Adductor Canal Block Provides Noninferior Analgesia and Superior Quadriceps Strength Compared with Femoral Nerve Block in Anterior Cruciate Ligament Reconstruction

ANTERIOR cruciate ligament (ACL) reconstruction is one of the most frequently performed orthopedic procedures.¹  The ability to perform ACL reconstruction on outpatient basis is largely dependent on an effective analgesic regimen that minimizes the role of systemic analgesics yet provides adequate postoperative pain control and eliminates the need for overnight stay or readmission.²  Femoral nerve block (FNB) is an effective analgesic technique for ambulatory ACL reconstruction^3–6 ; however, it weakens the quadriceps muscle,⁷  an outcome that is preferably avoided. In addition, recent evidence of persistent strength deficits in patients receiving FNB further underscores the need for effective alternatives.⁸ 

Adductor canal block (ACB), a distal block of the femoral nerve performed in the mid-thigh,⁹  is becoming an attractive alternative to FNB as the peripheral nerve block of choice for total knee arthroplasty.¹⁰  By virtue of the distal anatomical location where it is performed, ACB offers an important advantage, namely preserving the strength of the quadriceps femoris muscle,^9,11  while providing pain relief to the knee that is comparable to FNB.¹²  However, generalizing these benefits of ACB to outpatient ACL reconstruction is problematic due to the differences in pain generators between knee arthroplasty^13,14  and ACL reconstruction,¹⁵  as well as the unique requirements of short stay and immediate mobilization²  for outpatient ACL reconstruction.¹⁶  Importantly, our understanding of the functional anatomy within the adductor canal that is relevant to ACL reconstruction,¹⁷  including the relationship between the saphenous and medial femoral cutaneous nerves,¹⁷  remains limited. Indeed, preliminary evidence from trials of ACB in the setting of ACL reconstruction^18,19  is inconsistent; however, these studies suffer from several important methodological shortcomings. Therefore, the question of potential analgesic and motor-sparing benefits of ACB in the setting of ACL reconstruction remains unanswered.

We believe that ACB would be the preferred technique for ACL reconstruction if it provides postoperative analgesia that is at least as good as that of FNB, while preserving motor strength around the knee. Therefore, this randomized controlled trial aims to test the joint hypothesis that ACB provides noninferior analgesia, as measured by the cumulative opioid consumption and pain scores during the first 24 h postoperatively, and preserves quadriceps femoris muscle motor strength compared with FNB in adult patients undergoing ambulatory ACL reconstruction.

This trial received Research Ethics Board approval at Women’s College Hospital (2012-0077-B) and was registered on www.ClinicalTrials.gov (NCT01791036; registered on February 12, 2013). The study was conducted at Women’s College Hospital, an ambulatory center in Toronto, Ontario, Canada, fully affiliated with the University of Toronto, between May 2013 and March 2015. This study manuscript was compiled in accordance with the Consolidated Standards of Reporting Trials (CONSORT) guidelines^20,21  and the CONSORT extension for noninferiority trials.²² 

We enrolled adult patients aged 18 to 50 yr, with American Society of Anesthesiologists (ASA) physical classification I to II and body mass index 35 kg/m² or less, who were scheduled for elective unilateral ACL reconstruction. All subjects provided written informed consent to participate in this prospective, randomized, parallel-arm, patient- and assessor-blinded, placebo-controlled, superiority clinical trial. Patients were excluded in case of pregnancy; significant psychiatric or mental disorders precluding assessment; baseline neuropathy or neurological deficits involving the lower extremities; chronic pain or requirement of 30 mg or greater oxycodone (or equivalent) daily; nerve block contraindications, including coagulopathy or bleeding diathesis, local skin infections, and allergy to local anesthetics; or any component of multimodal analgesia. Potential study participants were identified from individual surgeons’ booking lists in advance of the surgery date. During the preadmission clinic visit before the day of surgery, the research coordinator interviewed eligible patients, introduced the study, and provided an information package describing the study procedures.

