Aberration-Free Optics Enhance Performance

By Louis D. ‘Skip’ Nichamin, MD

As the potential benefit of aspheric optics for IOLs has become more widely appreciated, manufacturers have been able to incorporate these advanced designs into just about every platform, including toric, multifocal, and accommodating lenses, as well as the latest generation of implants that are comprised of newer bio-materials. One such aspheric design is incorporated into the recently FDA-approved single-piece enVista IOL (Bausch + Lomb), which is made of a novel hydrophobic acrylic material that has been shown in clinical trials to be glistening-free. (See “Introducing the enVista IOL,” on page 2.)

A substantial number of studies in both model eyes and patients have been conducted over the past decade evaluating the performance of aspheric IOLs.^1-5 The preponderance of these studies demonstrate that IOLs with aspheric optics provide better quality of vision than traditional spherical implants.^1-5 These lenses improve image contrast that is projected upon the retina by reducing vision-degrading spherical aberration that has traditionally been exacerbated by the positive spherical aberration inherent in standard spherical IOLs.^1-5

The new enVista IOL incorporates Bausch + Lomb aberration-free aspheric Advanced Optics (AO). With this unique approach, both the anterior and posterior surfaces are aspheric, making the lens free of inherent spherical aberration. As a result, an AO lens will not introduce additional spherical aberration into the eye and is designed to achieve its goal of providing better contrast sensitivity, particularly in low-light conditions, than spherical lenses.^2,6 Other aspheric IOLs (such as the Tecnis model from Abbott Medical Optics and the AcrySof IQ model from Alcon) contain negative spherical aberration aimed at counteracting the positive spherical aberration in the average cornea in order to reduce total spherical aberration to near zero. Aspheric lenses with negative spherical aberration have been shown to improve contrast sensitivity but may decrease depth of field compared to other IOLs.^7,8 The AO aspheric concept is based upon the underlying principle that maintaining the eye’s natural corneal positive spherical aberration will enhance contrast sensitivity without compromising depth of field as compared to traditional spherical IOLs.

Studies confirm the effectiveness of the AO approach. For example, in a four-site, prospective, double-masked, randomized study, 80 patients received an AO lens in one eye and a Tecnis lens in the other. Patient satisfaction was high with both lenses, and both produced similar high- and low-contrast visual acuity and photopic and mesopic contrast sensitivity. The Tecnis eyes had lower spherical aberration, but a larger depth of field was calculated in the AO eyes. Also, twice as many patients reported that they preferred the vision in the AO eye. The researchers concluded that the greater depth of field in the AO eyes may have contributed to the preference, and wrote, “ … it would appear that maximum reduction of spherical aberration does not correlate with the perceived visual quality of the eye having surgery.”⁹

Because they are aberration-free, lenses with AO optics are suitable for all patients regardless of their corneal shape or whether that shape deviates from the refractive profile of the average cornea. Furthermore, because AO lenses have uniform power from center to edge, their performance is unaffected by optical misalignment, pupil decentration or IOL decentration or tilt.¹⁰ In contrast, aspheric lenses with negative spherical aberration may compromise vision in such scenarios. A study using a model eye and ray tracing illustrates this theory; the optical performance of a model eye was unaffected by decentration of an implant with AO optics, and performance was better with a decentered AO lens compared to that of a de-centered spherical lens, and was also better than an equally decentered negative aspheric lens.¹¹

In comparing currently available optic designs, an IOL that utilizes aberration-free aspheric technology such as Bausch + Lomb’s Advanced Optics may provide patients with the best overall quality vision after cataract surgery, under the most varying conditions. This, after all, is our main goal of modern implant surgery.

References

1. Shentu X, Tang X, Yao K. Spherical aberration, visual performance and pseudoaccommodation of eyes implanted with different aspheric intraocular lens. Clin Experiment Ophthalmol 2008;36(7):620-624.

2. Pepose JS, Qazi MA, Edwards KH, et al. Comparison of contrast sensitivity, depth of field and ocular wavefront aberrations in eyes with an IOL with zero versus positive spherical aberration. Graefes Arch Clin Exp Ophthalmol 2009;247:965-973.

3. Morales EL, Rocha KM, Chalita MR, et al. Comparison of optical aberrations and contrast sensitivity between aspheric and spherical intraocular lenses. J Refract Surg 2011;27(10):723-728.

4. 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.

5. 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.

6. Santhiago MR, Netto MV, Barreto J Jr, et al. Wavefront analysis, contrast sensitivity, and depth of focus after cataract surgery with aspherical intraocular lens implantation. Am J Ophthalmol 2010;149:383-389.

7. Rocha KM, Soriano ES, Chamon W, et al. Spherical aberration and depth of focus in eyes implanted with aspheric and spherical intraocular lenses. Ophthalmology 2007;114:2050-2054.

8. Nanavaty MA, Spalton DJ, Boyce J, et al. Wavefront aberrations, depth of focus, and contrast sensitivity with aspheric and spherical intraocular lenses: Fellow-eye study. J Cataract Refract Surg 2009; 35:663-671.

9. Johansson B, Sundelin S, Wikberg-Matsson A, et al. Visual and optical performance of the Akreos Adapt Advanced Optics and Tecnis Z9000 intraocular lenses, Swedish multicenter study. J Cataract Refract Surg 2007; 33:1565-1572.

10. Eppig T, Scholz K, Löffler A, et al. Effect of decentration and tilt on the image quality of aspheric intraocular lens designs in a model eye. J Cataract Refract Surg 2009; 35:1091-1100.

11. Altmann GE, Nichamin LD, Lane SS, Pepose JS. Optical performance of 3 intraocular lens designs in the presence of decentration. J Cataract Refract Surg 2005; 31:574-585.

Dr. Nichamin is medical director of the Laurel Eye Clinic, which serves patients in 10 locations in Pennsylvania, and the Laurel Laser & Surgery Centers. He is a consultant to Bausch + Lomb.