Which type of joint permits motion in only one plane?

The hinge joint is designed to allow motion primarily in one plane, similar to the way a door opens and closes. This type of joint facilitates flexion and extension movements, which occur around a single axis. Examples of hinge joints in the human body include the knee and elbow. Their structure includes a convex surface of one bone fitting into a concave surface of another, which limits the motion to forward and backward movements, ensuring stability while allowing this specific range of motion.

The other joint types have different functionalities; for instance, ball-and-socket joints allow for a wider range of motion in multiple planes, pivot joints enable rotational movement around a single axis, and saddle joints permit movement in two planes but are more versatile than hinge joints. Thus, the unique design and function of hinge joints make them the correct answer when identifying joints that permit motion solely in one plane.