Which joint moves in two planes and is exemplified by the thumbs, allowing movement in sagittal and frontal planes?

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

Answer: (C)

Movement in two planes indicates a biaxial joint. The thumb’s carpometacarpal joint is a saddle joint, formed where the first metacarpal and the trapezium meet. Its saddle-shaped surfaces are concave in one direction and convex in the other, allowing flexion and extension in the sagittal plane and abduction and adduction in the frontal plane. This combination enables the thumb’s versatile motion, including opposition. Other joints either move in a single plane (hinge or pivot) or in three planes with rotation (ball-and-socket), so the saddle joint best fits the two-plane motion described for the thumb.