The shoulder is a ball and socket joint where the socket is shallow and points to the side. Above the ball is an arch of bone called the acromion and between the ball and the acromion is a fluid filled sack called a bursa which allows smooth movement of the ball under the arch of bone
Video from Shoulderdoc.co.uk
The big muscles that lift the arm up and out to the side also have the effect of lifting the ball up towards the roof of the acromion above it. To prevent this, a special group of muscles called the Rotator Cuff help to keep the ball in the middle of the socket by pulling it down and in to allow a lot of movement to occur in the shoulder
WHAT CAN GO WRONG?
Anything that effects the way the Rotator Cuff muscles work can lead to the head of the humerus (ball) to rise up and catch or impinge against the roof of the Acromion.
This can include:
Strain to the tenons where the cuff muscles attach to the bone
Tear to the tendons
Rotator Cuff Tendonopathy
Poor movement of the shoulder blade
Instability of the shoulder joint due to naturally stretchy ligaments or trauma
The shape of the Acromion has an effect on the size of gap between the top of the humeral head (ball) and the undersurface of the Acromion. Even in healthy, normal shoulders, this gap is only 8-11mm.
There are 3 main shapes of Acromion:
Type 1 - flat
Type 2 - curved
Type 3 - hooked
People with type 2 and 3 have a greater risk of impingement as the gap becomes smaller. However, this does not mean that if you have got a curved or hooked Acromion that you will definitely get Shoulder Impingement
Physiotherapy can help to correct the way the shoulder works and moves to reduce the chance of impingement. It can also help strengthening the Rotator Cuff muscles and improve your shoulder posture
Sometimes a steroid injection is needed to reduce the inflammation in the bursa. This provides some pain relief to allow you to work on Physiotherapy strengthening exercises
Very occasionally surgery is required to make the space bigger. This is key hole surgery but is only used if other techniques have not worked