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ANATOMY

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The shoulder is designed for mobility in all directional planes, but stability is less than other joints. To meet the many demands placed on it, the shoulder uses three bones, four joints, and a specialized set of soft tissues consisting of muscles, tendons, ligaments, and bursae. The most common causes of nontraumatic shoulder pain in descending order of frequency are rotator cuff tendinopathy, acromioclavicular joint disease, adhesive capsulitis, and referred pain.1

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BONES AND JOINTS

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The humerus, clavicle, and scapula are the bones of the shoulder complex. The scapula consists of the body plus three bony extensions: the glenoid, the coracoid, and the acromion.

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The four joints of the shoulder are the glenohumeral, acromioclavicular, sternoclavicular, and scapulothoracic. The glenohumeral joint is a ball-and-socket joint and is the central axis of shoulder motion. The glenohumeral joint is the most mobile and least sTable joint in the body. Stability is derived from three components. The first is the glenoid labrum, which is a fibrous ring of tissue encircling the glenoid cavity. The glenoid labrum increases the surface contact area of the humeral head within the relatively shallow glenoid fossa. The second component consists of three glenohumeral ligaments, which aid stability by reinforcing the joint capsule. Finally, four specialized muscles, known as the rotator cuff, encompass the glenohumeral joint and provide stability during motion.

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The sternoclavicular and acromioclavicular joints together contribute to glenohumeral motion, but their primary function is to suspend and stabilize the shoulder girdle. Rotation at the acromioclavicular joint and elevation at the sternoclavicular joint allow complete arm elevation. The scapulothoracic joint represents the articulation of the scapula on the posterior wall of the thorax. Scapular motion is essential for overall shoulder motion: every degree of scapulothoracic motion allows 2 degrees of glenohumeral motion.

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SHOULDER MUSCLES

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The deltoid, which drapes the shoulder complex and forms its contour, acts as a powerful and independent elevator of the arm. Along with the pectoralis, the deltoid is the primary source of movement of the upper extremity.

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The rotator cuff consists of four muscles: supraspinatus, infraspinatus, teres minor, and subscapularis (Figures 280-1 and 280-2). All originate on the scapula, traverse the glenohumeral joint, and insert on the proximal humerus. The rotator cuff muscles also contribute to the power of the upper extremity, providing 30% to 50% of the power in abduction and 90% in external rotation.

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The supraspinatus muscle originates on the posterior and superior aspect of the scapula and passes beneath the acromion, inserting onto the great tuberosity of the humeral head. It initiates arm elevation and abducts the shoulder. It also balances the power of the deltoid, keeping the humerus centered in the glenoid during deltoid contraction. The infraspinatus originates on the posterior scapula just inferior to the scapular spine. It inserts on the posterior aspect ...

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