January 22, 2021
3 min read
January 22, 2021
3 min read
For cataract surgery, when we implant the IOL, the closer the optic is to the center of vision, the better it is for visual outcomes.
This is particularly true for IOLs that have diffractive rings such as in trifocal and bifocal IOLs, but also for newer extended depth of field IOLs that have central elements designed to alter the aberration profile. The optical zones of these IOLs perform well when they are centered in the eye.
When it comes to centration of an IOL, we must first define what we mean. There is the optical center of the eye, which is the geometric center of the cornea, and the line that connects this central point to the retina is the optical axis. The pupil center is not often perfectly aligned with the optical center because of the asymmetric pupil position, which is typically slightly nasally shifted. The Purkinje reflection from the cornea defines the visual axis when the patient is looking directly at a fixation light (Figure 1).
When examining a patient at the cataract consultation, it is easier and more natural for ophthalmologists to think about these angles while looking directly at the cornea. In the example shown here, angle alpha is noted to be the distance between the visual axis and optical center (Figure 2). Angle kappa is the angle between the pupillary axis and the visual axis and tends to be smaller (Figure 3). Our diagnostic machines often give us the value in fractions of a millimeter instead of the degree of the angle.
For monofocal IOLs, the sensitivity to centration depends on the dioptric power as well as the spherical aberration profile. If the IOL has zero power, its placement in the eye makes little difference. The IOL can sit a little more anterior or a little more posterior, and the optical effect is the same. Similarly, a little decentration will not likely produce a noticeable result because the dioptric power is so low. If we increase the IOL power to +28 D such as in a small, hyperopic eye, then any movement of the IOL will induce a change in the power as well as optical aberrations. If this high-powered optic moves anterior, then a shift to postoperative myopia will be induced, and if it sits more posterior, then it will shift to postoperative hyperopia.
If the IOL has positive or negative spherical aberration, then the dioptric power of the optic varies from center to edge compared with a zero spherical aberration design, which has a uniform power from center to edge. Decentering a zero spherical aberration IOL makes little difference in aberrations, but decentering a negative spherical aberration IOL can induce aberrations such as coma. For optimal visual performance, we want our monofocal IOLs to be well-centered in the visual axis.
For trifocal, bifocal and other designs with diffractive rings, centering the optic is even more critical. The pupil now plays a significant role because it determines how many of the diffractive rings are active. Ideally, we want to the diffractive rings to be concentric with the pupil and the center of the optic lined up with the visual axis. This will give the best visual performance with the IOL and the best satisfaction with patients. The key is to select patients who have a small angle alpha and angle kappa. That means that the optical center, the pupil center and the visual axis are all tightly placed in the same central zone. We knew from preoperative testing that this patient would be a great candidate for a trifocal IOL because her angle alpha and angle kappa measurements were small (Figure 4).
The way that we prefer to align the diffractive rings is by lining up the first and fourth Purkinje reflections with the patient’s fovea. The first Purkinje reflection is from the anterior surface of the cornea while the fourth Purkinje image is from the posterior surface of the IOL. These are placed in alignment with each other while the patient is asked to fixate on the coaxial lights of the surgical microscope. These reflections are placed in the center of the innermost diffractive ring, and the anterior chamber is sealed while the IOL optic is seated against the capsular bag.
While there is no exact cut-off point at which a multifocal IOL should be avoided, smaller angle alpha and angle kappa measurements tend to produce better visual outcomes.
Go to https://cataractcoach.com/2020/10/07/angle-alpha-and-angle-kappa-with-iols to see surgical video of this case at CataractCoach.com.