Providing pain relief with a minimum of side effects after total knee arthroplasty (TKA) remains challenging. Peripheral nerve blocks are often included in modern multimodal analgesic protocols. One of these techniques is the widely used femoral nerve block (FNB), which has a well-proven analgesic effect.1 Unfortunately, a substantial decrease in quadriceps muscle strength follows the FNB,2,3 and it has been claimed to impede mobility and to be associated with the risk of falling.4 Hence, it seems apparent that providing pain relief without the same degree of motor block would be preferable.
The adductor canal block (ACB) is a promising, motor-sparing alternative to the FNB, which targets mainly sensory nerves. There is accumulating evidence that the ACB results in less reduction in quadriceps muscle strength compared with the FNB,5–7 while still providing a comparable analgesic effect7–10 in the setting of multimodal analgesia.
Most studies have examined the effect of the ACB and the FNB on quadriceps muscle strength without controlling for postsurgical preblock baseline values. Consequently, these studies provide valuable information about differences between treatments, but, because knee surgery itself greatly reduces quadriceps muscle strength,11,12 we gain little knowledge of the isolated effect of the block. In a recent trial,13 the effects of the ACB and the FNB on quadriceps muscle strength were compared with postoperative preblock values, thereby enabling an evaluation of the isolated block effect. This study showed that the maximum voluntary isometric contraction (MVIC) decreased to 16% in the FNB group, but that the MVIC values almost doubled (193%) compared with preblock values in the ACB group. A plausible explanation is that the pain relief provided by the ACB decreases the centrally mediated inhibition of the voluntary contraction, thereby “allowing” the patient to use his/hers muscles. A weakness of our previous trial was that we included only patients in severe pain; therefore, it is unknown whether the increase in MVIC can also be found in patients with mild and moderate pain.
The aim of the present study was to examine whether the ACB can increase functional muscle strength in a broad population of TKA patients with a planned subgroup analysis (for the primary end point), dividing patients according to postoperative preblock pain scores. We hypothesized that the ACB increases quadriceps muscle strength when applied to a broad population of TKA patients, regardless of pain scores. The primary outcome was the difference between groups, 1 hour postblock in MVIC of the quadriceps muscle, expressed as a percentage of the preblock value. Secondary outcomes were pain scores at rest, during active knee flexion, and during the Timed Up and Go (TUG) test and time to complete the TUG test.
Patients and Design
This single-center, randomized, blinded, placebo-controlled trial was approved by the local Regional Ethics Committee (H-4-2014-062), the Danish Medicines Agency (EudraCT no. 2014-002245-21), and the Danish Data Protection Agency, and was registered at ClinicalTrials.gov (NCT02242591) before enrollment. The Good Clinical Practice Unit, Copenhagen University Hospital, monitored the trial, which was performed in accordance with the principles of the Declaration of Helsinki. Data are presented in accordance with the Consolidated Standards of Reporting Trials (CONSORT) statement.14
From September 2014 to November 2014, all patients scheduled for elective TKA at Gentofte University Hospital were screened for inclusion and informed about the study. Inclusion criteria were as follows: age 18 to 85 years; ASA physical status I to III. Exclusion criteria were as follows: inability to cooperate; inability to perform a TUG test before surgery; peripheral or central nerve block after surgery; inability to read or speak Danish; allergy to ropivacaine; alcohol or drug abuse; neuropathy in lower extremities; and pregnancy. Written informed consent was obtained from all subjects before enrollment. The study was performed on the first postoperative day.
Surgery, Anesthesia, and Preoperative and Postoperative Analgesia
TKA was performed in general or spinal anesthesia with insertion of tricompartmental prosthesis using a medial parapatellar approach. Cruciate retaining, as well as cruciate substituting designs, were inserted in a bloodless field (tourniquet). A compression bandage, from toe to mid-thigh, was applied at the end of surgery.
