AC Joint Osteophytosis

Sagittal T1 weighted image of the shoulder (with magnification view) showing an osteophyte projecting inferiorly from the clavicle to impinge the supraspinatus at the musculotendinous junction.
It is easy to see how repeated rubbing of the supraspinatus on the osteophyte (on arm abduction) can lead to degeneration of the tendon. A cause of the so called supraspinatus outlet impingement syndrome.