Briefly, X-rays show bony structures of the knee, but do not give direct information about the soft tissue structures inside the knee such as cartilage and ligaments. However, indirect information gained from X Rays (such as narrowed joint space or bone spur formation) can give us more than enough information to make judgements about the condition of the soft tissues. In these cases, MRI's aren't usually that helpful. However, when the X Rays are essentially normal, and symptoms and your exam point to a soft tissue injury, MRIs can be extremely valuable. Again, while the correct answer is multifactorial, generalizations can be made about who would benefit from an MRI and who may not.
MIDDLE AGE GOLFER: "I did something to my knee and now it hurts. Shouldn't I get an MRI?"
ANSWER: While there are several factors that must be considered, the quickest answer is… An MRI probably won’t be helpful, at least not at first. Even if the MRI shows a “degenerative meniscus tear,” “cartilage defects,” or other unpleasant sounding things, the best care for most common knee pains in this population is a course of regular anti-inflammatory use, physical therapy, +/- steroid injections. Now, if those methods have not yielded good results, and x-rays don’t show arthritis, then an MRI makes a lot of sense because you and your doctor may use that information to make decisions about your course of treatment.
HIGH SCHOOL SOCCER PLAYER: "I twisted my knee and now it’s really swollen and hurts. Shouldn’t I get an MRI?"
ANSWER: For young people who shouldn’t have much degeneration inside their knees, MRI’s frequently make more sense. The reason for this is that the pathologies found in this age group have a higher likelihood of requiring intervention to prevent further damage. Here are the most common reason to order an MRI of the knee after an injury
1) Persistent symptoms despite ibuprofen and physical therapy
2) Swelling and pain after a pivoting injury
3) Painful clicking or buckling of the knee
4) Sudden or intermittent inability to fully straighten your knee
5) Feelings of instability and slipping of the knee