Unless contraindicated, all patients received a standardized analgesic regimen: preoperatively: oral celecoxib 400 mg, acetaminophen 1 g, and gabapentin 600 mg; perioperatively: a single IV dose of methylprednisolone 125 mg and local infiltrating analgesia, as described by Kerr and Kohan,15 using 150 mL of ropivacaine 0.2% with epinephrine (10 μg/mL); postoperatively: oral acetaminophen 1 g ×4, ibuprofen 400 mg ×3, gabapentin 300 mg (7:00 AM) and 600 mg (10:00 PM), and opioids according to age, weight, and concomitant medication. Slow-release opioids were administered at 7:00 AM and 7:00 PM. Oral immediate-release opioids were self-administered.
Randomization and Blinding
The pharmacy performed a block-randomized allocation sequence (blocks of 8) and prepared 64 consecutive bags containing the study medication. Each bag contained 2 boxes, one marked “Injection no. 1” and the other marked “Injection no. 2.” For group A, the Injection no. 1 box contained 2 × 20 mL ampules with ropivacaine 0.75% and the Injection no. 2 box contained 2 × 20 mL ampules with isotonic saline. For group B, the Injection no. 1 box contained 2 × 20 mL ampules with isotonic saline and the Injection no. 2 box contained 2 × 20 mL ampules with ropivacaine 0.75%. Ropivacaine and isotonic saline are visually indistinguishable, and the ampules were of identical appearance; hence, the content was blinded and the allocation concealed. From each box, 30 mL of study medication was used for each block, and the remaining 10 mL was discarded.
On the first postoperative day, preblock (but postoperative) baseline values were obtained in the morning, at t0. Shortly thereafter, the patient received an ACB with 30 mL of study medication marked Injection no. 1. Immediately after the 60-minute assessments (t60), the patient received the second ACB with 30 mL of study medication marked Injection no. 2. Final assessments were done 60 minutes after the second ACB (t120). Thus, we had a placebo-controlled trial at t60, and the disappearance of a possible difference at t120 (when both groups had received an active ACB) would support our primary findings. The ACBs were performed at the mid-thigh level using real-time ultrasonography (short-axis view with an in-plane needle advancement), with the needle tip placed in the corner between the superficial femoral artery, the sartorius muscle, and the vastus medialis of the quadriceps muscle.
Assessments were done at t0, t60, and t120 in the following order: (1) visual analog scale (VAS) pain score at rest; (2) VAS pain score during 45° active knee flexion; (3) MVIC of quadriceps femoris; (4) a TUG test; and (5) worst VAS pain score experienced during the TUG test.
VAS pain scores were assessed with a 0- to 100-mm ruler. MVIC was measured using a handheld dynamometer (Lafayette Instrument Company, Lafayette, IN), with the patient in a seated position with both feet off the ground. The dynamometer was placed approximately 5 cm proximal to the transmalleolar axis. Patients were instructed to straighten their leg against the dynamometer, achieving maximal force within 2 seconds and to hold this for 3 seconds followed by a 30-second pause. Three measurements were obtained, and the mean value was used. Patients practiced with the nonoperated leg before baseline measurements to familiarize them with the procedure. In the TUG test, we recorded the time taken by the patient to get up from a chair using the armrests, walk 3 m, and return to the sitting position in the chair. All patients used a high 4-wheel walker (Hydraulic Topro Taurus Care Walker) with arm support. No help was provided. Immediately afterward, they reported the worst VAS pain score experienced during the test. To determine the success rate of the block, we tested the cutaneous sensory function of the saphenous nerve by swiping a cold gauze on the medial middle aspect of the tibia at t0 and at t120. One investigator did all assessments.
The primary outcome was the difference between groups in MVIC of the quadriceps femoris muscle, expressed as a percentage of the preblock values at t60. Secondary outcomes were differences between groups at t60 and t120 in VAS pain score at rest, during active knee flexion, and during the TUG test and time to complete the TUG test.
This study was designed as a superiority study. A difference of 25%, between the groups, in quadriceps MVIC was considered clinically relevant.16,17 With a 2-sided α = 0.05, a power of 90% and an assumed SD of 30% in MVIC, 2 × 30 patients were required. Because of uncertainty in predicting the actual SD, we chose to include 2 × 32 patients.
