Over the top of the shoulder joint is an arch made up of bone and ligament- this arch is known as the coracoacromial arch. Underneath this arch runs the rotator cuff tendon on its way to attaching to the humerus. Normally, there is very smooth movement between the arch and rotator cuff, which is facilitated by a fluid filled sac known as the subacromial bursa. As the arm and rotator cuff are moved underneath the arch, the fluid sac rolls around between the two and provides for a very low friction movement interface. If there is a problem with any of these structures, then the surfaces can start rubbing on one another. For example, if the tendon is swollen because it has been injured (tendinosis) or partially torn, it will rub underneath the bone and ligament arch. This can cause pain, and is known as “IMPINGEMENT”, or “BURSITIS”.
Classically, impingement pain is worst when the arm is brought into an overhead position. This is because the swollen injured tendon is being jammed up against the coracoacromial arch, and hence causing pain.
A common primary cause for the impingement is degeneration or tendinosis in the rotator cuff which causes acute swelling in the cuff which then contributes to the impingement. With time, the tendinosis and swelling settles down and the impingement pain then goes away. However, during this time when the patient has impingement pain, there are a number of measures that can help to maintain motion and improve the pain in the shoulder. These measures include activity modification, analgesics, and sometimes cortisone injections. A graded physiotherapy program will also help, by restablishing good tone and control of the rotator cuff muscles, which will then help to achieve better control the humeral head and lower the chance of impingement.
Most cases of shoulder impingement respond to these non-operative measures. Sometimes, a patient will continue to experience impingement pain. Fixing the cause of the impingement pain may solve this problem- e.g. fixing a rotator cuff tear. Sometimes however, the patient may only have rotator cuff tendinosis that is contributing to the impingement. In this scenario, the patient may benefit from having more room made above the swollen tendon. This is known as a subacromial decompression, and involves removing any spurs or prominences under the arch to make more room for the rotator cuff and lessen the pain of impingement. This procedure can be done arthroscopically, or with "minimally invasive" surgery.