The main finding of this study was that continuous ACBs performed at the proximal end of the AC in comparison to that at the middle of the AC showed a superior opioid-sparing effect after TKA; in addition, both ACB locations had a similar influence on the strength of the quadriceps.
This is the first RCT which compares a continuous ACB performed at the proximal end of the AC (where the medial border of the SM intersects the medial border of the ALM) with a middle AC injection. The result indicated that an injection at the proximal end of the AC may affect more comprehensive terminal sensory nerves that innervate the knee joint as compared to the middle AC placement. Indeed, the result of a more recent anatomical study by Tran published after the initiation of the present trial provided some insights into the mechanism of analgesia to the knee following ultrasound-guided proximal end AC injection and its motor sparing properties.24 Following a proximal end AC injection with 10 ml of dye in seven lightly embalmed specimens, they found that the dye spread consistently stained the SN, posteromedial branch of the VMN, superior medial genicular nerve, and the genicular branch of the obturator nerve, which were captured in all specimens at the proximal end of the AC. There was minimal to no dye spread to the distal FT, anterior division of the obturator nerve, and the anterior branches of the VMN. Previous cadaveric studies using a distal AC injection did not report staining of the posteromedial branch of NVM and/or its distal branch, the superomedial genicular nerve.19, 20 The result of the current study is also inconsistent with a similar RCT by Sztain,8 who found that the analgesic effect of continuous ACB after TKA with a catheter inserted at the mid-thigh level (termed “proximal AC” in their study) was improved compared with a more distal insertion closer to the adductor hiatus. The mid-thigh level, defined as the midpoint between the anterior superior iliac spine and the patella, was a commonly used surface anatomical landmark of the AC; however, it has recently been shown to actually indicate a cranial location to the proximal end of AC and inside the distal FT in most subjects.11, 12, 25, 26 Nevertheless, both the study by Sztain8 and the current study provided clinical evidence supporting previous speculation and advocation by others that a true AC (the distal locations of the study by Sztain and our study) may not produce the same analgesia effectiveness after TKA.10, 13, 20 Instead, a distal TF or a proximal end AC would be the more suitable block to alleviate pain after knee surgery.
In previous studies which aimed to clarify the optimal location to maintain ACB after TKA, three previously published RCTs 5–7 which investigated the “proximal AC” and “distal AC” failed to detect significant differences in regard to 24 h postoperative opioid consumption, as well as in quadriceps strength or motor function. The discrepancies between the present study and the previous RCTs can likely be attributed to the different definitions of the AC. All three RCTs5–7 designated the point at which the superficial FA passed underneath the medial border or medial part of the SM as the “proximal AC.” However, the true AC is now known to lie at approximately the level of the apex of the iliopectineal fossa, a proximal subset of the FT, based on recent anatomic descriptions of knee and thigh neuroanatomy,13 The “distal AC” was designated as the midpoint between the anterior superior iliac spine and the patella in the study by Mariano,5 the midpoint between the inguinal crease and top of the patella (somewhere in the AC based on a classic textbook of Gray’s Anatomy3, 27) in the study by Romano,6 and the point where the superficial FA is underneath the midpoint of the SM (supposed to be close to the proximal end/entrance opening of the AC based on the ultrasound image) in the study by Meier.7 In other words, these studies compared the distal FT or the proximal AC with a more cephalad injection in the FT, instead of the distal AC with the proximal AC. Since there is a current trend which suggests that a true continuous (distal) FT block is used instead of a true continuous ACB after TKA,10, 13, 20 further studies which explore the optimal catheter location and local anesthetic infusion regiment for an FT block may be needed.
The ideal location for continuous ACB after TKA is supposed to be where it achieves maximum analgesia with minimal quadriceps weakness. This is because both pain and motor weakness could affect the patient’s rehabilitation.5 Although the site of the proximal end of the AC is far from the inguinal increase, there is still a concern that a more proximal block runs the theoretical risk of quadriceps weakness caused by motor components of the femoral nerve blockade from more cephalad spread.10, 12 Regardless, the current study did not show a significant difference in the effect of catheter locations on quadriceps strength measured manually by a physiotherapist on a Lovett’s scale. Thus, proximal spread of the anesthetic to the femoral nerve was not likely to be a significant factor in this study population. This result can be explained by the finding of the latest cadaveric study by Tran,24 where the proximal end AC injection (10 ml, which is the same volume as the loading dose in the present study) was found to spare the anterior branches of the NVM which would likely preserve greater vastus medialis activation, contributing to the quadriceps motor sparing characteristic of the proximal ACB. Another non-negligible contributor to this result could be the following blockade infusion (at a rate of 6 ml/h) regimen adopted in the current study which may avoid further cephalad spread of the local anesthetic following the initial dose to the motor component of the femoral nerve.28 Similarly, RCTs which explored the distal TF area also did not detect a trend that a more proximal catheter location (the apex of the iliopectineal fossa) worsened the effect on quadriceps strength or motor function.5–7 However, caution should be used in drawing any definitive conclusions since previous RCTs and the present study were not powered to detect differences in these secondary outcomes. A further study powered to explore the effect of catheter location on quadriceps motor function is needed.
All other secondary outcomes from our study also illustrate similarities between a proximal end and a middle approach to the continuous ACB. These results demonstrate no statistical differences in patient-reported NRS scores, time to ambulation, episodes of PONV, patient satisfaction with anesthesia, block-related adverse effects, or hospital length of stay. Although these secondary outcomes are likely underpowered to detect statistical significance, differences between the two groups for these outcomes also do not seem to be clinically significant.
The current study had some limitations. First, the quadriceps muscle strength was only evaluated manually by a physiotherapist on a Lovett’s scale, which is not as precise as by using the force dynamometer such as the measurement of maximum voluntary isometric contraction.7, 28 In addition, we did not implement a validated test to measure patient mobilization ability, such as the Timed “Up and Go” measurement,29 which could directly reflect the balance between “pain-control during movement” and “preserving strength” that is important for effective pain management after TKA.30 In addition, the results apply only to the specific local anesthetic type, concentration, bolus volume, and basal rate of the present study. Similarly, results for single injection ACBs may differ from this investigation involving a perineural infusion (after the initial bolus of intermediate-acting local anesthetic). Finally, as this is a single-center study with a small sample size which is limited to TKA patients, the results may not be generalizable to other types of knee procedures.