An investigator with no further involvement in the study generated a list of random numbers in varying block sizes by using an online computer randomization service (www.Randomization.com). The unique randomization code was used to randomize consenting study participants on a 1:1 ratio with no restrictions to either of the two study groups: ACB group or FNB group. The results of the allocation were concealed in sealed opaque envelopes and kept with the research coordinator. On the day of surgery, the research coordinator handed an envelope to the attending anesthesiologist or a directly supervised regional anesthesia fellow in the block procedure room immediately before administering the study block to the participant. The staff anesthesiologist or fellow performing the block had no further role in the study.

Oral acetaminophen 1,000 mg and celecoxib 400 mg were administered to all study participants 1 h before surgery, unless contraindicated. IV access was secured and pulse oximetry, blood pressure cuff, and electrocardiogram were applied upon patient arrival to the block procedure room. Patients received fentanyl 25 μg IV and/or midazolam 1 to 4 mg IV for analgesia and anxiolysis, respectively, as needed, before block performance.

Patient positioning for the block performance was identical regardless of group allocation. While lying supine, the operative limb was externally rotated at the hip and the knee was flexed. Block sites were similarly prepped using chlorhexidine gluconate and isopropyl alcohol swabs in all patients; and all blocks were performed using 20 ml ropivacaine 0.5% with epinephrine 1:200,000.

After sterile skin preparation with chlorhexidine and skin infiltration with 1% lidocaine, a high-frequency linear array transducer (6 to 13 MHz; SonoSite M-Turbo; SonoSite™, USA) probe protected by a 3M Tegaderm^® (3M Health Care, USA) dressing was placed in the transverse position on the medial aspect of the mid-thigh. This ACB location has been shown to spare the innervation of the quadriceps muscle.^11,23–25  The femoral artery, vein, and sartorius muscle were identified. The saphenous branch of the femoral nerve is difficult to visualize, but it is usually located within the triangular hyperechoic region bound by the sartorius muscle superiorly, the vastus medialis laterally, and the adductor muscles medially.⁹  The ACB was performed using a 5- or 8-cm 22-gauge insulated needle (B. Braun Medical Inc., USA) inserted in plane with the ultrasound probe and advanced a lateral-to-medial approach until the needle tip was adjacent to the femoral artery, and 20 ml ropivacaine 0.5% with epinephrine 1:200,000 was injected in between the femoral artery and the sartorius muscle in 5-ml aliquots after negative aspiration.²⁶  A sham subcutaneous injection of 0.5 ml sterile normal saline injection was subsequently performed at the FNB site using ultrasound guidance with transducer pressure intended to simulate a real block procedure.

After sterile skin preparation with chlorhexidine and skin infiltration with 1% lidocaine, a high-frequency linear array transducer (6 to 13 MHz; SonoSite M-Turbo) probe protected by a 3M Tegaderm^® dressing was placed parallel and slightly caudad to the inguinal crease and adjusted as necessary to visualize the femoral nerve in short axis. The femoral artery, vein, and iliopsoas muscle were identified. The femoral nerve was sought within a triangular hyperechoic region, deep to the fascia iliaca, lateral to the femoral artery, and superficial to the iliopsoas muscle. The FNB was performed using a 5- or 8-cm 22-gauge insulated needle (B. Braun Medical Inc.) inserted in plane with the ultrasound probe and advanced a lateral-to-medial approach until the needle tip was adjacent to the femoral nerve. Twenty milliliter ropivacaine 0.5% with epinephrine 1:200,000 was injected in 5-ml aliquots after negative aspiration for blood to achieve a spread above the femoral nerve, below the fascia iliaca. A sham subcutaneous injection of 0.5 ml sterile normal saline was subsequently performed at the ACB site using ultrasound guidance with transducer pressure intended to simulate a real block procedure.

The blinded research coordinator performed baseline and postblock assessment of the sensory and motor blockade. Sensory blockade was assessed up to 30 min by comparing to the nonoperative limb, whereas motor blockade was assessed up to 60 min by comparing the operative limb to its own baseline.

