A Randomized Comparison of Pain Control and Functional Mobility between Proximal and Distal Adductor Canal Blocks for Total Knee Replacement.

BACKGROUND: Adductor canal blocks (ACBs) have become a popular technique for postoperative pain control in total knee arthroplasty patients. Proximal and distal ACB have been compared previously, but important postoperative outcomes have yet to be assessed. AIMS: The primary objective of this study is to compare postoperative analgesia between proximal and distal ACB. Secondary outcomes include functional mobility, length of stay (LOS), and adverse events. SETTINGS AND
DESIGN: This study was a single-center, assessor-blinded, randomized trial. SUBJECTS AND METHODS: Fifty-seven patients were randomly assigned to receive a proximal (n = 28) or distal (n = 29) ACB. A 20 mL bolus of 5 mg/mL ropivacaine was injected at the respective location followed by 2.0 mg/mL ropivacaine infusion for 24 h. STATISTICAL ANALYSIS: The primary outcome was intra- and postoperative 24-h opioid consumption in intravenous (IV) morphine equivalents. Secondary outcomes include percentage change in timed "Up and Go" (TUG) times, LOS, and average postoperative pain scores. Continuous variables were compared using Student's t-test.
RESULTS: The mean (±standard deviation) 24-h intra-and postoperative opioid consumption showed no difference between the proximal and distal groups (39.72 ± 23.6 and 41.28 ± 19.6 mg IV morphine equivalents, respectively, P = 0.793). There was also no significant difference in the median [minimum, maximum] percentage change in TUG times relative to preoperative performance comparing proximal and distal ACB (334.0 [131, 1084] %-change and 458.5 [169, 1696] %-change, respectively, P = 0.130). In addition, there were no differences in postoperative pain scores or LOS.
CONCLUSIONS: ACB performed at either proximal or distal locations shows no difference in postoperative pain measured by opioid consumption or pain scores. Better TUG performance seen in the proximal group was not statistically significant but might represent a clinically important difference in functional mobility.

INTRODUCTION

Adductor canal blocks (ACBs) are widely used for postoperative pain control because of their efficacy and proposed quadriceps motor-sparing effects in total knee arthroplasty (TKA) patients.[123456] Retrospective cohort studies and current meta-analyses suggest that compared to femoral nerve blocks, ACB allow for better early functional mobility.[5789] At the same time, ACB provides similar or better postoperative analgesia in TKA patients.[136910]

While there is widespread agreement on effectiveness of ACB, there is considerable debate in the literature over the optimal placement of the catheter along the adductor canal.[1211] Both distal (mid-thigh) and proximal placements have been described with each approach having benefits and drawbacks.[911] One anatomical study showed that distal (mid-thigh) ACB may be superior because of decreased risk of cephalad spread to the common femoral nerve and subsequent quadriceps weakness. At the same time, distal ACB effectively targets key sensory nerves to the knee capsule essential for effective pain control after TKA.[12] In a recent case report, this concern for cephalad spread was realized in a patient who reportedly developed severe quadriceps weakness after an ACB.[13] On the other hand, another study using cadaveric models showed sparing of the motor nerves to the sartorius and quadriceps muscles with a proximal ACB. Moreover, in the same study, the small clinical population receiving proximal ACB showed no evidence of motor weakness.[14] In the first randomized comparison of proximal and distal ACB, Mariano et al. found that the proximal ACB may offer better functionality during surgery without any increased quadriceps weakness. Further, those with proximal ACB were more likely to require morphine in the first 24 h.[11] Currently, only one small study has compared the effect of each block on 24-h opioid consumption as a primary measure.[15] Further, no studies have utilized the Timed “Up and Go” (TUG) assessment to compare differences in functional mobility. Importantly, the TUG assessment allows for objective measurement of functional mobility as a component of effective pain control during movement and has been validated for inpatient orthopedic rehabilitation purposes.[16]

Thus, the primary objective of this study is to compare the analgesic effects of proximal ACB to distal ACB using 24-h intra- and postoperative opioid consumption in TKA patients. The secondary objectives include assessing functional mobility using the TUG assessment, pain scores, adverse events, and length of stay (LOS). We hypothesized that proximal ACB would provide superior analgesia with a 20% less postoperative opioid consumption compared to distal ACB.

