Relevant Anatomy of Glenohumeral Instability.

Last updated Friday, February 04, 2005

The first muscle layer

Anatomy of the deltoid

The anterior deltoid extends to a line running approximately from the midclavicle to the midlateral humerus. This line passes over the cephalic vein, the anterior venous drainage of the deltoid, and over the coracoid process. The deltoid is innervated by the axillary nerve, whose branches swoop upward as they extend anteriorly. The commonly described "safe zone" 5 cm distal to the acromion does not take into account these anterior branches, which may come as close as 2 cm to the acromion. At the deltopectoral groove, the deltoid meets the clavicular head of the pectoralis major which assists the anterior deltoid in forward flexion. The medial and lateral pectoral nerves are not in the surgical field of shoulder stabilization. Splitting the deltopectoral interval just medial to the cephalic vein preserves the deltoid's venous drainage and takes the surgeon to the next layer. It is important to note that extension of the shoulder tightens the pectoralis major and the anterior deltoid as well as the coracoid muscles, compromising the exposure. Thus assistants must be reminded to hold the shoulder in slight flexion to relax these muscles and facilitate access to the joint.

Posteriorly, the medial edge of the deltoid is too medial to provide useful access to the glenohumeral joint. Access must be achieved by splitting the deltoid, which is most conveniently done at the junction of its middle and posterior thirds. This junction is marked by the posterior corner of the acromion. The site is favorable for a split because it overlies the joint and also because the axillary nerve exiting the quadrangular space divides into two trunks (its anterior and posterior branches) near the inferior aspect of the split.

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