‘Sensory’ block promise for postop knee pain

By Eleanor McDermid, Senior medwireNews Reporter

Using adductor canal block rather than femoral nerve block for analgesia after knee surgery could minimize muscle weakness and risk for falls, a study in Anesthesiology suggests.

The research in healthy volunteers shows that femoral nerve block reduced quadriceps strength by 49%, whereas adductor canal block reduced it by just 8% and there was a 6% increase in strength after placebo block.

In an accompanying editorial, Brian Ilfeld (University of California, San Diego, USA) and Admir Hadzic (Columbia University, New York, USA) say: "Of note, an 8% reduction with the adductor canal block is probably clinically irrelevant, given that a 10% side-to-side strength difference is common, yet functionally unnoticeable in healthy individuals."

But the editorialists caution against overinterpreting the findings, noting in particular that the study cannot compare the quality of postoperative analgesia provided by the two blocks.

They say it is too early to tell whether the results "signal a transformation in postknee surgery analgesic management, or, rather, represent an important addition to our understanding of functional regional anesthesia, but without a prodigious impact on clinical practice."

The study included 11 volunteers who received femoral nerve block with ropivacaine in one limb and with placebo (saline) in the other, in a randomized, double-blind fashion. On a separate day, they received adductor canal block to the same criteria.

The researchers, led by Pia Jæger (Copenhagen University Hospital, Denmark), then assessed the volunteers' maximum voluntary isometric contraction for quadriceps and adductor muscles. Adductor muscle strength was only slightly affected by the blocks, being reduced by 5% after adductor canal block, 10% after femoral nerve block, and 1% after placebo block.

All volunteers could complete mobilization assessments, including the timed up-and-go test and the 10 meter walk, after adductor canal block, whereas up to six of them failed to complete these tests at 1 and 6 hours after femoral nerve block.

Only one motor nerve - that leading to the vastus medialis - passes through the adductor canal, but many sensory nerves that innervate the knee pass through the canal.

"This study confirms that the [adductor canal block] is mainly a sensory block, which may be a useful analgesic adjuvant for acute pain management after knee surgery," says the team.

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