Being told the X-ray is normal when the knee clearly hurts is one of the more frustrating experiences in orthopedics. It is not a dismissal, and it is not a dead end. It narrows the diagnosis rather than closing it.
What an X-ray can and cannot see
An X-ray shows bone. It shows joint space narrowing, which is an indirect measure of cartilage loss, along with osteophytes, sclerosis, alignment and fractures.
It does not show cartilage directly, nor menisci, ligaments, tendons, bursae or the joint lining. A knee can hurt a great deal from any of those with a completely normal radiograph.
The commonest causes of a painful knee with a normal X-ray
Patellofemoral pain is the most frequent, particularly in younger and middle aged patients. Pain at the front of the knee, worse going downstairs, worse after sitting, with a normal X-ray, is a recognisable pattern.
Meniscal tears often have normal radiographs, especially in younger patients. Catching, locking, a sharp pain on twisting and joint line tenderness point this way.
Tendon problems around the knee, including patellar and quadriceps tendinopathy, and iliotibial band irritation on the outer side, are clinical diagnoses with normal films. Early cartilage damage and bone marrow oedema are invisible on X-ray but visible on MRI.
Pain referred from elsewhere
Hip disease commonly refers pain to the knee, and a patient with knee pain and a completely normal knee examination should always have the hip examined. The same applies to the lumbar spine, where a nerve root problem can present as knee pain with normal knee imaging.
Why a weight-bearing X-ray matters
A knee X-ray taken lying down can look considerably better than the same knee taken standing. Joint space narrowing often appears only when the joint is loaded.
If you have been told your X-ray is normal, it is reasonable to ask whether it was taken standing, and whether a skyline view of the kneecap was included. Both change what is visible.
When MRI is genuinely useful
MRI is the right next step when mechanical symptoms suggest a meniscal tear or loose body, when there is a history of significant injury, when pain is disproportionate and unexplained, or when the clinical picture does not fit the plain films.
It is not a routine screening test. A substantial number of people without any knee pain have meniscal changes on MRI, so a scan read without a clinical examination can produce the wrong operation.
A detailed guide on this situation is available here: https://www.mayurajcc.com/knee-pain-guides/knee-pain-with-normal-x-ray
What to do next
Ask for a proper clinical examination of the knee, the hip and the back, confirm that the X-ray was weight-bearing, and expect the diagnosis to come from the examination with imaging used to confirm it. A normal X-ray means the bones are fine, not that the knee is.