The quest for accurate intraocular lens (IOL) power calculation has existed ever since the first IOL was implanted by Sir Harold Ridley in 1949. The surgery went well, but the postoperative refraction was found to be – 20 D. Ridley soon realized that the error was due to the higher refractive index of Perspex (polymethyl methacrylate [PMMA]) as compared to the crystalline lens, the dimensions of which he had been trying to copy.1
Nowadays a prediction error of 0.5 D, be it spheric or cylindrical, can make the difference between success and failure, especially with multifocal and toric implants. Hence the question of improved accuracy to the first decimal point is not just of academic interest but is also clinically relevant for the patient’s quality of life.
This chapter discusses the principles and the sources of error behind IOL power calculation, beginning with the fundamentals of the “thin-lens” IOL power calculation formula. Next, the text describes the conditions for improved IOL power calculation using ray tracing and newly developed methods for the prediction of the IOL position. The benefit of using physiological optics is stressed. However, the calculation of IOL power is not just a question about optics. The eye is a biological tissue of small dimensions, which requires an understanding of how clinical measurements combine with physical optics to meet the clinical reality. As the demands and complexity of patients seem to rise in parallel, practitioners face significant challenges in future IOL power calculations.