Thumb CMC osteoarthritis

Thumb CMC (carpometacarpal) osteoarthritis — also called basal joint arthritis — is the second most common arthritis of the hand, after DIP joint arthritis. The CMC joint sits at the base of the thumb where the metacarpal meets the trapezium. Its saddle shape gives the thumb its wide range of motion but also makes it prone to wear when the joint is repeatedly loaded in pinch and grip.

It is far more common in women (estimated 10:1) and typically appears after age 40. Symptoms include aching pain with pinching or jar-opening, swelling at the thumb base, and a characteristic "shelf sign" or bony prominence where the metacarpal base subluxes dorsally.

The grind test — axial compression and rotation of the thumb metacarpal — reproduces pain and sometimes crepitus. Weight-bearing (stress) X-rays of the CMC joint show joint space narrowing, sclerosis, and osteophytes at the base of the first metacarpal and the trapezium. X-rays also rule out scaphotrapeziotrapezoid (STT) joint involvement.

Advanced thumb CMC arthritis — two views

1. Corticosteroid injection

   Ultrasound-guided injection into the CMC joint relieves inflammation, with duration of benefit varying by patient.

Surgery is considered for moderate-to-severe arthritis with persistent pain unresponsive to a structured non-operative trial.

External patient-education references and related OSI pages for additional background:

• Britannica — Arthritis
• Britannica — Hand (anatomy)
• AAOS OrthoInfo — patient education library
• OSI glossary — orthopedic terminology in plain language