In this issue of the Journal, Oliver and colleagues report that photorefractive keratectomy (PRK) alters the shape of the cornea, increasing corneal aberrations and consequently decreasing the modulation transfer function of the cornea, and presumably the eye.1 Their article adds to the growing body of evidence that PRK2 and radial keratotomy36 decrease refractive error and increase ocular aberrations that reduce optimal visual performance for larger pupils.6 We use this collective body of work as a spring board to discuss refractive surgery, ocular aberrations, and visual performance with the goal of moving the field toward designing and creating the ideal optical correction for the eye.
In principle, refractive surgery is designed to optimize the optical performance of the eye without the aid of glasses or contact lenses. Currently, refractive surgical procedures have focused on eliminating spherical and cylindrical defocus (Fig 1), the most important ocular optical aberration to correct. However, such an approach ignores the fact that the eye has significant higher order aberrations (Fig 2). These naturally occurring higher order aberrations, combined with large increases in the eye's higher order aberrations induced by refractive surgery1,3,6, can decrease visual performance despite the elimination of spherocylindrical errors.5,6
Surgically induced higher order aberrations have not been entirely ignored but they have received less attention than the correction of defocus errors, despite the fact that they are important to patient acceptance and to optimal visual performance. For instance, it was recognized a decade ago that visual outcomes were improved by correctly centering the procedures on the entrance pupil and using the largest central zone possible.7'10 Both of these steps significantly reduced the visual impact of surgically induced higher order aberrations (Fig 3) but they are not sufficient to create the ideal compensating optic.
What is the ideal compensating optic?
An ideal compensating optic would perfectly correct all optical aberrations of the eye for all pupil sizes. For a variety of reasons, including the viewing distance and changes in accommodative status, no fixed optic could meet this strict requirement for all retinal locations simultaneously. Fortunately in the eye, only the fovea has the spatial resolution necessary to take advantage of aberration-free optics and the design of the optic need only provide minimized aberration imagery over a limited area. Thus, the ideal compensating optic for best vision- refractive surgery, aphakic or phakic intraocular lens (IOL), contact lens or other optical device- should minimize optical aberrations for foveal distance viewing.
Figure 1: We generally conceive of the ametropic eye (in this case the simple myopic eye) as perfectly focusing light from an infinitely distant object point to a point image in front of the retina (A) and that this refractive error can be corrected with a simple spherical lens of minus power that diverges light before entering the eye such that when combined with the dioptrics of the eye, light from a distant object point is focused on the retina (B). Dashed rays are blocked by the pupil and do not participate in image formation.
Creating the Ideal optical correction
The cornea and/or crystalline lens for an individual eye may have relatively high aberrations as an optical element by itself, but when placed together to form the optical system of the eye, the total system may be relatively free of aberrations. This means the ideal shape of the cornea to minimize the aberrations of the eye cannot be determined from corneal topography. It also means that calculations of the cornea's higher order aberrations from corneal topographical measurements alone, as presented by Oliver and colleagues1 and in our previous work3,6,11,12, are based on carefully chosen reference surfaces. Due to lack of information concerning the total aberrations of the eye, these reference surfaces do not represent the shape of the cornea that would minimize ocular aberrations at the fovea. That is, the ideal shape of the cornea of any individual eye to render the eye aberration free at the fovea must be based on measurements of the total aberration structure of the eye and not the cornea alone. In principle, creating the ideal optical correction is straightforward- measure the aberration structure of the eye and use the measurements to design a compensating optic for placement in perfect registry with the existing optics, where it must remain. Such ideal corrections might be implemented using a contact lens, refractive surgery, or implantation of a phakic or aphakic IOL.
There are several methods for measuring the aberration structure of the eye. Clinically, the three most promising techniques appear to be the Hartmann Shack13, the objective aberroscope14, and the spatially resolved refractometer.1616 Although it is beyond the intended scope of this article to discuss the operating principles of each of these devices, it is worth emphasizing the importance of having a consensus on a fixed reference location for aberration measurement and correction to allow comparison and evaluation among the measurement and correction techniques. By convention, aberration measurements are referenced to the center of the entrance pupil. For an eye that has nonrotationally symmetric optics and a decentered fovea, it is important to use the line of sight (in object space, the line joining the object point of regard and the center of the entrance pupil and in image space, the line that joins the center of the exit pupil and the foveola) to define the center of the entrance pupil and not the visual axis (in object space, the line joining the object of regard and the first nodal point and in image space, the line that joins the second nodal point and the foveola).
