ULTRASOUND ARTICLE
A Comparison of 2 Ultrasound-Guided Approaches to the Saphenous Nerve Block: Adductor Canal Versus Distal Transsartorial A Prospective, Randomized, Blinded, Noninferiority Trial Anil A. Marian, MD, FRCA, Yatish Ranganath, MD, Emine O. Bayman, PhD, Jeffrey Senasu, MD, and Timothy J. Brennan, MD, PhD Background and Objectives: Saphenous nerve blocks can be technically challenging. Recently described ultrasound techniques have improved the success rate of saphenous nerve blocks, but randomized controlled trials comparing these ultrasound-guided techniques are lacking. We compared 2 common ultrasound-guided approaches for saphenous nerve block: saphenous nerve block at the adductor canal (ACSNB) versus block by the distal transsartorial (DTSNB) approach. Methods: Patients received either ACSNB or DTSNB in this prospective, randomized, blinded, noninferiority clinical trial. The primary objective was to show the noninferiority of ACSNB to DTSNB in terms of block success. Secondary outcome measures were time required to perform the block, time to onset of successful block, and the visibility of the nerve using ultrasound. Results: One hundred twenty patients were randomized to receive DTSNB (n = 62) or ACSNB (n = 58). There were 9 failures in the DTSNB group (85% success) and no failures in the ACSNB group (100% success), 90% confidence interval of difference in success rates (DTSNB - ACSNB) was −0.195 to −0.031. Given that the upper confidence bound (−3.1%) was less than 10%, the success rate of ACSNB was noninferior to DTSNB. After satisfying noninferiority and observing a greater success rate of ACSNB compared with DTSNB, we also determined that ACSNB was superior to DTSNB (P = 0.003). The median time to success was significantly less for the ACSNB group: 9 minutes versus 3 minutes (P < 0.001). The grade of the ultrasound image, as judged by the provider, was significantly better in the ACSNB group (P = 0.001). Conclusions: Ultrasound-guided block of the saphenous nerve at the adductor canal is not only noninferior but also superior to block at the distal transsartorial level in terms of success rate, with additional advantages of faster block onset time and better nerve visibility under ultrasound. (Reg Anesth Pain Med 2015;40: 623–630)
From the Department of Anesthesia, University of Iowa Hospitals and Clinics, Iowa City, IA. Dr Yatish Ranganath is now with Georgia Regents University, Augusta, GA, and Dr Jeffrey Senasu is now with UnityPoint/Trinity, Rock Island, IL. Editor's Note: Regional Anesthesia and Pain Medicine now requires that all clinical trials be registered before patient enrollment. The submission was accepted before this requirement was stated explicitly in our Instructions for Authors. Address correspondence to: Anil A. Marian, MD, FRCA, Clinical Associate Professor, Department of Anesthesia, University of Iowa Hospitals and Clinics, Iowa City, IA 52242 (e‐mail: [email protected] ). Attribution: Department of Anesthesia, University of Iowa Hospitals and Clinics, Iowa City, IA. This work was supported by funding from the Department of Anesthesia, University of Iowa Hospitals & Clinics. This study has not been presented at any meetings. Accepted for publication May 1, 2015. Copyright © 2015 by American Society of Regional Anesthesia and Pain Medicine ISSN: 1098-7339 DOI: 10.1097/AAP.0000000000000277
T
he saphenous nerve, a terminal branch of the femoral nerve, is a pure sensory nerve that supplies the skin anterior to the patella and to the anteromedial aspect of the leg and proximal part of the medial foot. A saphenous nerve block, in combination with a sciatic nerve block, is commonly performed for patients undergoing surgery of the lower leg, ankle, or foot either as the primary anesthetic or for postoperative pain relief.1 Numerous techniques for selectively blocking the saphenous nerve have been described based on landmark-guided, nerve stimulation–guided, and ultrasound-guided techniques above and below the knee.2–10 In 2007, Krombach et al5 described ultrasound-guided distal transsartorial saphenous nerve block (DTSNB) by injecting at the fascial plane under the sartorius muscle in the distal thigh (with or without imaging of the saphenous nerve and the saphenous branch of descending geniculate artery). In 2009, Manickam et al11 described ultrasound-guided saphenous nerve block at the distal adductor canal. Subsequently, Kirkpatrick et al12 described a more proximal approach within the adductor canal, with the superficial femoral artery (SFA) as the key landmark. These 2 approaches (blocking the saphenous nerve within the adductor canal and blocking distal to the adductor canal in the lower third of the leg) are currently the most commonly used ultrasound-guided techniques for saphenous nerve block. A recent volunteer study compared landmark-based below-knee field block to ultrasound-guided techniques and proved the superiority of ultrasound-guided techniques.13 However, there have not been any randomized controlled clinical trials specifically designed to compare the different ultrasound-guided techniques to block the saphenous nerve. This prospective, randomized, controlled, noninferiority trial compared the mid-thigh adductor canal saphenous nerve block (ACSNB) with the DTSNB. The distal transsartorial approach is the standard ultrasound-guided technique used at our hospital for blocking the saphenous nerve. We hypothesized that ACSNB is noninferior to DTSNB in terms of loss of pinprick sensation among patients undergoing lower leg, ankle, or foot surgery. Secondary end points were time taken to perform the block, time to onset of successful nerve block, and grade of the nerve visibility on the ultrasound image.
