The shoulder complex consists of three bones and three separate true joints.
The Bones of the Shoulder
humerus (upper arm bone)
scapula (shoulder blade)
Parts of the scapula (shoulder blade)
glenoid cavity: a part of the scapula; a shallow socket on one end of the scapula.
acromion: a part of the scapula that projects over the top of the shoulder.
coracoid process: a part of the scapula that projects in front of the shoulder.
The Joints of the Shoulder
glenohumeral joint: upper arm and shoulder blade (humerus and glenoid cavity)
acromioclavicular joint: shoulder blade and collarbone (scapula and collarbone)
sternoclavicular joint: collarbone and breastbone (sternum and clavicle)
scapulothoracic joint (not a true joint): shoulder blade and back of rib cage (scapula and thorax)
Glenohumeral Joint (Shoulder Joint)
Often referred to as simply the shoulder joint, the glenohumeral joint connects the upper arm bone to the shoulder blade. It is a ball and socket joint (the head of the upper arm is shaped like a ball which sits in a shallow socket at one end of the shoulder blade). The socket of the shoulder blade (glenoid cavity) is very shallow but has a lip of fibrous tissue (the glenoid labrum) attached to it that makes it deeper and helps stabilize the shoulder joint. The ends of the bones in a joint (in this joint, the head of the humerus and the glenoid) are covered in smooth cartilage (articular cartilage) that helps reduce friction. The glenohumeral joint is the most flexible joint in the body. Movement at this joint allows the arm to be raised and rotated.
The Acromioclavicular Joint connects the collarbone with the shoulder blade.
The Sternoclavicular joint connects the collarbone with the breastbone.
The Scapulothoracic Joint is not a a true joint. The shoulder blade sits on the back of the thorax (rib cage) and moves up and down and rotates with shoulder movement (scapulothoracic motion).
All three joints work together to allow movement of the arm in all directions. When moving the arm out to the side, motion occurs at the glenohumeral joint. As the arm is raised above shoulder level a small amount of movement also occurs at the acromioclavicular joint and the shoulder blade moves up. Finally, a small amount of movement occurs at the sternoclavicular joint and the collarbone tilts up.
The Shoulder Joint Capsule
The shoulder joint capsule encloses the glenohumeral joint. A joint capsule is a sac that encloses a freely movable joint. It is attached to the bones of the joint. It has a tough, fibrous outer membrane and an inner synovial membrane, which produces joint fluid (synovial fluid that lubricates and nourishes the joint). Ligaments reinforce the joint capsule. The shoulder joint capsule is loose enough to allow a wide range of motion – it allows the bones to separate by almost an inch. A joint capsule is also called an articular capsule.
Ligaments, Tendons and Muscles
Ligaments are tough bands of connective tissues that the hold bones of the joint together. Several ligaments hold the shoulder joint together. The ligaments of the shoulder joint (the glenohumeral joint) help limit the amount of movement of the shoulder joints but it is the tendons of the rotator cuff that keep the bones of the shoulder joint in place).
A tendon is a band of tough, fibrous tissue that connects muscle to bone. Tendons are quite inelastic. The biceps tendon that attaches the biceps muscle to the shoulder runs over the top of the humerus and attaches to the top of the glenoid labrum.
Muscles pull on tendons (by contracting) to move bones. Muscles have much more elasticity than tendons. There are several muscles that work together to move and stabilize the shoulder.
The Rotator Cuff
The rotator cuff is a group of four small muscles and their tendons, which envelope the glenohumeral joint. The rotator cuff stabilizes the shoulder joint while in motion (keeps the head of the humerus centered in the glenoid cavity). The rotator cuff muscles also allow the upper arm to move in all directions.
supraspinatus: abducts the arm (moves it up and away from the body)
subscapularis: internally rotates the humerus
infraspinatus: externally rotates the humerus
teres minor: externally rotates the humerus
A bursa is a sac containing a small amount of synovial fluid, located between moving parts of a joint to reduce friction. In the shoulder joint, the subacromial bursa is located between the acromion and joint capsule; the subscapular bursa is located between the subscapularis rotator cuff tendon and the joint capsule.