Background: Deltoid splitting is one of common and useful approaches in proximal humerus surgery. The disadvantage of this approach is high risk of axillary nerve injury. Nowadays, there is no consensus in the proper mean of the individual’s axillary nerve location.
Objective: The present study aim to determine the correlation between the axillary nerve and the upper arm length that may create equations to calculate the nerve location.
Material and Method: Seventy shoulders were measured the distance from lateral acromial edge to axillary nerve and
compared with upper arm length in three positions of the shoulder (adduction 30°, abduction 45° and 90°). The authors used distance of lateral acromial edge to lateral epicondyle and tip of greater tuberosity to lateral epicondyle as the upper arm length.
Results: The average distance of shoulder adduction 30°, abduction 45° and 90° were 57.9, 57.1 and 52.9 mm, respectively. All of three positions showed linear correlation to upper arm length from both of reference sites. From acromion edge reference, where “Y” is axillary nerve distance in mm and “x” is upper arm length in cm, the relation were Y = 2.3x-10, Y = 2x-2 and Y = 2x-7 with the accuracy rate were 88.57%, 85.71% and 81.43%, respectively. From greater tuberosity reference, the relations were Y = 2.54x-14, Y = 2x and Y = 2.3x-12 with the accuracy rates 87.14%, 80% and 84.29%, respectively.
Conclusion: There is linear correlation between distance from the lateral acromial edge to axillary nerve and the upper arm length. The authors can predict the danger zone in the location of the anterior upper branch of the axillary nerve. However, further clinical study may helpful to prove the equations. Level of evidence: Basic science anatomy study.

Keywords: Axillary nerve, Safe zone, Deltoid split approach, Axillary nerve injury, Shoulder, Correlation, Proximal humerus