Health

The shoulder joint is comprised of four articulations that work together in a synchronized fashion. An articulation is the point where two bones come together. These include the glenohumeral, acromioclavicular, sternoclavicular, and scapulothoracic articulations. The major bones involved in shoulder mechanics include the humerus (long bone of the upper arm), the scapula (shoulder blade), the clavicle (collar bone), the sternum (breast bone), and thoracic wall (rib cage). The scapula is notable for several landmarks. Finger-like projections at the top are called the acromion and coracoid process. The bony ridge on the back is called the scapular spine. The cup-like projection to the side is called the glenoid fossa, which will be discussed in a later section. 

The most easily identified articulation is the glenohumeral joint, a ball and socket joint The ball portion of the joint is the head of the humerus (the rounded top of the bone in the upper arm). The socket portion of the joint is found on the scapula and is the called glenoid fossa. The socket created by the glenoid fossa is a relatively shallow cup. The cup is reinforced by a ring of cartilage called the labrum. The glenohumeral joint is the most mobile joint in the body, but this mobility comes at the cost of stability. 

The acromioclavicular (AC) joint is where the bony projection of the shoulder blade, called the acromion, meets the clavicle. The joint is surrounded by a lax capsule and reinforced by ligaments. This joint is further reinforced by “coracoclavicular” ligaments, which connect the coracoid process of the scapula to the collar bone. The joint may contain a disc, which may be incomplete or absent altogether. The disc is prone to degeneration with age. The lower aspect of the joint is in contact with the rotator cuff. When people refer to a “separated shoulder,” it is often the AC joint that is involved. 

The sternoclavicular joint is where the sternum, or breast bone, meets the clavicle. This is where the appendicular skeleton (in this case, the arm) connects to the axial skeleton (trunk and spine).  

The scapulothoracic articulation is the interface between the scapula, or shoulder blade, and thoracic cavity, or ribcage. Rotating the scapula changes the orientation of the glenoid fossa, which is on the side of the scapula. In this way, this articulation contributes to the available range of motion of the shoulder. When elevating the arm, motion occurs at both the glenohumeral and scapulothoracic articulations in approximately a 3 to 2 ratio.  

Because there is the potential for great mobility, the shoulder requires extra support from non-bony, soft tissue structures. The potential for dislocation of the humeral head is greatest in the anterior, or forward, direction. Preventing this are several tough bands of connective tissue, or ligaments, covering the front of the joint. Together, these ligaments are called the anterior capsule. Perhaps the most important of these ligaments is the Anterior Inferior Glenohumeral Ligament (or AIGHL). This ligament acts like a sling to support the humeral head. When the arm is raised and rotated outward, as in throwing overhand, the sling tightens and stabilizes the humeral head. If there is laxity in this complex, the humeral head may dislocate from the glenoid fossa.  

The glenohumeral is also reinforced by an envelope of tendons called the rotator cuff. Tendons are the connective tissues that connect muscle to bone. Four separate muscles converge to form the rotator cuff which inserts on the humerus. The tendons are part of 4 muscles that move the arm with respect to the shoulder blade. The supraspinatus muscle lifts the arm, the infraspinatus and teres minor rotate the arm outward, and the subscapularis rotates the arm inward. Together these muscles prevent upward migration of the humeral head caused by the deltoid muscle. Together, these muscles may be remembered by the acronym S.I.T.S. Some consider the tendon of a fifth muscle, the long head of the biceps, as part of the rotator cuff as well. This muscle lies between the subscapularis and supraspinatus tendons.  

The movement of the scapula is controlled by a group of muscles that connect the scapula to the spine and chest wall. These muscles together stabilize the shoulder blade when the shoulder moves. Weakness of the scapular stabilizers can lead to altered joint mechanics, dysfunction, and pain.  

The subacromial space is the site of interaction between the “coracoacromial arch” which includes the acromion, the AC joint, the coracoacromial ligament, and the rotator cuff¹. This space can be the site of impingement, or pinching, usually due to the orientation of the acromion and/or inflammation of the soft tissues occupying the subacromial space. A bursa (or fluid filled sac) is located within this space. 

The nerves serving the shoulder, as well as those of the arm, forearm, and hands, leave the spinal cord at levels C5 through T1 (5^th cervical through 1^st thoracic, see spinal cord injury section). These nerves leave the spinal cord separately, joining and intermingling in a complex network called the brachial plexus. From there, the nerves continue as “peripheral nerves” to supply the muscles of the upper extremity.