Severe pain and the need for early mobilization pose a challenge

Transcription

1 ORIGINAL ARTICLE Adductor Canal Block Versus Femoral Nerve Block for Analgesia After Total Knee Arthroplasty A Randomized, Double-blind Study Pia Jæger, MD,* Dusanka Zaric, MD, DMSci, Jonna S. Fomsgaard, MD, Karen Lisa Hilsted, RN,* Jens Bjerregaard, MD, Jens Gyrn, MD, Ole Mathiesen, MD, PhD, Tommy K. Larsen, MD, and Jørgen B. Dahl, MD, DMSci, MBAex* Background and Objectives: Femoral nerve block (FNB), a commonly used postoperative pain treatment after total knee arthroplasty (TKA), reduces quadriceps muscle strength essential for mobilization. In contrast, adductor canal block (ACB) is predominately a sensory nerve block. We hypothesized that ACB preserves quadriceps muscle strength as compared with FNB (primary end point) in patients after TKA. Secondary end points were effects on morphine consumption, pain, adductor muscle strength, morphine-related complications, and mobilization ability. Methods: We performed a double-blind, randomized, controlled study of patients scheduled for TKA with spinal anesthesia. The patients were randomized to receive either a continuous ACB or an FNB via a catheter (30-mL 0.5% ropivacaine given initially, followed by a continuous infusion of 0.2% ropivacaine, 8 ml/h for 24 hours). Muscle strength was assessed with a handheld dynamometer, and we used the percentile change from baseline for comparisons. The trial was registered at clinicaltrials.gov (Identifier: NCT ). Results: We enrolled 54 patients, of which 48 were analyzed. Quadriceps strength as a percentage of baseline was significantly higher in the ACB group compared with the FNB group: (median [range]) 52% [31 71] versus 18% [4 48], (95% confidence interval, 8 41; P = 0.004). There was no difference between the groups regarding morphine consumption (P = 0.94), pain at rest (P = 0.21), pain during flexion of the knee (P = 0.16), or adductor muscle strength (P = 0.39); neither was there a difference in morphine-related adverse effects or mobilization ability (P > 0.05). From the *Department of Anaesthesia, Centre of Head and Orthopaedics, Copenhagen University Hospital, Rigshospitalet, Copenhagen; Department of Anaesthesia, Frederiksberg Hospital, Copenhagen University Hospital, Frederiksberg; Department of Anaesthesia, Glostrup Hospital, Copenhagen University Hospital, Glostrup; Section of Acute Pain Management, Department of Anaesthesia, Centre of Head and Orthopaedics, Copenhagen University Hospital, Rigshospitalet, Copenhagen; and Department of Surgery, Glostrup Hospital, Copenhagen University Hospital, Glostrup, Denmark. Accepted for publication August 20, Address correspondence to: Pia Jæger, MD, Department of Anaesthesia, 4231, Centre of Head and Orthopaedics, Rigshospitalet, Blegdamsvej 9, DK-2100 Copenhagen Ø, Denmark (e mail: pia.therese.jaeger@regionh.dk). The authors declare no conflict of interest. The study was awarded the European Society of Regional Anaesthesia and Pain Therapy Research Grant (Brussels, Belgium). The department and institution to which the work should be attributed: Department of Anaesthesia, Centre of Head and Orthopaedics, Copenhagen University Hospital, Rigshospitalet, Copenhagen, Denmark. Abstract was presented at Euroanaesthesia 2013 Congress, Barcelona, Spain, June 1 4, Supplemental digital content is available for this article. Direct URL citations appear in the printed text and are provided in the HTML and PDF versions of this article on the journal s Web site ( Copyright 2013 by American Society of Regional Anesthesia and Pain Medicine ISSN: DOI: /AAP Conclusions: Adductor canal block preserved quadriceps muscle strength better than FNB, without a significant difference in postoperative pain. (Reg Anesth Pain Med 2013;38: ) Severe pain and the need for early mobilization pose a challenge for postoperative pain treatment after total knee arthroplasty (TKA). Peripheral nerve blocks are commonly used to relieve pain and to reduce opioid requirements and opioid-related adverse effects. Nerve blocks involving the femoral nerve, however, lead to femoral quadriceps muscle weakness. 1,2 Quadriceps weakness, in turn, results in functional impairment and is associated with an increased risk of falling postoperatively. 