They Said the Joint Has Worn Down
The short answer
“Your cartilage has worn down a lot” — so why is it that some people whose X-rays look bad feel no pain, while others whose X-rays look fine have been hurting for years? Cartilage has no pain nerves, so the tissue an X-ray measures and the tissue that generates pain are not the same. That sentence only tells you about the cartilage. If a joint swells and grows hot overnight, or chills come with it, see a doctor first.
Contents
You get your knee X-rayed and walk out holding a sheet of paper. There is a black-and-white image with bones on it, and a note on how much the space between the bones has narrowed. And you hear one sentence: "Your cartilage has worn down a lot."
That sentence outlasts the knee. Standing in front of a flight of stairs, stepping onto a bus — the words come back before the knee even starts to ache.
But for many people, this sentence and what they actually experience do not line up well.
How much it has worn down and how much it hurts do not move together
Some people have clear arthritis on the X-ray yet find their knee not particularly bothersome. Others have an X-ray that is not so bad yet have been in pain for a long time.
That the range is this wide is itself the answer. It means that knowing one side tells you little about the other.
Pain does not grow in step with how much worse the X-ray looks, and an unchanged X-ray does not mean unchanged pain.
Pain comes from what is next to the cartilage
Why does this mismatch happen? Let me break down what is inside the knee.
Cartilage contains no nerves that sense pain. It is the tissue that absorbs the compressive force between the two bones, and doing that job does not require nerves.
So where does the pain come from? From the structures that surround the cartilage. The synovium, which lines the inside of the joint and secretes the fluid that keeps it slick; the bone just beneath the cartilage and the membrane that wraps it; the ligaments and joint capsule that hold the joint together; the meniscus sitting in between; and the muscles and tendons that move the joint. These contain the nerves that carry the pain signal.
These tissues do a different job. They have to sense how much the joint is being compressed right now, how far it has stretched, whether there is inflammation there. Sensing that requires nerves.
So the tissue that the X-ray measures and the tissue that generates the pain are not the same tissue.
What is more, an X-ray cannot even photograph cartilage directly. Cartilage does not show up on an X-ray, so the image infers how much remains by looking at how wide the space between the two bones is. What the X-ray measures is that gap. It does not capture the condition of the tissues next to it.
Still, wear is not nothing
When cartilage thins, the joint has less room to distribute the load. The force does not disappear — it moves sideways. It can be redistributed to the bone under the cartilage, to the ligaments, to the muscles holding the joint together. And those places do have nerves.
When the space between the two bones narrows, the amount of contact within the joint also increases. The nerve serving that joint responds more readily to the same stimulus. The walking has not changed, but what receives it has.
This actually shows up in practice. In a study that examined 401 knees by MRI, the proportion showing changes in the bone just under the cartilage was 78 percent in painful knees and 30 percent in pain-free knees. It was the bone beneath the cartilage, not the cartilage itself, that tracked with the pain.
This is not to say wear and pain are disconnected. They are connected one step removed. And where that one step leads differs from person to person.
That sentence only tells you about the cartilage
How much the cartilage has worn down is something an X-ray reads well. For measuring that gap, an X-ray is reliable, and that assessment belongs to the X-ray.
But the question the X-ray answers and why it hurts this much today are two different questions. You probably had the same kind of X-ray taken last year too, and if that time and this time feel different, it is hard to explain what changed in between by cartilage alone.
The knee is a joint where compressive force tends to concentrate. The muscle wrapping the joint is thin, so it leans heavily on the ligaments, while the amount of use is high. When weight is loaded onto a deeply bent knee, the pressure inside the joint rises, and if that pressure continues long enough, the cartilage wears down that much more. Wear itself also comes out of conditions.
These circumstances move independently of the gap noted on the X-ray. So it is not right to narrow pain down to muscle and tendon alone, nor to pin it all on the degree of wear. This rests on the idea that pain comes from conditions more than from tissue.
When to see a doctor first
If, without any injury, one knee alone swells overnight, feels hot to the touch, comes with chills, or locks at a certain angle so it will not straighten — this may be a different problem than the one discussed here. In that case, see a doctor first.
Hearing "the joint has worn down" tends to send people down one of two paths. Either they conclude this is it, it's over for me and give up, or they think that's supposedly nothing much and brush it off.
Both paths hand that one sentence more than it can carry. That sentence tells you how far the cartilage has gotten. That is as far as it goes.
Why it hurts today lives next door to that.
Written by Dr. Heo Ji-young (PhD in Korean Medicine Pathology, Kyung Hee University · former Research Professor of Herbology, Kyung Hee University)
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