The shoulder is composed of the proximal humerus, clavicle, and scapula. The joints of the shoulder include the sternoclavicular, the acromioclavicular, and the glenohumeral. There is also an articulation between the scapula and the thorax. Figures 16–1, 16–2, and 16–3 provide the essential anatomy, both osseous and ligamentous, which must be understood to comprehend the disorders involving the shoulder. Superficial to the ligaments are the muscles that support the shoulder and provide for its global range of motion. The rotator cuff surrounds the glenohumeral joint and is composed of the teres minor, infraspinatus, and supraspinatus muscles (insert on the greater tuberosity) and the subscapularis muscle (inserts on the lesser tuberosity) (Fig. 16–4). Superficial to these muscles is the deltoid, which functions as an abductor of the shoulder.
The essential anatomy of the shoulder.
The ligaments around the shoulder.
The ligamentous attachments of the clavicle to the sternum medially and the acromion laterally.
The rotator cuff.
The clavicle is an oblong bone, the middle portion of which is tubular and the distal portion, flattened. It is anchored to the scapula by the acromioclavicular and the coracoclavicular ligaments. The sternoclavicular and the costoclavicular ligaments anchor the clavicle medially (see Fig. 16–3). The clavicle serves as points of attachment for both the sternocleidomastoid and the subclavius muscles. The ligaments and the muscles act in conjunction to anchor the clavicle and, thus, maintain the width of the shoulder and serve as the attachment point of the shoulder to the axial skeleton.
The scapula consists of the body, spine, glenoid, acromion, and coracoid process. The bone is covered with thick muscles over its entire body and spine. On the posterior surface, the supraspinatus muscle covers the fossa superior to the spine, whereas the infraspinatus muscle covers the fossa below the spine. The anterior surface of the scapula is separated from the rib cage by the subscapularis muscle. These muscles offer protection and support for the scapula. The scapula is connected to the axial skeleton only by way of the acromioclavicular joint. The remainder of the scapular support is from the thick investing musculature surrounding its surface.
When examining the shoulder, start by assessing neurovascular structures. Neurovascular injuries frequently accompany traumatic shoulder injuries. The structures in closest proximity to the shoulder include the brachial plexus, axillary nerve, and axillary artery (Fig. 16–5).
The course of the important neurovascular structures surrounding the shoulder.
The range of motion of the shoulder can be assessed by testing internal and external rotation, as well as abduction. External rotation to 90 degrees is normal. To test internal rotation, have the patient put their hand on their back and gradually walk up the spine. Normal internal rotation allows the patient to reach the base of the scapula. Normal shoulder abduction and forward flexion is to 180 degrees.
The glenohumeral joint and scapulothoracic articulation function as a unit in abducting the humerus. The ratio of scapular to glenohumeral movement is 1:2; therefore, for every 30 degree of abduction of the arm, the scapula moves 10 degree and the glenohumeral joint moves 20 degree (Fig. 16–6). If the glenohumeral joint is completely immobilized, the scapulothoracic articulation is capable of providing 65 degree of abduction on its own. This “shrugging” mechanism is important for the physician to be aware of in assessing the movements at the shoulder joint that are hampered by certain pathologic entities.
The ratio of glenohumeral to scapulothoracic motion is 2:1. At 90 degree of abduction, 60 degree occurs at the glenohumeral joint and 30 degree at the scapulothoracic articulation. With the shrugging mechanism one can abduct the shoulder 65 degree because of scapulothoracic movement even though there is no motion at the glenohumeral joint.
At the sternoclavicular joint, the clavicle is elevated 4 degree for every 10 degree of shoulder abduction. This elevation continues until 90 degree of abduction has been obtained. The range of motion at the acromioclavicular joint is approximately 20 degree. This motion occurs during the first 30 degree and after 100 degree of abduction.
A number of structures can be palpated around the shoulder that are common sites of pathology. Palpation of the shoulder begins at the suprasternal notch. Find the sternoclavicular joint just lateral to the notch. The clavicle is slightly superior to the manubrium, and one is actually palpating the proximal end of the clavicle at this point. The clavicle is superficial in its entire course and can be palpated easily.
The acromioclavicular joint is palpated by pushing in a medial direction against the distal end of the clavicle as it protrudes above the flattened acromion process. The acromioclavicular joint is more easily palpated if the patient is asked to move the shoulder several times while the examiner palpates the joint. The greater tuberosity of the humerus lies lateral to the acromion process and can be palpated by following the acromion process to its lateral edge and then sliding the fingers inferiorly. A small step-off exists between the lateral acromion border and the greater tuberosity.
The bicipital groove is located anterior and medial to the greater tuberosity and is bordered laterally by the greater tuberosity and medially by the ...