This is a common painful condition of the shoulder and affects people in their late thirties up to sixty years of age. Symptoms include pain and weakness when lifting your arm above shoulder height. Weakness can also be associated with an underlying rotator cuff tendon tear.

The pain can be severe enough to cause sleeplessness. It is caused by a prominence on the humeral head called the greater tuberosity rubbing on the undersurface of the acromion. This occurs when the shoulder movements are abnormal due to the rotator cuff muscles not working properly. The rotator cuff tendons insert onto the humeral head and control the movements of the head in the glenoid (socket). They may not function properly due to pain, tendon degeneration or tears and also an inflammation within the subacromial space. The space between the undersurface of the acromion and the rotator cuff is called the subacromial space.
Pain is also caused by abnormal acromion shapes. Some are pointed or hooked and dig into the tendon which can cause tears and make the impingement worse. An injection of steroid and local anaesthetic into the subacromial space usually helps with the diagnosis and can control the pain in order to strengthen the rotator cuff muscles around your shoulder.

An injection is usually given after a rotator cuff tendon tear is ruled out using either an ultrasound or MRI of the shoulder. If the shoulder pain does not respond to this treatment then a "keyhole" or arthroscopic operation is performed to remove the hook of bone and at the same time any other causes of your pain or weakness can be addressed such as a repair of a tear in your rotator cuff tendon. (less info)