Cataract removal is the most commonly performed conventional surgical procedure performed in America. Medicare alone reimburses for over 2 million operations yearly.
Removing the lens from an expensive camera renders it useless – the same goes for the eyeball. Cataract surgery subtracts a whopping 18 diopters of focusing power from the normal adult eye. That is approximately one-third of the eye’s focusing ability. Eye surgeons compensate for this loss with implantation of an artificial intraocular lens (IOL).
Over the last 30 years there have been tremendous improvements in IOL materials and optical design, so much so that implantation of an ordinary IOL generated dissatisfaction among some recipients. These unhappy individuals could see the 20/20 line on the eye chart but claimed the vision was inferior to the kind of eyesight they experienced prior to cataract formation. Sometimes they said unkind things to the doctor. Were they just too picky?
Today we recognize that patient post-op expectations outpaced IOL technology. Those cranky patients were right! Sure they could see the 20/20 line but their vision fluctuated in different lighting conditions. Contrast sensitivity, a superior method of assessing visual function, was markedly diminished. Back in 1980 anything approaching a 20/20 outcome was cause for celebration. Today it takes more to make folks happy.
What was the explanation for the unsatisfactory post-op vision? A lot of it is attributed to something called spherical aberration. No man-made lens is perfect (spectacle, contact lens, IOL). The optics may be precise at dead center but as you travel towards the outer edge of the lens the focusing can change dramatically. Spherical aberration (SA) distorts and degrades the visual image. To make things worse SA is magnified in high-power lenses like an +18 diopter IOL. In dim light the pupil enlarges and that also exposes the eye’s visual system to more SA. Finally, if the IOL shifts the tiniest bit (decentration or IOL tilt) there can be an abrupt loss of crisp vision because the center of the IOL is no longer aligned with the visual axis.
Mother Nature’s lenses aren’t much better. The natural lens has measurable minus-power SA; however the clear cornea compensates for this situation with it’s own plus-power SA. So, even if the surgeon precisely implants an optically perfect IOL the eye still has to deal with leftover positive-power SA from the cornea. Getting bleary-eyed yet?
The current solution for this dilemna is Aspheric IOLs. They have brand names like Technis, Acrysof IQ, and Sofport AO. These IOLs have restored the minus-powered SA that was present in the natural pre-cataract lens so that the net effect is minimal post-op spherical aberration.
If you got this far I must congratulate you for your intellectual curiosity! Patients receiving Aspheric IOLs report an extraordinarily crisp visual image. All they want to know is, “Doc, when can you do my other eye?” That’s the nicest thing anyone can say to an ophthalmologist!