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  • Pseudophakic Correction of Spherical Aberration

    Cataract/Anterior Segment
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    Because the optical wavefront of the cornea remains essentially stable throughout life,1 refractive lens exchange appears to represent a permanent solution to the challenges of restoring accommodation and achieving youthful quality of vision. To that end, recent advances in aspheric monofocal lens design have lent themselves to improvements in multifocal and accommodative intraocular lenses (IOLs), as attention has turned to the development of aspheric IOLs to remedy the increase in total optical aberrations that result from the positive spherical aberration of a spherical pseudophakic IOL.2 These designs are intended to reduce or eliminate the spherical aberration of the eye, improve modulation transfer function as compared with a spherical pseudophakic implant, and enhance functional vision. A variety of aspheric IOL designs are currently marketed in the United States: the Tecnis Z9000 IOL, the AcrySof IQ IOL, and the SofPort AO IOL.

    Comparing Lenses

    Tecnis Z9000. The Tecnis IOL was designed with a modified prolate anterior surface to compensate for the average corneal spherical aberration found in the adult eye. It shares basic design features with the CeeOn 911A IOL including a 6-mm, biconvex, square-edge optic and angulated “capsular C” polyvinylidene fluoride (PVDF) haptics. The Tecnis Z9000 is a multi-piece lens. It is available in both second-generation silicone and acrylic. The silicone IOL has a refractive index of 1.46, and the acrylic lens has a refractive index of 1.47. The Tecnis Z9000 introduces -0.27 µ of spherical aberration to the eye (Figure 1).

    Courtesy Advanced Medical Optics.
    Figure 1. Tecnis IOL.

    The clinical investigation of the Tecnis IOL submitted to the U.S. Food and Drug Administration (FDA) demonstrated elimination of mean spherical aberration as well as significant improvement in functional vision when compared to a standard spherical IOL.3 The U.S. Centers for Medicare and Medicaid Services announced New Technology IOL Status for the Tecnis IOL on January 26, 2006.

    AcrySof IQ. The AcrySof IQ shares the ultraviolet (UV) and blue light-filtering chromophores as those found in the single-piece, acrylic AcrySof Natural IOL. The special feature of this IOL is the posterior aspheric surface, which is designed to compensate for spherical aberration by addressing the effects of over-refraction at the periphery. The AcrySof IQ is a single-piece lens made of hydrophobic acrylic, and it has a refractive index of 1.55. It adds -0.20 µ of spherical aberration to the eye (Figure 2).

    Courtesy Mark Packer, MD.
    Figure 2. AcrySof IQ.


    SofPort Advanced Optics. The SofPort Advanced Optics (LI61AO) IOL is an aspheric IOL that has been specifically designed with zero spherical aberration, so that it will not contribute to any preexisting higher-order aberrations. It is a foldable silicone IOL with polymethylmethacrylate (PMMA) haptics and square edges. It was specifically designed for use with the Bausch & Lomb SofPort System, an integrated, single-use, single-handed planar delivery IOL insertion system. The SofPort lens is a multi-piece lens made of second-generation silicone. It has a refractive index of 1.43, and it introduces no spherical aberration to the eye (Figure 3).

    Courtesy Mark Packer, MD.
    Figure 3. Advanced Optic IOL.

    Comparing Outcomes

    Peer-reviewed, prospective, randomized scientific publications have demonstrated reduction of spherical aberration and excellent contrast sensitivity and contrast acuity with the Tecnis modified prolate IOL in comparison to a variety of spherical IOLs.4-11 For example, Mester compared the quality of vision obtained with the Tecnis IOL and a spherical silicone IOL (SI 40) at the American Society of Cataract and Refractive Surgery (ASCRS) symposium (June 2, 2002). A total of 45 patients were enrolled and randomized to receive the Tecnis IOL in 1 eye and the SI 40 in the fellow eye. The average photopic contrast sensitivity values demonstrated a statistically significant advantage for the Tecnis IOL at all spatial frequencies.

    The contrast sensitivity curves show an even greater difference under mesopic conditions, an expected result due to the larger pupil size and consequent greater contribution from spherical aberration in dim light. A comparison of corneal and total ocular aberrations demonstrates the improved wavefront of the eye with the Tecnis IOL. This improvement in total aberrations demonstrates the critical compensatory relationship of cornea and lens in reducing spherical aberration. As of this writing, there are no peer-reviewed publications evaluating clinical results with either of the other 2 aspheric IOLs available in the United States.

    Regarding visual acuity, most studies have shown a slight although not statistically significant improvement in best corrected Snellen acuity with aspheric IOLs when compared to traditional IOLs.  The FDA-monitored study of the Tecnis IOL did, however, show a statistically significant advantage for the Tecnis IOL. This was a double-masked, prospective, randomized, multicenter trial.

