Final answer:
The proper collateral ligament of the thumb prevents radial deviation, particularly when the thumb is extended or abducted.
Step-by-step explanation:
The proper collateral ligament of the thumb, also known as the ulnar collateral ligament, provides stability to the thumb's metacarpophalangeal (MCP) joint, which is the articulation between the distal end of the metacarpal bone of the thumb and the thumb's proximal phalanx. This ligament prevents radial deviation, which is the movement of the thumb towards the radius bone or the lateral aspect of the forearm. The function of the proper collateral ligament is particularly important when the thumb is extended or abducted, such as when holding an object with the thumb opposed to the other fingers.