SHOULDER - OVERVIEW

1. Humerus: Upper arm bone. The "ball" of the shoulder's "ball and socket" joint.
2. Scapula: Flat, triangular bone called the shoulder blade. Prominent areas serve as attachment points for many muscles and ligaments:
 - glenoid: shallow "socket" on the side of the scapula that receives the 'ball' of the humerus.
 - Scapular spine: horizontal ridge along the back of the scapula that divides it into upper and lower regions.
 - Acromion: end of the scapular spine. It projects up to form the top of the shoulder.
 - Coracoid Process: a projection towards the front of the scapula and is an attachment site for several muscles and ligaments.
3. Clavicle is the collarbone. Although it appears to be straight, it actually forms an S-shape when seen from above.
4. Thorax or rib cage: an anchor for several muscles and ligaments. Although the ribs do not physically attach to the scapula, the thorax stabilizes and maintains proper positioning of the scapula so that the arm can function to its fullest capacity.
 

• The main joint of the shoulder.

• Here, the Glenoid on the scapula and the head of the Humerus come together.

• The fairly flat socket of the Glenoid surrounds only 20% - 30% of the Humeral head.

• Because of its poor fit, this joint relies heavily on the surrounding soft tissue for support.

• The Labrum, a ring of fibrocartilage tissue, attaches to the Glenoid and deepens the socket to encircle more of the humerus.

• The bony point on the top of the shoulder.
• It stabilizes the scapula to the chest, by connecting the Acromion on the scapula to the Clavicle, or "collarbone".
• A thick disk of fibrocartilage acts as a shock absorber between the two bones.
• The surrounding capsule and ligaments give this joint great stability.

• Connects the other end of the Clavicle to the Sternum, or "breastbone".
• Like the AC joint, this joint contains a fibrocartilage disk that helps the bones achieve a better fit.
• It also gets excellent support from its joint capsule and surrounding ligaments.

• Area where the Scapula, embedded in muscle, glides over the Thoracic rib cage.

• Surrounding muscles and ligaments keep the scapula properly positioned so that the arm can move correctly.

The thick tissue that forms the disks of the AC and SC joints and the labrum, the ring that deepens the glenoid. Fibrocartilage has three roles:
   - To act as a cushion in shock absorption
   - To help stabilize the joint by improving the fit of the bones
   - To act as a spacer and improve contact between the articular cartilage surfaces
 

Ligaments attach bone to bone and provide the "static" stability in a joint. Ligaments will alternately become tight and loose with normal motion. They keep the joint within the normal limits of movement.

1. Glenohumeral ligaments attach in layers from the glenoid labrum to form the joint capsule around the head of the humerus.
2. Coracoacromial arch is the group of ligaments that spans the bony projections of the coracoid process and the Acromion.
3. Coracoclavicular ligaments and the acromioclavicular ligament provide most of the support for the AC joint.
 

 There are four muscle groups in the shoulder:

• Rotator cuff muscles are the supraspinatus, infraspinatus, teres minor and subscapularis (s.i.t.s). They are the primary stabilizers that hold the "ball" of the humerus to the glenoid "socket". The socket is too shallow to offer much security for the humerus. These four muscles form a "cuff" around the humeral head, securing it firmly in the socket. As its name implies, this group of muscles also rotates the arm. The rotator cuff protects the glenohumeral joint from dislocation, allowing the large muscles that control the shoulder to power the arm with great mobility.

• The biceps tendon complex also helps keep the humeral head in the glenoid and helps raise the arm.

• The scapulothoracic muscles attach the scapula to the thorax. Their main function is to stabilize the scapula to allow for proper shoulder motion.

• The superficial muscles of the shoulder are the large, powerful outer layer of muscles that are important to the overall function of the shoulder. This group includes the deltoid muscle, which covers the rotator cuff muscles.

• A bursa is a pillow-like sac filled with a small amount of fluid.

• Bursae (plural) reduce friction and allow smooth gliding between two firm structures, like bone and tendon or bone and muscle.

• There are over 50 bursae in the human body

• the largest is the subacromial bursa (under the acromion) in the shoulder.

• The subacromial bursa and the subdeltoid bursa (under the deltoid muscle) are often considered as one structure. This bursa separates the rotator cuff and the deltoid muscle, from the acromion.