    Patient and Lens Selection

    Data show that healthy, normal subjects undergoing uncomplicated cataract surgery benefit from an aspheric IOL that balances the spherical aberration of the cornea.  Patients with compromised eyes due to pathologic processes other than cataract have not been studied in this regard.  In particular, any process which may significantly increase the risk of decentration or tilt of the IOL (greater than 0.5 – 0.8 mm) represents a relative contraindication to IOLs with negative spherical aberration .

    Previous refractive surgery generally alters the spherical aberration of the cornea and may alter the choice of IOL.  Procedures for myopia generally increase the positive spherical aberration of the cornea and generally indicate an IOL with negative spherical aberration. The spherical aberration of the cornea can be ascertained with topography by using available software; the surgeon can then match the IOL to the cornea.  Besides matching the IOL to the cornea, there may be other considerations which affect IOL choice: material—silicone vs. acrylic; clear vs yellow; and design—single piece vs. multiple piece. 

    Conclusion

    The youthful, emmetropic, minimally (or perhaps optimally) aberrated eye has become the standard by which the results of cataract and refractive surgery are evaluated. The erosion of accommodation and the decline in functional vision that occurs with age have both been linked to changes in the human lens.12,13 Lens replacement surgery, therefore, offers a natural avenue for the correction of presbyopia as well as for the reversal of increasing lenticular spherical aberration, and the lens has started to come into its own as the primary locus for refractive surgery.

    As the weight of evidence demonstrating superior functional vision and contrast sensitivity with the modified prolate IOL has continued to grow, the reversal of age-related decline in contrast sensitivity through the pseudophakic elimination of spherical aberration confirms the hypothesis that decreased functional vision results primarily from aging changes in the human lens.

    References

    1. Wang L, Dai E, Koch DD, Nathoo A. Optical aberrations of the human anterior cornea.J Cataract Refract Surg. 2003;29(8):1514-1521.
    2. Holladay JT, Piers PA, Koranyi G, van der Mooren M, Norrby NE. A new intraocular lens design to reduce spherical aberration of pseudophakic eyes.J Refract Surg. 2002;18:683-691.
    3. Tecnis Foldable Ultraviolet Light-Absorbing Posterior Chamber IOL [package insert]. Santa Ana, CA: Advanced Medical Optics, Inc.; 2005.
    4. Packer M, Fine IH, Hoffman RS, Piers PA. Prospective randomized trial of an anterior surface modified prolate intraocular lens.J Refract Surg. 2002;18:692-696.
    5. Mester U, Dillinger P, Anterist N. Impact of a modified optic design on visual function: clinical comparative study.J Cataract Refract Surg. 2003;29(4):652-660.
    6. Packer M, Fine IH, Hoffman RS, Piers PA. Improved functional vision with a modified prolate intraocular lens.J Cataract Refract Surg. 2004;30:986-992.
    7. 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(4):712-717.
    8. Kennis H, Huygens M, Callebaut F. Comparing the contrast sensitivity of a modified prolate anterior surface IOL and of two spherical IOLs.Bull Soc Belge Ophtalmol. 2004;294:49-58.
    9. Kershner RM. Retinal image contrast and functional visual performance with aspheric, silicone, and acrylic intraocular lenses. Prospective evaluation.J Cataract Refract Surg. 2003;29:1684–1694.
    10. Ricci F, Scuderi G, Missiroli F, Regine F, Cerulli A. Low contrast visual acuity in pseudophakic patients implanted with an anterior surface modified prolate intraocular lens.Acta Ophthalmol Scand. 2004;82(6):718-722.
    11. Martinez Palmer A , Palacin Miranda B, Castilla Cespedes M, Comas Serrano M, Punti Badosa A. Spherical aberration influence in visual function after cataract surgery: prospective randomized trial [in Spanish].Arch Soc Esp Oftalmol. 2005;80(2):71-77.
    12. Artal P, Berrio E, Guirao A, Piers P. Contribution of the cornea and internal surfaces to the change of ocular aberrations with age.J Opt Soc Am A Opt Image Sci Vis. 2002;19:137-143.
    13. Glasser A, Campbell MC. Biometric, optical and physical changes in the isolated human crystalline lens with age in relation to presbyopia.Vision Res. 1999;39(11):1991-2015.

    Author Disclosure

    Dr. Packer states that he is a consultant for Advanced Medical Optics (AMO) and Bausch & Lomb, and that he has received travel support and honoraria from Alcon. He has no financial interest in any of the products or procedures described in this article.