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.
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.
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.
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.
Two significant shoulder difficulties are at least partially prevented by the action of a rotator cuff of normal power. As the arm lifts at the shoulder the head of the humerus is pulled upwards by the deltoid muscle, counteracted by the rotator cuff keeping the ball down and centred on the socket. If not well supported a joint can sublux, a partial dislocation as one side of the joint slides off the other to some degree, and this is also counteracted by the cuff. Complete dislocation always occurs with trauma except in those who have abnormal collagen structure and so highly hypermobile joints.
Scapular mobility around the chest wall and ribs at the back make this bone the mobile, stable base for use of the arms, and it adds its own significant range of movement to the shoulder girdle before thinking about the glenohumeral joint. Losing some of the glenohumeral range and a loss of the ability to stabilise the scapula begin the processes of shoulder problems.
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.
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.
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.
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.
Two significant shoulder difficulties are at least partially prevented by the action of a rotator cuff of normal power. As the arm lifts at the shoulder the head of the humerus is pulled upwards by the deltoid muscle, counteracted by the rotator cuff keeping the ball down and centred on the socket. If not well supported a joint can sublux, a partial dislocation as one side of the joint slides off the other to some degree, and this is also counteracted by the cuff. Complete dislocation always occurs with trauma except in those who have abnormal collagen structure and so highly hypermobile joints.
Scapular mobility around the chest wall and ribs at the back make this bone the mobile, stable base for use of the arms, and it adds its own significant range of movement to the shoulder girdle before thinking about the glenohumeral joint. Losing some of the glenohumeral range and a loss of the ability to stabilise the scapula begin the processes of shoulder problems.
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 Sheffield visit his website.
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