Peripheral Nerve Blocks Improve Analgesia After Total Knee Replacement Surgery

Abstract

Total knee replacement (TKR) produces severe postoperative pain.Peripheral nerve blocks can be used as analgesic adjuncts for TKR, but the efficacy of femoral nerve blocks alone is controversial. The sciatic nerve innervates posterior regions of the knee; thus, performance of both sciatic and femoral nerve blocks may be necessary to improve analgesia after TKR. We performed this study to determine whether peripheral nerve blocks improve analgesia after TKR. In a randomized, double-blind fashion, 36 patients undergoing TKR received either femoral, sciatic-femoral, or sham nerve blocks after a standardized spinal anesthetic. Further postoperative analgesia was provided by patient-controlled IV morphine and ketorolac. Pain at rest and with physical therapy, morphine use, nausea, pruritus, sedation, and patient satisfaction were assessed. Patients receiving peripheral nerve blocks reported better analgesia at rest for at least 8 h after transfer to the hospital ward (P < 0.05). Morphine use was decreased by approximately 50% in the peripheral nerve block groups until the second postoperative day (P < 0.02). Side effect profiles and patient satisfaction were similar between groups. We conclude that femoral nerve blocks improve analgesia and decrease morphine use after TKR. The addition of a sciatic nerve block to the femoral nerve block did not further improve analgesic efficacy. Implications: Performance of femoral nerve blocks improves analgesia and decreases the need for morphine after total knee replacement surgery. The addition of a sciatic nerve block to the femoral nerve block does not provide additional benefits.

(Anesth Analg 1998;87:93-7)

Pain after total knee replacement (TKR) is marginally treated with patient-controlled IV opioids ^[1-3]. After TKR, visual analog pain scale scores range from 40-80 (of 100) during the immediate postoperative period and slowly decline by the first postoperative day ^[1-3]. The addition of nonsteroidal antiinflammatory drugs (NSAID) improves analgesia after TKR, but pain control during the immediate postoperative period remains difficult ^[2,4].

The knee is innervated by the lumbosacral plexus. The femoral and obturator nerves innervate the anterior aspect of the knee, and the sciatic nerve innervates the posterior aspect ^[3]. Previous studies have investigated the use of femoral and obturator nerve blocks (3-in-1 block) as analgesic adjuncts after TKR with conflicting results ^[1,3,5]. The variable efficacy of femoral nerve blocks may be the result of variable block of all three nerves and/or intact posterior sensation from the sciatic nerve. Blocking of both the sciatic and femoral nerves may be required to consistently provide postoperative analgesia after TKR. However, no study has compared the effects of femoral and sciatic-femoral nerve blocks for postoperative analgesia after TKR. This study was performed to determine the analgesic efficacy of femoral, sciatic-femoral, and sham nerve blocks after TKR in a randomized, double-blind fashion.

Methods

With approval from our institutional review board and written, informed patient consent, 36 patients undergoing unilateral TKR participated in this prospective, randomized, double-blind, placebo-controlled study. Exclusion criteria included age <40 yr or >80 yr; ASA physical status more than III; allergy to local anesthetics; history of opioid dependence; contraindications to spinal anesthesia, femoral nerve block, or sciatic nerve block (coagulation defects, infection at puncture site, preexisting neurological deficits in the lower extremities); inability to use patient-controlled analgesia (IV PCA), contraindications for NSAID use (NSAID or aspirin allergy, severe liver disease, or serum creatinine >or=to1.7 mg/dL), weight >140 kg, and recent steroid use (<or=to3 mo).

Premedication was limited to midazolam 40 [micro sign]g/kg and fentanyl 1.4 [micro sign]g/kg. Spinal anesthesia was induced with 15 mg of hyperbaric bupivacaine 0.75% with 8.25% dextrose at the L2-4 interspace. Further intraoperative sedation consisted of midazolam in increments of 0.5 mg and fentanyl in increments of 25 [micro sign]g up to 2 [micro sign]g/kg at the discretion of the anesthetist.

On arrival in the postanesthesia care unit, each patient was randomized to receive femoral, sciatic-femoral, or sham nerve blocks. All blocks were performed before resolution of spinal anesthesia using a nerve stimulator. In the femoral nerve block group, the femoral artery was palpated in the inguinal area, and a stimulator needle placed lateral to pulsations. The femoral nerve was identified by eliciting quadriceps contractions with nerve stimulator settings at 2-Hz frequency and current between 0.2 and 0.8 mA. Then, 30 mL of 0.25% bupivacaine with 1:400,000 epinephrine was injected with manual pressure applied distal to the needle (3-in-1 block) ^[6]. The patient was then turned lateral, and a sham sciatic nerve block was performed. The patient's gluteal region was prepared, draped, and palpated, and the foot was manually manipulated to simulate nerve stimulation.

