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Carpometacarpal Joint: Thumb (Detail)

articulatio carpometacarpalis pollicis (detail)

The first CMC joint is the most mobile joint of the hand and the foundation of human thumb opposition. Its reciprocally curved saddle surfaces allow the biaxial motion that, combined with the conjunct metacarpal rotation, positions the thumb pulp to face all four fingers. The beak ligament (anterior oblique ligament) is the primary CMC stabiliser against dorsal subluxation, its attenuation from degeneration initiates trapeziometacarpal arthritis.

Anatomical Data

Structure

Bones	Trapezium (reciprocal saddle surface) First metacarpal base (reciprocal saddle surface)
Ligaments	Anterior oblique (beak) ligament, the primary stabiliser against dorsal translation Dorsal radial ligament First intermetacarpal ligament Posterior oblique ligament

Biomechanics

Function & Actions

Movements	Palmarflexion-dorsiflexion (CMC flexion-extension) 50 degrees Abduction-adduction 40 degrees Conjunct rotation (pronation of the metacarpal during opposition), not an independent movement but occurs with combined motions

Clinical Relevance

Clinical Notes

The beak ligament (AOL) is the primary structure that fails in CMC arthritis. Its attenuation allows the first metacarpal to dorsally sublux, producing the adducted thumb ray deformity of basal joint arthritis. Trapeziectomy with LRTI (ligament reconstruction and tendon interposition using FCR strip) eliminates the arthritic joint and reconstructs the beak ligament function simultaneously.

Pathology

Common Injuries & Conditions

Bennett Fracture

First metacarpal base intra-articular fracture-dislocation from axial thumb loading managed with closed reduction and percutaneous pinning or ORIF.

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