Of course, the symptoms vary depending on your specific knee injury, but things to look out for are: Pain, often when bending or straightening the knee Swelling If you have either of these symptoms, seek medical advice. Together, you can work out the cause. To diagnose what's causing your knee pain, your doctor will need to do a thorough examination. You may need to go to hospital for X-rays, MRIs (magnetic resonance imaging), or MR arthrogram -- a special type of MRI in which a dye is injected into the knee to see more detail of the joint.