In pseudophakic wavefront correction, there are three basic elements to control: sphere, cylinder, and spherical aberration. Quite obviously, manipulating all three areas to offset their impact in the aging eye would provide the optimal quality of vision and depth of focus. The classical correction of ammetropia, however, is aimed at fixing sphere and sometimes cylinder—or rather just first-order aberrations. The only current and available method for pseudophakic wavefront-guided correction is aspheric IOLs.

Current aspheric IOL models offer a range of possible precise corrections for the first-order aberrations of sphere and sometimes cylinder, because it is possible to calculate these preoperatively. The problem we run into is that some measurements, such as spherical aberration, cannot be properly calculated preoperatively. This is because the crystalline lens influences the outcome of spherical aberration, so once it is removed, the spherical aberration may change again. Except in some particular highly aberrated eyes (eg, complicated eyes after corneal refractive surgery, complicated eyes after penetrating keratoplasty), other higher-order aberrations including coma and trefoil are not, from a quantative point of view, important. Spherical aberration is, however, important, and current models correct for 0.27 µm of spherical aberration (ie, the average spherical aberration of the cornea).

TOO COSTLY
Although this correction is adequate to provide most patients with good functional vision, it seems to provide the best visual results in hyperopes and post-myopic LASIK patients. If we could find the means to develop customized aspheric IOLs, offering our patients a level of postoperative spherical aberration equal or close to zero, we could treat an unending number of patients with the best possible quality of vision and depth of focus. At this time, however, it is too costly to manufacture a lens with the upward of 10 different asphericities for each power of sphere—and cylinder when it is available.

Finding a method to correct for sphere, cylinder, and spherical aberration may mean that we would not need to use multifocality in some of our patients to increase depth of focus. One promising method of manipulation is UV light, which is possible with the new Calhoun Light Adjustable Lens (LAL; Calhoun Vision, Inc., Pasadena, California) and the Digital Delivery Light System (DLD; collaboratively designed by Calhoun Vision, Inc., and Carl Zeiss Meditec AG, Jena, Germany). The lens is a polymer of silicone, and the number of molecules that are inside the lens are sensible to UV light, providing their precise distribution in the lens. (See Light Adjustable Lens Offers Noninvasive Treatment for more information.)

CHANGES TO THE IOL SURFACE
Although this technology does not yet individualize the spherical aberration, it does precisely change the IOL surface shape by using UV light to distribute the molecules in a submicron manner. Approximately 1 to 4 weeks after the LAL is implanted during cataract surgery, the surgeon irradiates the lens with the DLD, which spatially differentiates UV light and causes selective polymerization and diffusion of macromers. Waiting 1 to 4 more days, the surgeon again irradiates the lens to obtain the specified power for sphere and cylinder. Manipulating the LAL should be relatively easy, especially in regard to asphericity because it is a symmetrical aberration.

There are other options beside the LAL, although this may be conceptually the easiest method at the current time. One option is to improve spherical aberration calculation methods, much to the same way that we improved upon our biometry calculations for the final outcome in sphere. If we could devise a new instrument to preoperatively calculate the ideal cylinder and spherical aberration for a given eye as well as the influence of the natural crystalline lens for both aberrations, we may be able to better apply the existing aspheric IOL models in our patients.

If we can individualize our final outcome in these three aberration categories (ie, sphere, cylinder, aspherical), we would be in a much better situation than we are in today.

The take-home message is that we are all focused—and need to continue to be focused—on improving not only visual acuity but also visual quality. For this particular goal, the use of wavefront pseudophakic IOLs and new instruments to evaluate visual function is critical. We cannot consider wavefront pseudophakic lenses as we have considered wavefront-guided excimer laser correction. Practically speaking, we must find a way to adjust the three main aspects of aberration once the IOL is inside the eye. Until the time that companies can find the financing and means to offer aspheric IOL customization—where the surgeon tells the manufacturer the particular asphericity and it is built accordingly—I believe that the LAL represents a promising development in the future of pseudophakic wavefront correction.

José L. Güell, MD, PhD, is Director of the Cornea and Refractive Surgery Unit at the Instituto de Microciruglía, in Barcelona, Spain, and Associate Professor of Ophthalmology at the Autonoma University of Barcelona. Dr. Güell owns stock in Calhoun Vision, Inc., is the European Medical Director for the LAL project. He may be reached at +34 93 253 15 00; guell@imo.es.