METHODS The institutional review board of the University of Iowa Hospital and Clinics, Iowa City, Iowa, approved this study. This trial was not prospectively registered on clinicaltrials.gov but currently registered as NCT02383615. Patients were enrolled from April 2013 to August 2014. All patients provided written informed consent. All patients were undergoing elective foot and/or ankle surgery and were receiving sciatic (by either popliteal or subgluteal approach) and saphenous nerve blocks as part of the routine regional anesthesia practice at our hospital. Other eligibility criteria included age 17 to 70 years and American Society of
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Regional Anesthesia and Pain Medicine • Volume 40, Number 5, September-October 2015
Anesthesiologists Physical Status (ASA PS) classification 1, 2, or 3. Exclusion criteria included preexisting neuropathy in the operative leg, allergy to local anesthetics, and ASA PS 4 and 5. All patients also received general anesthesia, spinal anesthesia, or sedation as needed for the surgical procedure based on type of procedure, tourniquet site, and patient preferences. A randomized block design with variable block sizes was used to create the randomization sequence before the study began. The randomization was stratified by 4 providers to ensure a similar number of saphenous nerve blocks performed by each provider in each of the 2 groups. The total number of nerve blocks performed by each provider was not required to be equal. Group assignments were sealed in opaque envelopes that were opened immediately before performance of the blocks. All providers were regional anesthesiologists with experience in ultrasound-guided nerve blocks.
Block Performance Before receiving the block, a research assistant recorded baseline patient demographics. After time-out verification of the procedure and consent, electrocardiogram, pulse oximeter, and noninvasive blood pressure monitors were placed and were used to monitor patients throughout and after the procedure. Sedation and oxygen were administered by provider preference. Patients received intravenous Ringer’s lactate solution at a maintenance rate. The 3 areas for testing the success of the saphenous nerve block (infrapatellar, medial leg, and medial ankle) were marked with a skin marker and sharp sensation to pinprick with a 22-gauge needle was confirmed in all 3 areas before performing the block. The blocks were performed with the patient in the supine position, with the leg slightly externally rotated at the hip. The blocks were performed under ultrasound guidance (SparqPhilips, Venue 40, GE Healthcare; M-turbo, Sonosite; or Flex Focus 400, BK Medical) with a corresponding linear probe (6–18 MHZ) or the curvilinear probe (2–6 MHZ) based on patient characteristics and provider preference. A 21-gauge 90-mm insulated needle (StimuQuick, Arrow) was used for all blocks. After identifying the structures and sterile prep with 2% chlorhexidine and 70% isopropyl alcohol, skin and subcutaneous tissue were anesthetized with 1% lidocaine using a 25-gauge needle. The nerve block needle was then introduced in the plane of the ultrasound beam. On reaching the targeted location, a test dose of 2 to 5 mL of 5% dextrose with 1:200,000 epinephrine was injected to rule out intravascular placement of the needle. The local anesthetic drug, 10 mL of 0.5% ropivacaine, was then injected in 3- to 5-mL increments until the total volume was injected. A research assistant, who was blinded to the type of the block, recorded the time taken to perform the block, relying on verbal commands from the block provider. The time to perform the block was defined as the time from the start of local anesthetic infiltration of skin to the time the block needle was removed from the patient. At the end of the block performance, a stopwatch was started and the blinded research assistant tested pinprick sensation in the predefined areas every 3 minutes. Success was defined as loss of sharp sensation to pinprick (dull or can’t feel) in 2 of the 3 areas of saphenous nerve innervation at the previously marked areas. Once success was confirmed, testing was stopped. If the block was not successful by 30 minutes, the block was considered to be a failure and the testing stopped. The operating room anesthesia team was notified about these block failures, and additional pain relief measures including local infiltration at the wound site were performed to make sure patients were comfortable in the postoperative period. The provider performing the block graded the visibility of the nerve under
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ultrasound and it was recorded (grade 1, if the saphenous nerve was clearly identifiable even before injecting local anesthetic; grade 2, if saphenous nerve was visible after injecting the local anesthetic; and grade 3, if saphenous nerve was not identifiable even after injection of the local anesthetic). Success of the sciatic nerve block was not assessed as part of this study.
