First, we note that this study was a double-blind trial because the anesthesiologist, investigator, and patient were blinded to the exact solution administered (saline or ropivacaine). Additionally, data collection investigators were different across the recovery room and surgical ward. With respect to patient blinding, patients were not educated on whether to attribute muscular weakness or loss of cold sensation to the surgery or to the analgesic block. Indeed, as noted in the Results section, 12 patients (24%) in the ropivacaine group did not report a loss of cold sensation in the hip area. Additionally, at 2 h after extubation, significant motor weakness was observed in one patient in both groups.
Second, the final solution spread on ultrasound was recorded on an anatomical diagram at the end of the injection. We have found that this ultrasound image of the final injectate deposition was dynamic and not visibly maintained after block completion, due to rapid resorption of the injectate. In all the fascia plane blocks, the positive pressure ensures the macroscopic spread of the solution toward a zone of lower resistance. In our study, we observed that the macroscopic dispersion of injectate into a virtual space tended to depend on tissue compliance rather than needle tip position. The macroscopic dispersion reported in our anatomical diagram, called bulk flow or mass flow, is rapidly followed by a microscopic dispersion,3 invisible to ultrasound. Local anesthetic can cross fascial layers even in the absence of macroscopic perforations.3 Finally, there is resorption of the injectate by both lymphatic and vascular systems.3 Consequently, as pointed out by Chin et al.,3 the potential mechanisms of analgesic action of fascial plane blocks can be divided into (1) a local effect on nerves in the vicinity of injection and (2) a systemic effect secondary to vascular dispersion. Separate from the mechanism of action, our study focused on contributing to the literature with respect to this question, “Is a quadratus lumborum block effective for pain relief after hip surgery?” Our study,2 and similarly, a recent trial by Haskins et al.,4 reported that respectively a posterior or an anterior quadratus lumborum block, when combined with multimodal analgesia, does not decrease morphine consumption or pain scores after hip surgery.
Third, as noted in our figure, there was no loss of cold sensation superior to the 12th rib margin or inferior to the upper third of the thigh with a posterior quadratus lumborum block. The subdivision of loss of cold sensation in nine areas allowed us to quantify the frequency of variously distributed sensory loss. We acknowledge the comment from Hu et al.1 that the innervation of the anterior capsule is primarily provided by the articular branches of the obturator and femoral nerves. In our study, we note that there was no femoral nerve blockade (both sensory and motor), and we postulate that this may explain the lack of analgesic effect reported in our study. Our findings contrast with other quadratus lumborum blocks described. Diffusion to the lumbar plexus after an anterior quadratus lumborum block has been reported in two cadaveric studies,5,6 and a case report described a femoral blockade after a lateral quadratus lumborum block.7
Dr. Choquet receives funding from Milestone Scientific (Livingstone, New Jersey) and General Electrics (Boston, Massachusetts) for research unrelated to this work. The remaining authors declare no competing interests.