Shoulder Joints

The glenohumeral joint attaches the upper limb to the scapula. The rounded head of the humerus articulates with the slightly concave glenoid fossa (aka cavity) of the scapula. The glenoid labrum deepens the articulation between the humerus and scapula, which is otherwise shallow and incongruent; muscles and ligaments hold the bones in place. The glenohumeral joint is a synovial, ball-and-socket joint that allows for a wide range of motion (it is multiaxial): flexion, extension, abduction, adduction, external and internal rotation, and circumduction of the arm. The drawback to being highly mobile is reduced stability; thus, the glenohumeral joint is one of the most commonly injured joints in the body. in anterior view, we draw the lateral scapula in articulation with the humerus, and show the clavicle. Then, we outline the joint capsule, which is lined by a synovial membrane. The joint capsule is loose and spans from the anatomical neck of the humerus to the neck of the scapula. The looseness of the capsule is especially apparent inferiorly, which accommodates arm abduction. As we'll soon see, tendons of the rotator cuff blend into the external surface of the fibrous capsule. The anterior capsule is thickened by the superior, middle, and inferior glenohumeral ligaments. The glenohumeral ligaments limit the range of movement of the arm; they are taut during external rotation of humerus and relaxed during internal rotation. Together, they resist anterior translation of the humeral head. Additionally, the middle and inferior ligaments are taut during abduction, whereas the superior is relaxed. The superior glenohumeral ligament prevents inferior translation of humeral head, especially during shoulder adduction. The middle glenohumeral ligament lies deep to and blends with the tendon of the subscapularis muscle. It stabilizes the anterior capsule and limits external rotation, especially when arm is in the mid-range of abduction. The inferior glenohumeral ligament is sling-like; it splits into anterior and posterior bands on either side of the axillary pouch. The inferior stabilizes the humeral head when the arm is abducted above 90 degrees; the anterior band limits external rotation of the arm, whereas the posterior band limits internal rotation. The transverse humeral ligament extends horizontally between the greater and lesser humeral tubercles; it covers the intertubercular sulcus and holds the tendon of the long head of biceps brachii in place. The coracohumeral ligament extends between the coracoid process of scapula to the tubercles of the humerus; it limits inferior translation and excessive humeral external rotation. We draw the scapula and the glenoid cavity, which is covered in articular cartilage. The glenoid labrum is the fibrocartilaginous ring along the edge of the glenoid fossa of the scapula. It is continuous with the tendon of the long head of biceps brachii. The fibrous joint capsule, has thickened areas that comprise the superior, middle, and inferior glenohumeral ligaments. The rotator cuff muscles contribute to the external layers of the capsule as follows: Anteriorly, subscapularis; superiorly, supraspinatus; posteriorly, infraspinatus and teres minor. The synovial membrane it attaches to the glenoid labrum; it continues over the tendon of the long head of biceps brachii, which reduces friction between the tendon and bone. The synovial membrane protrudes though openings in the fibrous capsule to form bursae that reduce friction between the tendons and bones. The synovial membrane creates a pocket between the subscapularis tendon and the fibrous capsule (specifically the middle glenohumeral ligament); this is the subscapular bursa. This bursa is clinically important because, since it communicates with the joint cavity, opening the subscapularis bursa means entering the joint cavity. The subacromial bursa is superior; it is sometimes called the deltoid bursa, or the two are combined and referred to as the subacromial/subdeltoid bursae. There is a bursa between the joint capsule and the coracoid process. Bursae can become inflamed due to joint overuse or injury, which can cause pain and limit the range of joint movement. Septic bursitis is the result of infection, and requires antibiotic treatment. Two important weak spots in the glenohumeral joint capsule: The inferior capsule lacks tendinous or muscular support and is the weakest area of the capsule. The area between the supraspinatus and subscapularis tendons is also relatively weak. Innervation: subscapular nerve (C5-C6) serves the joint; the suprascapular nerve, axillary nerve, and lateral pectoral nerve serve the joint capsule. Blood supply: Anterior and posterior circumflex humeral, circumflex scapular and suprascapular arteries. Movements of the arm relies on glenohumeral joint and its positioning, which in turn rely on changes in the acromioclavicular and sternoclavicular joints. Because movement of these joints is coordinated for arm movement, they are often combined and referred to as the "scapulothoracic joint." We'll address them separately so we can see their anatomy. The acromioclavicular joint is between the clavicle and scapula; it is a plane type joint. The articular surfaces are covered with fibrocartilage and separated by incomplete articular disc. The AC has a sleeve-like and loose joint capsule, which is strengthened superiorly and inferiorly by the acromioclavicular ligaments that extend from the acromion to the lateral end of the clavicle. This joint allows the acromion of the scapula to rotate on the acromial end of clavicle; this is a passive joint, as no muscles act directly on it. As a plane joint, it allows gliding movement; however, as part of the scapulothoracic joint, acromioclavicular joint movements contribute to protraction/retraction, elevation/depression, and axial rotation of the acromion. Additionally, it allows for the transmission of force from the upper limb to the clavicle. The coracoacromial ligament connects the coracoid process and acromion; it forms the coracoacromial arch, aka the roof of the shoulder joint, which prevents superior displacement of the humeral head. The coracoclavicular ligament comprises two bands that anchor the clavicle to the coracoid process. The conoid ligament, which is vertical and cone-shaped, and the trapezoid ligament, which is more horizontal. In life, there may be a bursa between these two ligaments. The coracoclavicular ligament stabilizes the acromioclavicular joint. Acromioclavicular joint separation (aka shoulder separation) occurs when there is a direct force on the shoulder, as in contact sports or as a result of falling on an outstretched arm (FOOSH – fall on outstretched hand). Innervation: lateral pectoral nerve, suprascapular nerve. Blood supply is via the thoracoacromial artery and suprascapular arteries. The sternoclavicular joint comprises articulation between the sternal end of the clavicle and the manubrium and first rib. This is a synovial saddle joint when classified by the shape of its articular surfaces, with ball-and-socket functions. On one side, we'll show deeper structures, and on the other, more superficial structures. This joint is divided into two compartments by the articular disc, which attaches superiorly to the interclavicular ligament, which, as its name suggests, runs between the clavicles. The anterior sternoclavicular ligament is a thickened portion of the fibrous layer of the joint capsule; be aware that there is also a posterior sternoclavicular ligament, which we can't see in this view. The costoclavicular ligament anchors inferior surface of the sternal end of clavicle to the first rib and its costal cartilage; this limits the elevation of the pectoral girdle. The sternoclavicular joint is very strong, and dislocation is rare despite the clavicular fraction being common. The movements of this joint use the lateral end of clavicle as reference point, and include: elevation/depression, protraction/retraction, and axial rotation. Innervation is via the medial supraclavicular nerve and by nerve to subclavius. Blood supply comes from branches of suprascapular and internal thoracic arteries.