Adopting New Technology IOLs
©2004. Robert M. Kershner, MD, MS, FACS
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.
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