SUBJECTS AND METHODS

Study design

This study was a single-center, prospective, assessor-blinded, randomized trial comparing 24-h intra- and postoperative opioid consumption between proximal and distal ACB in TKA patients. Functional mobility (measured using the TUG assessment), LOS, pain scores, and adverse events were also compared. This study was approved by the Institutional Review Board at Montefiore Medical Center. All patients received detailed oral and written information before surgery and gave consent for the study.

Study population

From November 2013 to March 2015, adult patients ≥18 years old scheduled for primary TKA were considered eligible for the study. Patients unable to give consent or with any known contraindications to medications, regional, or neuroaxial anesthesia were excluded. Patients with daily intake of strong opioids (morphine, methadone, fentanyl, hydromorphone), a history of intravenous (IV) drug abuse, alcohol abuse, and those unable to perform the TUG test preoperatively were also excluded from the study.

Procedures

After obtaining informed consent, patients were assigned randomly to either proximal or distal (mid-thigh) groups using a computer-generated randomization sequence. All procedures were performed by an attending regional anesthesiologist or by a fellow under supervision of a regional anesthesia attending.

Before IV and monitor placement, all patients performed the baseline preoperative TUG assessment as described in Yeung et al.[16] In the TUG assessment, patients were asked to stand up from a chair, walk 3 m to a marked line, return to the chair, and sit down. All TUG assessments were performed with the assistance of a walker. The time taken from start to finish was recorded in seconds. All TUG assessments were performed by residents not involved in performing the ACBs and were blinded to the catheter placement site.

After the baseline preoperative TUG assessment, standard IV lines (20 or 22 gauge), monitors (noninvasive blood pressure cuff, pulse oximetry, 3 lead electrocardiography), and supplemental oxygen at a rate 2–5 L/min were placed. Fentanyl and midazolam (IV) were titrated to patient comfort before performing ACB and catheterization. The ultrasound machine (M-Turbo, HFL38; Sonosite, Inc., Bothell, WA, USA) with a 13-6 MHz linear array ultrasound transducer was protected with a sterile covering, and the procedural insertion site was cleaned with chlorhexidine (Chloraprep) and draped with sterile towels.

Proximal block

The ultrasound probe was first placed for a transverse cross-sectional view of the patient's groin and thigh. The femoral nerve was identified in the short axis near the inguinal crease, and the ultrasound transducer was positioned caudally beyond the femoral triangle. We designated the position of the proximal block at the site at which the superficial femoral artery passed beneath the medial border of the sartorius muscle (generally 8–12 cm distal to the inguinal crease). These measurements, as well as the length of the thigh from inguinal crease to the top of the patella and the width of the thigh at mid-thigh, were recorded. An 18-gauge Tuohy-tip needle (Braun Medical, Melsungen, Germany) was inserted through the skin wheal and directed in-plane under ultrasound guidance. The needle was advanced toward the target until the tip of the needle crossed the sartorius muscle and passed into the adductor canal lateral to the superficial femoral artery. A 21-gauge catheter was then inserted 3–5 cm beyond the needle cannula. After placement, a bolus of 20 mL of 5 mg/mL (5% w/v) ropivacaine was injected incrementally (aspiration every 3–5 mL) to expand the adductor canal space. To verify the correct position of the catheter tip, injection of an additional 1 mL of air under direct ultrasound visualization was performed. To confirm successful block, sensory function was assessed along the saphenous nerve distribution by comparing pinprick sensation to the unaffected limb. All adductor canal catheters were attached to an infusion device (OnQ, Irvine, CA) which infused 2.0 mg/mL (0.2% w/v) ropivacaine at a rate of 6–8 ml/h for at least 24 h in all patients. All catheters were secured to the thigh using Mastisol (Eloquest Health Care) steri-strips and a clear, occlusive dressing (Tegaderms, 3M). The insertion site was covered with opaque silk tape to the level of the mid-thigh to ensure that postoperative assessors were blinded to the placement of the catheter.