In the sciatic-femoral nerve block group, a femoral nerve block was performed as described above. The patient was then turned lateral, and landmarks for the sciatic nerve block (greater trochanter, posterior superior iliac spine, and sacral hiatus) were identified ^[7]. The sciatic nerve was identified by eliciting foot movements with nerve stimulator settings at 2-Hz frequency and current between 0.2 and 0.8 mA. Then, 30 mL of 0.25% bupivacaine with 1:400,000 epinephrine was injected. The control group had sham injections (prepared, draped and palpated, and manual manipulation to simulate nerve stimulation) performed at both the inguinal and gluteal regions. All procedures were performed behind a drape to block patient's view of procedure. Dressings were placed on all injection or sham injection sites. Blocks were performed by investigators (HWA, SSL, BDO) who were not involved with subsequent data collection for that patient. All patients were told to expect a variable amount of numbness and motor weakness after their peripheral nerve blocks. A blind investigator assessed sensory block to pinprick in the distribution of the saphenous (femoral block) and sural (sciatic block) nerve 4 h after the blocks and in the morning and evening of Postoperative Day (POD) 1. Another blind investigator collected additional data.

All patients received additional analgesia with IV morphine via the PCA device. After the peripheral nerve block procedures, patients were given morphine in 2- to 5-mg increments to provide a verbal pain scale score (0 = no pain, 10 = worst pain) of <or=to3. Initial PCA settings were a bolus of 1 mg and a lockout interval of 10 min with no limit or background infusion. Further adjustments were made at the discretion of the orthopedic surgery service. All patients received ketorolac either 15 (<65 yr or <50 kg) or 30 mg IV every 6 h. The first dose of ketorolac was given on arrival in the postanesthesia care unit and continued for at least 24 h. If oral medications were tolerated after 24 h, then oral ibuprofen 600 mg every 8 h was substituted for ketorolac.

Data collection included patient demographics, visual analog pain scale scores (0 = no pain, 10 = worst pain), sedation scores (1 = awake, 2 = drowsy but responsive to verbal stimulus, 3 = drowsy but arousable to physical stimulus, 4 = unarousable), presence of pruritus and nausea, and morphine use. These data were collected preoperatively, 1 h postoperatively, on arrival on the ward, every 4 h after arrival on the ward (data were not collected after 10 PM or if patients were asleep), and in the morning (8:00-10:00 AM) and evening (3:00-5:00 PM) of PODs 1-5. In addition, pain scores during physical therapy were collected in the morning and evening of PODs 1-5. Patients were contacted approximately 2 wk after discharge and asked to rate their satisfaction with the analgesic (0 = most satisfied, 10 = least satisfied) and whether they would agree to the same method of analgesia in the future.

Power analysis indicated that 12 subjects per group would allow us to detect a 50% difference in morphine use (power 0.8, P = 0.05). Demographics, pain scores, and morphine consumption were analyzed by using analysis of variance with Scheffe's test for post hoc comparisons. A contingency Table andnonparametric analyses were used for side effect and patients satisfaction data. A P value <0.05 was considered significant.

Results

Patient characteristics were similar among the groups (Table 1). Pain scores at rest were significantly lower for at least 8 h after transfer to the hospital ward in the groups receiving peripheral nerve blocks (Figure 1). Data were incomplete after the 8-h measurement and are not presented. Pain scores during physical therapy were also similar among the groups (Figure 2). Morphine consumption was significantly decreased to a similar degree in both peripheral nerve block groups until POD 2 (Figure 3). The incidence of side effects was similar among the groups (Table 2). No patient had a sedation score >2. Sensory block to pinprick from peripheral nerve blocks had mostly dissipated by the morning of POD 1 (Table 2). Patient satisfaction with analgesia was similar among the groups (Table 3).

Table 1: Patient Characteristics

Figure 1: Visual analog pain scale scores at rest. Mean +/- SE. *Different from both femoral and sciatic-femoral nerve block groups (P < 0.05). PACU = postanesia care unit, POD = postoperative day.

Figure 2: Visual analog pain scale scores during physical rehabilitation. Mean +/- SE. There were no differences among the groups. POD = postoperative day.

Figure 3: Morphine use via patient-controlled device throughout each measurement period. Mean +/- SE. *Different from both femoral and sciatic-femoral nerve block groups (P < 0.02). POD = postoperative day.

