BASIC SCIENCE AND ELBOW Effects of shoulder position on axillary nerve positions during the split lateral deltoid approach Sunny Cheung, MD, Michael Fitzpatrick, MD, Thay Q. Lee, PhD* Orthopaedic Biomechanics Laboratory, VA Long Beach Healthcare System and University of California, Irvine Background: The axillary nerve may be injured during percutaneous fixation of proximal humerus frac- tures. This study investigated the kinematic behavior of the superior and inferior borders of the axillary nerve under varying shoulder positions. This information may reduce iatrogenic neurologic injury during fracture reduction and hardware placement. Methods: The lateral deltoid approach was performed on 7 fresh frozen shoulders. The inferior and supe- rior borders of the axillary nerve were tagged. Screws were placed in the anterior, middle, and posterior acromion as landmarks. Three-dimensional distances of the inferior and superior border of the nerve were measured to the mid-acromion while the shoulder was placed in combinations of forward flexion, vertical abduction, and humeral rotation. The distances were compared by repeated measures ANOVA statistical analysis. Results: The distance from the mid-acromion to the superior border of the axillary nerve was 66.6 mm (5.7), and to the inferior axillary nerve was 75.7 mm (5.8) with the shoulder in neutral position. Vertical abduction to 60  significantly moved the superior and inferior borders of the axillary nerve to a distance of 53.9 (7.7) and 61.6 mm (8.1), respectively (P < 0.005). Forward flexion had no significant effect on the position of the axillary nerve (P > 0.5). The longest distance from the mid-acromion to the inferior border of the axillary nerve was 86 mm with the arm forward flexed. Conclusions: The main determinant of axillary nerve position with respect to the acromion is vertical abduction. Axillary nerve position is essentially unaffected by varying degrees of humeral rotation and forward flexion. Vertical glenohumeral abduction to 60  is required to move the nerve significantly closer to the acromion. Level of Evidence: Basic science anatomic study Ó 2009 Journal of Shoulder and Elbow Surgery Board of Trustees. Keywords: Axillary nerve; proximal humeral fractures; iatrogenic nerve injury; fracture reduction; hardware placement Most surgeons are familiar with the split lateral deltoid approach to the shoulder for addressing fractures of the proximal humerus and lesions of the rotator cuff. The dangers of iatrogenic injury to the axillary nerve during this approach is widely recognized. 12,16 As a result, the anatomy of the axillary nerve within the deltoid has been well studied. 3-5,8,9 The distance from the nerve to the tip of the acromion has often been cited to be approximately 5 cm. 4,5,14,16 A cadaveric study found that the nerve is about 6 cm inferior to the top of the humeral head. 2 Recently, Cetik et al characterized a quadrangular ‘‘safe zone’’ *Reprint requests: Thay Q Lee, PhD, Orthopaedic Biomechanics Laboratory, VA Long Beach Healthcare System (09/151), 5901 East 7th. Street, Long Beach, CA 90822. E-mail address: tqlee@med.va.gov, tqlee@uci.edu (T.Q. Lee). J Shoulder Elbow Surg (2009) 18, 748-755 www.elsevier.com/locate/ymse 1058-2746/2009/$36.00 - see front matter Ó 2009 Journal of Shoulder and Elbow Surgery Board of Trustees. doi:10.1016/j.jse.2008.12.001