The shoulder is one of the largest and most complex joints in the body. The shoulder joint is formed where the humerus (upper arm bone) fits into the scapula (shoulder blade), like a ball and socket.
- The acromion is a bony projection off the scapula.
- The clavicle (collarbone) meets the acromion in the acromioclavicular joint.
- The coracoid process is a hook-like bony projection from the scapula.
- The rotator cuff is a collection of muscles and tendons that surround the shoulder, giving it support and allowing a wide range of motion.
- The bursa is a small sac of fluid that cushions and protects the tendons of the rotator cuff.
- A cuff of cartilage called the labrum forms a cup for the ball-like head of the humerus to fit into.
The different layers of the shoulder are built and connected can help understand how the shoulder works and is affected by injury and overuse.
- The deepest layer includes the bones and the joints of the shoulder.
- The next layer is made up of the ligaments of the joints.
- The tendons and the muscles come next.
- The nerves supply all the above structures and make them work.
The scapula is a triangular-shaped bone that functions mainly as a site for muscle attachment. Four rotator cuff muscles that act on the shoulder take their origin from the scapula. These are the supraspinatus, infraspinatus, teres minor, and subscapularis.
The scapula hasThe glenoid cavity (or, alternatively, the glenoid fossa) is set on the expanded aspect of the lateral angle of the scapula.
The clavicle is an S-shaped bone that forms the anterior portion of the shoulder girdle that keeps the arm away from the trunk, allowing it to move freely. The clavicle has 2 articulations, the sternoclavicular joint and the acromioclavicular joint.
The proximal articular surface of the humerus is termed the humeral head. The humeral head articulates against the shallow glenoid cavity.
The sternoclavicular joint is the sole connection between the axial skeleton and the upper extremity. The sternoclavicular joint allows upward elevation, anteroposterior movement, and rotation about the long axis of the clavicle.
The acromioclavicular (AC) joint is the only articulation between the clavicle and scapula. The AC joint is an encapsulated diarthrodial joint held together by its joint capsule and the coracoacromial ligaments: the trapezoid and conoid ligaments.
The glenohumeral joint is the main articulation of the shoulder joint. It is the multi-axial ball-and-socket synovial joint formed by the articular surfaces of the glenoid cavity and the head of the humerus.
The glenoid labrum is a ring composed of mostly dense fibrous tissue. The average depth of the glenoid cavity is 2.5 mm, but the labrum serves to increase this depth.
The conoid and trapezoid ligaments comprise the coracoclavicular ligaments (CCLs). They function to maintain the articulation of the clavicle with the coracoid process of the scapula.
Three glenohumeral ligaments exist: (1) the superior glenohumeral ligament (SGHL), (2) the middle glenohumeral ligament (MGHL), and (3) the inferior glenohumeral ligament (IGHL).
The coracohumeral ligament (CHL) originates on the base and lateral border of the coracoid process of the scapula and inserts on the greater tubercle.
The supraspinatus, infraspinatus, teres minor, and subscapularis muscles comprise the rotator cuff. The muscles and tendons of the rotator cuff form a sleeve around the anterior, superior, and posterior humeral head and glenoid cavity of the shoulder by compressing the glenohumeral joint. In addition to stabilization, the rotator cuff provides the shoulder with tremendous mobility.
The subacromial bursa lies on the superior aspect of the supraspinatus tendon. The bursa 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 laterally to be continuous with the subdeltoid bursa.
- Frozen shoulder: Inflammation develops in the shoulder that causes pain and stiffness. As a frozen shoulder progresses, movement in the shoulder can be severely limited.
- Osteoarthritis: The common “wear-and-tear” arthritis that occurs with aging. The shoulder is less often affected by osteoarthritis than the knee.
- Rheumatoid arthritis: A form of arthritis in which the immune system attacks the joints, causing inflammation and pain. Rheumatoid arthritis can affect any joint, including the shoulder.
- Gout: A form of arthritis in which crystals form in the joints, causing inflammation and pain. The shoulder is an uncommon location for gout.
- Rotator cuff tear: A tear in one of the muscles or tendons surrounding the top of the humerus. A rotator cuff tear may be a sudden injury, or result from steady overuse.
- Shoulder impingement: The acromion (edge of the scapula) presses on the rotator cuff as the arm is lifted. If inflammation or an injury in the rotator cuff is present, this impingement causes pain.
- Shoulder dislocation: The humerus or one of the other bones in the shoulder slips out of position. Raising the arm causes pain and a “popping” sensation if the shoulder is dislocated.
- Shoulder tendonitis: Inflammation of one of the tendons in the shoulder’s rotator cuff.
- Shoulder bursitis: Inflammation of the bursa, the small sac of fluid that rests over the rotator cuff tendons. Pain with overhead activities or pressure on the upper, outer arm are symptoms.
- Labral tear: An accident or overuse can cause a tear in the labrum, the cuff of cartilage that overlies the head of the humerus. Most labral tears heal without requiring surgery.