Is this IOL centered? Angle kappa and multifocal IOLs

The art of advanced technology IOLs

Is this IOL centered? Angle kappa and multifocal IOLs

by Richard Tipperman, MD

Richard Tipperman, MD

Almost every clinician has had the experience of seeing a patient with a multifocal IOL where the rings on the optic are "perfectly centered" referable to the pupil and yet the patient is unhappy with his/her visual function; another patient will have a multifocal IOL where the rings are clearly "decentered" in relation to the pupil yet the patient is very pleased. How can this be possible? Is one patient just "high maintenance" while the other is easygoing? A better explanation than "baseline personality" rephrases the question of "Is the IOL centered?" to "Is the IOL centered in reference to what?"

        It is likely much more important that the IOL be centered in relation to the patient's true visual axis rather than the geometric center of the pupil. To understand this more fully we need to explore the concept of angle kappa.

        In this column I have been able to cover aspects of management and evaluation of patients receiving advanced technology IOLs. Many of these issues have dealt with the overall "psychology" of the physician-patient relationship and the patient's decision-making process. In this article, I would like to review the importance of evaluating angle kappa in patients preoperatively. Although this is a biometric variable rather than a clinical counseling issue, it is nonetheless a critical part of success with multifocal IOLs and one that is often overlooked by many clinicians.

Angle kappa defined

I believe the simplest way to understand angle kappa is to recognize that a point from the geometric center of the pupil to the retina will not, in most cases, be coincident with a line drawn from the fovea to the line of sight. The difference between the geometric center of the pupil and the line of sight is termed angle kappa. In most patients the fovea lies slightly temporal and inferior to a line drawn from the retina, which is orthogonal with the cornea and traverses the geometric center of the pupil. As a result the corneal light reflex will be slightly nasal and superior to the geometric enter of the pupil, producing what is termed a positive angle kappa.

Clinical importance of angle kappa

If an IOL is decentered there is the potential for it to induce coma or other higher order aberrations since the IOL optic will be off axis from the true visual axis ("line of sight"). If a patient has significant angle kappa his/her pupillary axis (the geometric center of the pupil) will not be in alignment with the visual axis. In these patients the IOL may be "perfectly centered" with its rings symmetric with the pupil—however a ray of light from the visual axis will not pass through the center of the IOL but instead will strike the optic obliquely. This can lead to poor image quality or even glare symptoms for the patient.

In this instance it would be better to center the IOL over the patient's true visual axis. Even though the rings of the IOL would not be centered referable to the pupil by having the center of the optic coincident with the true visual axis, the overall image quality should be better.

Editors' note: Dr. Tipperman is affiliated with the Wills Eye Institute, Philadelphia.

Contact information

Tipperman: rtipperman@mindspring.com