Description of Techniques Distal Transsartorial Saphenous Nerve Block The saphenous nerve was blocked at the lower third of the medial thigh, where it lies in the subsartorial fascial plane between the sartorius muscle and the vastus medialis muscle. Patients were placed in supine position, with the leg slightly rotated externally. The ultrasound probe was placed perpendicular to the long axis of the leg about 5 to 10 cm (3-finger breadth) proximal to the popliteal crease on the medial thigh. The sartorius and vastus medialis muscles were identified. The saphenous nerve was then identified if possible, along with the saphenous branch of the descending geniculate artery in some cases, under the sartorius muscle (Fig. 1). If the nerve was not clearly visualized, the fascial plane between the sartorius and the vastus medialis (closer to the deep part of the sartorius muscle) was chosen as the end point. After negative aspiration and test dose, the local anesthetic was injected as previously described.
Adductor Canal Saphenous Nerve Block The saphenous nerve was blocked at the level of the midthigh during its course through the adductor canal (Hunters canal), adjacent to the SFA. The sartorius muscle and SFA were located by ultrasound at the medial aspect of upper mid-thigh, with the probe perpendicular to the long axis of the leg. The saphenous nerve appears as a hyperechoic circle anterolateral to the SFA (Fig. 2). Farther down its course, the nerve crosses the SFA and lies anteromedial to the SFA before it exits the adductor canal. The end point was the needle position at the anterolateral corner of the SFA under the sartorius muscle in the proximal adductor canal. After negative aspiration and test dose, the local anesthetic solution was injected as previously described.
Statistical Analysis: Sample Size Calculation To address the primary objective of the study—to show noninferiority of the ACSNB to DTSNB in terms of success as previously defined—we used the test of noninferiority of 2 proportions. The DTSNB is the primary technique of doing saphenous nerve block at our institution. From chart review of the electronic records at our institution from 2011 to 2012, we found that there was a 3% incidence of rescue blocks among saphenous nerve blocks (19 out of 667 blocks). This assumed a success rate of approximately 97%, although not all failed blocks would have received a rescue block. Saranteas et al7 reported a success rate of 95.6% in healthy volunteers undergoing saphenous nerve block just after it exited from the adductor canal. Based on these 2 observations, we expected the success rate of DTSNB to be 95%. To establish the noninferiority of ACSNB to DTSNB, we predetermined that the success rate of ACSNB should not be lower than 85% (the expected difference in proportions or delta should be less than 95%–85% = 10%). Under these assumptions, to achieve 80% power, with a 1-sided type I error rate of 0.05, we needed 59 subjects per group; 118 patients total. We enrolled 120 patients for this study. To show the noninferiority of ACSNB to DTSNB; the upper 95% confidence bound of the success rates (πDTSNB – πACSNB) should be less than 10%. © 2015 American Society of Regional Anesthesia and Pain Medicine
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Regional Anesthesia and Pain Medicine • Volume 40, Number 5, September-October 2015
Ultrasound-Guided Saphenous Blocks
FIGURE 1. Ultrasound image of the DTSNB. The saphenous nerve lies close to the saphenous branch of the descending geniculate artery (descending branch) between the sartorius and vastus medialis muscles at the distal thigh.
FIGURE 2. Ultrasound image of the ACSNB. The saphenous nerve lies anterolateral to the SFA below the sartorius muscle at the mid-thigh. © 2015 American Society of Regional Anesthesia and Pain Medicine
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The normality of continuous variables was tested by the Shapiro-Wilk test. Based on the normality assumption, 2 independentsample t tests or Mann-Whitney U tests were performed for continuous variables. The χ2 test or Fisher exact test was performed for categorical variables. Because of observing 100% success rate in the ACSNB group, to calculate the confidence interval (CI) of the difference, 2 successes and 2 failures were added to the ACSNB group.14 Statistical analyses were performed on Wizard for Mac software version 1.5 (Evan Miller). The CI of the difference on success rates was calculated with the SAS software version 9.3 (Cary, North Carolina).
