Relevant Anatomy of Glenohumeral Instability.

Last updated Friday, February 04, 2005

Figure 1

Figure 2

Figure 3

Figure 4

The humeroscapular motion interface

Location of gliding

The humeroscapular motion interface (see figures 1 and 2) separates the structures that do not move on humeral rotation (the deltoid, coracoid muscles, acromion, and coracoacromial ligament) from those that do (the rotator cuff, long head of the biceps tendon, and humeral tuberosities). During shoulder motion, substantial gliding takes place at this interface (see figure 3). The humeroscapular motion interface provides a convenient plane for medial and lateral retractors and is also the plane in which the principal nerves lie.

The axillary nerve runs in the humeroscapular motion interface, superficial to the humerus and cuff and deep to the deltoid and coracoid muscles (see figure 4). Sweeping a finger from superior to inferior along the anterior aspect of the subscapularis muscle catches the axillary nerve, hanging like a watch chain across the muscle belly. Tracing this nerve proximally and medially leads the finger to the bulk of the brachial plexus. Tracing it laterally and posteriorly leads the finger beneath the shoulder capsule toward the quadrangular space. From a posterior vantage the axillary nerve is seen to exit the quadrangular space beneath the teres minor and extending laterally, where it is applied to the deep surface of the deltoid muscle. By virtue of its prominent location in close proximity to the shoulder joint anteriorly, inferiorly, and posteriorly, the axillary nerve is the most frequently injured structure in shoulder surgery.

The musculocutaneous nerve lies on the deep surface of the coracoid muscles and penetrates the coracobrachialis with one or more branches lying a variable distance distal to the coracoid. (The often-described 5 cm "safe zone" for the nerve beneath the process refers only to the average position of the main trunk and not to an area that can be entered recklessly.) The musculocutaneous nerve is vulnerable to injury from retractors placed under the coracoid muscles and to traction injury in coracoid transfer. Knowledge of the position of these nerves can make the shoulder surgeon both more comfortable and more effective.

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