The human shoulder is a ball-in-socket joint and in turn, is the most mobile joint in the body. This mobility allows you to move through a tremendous range of motion in a variety of planes.
Commonly thought of as a single joint, the shoulder is actually made up of two separate joints- the glenohumeral and acromioclavicular joints.
These two joints work together at the arm to allow the shoulder to move in a large circle and to rotate around its axis. This extensive range of motion makes the shoulder joint fairly unstable.
This instability is countered by the strength of the rotator cuff muscles, tendons, ligaments, and the glenoid labrum. The bones of the shoulder consist of the humerus (upper arm bone), the scapula (shoulder blade), and the clavicle (collar bone). The shoulder’s relationship with the scapula allows for functional movement of the shoulder blade. Let's dive into the shoulder anatomy.
Sometimes it is easier to understand anatomy when you look at a visual representation. Click here to watch an anatomy video about the shoulder joint anatomy.
The scapula is a triangular-shaped bone that functions mainly as a site for muscular attachment. Four rotator cuff muscles that act on the shoulder begin at the scapula.
These muscles are the supraspinatus, infraspinatus, Teres minor, and subscapularis. In addition, the trapezius, serratus anterior, rhomboids, and Levator scapulae insert on the scapula and are responsible for scapular mobility and stability.
The glenoid fossa is the socket of the shoulder joint. This part of the shoulder is a ball and socket joint. The head of the Humerus fits into the Glenoid fossa, though it is a shallow socket.
The clavicle is an S-shaped bone that forms the front portion of the shoulder girdle and keeps the arm away from the trunk, allowing it to move freely. The clavicle consists of two joint articulations: the sternoclavicular joint and the acromioclavicular joint.
The sternoclavicular joint is the only skeletal connection between the axial (main) skeleton and the upper extremity. Furthermore, the clavicle provides protection to the important underlying veins, arteries, and nerves that run in the front and back of the structure.
The top portion of the humerus is referred to as the humeral head. The humeral head articulates with the glenoid fossa of the scapula. However, only 25% of the humeral head surface makes contact with the glenoid surface at any point in time. The glenoid labrum, a fibrocartilaginous ring, attaches to the outer room of the glenoid fossa and provides additional depth and stability.
The glenoid labrum is a ring composed of mostly dense fibrous tissue. The average depth of the glenoid fossa is 2.5 mm, however, the labrum helps to increase this depth. Although the labrum increases the depth and volume of the glenoid fossa, it does not seem to increase the stability of the glenohumeral joint.
The conoid and trapezoid ligaments make up the coracoclavicular ligaments. These ligaments are the primary restraint for upward and backward movement of the clavicle.
There are three glenohumeral ligaments: The superior glenohumeral ligament, middle glenohumeral ligament, and inferior glenohumeral ligament. These ligaments provide stability to the glenohumeral joint and protect from numerous translations of the humerus within the glenoid.
The coracohumeral ligament originates on the base and outside the border of the coracoid process of the scapula and attaches to a landmark on the humerus called the greater tuberosity. While the function of this ligament is not fully understood, it appears to perform a suspensory function to the humeral head.
The Rotator Cuff is made up of 4 muscle including the:
The muscles and tendons of the rotator cuff form a cover around the anterior, superior, and posterior humeral head and glenoid cavity of the shoulder by compressing the gleno-humeral joint. In addition to stabilization of the joint, the rotator cuff provides the shoulder with tremendous mobility. Strained rotator cuff muscles are very common in sports. If you want to learn more about the rotator cuff, please watch this rotator cuff injuries video.
The subacromial bursa lies on the top portion of the supraspinatus tendon. The bursa is a fluid-filled sac and acts to cushion and reduce friction during motion between the overlying bone of the acromion and the soft rotator cuff muscles below. It often extends out to be continuous with the sub-deltoid bursa.
The muscles in the back of the shoulder include the:
These muscles work to extend the arm and rotate the shoulder outward or externally. The layer deeper to the muscles includes the labrum and joint capsule. These structures help to stabilize the shoulder from the back.
Also if you have recently had a shoulder surgery watch our video on How to Properly Put on an UltraSling 4 or Dressing the Upper Body After an Injury or Surgery
If you would like to learn more about the anatomy of the shoulder, go to: 5 Shoulder Symptoms You Should Not Ignore.
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