The MRI below is taken in the frontal view and depicts the acromioclavicular (A/C) joint. Notice the supraspinatus tendon as it passes under the spur at the end of the clavicle. This is the classic supraspinatus impingement due degenerative joint disease.
Below is a side view of shoulder joint without the humerus. The blue dotted lines represent the supraspinatus muscle as it passes under the A/C joint. Notice that the top portion is pushed downward creating friction which causes weakening of the supraspinatus muscle and pain in the shoulder.
Treatment for shoulder conditions such as this can sometimes be completed with soft tissue therapy including active release, passive stretching and specific exercises. However, in some cases surgery is the only treatment that can reduce the impingement.