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 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.
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.
The stabilising muscles and the joint capsule join the arm bone to the scapula but we must recognise that the scapula itself is not fixed to the thorax but lies over the ribs with only a muscular attachment to the trunk. The shoulder is more precisely called the glenohumeral joint and the movements which the shoulder blade is able to perform add to the already considerable movements of the glenohumeral joint. This permits us to place our arms and thereby our hands, the tools we use to manipulate objects, in a huge range of positions. The arm is a long lever and develops significant forces in use and its muscles do not seem particularly large.
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.
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.
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.
The stabilising muscles and the joint capsule join the arm bone to the scapula but we must recognise that the scapula itself is not fixed to the thorax but lies over the ribs with only a muscular attachment to the trunk. The shoulder is more precisely called the glenohumeral joint and the movements which the shoulder blade is able to perform add to the already considerable movements of the glenohumeral joint. This permits us to place our arms and thereby our hands, the tools we use to manipulate objects, in a huge range of positions. The arm is a long lever and develops significant forces in use and its muscles do not seem particularly large.
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.
About the Author:
Jonathan Blood Smyth, editor of the Physiotherapy Site, writes articles about Physiotherapy, back pain, orthopaedic conditions, neck pain, injury management and physiotherapists in Brighton. Jonathan is a superintendant physiotherapist at an NHS hospital in the South-West of the UK