To understand rhizarthrosis, you first need to understand the joint it affects. The trapeziometacarpal (CMC) joint at the base of your thumb is one of the most remarkable — and most vulnerable — joints in the human body.
This guide explains the anatomy and biomechanics of the thumb CMC joint in plain language, helping you understand why arthritis develops here and how it causes pain and disability.
The CMC Joint: A Saddle Joint
The first carpometacarpal joint is a saddle-shaped joint formed by:
- Trapezium — A small carpal bone at the base of the thumb
- First metacarpal — The long bone of the thumb
The saddle shape allows the thumb to move in multiple planes — flexion/extension, abduction/adduction, and the critical movement of opposition (touching your thumb to your fingertips). This unique mobility is what makes the human hand so dexterous, but it also makes the joint inherently unstable.
Why This Joint Is Vulnerable to Arthritis
Several anatomical factors make the CMC joint prone to osteoarthritis:
- Small surface area — The contact area is only about 1 cm², yet it bears forces 12× greater than the force applied at the thumb tip
- Ligament dependence — Stability relies heavily on 4 ligaments, especially the anterior oblique ligament (AOL/beak ligament)
- Constant use — Pinching, gripping, and opposing involve this joint in nearly every hand activity
- Force multiplication — A 1 kg pinch at the fingertip generates up to 12 kg of compressive force at the CMC joint
Research & Articles
Browse all 4 evidence-based articles in this category:
CMC Joint Arthritis: Stages, Symptoms, and Treatment by Stage
Key Takeaways If your doctor has mentioned “Eaton stage” or “CMC arthritis staging” during your visit, you are probably…
Does Your Thumb Muscle Connect to Arthritis Pain?
If you have thumb pain or arthritis at the base of your thumb, you may wonder if something in your […]
Is Your Thumb Pain Linked to How a Muscle Attaches?
If thumb pain or arthritis is bothering you, you’re not alone. A recent study looked at 104 hands from people […]
Why does thumb arthritis cause pain & deformity?
Thumb pain can make simple daily tasks like gripping a cup or buttoning a shirt surprisingly hard. If you’ve noticed […]
Frequently Asked Questions
What is the trapezium bone?
The trapezium is a small, irregularly shaped carpal bone at the base of the thumb. It articulates with the first metacarpal to form the CMC joint. In a trapeziectomy, this bone is partially or completely removed.
Why does my thumb deform with arthritis?
As cartilage wears away, the metacarpal bone slides off the trapezium (subluxation). The surrounding muscles then contract to compensate, pulling the thumb into a characteristic Z-shaped deformity with hyperextension at the MCP joint.
What are the Eaton stages of thumb arthritis?
The Eaton-Littler classification divides rhizarthrosis into 4 stages: Stage I (normal X-ray, widened joint space), Stage II (mild narrowing, small osteophytes), Stage III (significant narrowing, large osteophytes, subluxation), Stage IV (pantrapezial arthritis, affecting adjacent joints too).
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Medical Disclaimer: This information is based on published scientific research and is intended for educational purposes only. It does not replace professional medical advice. Always consult your doctor or hand surgeon for personalised treatment recommendations.