There are many different indicators to help patients and surgeons make this decision. Pain that is chronic and disabling is the first measure used by many patients. Loss of shoulder motion combined with pain often leads to loss of function.
When you can no longer do everyday activities easily, then it may be time to give a joint replacement a second look. X-rays are helpful to show how much and what kind of joint damage is present. The joint space is measured and the presence of any bone spurs is noted. Condition of the bone (mass and density) is also assessed.
If you haven’t tried conservative care (exercise, physical therapy, antiinflammatories), then a four to six month trial is always recommended first. Your age, general health, and activity level are all considered when making this decision.