TL;DR: Most knee X-ray reports use the Kellgren-Lawrence scale, which runs from grade 0 to grade 4 and describes four features: joint space narrowing, osteophytes (bone spurs), sclerosis (bone hardening) and changes in bone contour. It grades the picture, not the person. Plenty of grade 3 knees walk comfortably and plenty of grade 1 knees hurt a great deal. Films taken standing show the joint space very differently from films taken lying down, which is why the position you were in matters as much as the number.
What the scale actually describes
Cartilage does not show up on a plain X-ray. What an X-ray shows is the gap between the bones, and cartilage loss is inferred from that gap narrowing. Around it, the report looks for three other features: osteophytes at the joint margins, sclerosis where bone has thickened under load, and any change in the shape of the bone ends.
Those four features are what the Kellgren-Lawrence grades combine:
- Grade 0 — none of the features. A normal-looking knee on film.
- Grade 1 — doubtful narrowing of the joint space, with possible early osteophyte formation. This is the grade radiologists disagree about most.
- Grade 2 — definite osteophytes, with possible joint space narrowing. This is usually where a report starts using the word osteoarthritis without hedging.
- Grade 3 — multiple osteophytes, definite joint space narrowing, some sclerosis, and possibly some change in bone contour.
- Grade 4 — large osteophytes, marked joint space narrowing, severe sclerosis and definite deformity of the bone ends. This is the grade people are describing when they say “bone on bone.”
Why the grade and the pain disagree so often
This is the single most useful thing to understand about your report. The correlation between radiographic grade and symptoms is loose in both directions. A knee can look dramatic and function well; a knee can look mild and be genuinely disabling.
The reason is that an X-ray reports structure, and pain is produced by a longer list of things — the inflamed joint lining, the load going through the joint, the strength and endurance of the muscles controlling it, sleep, and how sensitized the system has become over months of hurting. Only one of those items appears on the film.
That matters practically, because the parts that do not appear on the film are the parts you can change. Quadriceps strength is the clearest example — see quadriceps weakness and knee pain.
Standing versus lying down
A knee X-ray taken while you are lying on the table unloads the joint. A weight-bearing film taken while you stand loads it, and the space between the bones can look substantially narrower as a result. Two films of the same knee taken the same afternoon can support different-sounding conclusions purely because of position.
If your report mentions joint space and does not say the film was weight-bearing, it is a fair question to ask. It is also a reason not to compare a film taken at one clinic with a film taken at another without checking how each was taken.
What a grade should and should not change
It should change the realistic goal. A grade 4 knee and a grade 2 knee are not aiming at the same thing, and pretending otherwise sets people up to feel like failures.
It should not decide whether a program is worth doing. Exercise is a strong recommendation across the range in the 2019 ACR and Arthritis Foundation osteoarthritis guideline, and a knee heading toward a replacement does better going into the operation stronger than weaker. The grade informs the plan; it does not cancel it.
It should not be the whole conversation about surgery either. Surgical decisions are made on function, symptoms, examination findings and what you have already tried — not on a number alone.
What we do with your report
Bring it. At a knee evaluation we go through the report line by line, separate the findings likely to be relevant to your symptoms from the ones that are simply common at your age, and use the examination to decide which story fits. Our glossary covers the other terms your report is likely to contain.
Does a high grade mean I need a knee replacement?
No. It means the structural change is advanced. Whether a replacement is the right answer depends on how much the knee is limiting your life, what conservative care has already been tried properly, your general health and what you want to get back to. Wanting a second look at a surgical recommendation is entirely reasonable — see who our program suits.
Can the grade go backward?
The structural changes an X-ray describes are not expected to reverse. Symptoms and function are a different question, and they can and do change. Those are the two things worth measuring, and they are what we re-measure at reassessment rather than repeating the film.
