The deltoid split approach is usually used for intramedullary nailing or minimum invasive plate osteosynthesis for proximal humerus fractures. The risk of axillary nerve injury has been reported with this approach when the incision exceeds 5cm distal from the anterior corner of the acromion. We carried out a cadaveric study on the axillary nerve anatomy in order to determine a safety zone for the deltoid split approach. We dissected and observed how the axillary nerve runs beneath the deltoid muscles in twenty three embalmed cadavers. The average distance from the anterior corner of the acromion to the axillary nerve in the anterior raphe was 4.67cm. Only one branch of the axillary nerve was noted crossing the anterior raphe in all shoulders. After crossing the anterior raphe, the axillary nerve was found to enter from beneath the muscularis, between the muscularis layers and after that many branches of it spread medially and cephalad. It may be difficult in actual operations to directly observe the axillary nerve in the area from the entry point medial. The distance from the point that the axillary nerve crosses the anterior raphe to its entry point between the muscularis layers was sometimes very close. (less than 1cm) These findings indicated that special care must be taken not to injure the axillary nerve in splitting the deltoid muscle at the antero-medial area beyond the anterior corner of the acromion.