3 5 So far, attempts to reduce quadriceps involvement after femoral nerve block (FNB) without compromising analgesia have not succeeded. 1,6 8 Unlike FNB, adductor canal block (ACB) is predominantly a sensory block, 9 which preserves quadriceps muscle strength and ambulation ability better than FNB. 2 A study in healthy volunteers showed that FNB reduced quadriceps strength by 49% from baseline, compared with only 8% with ACB. 2 An 8% reduction is probably not clinically relevant, as a side-to-side difference of 10% is common in healthy individuals without functional importance. 10,11 Adductor canal block has also been shown to reduce pain and morphine consumption compared with placebo after TKA. 12,13 The effect of ACB on pain and muscle strength in a postsurgical population has not, however, been directly compared with that of the FNB. The aim of this study was to compare the effects of ACB and FNB on muscle strength, pain, and mobilization in patients after TKA. We hypothesized that ACB preserves quadriceps muscle strength better than FNB (primary end point) without compromising analgesia. Secondary end points were adductor muscle strength, pain, morphine consumption, morphine-related adverse effects, and mobilization ability. METHODS This prospective, randomized, double-blind trial was approved by the Regional Research Ethics Committee and the Danish Data Protection Agency and registered at clinicaltrials. gov (NCT ). The study was conducted at Frederiksberg Hospital, Copenhagen University Hospital, Frederiksberg, Denmark, and at Glostrup Hospital, Copenhagen University Hospital, Glostrup, Denmark. All subjects provided written informed consent before participating. Patients scheduled for TKA with spinal anesthesia between November 14, 2011, and November 19, 2012, were included. Eligible participants were patients scheduled for primary TKA with spinal anesthesia, aged between 50 and 85 years, with an American Society of Anesthesiologists physical status 526 Regional Anesthesia and Pain Medicine Volume 38, Number 6, November-December 2013

2 Regional Anesthesia and Pain Medicine Volume 38, Number 6, November-December 2013 ACB Versus FNB FIGURE 1. Model photograph illustrating the assessment of quadriceps muscle strength with an HHD. The subject was placed in a seated position with the knees flexed 60 degrees. A nonelastic strap with Velcro closures was used to fix the HHD; it was attached to a chair and around the subject s ankle, perpendicular to the lower leg. The HHD was placed under the Velcro strap, on the anterior surface of the tibia, 5 cm above the transmalleolar axis. classification of I to III, and a body mass index of 18 to 40 kg/m 2. Exclusion criteria were inability to cooperate, inability to speak or understand Danish, allergy to any drug used in the study, a daily intake of steroids or strong opioids (morphine, oxycodone, methadone, fentanyl, or ketobemidone), alcohol or drug abuse, rheumatoid arthritis, and if the spinal anesthesia had resolved before conducting the block. Randomization was based on a computer-generated block randomization list (each block containing 10 numbers), in a 1:1 ratio. Upon inclusion in the study, subjects received the treatment assigned according to the randomization list, in consecutively numbered, sealed, opaque envelopes. All subjects, outcome assessors, and clinical personnel were blinded to the intervention except for the investigators performing the blocks (D.Z. and J.S.F.). These investigators were not involved in data collection or in handling the data. Care was taken to assure blinding of the subject and other clinical personnel. During block performance, the patient was shielded from other patients and staff, and the patient s view of the injection site was blocked by blankets. Each subject received both the assigned treatment catheter and a sham catheter to facilitate blinding of the patient and staff. Premedication consisted of 1-g acetaminophen and 400-mg ibuprofen given 1 hour preoperatively. In addition, subjects were given postoperative antiemetic prophylaxis intravenously, including 8-mg dexamethasone, 4-mg ondansetron, and mg droperidol. Spinal anesthesia was induced with 2- to 2.5-mL 