Sensory assessment was performed every 5 min using a pinprick (25-gauge needle) applied to the sensory distribution of the saphenous nerve from medial aspect of the knee to the medial mid-calf in comparison to the nonoperative limb. The degree of sensory blockade was graded on a 3-point scale, where a score of 2 was given for intact sensation, 1 for loss of sensation to pinprick, and 0 for loss of sensation to light touch. A block was considered successful if complete loss of sensation (sensory score = 0) was achieved within 30 min from the end of local anesthetic injection. If block success was not achieved after 30 min, we reported block failure and the corresponding data using an intention-to-treat approach.

Power assessment was performed every 15 min using a handheld isometric force electromechanical dynamometer (MicroFET2; Hoggan Health Industries Inc., USA) that allowed evaluating the variation in quadriceps femoris motor strength (pound-force) in the operative limb during knee extension compared with the baseline. Patients were seated upright at the edge of the stretcher with their thighs horizontal and their legs dangling vertically relative to the floor. The dynamometer was placed on the anterior aspect of their operative leg, on the tibial crest, just above the ankle.²⁷  Patients were instructed to extend their knee against the dynamometer and to maintain the maximal force for 3 s. The maximal force was documented, and the mean of three consecutive measurements was considered as representative of the maximal voluntary isometric contraction (MVIC). These postblock measurements were repeated every 15 min up to 60 min before surgery, and the preserved percentage of the MVIC when compared with baseline was calculated.

Patients were transferred to the operating room after block assessment, where standard monitoring including pulse oximetry, electrocardiogram, blood pressure cuff, and a skin temperature probe was applied. Patients were then administered a standardized general anesthesia by a blinded anesthesiologist, including propofol 2 to 4 mg/kg IV and fentanyl 1 to 3 μg/kg IV, followed by insertion of a laryngeal mask airway. Patients were allowed to breathe spontaneously using a 40:60 oxygen:air mixture with desflurane 5 to 7%. Supplemental analgesia was provided as needed in the form of fentanyl 1 to 2 μg/kg IV and/or morphine 0.05 to 0.1 mg/kg IV if heart rate and/or mean arterial pressure increased by 20% above the measured baseline. Ondansetron 4 mg IV dose was administered for postoperative nausea and vomiting (PONV) prophylaxis 30 min before the end of the case; dexamethasone use was avoided due to its potential confounding effect on the block characteristics.²⁸  No further local anesthetics were injected into the knee joint or the site of harvested graft.

Postoperatively, patients were transferred to the postanesthesia care unit (PACU) and were subsequently discharged home once they met the hospital discharge criteria.²⁹  Patient request for additional analgesics or pain in the PACU, defined as visual analog scale (VAS; 10-cm scale, where 0 = no pain and 10 = worst pain) pain severity score 4 or greater, was treated with fentanyl 25 to 50 μg IV every 5 min up to a total of 200 μg, followed by morphine 2 to 4 mg IV to a maximum of 20 mg, or hydromorphone 0.2 to 0.4 mg IV to a maximum of 3 mg, administered by blinded PACU nursing staff. Once oral intake was allowed, patients were able to receive oral analgesics, either Tylenol No. 3^® (codeine 30 mg/acetaminophen 300 mg/caffeine 15 mg; Janssen-Ortho, Canada) or Percocet^® (oxycodone hydrochloride 5 mg/acetaminophen 325 mg; Bristol-Myers Squibb, Canada) if they were allergic to codeine, as needed. Discharged patients received a prescription for Tylenol No. 3^® or Percocet^® if intolerant to codeine to use for pain control, as needed. PONV in the PACU was treated sequentially with ondansetron 2 to 4 mg IV, followed by dimenhydrinate 12.5 to 25 mg IV, and then metoclopramide 10 mg IV, as needed.

Discharged patients were provided with a postoperative home diary to document the time when they first experienced pain at the surgical site; pain severity scores; analgesic consumption; opioid-related side effects (nausea, vomiting, itching, or constipation); and satisfaction with pain relief received. The diary was returned to the investigators using a prestamped, self-addressed envelope.

All study participants received a scripted phone call from the research coordinator on postoperative days 1, 2, and 7 to remind them to complete and return their home diary and to inquire about any block-related postoperative neurologic symptoms (persistent numbness or paresthesia, weakness, or nonsurgical pain in the operative extremity). Any complications that were potentially block related were followed until resolution.

