What Is the CMC Joint and How Does Arthritis Affect It?

The carpometacarpal (CMC) joint of the thumb sits at the very base of the thumb, where the first metacarpal bone meets a small wrist bone called the trapezium. It is a saddle-shaped joint that lets you grip, pinch, and oppose your thumb to your fingers, and it is the second most common site for osteoarthritis in the hand. Because the joint bears surprisingly large forces during everyday tasks like turning a key or opening a jar, its cartilage wears down over time in many people, producing pain, weakness, and eventually visible deformity of the thumb column.

Anatomy of the Thumb CMC Joint

The thumb CMC joint gets its unusual range of motion from its shape. Both the base of the first metacarpal and the top surface of the trapezium are curved in two directions at once, creating a saddle-like fit. This design allows the thumb to move in three planes while using only two axes of rotation, which is what makes it possible to swing the thumb across the palm, pull it away from the hand, and rotate it so the pad can press against the other fingertips.¹ No other joint in the hand has that combination of mobility and precision. The trade-off is that the bony surfaces are not deeply interlocked, so the joint relies heavily on ligaments and muscles for stability rather than on the shape of the bones themselves.

How Ligaments Hold It All Together

Because the bones do not lock snugly the way a ball fits into a socket, the thumb CMC joint depends on a web of ligaments to keep it from sliding out of place. Two ligaments get the most attention in research: the anterior oblique ligament (AOL) on the palm side and the dorsoradial ligament (DRL) on the back-and-thumb side. Cadaver studies show that these two structures play complementary roles. The AOL primarily resists the thumb slipping toward the palm side, while the DRL primarily resists it sliding toward the back and the radial (thumb) side.²

When researchers cut the AOL in cadaver specimens, the force needed to push the joint out of alignment in one key direction dropped dramatically, from roughly 7 newtons to just over 1 newton.³ Cutting the DRL in a separate set of experiments produced the largest increases in translation during lateral pinch, with the metacarpal shifting over a millimeter in three-dimensional space.⁴ Those numbers sound small, but millimeter-scale shifts at the base of the thumb translate into meaningful instability during pinching and gripping. This is why ligament loosening is thought to be an early step on the road to arthritis.

Why This Joint Is So Prone to Arthritis

Thumb base osteoarthritis is the second most common form of hand arthritis overall, and it disproportionately affects women over 50.⁵ The leading explanation centers on a vicious cycle: as ligaments loosen, the metacarpal begins to slide slightly out of position during use. That abnormal sliding, called subluxation, concentrates force on a small patch of cartilage instead of spreading it evenly across the joint surface. Over years, the cartilage in that high-stress zone wears through, bone remodeling begins, and the joint progressively degenerates.

The sex difference in prevalence has been a puzzle. A systematic review found that in healthy young women, peaks of the hormone relaxin during the menstrual cycle corresponded with measurable thumb CMC joint instability, even without symptoms. In tissue samples from women who had gone on to need surgery, relaxin receptors were far more active in the ligaments around the joint than in equivalent tissue from men.⁵ The implication is that hormonal cycling may subtly loosen ligament support around the thumb base decades before arthritis appears, giving the joint a head start on instability. Women who eventually needed surgical reconstruction were also, on average, younger at the time of surgery than men, reinforcing the idea that the process begins earlier in female patients.

From an evolutionary perspective, the thumb CMC joint is something of an ironic achievement. The same saddle-shaped design that gave humans an opposable thumb and set us apart from other primates also created a mechanically vulnerable joint. Researchers have described it as a paradox: the structure that defines our hand’s extraordinary dexterity is also the most common source of arthritis requiring surgical reconstruction in the upper limb.⁶

What Thumb CMC Arthritis Feels Like

The typical presentation is an aching pain at the base of the thumb that worsens with pinching and gripping activities. Opening a jar, turning a key, or even unscrewing a bottle cap can become painful enough to make people switch hands or avoid the task entirely. Over time, pinch strength drops, and the thumb may begin to look different. As the metacarpal subluxes dorsally (toward the back of the hand), a bony bump develops at the base of the thumb, and the thumb column may develop a zigzag deformity as the joints farther along compensate for the shifting base. In advanced cases, the web space between the thumb and index finger narrows, and the thumb becomes stiff.

