Humerus: the upper arm bone
Scapula: the shoulder blade
Clavicle: the collarbone
The scapula, known as the shoulder blade, is an unusual shaped bone and includes:
Glenoid Cavity: 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.
Glenohumeral Joint (Shoulder Joint):
You’ll often hear it 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 the lateral end of the shoulder blade.
The socket of the shoulder blade, the glenoid cavity, is very shallow but has a lip of fibrous tissue. The glenoid labrum that it’s attached to makes it deeper and helps stabilise the shoulder joint. The glenohumeral joint is the most flexible joint in the body. Movement at this joint allows the arm to be raised and rotated simultaneously.
The Acromioclavicular Joint:
The acromioclavicular joint connects the collarbone with the shoulder blade.
The Sternoclavicular joint:
The sternoclavicular joint connects the collarbone with the breastbone.
The Scapulothoracic Joint:
This is not really a joint. The shoulder blade sits on the back of the rib cage and moves up and down and rotates with shoulder movement.
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, movement also takes place at the acromioclavicular joint and the shoulder blade moves up. A little further, and movement happens at the sternoclavicular joint raising the collarbone tilts.
The Shoulder Joint Capsule:
A joint capsule is a pouch 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.
The shoulder joint capsule encloses the glenohumeral joint, and is reinforced by ligaments. The shoulder joint capsule is loose enough to allow a wide range of motion and it allows the bones to separate by about 25 mm.
A joint capsule is also called an articular capsule. The ends of the bones of the heads of the humerus and the glenoid which form the glenohumeral joint are covered in smooth cartilage (aka articular) to reduce friction.
Ligaments are sturdy strips of connecting tissues that the keep bones of the joint together. The shoulder joint uses several. The ligaments of the shoulder joint (aka the glenohumeral joint) help limit the movement of the shoulder joints, but it is the tendons of the rotator cuff that keep the bones of the shoulder joint in position.
Tendons: A tendon is a band of tough, fibrous tissue that join muscles to bones and doesn’t stretch.
Muscles: Muscles contract to pull on tendons to move bones. Muscles have much more elasticity than tendons. The shoulder is stabilised by several muscles.
The Rotator Cuff: The rotator cuff is a group of four small muscles and along with their tendons, envelope the glenohumeral joint. The rotator cuff stabilises the shoulder joint movement, and keeps the head of the humerus lined up with the glenoid cavity. The muscles of the rotator cuff allow the upper arm to move in all directions.
Contraction of the supraspinatus muscle moves the arm up and away from the body at the shoulder joint. It is the main agonist muscle for this movement during the first 30 degrees of its arc.
The Subscapularis rotates the head of the humerus medially (internal rotation). When the arm is raised, it draws the humerus forward and downward. It is a powerful defence to the front of the shoulder joint, preventing displacement of the head of the humerus.
It is an external rotator of the glenohumeral joint and adductor of the arm.
Teres Minor: The Infraspinatus and Teres minor laterally rotate the head of the humerus. They also help hold the humeral head in the glenoid cavity of the scapula.
The Biceps Tendon:
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.
This is a pouch containing a small quantity of lubricant known as synovial fluid. It’s located between moving parts of a joint to reduce friction.
In the shoulder joint, the subacromial bursa is found between the acromion and joint capsule. It’s the acromion that forms the summit of the shoulder, and is a large, somewhat triangular or oblong process, flattened from behind forward, projecting at first outward, and then curving forward and upward, so as to overhang the glenoid cavity. The subscapular bursa is located between the subscapularis rotator cuff tendon and the joint capsule.
The shoulder is a very complex part of our anatomy and often bears the brunt of some punishing treatment we are inclined to inflict on ourselves in the name of sport or fitness.
If you’re experiencing pain in the shoulder and haven’t tried massage therapy make a call today. Arrange an appointment and experience the many benefits that massage has to offer.