localization of the nerve is not essential for block effectiveness (11). Injections of larger volumes of local anesthetic (ie, 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 assuring consistent and dense blockade of the entire popliteal nerve (12).

In summary, the lateral approach popliteal block for ankle and foot surgery resulted in reliable anesthesia, comparable to that of the popliteal block using the posterior approach. The 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 popliteal block in patients who can not 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.

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