The shoulder is a very special joint. It allows a very great degree of movement to occur at the important junction between the torso and the arm. Notionally a ball and socket joint, the shoulder has been modified so this structure is much less clear than in the hip. The top of the arm bone or humerus is expanded into a large rounded knuckle which is like a ball but the socket is different. Unlike the deep hip socket which holds the head of the femur, the shoulder socket is very small in comparison to the head and very shallow.
The shoulder blade or scapula is a flat bony structure which is placed over the upper posterior ribs on each side, and its outer ends are modified into the shoulder socket or glenoid cavity. There is a fibrous bag around the shoulder as in all synovial joints, called the capsule and in the shoulder this is less supportive than commonly and is baggy and slack to allow movement. The origin of the rotator cuff muscles is on the scapula and they run laterally from there to insert (stick onto) just lateral to the ball of the joint in an area called the lesser tuberosity.
The end of the shoulder blade, a bony process called the acromion, joins the lateral end of the clavicle to form the acromioclavicular joint, a bony structure which lies immediately above the humeral head. The acromioclavicular joint is a stability joint a little like a car suspension strut, holding the shoulder away from the chest when forces are being taken by it. The acromioclavicular joint can be injured by a fall on the hand, shoulder or elbow such as in sport or skiing, leading to a very painful injury which is difficult to treat and which often cannot be restored to the original stability of the joint.
While the arm bone is attached by the capsule and the supporting muscles to the scapula it is important to realise that the scapula is not a fixed point and is not attached to but lies over the upper ribs at the back. The glenohumeral joint is the proper name for the shoulder, and its range of movement is enhanced by scapular movements which allow us to place our hands in a huge variety of positions so we can perform object manipulation. The deltoid and the rotator cuff muscles seem to have insufficient bulk to manage to the forces which use of the long lever of the arm can generate.
The rotator cuff has several functions in the movement and stability of the arm. Firstly it compresses the ball of the humeral head against the glenoid socket whilst the larger deltoid muscle exerts its force, centring the ball on the socket. Secondly it stops the arm bone from dragging the ball down, allowing a loss of contact of the joint. Thirdly they rotate the joint and add to the lifting forces for the arm. Shoulder problems roughly divide into those related to stiffness and/or pain at the shoulder which are typically accompanied by poor scapular control, and similar problems which relate to abnormal mobility at the glenohumeral joint and again likely poor scapular control.
If the rotator cuff is of sufficient strength it will help reduce the chance of suffering from a couple of shoulder problems. Lifting the arm above the head pulls the ball of the arm bone upwards towards the acromion and can cause impingement, which is prevented by the cuff muscles pulling the ball down and keeping it centred on the small socket. Subluxation of the joint, a part dislocation where one surface slips off the other to a degree, is also guarded against by the rotator cuff. Trauma is always necessary for full dislocation unless the person has abnormal collagen and so abnormal joint mobility.
The scapula is mobile around the ribs and back of the thorax, adding some considerable range of movement to the shoulder before we even consider the large movement capabilities of the glenohumeral joint itself. Shoulder problems develop as the joint loses some of its mobility and the scapula is less well stabilised, allowing a biomechanical imbalance to develop.