Which joint is described as synovial with two planes of motion (sagittal and frontal), such as the thumb's joint on the hand?

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

Answer: (B)

Two-plane motion in a synovial joint is best demonstrated by a saddle joint, which has articular surfaces that are concave in one direction and convex in the other. This shape lets the thumb’s carpometacarpal joint move in two perpendicular planes—flexion-extension in the sagittal plane and abduction-adduction in the frontal plane—enabling opposition. The joint is synovial, with a capsule and lubricating fluid. By contrast, a hinge joint is limited to one plane (flexion and extension), a pivot joint mainly rotates around a single axis, and a ball-and-socket joint, while capable of multiple directions, does not describe the specific saddle shape that gives the thumb its unique two-plane movement.