Data were analyzed using IBM SPSS Statistics version 20.0 (IBM Corp., Armonk, NY). All end points are continuous data. MVIC measurements were found to be skewed (visual examination of histograms, Q-Q and box plots, and Shapiro-Wilk test with a P < 0.05) and therefore analyzed as nonparametric data using the Mann-Whitney U test. MVIC is presented as percentage of preblock values (median with a bootstrapped 95% confidence interval [95% CI], 1000 iterations). One hundred percent thus represents “no change.” The treatment effect, difference between groups, was quantified using the Hodges-Lehmann estimator with a constructed 95% CI. Time to perform the TUG test and VAS pain scores were found to have an unexplained difference in baseline values (especially VAS flexion), therefore we used analysis of covariance with the baseline values as a covariate. Categorical data (number of patients able to mobilize, success rate of the block, etc.) are reported only as actual numbers and not compared using statistical tests because they were not end points.
All data were entered into a spreadsheet and checked for typos by 2 investigators. The pharmacy delivered a list allocating each patient to 1 of the 2 groups (“y” and “z”) without revealing the identity of the groups. After data analysis was completed and conclusions were drawn, the pharmacy reported which of the primary groups y and z matched to group A and group B.
A total of 107 patients were screened for participation in the study. Sixty-four patients were enrolled and randomly assigned. One patient in group B was excluded before analysis, because we became aware that he had received an ACB immediately after surgery (exclusion criteria; Fig. 1). Data from 63 patients were analyzed. Table 1 shows similarity between the groups with respect to demographics and perioperative data. To avoid potential influence, no medications were administered at least 1 hour before baseline measurements. Although optional, no patients used analgesics during the 3-hour study period.
At t60, quadriceps MVIC was significantly higher in group A = 170% (95% CI, 147–231) of preblock values compared with 93% (95% CI, 82–98) in group B (P < 0.0001). After both groups had received an active ACB, at t120, MVIC increased to 160% (95% CI, 126–188) in group B and the difference between groups disappeared (P = 0.85; Fig. 2). Results from the planned subgroup analysis can be seen in Table 2, and individual patients’ absolute MVIC values at t60 can be seen in Figure 3.
There were no statistically significant differences between groups in time to perform the TUG test at any time points. At t60, group A performed the test in 22 seconds (95% CI, 19–24) compared with 24 seconds (95% CI, 22–27) for group B (P = 0.13).
VAS Pain Scores
All differences in VAS scores, at t60, were in favor of group A; 18 mm (95% CI, 10–26) during the TUG test (P = 0.002), 12 mm (95% CI, 6–18) at rest (P < 0.001), and 14 mm (95% CI, 5–22) during knee flexion (P < 0.001). At t120, all differences had disappeared (Fig. 4).
Success Rate of the Block
Two patients could not sense cold in the saphenous area before the first block and were excluded from the block success evaluation. After the study period, 2 patients in group A had no change in cold sensation (failed blocks), thus resulting in a success rate of 60/62 = 97% (95% CI [modified Wald interval], 88%–100%). The 2 patients with failed blocks were included in the outcome analyses (intention to treat).
At t60, 2 patients in group A (black and blue dot close to the x-axis in Fig. 3) had a substantial decrease in MVIC and were no longer able to perform the TUG test. Another patient also became unable to perform the TUG test at t60, despite an increase in MVIC to 347% (she felt no control over her knee). At t120, when both groups had received 30 mL saline and 30 mL ropivacaine, the number of patients with a substantial decrease in MVIC (defined as MVIC <75% of preblock values) increased to 5, and 4 of these were unable to perform the TUG test. Furthermore, one more patient became unable to perform the TUG test, despite no decrease in MVIC (she felt no strength in the leg).
