The shoulder is an extremely complex joint, designed to provide maximum mobility and 3 dimensional range of motion, more than any other joint in the body.
Its function includes strong power to lift heavy weights even at arm’s length (which requires much more force at the end of a long “lever arm”, than close to the body); the shoulder joint is also responsible for getting the hand in the right position in space for whole range of functions.
When you consider all the different functions and positions we use our hands in, every day, it is easy to understand how hard daily life can be when the shoulder isn’t working well.
The shoulder complex is formed of 3 bones, the scapula, the humerus and the clavicle. There are 2 joints linking the bones.
a) The glenohumeral joint linking the scapula and the humerus.
b) The acromioclavicular joint linking the scapula and clavicle.
The shoulder joint is the most mobile in the body. It allows the arm to be positioned in a range greater than a hemisphere and can be rotated at any point in the hemisphere.
The glenohumeral joint itself is a ball and socket joint, the spherical head of the humerus sits into the concave socket of the glenoid.
The bony glenoid is thickened by a rim of cartilage called the glenoid labrum to increase the stability of the joint.
RC GLENOID LABRUM
The joint is covered by a thick joint capsule which is enforced by several ligaments.
Overlying this is a layer of small muscles called the rotator cuff, composed of – supraspinatus, infraspinatus, subscapularis and terres minor. They form a “cuff” anchoring the humerus into the glenoid socket and although anatomically small, play a vital role.
Overlying these muscles are the larger power muscles which move the arm once the rotator cuff muscles have contracted and anchored the joint. They are the biceps (flexes the elbow and shoulder), triceps (extends the elbow and shoulder), pectoralis major ( adducts the arm and moves the body on a fixed arm eg press ups), latissimus dorsi ( extends the arm and raises the body on a fixed arm eg pull ups) and deltoid (abducts the arm).
Sub-Acromial / Sub- Deltoid Bursa
Sandwiched between the rotator cuff muscles and the outer layer of large bulky shoulder muscles are structures known as bursae.
They are present in many parts of the body. They are found wherever two body parts move against one another and there is no joint to reduce the friction. A single bursa is simply a sac between two moving surfaces that contains a small amount of lubricating fluid, which allows the movement to occur with less friction.
There are many instances however when a bursa can become inflamed; this may cause pain, tenderness or the sac to become distended with excessive fluid. The condition is known as a bursitis.