American Society for Peripheral Nerve

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The SPA Arrangement of the Branches of the Upper Trunk of the Brachial Plexus
Amgad S. Hanna, MD
Neurosurgery, University of Wisconsin, Madison, WI

Introduction: All brachial plexus diagrams in every textbook or paper draw the branches/divisions of the upper trunk in the following sequence from cranial and posterior to caudal and anterior: Suprascapular nerve (S), anterior division (A), then posterior division (P), respectively. This is such a wrong concept.

Method: Twelve cadavers (24 brachial plexuses) were dissected. We performed both supra and infraclavicular exposures. The clavicle was resected to identify the divisions of the brachial plexus.

Result: In all dissections the origin of the posterior division was in a more cranial and dorsal plane in relation to the anterior division, which was more caudal and ventral. In most dissections, the supra scapular nerve branched off distally from the upper trunk (UT) giving it the appearance of a trifurcation, and taking off just cranial and dorsal to the posterior division.

Conclusion: The branching pattern of the UT most commonly takes the form of a trifurcation with the following arrangement from cranial and posterior to caudal and anterior: Suprascapular nerve (S), posterior division (P), and anterior division (A), respectively. Hence the nemonic SPA can be used to avoid confusion. The application of this is huge. Supraclavicular exposure of the brachial plexus exposes only the trunks and divisions. Recognizing the 'SPA' arrangement of the branches helps identifying the correct targets for neurotization, especially that these 3 branches are the most common targets for brachial plexus repair. Understanding the anatomy means better surgical planning, and better patient outcomes. There is nothing worse than grafting to the wrong targets if we don't understand the anatomy.
Key Words: brachial plexus, suprascapular nerve, upper trunk.
Figure Legend: DS: dorsal scapular, Lt: long thoracic, UT: upper trunk, MT: middle trunk, LT: lower trunk, SC: subclavius, SSc: suprascapular, PD: posterior division, AD: anterior division, PC: posterior cord, LC: lateral cord, MC: medial cord, U&L Ssc: upper and lower subscapular, TD: thoracodorsal, Ax: axillary, R: radial, LP: lateral pectoral, MC: musculocutaneous, M: median, U: ulnar, MP: medial pectoral, MBC: medial brachial cutaneous, MABC: medial antebrachial cutaneous.

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