We sequentially³⁰  examined three primary outcomes: (1) the cumulative 24-h analgesic consumption (converted to oral morphine equivalent)³¹  at home; (2) the area under the curve (AUC) for the postoperative rest pain severity VAS scores measured at PACU admission, at 30, 60, 90, and 120 min, and at 6, 12, and 24 h; and (3) the mean dynamometer reading of MVIC during knee extension of the operative limb at 60 min postblock, as a measure of quadriceps femoris muscle strength. Postblock presurgical assessment of MVIC was the most realistic measure in the outpatient surgical setting as postoperative movement restrictions and discharge preclude postsurgical assessment.

Block-related outcomes included (1) block procedure time, defined as time (minute) between applying the ultrasound probe to the patient’s skin and needle withdrawal after local anesthetic injection; (2) block procedural pain, reported by patients as no pain, mild–moderate pain, and moderate–severe pain; (3) the difference in MVIC between the two study groups at 15, 30, 45, and 60 min after nerve block; (4) incidence of falls or near-falls during the first 24 h, as reported by patients; and (5) incidence of postoperative neurologic symptoms (persistent numbness or paresthesia, weakness, or nonsurgical pain in the operative extremity) at 7 days postoperatively.

Analgesic outcomes assessed included (1) intraoperative morphine equivalent requirements (milligram); (2) time (minute) to first analgesic request; (3) time (minute) to PACU discharge²⁹ ; (4) postoperative pain severity (at rest) VAS scores (centimeter) at PACU admission, at 30, 60, 90, and 120 min, and at 6, 12, and 24 h; (5) interval postoperative analgesic consumption during the PACU stay, at 24 and 48 h, and at 7 days (converted to oral morphine equivalent)³¹ ; (6) incidence of PONV during the PACU stay and the first 24 h postoperatively; and (7) patient satisfaction with analgesia measured on a VAS (10-cm scale, where 0 = least satisfied and 10 = most satisfied) at 24 h postoperatively.

Apart from assessment of block onset, which was performed by the anesthesiologist administering the block, all outcome data were collected by a blinded research coordinator. To assess the success of blinding, all patients were asked which block they think they had received.

We aimed to test the hypothesis³⁰  that ACB produces postoperative analgesia that is at least as good as FNB while maintaining quadriceps motor strength compared with FNB. The choice of hypotheses and the specific order of their testing were informed by our contemporary clinical and anatomical knowledge of the interventions performed and the outcomes measured. We recognized a priori that (1) the ACB was unlikely to produce superior analgesia to FNB in knee surgery³² ; (2) neither opioid consumption nor pain severity scores in isolation were sufficient measures³³  to demonstrate noninferiority of postoperative analgesia; and (3) FNB per se may not provide sufficient analgesia in the setting of ACL reconstruction.^34,35  Proving that ACB provides noninferior analgesia was thus a necessary prerequisite before examining the effect on motor strength. We chose the joint hypothesis testing approach with the serial gatekeeping procedure described by Mascha and Turan.³⁶  We sought to sequentially demonstrate that ACB was noninferior to FNB for postoperative analgesic consumption and then for postoperative pain scores, followed by attempting to prove the superiority of ACB in preserving motor strength. The serial gatekeeping technique allowed us to test several hypotheses while maintaining the level of significance (α) for the one-sided test of noninferiority at 0.025 and at 0.05 for the two-sided test of superiority provided that each hypothesis in the series is proven before proceeding to the next one.^36,37 

Institutional pilot (R.B., unpublished data, April 2012) data from 12 patients undergoing ACL reconstruction with FNB and using a multimodal analgesic regimen inclusive of FNB with 20 ml ropivacaine 0.5% were used to power this trial. We based our sample size estimate on testing the one-sided ACB noninferiority hypothesis of the mean cumulative oral morphine equivalent consumption at 24 h, our first primary outcome. Our institutional data suggest that the cumulative oral morphine equivalent consumption for the first 24 h postoperatively in patients undergoing ACL reconstruction is 41.5 ± 19.6 mg. Considering a noninferiority margin (Δ) of 25% (i.e., 10 mg morphine), and assuming that the true difference in 24-h oral morphine equivalent consumption between the ACB and FNB treatment groups is 0%, a power analysis using a type I error estimate of 5% (α = 0.05), a power (1-β) of 80% indicated that a sample of 45 patients per group would be needed. To account for attrition resulting from incomplete follow-up or dropout of recruited patients, we chose to inflate the sample size by 10%. Consequently, we aimed to enroll a total of 50 patients per group or a total of 100 patients for this study.

