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The optics of IOL decentration
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In the last blog entry, I showed a case in which I performed an iris repair and IOL exchange. A reader asked why I removed the old IOL instead of simply repositioning it. That's a great question.
The old IOL was clearly decentered, and from the patient's chart, it was noted to be a lens with negative spherical aberration. This type of lens requires excellent centration with the patient's visual axis in order to properly offset the positive spherical aberration of the cornea. If I could be sure that the original IOL was in good condition and without damage, then I could re-center the lens in the visual axis. This would not be an easy task in this eye because the reconstructed pupil would shift to some degree due to the iris sutures. Additionally, at the beginning of the surgery, a chopper was used to lift the nasal iris, and it was noted that the nasal haptic of the lens was bent. The old IOL was removed, and it was replaced with an IOL with zero spherical aberration, which is relatively immune to decentration.
 Negative spherical aberration IOLs work best when they are centered with the visual axis. Click image for larger view |
In the diagram shown here, a negative spherical aberration IOL has a different power in the center than in the periphery — the opposite of the cornea. When the negative spherical aberration IOL is aligned with the cornea, the total spherical aberration of the eye is cancelled out and the patient achieves excellent visual quality. But when the negative spherical aberration IOL is decentered with respect to the visual axis, it does not evenly address the corneal positive spherical aberration and, as such, it induces coma. Coma is where the power is higher on one side vs. the other, and the result is a comet-shaped aberration of the image.
In routine cases, aspheric IOLs with negative spherical aberration are good choices because they tend to improve image quality. However, in eyes in which you think that there is a chance for IOL decentration, you may be better off using a zero spherical aberration IOL instead.
See Dr. Devgan share more expert insight live at OSN New York 2010, to be held November 19-21, 2010 at the Sheraton New York Hotel & Towers. Learn more at OSNNY.com.