Shifting Paradigms:
Adopting New Technology IOLs
 
©2004. Robert M. Kershner, MD, MS, FACS
Boston, Massachusetts
 
In September of 1984, I predicted that twenty years from now, we won’t be cutting, slashing, burning or otherwise modifying the human cornea to effect refractive change. Twenty years have passed and now that new technology IOLs are FDA approved and readily available, the time is right for surgeons to reexamine our paradigms for IOL use in cataract and refractive surgery.
 
The most powerful refracting surface of the human eye is the cornea. It is a living, breathing structure with complex anatomy. Unfortunately, it is far from perfect. Despite being responsible for over 85% of the refractive needs of the eye, it contributes a positive spherical aberration to the optical system. This is not much of a problem when we are young and our crystalline lens has an overall negative spherical aberration, the effect of the two are neutralized. But as the lens grows, it induces an overall positive spherical aberration compounding that of the cornea and visual quality suffers. That is why as we age, we have more difficulty reading a menu in a restaurant, walking at dusk and driving at night. It is not just presbyopia, a cataract or a failing retina that are at fault, the spherical aberration of the system has increased.
 
LASIK, LASEK and PRK fail to improve visual quality despite improving uncorrected acuity. It is simply impossible to improve functional vision when additional spherical aberration is induced into the system whether it be by excimer laser, incisional keratotomy, implanted rings, conductive or thermal keratoplasty.  In IOL surgery the same problem exists, the spherical lenses commonly in use today, which have not changed since Ridley’s time, simply do not take into account the positive spherical aberration within the optical system. Having a positive spherical aberration of their own, all IOLs add to the worsening optical quality. Spherical aberration degrades image contrast, and although an image can still be discerned, the ability to detect it can be severely limited when contrast is low. That is why, following cataract surgery, a person with 20/20 vision on a high contrast Snellen Chart in the office (viewing black letters on a white background) can still complain of poor vision when reading or detecting objects, especially at night, when contrast is low.
 
Today we no longer have a choice of simple spherical IOLs. There are IOLs that split light to provide a range of focus for near vision, so-called multifocal designs such as the AMO Array™, and possibly a new addition the Alcon ReStor™, lenses that correct astigmatism, STAAR Toric, accommodative IOLs such as the eyeonics Crystalens™, and nearing approval are the phakic IOLs, the anterior chamber iris supported lens AMO Verisyse™, and posterior chamber, the STAAR Visian™. It is interesting to note that reports have already surfaced from numerous investigators demonstrating night vision problems, glare, and halos in many of these designs. Sound familiar?
 
What is the solution? Optical engineers can place a modified prolate surface to the anterior of an IOL optic that exactly neutralizes the spherical aberration of the average cornea (AMO Tecnis™ Z-9001 IOL). These lenses work because we have learned what we needed to fix. Wavefront technology, by measuring all those corneas that have been surgically altered and all those lenses we have eagerly implanted, have shown us the way. Neutralize the corneal aberration rather than alter it surgically and better vision will invariably result.
 
The time has come for cataract and refractive surgeons to reconsider this spherical paradigm. The human eye is not spherical. Optics should not be either. The only correction we have for sphere is sphere itself and that correction should be nonspherical.
 
©2004. Robert M. Kershner, MD, MS, FACS is director of Eye Laser Consulting in Boston, Massachusetts. He holds no financial or proprietary interest in any product or company. Dr. Kershner can be reached at Kershner@EyeLaserConsulting.com