Our pilot data suggest that the AUC of postoperative pain VAS scores during the first 24 h for patients who undergo ACL reconstruction with FNB to be 743 ± 312 mm h. Considering that a 30% relative reduction in AUC would be clinically significant,^38,39  we selected a 25% relative reduction as a noninferiority margin (Δ). Assuming the true difference in AUC to be 0%, the above calculated sample size provides 90% power for a one-sided test of noninferiority using α = 0.05.

Our pilot data suggest that baseline quadriceps motor strength in patients before receiving FNB is equivalent to 30.3 ± 10.3 pound-force during MVIC. Based on evidence that up to 19% difference in MVIC values between the two lower extremities in the same individual is considered normal,^40–43  we assumed that a 25% reduction in MVIC at 60 min after a nerve block to be clinically significant. Although other studies have considered a 50% reduction to be clinically significant,^44,45  we felt that a much smaller difference would be clinically relevant in the immediate postoperative period after ACL reconstruction. As such, the above calculated sample size provides 84% power at α = 0.05 to detect this difference between the ACB and FNB groups.

The SPSS for Windows statistical package (Version 22; IBM, USA) was used in our calculations. We used the Kolmogorov–Smirnov test to confirm the normality of data distribution. All of our analyses were performed using an intention-to-treat approach. Continuous data are presented as mean (SD) or mean (95% CI) and were analyzed by using the Student’s t test. Categorical data are presented as numbers or percentages and were analyzed using the chi-square or Fisher exact test, as appropriate. Ordinal data are presented as median (95% CI) and were analyzed by using the Mann–Whitney–Wilcoxon U test. We performed noninferiority testing for a particular outcome by comparing the 95% CI of the difference between groups ACB and FNB to the predetermined noninferiority margin (Δ) for this outcome. The rest pain VAS scores are presented as AUC; the area was calculated using the weighted mean over a fixed time (trapezoid role) and analyzed by using the Mann–Whitney–Wilcoxon U test. We used the Student’s paired t test to analyze the preblock and postblock motor strength comparisons within the same study group. We used the Kaplan–Meier survival analysis combined with the log-rank test for time-to-event outcomes. The threshold of statistical significance (P value) was set at 0.05, except if repeated measures of the same outcome were performed, in which case the Bonferroni–Holm correction was used.⁴⁶ 

We assessed 286 patients for eligibility, of which 115 did not meet the inclusion criteria, 69 declined, 1 had a change of surgical procedure, 1 had a change in the anesthetic plan, and 100 were recruited. Figure 1 depicts the CONSORT^20,21  flow diagram of patient progress through the study. All enrolled patients were randomized (ACB group: n = 52; FNB group: n = 48) and completed the study, and their data were analyzed. Data were complete for the primary outcomes assessed; only minimal secondary outcome data were missing. However, due to time constraints in the block procedure room and/or operating theater, evaluation of the MVIC at 60 min was not feasible, and this measurement was omitted. Enrolled patients had similar demographic characteristics, and no clinically important differences existed between the study groups (table 1). Block success was confirmed in all study participants.

The difference (95% CI) in cumulative 24-h oral morphine equivalent consumption between the study groups (ACB–FNB) was −4.8 mg (−12.3 to 2.7). The upper CI of the difference was significantly smaller (P = 0.03) than the predetermined noninferiority margin (Δ = 10 mg) (fig. 2). Consequently, ACB was found to be noninferior to FNB in cumulative 24-h postoperative analgesic consumption after ACL reconstruction. Further testing of potential superiority of ACB for this outcome found no difference between the two study groups (P = 0.21) (table 2).

