The shoulder complex is a term that is often used to describe all the structures that are involved in the moving of the shoulder. Movements of the shoulder girdle include abduction, adduction, internal rotation, external rotation, flexion, and extension. There are many parts in the shoulder complex, muscles, bones, and the joints they form. The scapula, clavicle, sternum, humerus, and rib cage make up the five bones of this complex. Three of the joints in the shoulder complex are the sternoclavicular joint, acromioclavicular joint, and the glenohumeral joint. These, along with several different muscles, allow for the range of motion the shoulder complex has.
The sternoclavicular joint consists mainly of the sternum and clavicle. This joint provides the shoulder girdle with its only direct attachment to the trunk. The sternum is a flat bone, it’s inferior tip being the xiphoid, the superior tip being the manubrium, where the clavicle and the first rib attach. The ligament which joins the clavicle to the sternum is called the interclavicular ligament. To help absorb shocks to the clavicle, there is cartilage, called the articular disk, in between the sternum and the clavicle. The ligament that runs between the medial ends of the clavicle and across the manubrium is called the interclavicular ligament. The final ligament in this joint connects the clavicle to the first rib, this ligament is called the costoclavicular ligament. This ligament limits the amount of clavicular elevation.
The next joint I will discuss is the acromioclavicular joint. This joint connects the lateral end of the clavicle with the acromion process of the scapula. The scapula is another flat bone, but has many different parts to it. On the anterior side is located the coracoid process, while on the posterior side is the spine of the scapula; these are two of the major ways to tell which side of the scapula you are looking at. The glenoid fossa is located on the lateral side of the scapula. This is where the head of the humerus fits. The acromion process extends from the spine of the scapula (it is the tip of the spine of the scapula.) The superior angle and inferior angle are opposite each other, one being on the medial side of the scapula, the other on the lateral side, respectively. The vertebral border is the edge of the scapula closest to the spine, and the axillary border is on the lateral side.
Now that you have and idea of the parts of the scapula, I’ll move on to the ligaments that connect the scapula and clavicle. The acromioclavicular ligament connects the acromion process and the clavicle. It lends support to the joint and helps prevent dislocation. The coracoclavicular ligament runs from the coracoid process to the acromion process and helps provide the joint with stability, though not directly located in the acromioclavicular joint itself. There are two part to the coracoclavicular ligament, which runs from the coracoid process to the clavicle. The first is the conoid portion, and the second is the trapezoid portion. These ligaments help prevent backward motion of the scapula and limit scapular rotation.
Muscles in the shoulder girdle include the rhomboids, deltoid, trapezius, and serratus anterior. The trapezius muscle consists of three parts, the upper middle and lower muscles. The three muscles originate from different vertebra, and all insert somewhere along the scapular spine. The rhomboids are actually two muscles, but since it is nearly impossible to separate the two and they have basically the same functions and actions, they are commonly considered as one. These muscles are a prime mover in retraction and only assist in elevation. The serratus anterior muscle gets its name from its serrated-like edge. The serratus anterior is a major mover in scapular protraction. It also helps keep the vertebral border of the scapula against the rib cage. The deltoid muscle almost completely covers the shoulder joint, covering three sides and giving the joint its rounded shape. This muscle has three parts, the anterior, middle, and posterior.
Going back to the scapula, there are two more muscles that are part of the shoulder joint. The supraspinatus, and the infraspinatus. The supraspinatus muscle is located above the spine of the scapula, while the infraspinatus muscle is located below the scapular spine. The supraspinatus muscle helps initiate shoulder abduction as well as aids in stabilizing the head of the humerus against the glenoid fossa. The infraspinatus muscle has the ability to extend and shoulder joint, but has a horizontal line of pull, as well.
There are two more ligaments in the shoulder joint that were not mentioned earlier. They are the glenohumeral ligaments and the coracohumeral ligament. The glenohumeral ligaments are really just pleated folds of the joint capsule, a thin-walled, spacious container that attaches around the rim of the glenoid fossa. The glenohumeral ligaments help to reinforce this capsule. The coracohumeral ligament attaches to the lateral side of the coracoid process and spans the shoulder joint anteriorly to the medial side of the greater tubercle. This ligament helps strengthen the upper part of the joint capsule.
The final two muscles I will discuss in this essay are the biceps and triceps muscles. These are not major muscles in the shoulder joint, so I will only mention them briefly. The biceps brachii muscle has two heads, both of which attach on the scapula. The triceps brachii muscle runs form the inferior rim of the glenoid fossa, through the teres minor and major muscles, and attaches to the posterior surface of the humerus. Both the biceps and triceps brachii muscles are located mainly on the arm and only aid in the movements of the shoulder joint at best.
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