The Shoulder Joint

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 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.

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.

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.

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.