A new software platform, called Okulix (Ingenieurbüro der Leu, Hillerse, Germany), enables the clinician to integrate corneal topography into IOL calculations and evaluate the retinal image quality based on the individual's IOL parameters. These capabilities set the stage for producing truly customized IOLs.
By using numerical ray tracing of the entire pseudophakic eye and including manufacturers' original IOL data (radius, thickness, and refractive index), the Okulix software program can generate blurred Landolt's rings that are superimposed on the retinal photoreceptor grid in order to demonstrate the actual image quality produced. Clinical applications of this software include determining IOL calculations after corneal refractive surgery and improving vision by customizing the IOL's parameters. The program can even calculate the specifications for an IOL to balance the aberrations inherent in an eye with a decentered PRK ablation in order to produce an image of very high definition.
IMPETUS FOR CHANGE
Customizing IOLs and what is called just-in-time production could save manufacturers large sums of money by reducing their amount of idle inventory while simultaneously improving patient outcomes. For example, in the case of a toric IOL with a spherical power from 10.00 D to 30.00 D in 0.50-D increments and an astigmatic power from 0 D to 6.00 D in 0.50-D steps, the whole stock volume is 31 X 8 = 248 pieces. Most hospitals and surgery centers, however, keep more than one IOL of each power on hand, a step that doubles or triples the number of idle lenses they maintain. Just-in-time production eliminates inventory stocks by having customers directly communicating a patient's ocular measurements to the manufacturer, which then creates and ships the IOL.
Although most IOLs today are spherical, clinical studies show an improvement in functional vision in eyes implanted with the new Tecnis lens (Pharmacia Corporation, Peapack, NJ).1 This modified prolate IOL is designed to correct spherical aberration, a higher-order optical aberration that increases with age. The IOL's design is based on an average cornea model so that it corrects for the mean spherical aberration found in the general population of cataract patients. However, because corneal asphericity varies among individuals (especially following keratorefractive surgery), it makes sense that further improvements in visual acuity can be obtained by producing IOLs with a range of corrections for spherical aberration. The postmyopic LASIK patient could receive an IOL with greater negative spherical aberration; the average patient could receive a lens with midrange negative spherical aberration; and the posthyperopic LASIK patient could receive an IOL with less negative spherical aberration still.
Adding a new parameter to IOL production under the current inventory model would drive up production costs immensely by requiring even more IOLs to remain idly on shelves around the world. The only cost-effective way to begin customizing IOLs with higher-order corrections is through just-in-time production. Fortunately, business sense and patient benefits create a win-win situation as just-in-time production of IOLs sets the stage for truly individualized IOL customization.
CALCULATING A CUSTOMIZED IOL
The Okulix program relies on ray-tracing the entire pseudophakic eye to create a customized IOL.2,3 Using Snell's law, the program can precisely calculate the refraction of individual rays. To save computing time, it performs the calculations backwards, starting at the fovea. The program refracts the rays on the anterior and posterior surfaces of both the IOL and cornea and then calculates these four refractions in three-dimensional geometry using corneal topography to represent the anterior corneal surface.
Creating the simulated visual impression (the Landolt's ring) entails tracing approximately 1,000 rays, depending on pupil size. The rays, which are purely mathematical constructs, have a uniform distribution in the pupillary plane. The calculations account for the diffraction from the pupil aperture.
The customized IOL is uniquely determined by a set of parameters that may be used to specify its production. These parameters include one anterior and two posterior vertex radii perpendicular to each other, the numerical eccentricity of the anterior surface, the central thickness of the IOL, the diameter of the optical zone, and the index of refraction.4,5
The corneal asphericity plays a key role in the design of a customized IOL. Measurements of asphericity normally differ from meridian to meridian, however, so that calculating the mean of these values produces mathematical nonsense. Therefore, the Okulix method extracts from the topographic data the corneal vertex radii and best-fit numerical eccentricity of the entire cornea in a consistent, three-dimensional model.6
Demonstrating the benefits of a customized IOL requires comparing it with a standard lens. Figure 1 shows the optical properties of an eye implanted with a standard IOL, while Figure 2 depicts the optical properties of the same eye after receiving a customized lens implant. Figure 3 demonstrates a clear improvement in visual outcome with a customized versus standard IOL under the same conditions. These computerized simulations demonstrate the promise of optically customized IOLs.
New technology always demands more of the surgeon. In theory, functional vision with a customized IOL will always be better than vision with a standard IOL, even with additional spectacle or contact lens correction. The customized IOL, however, must remain centered and oriented correctly. If it is not, the outcome may be worse than with a standard lens implant. Decentration of an aspheric IOL has a greater negative impact than decentration of a spherical IOL, and rotation is more problematic with toric lenses. Misalignment becomes an even more significant problem with IOLs designed to correct higher-order aberrations, such as coma and trefoil.
Because postoperative shrinkage of the capsular bag always causes IOL rotation and sometimes decentration, my colleagues and I are currently developing a new type of capsular tension ring. Its fixed diameter after implantation prevents any shrinking of the capsular bag, even if capsular fibrosis occurs.
As advances in technology allow cataract and refractive surgeons to address higher-order optical aberrations, the measurement of functional vision becomes increasingly critical as a gauge of our progress. Sine wave grating contrast sensitivity testing is assuming a prominent place in our evaluation of surgical modalities, because it reflects functional vision, correlates well with visual performance, and provides a key to understanding the optical and visual processing of images.
Customized IOLs hold the promise of improved contrast sensitivity, better functional vision, and superior patient outcomes. These benefits will entice surgeons to continue riding the wavefront of progress in lenticular refractive surgery, while the potential economic benefits of just-in-time production will hold industry's attention. The Okulix program represents an important advance in the individual customization of IOLs.
Paul-Rolf Preussner, MD, PhD, is a physicist and ophthalmic surgeon, as well as an assistant professor at the University Eye Hospital in Mainz, Germany. He holds a financial interest in the Okulix program package, which was used for the optical calculations in this article, and he holds a patent for the fixable capsule tension ring manufactured by Acri.Tec GmbH (Berlin). Dr. Preussner may be reached at +49 61 31 17 22 14; firstname.lastname@example.org.
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2. Preussner PR, Wahl J. Consistent numerical calculation of optics of the pseudophakic eye. Ophthalmologe. 2000;97:2:126-141.
3. Preussner P, Wahl J, Lahdo H, et al. Ray tracing for intraocular lens calculation. J Cataract Refract Surg. 2002;28:8:1412.
4. Preussner PR, Wahl J. Simplified mathematics for customized refractive surgery. J Cataract Refract Surg. In press.
5. Olsen T, Corydon L, Gimbel H. Intraocular lens power calculation with an improved anterior chamber depth prediction algorithm. J Cataract Refract Surg. 1995;21:3:313-319.
6. Preussner PR, Wahl J, Kramman C. Corneal model. J Cataract Refract Surg. In press.