The CMC (carpometacarpal) joint is where the bones of your fingers or thumb meet the small bones of your wrist. There are five CMC joints in each hand, but when people talk about “the CMC joint,” they almost always mean the one at the base of the thumb. This joint sits between the trapezium (a wedge-shaped wrist bone) and the first metacarpal (the long bone running through the fleshy part of your thumb). It’s one of the most mobile and most heavily used joints in your hand, and it’s a common source of pain as people age.
Where the CMC Joint Sits
If you press into the meaty area where your thumb meets your wrist, you’re right on top of the first CMC joint. It’s formed by two bones: the trapezium on the wrist side and the base of the first metacarpal on the thumb side. The joint surfaces are shaped like two saddles interlocking at right angles, which is why anatomists call it a “saddle joint.” That shape is what gives the thumb its extraordinary range of movement compared to the other fingers.
The four CMC joints of the fingers (index through pinky) are far less mobile. They sit in a tight row where the second through fifth metacarpals connect to the wrist bones, and their main job is providing a stable platform for your palm. The thumb CMC joint is different. It sacrifices some stability for the freedom of movement that lets you grip, pinch, and oppose your thumb to each fingertip.
How the Thumb CMC Joint Moves
The saddle shape allows movement in multiple directions. Your thumb can swing away from your palm (abduction), pull back toward your index finger (adduction), flex forward, extend backward, and rotate inward to touch each fingertip. That last motion, called opposition, is what makes human hands so dexterous. It’s the reason you can button a shirt, turn a key, or unscrew a jar lid.
In terms of actual range, the thumb CMC joint typically allows about 61 degrees of palmar abduction (swinging the thumb out perpendicular to your palm) and roughly 63 degrees of radial abduction (fanning the thumb away from the index finger in the plane of your palm). Adduction, pulling the thumb tight against the hand, covers a smaller range of about 10 degrees. These numbers vary from person to person, but they give a sense of how much work this small joint does every day.
Why the CMC Joint Is Prone to Problems
The thumb generates a surprising amount of force. During a simple pinch grip, the compressive load at the CMC joint can reach several times the force applied at your fingertips. Over decades, that repeated stress wears down cartilage and loosens the ligaments that hold the joint together. The result is osteoarthritis at the base of the thumb, one of the most common forms of hand arthritis.
A large population study (the Rotterdam Study) found that about 25% of participants had radiographic signs of thumb CMC osteoarthritis. Women are roughly twice as likely to develop it as men, and the risk climbs with every year of age. Hormonal changes after menopause likely play a role, though repetitive hand use over a lifetime is also a major contributor.
What CMC Arthritis Feels Like
The hallmark symptom is a deep, aching pain right at the base of the thumb, especially during gripping, pinching, or twisting motions. Opening jars, turning doorknobs, and snapping buttons often become painful early on. As the condition progresses, you may notice weakness in your grip, a grinding sensation when you move the thumb, and visible swelling or a bony bump at the joint.
Doctors sometimes use a “grind test” during a physical exam, compressing and rotating the thumb into the joint to see if it reproduces pain or a gritty feeling. While helpful, this test has limited sensitivity (catching only about 42% to 53% of confirmed cases depending on the examiner), so X-rays are the standard way to confirm the diagnosis and determine how far the arthritis has progressed.
Stages of CMC Joint Arthritis
Radiographic severity is commonly graded using a four-stage system. In Stage I, the joint may actually look wider than normal on X-ray because of swelling or ligament looseness, but there’s no cartilage loss yet. Stage II shows early narrowing of the joint space with small bone spurs under 2 mm. By Stage III, the joint space is significantly narrowed, bone spurs exceed 2 mm, and the metacarpal may have shifted partially out of position. Stage IV looks like Stage III but with arthritis spreading to the neighboring joint between the trapezium and the scaphoid (another wrist bone).
These stages help guide treatment decisions, but they don’t always match how much pain someone feels. Some people with Stage III changes on X-ray manage well with conservative care, while others with milder imaging findings have significant difficulty using their hand.
Nonsurgical Treatment Options
For early to moderate CMC arthritis, the first line of treatment is almost always nonsurgical. A thumb splint or brace that immobilizes the base of the thumb can take stress off the joint during activities that provoke pain. Many people wear a splint at night and during heavy hand use, then remove it for lighter tasks.
Exercise programs designed to strengthen the muscles around the thumb have shown measurable benefits. A meta-analysis of randomized controlled trials found that exercise-based interventions are effective for managing thumb CMC osteoarthritis. Typical programs include isometric pinch holds (squeezing for 5 to 10 seconds, 10 repetitions, three times daily), strengthening the muscle between the thumb and index finger, and functional exercises like pinching, turning, and twisting small objects. Some programs use a stress ball or chopsticks for resistance.
Joint injections are another option when splinting and exercise aren’t enough. Corticosteroid injections provide relatively quick pain relief, though the effect tends to fade over weeks to months. Hyaluronic acid injections, which supplement the joint’s natural lubricant, appear to offer longer-lasting benefits for people with more severe symptoms. In a six-month trial comparing the two, hyaluronic acid showed significantly better improvement in both pain and hand function at 90 and 180 days for patients who started with higher pain levels.
When Surgery Becomes an Option
If pain persists despite months of conservative treatment and limits your ability to use your hand, surgery may be recommended. The most well-established procedure is ligament reconstruction with tendon interposition, commonly called LRTI. The surgeon removes the worn-out trapezium bone, then uses a piece of nearby tendon to fill the gap and reconstruct the stabilizing ligament. This eliminates the bone-on-bone contact causing pain while preserving thumb mobility.
Long-term outcomes are encouraging. In the longest follow-up study of the procedure (averaging 9 years after surgery), 95% of patients were satisfied and reported excellent pain relief. Grip strength improved by about 93%, and pinch strength increased by roughly 65%. Most patients regained the ability to flex their thumb tip all the way to the base of the small finger. Recovery does take time, typically involving several weeks in a cast or splint followed by months of hand therapy, but the end result for most people is a functional, pain-free thumb.
Joint fusion is occasionally used for younger, physically demanding patients, though it eliminates movement at the joint in exchange for complete stability. Joint replacement with an artificial implant is another option, though it’s less commonly performed at this joint than LRTI.