How It Is Diagnosed

A clinician can often identify thumb CMC arthritis with hands-on examination before any imaging is ordered. Several provocative tests exist, and they vary in how well they perform. The classic “grind test,” where the examiner pushes the thumb into the joint while rotating it, is highly specific but misses a fair number of actual cases. A study comparing four common maneuvers found that the metacarpal pressure-shear test had the best combination of accuracy and sensitivity, correctly identifying arthritis in about 99% of affected thumbs while still being 95% specific.⁷ A separate study found that the lever test, a different maneuver, produced the greatest reproduction of the patient’s typical pain and had the highest sensitivity of the tests it compared, while the grind test again came out with the lowest sensitivity.⁸

When imaging is used, plain X-rays remain the standard. The most widely used classification for radiographic severity was developed by Eaton and Littler and ranges from stage I (early) through stage IV (severe, with joint destruction and adjacent joint involvement).⁹ The system is simple and familiar, but it has a weakness: different readers looking at the same X-ray often disagree on the stage. A systematic review found that agreement between observers was poor to fair, with kappa values as low as 0.11 in some studies.¹⁰ This matters because treatment decisions sometimes hinge on the stage, and two surgeons may categorize the same thumb differently.

The Role of Ultrasound

Ultrasound is starting to gain ground as a supplementary tool. It can detect bone spurs, joint narrowing, and erosion, and its findings correlate with what X-rays show. More interestingly, ultrasound can pick up active inflammation through power Doppler signal, a measure of blood flow in the tissue. In one study, the presence of power Doppler signal at the thumb base was significantly associated with pain levels, even after adjusting for other factors. Other structural findings on ultrasound, like osteophytes or effusion alone, did not show the same link to how much pain patients reported.¹¹ That distinction could eventually help clinicians figure out whose pain is driven by active inflammation (and might respond to anti-inflammatory treatment) versus whose pain is more structural.

Non-Surgical Treatment

Most people with thumb CMC arthritis start with conservative measures, and many get meaningful relief without surgery. Splinting is the most studied intervention. The idea is to reduce abnormal movement at the arthritic joint, lower the mechanical stress on damaged cartilage, and give inflamed tissue a chance to quiet down. A randomized trial found that four weeks of thumb splinting reduced pain scores by about 20%, compared to only 3% in a control group.¹² Another study showed that after 30 days of custom splint use, pain had already dropped, and improvements in grip strength, pinch strength, and hand function continued to accumulate through 60 days.¹³ Both prefabricated and custom-made splints appear to work, though more rigid designs that also immobilize the metacarpophalangeal joint have shown benefits for pain, function, and strength.¹⁴

Exercise, particularly exercises aimed at strengthening the muscles that stabilize the thumb base, is often combined with splinting. A pilot randomized trial compared specific CMC-stabilizing exercises with general hand exercises and found both approaches worth studying further, though the small sample made definitive conclusions difficult.¹⁵ A multimodal program combining splinting, exercises, joint protection education, and activity modification produced clinically meaningful improvements in pain, range of motion, and patient-rated function and satisfaction at six weeks.¹⁶ In practice, hand therapists typically combine several of these elements rather than relying on any single intervention.

Do Injections Help?

Corticosteroid injections into the thumb CMC joint are among the most commonly offered treatments, but the evidence for them is weaker than many patients and clinicians assume. A network meta-analysis comparing corticosteroid injections, hyaluronic acid, platelet-rich plasma (PRP), and placebo found that in the short term, none of the injections was better than placebo at reducing pain. At a medium-term follow-up, PRP showed superiority over both placebo and corticosteroids in a sensitivity analysis restricted to randomized trials, but no injection type improved function at either time point.¹⁷ That does not mean injections are useless for every patient, but it does mean the expected benefit is modest, and patients should understand that a shot is unlikely to produce lasting improvement on its own.

