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 or scapula is a flat bony structure which is placed over the upper posterior ribs on each side, and its outer ends are modified into the shoulder socket or glenoid cavity. There is a fibrous bag around the shoulder as in all synovial joints, called the capsule and in the shoulder this is less supportive than commonly and is baggy and slack to allow movement. The origin of the rotator cuff muscles is on the scapula and they run laterally from there to insert (stick onto) just lateral to the ball of the joint in an area called the lesser tuberosity.
Above the shoulder joint is an arch of bone made up of two parts, the end of the clavicle or collar bone and part of the scapula known as the acromion process. The junction between these two structures is known as the acromioclavicular joint, a stable, non-moving joint which acts like a stabilising suspension strut in a car, keeping your shoulder out to the side when you are doing something. The acromioclavicular joint is injured moderately often by a direct fall on the hand, elbow or shoulder which can rupture the stabilising ligaments. This is a difficult injury to treat and very painful at the time.
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 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.
Jonathan Blood Smyth, editor of the Physiotherapy Site, writes articles about
Physiotherapists, physiotherapy,
physiotherapists in London, back pain, orthopaedic conditions, neck pain and injury management. Jonathan is a superintendant physiotherapist at an NHS hospital in the South-West of the UK.
