Background and objectives Recent reports suggest that a selective trunk block (SeTB) can produce sensorimotor blockade of the entire upper extremity, except for the T2 dermatome. There are no data demonstrating the anatomic mechanism of SeTB. This cadaver study aimed to evaluate the spread of an injectate after a simulated ultrasound-guided (USG) SeTB.

Methods USG SeTB (n=7) was performed on both sides of the neck in four adult human cadavers with 25 mL of 0.1% methylene blue dye. Anatomic dissection was performed to document staining (deep, faint, and no stain) of the various elements of the brachial plexus from the level of the roots to the cords, including the phrenic, dorsal scapular, and long thoracic nerves. Only structures that were deeply stained were defined as being affected by the SeTB.

Results All the trunks and divisions of the brachial plexus, as well as the ventral rami of C5–C7 and suprascapular nerve, were deeply stained in all (100%) the simulated injections. The ventral rami of C8 and T1 (86%), dorsal scapular and long thoracic nerve (71%), and the phrenic nerve (57%) were also deeply stained in a substantial number of the injections.

Conclusion This cadaver study demonstrates that an USG SeTB consistently affects all the trunks and divisions of the brachial plexus, as well as the suprascapular nerve. This study also establishes that SeTB may not be phrenic nerve sparing. Future research to evaluate the safety and efficacy of SeTB as an all-purpose brachial plexus block technique for upper extremity surgery is warranted.

Trial registration number Registered at https://www.thaiclinicaltrials.org on December 13, 2021 under the trial registration number TCTR20211213005.