The shoulder joint, or glenohumeral joint, is a synovial ball and socket joint, which involves the articulation between the shoulder blade, and the upper arm bone. It is the most mobile joint in the human body.
The glenoid fossa, or the glenoid cavity, is the part of the shoulder joint that interacts with the upper arm bone. The glenoid labrum is a part of the glenoid fossa, which helps with the stability of the shoulder. As we said before, the glenohumeral joint is the most mobile joint in the human body, with 120 degrees of flexion, without assistance. The Scapulohumeral rhythm, which is the movement of the scapula across the thoracic cage, helps with achieving even more movement. Anything that can change the position of the scapula, can compromise the Scapulohumeral rhythm. An imbalance in the muscles (upper and lower trapezium) that hold the scapula in place, can also compromise the Scapulohumeral rhythm. There are various movements of the shoulder joint, like flexion, extension, abduction, adduction, lateral rotation, and medical rotation. All of these movements have different starting and ending points, and different muscles that are involved, in order to complete the movement.
The shoulder joint also has a loose capsule, which is an envelope that surrounds the synovial joints. The glenohumeral joint a type of synovial joints, and they are the most common type of joints in the human body. Because the capsule is lax inferiorly, it stands the risk of dislocation. The head of the biceps muscle goes inside the capsule, and it attaches itself to the supraglenoid tubercle. Because of the fact that the tendon is located inside the capsule, it requires certain lubrication, to minimize the friction. The numerous sacks filled with fluid, called bursae (plural), help with the mobility of the capsule. They are the subdeltoid bursa (located between the deltoid muscle and the join capsule), the subcoracoid bursa (located between the coracoid process of the scapula and the joint capsule). Then, there are the coracobrachial bursa (between the tendon of the coracobrachialis muscle and the subscapularis muscle), the subacromial bursa (between the acromion of the scapula and the joint capsule), and the subscapular bursa (located between the subscapularis muscle and the joint capsule). The synovial membrane of the joint capsule is the one that forms the bursae. The shoulder joint depends on the muscles surrounding it, because it lacks strong ligaments.
The shoulder joint has three types of ligaments. They are the GHL (glenohumeral ligaments), the coracohumeral ligament, and the transverse humeral ligament. The glenohumeral ligaments are three ligaments, which can be found on the anterior side of the shoulder (between the glenoid cavity of the scapula, and the head of the upper arm bone). The superior, middle, and inferior GHL affect the stability of the upper arm bone, depending on the arm position. The coracohumeral ligament originates from the lateral border of the coracoid process, and is a broad ligament that strengthens the upper part of the joint capsule. The transverse humeral ligament is a broad ligament, and it lies between the lesser and the greater tubercle of the upper arm bone.