RESULTS Patient recruitment and flow through the protocol were as described by the CONSORT (Consolidated Standards of Reporting Trial) diagram (Fig. 3). Two patients were excluded after consent because of time constraints. Of the 120 patients enrolled, 62 were randomized to the DTSNB group and 58 were randomized to the ACSNB group. One patient received DTSNB after being randomized to the ACSNB group. In this case, the provider felt that there was high risk of intravascular needle placement because of the anatomy of the adductor canal. Analysis was done as intent-totreat basis. Furthermore, sensitivity analyses were performed
a) on as-treated basis and b) with exclusion of this 1 patient as a randomization failure. Baseline demographics were comparable in each group except for sex (Table 1). There were 9 failures and 53 successes in the DTSNB (85% success) group and no failures in the ACSNB group (100% success); 90% 2-sided CI of the difference in success rates (DTSNB-ACSNB) was −0.195 to −0.031. Given that the upper 95% confidence bound (the upper limit of the 2-sided 90% CI, −3.1%) is less than 10%, the success rate of ACSNB was noninferior to DTSNB (Fig. 4). After satisfying the noninferiority criterion and observing a greater success rate of ACSNB compared with DTSNB, we also examined the superiority of ACSNB to DTSNB and found that ACSNB is superior to DTSNB (P = 0.003; Table 2). The median time to success was significantly shorter for the ACSNB group: 9 minutes (DTSNB) versus 3 minutes (ACSNB) (P < 0.001; Table 3). The grade of the ultrasound image, as judged by the provider, was significantly better in the ACSNB group: grade 1 (nerve visualized before injecting local anesthetic), 58.6%; grade 2 (nerve visualized after injecting local anesthetic), 27.6%; and grade 3 (nerve unable to be visualized), 13.8% (P = 0.001; Table 3). The median time to perform the block was 112 seconds in the DTSNB group versus 131 seconds in the ACSNB group (P = 0.008).
FIGURE 3. CONSORT diagram showing patient recruitment and flow through the study protocol.
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TABLE 1. Demographics
TABLE 2. Primary Outcome DTSNB (n = 62)
Age, mean ± SD, y Sex Female Male Race White African American BMI, mean ± SD ASA PS 1 grade 2 3
Ultrasound-Guided Saphenous Blocks
ACSNB (n = 58)
P
47.1 ± 13 44 (71%) 18 (29%) 60 (96.8%) 2 (3.2%)
45.2 ± 14.7 0.437 (t test) 26 (44.8%) 0.004 (χ2 test) 32 (55.2%) 55 (94.8%) 0.594 (χ2 test) 3 (5.2%)
30.2 ± 6.5 12 (19.4%) 43 (69.4%) 7 (11.3%)
31.3 ± 7.2 0.410 (t test) 18 (31.0%) 0.333 (χ2 test) 34 (58.6%) 6 (10.3%)
BMI indicates body mass index.
Block success
Success Failure
DTSNB (n = 62)
ACSNB (n = 58)
P
53 (85.5%) 9 (14.5%)
58 (100%) 0 (0.0%)
0.003 (Fisher exact test)
The 95% confidence interval of the difference on success rates (DTSNB - ACSNB) is −0.195 to −0.031.
Two of the 9 patients with failed blocks in the DTSNB group received rescue saphenous nerve blocks by the primary anesthesia team in the postanesthesia care unit. Standard postoperative pain medications were used to treat any pain as needed for the rest of the patients.
Adverse Events Analysis by Provider The randomization sequence was stratified among 4 providers as previously described. Distribution of blocks by providers is presented in Table 4. Provider 3 dropped out of the study after performing 2 blocks because of other research commitments unrelated to the study. There was no statistically significant difference among the providers in terms of success rate of DTSNB (P = 0.909; Table 4). There was 1 randomization failure. Sensitivity analyses were performed based on “as-treated basis” and with exclusion of data from the patient who was a randomization failure. The results were consistent with the “intention-to-treat” results (Tables 5 and 6).
One patient in the ACSNB group had intravascular placement (SFA) of the block needle. It was promptly identified on aspiration, the needle was repositioned, and the block was performed as usual. None of the study patients reported any saphenous block–related complications in the postsurgical follow-ups and phone calls. One patient had persistent neurological symptoms in the popliteal nerve block area (common peroneal), which resolved in the 1-month follow-up.
