The CMC joint, short for carpometacarpal joint, is the joint at the base of your thumb where the thumb’s long bone (the first metacarpal) meets a small wrist bone called the trapezium. It’s one of the most mobile joints in your hand and the one most likely to develop arthritis. While there are technically CMC joints at the base of every finger, “the CMC joint” almost always refers to the thumb.
How the CMC Joint Works
The thumb CMC joint has a distinctive saddle shape: each bone surface curves inward in one direction and outward in the other, like two saddles interlocking at right angles. This design gives your thumb an extraordinary range of motion. It can swing away from your palm by about 61 degrees, spread sideways by about 63 degrees, and rotate inward to touch each fingertip, a movement called opposition. No other finger joint comes close to this versatility.
That range of motion is what lets you grip a doorknob, pinch a key, button a shirt, and hold a pen. These are movements unique to humans, and they all depend on the CMC joint functioning well.
Why It’s Vulnerable to Injury and Arthritis
The same design that makes the thumb so mobile also makes this joint inherently unstable. The two bone surfaces are shallow and only fully match up at the extremes of motion. The skeleton itself provides very little built-in stability, so ligaments do most of the work holding the joint together during everyday tasks.
The forces involved are surprisingly large. During a simple key pinch, the reactive force at the CMC joint is 12 times greater than the force at the tip of your thumb. A forceful grasp can generate compressive loads of up to 120 kilograms at the base of the thumb. Over years, this combination of high forces and ligament-dependent stability wears the joint down. Research points to gradual degeneration of a key ligament on the front of the joint as the likely starting event, which shifts how the bones contact each other and accelerates cartilage loss.
CMC arthritis is the most common form of arthritis in the hand. Radiographic evidence of it shows up in about 7% of women and 6% of men by age 50. By age 80, those numbers climb to roughly 39% of women and 33% of men.
Signs of CMC Joint Problems
Pain at the base of the thumb is the earliest and most common symptom. It typically flares when you pinch or grasp something: turning a key, opening a jar, snapping buttons, or gripping a tool. Over time, you may notice:
- Swelling and tenderness right where the thumb meets the wrist
- Weakened grip and pinch strength, making it hard to hold objects securely
- Stiffness and reduced range of motion in the thumb
- A bony, enlarged appearance at the base of the thumb as the joint changes shape
These symptoms often develop gradually. Many people adapt by unconsciously changing how they use their hand, which can mask how much function they’ve lost until daily tasks become genuinely difficult.
How CMC Problems Are Diagnosed
A physical exam is usually the starting point. One common test is the grind test, where a clinician pushes the thumb bone into the wrist bone while rotating it to see if it reproduces pain. However, the grind test misses about half of confirmed cases. A newer approach called the lever test, where the examiner grasps the base of the thumb and rocks it side to side, is more sensitive and better at reproducing the kind of pain people actually feel during daily activities. X-rays confirm the diagnosis by showing cartilage loss and bone changes at the joint.
Nonsurgical Treatment
Most people start with conservative measures, and many get meaningful relief without surgery. Thumb splints are one of the most effective first-line options. Two main types exist: prefabricated soft splints you can buy off the shelf, and custom-molded rigid splints made from thermoplastic by a hand therapist. Both types reduce pain and improve function. Research comparing the two found no significant difference in pain relief, grip strength, or pinch strength, though prefabricated splints showed a slight edge in reducing overall disability. Either type is a reasonable choice.
Exercise-based therapy also helps. Typical programs focus on isometric strengthening (contracting muscles without moving the joint) of the muscles around the thumb and the side of the hand. A common routine includes pinch holds of 5 to 10 seconds, repeated 10 times, three times daily, along with gentle stretching of the thumb’s web space. These exercises help stabilize the joint by building up the surrounding muscles that compensate for weakened ligaments. Systematic reviews of clinical trials confirm that exercise programs meaningfully improve pain and function in CMC arthritis.
When Surgery Becomes an Option
If splinting, therapy, and other conservative measures stop providing adequate relief, surgery is the next step. Several techniques exist, including removing the trapezium bone entirely, reconstructing the ligaments, or using implants. The most widely performed procedure is called ligament reconstruction and tendon interposition, or LRTI, which removes the trapezium and uses a piece of nearby tendon to cushion and stabilize the space. Newer techniques using strong sutures instead of tendon grafts have shown comparable early results.
Interestingly, comparative studies have repeatedly found little difference in long-term outcomes between the various surgical approaches. The choice often comes down to surgeon preference and the specifics of each case.
What Recovery Looks Like After Surgery
Recovery follows a predictable timeline. For the first 10 to 14 days, your hand stays in a bulky bandage with a plaster splint covering the thumb, wrist, and forearm. At the first follow-up visit, stitches come out and you’re fitted with a removable custom brace.
You wear the brace full-time for four more weeks, removing it only for showering and gentle exercises prescribed by your therapist. At six weeks after surgery, the brace comes off and more intensive hand therapy begins, typically twice a week, focusing on reducing swelling and restoring range of motion. Throughout this period, you can handle light daily tasks like eating, writing, and getting dressed, but heavy gripping and pinching are off-limits.
Three months after surgery marks the real turning point. Strengthening exercises begin, activity restrictions are lifted, and braces are no longer needed. During the earlier weeks, light use of the hand is encouraged, but it takes that full three-month window before you can return to forceful gripping, heavy lifting, and unrestricted use of the thumb.