Table 2: Incidences of Side Effects and Sensory Block to Pinprick

Table 3: Patient Satisfaction with Analgesia

Discussion

Systemic opioids are a popular postoperative analgesic regimen for TKR because they are relatively simple to administer. Delivery of opioids with PCA devices results in better analgesia, decreased opioid use, and better satisfaction than their administration by nurses ^[8]. Despite the use of IV PCA opioids, postoperative pain after TKR remains severe, and side effects of sedation, nausea, and pruritus are common ^[1-3]. NSAIDs (ketorolac) are valuable analgesic adjuncts to systemic opioids for postoperative analgesia after TKR. The addition of ketorolac improves analgesia, reduces morphine use, reduces opioid related side effects, and improves patient satisfaction ^[2,4]. Thus, a combination of IV PCA opioid and NSAIDs represents current optimal systemic analgesic therapy after TKR.

Studies examining the use of peripheral nerve blocks as analgesic adjuncts to systemic opioids after TKR have produced conflicting results. One randomized controlled study examining local anesthetic infusions into the femoral nerve sheath after TKR reported better analgesia and opioid sparing compared with systemic opioid alone ^[5]. Two other studies using either continuous infusions or single-injection blocks of the femoral nerve reported no difference in analgesia ^[1,3], and only one study noted opioid sparing compared with opioid alone. Inconsistent analgesic effects of femoral nerve blocks alone could be explained by intact posterior sensation from the sciatic nerve, but none of these previous studies included a study group receiving a sciatic nerve block. In addition, none of these studies used NSAIDs combined with IV PCA opioids to provide an optimal systemic analgesia control group. Thus, the value of peripheral nerve blocks as analgesic adjuncts after TKR was unclear.

Our data indicate that peripheral nerve blocks are useful analgesic adjuncts for the immediate postoperative period after TKR even when added to IV PCA opioids plus NSAIDs. Both femoral and sciatic-femoral blocks improved analgesia at rest for at least 8 h after patients were transferred to the hospital ward. Peripheral nerve blocks had mostly resolved by the morning of POD 1, which probably explains the lack of difference in analgesia at rest or with physical therapy by that time. Although prolonged analgesia may be desirable, this duration encompasses the most painful period after TKR ^[1]. Furthermore, prolonged motor block from a prolonged block of femoral and sciatic nerves may delay physical therapy.

We observed an approximately 50%-67% decrease in morphine usage in the peripheral nerve block groups. It is interesting that morphine use was decreased longer (until POD 2) than the duration of improved analgesia from peripheral nerve blocks. We speculate that prolonged opioid sparing may reflect preemptive analgesic effects of the peripheral nerve blocks ^[9,10]. Femoral and sciatic-femoral nerve blocks were performed before resolution of spinal anesthesia, and a prolonged duration of suppression of painful afferent input to the spinal cord may have produced prolonged opioid sparing. Although we may have provided preemptive analgesia, our decision to perform nerve blocks before the resolution of spinal anesthesia was based on a desire to maximize double blinding in our study. It is unclear whether such a practice may result in a greater risk of direct needle trauma to the peripheral nerve. We attempted to minimize risks of peripheral nerve injury by using a nerve stimulator and by ensuring that nerve stimulation ceased at a low level of current. We expect that analgesia would still improve if peripheral nerve blocks were performed after resolution of spinal anesthesia or with a different nerve block technique.

We observed equal analgesic efficacy with either femoral or sciatic-femoral nerve blocks. This observation suggests that sciatic innervation of the posterior knee is a relatively minor contribution to postoperative pain after TKR. Because the performance of an additional sciatic nerve block is time-consuming and adds little to a femoral nerve block, we recommend performing only femoral nerve blocks after TKR.

We did not observe a reduction in nausea, pruritus, or sedation with peripheral nerve blocks, despite marked reductions in morphine use. It is likely that we did not study enough patients to detect a difference in side effects, as our sample size was chosen to detect a difference in opioid consumption. In addition, the use of NSAIDs reduces opioid side effects ^[2] and so may have contributed to the lack of difference in the incidence of observed side effects. Nonetheless, most systemic opioid side effects seem to be dose-related ^[11], and an observed reduction in opioid related side effects would be expected with an adequate study sample size.

The addition of femoral or sciatic-femoral nerve blocks improved analgesia from IV PCA morphine and ketorolac after TKR. The analgesic efficacy of sciatic-femoral nerve blocks was similar to that of femoral nerve blocks alone; thus, we recommend performance of femoral nerve blocks alone after TKR.