What structures are most commonly injured?
The bones of the shoulder region are the sternum (breastbone), the clavicle (collarbone), the scapula (shoulder blade), and the humerus (arm bone). The joint between the sternum and the clavicle is called the sternoclavicular joint. Movement at this joint allows you to shrug your shoulder, pull the shoulder back, or bring it forward.
The tip of the shoulder blade is called the acromion process. This piece of bone juts out over the shoulder and forms a joint with the collarbone - the acromioclavicular (AC) joint. The AC joint supports the shoulder blade and allows it to pivot and rotate. Most of the movement at the shoulder takes place at the joint between the shoulder blade and the humerus.
The scapula forms a shallow cup called the glenoid cavity. The round head of the humerus fits into this cup and forms the glenohumeral joint. This is what is referred to as the shoulder joint. This joint allows a large amount of movement - forward elevation (bringing the arm forward), abduction (bringing the arm out to the side), and rotation (allowing the hand to be rotated inwards 90° to the waist and outward to 90°). Although these movements take place at the glenohumeral joint they are supplemented by movement at other joints.
Arm movement can be compared to a crane where the main movement occurs at the end of a lever. Most shoulder movements involve a fluent combination of movements at all three joints of the shoulder. This can be appreciated by feeling the movement of the joints of the shoulder when the arm is moved.
You can easily feel the bones of the shoulder girdle. The sternum is in the front of the chest just under the skin. It passes from the base of the neck at mid-line down between the breasts. The sternoclavicular joint is at the upper end of the sternum just off mid-line. Movement at this joint can be appreciated if you shrug your shoulder. The collarbone also runs just under the skin for its whole length so it can be easily felt. It is most prominent if the arm is allowed to hang down.
The acromioclavicular joint is not easy to feel. The acromion process is a bony prominence of the shoulder-blade situated at the upper and outer part of the shoulder, just under the skin. If you place two fingers (of the opposite hand) on top of the acromion the joint will be between them. If you now swing the arm forward and back you will appreciate movement (rotation) between the acromion and the collarbone.
The other parts of the shoulder blade that can readily be felt are the the coracoid process, the spine of the scapula, and the tip or distal pole of the shoulder blade. The blade itself is mostly covered with muscle. The coracoid process is the site of origin of muscles and ligaments. If you lay your opposite hand's index finger along the collarbone and press with the long finger you will feel a hard lump of bone which does not move when you rotate the shoulder. This is the coracoid process; if you bend the elbow you can feel the flexor muscles contracting. These muscles can overact and contract hard enough to pull off the coracoid process. This is called an avulsion fracture.
The spine of the scapula can be felt by placing the heel of your opposite hand on the collarbone. Your fingertips will rest on a hard ridge of bone at the back of the shoulder blade with softer tissue (muscle) above and below. This is the spine of the scapula, the boundary between two important muscles which rotate and stabilize the shoulder. Above is the supraspinatus muscle and below the spine is the infraspinatus muscle. They can be felt contracting when you rotate the arm out.
The tip of the scapula can be felt if you put your other hand behind your back and reach up towards the shoulder. Normally you can just touch the lower tip of the opposite shoulder blade. If you now swing your arm out to the side (abduction) you will feel the tip of the blade rotate outwards over the chest wall.
This scapulothoracic motion is an important part of shoulder movement and must be recovered if you are to regain full range of motion after an injury. The upper end of the humerus is covered with thick muscle so it is difficult to feel. If you feel the acromion process and move your finger forwards to the front end of the bone then rotate your arm, you will feel the rotation of the head of the humerus through the muscle. The glenohumeral joint is too deep to feel.
The nerve and blood supply of the arm is closely related to the bony structures of the shoulder girdle. The nerves exit the spine at the neck and join together to form the brachial plexus. The brachial plexus then runs into the upper arm in the arm pit just under the shoulder joint. The brachial plexus then separates into the main nerves of the arm. The main nerves are called the radial, median, ulnar nerves, and together with the musculocutaneous nerves they supply sensation and movement in the hand and arm. Injuries severe enough to cause fractures can also damage these nerves causing severe functional defects. The major artery to the arm, the brachial artery, lies under the collarbone and passes into the armpit region close to the nerves of the brachial plexus. It, too, can be injured in major trauma to this region.
All these structures suffer injuries, tears, fractures, dislocations, and fracture dislocations. In the elderly, fractures of the upper end of the humerus may be the most common fracture of the region but clavicle fractures are also common as are fractures associated with dislocation of the glenohumeral joint.