Your knee X-ray and your knee pain often disagree

How it works

There is an X-ray of my knees in a brown envelope in my desk drawer, and the knee that looks worst on it is the one that does not hurt.

Last winter my left knee began to ache on the stairs down to the cellar. I went to my doctor in Rockland, and she sent me for an X-ray of both knees. An X-ray is a flat black and white picture of bone. Cartilage, the smooth, rubbery coat on the ends of the bones, does not show up on it at all. So the doctor reads the gap between the bones. A wide gap means plenty of cartilage. A narrow gap means it has worn thin.

My left knee, the sore one, looked fair. A small bit of narrowing, she said, about what you would expect at 67. My right knee, the one that felt fine, looked worse. The gap was tighter. It had a couple of bone spurs, which are small lumps of extra bone that grow at the edge of a worn joint. I have hauled that right knee up and down a dock in Maine for forty years, and it has never said a word.

Carol asked the plain question. Which picture is lying? I did what I always do. I went home and read the studies. It turns out neither one is lying. The picture and the pain just measure two different things.

What the X-ray can tell you

In 2008 two researchers at Keele University in England, John Bedson and Peter Croft, pulled the studies on this together. Their review ran in the journal BMC Musculoskeletal Disorders. First they looked at people whose X-rays showed osteoarthritis. That is the common kind of arthritis, where cartilage wears thin and the bone under it starts to change. The share of those people who had knee pain ran from 15 percent… all the way up to 81 percent, study by study.

Then they turned it around. Of people with knee pain, between 15 and 76 percent had arthritis that showed on the film. Those are wide ranges. Part of the spread comes from how each study took its pictures and how it defined pain. But the verdict was plain. An X-ray is a rough guide to who will hurt. And pain is a rough guide to what an X-ray will show.

In other words, lots of worn knees feel fine. And lots of sore knees look clean.

The scan that sees too much

An MRI is a scan that uses a big magnet to take pictures of soft parts, not just bone. It can see the meniscus. That is a C-shaped pad of tough tissue between the thigh bone and the shin bone. Each knee has two. They work like shock pads. A torn meniscus is a common reason people get sent for knee surgery.

In 2008 a team led by Martin Englund wrote up a study in the New England Journal of Medicine. They scanned the right knee of 991 people aged 50 to 90 in Framingham, Massachusetts. These were folks from the town, not patients. Tears were common. They ran from 19 percent of women in their fifties to 56 percent of men aged 70 to 90.

Here is the part that stopped me. Of the people with a torn meniscus, 61 percent had felt no pain, aching, or stiffness in that knee in the past month. And in people whose X-rays showed arthritis, a tear was about as common in knees that hurt as in knees that did not. It was 63 percent in the sore knees… and 60 percent in the quiet ones. So a tear on a scan does not, by itself, prove it is the cause of your pain.

The other half

Now, I promised you both halves. The picture is not useless. In 2009 Tuhina Neogi and her team at Boston University ran a smarter test, and wrote it up in the BMJ. They found people with one knee that hurt and one that did not. Then they put the two knees side by side, inside the same person. That clears out a lot of noise, like mood, weight, sleep, and how tough a person is about pain.

Seen that way, the worse knee on the X-ray was far more often the sore one. In one of the two groups, the most worn knees had odds of frequent pain about 151 times those of knees with no wear. That number is shaky… the range ran from 43 to 526. But even the low end is large. The narrowing of the gap mattered more than the bone spurs did.

So which is it? Both. Inside your own body, a worse knee tends to be the sorer knee. But from one person to the next, the match is loose. Two people with the same film can live two very different lives. My right knee is one of the odd ones… worse on film, quiet on the dock.

Why worn does not always mean sore

Here is one reason the two can part ways. Cartilage has no nerves. It cannot feel a thing. The ache comes from the parts around it that do have nerves. That means the bone under the cartilage, the lining of the joint, and the tough bands that hold the knee together. The lining is called the synovium. When it swells, it can hurt a good deal.

So a knee can lose a lot of cartilage and stay calm, as long as the parts around it stay calm. A knee with less wear can flare up when the lining is angry. Or put another way, the film shows the road. It does not show the traffic.

What I do with this

NICE, the group that writes health guidance for England, goes further. Its 2022 guidance says a doctor can name knee osteoarthritis with no picture at all. That holds if you are 45 or over, the knee hurts when you use it, and any stiff spell in the morning lasts 30 minutes or less. It says not to use scans as a matter of course, unless something looks odd. In other words, how you feel and what you can do count for more than the film.

So I keep the brown envelope, but it does not run my life. The left knee gets Monday's slow chair stands. The right knee gets left alone, since it seems to like that. And if a scan ever shows me a tear, I will ask one thing before anyone talks about surgery. Is this tear the cause of my pain, or has it been there for years?

I may never know for sure… and bear in mind, the scan may not know either.

Here is to a knee that feels better than its X-ray,
Walt

The Health Brief is for education only. It is not medical advice. Always check with your own doctor about your own situation.