0.5% hyperbaric bupivacaine at the L3/4 interspace (alternatively at the L2/3 or L4/5 interspaces). Sedation with propofol and intraoperative fluid therapy were administered at the discretion of the anesthetist. All patients received a femoral tourniquet perioperatively. Peripheral nerve blocks were performed in the postanesthesia care unit, immediately postoperative and before the spinal anesthesia had worn off. The subjects received an active adductor canal or FNB according to randomization, as well as a sham procedure for the alternative treatment. All blocks were performed by one of the 2 investigators, D.Z. or J.S.F., who have experience in ultrasound-guided nerve block techniques and who subsequently refrained from any further contact with the patient. A high-frequency linear ultrasound transducer (a 6 18 MHz, Flex Focus unit, BK Medical, Herlev, Denmark, at Frederiksberg Hospital and a 5 10 MHz, GE LOGIQ unit, Waukesha, Wisconsin, at Glostrup Hospital) was used to scan the block area before needle insertion. The study medication was administered as a bolus of 30 ml of ropivacaine 0.5% via the catheter initially, followed by an infusion of 0.2% ropivacaine at a rate of 8 ml/h during the next 24 hours. For the ACB, we performed an ultrasound survey at the medial part of the thigh, halfway between the superior anterior iliac spine and the patella. In a short axis view, we identified the femoral artery underneath the sartorius muscle, with the vein just inferior and the saphenous nerve just lateral to the artery. The needle was introduced in-plane and 2 to 3 ml of saline was used to ensure correct placement of the needle in the vicinity of the saphenous nerve in the adductor canal, as described previously. 9,12,14 The catheter was then introduced and advanced 1 to 2 cm beyond the tip of the needle. The correct spread of the ropivacaine bolus injection in a semicircular form around the artery was observed. For the FNB, the catheter was inserted in-plane with the probe parallel to the inguinal crease, to obtain a short-axis view of the nerve. The correct needle placement was confirmed by injecting 2 to 3 ml of saline to cause tissue expansion below the iliac fascia, lateral to the femoral artery, and in the vicinity of the femoral nerve. The catheter was introduced 1 to 2 cm beyond the tip of the needle and adequate spread of the ropivacaine bolus injection around the femoral nerve was observed. The sham procedure involved an ultrasound survey, marking of the injection site with a stump needle not penetrating the skin threading of a catheter, fixating the catheter with opaque plaster, and injection of 30-mL saline through the catheter into an absorbing cloth. Then, the 2 catheters were joined under an opaque plaster, with only the active catheter emerging through the plaster and, finally, both injection sites were covered with bandage. Postoperative pain treatment included intravenous patientcontrolled analgesia (PCA) with morphine (bolus 2.5 mg, lock-out time 10 minutes, and no background infusion) in addition to 1-g acetaminophen and 400-mg ibuprofen administered orally at 6-hour intervals, initiated 2 hours postoperatively. If analgesia was inadequate, patients received additional boluses of 2.5-mg morphine intravenously until adequate analgesia was obtained. In the case of moderate to severe nausea or vomiting, the preoperative antiemetic prophylaxis treatment was repeated. The primary end point was the difference in quadriceps muscle strength between the groups, assessed as maximum voluntary isometric contraction (MVIC) as percent of baseline at 24 hours postoperative. Secondary end points included adductor muscle strength, pain during flexion of the knee and at rest (area under the curve 2 24 hours), cumulative morphine consumption (0 24 hours), morphine-related adverse effects (nausea, vomiting, sedation, and postoperative need for antiemetics), and mobilization ability assessed with the Timed- Up-and-Go (TUG) test and on a 10-point mobility scale. We assessed muscle strength and the TUG test preoperatively and at 24 hours postoperative. Pain, morphine consumption, and morphine-related adverse effects were assessed at 2, 4, 8, and 24 hours postoperative. We used a handheld dynamometer (HHD; Lafayette Instrument, Lafayette, Indiana) for measuring muscle strength. The HHD is considered a reliable and valid instrument, 15 and we 2013 American Society of Regional Anesthesia and Pain Medicine 527

