Continuous Adductor Canal Nerve Blocks: Relative Effects of a Basal Infusion v. Hourly Bolus Doses

Patients usually experience moderate-to-severe pain following the knee replacement that is often treated with a femoral nerve block (injection of numbing medicine placed around the main nerve of the knee joint). To make the nerve block last longer, a tiny tube is often placed next to the nerve and numbing medicine is infused for multiple days. However, while the numbing medicine takes away pain, it also decreases sensations, muscle strength, and proprioception (knowing where the leg is in space without looking at it) which greatly increases the risk of falling. Since falling can be catastrophic following major surgery, a femoral nerve blocks are being phased out by surgeons and anesthesiologists. The most-promising replacement is called an adductor canal nerve block. For this new type of block, a perineural catheter is inserted into a small canal in the middle of the upper leg. This canal contains the sensory nerve fibers leading to the knee, and only a single nerve that serves a relatively small muscle. Multiple studies have demonstrated a dramatic increase in muscle strength using the new adductor canal block compared with the traditional femoral block. However, practitioners perceptions of the new block is that it provides insufficient pain control following knee arthroplasty, even though all of the sensory nerves affected with the femoral block are also-theoretically-affected with the adductor canal block. One reason for this difference may be the small canal of the latter which is a relatively tight area in which the numbing medicine might not spread particularly well (due to pressure from surrounding tissues). One way to possibly counter this issue is by providing repeated boluses of the numbing medicine that will improve the medicine's spread relative to a more-traditional slow, continuous ("basal") infusion. This study seeks to compare these two techniques of medication administration through perineural adductor canal catheters: Our primary aim is to test the hypothesis that, for continuous adductor canal blocks, providing local anesthetic as repeated, hourly bolus doses results in an increased sensory block compared with providing local anesthetic as a continuous basal infusion at an equivalent hourly dose. As a secondary aim, we hypothesize that, for continuous adductor canal blocks, providing local anesthetic as repeated, hourly bolus doses results in either equivalent or less motor block compared with providing local anesthetic as a continuous basal infusion at an equivalent hourly dose.