How The Shoulder Works
This article will help you understand:
The important structures of the shoulder can be divided into several categories.
Bones and Joints
The bones of the shoulder are the humerus (the upper arm bone), the scapula (the shoulder blade), and the clavicle (the collar bone). The roof of the shoulder is formed by a part of the scapula called the acromion.
There are actually four joints that make up the shoulder. The main shoulder joint, called the glenohumeral joint, is formed where the ball of the humerus fits into a shallow socket on the scapula. This shallow socket is called the glenoid.
The acromioclavicular (AC) joint is where the clavicle meets the acromion.
Acromioclavicular and Glenohumarel Joints
The sternoclavicular (SC) joint supports the connection of the arms and shoulders to the main skeleton on the front of the chest.
Sternoclavicular and Scapulothroacic Joints
A false joint is formed where the shoulder blade glides against the thorax (the rib cage). This joint, called the scapulothroacic joint, is important because it requires that the muscles surrounding the shoulder blade work together to keep the socket lined up during shoulder movements.
Articular cartilage is the material that covers the ends of the bones of any joint. Articular cartilage is about one-quarter of an inch thick in most large, weight-bearing joints. It is a bit thinner in joints such as the shoulder, which don't normally support weight. Articular cartilage is white and shiny and has a rubbery consistency. It is slippery, which allows the joint surfaces to slide against one another without causing any damage. The function of articular cartilage is to absorb shock and provide an extremely smooth surface to make motion easier. We have articular cartilage essentially everywhere that two bony surfaces move against one another, or articulate. In the shoulder, articular cartilage covers the end of the humerus and socket area of the glenoid on the scapula.
Ligaments and Tendons
There are several important ligaments in the shoulder. Ligaments are soft tissue structures that connect bones to bones. A joint capsule is a watertight sac that surrounds a joint. In the shoulder, the joint capsule is formed by a group of ligaments that connect the humerus to the glenoid. These ligaments are the main source of stability for the shoulder. They help hold the shoulder in place and keep it from dislocating.
Ligaments attach the clavicle to the acromion in the AC joint. Two ligaments connect the clavicle to the scapula by attaching to the coracoid process, a bony knob that sticks out of the scapula in the front of the shoulder.
A special type of ligament forms a unique structure inside the shoulder called the labrum. The labrum is attached almost completely around the edge of the glenoid. When viewed in cross section, the labrum is wedge-shaped. The shape and the way the labrum is attached create a deeper cup for the glenoid socket. This is important because the glenoid socket is so flat and shallow that the ball of the humerus does not fit tightly. The labrum creates a deeper cup for the ball of the humerus to fit into.
The labrum is also where the biceps tendon attaches to the glenoid. Tendons are much like ligaments, except that tendons attach muscles to bones. Muscles move the bones by pulling on the tendons. The biceps tendon runs from the biceps muscle, across the front of the shoulder, to the glenoid. At the very top of the glenoid, the biceps tendon attaches to the bone and actually becomes part of the labrum. This connection can be a source of problems when the biceps tendon is damaged and pulls away from its attachment to the glenoid.
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Dr. Paul Hendricks