3 Jæger et al Regional Anesthesia and Pain Medicine Volume 38, Number 6, November-December 2013 FIGURE 2. Flow diagram of patient distribution. ASA indicates American Society of Anesthesiologists; GA, general anesthesia. used standardized and recommended procedures to obtain valid measurements. 16 For quadriceps muscle strength evaluation, we placed the subject in a seated position with the knees flexed 60 degrees (see Fig. 1). 17 To avoid reduced interrater reliability when the strength of the subject overcomes the strength of the tester, it has been suggested to fix the HHD for quadriceps evaluation. 18 We used a nonelastic strap with Velcro closures to fix the HHD. We attached the Velcro strap to a chair and around the subject s ankle, perpendicular to the lower leg. The HHD was placed under the Velcro strap, on the anterior surface of the tibia, 5 cm above the transmalleolar axis. As the adductor muscles are far weaker than the quadriceps muscle, we applied a procedure without fixation of the HHD that has been validated in a previous study, 19 placing the subject in the supine position with the operated leg abducted 30 degrees from midline. The HHD was placed on the medial tibia, 5 cm above the medial malleolus. Muscle strength was assessed as MVIC. Subjects were familiarized with the procedure before outcome assessments. We instructed the subjects to take 2 seconds to reach maximum effort, maintain this force for 3 seconds, and then relax. A standardized verbal command was issued during the testing: pushpush-push-pause. For each assessment, the subjects performed 3 consecutive contractions, separated by a 30-second pause between each trial. We used the mean value at each time point for calculations, and calculated muscle strength as percent of baseline value American Society of Regional Anesthesia and Pain Medicine

4 Regional Anesthesia and Pain Medicine Volume 38, Number 6, November-December 2013 ACB Versus FNB TABLE 1. Patient Characteristics and Perioperative Data for Randomized Patients ACB Group FNB Group No. patients Sex (male/female) 5/18 14/13 Age, y 70 (8) 66 (9) Height, cm 167 (8) 174 (9) Weight, kg 75 (13) 87 (16) Preoperative VAS pain 9 (16) 3 (6) score at rest, mm Preoperative VAS pain score at 13 (15) 7 (9) 45 degrees flexion of the knee, mm Preoperative TUG test score, s 13 (6) 11 (5) Habitual analgesics None 14/23 (61%) 11/27 (41%) Paracetamol and/or ibuprofen 7/23 (30%) 11/27 (41%) Weak opioids* 2/23 (9%) 5/27 (18%) Operated side (right/left) 8/15 14/13 Duration of surgery, min 82 (20) 75 (15) Blood loss, ml 39 (60) 28 (45) Isotonic sodium chloride, ml 1341 (300) 1265 (399) Hydroxyethyl starch colloids, ml 0 (0) 0 (0) Values are reported as number of subjects, proportions (percentage), or mean (SD). Two patients (1 ACB and 1 FNB) were excluded after randomization due to technical problems with the PCA infusion pumps. *Weak opioids are codeine or tramadol. VAS indicates visual analog scale. Pain was evaluated on a visual analog scale (0 100 mm; 0, no pain; 100, worst imaginable pain), at rest and during 45-degree passive flexion of the knee. We assessed nausea and sedation on a 4-point scale: 0, no nausea/sedation; 1, light; 2, moderate; and 3, severe. Vomiting was assessed as number of vomiting episodes with a volume greater than 10 ml. The need for antiemetics in the first 24 hour postoperative was recorded. Mobilization ability was assessed with a validated ambulation test, the TUG test. 20 The TUG test measures the time it takes a person to get up from a chair, walk 3 m, and return to the chair. Postoperatively, all subjects used a high walker with arm support, as an assisted walking aid, for the test. Furthermore, mobilization ability was evaluated by the department s physiotherapists on a 10-point mobility scale, together with the degree of knee flexion. The 10-point mobility scale evaluates whether the subject can achieve 5 predefined goals of mobilization with or without the help of the physiotherapist (sit up on the edge of the