Although the anterolateral deltoid-splitting approach can ensure direct access and excellent visualization of the plating area in the management of proximal humerus fractures [2, 3], there is an increased risk for axillary nerve injury, which is the most common neurological complication associated with surgery of proximal humerus fractures [6, 18, 19]. Accordingly, defining the safe zone for the axillary nerve is important to avoid iatrogenic injury. However, various anatomical studies have defined a broad range of safe zones for deltoid-splitting approaches, varying from 30 to 70 mm distally to the acromion [6, 9–11]. Because of the broad anatomical variation in the course of the axillary nerve, the acromion-axillary nerve distance and its association with the humeral length were investigated to predict the axillary nerve location [13–16] in some cadaveric studies. Nonetheless, according to our review of the literature, the relationship between the axillary nerve location and humeral length has not been investigated in a clinical setting to date.
The present study aimed to describe a safe area for executing the anterolateral deltoid split approach during open reduction–plate fixation for managing patients with proximal humerus fractures. We found that ANND was 6.0 ± 0.36 cm, which was moderately correlated with AL. However, ANND could be predicted according to AL in only 18% of the patients.
Numerous studies have attempted to measure ANND and found significant variations with a range of 4.5 to 7.5 cm [10, 13–15, 20, 21]. Kongcharoensombat et al.  calculated the mean distance of the axillary nerve from the anterolateral acromion as 6.39 cm (ranging from 4.6 to 8.2 cm), and Cetik et al.  found the distance of the axillary nerve from the anterolateral acromion to be 6.08 cm (ranging from 5.20 to 6.90 cm). Both previous studies observed significant correlation between the distance of the axillary nerve from the anterolateral acromion and humeral length. In contrast to the cadaveric studies of Kongcharoensombat  and Cetik et al. , the present study was conducted in a clinical setting, and all measurements were performed intraoperatively after the anatomic reduction and fixation were completed. In this regard, our study is advantageous over the existing previous cadaveric studies in the literature.
While using the anterolateral approach for proximal humeral fractures, the plate should be inserted under the axillary nerve so that the nerve could be dissected carefully, and potential injury could be prevented. Also, the shortest distance should be taken into consideration during dissection to minimize the risk of probable axillary nerve injury. We measured the minimum distance of the axillary nerve to be 5.5 cm from the acromion. Hence, this distance could be considered as a safe zone according to the findings of the present study. In the study by Cetik et al. , this distance was measured as 5.2 cm. However, this data contradicts the findings of Kongcharoensombat et al.  because the axillary nerve was found located at <5 cm in 13% of the cadaver shoulders.
In our study, the calculated mean axillary nerve index was lower than that given by Cetik et al.  and Kongcharoensombat et al. . The exact prediction ratio of the location of the axillary nerve according to the humeral length of the patients was 18%, which was less than the expected value. Therefore, we believe that it would be safer to use the distance instead of the ratio.
Our study has several limitations. First, the number of patients who participated in the study was less. Second, the measurements were made using a manual caliper, thereby giving room for human errors. Third, all the measurements were performed after the anatomic reduction was completed. However, in case of deformity due to proximal humerus fracture before reduction was performed during the exposure, this distance is likely to be shortened.