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.
Although the scapula is attached to the upper arm bone or humerus the scapula is not a fixed point as it floats on a bed of muscle over the posterior ribs of the upper back. It has a range of movement of its own which complements the movement at the shoulder, more properly called the glenohumeral joint and with the slackness of the glenohumeral joint this allows placement of the hand in any number of useful positions so we can manipulate objects. The rotator cuff muscles and even the deltoid are relatively small muscles to cope with the forces developed in using the long lever of the arm.
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.
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.
- 0