bed, standing, transfer to a chair, walk with a high walker, and walk with crutches unassisted), in addition to 45-degree flexion of the knee on the first postoperative day (see Appendix, Supplemental Data Content 1, All patients were tutored by one of the investigators preoperatively in the visual analog scale, as well as trained in the TUG test and in the use of the PCA system. Data Handling and Statistics A side-to-side difference of 10% in quadriceps muscle strength is normal in healthy individuals 10,11 ; therefore, we considered a difference of 20% to be clinically relevant. A previous study in healthy volunteers reported an SD of 18% in quadriceps strength 1 ; in the current study of postsurgical patients, we assumed an SD of 23%. A trial with 22 subjects in each group was needed to detect a 20% difference in quadriceps strength with a power of 80% at the 5% significance level. To compensate for dropouts and uncertainty regarding our estimated SD, we planned for the inclusion of 55 subjects. The randomization key was first broken once enrollment of all patients was completed, the data had been computed and subsequently double-checked by 2 investigators, and decisions regarding exclusion of patients had been taken. Data were analyzed using SPSS version 18.0 (SPSS, Chicago, Illinois). We used the Kolmogorov-Smirnov test to assess whether variable distributions violated the assumption of normality. Data are presented as mean and SD, or with medians and 10th to 90th percentiles as appropriate. We used the independent samples t test for comparing data, except for quadriceps MVIC, which was not normally distributed and was compared using the Mann- Whitney U test for nonparametric data, and categorical data (need for antiemetics), which were analyzed using the χ 2 test. We used the mean value from 3 consecutive assessments, performed preoperatively and at 24 hours postoperative, to calculate the percentile change in MVIC from baseline. Pain scores were compared after calculating the area under the curve for the interval 2 to 24 hours [(t 2 t 1 )(y 1 + y 2 )/2]. For comparison of nausea and sedation scores, we calculated the arithmetic mean scores by attributing numerical values to the scores from each patient. The nature of the hypothesis testing was 2-tailed and a P value of less than 0.05 was considered statistically significant. All planned statistical analyses were reported on clinicaltrials.gov before inclusion into the study. RESULTS We enrolled 54 patients during a 13-month period beginning November Three patients were excluded after randomization but before receiving any treatment (FNB, n = 3; ACB, n = 0). The remaining 51 patients were randomized to FIGURE 3. Effects of ACB and FNB on quadriceps and adductor muscle strength. Muscle strength was assessed as MVIC at 24 hours postoperatively and presented in percentile change from baseline. Quadriceps MVIC was significantly reduced when comparing ACB with FNB (P = 0.004), but there was no difference between the groups in adductor muscle strength (P = 0.39). Data are expressed as median (horizontal bar) with 25th to 75th (box) and 10th to 90th (error bars) percentiles. *Indicates statistically significant difference American Society of Regional Anesthesia and Pain Medicine 529

5 Jæger et al Regional Anesthesia and Pain Medicine Volume 38, Number 6, November-December 2013 FIGURE 4. Effects of ACB and FNB on pain during 45 degrees flexion of the knee. Visual analog scores (0-100 mm, with 0 equal no pain and 100 being the worst imaginable pain). Comparisons between the groups were made as area under the curve (AUC) for the interval of 2 to 24 hours postoperatively, showing no difference between the groups (P = 0.16). Data are expressed as mean (SD). VAS indicates visual analog scores. receive either an ACB (n = 24) or an FNB (n = 27). Three patients (FNB, n = 1; ACB, n = 2) were excluded after randomization due to technical problems with the PCA infusion pumps. Available data for 2 of these subjects (0-8 hours) are included in demographics and figures, but the 24-hour data needed for analyses were not available. There were no available data for the last subject. For subjects flow through the study, see Figure 2. Subjects demographic and