Reply to the Letter to the Editor: Single-injection or Continuous Femoral Nerve Block for Total Knee Arthroplasty?

To the editor, We would like to thank Dr. Byrne for his comments regarding our recent publication exploring the effects of a single injection versus two types of continuous infusion for femoral nerve block [1]. We agree that Brodner and colleagues [3] previously have concluded that ropivacaine 0.2% and ropivacaine 0.3% were equivalent in terms of pain outcomes compared to ropivacaine 0.1% when used as continuous infusion for femoral nerve blocks. However, Brodner and colleagues designed their study to test the hypothesis that ropivacaine 0.3% was superior to ropivacaine 0.2% or 0.1%, and the authors’ conclusion that 0.1% was ineffective was based on an interim statistical analysis of only 20 patients. Although their study demonstrated that increasing the concentration of ropivacaine above 0.2% did not confer any advantage, we relied on another study, that of Paauwe et al [4], to address the effect of decreasing the concentration of ropivacaine below 0.2%. Indeed, Paauwe and colleagues [4] found no advantage in using concentrations of ropivacaine less than 0.1%. In designing our protocol, we were also mindful that Brodner and colleagues had not used a comprehensive multimodal analgesia strategy. The lack of benefits associated with increased concentrations of ropivacaine (ie greater than 0.1%) recently has been confirmed in another study [2]. We recognize that we used a lower total mass of local anesthetic for both the priming injection and for the continuous infusion compared to what has been published recently by Spangehl and colleagues [5]. Such differences in postoperative therapeutic regimens may explain the discrepancy in reported pain scores between institutions.