Novel anatomical mapping of the anterior branch of the axillary nerve across the anterior and middle part of the deltoid: a fresh-cadaver study defining safe zones.

PURPOSE: To map the surface projection of the anterior branch of the axillary nerve (ABAN) over the anterior and middle part of the deltoid and to define safe and high-risk zones for proximal humeral approaches.

METHODS

Fourteen whole adult fresh-frozen cadavers (28 paired shoulders) were dissected in this study. Acromial landmarks were defined as point A (anterolateral corner of acromion), point B (middle point of AC), point C (posterolateral corner of acromion), and insertion of middle part of the deltoid (point D) in a modified anatomical position. A delto-pectoral landmark connected D to the clavicular junction (point E). The BD line was quartered and extended to the deltoid margins, while quarter points of AD were projected onto ED, dividing the deltoid into 12 mediolateral zones; zones 1-6 (anterior and middle part of the deltoid) were each subdivided into quadrants a-d.

RESULTS

The risk ranking, defined as the probability of encountering the nerve, was as follows: 10 = 11 = 12 (0%) > 9 (7.14%) > 3 = 8 (14.28%) > 2 (17.85%) > 7 (28.57%) > 4 (78.57%) > 6 (92.86%) > 1 = 5 (100%, highest-risk zones). Safe zones were limited to proximal zones 2-3 (2a, 2b, 3a, and 3b) and distal zones 10-12; all other zones placed the nerve at risk, with zones 1 and 5 representing the highest risk. A curved surgical trajectory along Zone 6c → 5d → 5b → 4a → 1c, rather than an acromion-based approach, may reduce iatrogenic injury to this branch.

CONCLUSIONS

Mapping of the anterior branch of the axillary nerve (ABAN) across 12 zones, highlighting both safe and high-risk regions. Key findings indicate that only Zones 2a, 2b, 3a, and 3b in the proximal humerus are safe, whereas all other zones pose risk, with Zones 1 and 5 representing the highest risk. A curved surgical trajectory along Zone 6c → 5d → 5b → 4a → 1c, rather than an acromion-based approach, may reduce iatrogenic ABAN injury during surgery.