Murat V. Kalayoglu, M.D., Ph.D.
Contributing Editor
The human eye is truly a remarkable piece of work, capable of perceiving the slightest motion and the most subtle change in light or color. But the human quest for improvement and perfection is insatiable. Indeed, we have identified subtle imperfections in every part of the visual axis that, if fixed, could potentially result in super – human visual function. For example, the cornea has inherent higher order aberrations, such as coma and spherical aberration, that lead to optical degradation even when refractive error is corrected with myopic or hyperopic astigmatic lenses. Perhaps the most significant corneal “imperfection” is spherical aberration. On average, most corneas have some degree of positive spherical aberration. When patients undergo cataract surgery and receive a traditional spherical intraocular lens, the implanted lens’s spherical aberrations can cause additional optical degradation. In many patients who have good retinal ganglion cell function in the macula, this amplification may lead to a degradation of potential contrast sensitivity and acuity. Therefore in theory, if the implanted intraocular lenses were to have no or negative spherical aberration, the patient may perceive higher contrast sensitivity and better visual acuity.
Several companies have developed a new generation of intraocular lenses that have negative or no spherical aberrations. These aspheric IOLs could provide improved contrast sensitivity and visual acuity over traditional, spherical IOLs. The aspheric IOLs may balance or counteract the positive spherical aberrations found on the cornea. Implanting a truly aspheric IOL on a patient with some positive spherical aberration of the cornea may help maintain the eye’s natural spherical aberration instead of inducing additional aberrations onto the macula. Implanting an IOL with a slightly negative spherical aberration into an eye with positive corneal spherical aberrations may actually eliminate the overall spherical aberration of the visual axis.
Aspheric IOLs with negative spherical aberrations have potential advantages and disadvantages. When implanted on patients who have an average amount of positive corneal spherical aberration, these lenses counter the corneal aberration and lead to a neutralization of total spherical aberration. Two such lenses are currently available in the U.S: The AcrySof SN60WF (Alcon) and Tecnis Z9000 (AMO Inc.).
Aspheric IOLs with no spherical aberrations do not contribute to any preexisting higher order aberrations on the visual axis. Therefore, if the IOL decenters in the bag, it is less likely to induce astigmatism or higher order aberrations such as coma. There is also a theoretical benefit of maintaining the positive corneal spherical aberration: some patients may experience a greater depth of field. Currently, the SofPort AO (Bausch & Lomb) is the aberration – free IOL available in the U.S.
One important consideration in choosing traditional IOLs versus aspheric IOLs is to understand the degree to which a patient might benefit from the newer aspheric IOLs. The patient’s contrast sensitivity and visual acuity is ultimately determined by his or her retinal function. Since all individuals lose retinal ganglion cell function as they age, the degree to which a typical 75 year old patient may perceive an added visual benefit from an aspheric IOL may be minimal. However, in the United States, there is a trend towards patients opting to have cataract surgery earlier in life, which could translate to more patients perceiving added benefit from subtle improvements in spherical aberrations.
Also important to note is that most data gathered from these lenses are based on theoretical and physical eye models. There are relatively few clinical trials on patients that represent the overall U.S. cataract surgery population and that measure specific perceived performance metrics (i.e. contrast sensitivity and visual acuity). Many patients may not be able to perceive the subtle improvements in their vision afforded by the aspheric IOLs. Available data strongly suggest that the most significant added benefit of using aspheric IOLs may be perceived in mesopic environments. Contrast sensitivity under mesopic conditions declines with age, and aspheric IOLs restore mesopic contrast sensitivity to maximal levels.
The new generation aspheric IOLs bring us a step closer to custom – designed intraocular lenses. Integration of lens design with wavefront corneal measurements could open the way to manufacturing IOLs specifically designed to counteract corneal aberrations in each individual patient. One can envision a patient visiting an Ophthalmologist for cataract evaluation, receiving wavefront measurements, then having cataract surgery with implantation of a custom – designed aspheric IOL that negates only the higher order aberrations found on his or her cornea.