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 set on the lateral angle of the scapula and articulates with the head of the humerus. This relationship forms the glenohumeral joint, which serves as the main joint of the shoulder.
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 2.5mm, however, the labrum serves 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 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.
Rotator Cuff Injuries
The supraspinatus, infraspinatus, teres minor, and subscapularis muscles make up the rotator cuff. 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 glenohumeral joint. In addition to stabilization of the joint, the rotator cuff provides the shoulder with tremendous mobility. If you want to learn more about the rotator cuff, please watch this rotator cuff injuries video.
Subacromial Bursa/Subdeltoid Bursa
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 subdeltoid bursa.
If you would like to learn more about the anatomy of the shoulder, go to our Shoulder Trending Section.
If you need an appointment with a Jacksonville Orthopedic Institute Shoulder Specialist, please call JOI -2000