As is the case with most subjects in orthopedics, the best way to start a discussion of the shoulder is to begin with the basic anatomy. This leads to some confusion right from the start because there really is no single structure that is accepted to be the “shoulder joint.” In the shoulder, three bones come together to make two joints, the glenohumeral joint and the acromioclavicular (AC) joint. Technically, there is also a third articulation between the scapula and the chest wall known as the scapulothoracic joint, but pathology in the scapulothoracic joint is rare, so we will limit our discussion to the glenohumeral and AC joints. The three bones that come together to form these two joints are the clavicle, the humerus, and the scapula (Figure 2-1). Of the three bones, the scapula has the most complicated shape, so we will study it in more detail. Figure 2-2 shows the scapula viewed from three different angles: a posterior view, a lateral view, and an anterior view. Each of these views highlights a different feature of the scapula. The posterior view shows the scapular spine, a long, thin bony prominence that terminates as the acromion process. The scapular spine is a subcutaneous bony prominence that is palpable on even the heaviest patients. The lateral view shows the acromion process, the coracoid process, and the glenoid fossa. Looking at the anterior view of the shoulder gives us the best view of the coracoid process and the anterior edge of the acromion. On all of the views, we can see the thin body (blade) of the scapula. The body of the scapula is designed to provide a large surface area for the origins and insertions of various muscles, and, as such, it is broad and flat. The coracoid process projects anteriorly and laterally, like a hooked finger. Several tendons, including the short head of the biceps and the coracobrachialis, attach to the coracoid process.
The two joints formed by the humerus, clavicle, and scapula are stabilized by a series of firm, but flexible, ligaments (Figure 2-3). The ligaments that stabilize the glenohumeral joint are bands of collagen-rich connective tissue that are embedded in the substance of the joint capsule, the thick membrane that originates from the rim of the glenoid socket and inserts around the neck of the humerus. At the point of its origin along the edge of the socket rim, the joint capsule becomes thick and dense. This tissue is known as the labrum. The AC joint has this same system of stabilizing ligaments in its capsule, but it also has an interesting, second set of ligaments that anchor the clavicle to the coracoid process. Since the coracoid process and the acromion are both part of the same bone, anchoring the clavicle to the coracoid process stabilizes the AC joint.
The humerus, clavicle, and scapula form the two joints ...