The shoulder (glenohumeral joint) is very complicated. It is comprised of three bones, the shoulder blade (scapula), collar bone (clavicle) and upper arm bone (humerus). It is classified as a ball and socket joint (similar to the hip), but lacks the bony stability featured in the hip and other joints. In fact, the shoulder joint is often compared to a golf ball resting on a tee, with the ball representing the head of the humerus and the tee representing the shoulder blade. This depiction accurately shows how the shoulder sacrifices bony stability for exceptional range of motion.
The clavicle provides the shoulder with the only bony attachment to the rest of the body. One end of the clavicle is attached to the breastbone (sternum), forming the sternoclavicular joint (SC joint), while the other end attaches to the scapula at the acromion process, forming the acromioclavicular joint (AC joint).
The scapula serves as a broad, flat origin for the muscles of the rotator cuff as well as an attachment site for other back and shoulder muscles. The spine of the scapula eventually forms the acromion process, where the clavicle attaches. Just underneath (inferior) to the acromion process is the subacromial space, an area where tendons from the rotator cuff cross the shoulder to insert on the humerus. Just below that lies the glenoid fossa, where the humerus articulates.
The humerus attaches to the scapula at the glenoid fossa to make up the glenohumeral joint (GH joint). This is the main joint associated with the shoulder complex. The humerus features a greater and lesser tubercle, which serve as attachments for muscles of the upper arm. The biceps tendon runs between the two tubercles, forming the biceptial groove. The humerus features many muscular attachments, including the four muscles of the rotator cuff, pectorals, deltoid and other upper body muscles. The biceps and triceps originate on the humerus and scapula and attach past the elbow joint.
The shoulder gains stability from three major ligaments (pictured in blue): the superior, middle, and inferior glenohumeral (GH) ligaments. These GH ligaments start on the scapula and attach to the humerus and help to control motion at the shoulder as well as provide stability. Additionally, the shoulder is further stabilized by the labrum (pictured in green), a thin, cartilaginous ring around the glenoid fossa, and the muscles of the rotator cuff. Both the labrum and the rotator cuff will be described in further detail in future blog entries.
Knowledge of the bony anatomy of the shoulder is very important if one wishes to fully understand the shoulder joint and the injuries associated with it. This knowledge allows a person to visualize the shoulder, and how different problems affect the glenohumeral joint.
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