Neuraxiom Ultrasound Guided Nerve Blocks

Lately, there has been interest in a variation of the femoral nerve block at the adductor canal for total knee arthroplasty.  The idea being that blocking the femoral nerve further down the thigh can decrease post-operative quadriceps weakness common following femoral nerve block at the level of the inguinal ligament. This makes good sense from a standpoint of motor unit preservation as innervation of the motor units proceeds in a top down manner.  If the goal is early ambulation with some pain control then this block is attractive and makes good sense. If your goal is maximum post-operative pain control, then block of the femoral at the level of the inguinal ligament is more effective but sacrifices motor control for the length for the block. These seem to be your choices for the single shot block.
However, there may be a third option. All of the pain control with the duration of the single shot, at the least; the continuous adductor canal block.
So what’s wrong with the single shot adductor canal block? In my (humble) opinion, the problem is partly the position and partly the effect.
Consider this. Placing your local anesthetic solution at mid-thigh means that the tourniquet will be placed almost exactly over the injection site. External pressure like that will spread the local solution along the canal and the spread will increase the surface area of the solution bolus, an increase in surface area will increase the rate of absorption shortening the duration of the block.
Secondly, pursuant to the goal of the approach, the patient is up and ambulating early. Walking and therefore working the quadriceps will obviously put the thigh through a series of repeated compressions and relaxations, a pumping effect. That and the attendant increase in bloodflow and movement of interstitial fluid will cause the patient to “walk off” the block more quickly.
This may seem hyper-critical or nit-picking, but it’s not; you can cut hours off of a single-shot nerve block with active movement.
In order for this block to last through the first 24 hours post-operatively I think you should leave a catheter in place after the initial single shot bolus. Then either use a medication pump or periodically bolus the catheter with local.
More information on the  Adductor Canal block is found here.              May 25, 2013

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