Origin-Order Classification of Axillary Third-Part Branching: Donor-Based Dissection–CTA Correlation for Surgical Planning

Variations in the branching of the subscapular artery (SSA), anterior circumflex humeral artery (ACHA), and posterior circumflex humeral artery (PCHA) are directly relevant to reconstructive planning with subscapular-system flaps and to humeral-head perfusion in shoulder surgery. Evidence organized around an origin-order–based framework remains limited. We aimed to address this gap by comparing body-donor dissections and computed tomography angiography (CTA) to provide a clinically useful classification and morphometric reference. We performed a two-arm cross-sectional morphometric study: body-donor dissection (28 donors; bilateral, 56 sides) and CTA (25 patients; bilateral, 50 sides). In total, 104 sides were evaluated; 96 were classifiable. Classification used the sequential SSA–ACHA–PCHA origin order and common-trunk presence. Ostial diameters and SSA → CSA distances were measured; the radial nerve (NR)–SSA relationship was assessed in donors. origin_order__five_type_classif… In donors, Type-1, -2, -3, and -4 accounted for 36.5%, 32.7%, 23.1%, and 7.7%, respectively; Type-5 was absent. In CTA, Type-1, -4, and -5 comprised 93.2%, 2.3%, and 4.5% (one bilateral case); Types-2/-3 were not observed. Inter-modality comparison showed a longer SSA → CSA distance and smaller TDA/CSA diameters in CTA (all p < 0.001), while the SSA ostial diameter was similar. A posterior NR course relative to the SSA was associated with a longer SSA → CSA distance (p = 0.026). An origin-order–based classification, corroborated across dissection and CTA, yields a practical map for (i) single-pedicle harvesting within the subscapular system and chimeric flap design, and (ii) avoiding iatrogenic compromise of humeral-head vascularity during shoulder procedures. Incorporating presurgical CTA mapping of the SSA and its branches may enhance safety where variants (e.g., short/combined trunks, rare Type-5) are suspected.