Analgesic efficacy of ultrasound-guided adductor canal blockade after arthroscopic anterior cruciate ligament reconstruction

Editor, We read with interest the article by Espelund et al.1 entitled ‘Analgesic efficacy of ultrasound-guided adductor canal blockade after arthroscopic anterior cruciate ligament reconstruction: A randomised controlled trial’. The authors have compared the use of adductor canal block and multimodal analgesia with multimodal analgesia alone for analgesia following anterior cruciate ligament repair with ipsilateral hamstring graft. The methodology is exemplary and the results are novel in showing very low mean pain scores at rest, movement and standing at 2 h. The study deviates from others in demonstrating very low pain scores (median VAS of 20 mm on standing at 2 h) with similar levels of dispersion in both the study groups. Although adductor canal block has been sparsely evaluated for this surgery, previous studies evaluating femoral nerve blocks for anterior cruciate ligament repair have reported higher pain scores than that noted in the present study and a clear benefit from blocks.2,3 Dahl et al.4 reported similar pain scores as the current study without employing blocks wherein they evaluated COX-2 inhibitors with or without steroids, combined with a cryo-cuff unlike, the present study. Although not all patients require blocks for analgesia, factors predicting their need are still a topic of research and have even compelled some researchers to develop models based on machine-learning to predict who would benefit from them.5 The observed results, although difficult to explain as elaborated by the authors, the inadequate sample size, the method of block performance and the presence of a confounding variable need to be investigated further. First, the degree of reduction in pain that is clinically meaningful needs to be decided before choosing the sample size in order to determine the significance of nerve blocks. The authors assumed a 50% reduction in pain scores in the experimental group to be meaningful, which obviously resulted in a low sample size (22 patients per group), a skewed distribution and an inability to show a significant difference between the two groups. If the authors were to calculate the sample size to achieve a 25 or 30% improvement in pain scores, they would need 99 or 44 patients per group, respectively. What is really surprising is the very low pain scores noted in either group. Such pain scores are desirable and make performance of any regional technique unnecessary. Another possibility for a lack of difference between the groups may be the method of performing the block. Also, the success rate of the block was not tested, which is unusual in clinical practice. The authors of the study performed the blocks in the postoperative period; this may not be possible in the clinical practice due to the presence of braces and surgical dressings which make appreciation of anatomical structures difficult. Bushnell et al.6 did not comment on the site of pain; the graft donor site can be a significant source of pain even with an effective femoral nerve block. Hence, in the presence of a confounding factor such as pain in the nonblocked area (graft donor site), comparing and commenting on pain scores could be misleading. If the patients requested and received enough analgesics to achieve pain control at the donor site where the adductor canal block will have no effect, there will be no difference in pain scores or analgesic consumptions. Attractive strategies to cover the donor site such as a block of the anterior division of obturator nerve or intramuscular injections into the donor muscles near the motor end-plate zones similar to those performed for botulinum toxin injection7 need to be evaluated. Cumulative rescue analgesic frequency over time would have provided a better idea about the effect of block on analgesic consumption. Acknowledgements relating to this article Assistance with the letter: none. Financial support and sponsorship: none. Conflicts of interest: none. Presentation: none.

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