DISCUSSION In this study, blocking the saphenous nerve at the adductor canal was not only noninferior but was, in fact, superior to
FIGURE 4. Noninferiority diagram with observed difference in success rates (DTSNB - ACSNB). The blue dashed line at x = 0.1 indicates the noninferiority margin. Error bars indicate 2-sided 90% CI of the difference in success rates (DTSNB - ACSNB): −0.195 to −0.031. The upper 95% 1-sided confidence bound (the upper limit of the 2-sided 90% confidence interval) is −0.031 and less than 0.1 indicates that ACSNB is noninferior to DTSNB. Because the CI of the difference wholly lies on the left side of zero, ACSNB is not only noninferior but also superior to DTSNB. © 2015 American Society of Regional Anesthesia and Pain Medicine
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TABLE 3. Secondary Outcomes
Time to success, median (IQR), min Time to perform block, median (IQR), s Ultrasound image of nerve Grade 1 Grade 2 Grade 3
DTSNB (n = 62)
ACSNB (n = 58)
P
9 (6–18) 112 (89–137) 16 (25.8%) 29 (46.8%) 17 (27.4%)
3 (3–6) 131 (109–156) 34 (58.6%) 16 (27.6%) 8 (13.8%)
A Comparison of 2 Ultrasound-Guided Approaches to the Saphenous Nerve Block: Adductor Canal Versus Distal Transsartorial A Prospective, Randomized, Blinded, Noninferiority Trial Anil A. Marian, MD, FRCA, Yatish Ranganath, MD, Emine O. Bayman, PhD, Jeffrey Senasu, MD, and Timothy J. Brennan, MD, PhD Background and Objectives: Saphenous nerve blocks can be technically challenging. Recently described ultrasound techniques have improved the success rate of saphenous nerve blocks, but randomized controlled trials comparing these ultrasound-guided techniques are lacking. We compared 2 common ultrasound-guided approaches for saphenous nerve block: saphenous nerve block at the adductor canal (ACSNB) versus block by the distal transsartorial (DTSNB) approach. Methods: Patients received either ACSNB or DTSNB in this prospective, randomized, blinded, noninferiority clinical trial. The primary objective was to show the noninferiority of ACSNB to DTSNB in terms of block success. Secondary outcome measures were time required to perform the block, time to onset of successful block, and the visibility of the nerve using ultrasound. Results: One hundred twenty patients were randomized to receive DTSNB (n = 62) or ACSNB (n = 58). There were 9 failures in the DTSNB group (85% success) and no failures in the ACSNB group (100% success), 90% confidence interval of difference in success rates (DTSNB - ACSNB) was −0.195 to −0.031. Given that the upper confidence bound (−3.1%) was less than 10%, the success rate of ACSNB was noninferior to DTSNB. After satisfying noninferiority and observing a greater success rate of ACSNB compared with DTSNB, we also determined that ACSNB was superior to DTSNB (P = 0.003). The median time to success was significantly less for the ACSNB group: 9 minutes versus 3 minutes (P < 0.001). The grade of the ultrasound image, as judged by the provider, was significantly better in the ACSNB group (P = 0.001). Conclusions: Ultrasound-guided block of the saphenous nerve at the adductor canal is not only noninferior but also superior to block at the distal transsartorial level in terms of success rate, with additional advantages of faster block onset time and better nerve visibility under ultrasound. (Reg Anesth Pain Med 2015;40: 623–630)
From the Department of Anesthesia, University of Iowa Hospitals and Clinics, Iowa City, IA. Dr Yatish Ranganath is now with Georgia Regents University, Augusta, GA, and Dr Jeffrey Senasu is now with UnityPoint/Trinity, Rock Island, IL. Editor's Note: Regional Anesthesia and Pain Medicine now requires that all clinical trials be registered before patient enrollment. The submission was accepted before this requirement was stated explicitly in our Instructions for Authors. Address correspondence to: Anil A. Marian, MD, FRCA, Clinical Associate Professor, Department of Anesthesia, University of Iowa Hospitals and Clinics, Iowa City, IA 52242 (e‐mail: [email protected] ). Attribution: Department of Anesthesia, University of Iowa Hospitals and Clinics, Iowa City, IA. This work was supported by funding from the Department of Anesthesia, University of Iowa Hospitals & Clinics. This study has not been presented at any meetings. Accepted for publication May 1, 2015. Copyright © 2015 by American Society of Regional Anesthesia and Pain Medicine ISSN: 1098-7339 DOI: 10.1097/AAP.0000000000000277
T
he saphenous nerve, a terminal branch of the femoral nerve, is a pure sensory nerve that supplies the skin anterior to the patella and to the anteromedial aspect of the leg and proximal part of the medial foot. A saphenous nerve block, in combination with a sciatic nerve block, is commonly performed for patients undergoing surgery of the lower leg, ankle, or foot either as the primary anesthetic or for postoperative pain relief.1 Numerous techniques for selectively blocking the saphenous nerve have been described based on landmark-guided, nerve stimulation–guided, and ultrasound-guided techniques above and below the knee.2–10 In 2007, Krombach et al5 described ultrasound-guided distal transsartorial saphenous nerve block (DTSNB) by injecting at the fascial plane under the sartorius muscle in the distal thigh (with or without imaging of the saphenous nerve and the saphenous branch of descending geniculate artery). In 2009, Manickam et al11 described ultrasound-guided saphenous nerve block at the distal adductor canal. Subsequently, Kirkpatrick et al12 described a more proximal approach within the adductor canal, with the superficial femoral artery (SFA) as the key landmark. These 2 approaches (blocking the saphenous nerve within the adductor canal and blocking distal to the adductor canal in the lower third of the leg) are currently the most commonly used ultrasound-guided techniques for saphenous nerve block. A recent volunteer study compared landmark-based below-knee field block to ultrasound-guided techniques and proved the superiority of ultrasound-guided techniques.13 However, there have not been any randomized controlled clinical trials specifically designed to compare the different ultrasound-guided techniques to block the saphenous nerve. This prospective, randomized, controlled, noninferiority trial compared the mid-thigh adductor canal saphenous nerve block (ACSNB) with the DTSNB. The distal transsartorial approach is the standard ultrasound-guided technique used at our hospital for blocking the saphenous nerve. We hypothesized that ACSNB is noninferior to DTSNB in terms of loss of pinprick sensation among patients undergoing lower leg, ankle, or foot surgery. Secondary end points were time taken to perform the block, time to onset of successful nerve block, and grade of the nerve visibility on the ultrasound image.
