Relating intraocular lens tilt to the optical axes of the eye

An average intraocular lens (IOL) tilt of approximately 5 deg relative to the visual axis is consistently reported in the clinical literature, with the implication that it represents a lens malposition. Modern biometry equipment also now routinely generates "chord" values related to tilt of the eye (chord alpha), and to centration of the pupil (chord mu), with other equipment measuring tilts directly. In this work, clinical data were used to create raytrace eye models, and these show that decentration of the foveal center from the optical axis causes a 5 deg rotation from the optical axis to the visual axis (angle α), with similar values for both phakic and pseudophakic eyes, which explains the apparent lens tilt when viewing along the visual axis. Additional findings include that the iris is modestly decentered nasally from the optical axis, the cornea is typically measured along the keratometric axis but eye models are configured around the optical axis, and that lines drawn at 5 deg angles from the pupils meet at a distance corresponding to near vision. These findings clarify the geometric origin of commonly reported IOL tilt values and have important implications for clinical practice.