Which axis corresponds to torsional movements (intorsion/extorsion)?

• A. X axis
• B. Z axis
• C. T axis
• D. Y axis

Answer: (D)

Cyclotorsion is the rotation of the eye around its visual axis—the twist you feel when the top of the eye tilts toward or away from the nose. In this coordinate framework, that twisting is described around the Y axis, so intorsion and extorsion happen about the Y axis. Intorsion moves the top of the eye inward toward the nose; extorsion moves it outward. Because torsion is a rotation about the line of sight, this axis best captures that movement in this system, making it the correct association. The other axes describe movements in different planes (horizontal or vertical tilts), which are not torsional twists. Some schemes label a specific torsion axis as T, but in this convention the torsional rotation is defined about the Y axis.