The posterior oblique ligament of the thumb CMC joint runs from the dorsal-ulnar surface of the trapezium to the palmar-ulnar beak of the first metacarpal base, tightening with thumb flexion and adduction. Together with the anterior oblique (beak) ligament and the dorsoradial ligament, it forms the three-ligament stabilising complex of the first carpometacarpal joint.
Resists palmar subluxation and radial angulation of the first metacarpal, particularly during pinch loading when the thumb is adducted, and guides the metacarpal's arc of circumduction around the saddle surface of the trapezium.
The posterior oblique ligament becomes increasingly important for stability as the anterior oblique ligament attenuates in progressive CMC arthrosis. In arthroscopic assessment of the CMC joint, the posterior oblique is visualised from the volar radial portal as a distinct band on the ulnar floor of the joint. Ligament reconstruction procedures may incorporate both the anterior and posterior oblique to provide full circumferential stability to the metacarpal base.
Combined failure of the anterior and posterior oblique ligaments produces complete first CMC dislocation, rare without a fracture, requiring anatomic reduction and ligament repair or temporary pin fixation to maintain joint congruity.
