Adductor Canal Block Analgesia Unaffected by Injection Site

Author: Michael Vlessides

Anesthesiology News

The analgesic efficacy of adductor canal block in patients undergoing anterior cruciate ligament (ACL) repair appears consistent regardless of whether the injection was in the proximal thigh, midthigh or distal thigh, according to a team of Canadian researchers. Their study found that postoperative oral analgesic consumption and time to first reported pain were similar among groups, as was quadriceps motor strength.

“The ultrasound-guided adductor canal block is essentially a distal femoral nerve block performed in the mid-thigh region for patients undergoing major knee surgery,” said Jorge Mejia, MD, a fellow in anesthesia at Women’s College Hospital, in Toronto. “Interestingly, the adductor canal block is progressively replacing the femoral nerve block for anterior cruciate ligament repair surgery, largely because it provides comparable pain relief while preserving quadriceps strength.

“Nonetheless, it is unsure which exact location within the canal is the best injection site to maximize the benefits of the block, since different injection sites within the canal may produce different degrees of analgesia and motor impairment,” Dr. Mejia added. With that in mind, the investigators sought to evaluate the effects of three distinct injection sites—all relative to the sartorius muscle—on postoperative analgesia and motor function in these patients.

• proximal thigh (sartorius muscle before crossing over femoral artery; n=34);
• midthigh (sartorius superimposed over femoral artery; n=38); and
• distal thigh (sartorius just after crossing over femoral artery; n=36).

The researchers used ultrasound to examine the adductor canal by moving the probe cephalad or caudad to achieve one of the sartorius muscle configurations.

All patients received 20 mL of a 1:1 solution composed of 2% lidocaine and 1% ropivacaine. Primary outcomes included cumulative 24-hour oral morphine equivalent consumption and percentage decrease in quadriceps muscle strength at 30 minutes after the block. The three groups were similar on demographic characteristics.

Presenting the study at the 2018 Joint World Congress on Regional Anesthesia and Pain Medicine and annual meeting of the American Society of Regional Anesthesia and Pain Medicine (abstract 5406), Dr. Mejia noted that patients in group 1 consumed a mean of 55.73 mg of oral morphine equivalents, compared with 51.71 mg in group 2 and 55.83 mg in group 3. Time to first reported pain also was similar: 166.0 minutes in group 1, 177.5 minutes in group 2 and 167.1 minutes in group 3.

Quadriceps muscle strength was expressed as a percentage of preserved maximum voluntary isometric contraction at five minutes after the block was performed, and was found to be 89% in group 1, 88% in group 2 and 81% in group 3.

“The analgesic efficacy of the block and quadriceps motor strength [are] consistent with previous reports of the adductor canal block,” Dr. Mejia said. “We also saw that pain scores increased at six, 12 and 24 hours, which is also in accordance with previous studies of lower extremity blocks.”

These findings demonstrate parity between the three approaches. “It might seem like the raw statistics favor the midthigh approach,” he added, “but it doesn’t seem that there’s a big difference based on the site of injection.”

Not Everyone Convinced

Despite these findings, session moderator Timur Ozelsel, MD, remained unconvinced about the clinical applicability of the adductor canal block. “I’m still a disbeliever in the adductor canal block,” said Dr. Ozelsel, an associate clinical professor of anesthesiology and pain medicine at the University of Alberta, in Edmonton. “In fact, I keep telling everyone that in five years, someone will publish that the block actually doesn’t work at all.

“It’s interesting,” Dr. Ozelsel continued, “because about 10 years ago, we published a study about the midthigh saphenous approach [Reg Anesth Pain Med 2009;34(2):177-178]. We performed the trial because our surgeons were increasingly concerned that we were producing motor weakness with our blocks. And we found that the more distal you go, the less motor weakness you get and also the less analgesia you get.

“So, I’m a disbeliever that it’s the block, but more likely multimodal analgesia that we pump into our patients,” he said. “But time will tell.”