surgical characteristics are presented in Table 1. Quadriceps MVIC was significantly higher for subjects with an ACB, median, 52% (9 92) of baseline value, compared with 18% (0 69) for those with an FNB (median difference, 26; 95% confidence interval [CI], 8 41; P = 0.004; Fig. 3). There were no differences between the groups in any of the secondary end points. The mean (SD) adductor MVIC was 78% (35%) of baseline value in subjects receiving ACB, compared with 71% (24%) in subjects receiving FNB (mean difference, 7( 10 to 24), P = 0.39; Fig. 3). During flexion of the knee, mean (SD) pain scores (area under the curve for the interval 2 24 hours/22 hours) in the ACB group were 36 (15) mm compared with 29 (19) mm in the FNB group (mean difference, 7 ( 3 to11),p = 0.16; Fig. 4). At rest, mean pain scores (area under the curve for the interval 2 24 hours/22 hours) were 16 (12) mm in the ACB group compared with 12 (12) mm in the FNB group (mean difference, 4 ( 3 to 17), P = 0.21; Fig. 5). Cumulative mean (SD) total morphine consumption was comparable between groups, 22 (9) mg in subjects receiving ACB and 22 (21) mg in subjects receiving an FNB (P = 0.94, Table 2). There were no differences between the groups regarding morphine-related adverse effects: nausea (P = 0.49), vomiting episodes (P = 0.19), sedation (P = 0.31), or in the need for antiemetics (P =0.34). There were no differences in mobilization ability between the groups, neither in the TUG test (P = 0.59), in points achieved on the 10-point mobility scale (P = 0.28), or in degrees of active flexion (P = 0.65). For further details, see Table 2. Except for 1 overdose of morphine, which resolved quickly on naloxone, there were no other adverse events or fall episodes. DISCUSSION The most important finding of this study is that ACB preserved quadriceps muscle strength better than FNB, with no major differences in pain scores or morphine consumption. As quadriceps muscle strength is greatly reduced (60% 83%) after TKA, 17,21,22 an effective analgesic procedure preserving muscle strength is warranted. A previous study in healthy volunteers 2 showed that the ACB is predominately a sensory block, which reduced quadriceps strength by just 8%, compared with 49% with an FNB. The implication of quadriceps weakness was reflected in the ambulation tests, where subjects performed significantly better with an ACB than with an FNB. 2 Additionally, it was only possible to mobilize half of the subjects with an FNB, whereas all subjects could be mobilized with an ACB. 2 In the current study, we were not able to show a difference between the groups in mobilization ability 24 hours after surgery. The TUG test involves activities that require a functional quadriceps muscle (rising from a chair and walking), but a chair with armrest and a high walker were used for the test, thereby providing an opportunity to bypass the quadriceps function in the operated limb. Additionally, the physiotherapy evaluation was performed as part of the department s standard regimen and the 10-point mobility scale was not developed for evaluating quadriceps function. The mean score in both groups was 7. Of note, 7 subjects had a MVIC of 0 at 24 hours postoperative, but they still garnered scores from 6 to 8 on the mobility scale. Any activity involving the use of both legs contains an opportunity for the nonsurgical leg to compensate for the surgical leg, 23 which was only enhanced by the use of assistive devices. Unlike the study in healthy volunteers, where no gait aids were allowed for the ambulation tests, we would not risk mobilizing the patients without assistive devices in the current study. Although we were not able to demonstrate an effect on ability to mobilize 24 hours after surgery, other studies have shown that quadriceps weakness leads to functional impairment, 23 to lower points on the falls efficacy scale, 24 and to an increased risk of falls. 25 There were no fall episodes in this study population, but peripheral nerve blocks FIGURE 5. Effects of ACB and FNB on pain at rest. Pain was assessed with a VAS (0-100 mm, with 0 equal no pain and 100 being the worst imaginable pain). Comparisons between the groups were made as area under the curve (AUC) for the interval of 2 to 24 hours postoperatively, showing no difference between the groups (P = 0.21). Data are expressed as mean (SD) American Society of Regional Anesthesia and Pain Medicine