METHODS The institutional review board of the University of Iowa Hospital and Clinics, Iowa City, Iowa, approved this study. This trial was not prospectively registered on clinicaltrials.gov but currently registered as NCT02383615. Patients were enrolled from April 2013 to August 2014. All patients provided written informed consent. All patients were undergoing elective foot and/or ankle surgery and were receiving sciatic (by either popliteal or subgluteal approach) and saphenous nerve blocks as part of the routine regional anesthesia practice at our hospital. Other eligibility criteria included age 17 to 70 years and American Society of
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Copyright © 2015 American Society of Regional Anesthesia and Pain Medicine. Unauthorized reproduction of this article is prohibited.
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Regional Anesthesia and Pain Medicine • Volume 40, Number 5, September-October 2015
Anesthesiologists Physical Status (ASA PS) classification 1, 2, or 3. Exclusion criteria included preexisting neuropathy in the operative leg, allergy to local anesthetics, and ASA PS 4 and 5. All patients also received general anesthesia, spinal anesthesia, or sedation as needed for the surgical procedure based on type of procedure, tourniquet site, and patient preferences. A randomized block design with variable block sizes was used to create the randomization sequence before the study began. The randomization was stratified by 4 providers to ensure a similar number of saphenous nerve blocks performed by each provider in each of the 2 groups. The total number of nerve blocks performed by each provider was not required to be equal. Group assignments were sealed in opaque envelopes that were opened immediately before performance of the blocks. All providers were regional anesthesiologists with experience in ultrasound-guided nerve blocks.
Block Performance Before receiving the block, a research assistant recorded baseline patient demographics. After time-out verification of the procedure and consent, electrocardiogram, pulse oximeter, and noninvasive blood pressure monitors were placed and were used to monitor patients throughout and after the procedure. Sedation and oxygen were administered by provider preference. Patients received intravenous Ringer’s lactate solution at a maintenance rate. The 3 areas for testing the success of the saphenous nerve block (infrapatellar, medial leg, and medial ankle) were marked with a skin marker and sharp sensation to pinprick with a 22-gauge needle was confirmed in all 3 areas before performing the block. The blocks were performed with the patient in the supine position, with the leg slightly externally rotated at the hip. The blocks were performed under ultrasound guidance (SparqPhilips, Venue 40, GE Healthcare; M-turbo, Sonosite; or Flex Focus 400, BK Medical) with a corresponding linear probe (6–18 MHZ) or the curvilinear probe (2–6 MHZ) based on patient characteristics and provider preference. A 21-gauge 90-mm insulated needle (StimuQuick, Arrow) was used for all blocks. After identifying the structures and sterile prep with 2% chlorhexidine and 70% isopropyl alcohol, skin and subcutaneous tissue were anesthetized with 1% lidocaine using a 25-gauge needle. The nerve block needle was then introduced in the plane of the ultrasound beam. On reaching the targeted location, a test dose of 2 to 5 mL of 5% dextrose with 1:200,000 epinephrine was injected to rule out intravascular placement of the needle. The local anesthetic drug, 10 mL of 0.5% ropivacaine, was then injected in 3- to 5-mL increments until the total volume was injected. A research assistant, who was blinded to the type of the block, recorded the time taken to perform the block, relying on verbal commands from the block provider. The time to perform the block was defined as the time from the start of local anesthetic infiltration of skin to the time the block needle was removed from the patient. At the end of the block performance, a stopwatch was started and the blinded research assistant tested pinprick sensation in the predefined areas every 3 minutes. Success was defined as loss of sharp sensation to pinprick (dull or can’t feel) in 2 of the 3 areas of saphenous nerve innervation at the previously marked areas. Once success was confirmed, testing was stopped. If the block was not successful by 30 minutes, the block was considered to be a failure and the testing stopped. The operating room anesthesia team was notified about these block failures, and additional pain relief measures including local infiltration at the wound site were performed to make sure patients were comfortable in the postoperative period. The provider performing the block graded the visibility of the nerve under
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ultrasound and it was recorded (grade 1, if the saphenous nerve was clearly identifiable even before injecting local anesthetic; grade 2, if saphenous nerve was visible after injecting the local anesthetic; and grade 3, if saphenous nerve was not identifiable even after injection of the local anesthetic). Success of the sciatic nerve block was not assessed as part of this study.