After ACB and catheterization was complete, patients received a selective tibial nerve block as described by Sinha et al. with 5–10 mL of 5 mg/mL ropivacaine injected.[17] In the operating room, patients received spinal anesthesia with 2.5–3.0 mL of 5 mg/mL isobaric bupivacaine. The addition of intrathecal fentanyl was done at the discretion of the intraoperative anesthesiologist. Further use of propofol and IV fentanyl was also at the discretion of the intraoperative anesthesiologist. A femoral tourniquet was used at the discretion of the surgeon.

Distal block

The operative leg was exposed and measured as described above. The midpoint of the thigh was then determined as half the distance between the inguinal crease and top of the patella. After marking the mid-thigh mark with a sterile marking pen, the ultrasound transducer was positioned for a transverse view of adductor canal at mid-thigh. The femoral artery and saphenous nerve were identified under ultrasound visualization. The needle was guided through the sartorius muscle and placed lateral to the femoral artery and the saphenous nerve. Initial bolus of ropivacaine and catheter placement was performed as described above. This procedure follows what is described in Jenstrup et al.[1] Following the distal ACB, tibial nerve block, spinal anesthesia, and intraoperative sedation were performed as described above for the proximal ACB.

Postoperative pain management

Postoperatively, IV patient-controlled analgesia was provided with either morphine (bolus 1 mg, 6-min lockout time, no background infusion) or hydromorphone (bolus 0.1–0.2 mg, 6-min lockout time, no background infusion). If analgesia was inadequate in PACU, patients received additional bolus IV opioids. These additional bolus doses were included in totals. After leaving PACU, oral oxycodone was given for break through pain on an as-needed basis (pain score ≥7). Additional nonopioid analgesics were given per standardized protocol as follows to all patients: ketorolac 30 mg IV every 6 h and oral celecoxib 200 mg once a day; for those ages 65 and older, the dose of ketorolac was adjusted to 15 mg IV every 6 h to a maximum of 60 mg every 24 h. Postoperative TUG assessment was performed at 24 h after the surgery as described above.

Measurements/end points

The primary end-point was the amount of intra- and postoperative IV morphine equivalents given to the patients for 24 h. This included all opioids given intraoperatively and postoperatively. Twenty-four hour opioid consumption was converted to IV morphine equivalents using a standardized opioid equivalency table.[18] Opioid consumption was retrieved from electronic medical records by an assessor who was blinded to the catheter placement location. The secondary end-points included the percent change in TUG assessment times from baseline, average daily postoperative pain scores (postoperative day [POD] #0 and #1), and LOS. TUG percent change was calculated by comparing the preoperative and POD#1 times to perform the TUG assessment. Patient pain scores, LOS, and adverse events were also retrieved from electronic medical records.

Statistical analysis

All statistical analyses were performed using SPSS v. 24 (IBM Corp., Armonk, NY, USA). Descriptive statistics of demographic and clinical characteristics are presented with mean (standard deviation [SD]) for continuous scale variables. The difference between normally distributed continuous scale variables was examined using Student's t-test, while nonnormal variables were examined using Wilcoxon rank sum test. The association between categorical variables was examined using Pearson Chi-squared test or Fisher's exact test. All analyses were performed in accordance with intention-to-treat (ITT) principle.

Sample size calculation

Twenty-four-hour opioid consumption in IV morphine equivalents was the primary outcome used to power the study. Using the average 24 hr. opioid consumption reported in Jenstrup et al. for patient receiving ACB for TKA (50 mg, SD ±25%) and to detect a clinically-significant 20% change in opioid consumption, 28 patients were required in each treatment arm with 2-sided type I error protection of. 05 and power of 80%. Thus, we enrolled 28 patients in each treatment arm to adequately power the study.