6 Regional Anesthesia and Pain Medicine Volume 38, Number 6, November-December 2013 ACB Versus FNB TABLE 2. Morphine Consumption and Mobilization Ability ACB Group FNB Group Cumulative total morphine consumption, mg 2 h postoperatively 2 (4) 1 (3) 4 h postoperatively 4 (5) 2 (5) 8 h postoperatively 9 (7) 7 (11) 24 h postoperatively 22 (9) 22 (21) Mobilization ability TUG test at 24 h postoperative, s 37 (22) 39 (16) TUG test as a percentage 335 (220) 368 (171) of baseline, % 10-point mobility scale (points) 8.1 (1) 8.4 (1) Degrees of active flexion 61 (13) 63 (11) Values are reported as mean (SD). involving the femoral nerve are associated with the risk of falling postoperatively, 3 5 which might be avoided with the ACB. There were no statistically significant differences in pain scores between the groups at rest or during flexion of the knee (P = 0.16 and P = 0.21, respectively). In both groups, pain scores were relatively low, but albeit PCA-morphine usage was similar (Table 2), they were somewhat lower in the FNB group compared with the ACB group, in particular in the first few hours after surgery (Figs. 4 and 5). The mean difference for pain during flexion of the knee was fairly small, with rather narrow CIs (7 mm, 95% CI, 3 to 11), suggesting that no major differences in dynamic pain have been overlooked. We excluded 6/54 (6%) patients for various reasons (see Fig. 2). This might have distorted the result, as twice as many were excluded from the FNB group (n = 4) compared to the ACB group (n = 2). Notably, patients may have been excluded due to inferior treatment (excessive motor loss or pain) and this may potentially have underestimated the effect of the ACB. Furthermore, by chance, patients had higher preoperative pain scores in the ACB group (Table 1), which may explain some of the slight postoperative pain differences. It might be considered a limitation of the study that the anesthesiologists performing the blocks were not blinded to treatment, although they refrained from further contact with the patient. Blinding of the anesthesiologists could theoretically have been accomplished by administering a placebo treatment in one of the blocks. However, we did not find it appropriate to perform 2 invasive procedures in each patient and decided to use a sham procedure for one of the blocks, keeping patients, care providers, and outcome assessors blinded to treatment. All blocks were performed postoperatively under spinal anesthesia. This was done for the following 2 reasons: (1) to avoid entrapment of the catheter between the nerve and the tourniquet (double-crush concept) and (2) to avoid dislodging of the catheter during surgery. Many anesthesiologists are reluctant to provide a block in an area that is still anesthetized. Although we risked overlooking paresthesias, we excluded patients whose spinal had resolved. This was done solely to ensure blinding of the patients. As with other catheter-based techniques, withdrawal/displacement of the catheter is a possible limitation. We only had 1 patient with catheter withdrawal, albeit we did not assess for block failures. We applied a regimen often used for peripheral nerve blocks, with an initial bolus (30-mL 0.5% ropivacaine) followed by infusion (0.2% ropivacaine, 8 ml/h). As most assessments were performed at time points where the bolus was working and as the study period was relatively short, it is difficult to interpret whether the result is a reflection of the single injection or continuous infusion. To obtain more accurate pharmacodynamic data on this relatively new technique, further studies are needed to investigate single injection blocks alone or continuous infusions with a longer period of assessment. In earlier reports of ACB, local anesthetics were administered as repeated boluses via a catheter to ensure spread of local anesthetic throughout the entire aponeurotic canal, under the assumption that ACB is more than just a saphenous block in the adductor canal. 