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The Shoulder
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The Shoulder

The shoulder is thought of as one single joint, but is actually three joints, with three bones and a group of muscles, tendons and ligaments. The three joints are:

  • Glenohumeral – between the arm bone and the shoulder blade
  • Acromioclavicular – between the collar bone and shoulder blade
  • Sternoclavicular – between the collar bone and the sternum, or breast boneShoulder-Healthy_v1_20141126

The glenohumeral joint is a so-called ‘ball and socket’ joint between the shoulder blade and the arm bone. The socket, or glenoid, lies in the shoulder blade. It is very shallow and looks like a golf tee. The humerus has a ball on the upper end that rolls in the socket. As the socket is not deep, the ball can move freely, and ligaments and a group of tendons known as the ‘rotator cuff’ hold the ball in the socket. This means that the shoulder’s range of motion is much greater than for other joints, but it makes the shoulder more prone to injury. Inside the shoulder joint, there is a cartilage ring called the labrum that deepens the socket and increases stability without restricting movement.

The three bones of the shoulder are:

The main muscles of the shoulder are:

  • Deltoid – attaches the shoulder blade and collarbone to the arm and provides the power for lifting
  • Rotator cuff – the supraspinatus, infraspinatus, teres minor and subscapularis, which stabilise the head of the humerus and help movement
  • Biceps – attaches to the shoulder blade, and flexes the elbow and turns up the forearm

Other muscles around the shoulder include the latissimus dorsii in the back and the pectoralis major in the chest. These large muscles help to turn the arm bone.

Figures

Healthy Shoulder

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