RESULTS

Fifty-seven patients were originally enrolled in the study and randomly assigned to one of the two study groups. Six patients did not complete the postoperative TUG assessment for the following reasons: four patients had severe postoperative nausea and vomiting and opioids were discontinued; 1 patient refused the postoperative TUG test; and 1 patient with a known seizure disorder experienced a seizure on POD#0. Another 11 patients from one surgeon had a combination of local anesthetic and preservative-free morphine injected directly into the joint space at the end of the procedure. One 1 patient was discovered to have been taking oral tramadol preoperatively which had not been told to the anesthesiologist. This patient was not included in any analyses and was considered a screening failure [Figure 1]. No patients were converted to general anesthesia, had unusually long tourniquet times, and no catheters were known to be dislodged in the first 24 h.

Figure 1

Schematic representation of the adductor canal and surrounding musculoskeletal anatomy (Credit: Dr. Soo Yeon Kim)

The distribution of patient's demographic and clinical characteristics is presented in Table 1. The ITT analysis had 28 patients in both the proximal and distal block group. As shown in Table 1, the proximal and distal groups did not differ in age, sex, height, weight, body mass index, length of surgery, leg length, length circumference, or baseline TUG times.

Table 1

Distribution of demographic and clinical characteristics by adductor canal block

The primary and secondary outcomes are presented in Table 2. Mean (±SD) intra- and postoperative 24-h opioid consumption measured in IV morphine equivalents does show not any major difference between the proximal and distal groups (39.72 ± 23.6 mg and 41.28 ± 19.6 mg, respectively, P = 0.793). Percentage increase in time to perform the TUG test postoperatively relative to baseline preoperative time was derived for both groups and compared. Though not statistically significant, proximal group showed a lesser increase in time taken to complete the TUG test. TUG times increased 334 (131, 1084) % from baseline in proximal group compared to 458.5 (169, 1696) % in the distal group (P = 0.130). There was also no difference in LOS. The proximal average LOS in the proximal (2.67 ± 0.7 days) and distal (2.89 ± 1.2 days) did not show a statistically significant difference (P = 0.430). The daily average pain score for each patient collected for POD#0 in proximal and distal group also showed no difference (3.89 [2.4] and. 4.04 [2.6], respectively, P = 0.817). Similarly, there were no difference in POD#1 average daily pain scores (P = 0.13).

Table 2

Experimental outcomes by adductor canal block in intention-to-treat analysis

DISCUSSION

This study demonstrates that both proximal and distal ACB provide similar analgesia as demonstrated by intra- and postoperative 24-h opioid consumption. Measuring functional mobility, the percent-change TUG times between preoperative and POD#1 tests were not statistically significant between the proximal and distal groups, but there may be a clinically important benefit to proximal placement based on the underlying anatomy and trends in the data. There were no differences in LOS or average daily postoperative pain scores at POD # 0 and #1 between proximal and distal ACB.

Adductor canal blocks: Comparing proximal and distal

ACBs are an effective method of postoperative pain management in TKA patients.[12] The adductor canal primarily contains sensory nerves that branch from the femoral nerve and provide the major sensory innervation to the knee [Figure 2]. The only motor innervation through adductor canal is the nerve to vastus medialis.[1219] Thus, ACB provide effective sensory blockade and minimize quadriceps motor blockade. In contrast to other methods, ACB provide effective analgesia without related quadriceps weakness and increased fall risk.[342021222324]

Figure 2

Flowchart of patient enrollment, randomization, exclusions, and analysis

Proximal and distal ACB are both safely performed, but there is debate as to the best placement. Mariano et al. compared the two approaches and showed that proximal placement may provide better analgesia without additional motor blockade although the differences were not significant.[11] Our study confirms this result along with more recent studies.[15]

Timed “Up and Go” assessment and early ambulation

The current study is the first that directly compares functional mobility between proximal and distal placement using the TUG assessment. The use of physical assessment measures like the TUG assessment is important because it provides direct and objective measurement of the functional status of the patient.[25] Moreover, performance on the TUG assessment has clinical value as it is associated with fall risk, muscle strength, and other outcomes in various populations.[126] While the current study is not adequately powered to determine if there is or is not a difference between proximal and distal ACB in TUG performance, understanding the balance between (1) reducing pain during movement and (2) preserving strength is critical in understanding how ACBs affect functional mobility. The TUG assessment captures both of these critical components of functional mobility.