When Surgery Becomes the Conversation

Surgery is typically reserved for people whose pain and functional limitations persist despite months of conservative treatment. The most established procedure is trapeziectomy, which involves removing the trapezium bone entirely. With the arthritic surface gone, the pain source is eliminated, though the thumb loses some of its bony foundation. For decades, surgeons added soft-tissue reconstruction to trapeziectomy, using a tendon graft to fill the space left by the removed bone, stabilize the metacarpal, and theoretically preserve thumb length and pinch strength. This combined procedure, called ligament reconstruction and tendon interposition (LRTI), became one of the most popular operations in hand surgery.

The evidence, however, has not supported the added complexity. A meta-analysis of over 650 cases across eight studies found that while grip and pinch strength were slightly better in the LRTI group early on, there was no significant difference between the two groups in pain, disability scores, or complication rates at one year.¹⁸ A separate meta-analysis found moderate-certainty evidence that LRTI offers no long-term clinical benefit over simple trapeziectomy for either function or pinch strength.¹⁹ A systematic review went further, reporting that soft-tissue augmentation was associated with reduced thumb movement, greater complication rates including tendon rupture and sensory loss, and no offsetting benefit.²⁰ The trend in the literature is clear: simple trapeziectomy performs as well as or better than the more complex reconstructions, with fewer complications.

Joint replacement is an alternative that aims to preserve thumb length and restore alignment while avoiding the inherent shortening that comes with bone removal. Modern cementless, modular implants using a ball-and-socket design with metal-on-polyethylene bearings have achieved survival rates above 90% at ten years.²¹ The advantages include faster recovery of hand function and a cosmetically normal-looking thumb. The disadvantages include the technical difficulty of the surgery and a potentially higher complication rate. If the implant fails, it can be revised with a new implant or converted to a trapeziectomy, so it does not burn bridges. A systematic review noted that early results of joint replacement appeared promising compared to trapeziectomy with LRTI, but called for higher-level trials before drawing firm conclusions.²²

Joint fusion (arthrodesis), where the metacarpal is permanently fixed to the trapezium, eliminates pain by eliminating motion. It is sometimes considered for younger, high-demand patients who need a strong, stable thumb, but it comes with trade-offs: loss of the thumb’s natural range of motion, nonunion rates reported at 8% to 21%, and more frequent complications and repeat surgeries compared to trapeziectomy.²²

What Recovery Looks Like After Surgery

Patients undergoing trapeziectomy typically spend several weeks in a splint or cast before starting guided rehabilitation. Grip and pinch strength tend to be reduced in the first few months but improve steadily. In one long-term follow-up of suture-button suspensionplasty, a variation of trapeziectomy that uses a button device to maintain metacarpal height, grip and pinch strength at final follow-up reached about 116% and 111% of their three-month postoperative values, meaning patients continued to gain strength well beyond the early recovery window.²³ Recovery after trapeziectomy is generally described as slow but reliable, and most patients report satisfaction with pain relief and functional return by six to twelve months.

Arthritis in the Other CMC Joints

The thumb CMC joint gets almost all the attention, but the hand has four other CMC joints connecting the index through small finger metacarpals to the wrist bones. Arthritis in these joints is far less common and tends to follow a different path. Rather than developing from chronic wear and hormonal loosening, finger CMC arthritis most often appears after trauma: fractures or dislocations that damage the cartilage surface. The fourth and fifth CMC joints, which allow a small amount of flexion that helps the hand cup around objects, are the most frequently injured. When those injuries are missed or poorly treated, chronic instability and secondary joint degeneration can follow, producing persistent pain and reduced grip strength. Interestingly, symptomatic post-traumatic arthritis of the small finger CMC joint is less common than you might expect, possibly because the joint’s natural looseness makes it somewhat forgiving. Degenerative changes at the finger CMC joints can also show up as metacarpal bosses, bony lumps on the back of the hand, but these are rarely painful enough to need treatment.