The shoulder is the most flexible large joint in the body and enables us to orientate our hand in many different directions, giving us the freedom to manipulate the world around us in a highly flexible manner.
The human hand is amongst the most dextrous of the animal kingdom and so it is paramount to have the ability to deploy this incredible tool in as many different positions as possible. This is what the shoulder enables us to do.
Because it needs to be flexible the shoulder has a unique modified shallow ball and socket structure: the socket is actually a shallow pear shaped cup located on the lateral edge of the shoulder blade, technically known as the glenoid fossa. The ball is the uppermost inward facing part of the humerus (upper arm bone) which is roughly hemispherical.
Because these two joint surfaces do not fit together incredibly well there is a cartilage rim called the glenoid labrum which surrounds the edge of the glenoid fossa (socket) of the scapula. This rim helps to hold the ball of the humerus within the socket of the scapula during movement.
Because the labrum is key for shoulder stability it can be damaged during end of range high energy movements such as pitching and bowling in baseball and cricket.
To maintain the shoulders flexibility the ligaments surrounding the it are amongst the most elastic in the body. Because the ligaments are so stretchy, extra stabilisation of the shoulder joint is provided by a group of muscles known as the rotator cuff. These muscles move the joint but also create a nest of tension very close to the joint itself which provides stability in much the same way of an elastic knee support. These muscles are commonly injured due to their role of shoulder stabilisation. The rotator cuff group includes the muscles supraspinatus, infraspinatus, subscapularis and teres minor but the long head of biceps and triceps also have key stabilisation functions and probably should be loosely considered within this group.
Because of the shoulders flexibility dislocations of the arm (ball part of the joint) are common.
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