What Is the CMC Joint? The Hand Joint Explained

What is the CMC joint? It is the place where a long bone of your hand meets a small bone of your wrist, and you have five of them in each hand. As a teacher of the Feldenkrais Method®, I like to start with a picture you can feel, so this page keeps the Latin short and asks what each joint lets your hand do.

What is the CMC joint, in plain words?

CMC stands for carpometacarpal. "Carpal" bones are the small bones of the wrist. "Metacarpal" bones are the five long bones inside your palm. A CMC joint is simply the meeting point of one wrist bone and one palm bone.

You can find them yourself. Run a finger down the back of your hand from a knuckle toward the wrist. Near the wrist, the bone feels broader and then seems to disappear. That is roughly where the palm bones end and the CMC joints begin.

Each hand has five CMC joints, one for the thumb and one for each finger. They are not equal. Two of them barely move, and one moves a great deal.

The five CMC joints and what they do

• Index and middle finger. These two joints are firm and nearly still. They give the palm a steady centre that the fingers can work from.
• Ring finger. This one moves a little more. It starts to let the palm curve.
• Little finger. This joint moves more again. When you cup your hand to hold a glass of water, the little finger side of your palm swings forward. That is partly this joint.
• Thumb. This is the most mobile of all. It is the joint most people mean when they say "the CMC joint".

So the hand is built with a steady middle and a free edge. The still part gives you a base. The moving parts let you shape the hand around an object.

The thumb CMC joint is a saddle

The thumb CMC joint sits between the first palm bone and a small wrist bone called the trapezium. Because of this, you will also hear it called the trapeziometacarpal joint, the basal joint, or just the base of the thumb.

Its shape is the interesting part. Picture a horse saddle. It curves up from front to back and down from side to side. The end of the thumb bone has a matching saddle shape, turned crosswise, and sits on the wrist bone like a rider. The two surfaces fit, and each can slide and tilt on the other.

That shape lets the thumb do several things:

• Move across the palm and back out again.
• Lift away from the palm, the way it opens to wrap around a large glass.
• Turn a little as it moves, so the thumb pad faces the fingers.

Put them together and you get opposition: the thumb pad meeting the pad of another finger. This move is a large part of what makes the human hand so skilled. Threading a needle, holding a pen and pinching a key all depend on it.

Ligaments, which are tough bands that join bone to bone, wrap the joint and keep the rider on the saddle. A loose capsule gives the freedom. The ligaments and the small muscles at the base of the thumb give the control. How snug or loose these feel varies a lot from person to person, and I mention hypermobile thumbs in more detail in our explainer on hypermobility of the thumb.

How the CMC joint works in daily life

Think of the small jobs your thumb did this morning.

• Turning a key or a tap. The thumb pad and finger pad meet, and the CMC joint gives the pinch its direction.
• Holding a phone. The thumb sweeps across the screen. That is a lot of repeated small movement at the base.
• Opening a jar. The thumb braces against the lid while the palm turns. The load is shared by the base joint, the wrist and the forearm.
• Carrying a bag by the handle. The thumb wraps around and presses in. Here the joint is asked to hold, not to move.

Notice that a pinch does not end at the thumb. A small muscle group at the base of the thumb, called the thenar muscles, does much of the work. Longer muscles in the forearm send tendons into the thumb, and the wrist steadies the whole thing. The joint is a meeting point in a long chain, not a lone hinge.

Why the CMC joint is often in the news

Because the thumb CMC joint moves widely and takes the force of every pinch, it is a common place for wear to show up. Doctors call this osteoarthritis, a gradual change in the cartilage that cushions a joint. Thumb base arthritis is one of the most common types of hand arthritis. It is seen more often in women and becomes more common as the years pass. That is a statement about how often it appears in large groups of people, not about any one person.

Worldwide, the hand ranks third among joints affected by osteoarthritis, after the knee and the hip (WHO, 2023). Wear in a joint is not a failure. Many people with changes on an X ray feel little, and many feel more than the picture suggests.

If an ache at the base of your thumb is what brought you here, our guide to thumb joint pain covers what tends to cause it and what may ease it. Here the goal is simply to understand the joint. The Feldypedia article on osteoarthritis and joint discomfort gives the wider picture.

The Feldenkrais® angle: how the whole arm shares a grip

In Feldenkrais work, we do not look at a joint alone. We ask how a whole action is spread through the person. A grip that lives only in the thumb base is a grip with very few helpers.

Here is a small experiment. Pick up a light cup and hold it the way you usually do. Now notice where you feel the effort. In the thumb? The fingers? The forearm? The shoulder? Put the cup down.

Pick it up again, but let the cup be a little heavier in your palm than you thought it needed to be. Let your forearm and elbow take part. Does the thumb need to squeeze quite as much? Nothing here is a test. You are just finding out how many choices your hand has.

Many people notice that a lighter effort in the thumb is possible once the wrist, forearm and shoulder join in. The movements in an Awareness Through Movement® lesson are slow and small so you can sense those choices. If you would like a first gentle step, try the lessons on somatic exercises for beginners. Feldy offers this kind of learning as short audio lessons for home, and you can try them in the 7 day free trial.

Alongside clinical care and other methods

If you have thumb pain or a diagnosis, your doctor, hand therapist or physiotherapist is the right person to guide treatment, including splints. Learning to sense your hand sits alongside that care. It does not replace it.

Other movement methods are different options with a different mechanism. Strength training builds force in chosen muscles. Yoga and hand mobility routines explore range. A Feldenkrais approach adds a quiet way of noticing how effort is shared. Many people enjoy more than one. For ideas shaped around sore thumbs, see our page on exercises for thumbs with arthritis.

Common Questions about what is the CMC joint (FAQ)

Is it safe to move the CMC joint if it feels sore?

Slow, small and easy movement is usually welcome, but stay inside comfort and stop if pain sharpens. Take extra care if the joint is swollen, hot or unstable, or after an injury or surgery. Ask your doctor or a hand therapist first in those cases. A splint, if you were given one, still matters.

How often should I explore my thumb movement?

A few minutes once or twice a day is plenty. You can also add tiny pauses while you use your hands, such as when you hold a phone or open a jar. The movements are light, so a day off is not needed.

How long until my thumb feels different?

Some people notice a lighter grip within one short session, although it may fade by evening. Lasting change in how you use your hand usually needs a few weeks of regular, easy practice. If pain or stiffness has not changed after several weeks, check in with a clinician.

How is the CMC joint different from the other thumb joints?

The thumb has two more joints, the MCP in the middle and the IP at the tip. They mostly bend and straighten, like hinges. The CMC joint at the base is the one that swings freely in two directions and turns a little, which is why it matters so much for pinching and gripping.

When should I see a professional about my thumb base?

Book a visit if the base of the thumb puffs up, feels warm, changes shape, tingles or goes numb, or if pain began after a fall. Also get it checked if pain wakes you at night or keeps growing. A hand therapist can advise on splints and daily use.

Is the CMC joint the same as the basal joint?

Yes, in most conversations. The base of the thumb joint, the basal joint, the thumb CMC and the trapeziometacarpal joint all name the same place. It sits where the first long hand bone rests on a small wrist bone called the trapezium.

Why is the thumb CMC joint so often affected by arthritis?

It moves in many directions and carries a lot of load in every pinch and grip, which is thought to be one reason it shows wear more often than most hand joints. Large surveys find it more common in women and in later life. Having wear in the joint is not a reason to stop using your hand gently.