How does the thumb's carpometacarpal joint enable opposition?

The thumb's carpometacarpal joint is a sellar joint, which is characterized by its ability to perform two types of swings: flexion-extension and abduction-adduction.

This joint enables opposition—the movement that allows the thumb to come "face to face" with the other fingers—by combining these swings with a rotation of the moving bone around its long axis. During this process, the thumb swings across the palm, and the necessary rotation allows it to position itself effectively for tasks such as grasping objects. While the opponens pollicis muscle is specifically responsible for acting to oppose the thumb to the fingers, the unique structural mechanics of the sellar joint are what provide the range of motion required for this essential human capability.12