The SPA arrangement of the branches of the upper trunk of the brachial plexus: a correction of a longstanding misconception and a new diagram of the brachial plexus

Abstract

W e have all been taught to draw the brachial plexus (BP) with the anterior division (A) of the upper trunk (UT) more cranial than the posterior division (P). 4,5,7,9,[14][15][16]18 For the peripheral nerve surgeon familiar with BP reconstruction, this is anatomically incorrect; the correct arrangement should be P cranial to A. Dealing with BP trauma is already challenging because of scar tissue that sometimes masks the normal anatomy. Without the knowledge of normal anatomical concepts, one may be grafting into the wrong targets. In an effort to better understand the relationship of the divisions of the UT, 20 BPs were dissected in embalmed cadavers, looking specifically at the branching pattern of the UT. MethodsTen embalmed cadavers were used for this project.Both sides (20 BPs) were dissected, mostly via an anterior approach. A skin incision was made along the posterior border of the sternocleidomastoid (SCM) muscle or horizontally. The platysma was opened. The omohyoid, suprascapular, and transverse cervical vessels were transected. The supraclavicular pad of fat was reflected laterally from the posterior border of SCM. The trunks of the BP were then identified between the anterior and middle scalenes. Special attention was devoted to identifying the branches and divisions of the UT. They were also followed proximally to their origin from the C-5 and C-6 spinal nerves.Infraclavicular exposure was achieved through the deltopectoral groove. The pectoralis minor was transected, which allowed exposure of the cords of the BP and the axillary artery. For full exposure of the divisions, the clavicle was either retracted or transected. This concept contradicts what is seen in the operating room and is noticed by most peripheral nerve surgeons. This cadaveric study was conducted to look specifically at the exact pattern of branching of the upper trunk of the BP. Methods Ten cadavers (20 BPs) were dissected. Both supra- and infraclavicular exposures were performed. The clavicle was retracted or resected to identify the divisions of the BP. A posterior approach was used in 2 cases. results In all dissections the origin of the posterior division was in a more cranial and dorsal plane in relation to the anterior division. In most dissections the supra scapular nerve branched off distally from the UT, giving it the appearance of a trifurcation, taking off just cranial and dorsal to the posterior division. The branching pattern of the UT consistently had the following sequential arrangement from cranial and posterior to caudal and anterior: suprascapular nerve (S), posterior division (P), and anterior division (A), hence the acronym SPA. coNclusioNs Supraclavicular exposure of the BP exposes only the trunks and divisions. Recognizing the "SPA" arrangement of the branches helps in identifying the correct targets for neurotization, especially given that these 3 branches are the most common targets for BP repair. Understanding the anatomy means better surgical planning and better patient outcomes.

Subomohyoid Anterior Suprascapular Block versus Interscalene Block for Arthroscopic Shoulder Surgery

“…A superior trunk block per se may also be an option, but its proximity to the phrenic nerve and lack of knowledge of its respiratory effects favor more distal alternatives. In contrast, the suprascapular block, by virtue of its vicinity to the superior trunk, 15,16 is purported to be more effective than posterior suprascapular block. 13 Indeed, proximal spread involving the superior trunk was frequently observed when performing suprascapular block ( fig.…”

“…4,12 In their description of the anterior suprascapular block, Siegenthaler et al 13 and Laumonerie et al 14 have uncovered an indirect approach to block the superior trunk that is relatively far from the neck and phrenic nerve. Owing to anatomical proximity and proximal spread, 15,16 performing an anterior suprascapular nerve block under the inferior belly of the omohyoid muscle in the supraclavicular fossa seems to consistently block the superior trunk and brachial plexus, even when very small local anesthetic volumes are used. [15][16][17] Consequently, we thought that the subomohyoid anterior suprascapular block is another approach to superior trunk block that blocks the majority of shoulder innervation, thus providing postoperative analgesia that is not worse than the interscalene block for shoulder surgery.…”

Description and ultrasound targeting of the origin of the suprascapular nerve

“…The orSSN at the level of the proximal portion of the UTBP was described by Vesalius in 1555and Kerr in 1918(Vesalius, 1555Kerr, 1918;Boileau, 1972;MacKinnon and Dellon, 1988;Kline et al, 2001;Maniker, 2005;Slutsky et al, 2006;Standring, 2008;Tung and Moore, 2015;Hanna, 2016). This identified the SSN's micro-anatomical origin (i.e., SSN's fascicular origin stemming from C6, C5, or even C4) following systematic dissection of the mesoneurium (Kerr, 1918;Bonnel, 1984;Franco et al, 2008;Siqueira et al, 2010;Arad et al, 2014;Sinha et al, 2016).…”

Brachial Plexus Anatomy of Miniature Swine Compared to Human

“…The present study focuses on the WMS model and its applicability for researching BPI. Overall, this study demonstrates that the WMS brachial plexus closely resembles the human brachial plexus in comparison to other non-primate vertebrate models [29]. The similarities can be appreciated through previous research with swine model post-avulsion injury retaining more similarities to human models in terms of motor neuron death compared to small animal models [30,31].…”