Understanding The Shoulder

                                                                                         

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 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.

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.

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.

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.

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.

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.

About the author:
Jonathan Blood Smyth, editor of the Physiotherapy Site, writes articles about Physiotherapists, physiotherapy, physiotherapists in Leeds, 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.

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