The shoulder joint is a complex joint consisting of a large ball at the top of the arm bone (the ‘humerus’) and a small shallow socket (the glenoid) which is part of the shoulder blade. Sitting above the ball is a roof formed by a bony part of the shoulder blade at the back (the ‘acromion’) and a ligament at the front. The space in between the ball and the roof is called the sub-acromial space, See picture.
The rotator cuff muscles pass through the sub-acromial space. Their role is to move the arm and keep it sitting in the correct position on the socket as it moves. A fluid filled sac (the ‘bursa’) sits on top of the tendons of the rotator cuff muscles and protects them from the acromion. When the arm is at shoulder height the sub acromial space narrows. When the arm is in positions above and below this, the sub-acromial space is much larger.