The Shoulder Girdle

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.

Directly above the head of the humerus is a strut of bone made up of two parts, the acromion process which is part of the shoulder blade and the outer end of the collar bone. Where these two bones meet is called the acromioclavicular joint which is a joint without much movement and which stabilises the arm like a suspension arm on a car. This allows the shoulder to cope with force without it falling in towards the centre of the chest. A direct fall on the shoulder, the elbow or the hand can rupture the ligaments of this joint, an extremely painful injury and a difficult one to manage as the joint may not be restored.

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.

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.

A strong rotator cuff will allow the actions of the shoulder to prevent two major shoulder problems. One is impingement, where the head of the humerus impacts on the underside of the acromion above on lifting the arm, an occurrence which is prevented by the centring and holding down action of the cuff. The second is the tendency for the joint to sublux, which is the term for one joint surface partially sliding off contact with the other, a kind of partial dislocation. Full dislocation does not occur without trauma, except in  people who have highly increased joint mobility who may find their shoulder pops out without much fuss.

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.

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