Pain (rest) severity VAS scores for the two groups during the first 24 h postoperatively were plotted over time to examine the AUC (fig. 3). The difference in AUC between the study groups (ACB–FNB) was −71 mm h (−148 to 6). The upper CI of the difference was significantly smaller (P< 0.00001) than the predetermined noninferiority margin (Δ = 186 mm h) (fig. 4). Consequently, ACB was found to be noninferior to FNB in postoperative rest pain severity scores during the first 24 h after ACL reconstruction. Further testing of potential superiority of ACB for this outcome found no difference between the two study groups (P = 0.07) (table 2).

Starting from similar quadriceps femoris baseline strength in both groups (table 2), patients in ACB group were found to have significantly greater motor strength at 45 min postblock, with an MVIC of 26.6 pound-force (24.7–28.6) when compared with 10.6 pound-force (8.3–13.0) (P < 0.00001) for the FNB group (fig. 5). The decrease in quadriceps strength from baseline was statistically significant (paired t test) for both groups, with a 21.8 and 67.5% relative reduction in the ACB and FNB groups, respectively (table 2). In fact, patients in the ACB group had superior motor strength, as measured by the MVIC, at all of the time points examined, up to 45 min postblock. These findings of our joint hypothesis testing indicate that ACB provides noninferior analgesia up to 24 h postoperatively and preserves quadriceps muscle strength when compared with FNB in patients undergoing ACL reconstruction.

Both study groups had similar intraoperative opioid requirements as well as time-to-first postoperative analgesic request; but the patients in the ACB group were discharged from PACU 18 min earlier than those in the FNB group (P = 0.02) because they met the ambulation requirements of the discharge criteria earlier (table 2). The postoperative rest pain severity VAS scores were similar for both groups at all of the time points examined (fig. 3) The postoperative interval oral morphine equivalent consumption was also similar between the two groups at all of the time points examined (fig. 6). There was no difference between the two groups in the incidence of PONV during PACU stay or at 24 h after ACL reconstruction (table 2). The degree of satisfaction with postoperative analgesia received was also similar between the two groups. The proportion of patients providing the correct answer to the question “which block do you think you received” was similar between the two groups (table 2), suggesting that the measures taken to maintain patient blinding throughout the trial interventions were successful. Three patients reported falls or near-falls after hospital discharge in the FNB group when compared with none in the ACB group; but the difference did not reach statistical significance. None of the patients reported postoperative neurologic symptoms when contacted at 7 days after ACL reconstruction.

This study demonstrates that ACB is a superior analgesic modality to FNB in patients undergoing ACL reconstruction as it provides postoperative analgesia that is as effective as that of FNB while producing less weakness of the quadriceps femoris muscle than FNB. Both the analgesic consumption and pain control associated with ACB were noninferior to those associated with FNB during the first 24 h postoperatively, whereas the quadriceps weakness produced by ACB was significantly less compared with FNB.

In contrast to earlier work,^18,19,47  our findings favor the ACB in the setting of ACL reconstruction. Three studies^18,19,47  have previously attempted to explore the potential role of ACB in this population; however, each had important design and methodological limitations. Two trials performed blocks postsurgically^18,19  while patients were still under general anesthesia and did not ascertain block success and not performed an objective assessment of quadriceps motor strength. In the first study, Espelund et al.¹⁸  found no analgesic benefits when ACB was compared with placebo. Another major limitation of this trial is that it was designed to detect large changes (50%) in pain scores at a single time point (2 h postoperatively) while standing, in non–weight-bearing patients. Although opioid consumption and AUC of rest pain scores were examined, the trial lacked sufficient power to detect differences in these important outcomes. It is also noteworthy that the pain severity scores reported at 2 h in this study were unusually low and not commensurate with the findings of numerous other trials.^6,48–52  In the second trial, El Ahl¹⁹  compared ACB to FNB and observed similar pain scores up to 12 h postoperatively, yet he concluded that ACB provided inferior analgesia based on differences in pain scores and analgesic consumption that can be attributed to the 12- to 24-h interval when both blocks would have already worn off. Finally, a third trial involving a variety of arthroscopic knee surgeries resorted to post hoc subgroup analysis to tease out the analgesic benefits of postoperative ACB during the first 90 min specifically after ACL reconstruction and found ACB to be not better than placebo for rest pain.⁴⁷  Notably, patients undergoing ACL reconstruction with a hamstring graft were excluded from this study.

