The CMC joint, short for carpometacarpal joint, is where the base of your thumb meets a small wrist bone called the trapezium. It sits right at the fleshy part of your palm near the wrist, and it’s the joint that gives your thumb its remarkable range of movement. While there are technically five CMC joints in each hand (one for each finger), the term almost always refers to the thumb’s CMC joint, also called the basal joint or trapeziometacarpal joint. This joint is one of the most frequently affected by arthritis in the entire hand.
Where the CMC Joint Sits
If you feel along the base of your thumb toward your wrist, you’ll find a bony prominence where the thumb seems to “plug in” to the hand. That’s the CMC joint. It’s formed by two bones: the trapezium, a small, roughly cube-shaped bone in the wrist, and the first metacarpal, the long bone that runs through the fleshy part of your thumb.
What makes this joint unusual is its shape. Both bone surfaces are curved like a saddle, with two opposing saddles fitting together at right angles. The thumb metacarpal is concave in one direction and convex in the other, while the trapezium curves the opposite way. This interlocking saddle design is rare in the body, and it’s what allows the thumb to move in so many directions while still staying reasonably stable.
What the CMC Joint Does
The saddle shape of the CMC joint allows your thumb to move in ways no other finger can. It swings forward and backward (toward and away from your palm), fans out sideways away from the hand, and most importantly, rotates inward to touch the tips of your other fingers. That last movement, called opposition, is what lets you grip a pen, pinch a zipper, turn a key, or button a shirt.
The joint allows roughly 45 degrees of motion when the thumb swings toward and away from the palm, and about 60 degrees when it fans out sideways. These ranges combine to create a cone-shaped sweep of motion that makes human grip strength and dexterity possible. Nearly every task involving your hand, from opening a jar to texting, routes force through this single joint.
Why the CMC Joint Is Vulnerable
The same design that makes the thumb so versatile also makes the CMC joint prone to wear. Every pinch and grip compresses the two saddle-shaped surfaces together, and over decades, the smooth cartilage lining those surfaces begins to thin. Once enough cartilage wears away, bone contacts bone, causing friction, inflammation, and pain. Bone spurs often develop along the joint margins as the body tries to stabilize the deteriorating surface.
Prior fractures or injuries to the thumb accelerate the process, but most CMC arthritis develops simply from age and cumulative use. The condition is more common in women: females are about 30% more likely to develop it than males at any given age. At age 50, roughly 6% of men and 7% of women already show signs of it on X-rays. By age 80, those numbers climb to 33% of men and 39% of women.
Signs of CMC Joint Arthritis
The hallmark symptom is a deep, aching pain at the base of the thumb that worsens with pinching, gripping, or twisting motions. Opening jars, turning doorknobs, and snapping buttons become progressively harder. Some people notice a bony bump developing at the thumb base, and the joint may feel loose or unstable during heavier tasks.
To check for CMC problems, a clinician will often press and rotate the thumb into the joint while feeling for grinding or pain. This “grind test” has a specificity of 100%, meaning that if it produces the characteristic gritty, painful grinding, the diagnosis is almost certainly correct. However, it misses about a third of cases (sensitivity of 64%), so a normal grind test doesn’t rule arthritis out. A related pressure-shear test is far more sensitive at 99%, catching nearly every case. X-rays confirm the diagnosis by showing joint space narrowing and bone spurs.
How CMC Arthritis Is Staged
Doctors classify CMC arthritis into four stages based on what X-rays reveal. In stage I, the joint space actually appears slightly widened, a sign of early ligament looseness before cartilage loss is visible. Stage II shows mild narrowing with small bone spurs under 2 millimeters. By stage III, the narrowing is pronounced and spurs exceed 2 millimeters. Stage IV means the arthritis has spread beyond the CMC joint into neighboring wrist joints. These stages help guide treatment decisions, though symptoms don’t always match the X-ray appearance perfectly.
Non-Surgical Treatment
Early and moderate CMC arthritis is typically managed without surgery. A thumb stabilizing splint, usually made from neoprene or a lightweight thermoplastic, limits painful motion at the joint while still allowing use of the fingers. Both prefabricated and custom-molded designs are used. Wearing a splint during aggravating activities reduces stress on the worn cartilage and can noticeably improve comfort over weeks of consistent use.
Corticosteroid injections placed directly into the joint can provide meaningful relief. When the injection is accurately placed inside the joint, improvements in pain and function last up to six months. Injections that land outside the joint (which happens more often than you’d expect) still offer some short-term benefit, but that relief fades within about a month. Most people combine splinting, activity modification, and occasional injections to manage symptoms for years before considering surgery.
Surgical Options and Recovery
When conservative treatment no longer controls the pain, surgery becomes an option. The two most common approaches are trapeziectomy (removing the trapezium bone entirely and letting scar tissue fill the space) and joint replacement using a small implant. Both produce significant improvements in pain and function.
Joint replacement tends to score better on patient satisfaction, with about 89% of implant recipients saying they’d have the surgery again compared to 76% for trapeziectomy. Implant recipients also report better function scores and greater pain reduction. The tradeoff is a higher complication rate: around 14% of implant patients need additional surgery within two years, often for a dislocated or shifted implant. Trapeziectomy has fewer reoperations and remains the more established, time-tested procedure. Overall complication rates for both approaches range from 20% to 40%, though many of these are minor.
Recovery after surgery varies by technique. Traditional trapeziectomy typically involves six weeks of immobilization followed by several months of hand therapy. Newer ligament reconstruction methods using stronger suture materials have shortened immobilization to as little as three weeks, with full recovery in about three months. Hand therapy after any CMC surgery focuses on gradually restoring grip strength and thumb mobility so you can return to everyday tasks like cooking, writing, and carrying bags.