Description of Techniques Distal Transsartorial Saphenous Nerve Block The saphenous nerve was blocked at the lower third of the medial thigh, where it lies in the subsartorial fascial plane between the sartorius muscle and the vastus medialis muscle. Patients were placed in supine position, with the leg slightly rotated externally. The ultrasound probe was placed perpendicular to the long axis of the leg about 5 to 10 cm (3-finger breadth) proximal to the popliteal crease on the medial thigh. The sartorius and vastus medialis muscles were identified. The saphenous nerve was then identified if possible, along with the saphenous branch of the descending geniculate artery in some cases, under the sartorius muscle (Fig. 1). If the nerve was not clearly visualized, the fascial plane between the sartorius and the vastus medialis (closer to the deep part of the sartorius muscle) was chosen as the end point. After negative aspiration and test dose, the local anesthetic was injected as previously described.
Adductor Canal Saphenous Nerve Block The saphenous nerve was blocked at the level of the midthigh during its course through the adductor canal (Hunters canal), adjacent to the SFA. The sartorius muscle and SFA were located by ultrasound at the medial aspect of upper mid-thigh, with the probe perpendicular to the long axis of the leg. The saphenous nerve appears as a hyperechoic circle anterolateral to the SFA (Fig. 2). Farther down its course, the nerve crosses the SFA and lies anteromedial to the SFA before it exits the adductor canal. The end point was the needle position at the anterolateral corner of the SFA under the sartorius muscle in the proximal adductor canal. After negative aspiration and test dose, the local anesthetic solution was injected as previously described.
Statistical Analysis: Sample Size Calculation To address the primary objective of the study—to show noninferiority of the ACSNB to DTSNB in terms of success as previously defined—we used the test of noninferiority of 2 proportions. The DTSNB is the primary technique of doing saphenous nerve block at our institution. From chart review of the electronic records at our institution from 2011 to 2012, we found that there was a 3% incidence of rescue blocks among saphenous nerve blocks (19 out of 667 blocks). This assumed a success rate of approximately 97%, although not all failed blocks would have received a rescue block. Saranteas et al7 reported a success rate of 95.6% in healthy volunteers undergoing saphenous nerve block just after it exited from the adductor canal. Based on these 2 observations, we expected the success rate of DTSNB to be 95%. To establish the noninferiority of ACSNB to DTSNB, we predetermined that the success rate of ACSNB should not be lower than 85% (the expected difference in proportions or delta should be less than 95%–85% = 10%). Under these assumptions, to achieve 80% power, with a 1-sided type I error rate of 0.05, we needed 59 subjects per group; 118 patients total. We enrolled 120 patients for this study. To show the noninferiority of ACSNB to DTSNB; the upper 95% confidence bound of the success rates (πDTSNB – πACSNB) should be less than 10%. © 2015 American Society of Regional Anesthesia and Pain Medicine
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Regional Anesthesia and Pain Medicine • Volume 40, Number 5, September-October 2015
Ultrasound-Guided Saphenous Blocks
FIGURE 1. Ultrasound image of the DTSNB. The saphenous nerve lies close to the saphenous branch of the descending geniculate artery (descending branch) between the sartorius and vastus medialis muscles at the distal thigh.
FIGURE 2. Ultrasound image of the ACSNB. The saphenous nerve lies anterolateral to the SFA below the sartorius muscle at the mid-thigh. © 2015 American Society of Regional Anesthesia and Pain Medicine
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Regional Anesthesia and Pain Medicine • Volume 40, Number 5, September-October 2015
Marian et al
The normality of continuous variables was tested by the Shapiro-Wilk test. Based on the normality assumption, 2 independentsample t tests or Mann-Whitney U tests were performed for continuous variables. The χ2 test or Fisher exact test was performed for categorical variables. Because of observing 100% success rate in the ACSNB group, to calculate the confidence interval (CI) of the difference, 2 successes and 2 failures were added to the ACSNB group.14 Statistical analyses were performed on Wizard for Mac software version 1.5 (Evan Miller). The CI of the difference on success rates was calculated with the SAS software version 9.3 (Cary, North Carolina).