Our understanding of adductor canal anatomy, particularly nerves relevant to ACL reconstruction, remains limited and precludes an anatomically based theoretical assumption of analgesic effectiveness of the ACB.¹⁷  For example, descriptions of the anatomical communication between the saphenous and medial femoral cutaneous nerves in the adductor canal have been inconsistent.^17,53  Furthermore, the posterior branch of the medial femoral cutaneous nerve, which provides cutaneous innervation to the medial thigh and leg, may not pass in the adductor canal in up to 40% of cases, whereas its anterior branch, which provides cutaneous innervation to the medial and lateral knee, is never part of the adductor canal.^14,17,53  As such, our trial provides much needed clinical evidence regarding the functional anatomy of the adductor canal relevant to ACL reconstruction.

Our study is subject to several limitations. First, in terms of outcome selection, we did not include an assessment of posterior pain originating from the back of knee or the graft site¹⁵ ; neither FNB nor ACB may provide sufficient relief to this component of pain.⁵⁴  In fact, clinical evidence suggests that pain may occasionally arise in anatomical areas innervated by the sciatic⁵⁵  and obturator⁵⁶  nerves after ACL reconstruction. Accurate assessment of such pain requires thoughtful study designs that involve catheters and staged treatment of each pain component, according to its source, to permit isolation of the various contributors to pain.⁵⁷  However, in an attempt to explore the role of posterior knee pain in our results, we performed post hoc analysis according to the type of graft received; the results suggested no difference in pain severity between ACB and FNB (P = 0.06). In addition, our evaluation of overall the analgesic efficacy of both blocks may have been limited by the lack of a control group. We also did not evaluate the effect of either block on the occurrence of rebound pain.⁵⁰  Furthermore, our assessment of quadriceps strength was limited to 45 min presurgery, and postsurgical assessment was not possible because of immobilization and discharge. This precluded investigating any delayed-onset motor weakness after ACB as well as the duration of motor blockade or persistent weakness⁸  in either group. Nonetheless, recent evidence suggests that quadriceps weakness is caused by postsurgical pain as the analgesic effect of the blocks subsides.⁴⁵  We also excluded assessment of hip adduction as both ACB and FNB have been shown to similarly affect hip adduction.^11,12  In the absence of any formal postoperative strength testing, the clinical importance of our postblock strength data remains to be elucidated. However, we have recently demonstrated that despite superior analgesia compared with placebo, FNB causes an increased risk of falls in similarly young healthy outpatients discharged using crutches after hip arthroscopy.⁵⁸  Finally, our results are applicable to the specific settings described in this trial and may not necessarily be generalizable to different surgical procedures (e.g., revisions, medial or posterior cruciate reconstruction, and meniscectomy), ACB injection locations (e.g., more distal),⁵⁹  local anesthetic volumes,⁶⁰  or other analgesic modalities (e.g., infrapatellar saphenous block,⁶¹  intra- and periarticular local anesthetic infiltration,^62,63  hamstring block,¹⁵  and fascia iliaca block⁵¹ ).

In summary, our trial suggests that ACB is a better option compared with FNB in patients undergoing outpatient ACL reconstruction as it provides noninferior postoperative analgesia while preserving quadriceps motor strength.

Supported by the Merit Award Program, Department of Anesthesia, University of Toronto, Toronto, Ontario, Canada (to Drs. Abdallah and Brull). Research time support was received from the University Health Network (UHN; Toronto, Ontario, Canada) Anesthesia Associates and from the UHN Innovation Fund Plan (to Dr. Chan).

Dr. Chan is a member of the medical advisory board of Philips Medical Systems (Andover, Massachusetts) and Smiths Medical (St. Paul, Minnesota). The other authors declare no competing interests.

Full protocol available from Dr. Abdallah: abdallahf@smh.ca. Raw data available from Dr. Abdallah: abdallahf@smh.ca.