9,12,13 In addition to the saphenous nerve, the adductor canal contains the nerve to the vastus medialis, the medial femoral cutaneous nerve, the medial retinacular nerve, and finally, the articular branches from the obturator nerve, which enter the distal part of the canal. 26 Choosing infusion instead of intermittent boluses may have reduced the analgesic potential of ACB. This choice was based upon logistical reasons, as it was not possible to ensure proper administration of repeated boluses in the current study. On the contrary, a previous study has shown that reducing the total dose of local anesthetic for FNB was followed by insufficient pain relief. 7 Therefore, increasing the volume might have potentiated the analgesic effect of the FNB. The large volumes applied (bolus and infusion) might have increased the risk of motor blockade after both blocks. Davis and colleagues found that 30 ml injected into the adductor canal spreads proximally to the anterior and posterior divisions of the femoral nerve outside the canal. 27 Although reducing the volume for bolus injection may further spare quadriceps strength, the 48% reduction from baseline seen with the ACB at 24 hours postoperative in the current study is actually less than reported in previous studies. 17,21,22 At 4 weeks postoperative, quadriceps strength was reduced by 60% to 64%. 21,22 Strength loss was mainly due to a central inability to fully activate the muscle, with pain accounting for less than 25% of the change in voluntary activation. 21,22 In a third study, 17 quadriceps strength was reduced by 83% at discharge (mean, 2.4 days postoperative). This study further demonstrated that knee swelling explained 27% of the decrease in quadriceps strength (50% when excluding 2 outliers). Of note, patients received a 24-hour treatment with local infiltration analgesia, which might have contributed to an increase in knee swelling and may partly explain the larger reduction in strength relative to our study. 17 Our patients did not receive local infiltration analgesia but we applied a tourniquet, which may also have affected quadriceps muscle strength postoperatively. 28 Regarding FNB, Charous et al 1 reported similar motor loss after FNB in healthy volunteers as seen in the current study of postsurgical patients and, furthermore, with smaller volumes and lower concentration. If anything, our regimen may have contributed to underestimating the motor-sparing effect of ACB and, from a clinical point of view, overestimating the pain relief seen with the FNB, neither affecting the conclusion of the study. Future studies should try to reveal the volume needed to ensure distal spread to the articular branches from the obturator nerve without proximal spread to femoral branches. Ambulatory arthroscopic knee surgery and other procedures in which quadriceps muscle paralysis may be a concern before discharge are other potential applications for ACB. In conclusion, ACB preserved quadriceps muscle strength better than FNB, without demonstrating statistically or clinically significant inferiority in pain relief. The ACB did not, however, enhance mobilization ability assessed with the TUG test and the 10-point mobility scale. Further studies are needed 2013 American Society of Regional Anesthesia and Pain Medicine 531

7 Jæger et al Regional Anesthesia and Pain Medicine Volume 38, Number 6, November-December 2013 to investigate the functional importance of quadriceps weakness in the early postoperative period. ACKNOWLEDGMENTS The authors thank the staff at the Orthopaedic Ward and the entire Operating and Recovery room staff at Frederiksberg Hospital, Copenhagen University Hospital, Frederiksberg, Denmark, and Glostrup Hospital, Copenhagen University Hospital, Glostrup, Denmark, for the invaluable assistance. REFERENCES 1. Charous MT, Madison SJ, Suresh PJ, et al. Continuous femoral nerve blocks: varying local anesthetic delivery method (bolus versus basal) to minimize quadriceps motor block while maintaining sensory block. Anesthesiology. 2011;115: Jaeger P, Nielsen ZJ, Henningsen MH, Hilsted KL, Mathiesen O, Dahl JB. Adductor canal block versus femoral nerve block and quadriceps strength: a randomized, double-blind, placebo-controlled, crossover study in healthy volunteers. 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