UCL Anatomy and Function

The UCL proper runs from the head of the thumb metacarpal bone to the volar aspect of the proximal phalanx. Its action is to tighten in flexion and relax in extension of the MCP joint.

The accessory UCL lies anterior to the UCL proper and inserts into the volar plate. This ligament tightens in MCP joint extension and relaxes in flexion.

The adductor pollicis aponeurosis inserts to the dorsal thumb tendons and capsule of the MCP joint.


The MCP joint of the thumb maintains ulnar side stability by means of both static and dynamic mechanisms. Static stability is provided by the main and accessory UCLs, the volar plate, as well as the dorsal capsule.

The adductor pollicis muscle and tendon in the first web space of the hand also provide dynamic stability, ulnar support and restraint to the MCP joint of the thumb.


Due to the anatomical arrangement in this area, UCL injuries may also involve injuries to the accessory UCL, volar plate, dorsal capsule, adductor pollicis insertion, and may also have associated fractures of the proximal phalanx.