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INTRODUCTION

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The shoulder complex, composed of the clavicle, scapula, and humerus, is an intricately designed combination of three joints that links the upper extremity to the thorax. The articular structures of the shoulder complex are designed primarily for mobility, allowing us to move and position the hand through a wide range of space. The glenohumeral (GH) joint, which links the humerus and scapula, has greater mobility than any other joint in the body. Although the components of the shoulder complex constitute half of the mass of the entire upper limb,1 they are connected to the axial skeleton by a single joint, the sternoclavicular (SC) joint. As a result, muscle forces serve as a primary mechanism for securing the shoulder girdle to the thorax and providing a stable base of support for upper extremity movements.

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The contradictory requirements on the shoulder complex for both mobility and stability are met through active forces, or dynamic stabilization, a concept of which the shoulder complex is considered a classic example. In essence, dynamic stability exists when a moving segment or set of segments is limited very little by passive forces such as articular surface configuration, capsule, or ligaments and instead relies heavily on active forces or dynamic muscular control. Dynamic stabilization results in a wide range of mobility for the shoulder complex and provides adequate stability when the complex is functioning normally. However, the competing mobility and stability demands on the shoulder girdle and the intricate structural and functional design make the shoulder complex highly susceptible to dysfunction and instability.

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Case 7-1: Patient Case

Susan Sorenson is a 42-year-old dental hygienist who presents to the clinic with a chief complaint of right shoulder pain. She localizes the pain primarily at the lateral proximal humerus (C5 dermatome region) but also reports pain in the upper trapezius. Symptoms include pain and fatigue with elevating her arm and the inability to sleep on her right shoulder. Her medical history includes a diagnosis of early-stage breast cancer in the right breast 6 months ago. She had a lumpectomy with sentinel node biopsy, followed by radiation treatments for 5 weeks. She finished treatment 4 months ago. She reports feelings of tightness over the anterior chest region when she raises her right arm. Her history also includes a right acromioclavicular joint separation many years ago, for which she was immobilized in a sling for several weeks with no further treatment.

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COMPONENTS OF THE SHOULDER COMPLEX

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The osseous segments of the shoulder complex are the clavicle, scapula, and humerus (Fig. 7–1). These three segments are joined by three interdependent linkages: the sternoclavicular joint, the acromioclavicular (AC) joint, and the glenohumeral joint. The articulation between the scapula and the thorax is often described as the scapulothoracic (ST) "joint," although it does not have the characteristics of a fibrous, cartilaginous, or synovial joint. Instead, scapula motion on the thorax is ...

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