Discussion
As we all know, the success of peripheral nerve blocks depends on the
accuracy with which the nerves are localized and impregnated. However,
it has reported that other relevant factors can affect the success rate
and quality of nerve blocks, including the concentration and volume of
local anesthetic injected in proximity of the
nerves[20, 21]. In this concentration-finding
study, we found that the median EC50 was 0.113%(95%CI, 0.108% to
0.343%).
There are various approaches for sciatic nerve blocks, including
popliteal, subgluteal or mid-femoral approaches performed under US
guidance[22, 23]. Among these approaches, the
subgluteal approach showed following advantages. First of all, the depth
of the puncture needle through the subgluteal approach is shallow, and
ultrasound can more clearly show the course and structural
characteristics of the sciatic nerve. Secondly, it was shorter that the
onset time of subgluteal sciatic nerve block compared with that of
popliteal sciatic nerve block, but the effect of block was basically
comparable among the three groups[24].
Circumferential injection is beneficial for US-guided sciatic nerve
block[25]. Compared with the multiple injections
of LA used to achieve circumferential
spreading[26], a single-injection technique was
used in our study. It was found that, after injection of 20mL of LA, all
patients could achieve circumferential spreading around the sciatic
nerve. This is one of the reasons why the EC50 of ropivacaine
concentration is so low. Meanwhile, multiple injections for
circumferential spreading of LA should be used with caution, since it
might cause patient discomfort.
A previous study by Frost et al.[27] found that
femoral nerve blockade did not reduce postoperative analgesic
requirements in patients undergoing ACL reconstruction when a hamstring
graft was harvested. This is anatomically meaningful because femoral
blockade only covers the anterior thigh and knee, while hamstring graft
harvesting would likely lead to posterior thigh and knee pain, which is
the sciatic nerve sensory distribution. A retrospective study
demonstrated that the pain scores of patients with hamstring autograft
was higher than those patients who received
allograft[11]. However, in the study of Jansen et
al.[14], it showed no association between
hamstring autograft and higher PACU pain scores or increased opioid
consumption. In our study, the patients who were conducted femoral nerve
block combined with sciatic nerve block had low PACU pain scores and low
opioid consumption perioperatively. The posterior knee pain during ACL
reconstruction surgery is possibly unrelated to graft harvest site but
rather may be due to surgical factors (manipulation, drilling a hole in
the tibia), posterior knee edema, tourniquet pain, or a combination of
factors.
This study used a relatively high volume of local anesthetic (20mL of
ropivacaine) for the nerve blocks. Although lower amounts of local
anesthetic can be used, these volumes are used in our practice for
prolonging duration. There may be some adverse effects on motor function
and early mobilization due to the dose of local anesthetic used;
however, our surgeons do not want any knee mobilization on the day of
surgery. Despite the amount of local anesthetic used and use of FNB over
a more motor-sparing adductor canal block, we saw no decline in our
study. This is in accordance with a previous study by Memtsoudis et
al[28], which suggests that peripheral nerve
blocks are not the major culprit of falls after knee surgery.
This study has limitations. UDM allows determination of an EC50 for a
clinical variable with a binary outcome[29], while
in a smaller sample size. As we all know that the UDM is unreliable when
calculating small or large percentage points, such as the
EC95[30], which is a more relevant indicator for
clinical application. Although the EC95 level may be more clinically
useful, our simulation calculations results in 29 small samples were
significantly less accurate.
When calculating EC50 with UDM, the premise is that the
concentration-effect relationship is the traditional s-shaped curve,
which may be incorrect. It is not accurate to speculate the EC95.
Thus, we conclude that the median EC50 is 0.113%. Further
concentration-comparative studies are needed for other volume of
ropivacaine and multiple-injection techniques as well as to strengthen
the validity of the results of our study.