The shoulder joint is a ball and socket joint situated at the top end of the arm bone. There is a mismatch between the ball and the socket where the ball is much larger than the small saucer shaped socket. The advantage of this is that the shoulder is the most mobile joint in the body. This is in contrast to the hip joint which is also a ball and socket joint. The hip joint is an enclosed joint, which has a limited arc of motion as compared to the shoulder joint. The downside of having such a mobile shoulder joint is that it is not very stable. Hence dislocations of the shoulder are more common than hip dislocations.


The very mobile shoulder joint is made stable by a number of factors. It has a sac around it known as the capsule, which has a number of thickenings called ligaments. The shallow socket is deepened by a thick lip of cartilage around the border known as the labrum. Often when the shoulder dislocates the ligaments, labrum and the capsule can be damaged.


Above the shoulder is the collarbone. The collarbone forms a joint with a flat bone known as the acromion, which is part of the shoulder blade. This arch of the collarbone, the acromion and the joint between the two (acromioclavicular joint) form an arch above the ball and socket shoulder joint. The space in between is known as the subacromial space. This space is not vacant but is filled with a small sac of fluid known as a bursa and four tendons. These tendons join together and form a cuff over the ball of the shoulder joint. They are responsible for movement of the arm, particularly rotation and are therefore known as the rotator cuff. As the rotator cuff functions it moves the arm up and down, inwards and outwards underneath the coracoacromial arch. This movement is made smooth by the bursa.


The ball and socket are covered by soft, smooth cartilage. This is essential for easy, pain free movement of the shoulder joint.


The animations below demonstrate the normal range of motion for the shoulder and the name of each range of motion. You may find these terms useful when discussing you shoulder problem with Mr Kulkarni or his physiotherapists.


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