In Depth: Bones
The shoulder is the meeting point for three major bones in the upper body. Powered by muscles, the bones’ actions make the shoulder joints the most versatile in the body.
More commonly known as the shoulder blade, the scapula is a flat triangular bone located in the upper back. It connects with the collarbone in the front of the body. Several muscles, including the pectoralis minor, bicep, trapezius, deltoid, infraspinatus, and others connect to the scapula via fibrous tendons.
The scapula is capable of numerous movements, including elevating, rotating, tipping, and retracting at multiple angles.
Fracturing a scapula is uncommon because of its sturdy structure and protected location. Should a scapula fracture occur, it is often accompanied by severe chest trauma such as the kind during car accidents.
However, the clavicle, also known as the collarbone, is one of the most commonly broken bones. This “long bone” extends across the front of the shoulder from the sternum to the scapula. It helps stabilize the shoulder’s movement, but can be broken during falls or other high-impact situations.
The humerus is the largest bone of the arm. It connects to the scapula in the shoulder and the radius and ulna in the elbow. The head of the humerus has a ball-like knob on its end that acts as part of a ball-and-socket with the glenoid, a shallow socket in the scapula.
Three joints affect the shoulder movement.
The shallowness of the glenohumeral joint allows the arm to rotate circularly and to move up and out from the body. It is surrounded by soft tissue and strengthened by fibrous ligaments.
Slightly higher, the acromioclavicular (AC) joint forms the top of the shoulder and provides the ability to raise the arm above the head. It is created by the meeting of the top of the scapula and clavicle.
The third joint of the shoulder, the sternoclavicular joint, isn’t in the shoulder. This joint is at the center of the chest where the clavicle meets the sternum. It allows the clavicle to move.
At times of high impact, the head of the humerus can become dislocated from the glenoid and rotator cuff. Although the bone can normally be reset without professional assistance, the joint may be more prone to redislocation without proper medical treatment. Physical therapy is often recommended to prevent re-injury.