Figure 2: The myopic eye has higher order aberrations that cause light from an infinitely distant point not to be perfectly focused in front of the retina (A). When this typical myopic eye is corrected with a simple minus spherical correction, the less than perfect focus is simply moved to the retina (B). Dashed rays are blocked by the pupil and do not participate in image formation.
Once quantified, the measured aberration structure of each individual eye can be used to design the ideal compensating optic for the eye. The ideal optic will alter the aberrated wavefronts (or in terms of ray optics, the direction of each ray) such that, when combined with the eye's optics, aberration-free imaging is achieved over the foveal area for all physiological pupil sizes. If the spherical and cylindrical components of the ideal optic are low enough that available corneal thickness is not an issue, a large diameter optical zone can be designed and created in the cornea. For example, assuming that variability in healing can be controlled, a small beam scanning excimer laser with eye tracking could carve the desired corneal shape into the stroma using PRK or laser in situ keratomileusis (LASIK). If the spherical and/or cylindrical components are large enough to make corneal thickness an issue, a large aperture optic can be designed in the form of a phakic or aphakic IOL, or an IOL could be combined with corneal refractive surgery to achieve the desired outcome. The major limiting problems with such an approach will be the maintenance of proper optical registration and the desired corneal shape during and after the healing process.
Figure 3: When a simple myopic eye is corrected with refractive surgery that perfectly corrects the myopia over a small optical zone, the eye is highly aberrated when the pupil is large (A). When the perfect optical zone correction increases in size, the aberrations decrease accordingly (B).
What are the visual consequences of providing the eye with such an ideal compensating optic?
The visual consequences are pupil size dependent. For the typical eye, when the pupil size is smaller than approximately 2.5 to 3 mm, the eye is diffraction limited and the ideal compensating optic will not provide an optical advantage to the visual system. This fact provides a generous buffer for error in refractive surgery. As long as the defocus error (the residual refractive error) is minimal and the surgery is well centered on the eye's entrance pupil, most patients will be happy with their outcomes under bright light conditions when their pupil is small. For pupil sizes larger than 3 mm where the eye is aberration limited (as opposed to diffraction limited), the ideal compensating optic for all but the largest pupil sizes will provide the best possible image for the visual system to interpret. For pupils over approximately 8 mm, the perfect compensating optic may provide images with detail on the order of smaller than a single receptor, raising the potential for significant aliasing (false spatial information resulting from a mismatch between the spatial detail in the image and the spatial density of the receptors) within the fovea. In most circumstances, aliasing will never be realized because it is extremely unlikely that a perfect compensating optic could ever be fully realized from a surgical or contact lens correction over an 8-mm or larger pupil.
The human visual system is the world's most versatile and accomplished image processor, and is theoretically capable of seeing 20/8. This is the challenge to refractive surgeons. Conversely, if there is relevant information in the optical image, no matter how aberrated, we can generally learn to extract the information in a useful manner.
Viewing into the unaberrated eye
So far we have discussed methods to optomize vision in everyday life. There is another reason for reducing the aberrations of the eye and insuring that therapy does not aggravate the eye's aberration structure; that is, the clinicians view into the eye, just like our view to the outside world, is limited by the eye's aberration structure. If the eye were diffraction-limited or could be made diffraction-limited over a large pupil area (8 mm or greater), then clinicians and researchers could noninvasively view, m the living eye, retinal detail to the level of individual photoreceptors. It doesn't take much imagination to appreciate the opportunities this would create to increase our understanding of normal and pathological retinal function and anatomy.
In fact, the technology is already available and does not require touching the eye. Deformable mirrors can correct aberrations of the eye and provide a noninvasive view to the level of individual photoreceptors in the living eye.17 This exciting technology can operate in real time and in principle, could be coupled with clinical devices used to look into the eye, such as fundus cameras, laser scanning ophthalmoscopes, and slit-lamp microscopes.
Visual optics is taking on new clinical significance. Given that current refractive procedures can and do induce large amounts of higher order ocular aberration that often affects the patient's daily visual function and quality of life, we can no longer relegate the considerations of ocular aberrations to academic discussions. Instead, we need to move toward minimizing (not increasing) the eye's aberrations at the same time we are correcting the eye's spherical and cylindrical refractive error. These are exciting times in refractive surgery, which need to be tempered by the fact that after all the research, clinical, and marketing dust settles, the level to which we improve the quality of the retinal image will be guided by the trade-off between cost and the improvement in the quality of life that refractive surgery offers.
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