Thumb CMC (Basal Joint) Osteoarthritis

What is it?

The saddle-shaped thumb CMC joint degenerates under high axial loads, eroding cartilage.

Commonly affected: Thumb CMC (trapeziometacarpal) joint

How it develops

1. Normal Saddle JointSaddle joint allows multi-plane thumb motion
2. Repetitive Axial LoadGripping/pinching repeatedly axially loads joint
3. Ligament LaxityVolar ligament laxity causes instability
4. Cartilage LossCartilage wears, joint space narrows
5. Osteophytes/SubluxationOsteophyte formation and dorsal metacarpal subluxation

Symptoms

• Thumb Base PainPain with pinching/gripping at thumb base
• Grip/Pinch WeaknessDifficulty opening jars, pinching
• Thumb DeformityDorsal CMC prominence + shoulder sign
• MCP HyperextensionCompensatory MCP hyperextension (Z-deformity)

How it is examined

• Grind TestAxial load + rotation on thumb reproduces pain/crepitus
• CMC TendernessDirect tenderness at CMC joint base
• Shoulder SignDorsal CMC prominence resembles shoulder
• Pinch/Grip MeasurementFunctional assessment

Imaging

X-ray is standard — include stress views.

• CMC joint space narrowing
• Osteophyte formation
• Dorsal subluxation
• Eaton-Littler Stage I–IV

MRI rarely needed; selective for ligament assessment.

• Cartilage defects
• Ligament injury
• Peri-articular edema

When surgery is considered

6-month conservative failure, severe deformity/dysfunction

Procedures that may be discussed

• LRTI (ligament reconstruction + tendon interposition)
• CMC arthrodesis
• CMC arthroplasty

Outlook

70% improve with conservative care; 90% pain relief after LRTI.