Which joint allows rotation around a single axis, as seen in the forearm?

• A. Ball-and-socket joint
• B. Pivot joint
• C. Hinge joint
• D. Saddle joint

Answer: (B)

Rotation around a single axis is the characteristic of a pivot joint. In the forearm, the radius spins around the ulna at the radioulnar joints, with the radial head rotating within the annular ligament. This arrangement lets the forearm turn (pronation and supination) around its long axis while the ulna stays relatively fixed. Ball-and-socket joints move in many directions, not just one axis. Hinge joints mainly bend and straighten in one plane and don’t provide the rotation seen in the forearm. Saddle joints allow movement in two planes but not the pure single-axis rotation typical of a pivot joint. So, the joint type that best fits forearm rotation is the pivot joint.