A light pinch puts twelve times the load on your thumb base

How it works

There is a hard bump at the base of my right thumb, just where it meets the wrist, and I have been staring at it all week.

It showed up some time in the last year. It is a bit bigger than a pea, and it aches when I turn a key. Last week a jar of Carol’s pickles beat me because of it. I could not twist the lid, and the ache ran up into my wrist. So I went and read about the joint under the bump. I like to know how a thing works before I try to fix it.

Here is the odd part. My thumb does not hurt at the tip, where I press. It hurts down at the base, a good two inches away, where nothing seems to touch. That joint has a name. Doctors call it the CMC joint, short for carpometacarpal. In plain words, it is where the long bone of the thumb meets a small bone of the wrist.

Some of you know this ache well. Mayo Clinic lists the jobs that set it off: turning a key, opening a door, writing, and holding a fork. Women get it more than men, and it tends to start after 40. I am a man of 67, so I came late to this party... but I got here.

A saddle with a rider

The thumb sits on a joint unlike any other in the hand. The small wrist bone at its base is shaped like a saddle. It is called the trapezium. The end of the thumb bone sits on top of it, like a rider. A rider can lean forward, back, and side to side, and so can your thumb. That is how it swings across the palm to touch each fingertip. No finger can do that.

But a saddle is not a socket. Your hip is a ball in a deep cup, held in place by the shape of the bones. The base of the thumb is held mostly by ligaments, the short, tough straps that tie one bone to the next. In other words, the joint trades hold for range. It can move in more ways than any knuckle, and it pays for that with less built-in grip.

Mayo Clinic lists loose ligaments and an old thumb injury among the risk factors, the things that raise your odds. That fits the picture. If the straps go slack, the rider can slide toward the edge of the saddle. Then the load lands on a smaller patch of cartilage, the smooth pad on the ends of the bones. How much thumb arthritis starts this way is still argued over. As for my own hand... I have no idea which came first.

Why a light pinch is a heavy load

Now the number that stopped me. In 1977, two researchers, William Cooney and Edmund Chao, worked out the forces inside the thumb. Their paper ran in The Journal of Bone and Joint Surgery. They used five hands that people had left to science, and they mapped the pull of each tendon. Then they did the math for a plain pinch, the kind you use to hold a key.

Here is the result. Press with a little over 2 pounds at the tip of the thumb, and the joint at the base takes about 26 pounds. That is twelve times the load... The tip feels light. The base does the heavy work.

Why so much? It comes down to levers. The muscles that work the thumb pull close to the joint, on a short lever. The tip is far out, on a long one. So the muscles must pull hard to make even a soft pinch, and all that pull squeezes the base. Or put another way, the joint pays for the reach of the thumb.

Now, what that number is and what it is not. It came from a model built on hands that were held still. A live hand twists and moves, and no one put a gauge inside a living thumb. Later models found that some grips load the base even more, such as a full grasp around a handle. So I take the twelve as a rough guide, not a law. But the main point holds in every version I found. The base of the thumb carries far more than you press with.

How common it is

So it is no shock that this joint wears. In the Rotterdam Study, Dutch researchers took X-rays of the hands of 3,906 people aged 55 and up. More than a third of them, 35.8 percent, showed arthritis at the base of the thumb. Only the knuckles nearest the nails showed it more often.

But an X-ray is not a pain score. In that same study, the link between what the films showed and how much hands hurt was modest to weak. Plenty of people had worn joints and no complaint.

Pain is a different count. The Framingham Study is a long-running health study in a Massachusetts town. Its researchers looked at 1,041 people aged 71 to 100. They counted a hand joint only if it hurt and also showed arthritis on an X-ray. By that bar, 26.2 percent of the women had it, and 13.4 percent of the men. Their grip was 10 percent weaker. They had more trouble writing and handling small things, and more trouble carrying a 10-pound bundle.

What I do with this

Here is how I use it. If the load at the base rises with the force at the tip, then less force at the tip means less load at the base. That is arithmetic, not a trial. I have not found a study that shows it cuts pain. But it costs little, and I can see no harm in it.

So I have made a few swaps. I keep a rubber pad in the kitchen drawer for jar lids, so I can squeeze less and still turn them. I bought a key turner, a fat plastic grip that slips over the key and gives me a longer lever. I write with a thick pen now. Each one asks less force of the thumb tip... and so, by the math, less of the base.

Carol thinks this is a lot of fuss over a jar. She may be right... But I spent thirty years learning that small loads, done again and again, add up. A pinch is a small load. I make a great many of them each day. Will the swaps ease my thumb? We’ll see.

Here is to a lighter pinch,
Walt

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