What is the structure of the shoulder?

Edward Phillips
Physical Therapy
Although we refer to the shoulder as if it were a single joint, in reality four joints loosely connect several bones. Riding above the rib cage are four bones that form the shoulder girdle: a pair of collarbones (clavicles) at the front, and a pair of triangular shoulder blades or wing bones (scapulae) at the back. The inner end of each collarbone is linked to the breastbone (sternum). The outer end of the collarbone fits into a small joint meeting up with the front edge of the shoulder blade (forming the acromioclavicular, or AC, joint), so that the four bones largely float above the ribs, suspended by several strong muscles and ligaments.

The long bone of the upper arm (humerus) fits into a larger ball-and-socket joint at the shoulder blade. This allows the arm to move freely in many directions, making it possible to serve a tennis ball or crank down the window of your car. Yet it also makes the shoulder joint inherently unstable and easy to injure.

Tendons bridging four small muscles create the rotator cuff. The cuff covers the ball of the shoulder joint and permits you to rotate your arm and stabilizes the joint. Even a basic action, like lifting your arm, requires every part of the shoulder girdle to move in concert and calls into play rotator cuff muscles plus a raft of strong muscles of the shoulders, back, and chest.

The shoulder complex is a multi-joint structure that provides the link between the thoracic cage and upper extremities. The shoulder complex has the highest degree of mobility of any joint in the body and as a result, the shoulder complex is very unstable. Because the bony structures of the shoulder complex provide relatively little support, much of the responsibility for stabilizing this region falls on the muscles, the ligaments, the joint capsules and proper shoulder mechanics.

We will now look a little deeper into the area of the shoulder complex by breaking it down into the following two regions:

THE GLENOHUMERAL JOINT-The glenohumeral joint is a ball-and-socket joint which is the area of articulation between the head of the humerus and the glenoid fossa of the scapula. The glenoid surface is one third to one fourth the size of the humeral head, this produces a minimum area of contact which in turn results in minimal stability. The joint must rely on static and dynamic stabilizers for its stability and its motion. The static stabilizers include the following but are not limited to the glenoid labrum and the glenohumeral ligaments (anterior, posterior, and inferior). The dynamic stabilizers include muscles that directly move the arm and they include the Pectoralis Major, Latissimus Dorsi, Teres Major, Coracobrachialis, the Deltoid consisting of the front, side and rear head and the rotator cuff consisting of the Supraspinatus, Infraspinatus, Subscapularis, and Teres minor. The glenohumeral joint has a large range of motion and great mobility that sacrifices stability.

THE SHOULDER GIRDLE-The shoulder girdle region contains the scapula and the clavicle and is made up of three joints the Acromioclavicular joint (AC joint), Sternoclavicular joint (SC joint), and Scapulothoracic joint which is not a true synovial joint. The Scapulothoracic joint is formed by the articulation of the scapula with the thoracic cage. Any movement of the scapulothoracic joint results in movement of the Acromioclavicular, Sternoclavicular, and Glenohumeral joints. The scapulothoracic joint provides mobility and stability for the orientation of the glenoid fossa and the humeral head for arm movements in all planes. The muscles involved in shoulder girdle movement, mobility and stability include the following but are not limited to the Pectoralis Minor, Serratus anterior, Levator Scapulae, Rhomboid Major and Minor and the Trapezius consisting of the upper, middle and lower fibers.

Rick Olderman
Physical Therapy
The shoulder joint is unique in the human body because it is a floating system of muscles, bones, ligaments, and nerves that must function nearly perfectly to be pain free. Floating? How does something float on our bodies?

If you look at a human skeleton, you'll see that the bones are stacked on top of each other at almost every joint. Joint compression due to gravity is one way your body stays in alignment. But look more closely at the shoulder, and you'll see that its only connection to the rest of the skeleton is via a horizontal collarbone (clavicle) at the front of the rib cage. This joint acts as something of a fulcrum for shoulder function.

How can that tiny clavicle hold up the shoulder? It can't. The rest is just floating in space. What holds the shoulder in position is not joint compression but muscle tone, specifically that of the trapezius, levator scapula, serratus anterior, and rhomboid muscles as well as ligaments.

This is both the beauty and curse of the shoulder joint: It has unparalleled freedom of movement, but on the other hand, it must move precisely to be pain free. Unfortunately, this is a tall order for many people. In order for the shoulder to function, the scapula must work properly, resting and moving like clockwork to keep the shoulder and elbow joints pain free.
Fixing You: Shoulder & Elbow Pain: Self-treatment for rotator cuff strain, shoulder impingement, tennis elbow, golfer's elbow, and other diagnoses.

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Fixing You: Shoulder & Elbow Pain: Self-treatment for rotator cuff strain, shoulder impingement, tennis elbow, golfer's elbow, and other diagnoses.

The shoulders are a floating system on the trunk, held in place and moved by muscular control. The foundation of shoulder movement is the shoulder blade which has precise resting and moving...

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