RESULTS Patient recruitment and flow through the protocol were as described by the CONSORT (Consolidated Standards of Reporting Trial) diagram (Fig. 3). Two patients were excluded after consent because of time constraints. Of the 120 patients enrolled, 62 were randomized to the DTSNB group and 58 were randomized to the ACSNB group. One patient received DTSNB after being randomized to the ACSNB group. In this case, the provider felt that there was high risk of intravascular needle placement because of the anatomy of the adductor canal. Analysis was done as intent-totreat basis. Furthermore, sensitivity analyses were performed
a) on as-treated basis and b) with exclusion of this 1 patient as a randomization failure. Baseline demographics were comparable in each group except for sex (Table 1). There were 9 failures and 53 successes in the DTSNB (85% success) group and no failures in the ACSNB group (100% success); 90% 2-sided CI of the difference in success rates (DTSNB-ACSNB) was −0.195 to −0.031. Given that the upper 95% confidence bound (the upper limit of the 2-sided 90% CI, −3.1%) is less than 10%, the success rate of ACSNB was noninferior to DTSNB (Fig. 4). After satisfying the noninferiority criterion and observing a greater success rate of ACSNB compared with DTSNB, we also examined the superiority of ACSNB to DTSNB and found that ACSNB is superior to DTSNB (P = 0.003; Table 2). The median time to success was significantly shorter for the ACSNB group: 9 minutes (DTSNB) versus 3 minutes (ACSNB) (P < 0.001; Table 3). The grade of the ultrasound image, as judged by the provider, was significantly better in the ACSNB group: grade 1 (nerve visualized before injecting local anesthetic), 58.6%; grade 2 (nerve visualized after injecting local anesthetic), 27.6%; and grade 3 (nerve unable to be visualized), 13.8% (P = 0.001; Table 3). The median time to perform the block was 112 seconds in the DTSNB group versus 131 seconds in the ACSNB group (P = 0.008).
FIGURE 3. CONSORT diagram showing patient recruitment and flow through the study protocol.
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Regional Anesthesia and Pain Medicine • Volume 40, Number 5, September-October 2015
TABLE 1. Demographics
TABLE 2. Primary Outcome DTSNB (n = 62)
Age, mean ± SD, y Sex Female Male Race White African American BMI, mean ± SD ASA PS 1 grade 2 3
Ultrasound-Guided Saphenous Blocks
ACSNB (n = 58)
P
47.1 ± 13 44 (71%) 18 (29%) 60 (96.8%) 2 (3.2%)
45.2 ± 14.7 0.437 (t test) 26 (44.8%) 0.004 (χ2 test) 32 (55.2%) 55 (94.8%) 0.594 (χ2 test) 3 (5.2%)
30.2 ± 6.5 12 (19.4%) 43 (69.4%) 7 (11.3%)
31.3 ± 7.2 0.410 (t test) 18 (31.0%) 0.333 (χ2 test) 34 (58.6%) 6 (10.3%)
BMI indicates body mass index.
Block success
Success Failure
DTSNB (n = 62)
ACSNB (n = 58)
P
53 (85.5%) 9 (14.5%)
58 (100%) 0 (0.0%)
0.003 (Fisher exact test)
The 95% confidence interval of the difference on success rates (DTSNB - ACSNB) is −0.195 to −0.031.
Two of the 9 patients with failed blocks in the DTSNB group received rescue saphenous nerve blocks by the primary anesthesia team in the postanesthesia care unit. Standard postoperative pain medications were used to treat any pain as needed for the rest of the patients.
Adverse Events Analysis by Provider The randomization sequence was stratified among 4 providers as previously described. Distribution of blocks by providers is presented in Table 4. Provider 3 dropped out of the study after performing 2 blocks because of other research commitments unrelated to the study. There was no statistically significant difference among the providers in terms of success rate of DTSNB (P = 0.909; Table 4). There was 1 randomization failure. Sensitivity analyses were performed based on “as-treated basis” and with exclusion of data from the patient who was a randomization failure. The results were consistent with the “intention-to-treat” results (Tables 5 and 6).
One patient in the ACSNB group had intravascular placement (SFA) of the block needle. It was promptly identified on aspiration, the needle was repositioned, and the block was performed as usual. None of the study patients reported any saphenous block–related complications in the postsurgical follow-ups and phone calls. One patient had persistent neurological symptoms in the popliteal nerve block area (common peroneal), which resolved in the 1-month follow-up.
DISCUSSION In this study, blocking the saphenous nerve at the adductor canal was not only noninferior but was, in fact, superior to
FIGURE 4. Noninferiority diagram with observed difference in success rates (DTSNB - ACSNB). The blue dashed line at x = 0.1 indicates the noninferiority margin. Error bars indicate 2-sided 90% CI of the difference in success rates (DTSNB - ACSNB): −0.195 to −0.031. The upper 95% 1-sided confidence bound (the upper limit of the 2-sided 90% confidence interval) is −0.031 and less than 0.1 indicates that ACSNB is noninferior to DTSNB. Because the CI of the difference wholly lies on the left side of zero, ACSNB is not only noninferior but also superior to DTSNB. © 2015 American Society of Regional Anesthesia and Pain Medicine
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Regional Anesthesia and Pain Medicine • Volume 40, Number 5, September-October 2015
Marian et al
TABLE 3. Secondary Outcomes
Time to success, median (IQR), min Time to perform block, median (IQR), s Ultrasound image of nerve Grade 1 Grade 2 Grade 3
DTSNB (n = 62)
ACSNB (n = 58)
P
9 (6–18) 112 (89–137) 16 (25.8%) 29 (46.8%) 17 (27.4%)
3 (3–6) 131 (109–156) 34 (58.6%) 16 (27.6%) 8 (13.8%)
