If an IOL has zero power, such as in an extremely myopic patient, where we place it in the eye makes no difference. The IOL can sit a little more anterior or a little more posterior and the optical effect is the same. The IOL can even be somewhat decentered and it will again, have very little optical effect. With IOL powers that are higher, moving the lens anterior to posterior will definitely change the effective optical power and this is why we use the rule of 9s to adjust for sulcus IOL power. If the IOL has positive or negative spherical aberration then the dioptric power will vary slightly from center to edge, compared to an IOL with zero spherical aberration which tends to have the same power from edge to edge. This graph shows that a decentered negative spherical aberration IOL can induce coma aberration. For optimal visual performance we want to have our monofocal IOLs centered in the patient’s visual axis.
The IOLs with diffractive rings such as trifocal, bifocal, multifocal and some extended depth of field (EDOF) designs, we want to center these in the patient’s visual axis, but we must also understand that the pupil centration is also important since it influences the performance of the diffractive rings.
Angle Alpha and Angle Kappa are important considerations with IOLs that have diffractive rings such as trifocal, bifocal, multi-focal, and some extended depth of field (EDOF) designs. This video explains the concepts in detail and can serve as a guide to patient selection for the best optical results.
click below to learn from this detailed video about angle alpha and angle kappa: