The Shoulder
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 (scapula) is a large, flattened bone which overlies the posterior ribs on both sides of the upper thoracic spine, the outer end of which has been expanded to form the glenoid cavity or socket of the shoulder. All synovial joints have a fibrous bag or capsule surrounding and supporting them, but in the shoulder the capsule is slack and roomy, giving less support but allowing greater degrees of movement. The scapula holds the origin of the rotator cuff muscles on its flat surfaces and they travel outwards to insert just past the ball of the shoulder itself.
Directly above the head of the humerus is a strut of bone made up of two parts, the acromion process which is part of the shoulder blade and the outer end of the collar bone. Where these two bones meet is called the acromioclavicular joint which is a joint without much movement and which stabilises the arm like a suspension arm on a car. This allows the shoulder to cope with force without it falling in towards the centre of the chest. A direct fall on the shoulder, the elbow or the hand can rupture the ligaments of this joint, an extremely painful injury and a difficult one to manage as the joint may not be restored.
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.
In the shoulder girdle the rotator cuff has a series of functions to move and stabilise the region. First the humeral head is centred on the shallow socket by the cuff muscles to allow the major shoulder muscles to move the arm. Secondly it prevents the the ball from sliding off the lower edge of the shoulder socket. Thirdly they perform a degree of the lifting work of the arm and facilitate the rotatory control of the shoulder. Presenting shoulder difficulties include pain and stiffness which usually includes poor control of the scapular complex and pain and increased mobility which is again typically presenting with reduced scapular control.
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.
Scapular mobility around the chest wall and ribs at the back make this bone the mobile, stable base for use of the arms, and it adds its own significant range of movement to the shoulder girdle before thinking about the glenohumeral joint. Losing some of the glenohumeral range and a loss of the ability to stabilise the scapula begin the processes of shoulder problems.
