The anterior oblique ligament, also called the beak ligament, is the primary stabiliser of the thumb carpometacarpal joint. It runs from the palmar beak of the first metacarpal base to the palmar tubercle of the trapezium, tightening in abduction and extension to prevent dorsal subluxation of the metacarpal on the trapezium. It is thicker and stronger than the other thumb CMC ligaments.
Resists dorsal subluxation of the first metacarpal during pinch and grip, maintaining the metacarpal in its concave articular seat on the saddle-shaped trapezial surface. It is the primary restraint against the metacarpal sliding to the dorsal-radial quadrant of the trapezium.
The anterior oblique ligament is the ligament that fails progressively in thumb CMC osteoarthritis (Bennett joint arthrosis), allowing the metacarpal to sublux dorsally and radially, adduct (producing the Z-thumb deformity), and produce a metacarpal adduction contracture. Ligament reconstruction using a portion of flexor carpi radialis tendon (LRTI, Eaton-Littler procedure) is the most common surgical treatment, though trapeziectomy alone is increasingly favoured. The ligament is assessed arthroscopically by probing the palmar floor of the joint.
An avulsion fracture of the palmar beak of the first metacarpal maintains its attachment to the AOL and trapezium while the metacarpal shaft displaces dorsally and radially by the pull of abductor pollicis longus, requiring closed or open reduction and fixation.
Progressive attenuation of the AOL allows metacarpal subluxation, joint space narrowing, osteophytosis, and the Z-collapse deformity, producing pain at the base of the thumb with pinch and grip, initially managed with splinting and injection, ultimately with trapeziectomy or ligament reconstruction.
