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Imaging of the shoulder remains a challenge primarily related to the multiple layers of overlapping soft and bony tissues. Often an image reflects not only the single structure but rather that structure superimposed onto several other structures. This leads to numerous "special" projections which attempt to better isolate the desired structure. Much like imaging of the knee, clinicians increasingly have begun to use magnetic resonance imaging (MRI) as a more definitive modality again directly through its ability to provide both soft tissue and bony differentiations.
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The shoulder is the most mobile joint in the body and is composed of a true complex of both bony and soft tissue articulations. A very appropriate description is that the shoulder is designed to provide mobility with stability being secondary (at best). The bony components are the humerus, scapula, and clavicle, while the soft tissue articulation is that of the scapula and thorax (scapulothoracic joint).
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The clavicle serves as a crankshaft-strut assembly maintaining the ability of the arm to be positioned functionally while doing so in an efficient fashion. The clavicle is often the victim of falls onto the shoulder or direct trauma associated with vehicles, particularly bicycles.
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The scapula is positioned by soft tissues to permit appropriate function of the arm through orientation of the glenoid in relation to the humeral head. This finely tuned process is described as scapulohumeral rhythm and provides the harmonious functions of the upper extremity while enabling it to be anchored to the trunk. The scapulothoracic joint provides an upward rotation and sliding movement which requires a well-orchestrated sequence of proximal muscular actions in concert with humeral rotators (actually centering/compressing the humeral head onto the glenoid) and humeral movers culminating in upper extremity functional actions. The superior projection of the scapula includes the acromion which provides the "roof" of the glenohumeral joint proper, while the inferior projection is the coracoid process serving as an anchor for muscle and ligament insertions.
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The humerus provides the proximal rounded head which articulates with the rather flat glenoid fossa of the scapula (Figure 5–1). Thus the round head sitting/positioned onto a flat "saucer" provides an inherently unstable glenohumeral joint which is also dictated by the small size of the glenoid as well. The glenoid labrum is a fibrocartilagenous rim which helps increase the contact between these structures by enhancing the peripheral surface thickness much as does the meniscus of the knee. Although very dense, only special image modalities well define this wedge-shaped fibrous structure, with MRI being most typically being applied today.
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The space between the humeral head and the acromion is often referred to as the suprahumeral space and includes several soft tissues which can be "pinched" if inadequate muscular action, decreased space, or ...