X-rays aren't used to diagnose problems with soft tissues like tendonitis or a torn rotator cuff. MRIs and arthroscopic exams are better choices. In the case of a cuff tendonitis there are two important signs that guide the doctor's exam.

The first is a positive painful arc test. As the patient lifts the arm out to the side and up over the head, the pain is reproduced during the middle of the movement. This usually occurs somewhere between 60 and 120 degrees of motion.

Injecting a local numbing agent into the subacromial space is also a helpful test. Pain on movement goes away when this test is positive. It indicates that the tendon is getting pinched under the acromion as the arm is raised. The acromion is a curved bone that comes around from the shoulder blade over the top of the shoulder. One of the rotator cuff tendons (the supraspinatus) passes under this bone.

Other helpful tests include palpation and strength testing. Pressing on the painful tendon reproduces painful symptoms. Specific strength tests for each tendon may be weak pointing more to a rotator cuff tear than to tendonitis.

A short course of physical therapy can also help with the diagnosis. Exercises to improve your posture may help take pressure off the tendon and allow it to heal.