The shoulder complex is perhaps the most complicated of the body's articulations because it must provide extensive yet precise range of motion (ROM) in all anatomical planes. The relationship between the glenohumeral (GH) joint and the scapula allows the humerus to be placed in 16,000 positions that can be differentiated in 1-degree increments.1
The shoulder complex lacks the intrinsic bony and ligamentous stabilizers that occur in other joints. Relying on its musculature to provide most of its stability, the shoulder complex in general, and specifically the GH joint, is inherently unstable. The GH joint, because of its poor bony stability and weak capsular structures, depends more on the proprioceptive and stabilizing function of its musculature than any other joint in the body.2 Injury to the shoulder complex may occur from a direct force or secondary to forces transmitted proximally along the upper extremity. The shoulder complex also is predisposed to overuse conditions, especially in individuals participating in activities that require repeated overhead movements.
The upper extremity's only attachment to the axial skeleton is at the sternoclavicular (SC) joint. This configuration results in a mechanism whereby the arm is suspended from the torso by muscular attachments. The motions provided by the upper extremity arise from the intricate interactions of the four bones forming the shoulder girdle and the four articulations providing movement. Elevation describes the integrated movement of the glenohumeral, sternoclavicular, acromioclavicular, and scapulothoracic articulations as the arm is raised. Elevation can occur in the sagittal plane (flexion), frontal plane (abduction), and anywhere in between them.3 The large ROM provided by the shoulder complex, especially the GH joint, is achieved at the expense of joint stability. Unlike the hip joint, which gains its stability through a deep ball-and-socket joint and strong ligamentous support (at the expense of mobility), the GH joint is characterized by shallow articular surfaces, inconsistent ligamentous support, and an increased reliance on dynamic support through muscle activity.
The shoulder complex, formed by the sternum, clavicle, scapula, and humerus, may be likened to a series of hinges, pulleys, and levers working in unison to choreograph intricate motions in many anatomical planes (Fig. 15-1). A precise degree of ROM, strength, and coordination must be maintained to ensure efficient biomechanics.
Bones of the shoulder complex and glenohumeral joint.
The manubrium of the sternum serves as the site of attachment for each clavicle. Projecting above the body of the sternum, the superior surface of the manubrium is indented by the jugular (suprasternal) notch. Projecting off each side of the jugular notch is the clavicular notch, which accepts the medial head of the clavicle (Fig. 15-2).