We thank Drs. La Colla and Chelly for their comments on our article.1 We hereby address their comments in a point-by-point fashion, as received.
Femoral block with 0.5% ropivacaine does indeed produce a motor block, but so does femoral block with concentrations as low as 0.1% or 0.2% ropivacaine, as shown in numerous studies.2–4 The difference in motor power between the high- and low-concentration solutions is in fact minimal; dilute local anesthetic solutions are not necessarily motor preserving, but their analgesic efficacy is questionable.
While reducing the volume of local anesthetic used in adductor canal block is intuitively expected to reduce motor weakness, the evidence does not support this assumption. Indeed, studies indicate that the differences in motor power between small (10 ml) and larger (30 ml) volumes are not clinically significant.5,6 The optimal adductor canal block volume is not defined, as most evidence is derived from volunteer and cadaveric studies,5–7 not clinical trials in real patients.
We used 20 ml ropivacaine, 0.5%, for our blocks as this is the care standard at our institution. More importantly, while we contemplated using lower volumes and concentrations at the outset of our trial, we decided that using robust volumes and concentration is critical for confirming block success to allow documenting block onset within a reasonable period of time after injection. Whether an intent-to-treat or per-protocol analysis is used, confirming block success is important to attribute the observed benefits to the interventions performed.
Assessing motor function postoperatively would have been ideal, but this approach is not possible in this particular population. All patients who undergo anterior cruciate ligament (ACL) reconstruction at our institution receive immobilizing splints and cold compressors applied to their knees, precluding postoperative motor assessment.
Our study focused on evaluating the effect of femoral and adductor canal blocks on motor power and pain scores. Determining the exact duration of analgesia requires a different study design with more frequent documentation of pain scores and opioid consumption. That said, the potential effect of age on block duration is not relevant in this context, as ACL reconstruction, whether with a femoral or adductor canal block, is generally performed in younger patients. Therefore, it is unlikely that age has biased our results.
While evidence from 20 yr ago suggested that femoral block provides good pain relief for ACL reconstruction with hamstring grafts, the evidence from 2016 suggests that adductor canal block spills to the sciatic nerve territory and may possibly provide pain relief to the posterior knee,7,8 thus may be better matched to ACL reconstruction with hamstring grafts than femoral block. However, post hoc analysis of our analysis found that the type of graft did not affect the results.
While the volume and concentration of local anesthetics used in adductor canal or femoral blocks, as well as the type of graft received in ACL reconstruction, are thought to influence the outcomes evaluated in this study, namely pain and motor power, these assumptions are not supported by contemporary evidence, including our own.
The authors declare no competing interests.