The Shoulder Girdle
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.
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.
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.
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 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.
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