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 (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.
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.
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.
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 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. - 14130
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.
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.
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.
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 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. - 14130
About the Author:
Jonathan Blood Smyth is the Superintendent of Physiotherapists at an NHS hospital in the South-West of the UK. He writes articles about back pain, neck pain, and injury management. If you are looking for physiotherapists in Manchester visit his website.
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