Axillary nerve risk mapping and arm-length-based prediction in percutaneous proximal humeral fracture fixation

Detailed knowledge of axillary nerve anatomy and its relationship to a proximal humeral locking plate is important for reducing iatrogenic nerve injury during percutaneous minimally invasive plate osteosynthesis (MIPO) for proximal humeral fractures. Twelve adult upper-limb cadaveric specimens were dissected, and PHILOS plates (Synthes) were positioned using a standardized protocol. We recorded the distance between the axillary nerve and each plate hole, the acromion-to-axillary nerve distance (AAND), and arm length (AL), defined as the distance from the acromion to the lateral epicondyle. The axillary nerve index (ANI) was calculated as AAND/AL. Pearson correlation and ordinary least-squares linear regression were used to examine the association between AL and AAND. The six proximal holes (A1–C2) and three distal holes (F–H) were more than 5 mm from the axillary nerve in all specimens, whereas D, E1, and E2 were within 5 mm. Mean AAND was 6.4 ± 0.5 cm (range, 5.6–7.2 cm), mean AL was 29.4 ± 2.0 cm (range, 26.3–32.4 cm), and mean ANI was 0.22 ± 0.01 (range, 0.20–0.23). AL and AAND were positively correlated (Pearson r = 0.902, p < 0.001). The equation derived from this dataset was AAND = 0.2356 × AL − 0.5684 (in-sample R² = 0.813). In this cadaveric series, holes D, E1, and E2 were closest to the axillary nerve and require direct nerve identification and protection during screw insertion. The regression equation is exploratory and has not undergone internal or independent external validation. Although the minimum observed AAND was 5.6 cm, a 5-cm distance should be regarded only as an approximate anatomical reference rather than a universally safe surgical corridor.