In the current era of presbyopia-correcting, toric, and aspheric IOL technologies, the practice milieu is changing. Informed consent takes on new meaning when the surgeon and patient decide together what IOL technology represents the best fit for a particular lifestyle and its visual demands. Customizing IOL choice is no longer optional; it is essential to the practice of refractive lens surgery.

Because the positive spherical aberration of a spherical pseudophakic IOL tends to increase total optical aberrations, attention has turned to the development of aspheric IOLs.¹ These designs are intended to reduce or eliminate the spherical aberration of the eye and improve functional vision as compared with a spherical pseudophakic implant. Three aspheric designs are currently marketed in the United States (ie, Tecnis Z9000, Z9002, and Z9003 IOLs [Advanced Medical Optics, Inc., Santa Ana, California], AcrySof IQ IOL [Alcon Laboratories, Inc., Ft. Worth, Texas], SofPort AO [Bausch & Lomb, Rochester, New York]). Other aspheric IOL designs not yet available in the United States also show promise for the reduction of spherical aberration.²

ASPHERIC IOL MODELS
Tecnis. The Tecnis was designed with a modified prolate anterior surface to compensate for the average corneal spherical aberration found in an adult eye. It introduces -0.27 µm of spherical aberration into the eye, measured at the 6-mm optical zone. The clinical investigation submitted to the US Food and Drug Administration (FDA) demonstrated that the Tecnis IOL eliminated mean spherical aberration as well as significantly improved functional vision compared with a standard spherical IOL.³

AcrySof IQ. This lens shares the UV- and blue-light–filtering chromophores found in the single-piece acrylic AcrySof Natural IOL, but it also has an additional feature. Its posterior aspheric surface design helps reduce spherical aberration by addressing the effects of over-refraction at the periphery. A total of -0.20 µm of spherical aberration is added to the eye at the 6-mm optical zone.

SofPort AO (LI61AO). The Bausch & Lomb aspheric IOL is specifically designed with zero spherical aberration. The logic behind this principle is that there will be nothing to contribute to any preexisting higher-order aberrations.

COMPARISONS
Multiple peer-reviewed, prospective, randomized scientific studies have demonstrated a reduction or elimination of spherical aberration with the Tecnis modified prolate IOL when compared with a variety of spherical IOLs.^4-13 Data show that the mean spherical aberration in eyes implanted with the Tecnis IOL is, in the words approved by the FDA, "not different from zero." Studies have also documented superior functional vision with the Tecnis IOL. Patients in the FDA-monitored, randomized, double-masked, night driving simulation study of the Tecnis IOL performed functionally better in 20 of 24 driving conditions—and statistically better in 10 conditions—when using best-spectacle correction with the eye implanted with the Tecnis IOL, as compared to best-spectacle correction with the eye implanted with the AcrySof spherical IOL.² Data from the night driving simulation showed a significant correlation between reduction of spherical aberration and detection distance for the pedestrian target. This test was performed under rural conditions with glare (the most difficult target to discern).

More recently, peer-reviewed clinical studies have also supported reduction of spherical aberration and superior functional vision with the AcrySof IQ when compared with spherical IOLs.^14-17 In fact, the optical advantages of aspheric IOL technology have become fairly well accepted, although some controversy remains. Examples include functional benefit as it relates to pupil size, IOL decentration, depth of focus, and customization.¹⁸ Some studies have shown that aspheric IOLs offer little or no benefit in smaller pupils,^12,13 however, one laboratory study showed that the SofPort AO provided better optical quality than either a negatively aspheric or a spherical IOL under conditions of significant decentration.¹⁹ Another study showed a diminished distance-corrected near visual acuity, a surrogate measure for depth of focus, with the AcrySof IQ aspheric IOL as compared with the AcrySof SN60AT spherical IOL.¹⁷

TWO VALUES OF SPHERICAL ABBERATION
Regarding customization of the aspheric correction, it has been suggested that achieving zero total spherical aberration postoperatively provides the best quality of vision. Peers and coauthors utilized an adaptive optics simulator to assess letter acuity and contrast sensitivity for two values of spherical aberration. The first condition was the average amount of spherical aberration measured in pseudophakic patients with spherical IOLs. The second condition represented the complete correction of the individual's spherical aberration (Z [4,0] = 0). The researchers found an average improvement in visual acuity associated with the correction of spherical aberration of 10% and 38% measured in white and green light, respectively. Similarly, average contrast sensitivity measurements improved 32% and 57% in white and green light, respectively.

