Analyzing the Components of Aspheric Lenses

The introduction of the first aspheric IOL represents one of the most important innovations in ophthalmic surgery. The Tecnis lens (Advanced Medical Optics, Inc., Santa Ana, California) has been shown to improve contrast sensitivity and quality of vision by reducing spherical aberrations.^1-3

Since the Tecnis was introduced, many leading IOL manufacturers have developed a variety of aspheric lens designs, each differing according to the position of the aspheric surface and the extent of spherical aberration correction.
We have studied several aspheric models using a ray-tracing program we developed and a pseudophakic eye model. Our analysis simulated scotopic conditions of illumination through a 5-mm pupil. All lenses used in the simulation were 20.00 D. Lens feature data (ie, curvature rays, aspheric surface features, central and peripheral thickness, refractive index) were then entered into the computer.

Using this technique, we evaluated four aspheric IOLs for their performance in regard to depth of focus:

•Tecnis: a three-piece silicone lens with the aspheric surface on the anterior of the lens;

•AcrySof IQ (Alcon Laboratories, Inc., Fort Worth, Texas): a single-piece acrylic lens with the aspheric surface on the posterior of the lens and a yellow blue-blocker chromophore inside the optic;

•SofPort AO (Bausch & Lomb, Rochester, New York): a three-piece silicone lens with aspheric profiles on the anterior and posterior surfaces of the lens; and

•KS-3Ai (STAAR Surgical, Monrovia, California): a three-piece silicone lens with the aspheric profile on the anterior surface of the optic. This lens comes preloaded in its injector.

RESULTS
We tested the depth of focus to find the limit conditions of an acceptable point spread function (PSF). Each lens was subjected to a defocus of ±0.08 mm, followed by a monochromatic PSF evaluation for three color curves (ie, yellow, red, blue). Figure 1 represents the PSF for the three colors, with a defocus of -0.08 mm and 0.08 from the yellow best focus. The total focus depth was 0.16 mm, corresponding to an object distance varying from infinity to approximately 1.8 m. This is an interesting test to investigate a patient's reasonable capacity of driving a car under scotopic conditions.

LENS BEHAVIOR
To better understand the depth of focus for each of the four lenses, we determined the variation of Strehl ratio for the three colors found in each lens. Figure 1, which shows the change of Strehl ratio (ie, vertical axis, %), in terms of defocus (ie, horizontal axis, mm) for yellow, blue, and red, demonstrates that the larger the overlap of a curved color, the greater the depth of focus. This is particularly important for blue and yellow curves because we are working in scotopic conditions. It is also important to note that the value of the Strehl ratio determines the amount of light that reaches the retina for each wavelength. Both large overlapping curves and a high Strehl ratio show that there is a good depth of focus (Figure 2).

STREHL RATIO FOR EACH LENS
We found that the Tecnis lens presented a high Strehl ratio for all colors and a large overlap of the yellow and blue curves. There is some shift of the red curve, which partially overlaps the yellow and blue curves.

The AcrySof IQ shows higher Strehl ratios, but the overlap of the blue and yellow curves is poor, and only marginal with the red and yellow curves. No overlap occurs with the blue and red curves.

SofPort AO has low Strehl ratios with a large overlap of the three color curves. Lastly, the KS-3Ai shows intermediate Strehl ratios and a poor overlap.

We believe that at a glance, the results obtained in the depth of focus evaluation show that the Tecnis IOL may be the most balanced aspheric lens.

DEPTH OF FOCUS
To further validate our findings, we stated a threshold of Strehl ratio (eg, 10%) and computed the corresponding depth of focus for the three colors, either combined or in pairs (eg, yellow and blue). Using the Gaussian formula, we computed the depth of focus and determined the distinct vision range of each lens.

At a threshold Strehl ratio of 10%, the Tecnis presented a 0.16-mm depth of focus for yellow and blue that corresponded to a distinct vision range of infinity to approximately 1.829 m. Under the same conditions, the AcrySof IQ had a 0.17-mm depth of focus that corresponded to a distinct vision range of infinity to approximately 1.7 m. The SofPort AO showed a 0.03-mm depth of focus, and corresponded to a distinct vision range of infinity to approximately 9.65 m. The KS-3Ai lens showed a 0.065-mm depth of focus and corresponded to a distinct vision range of infinity to approximately 4.45 m.

If we consider all three colors, Tecnis—at a Strehl ratio threshold of 10%—presented a 0.09-mm depth of focus and corresponded to a distinct vision range of infinity to approximately 3.3 m.

Under the same considerations, the AcrySof IQ and SofPort lens demonstrated minimal depth of focus. Additionally, the KS-3Ai showed a 0.065-mm depth of focus and corresponded to a distinct vision range of infinity to approximately 4.45 m.

CONCLUSION
Our results showed that it is insufficient to design an aspheric IOL that maintains a degree of spherical aberration to obtain pseudoaccommodation. In fact, if the quantity of light focused along the optical axis in front of and behind the retina does not guarantee sufficient illumination, it actually decreases the quality of the main image.

Alessandro Franchini, MD, is Professor at the School of Ophthalmological Specialization, University of Florence, in Italy. Dr. Franchini states that he has no financial interest in the companies or products mentioned. He may be reached at alessandrofranchini@yahoo.it.

1. Sandoval HP, Fernández de Castro LE, Vroman DT, Solomon KD. Comparison of visual outcomes, photopic contrast sensitivity, wavefront analysis, and patient satisfaction following cataract extraction and IOL implantation: aspheric vs spherical acrylic lenses. Eye. 2007 [Epub ahead of print].
2. Denoyer A, Le Lez ML, Majzoub S, Pisella PJ. Quality of vision after cataract surgery after Tecnis Z9000 intraocular lens implantation: effect of contrast sensitivity and wavefront aberration improvements on the quality of daily vision. J Cataract Refract Surg. 2007;33:173-174.
3. Bellucci R, Scialdone A, Buratto L, et al. Visual acuity and contrast sensitivity comparison between Tecnis and AcrySof SA60AT intraocular lenses: A multicenter randomized study. J Cataract Refract Surg. 2005;31:712-717. Erratum in: J Cataract Refract Surg. 2005;31:1857.