The shoulder is a specialised joint which has an extreme range of motion at the connection between the upper limb and the trunk. The joint classification of the shoulder is as one of the ball and socket joints but this structure is much clearer in the hip than the shoulder. The humeral head, the upper end of the arm bone, is a large rounded ball-like structure with some obvious relationship to the ball of the hip. The shoulder socket however is quite different from the hip in that the joint surface is very flat and small compared to the head.
The scapula or shoulder blade is a large flat piece of bone that lies over the ribs each side of the upper back, and the outer ends of the scapula are formed into the glenoid or shoulder socket. The capsule of the shoulder joint, the fibrous bag which surrounds the joint, and supports it strongly in many joints, is particularly lax and baggy in the shoulder to allow a large range of motion. The rotator cuff muscles start (have their origin) from the flat plate of the scapula and pass across the humeral head to stick on (insert) to the upper outer part of the head of the humerus.
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.
The stabilising muscles and the joint capsule join the arm bone to the scapula but we must recognise that the scapula itself is not fixed to the thorax but lies over the ribs with only a muscular attachment to the trunk. The shoulder is more precisely called the glenohumeral joint and the movements which the shoulder blade is able to perform add to the already considerable movements of the glenohumeral joint. This permits us to place our arms and thereby our hands, the tools we use to manipulate objects, in a huge range of positions. The arm is a long lever and develops significant forces in use and its muscles do not seem particularly large.
There are several functions which the rotator cuff performs in the shoulder girdle. Firstly the cuff centres the large ball on the small socket by compression while the bigger shoulder muscles exert the power to move the arm. Secondly the cuff holds the ball up and stops it sagging down towards the edge of the small socket. Thirdly the cuff performs a degree of lifting of the arm and rotates it when required. Shoulder pathology may be related to stiffness and pain, usually with poor scapular control, or to increased mobility and pain with similar problems with scapular control. Pain and loss of movement is the commonest presentation.
A strong rotator cuff will allow the actions of the shoulder to prevent two major shoulder problems. One is impingement, where the head of the humerus impacts on the underside of the acromion above on lifting the arm, an occurrence which is prevented by the centring and holding down action of the cuff. The second is the tendency for the joint to sublux, which is the term for one joint surface partially sliding off contact with the other, a kind of partial dislocation. Full dislocation does not occur without trauma, except in people who have highly increased joint mobility who may find their shoulder pops out without much fuss.
The scapula moves around on the posterior chest wall and is the mobile base of support for the upper limbs, contributing significant mobility by itself before we start thinking about the large range of movement of the glenohumeral joint. Loss of shoulder power and movement begin to occur with shoulder joint stiffness and loss of scapular stability.