The aim of this study was to determine the isolated effect of an ACB on quadriceps muscle strength after TKA. The use of postoperative preblock baseline values circumvented the influence from the detrimental effect caused by surgery itself. The 170% increase in quadriceps MVIC contradicts former claims that nerve blocks are always followed by muscle weakness and supports the idea that the analgesic effect of an ACB can be converted into functional muscle strength.13 It is encouraging that this effect was found even in the subgroup of patients with low preblock pain scores. Although it seems reasonable that lower pain scores and increased muscle strength would translate into enhanced mobility, the lack of difference in time to perform the TUG test does not support this assumption. A possible explanation could be the use of a high 4-wheel walker, which most likely lowered the assay sensitivity, because the nonsurgical leg can compensate for the surgical leg.18 However, it is reassuring that we found a clinically relevant 18-mm decrease in pain score during mobilization. The finding of a nonsignificant difference between subgroups of patients with VAS pain score during knee flexion = 60 − 100 should not be overinterpreted, because only 13 patients were included in this subgroup analysis. The median values match the overall results.
Of concern, 3 of the 32 patients in group A temporarily lost the ability to perform the TUG test after receiving the first ACB. Two of these patients experienced a marked decrease in MVIC and were no longer able to rise from the sitting position without help. The decrease in MVIC was comparable to the motor impairment seen with FNBs,5,6,19 and we could trace (ultrasound) the local anesthetic from the adductor canal proximally where it surrounded the femoral nerve proximal to the division of the femoral artery—a dispersion that disappeared 1 hour later. This is interesting, because we saw no accidental FNBs in our previous study,13 where all the blocks were performed by the same 2 investigators using the same volume. Whether the excessive spread was caused by the high volume, high injection pressure, or anatomical variations remains unknown but adds to the apprehension raised by others.20–22 The third patient, who lost the ability to ambulate, actually had a major increase in MVIC to 374% of baseline, but this patient reported a total loss of control over the knee. Of note, examination of the leg revealed no signs that the sciatic nerve was affected. Furthermore, when group B received the second and active ACB, another 2 patients lost the ability to perform the TUG test. At this time, group B had already received an ACB with 30 mL of saline, which possibly could have “dissected” a pathway to the femoral nerve.
Clearly, our study has limitations; because we designed it to evaluate the immediate and isolated effect of the ACB on muscle strength, it tells us nothing about the duration of the effect. Most of the patients were anesthetized with a spinal (Table 1), and we do not know whether results apply for patients undergoing surgery with general anesthesia. However, we find it unlikely that the anesthetic technique should influence results for more than a few hours postoperatively. Furthermore, the 30-mL volume can be criticized, because our results indicate that it may be excessive and that we may have actually impeded mobility. We chose this volume because we still regard the ACB to be a high-volume block and to be consistent with our previous studies, allowing a comparison of the results. Apart from few case reports, we had no previous warnings of a volume-related effect on quadriceps strength, but future studies should focus on finding the optimal volume and injection pressure for the ACB, mandating noninferiority designs.
In conclusion, for most patients, the ACB provides pain relief that converts into increased functional muscle strength, but we must remain aware of a possible spread to motor branches of the femoral nerve.
Name: Johan Kløvgaard Sørensen, MS.
Contribution: This author helped in study design, conduct of the study, data collection, data analysis, and manuscript preparation.
Attestation: Johan Kløvgaard Sørensen attests to the integrity of the original data and the analysis reported in this manuscript.
Name: Pia Jæger, MD, PhD.
Contribution: This author helped in study design and manuscript preparation.
Attestation: Pia Jæger approved the final manuscript
Name: Jørgen Berg Dahl, MD, DMSci, MBA.
Contribution: This author helped in study design and manuscript preparation.
Attestation: Jørgen Berg Dahl approved the final manuscript.
Name: Bo Gottschau, MD.
Contribution: This author helped in study design, conduct of the study, and manuscript preparation.
Attestation: Bo Gottschau approved the final manuscript.
Name: Snorre Læssøe Stephensen, MD.
Contribution: This author helped in study design and manuscript preparation.
Attestation: Snorre Læssøe Stephensen approved the final manuscript.
Name: Ulrik Grevstad, MD.
Contribution: This author helped in study design, conduct of the study, data analysis, and manuscript preparation.
Attestation: Ulrik Grevstad attests to the integrity of the original data, the analysis reported in this manuscript, and is the archival author.
This manuscript was handled by: Terese T. Horlocker, MD.
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© 2016 International Anesthesia Research Society
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