In terms of movement, how does the 1st CMC joint function?

The first carpometacarpal (CMC) joint, located at the base of the thumb, primarily allows for opposition, which is a unique movement critical for grasping. Understanding the joint mechanics involves knowing the relationship between the articulating surfaces: the trapezium bone of the wrist (which is convex) and the base of the first metacarpal (which is concave).

For movements such as flexion and extension at the 1st CMC joint, the concave base of the first metacarpal glides in the same direction as the movement of the thumb. For instance, during flexion, the concave surface of the metacarpal rolls and glides towards the palm, indicating that the concave surface is moving over the convex surface in the same direction.

Thus, the correct answer points out that in terms of movement, the 1st CMC joint functions with a concave surface (the first metacarpal) moving on a convex surface (the trapezium) in the same direction, which is essential for its functional role in hand movements.