- The subacromial bursa separates the shoulder capsule from the coracoacromial ligament above.  The subacromial bursa does not communicate with the joint.  The tendon of supraspinatus lies in the floor of the bursa.  Inflammation of the bursa due to adjacent inflammation of the supraspinatus tendon caused by impingement of the supraspinatus tendon on the coracoacromial ligament gives rise to severe pain and limitation of shoulder abduction (classically between 60 degrees and 120 degrees) known as the painful arc syndrome.

Reference:
1. Anatomy at a Glance. Omar Faiz and David Moffat.  Blackwell Science. 2002.