In terms of (1) pain control during movement, the lesser percent change in the postoperative TUG times relative to baseline in the proximal group suggests that there may be some underlying benefit to proximal placement of the catheter in terms of pain control during movement. This might be explained by the fact that more proximal blocks affect more femoral nerve branches to the knee joint as compared to distal placement. Improved pain control during movement – not necessarily detected on 24-h opioid consumption or pain scores – could be responsible for faster relative TUG test performance in the proximal group. While this needs to be confirmed in future studies, the potential difference in functional mobility between proximal and distal ACB demonstrates the importance of using the TUG assessment. While 24-h opioid consumption did not demonstrate any differences, the TUG assessment was able to detect a potentially clinically significant difference between the groups. Future studies should be powered to detect differences in TUG percent change from baseline because of the potential to detect differences in effective pain control during movement that are missed in 24-h opioid consumption. Post hoc analysis of current data indicates that approximately 70 patients would be needed to detect a statistically significant difference in TUG performance.

For assessing (2) motor weakness, the TUG assessment is also useful. The trends in the data notwithstanding, it is notable that TUG times did not increase more in the proximal ACB relative to the distal ACB. With more proximal blocks, there have been concerns of proximal spread to the femoral nerve causing quadriceps weakness.[12132728] The results of this study, while underpowered, did not show that proximal block had worse functional mobility due to decreased muscle strength. Thus, proximal spread of anesthetic to the femoral nerve was not likely a significant factor in this study population. This allays concerns about quadriceps weakness and fall risk associated with more proximal ACB.

As noted, important components of effective pain management are controlling (1) pain specifically during movement and (2) preserving motor strength. Using performance assessments like the TUG assessment allows anesthesiologist to assess both parameters for restoration of functional mobility. Achieving functional mobility and early ambulation in the first 24 h is critical for achieving better outcomes after TKA. With improved functional mobility and thus early ambulation, there is decreased risk of thrombosis, earlier participation in physical therapy, and overall better outcomes.[729] Thus, assessing changes TUG performance is an important outcome to assess in the clinic and in future research comparing peripheral nerve blocks.

Study limitations

There are several limitations associated with this study. The most significant limitation of this study is our inability to separate the analgesic effects of the initial bolus of ropivacaine from the analgesic effect of continuous perineural catheter because of the relatively long half-life of ropivacaine. Thus, we had to limit our interpretation such that we attribute the analgesic effects to the ACB site itself (proximal or distal) rather than to the perineural catheter infusion, specifically. Another limitation is the inability to blind the anesthesiologist and surgeon regarding the randomization group of the individuals. However, we have taken all possible measures to blind the assessor who collected the study data. Next, the use of intraoperative analgesics in these individuals might be a confounding variable. Even though there were no standardized protocols for intraoperative anesthetic management, all anesthetics were delivered by the same group of anesthesiologists using similar approaches. Finally, as this is a single-center study with a small sample size and the study was limited to TKA patients, the results may not be generalizable for other types of knee procedures. The study is only powered to find a difference in the 24-h opioid consumption. All other results reported should be interpreted with caution.

CONCLUSIONS

In summary, our study confirms that there is no statistically significant difference in the average 24-h analgesic effects between the proximal and distal ACB. Percent change in TUG times showed a trend favoring the proximal group but was also not statistically significant. Insofar as the proximal ACB did not show a significant difference TUG times relative to the distal ACB, proximal blocks can be safely performed without concern of worse quadriceps weakness relative to more distal blocks. At the same time, we speculate based on the trends in the data that that proximal ACB may provide better analgesia, specifically during times of movement. Future studies are needed to confirm this hypothesis. Based on these trends, we speculate that proximal ACB may provide better analgesia, specifically during movement, without any increase in quadriceps weakness. Future studies are needed to confirm this hypothesis.

Financial support and sponsorship

This project was funded by the Department of Anesthesiology at Montefiore Medical Center.

Conflicts of interest

There are no conflicts of interest.