The most common location for shoulder bursitis to occur is in the subacromial space, an area on top of the shoulder that contains the acromio-clavicular joint, coraco-acromial ligament and acromion. This area creates the coracoacromial arch. The subacromial bursa, rotator cuff tendons, and the long head of the biceps tendon pass under this arch. The subacromial bursa is a fluid-filled sac that decreases friction between the rotator cuff muscles and the acromion in the shoulder and protects the rotator cuff from injury when moving under the acromion. If the space becomes narrowed for any reason, rubbing, friction, irritation and inflammation of the bursa can result. As the bursa becomes inflamed, it can thicken, leading to further impingement in the narrow subacromial space and continuing the cycle of pain and inflammation.