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Carpometacarpal Joint of the Thumb

articulatio carpometacarpalis pollicis

The carpometacarpal joint of the thumb is a saddle joint that allows the thumb its remarkable opposition ability, the movement that defines fine human hand function. Its two reciprocally curved surfaces allow multiplanar gliding and rotation, making it the most mobile joint in the wrist-hand complex. It is the second most common site of osteoarthritis in the hand after the DIP joints, producing the classic basal thumb arthritis that affects particularly postmenopausal women.

Anatomical Data

Structure

Bones	Base of the first metacarpal Trapezium
Ligaments	Anterior oblique ligament (beak ligament) Posterior oblique ligament Dorsoradial ligament Intermetacarpal ligaments

Biomechanics

Function & Actions

Movements	Flexion and extension in the plane of the palm Abduction and adduction perpendicular to the palm Circumduction Opposition combining the above movements with pronation of the metacarpal

Clinical Relevance

Clinical Notes

Thumb CMC osteoarthritis produces pain at the base of the thumb that worsens with pinching and gripping, a positive grind test (axial compression and rotation of the first metacarpal on the trapezium), and eventual squaring of the thumb base as the first metacarpal subluxes dorsally. The anterior oblique (beak) ligament is the key stabiliser, and its attenuation allows this subluxation. Management progresses from splinting and corticosteroid injection to trapeziectomy, with or without ligament reconstruction and tendon interposition (LRTI), which reliably relieves pain at some cost to pinch strength. Arthrodesis and joint replacement are alternatives in selected patients. In a Bennett fracture the small anteromedial fragment stays attached to the anterior oblique ligament while abductor pollicis longus pulls the metacarpal shaft proximally and radially.

Pathology

Common Injuries & Conditions

CMC Thumb Osteoarthritis

Degenerative arthritis at the base of the thumb producing pinch pain, positive grind test, and eventual subluxation, managed with splinting, injection, and trapeziectomy with or without LRTI in advanced cases.

Bennett Fracture

Intra-articular fracture-dislocation of the first metacarpal base from axial load; the shaft is pulled away by abductor pollicis longus while a small fragment stays on the anterior oblique ligament, so fixation is usually required.

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