Angle Alpha and Angle Kappa with IOLs

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.

the geometric center of the cornea (based on the limbal ring) is the optical center of the cornea.
the geometric center of the pupil is defined as the pupil center and it may not correspond to the optical center
the visual axis is when the patient is fixating on an object such as this microscope light. By having the patient foveate on the light, we can see the Purkinje images aligned with the visual axis.
In this particular patient, the optical center, the pupil center, and the visual axis are all very close to each other. This makes it easy to implant a trifocal IOL (Alcon PanOptix in this case) that is aligned with the visual axis while the diffractive rings are aligned with the pupil center.
In this patient, the pupil is nasally shifted in comparison to the optical center of the cornea. The visual axis is even farther away from the optical center and this means that a trifocal or multifocal IOL may not have the best level of performance in this patient.
Angle Alpha is the distance between the optical center (green) and the visual axis (yellow).
Angle Kappa is the difference between the pupil center (blue) and the visual axis (yellow). In this patient it is 0.7 mm at the lens plane and this will likely pose an issue with a diffractive ring-based IOL.
When we look at this diagram of the eye, we can see that Angle Alpha and Angle Kappa are actual angles (degrees), even though surgeons tend to refer to linear measurements (mm) as seen from the microscope surgical view.

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:


  1. UDay, I still remember a dozen years ago when you explained iris prolapse prevention and management so clearly to residents and fellows at UCSD. You have a gift for clarity and teaching. Nice cases and diagrams. Keep up the great work!

  2. Thank you. How can you measure alpha and kapa angles in clinic, so you can estimate who is a good candidate for defractive IOL?

Leave a Reply