When spherical aberration was corrected, visual performance was as good as or better than the normal spherical aberration case for a defocus as large as ±1.00 D. Therefore, it was concluded that a complete correction of ocular spherical aberration improves spatial vision in the best-focus position without compromising the subjective tolerance to defocus.²⁰

It has alternately been suggested that providing Z [4,0] = +0.1 µm of postoperative spherical aberration represents a better choice.²¹ This line of reasoning was demonstrated in a study of 35 young patients with a UCVA of 20/15 or better. The patients had a mean total spherical aberration of Z [4,0] = +0.110 ±0.077 µm,²² however, no logical basis infers that the spherical aberration is responsible for the supernormal visual acuity. In fact, the authors concluded that, "The amount of ocular [higher-order aberrations] in eyes with natural supernormal vision is not negligible, and is comparable to the reported amount of [higher-order aberrations] in myopic eyes."²² This conclusion is born out of a study performed by Wang and Koch that demonstrated a mean total spherical aberration of Z [4,0] = +0.128 ±0.074 µm in 532 eyes (306 patients) presenting for refractive surgery.²³ Nevertheless, Beiko²¹ used the Easygraph corneal topographer (Oculus, Lynnwood, Washington) to select patients with corneal spherical aberration of +0.37 µm, thus targeting a postoperative total ocular spherical aberration of +0.10 µm following implantation of the Tecnis IOL with -0.27 µm spherical aberration. This is possible because the Easygraph includes an optional software package that provides Zernike analysis. The selected patient group demonstrated significantly better contrast sensitivity than an unselected group of control patients under both mesopic and photopic conditions.

Recently, Beiko, Haigis, and Steinmueller presented data from a series of 696 eyes that confirmed the mean corneal spherical aberration of -0.27 µm used in the Tecnis IOL design.²⁴ They found a wide standard deviation of 0.089 µm (range, 0.041–0.632 µm) and significantly different corneal spherical aberration means in men and women. In some cases, the corneal spherical aberration differed significantly between fellow eyes. The authors concluded that "individuals should be measured to determine their unique value when considering correction of this aberration."²⁴ Additionally, they noted that keratometry and the corneal Q-value do not correlate well with spherical aberration, and that therefore, corneal spherical aberration must be measured directly with a topographer.

PROTOCOL FOR CUSTOMIZATION
One method of proceeding with customized selection of aspheric IOLs involves the following protocol: (1) preoperative testing to include corneal topography as well as axial length determination, anterior chamber depth, phakic lens thickness, and corneal white-to-white diameter; (2) use of the iTrace (Tracey Technologies, Houston) to determine topographically derived Zernike coefficients or application of a software package such as VOL-CT (formerly CTView; Sarver and Associates, Carbondale, Illinois) to transform the topography elevation data into preoperative corneal Zernike coefficients, with special attention to Z [4,0], fourth-order spherical aberration at the 6 mm-optical zone; (3) application of an IOL calculation formula, such as the Holladay 2 (available as part of the Holladay IOL Consultant & Surgical Outcomes Assessment Program; Holladay Consulting, Inc., Bellaire, Texas) to determine correct IOL power for desired postoperative spherical equivalent; and (4) determination of desired postoperative total ocular spherical aberration and selection of IOL type (eg, if the desired postoperative total ocular spherical aberration is zero and the preoperative corneal spherical aberration measures approximately 0.27 µm, the Tecnis with -0.27 µm would be selected). Generally, the aspheric IOL that comes closest to providing the desired correction should be selected (Figure 1).

Initial results of customizing the selection of aspheric IOLs have shown promise. Data from our series of 30 eyes (18 consecutive patients) were presented in Stockholm, Sweden, during the Congress of the European Society of Cataract & Refractive Surgeons (ESCRS).²⁵ The mean age was 72.8 ±6.2 years (range, 62–86 years), and the mean preoperative corneal spherical aberration measured at the 6-mm optical zone was 0.26 ±0.089 µm. The frequency distribution of the preoperative corneal spherical aberration is shown in Figure 2.

Postoperative BCVA measured 20/20 or better in 90% of eyes. The mean spherical equivalent measured -0.32 ±0.54 D; 93.3% of eyes measured within ±0.50 D of the targeted postoperative refraction. There was no significant tilt or decentration greater than 0.25 mm of any IOL as measured by Guyton's method.^26,27

The total postoperative ocular spherical aberration for the entire population measured -0.013 ±0.072 µm, and the total ocular spherical aberration for eyes implanted with the SofPort AO measured 0.025 µm. For the eyes implanted with the SN60WF and the Tecnis IOLs, the mean total postoperative ocular spherical aberration measured 0.010 ±0.053 µm and -0.015 ±0.052 µm, respectively. There was no statistically significant difference between the postoperative ocular spherical aberration of the SN60WF versus the Tecnis groups (two sample t-test assuming equal variances, P=0.22) (Figure 3).

Examining the difference between the predicted and measured postoperative spherical aberration for the entire group showed that the mean absolute error measured 0.058 ±0.056 µm. For the eye implanted with the SofPort AO, SN60WF, and Tecnis, the mean absolute errors measured 0.040 µm, 0.052 ±0.040 µm, and 0.063 ±0.066 µm, respectively. A two sample t-test assuming equal variances revealed no statistically significant difference between the mean absolute errors for the SN60WF and Tecnis eyes (P=0.63).

RESULTS
These results indicate that targeted postoperative spherical aberration can be achieved within a range close to the limits of surgically induced spherical aberration.²⁸ Future directions for research include psychophysical measures (eg, contrast sensitivity) to elucidate the real value of eliminating spherical aberration. It is important to realize that these psychophysical tests of functional vision are generally performed with best spectacle correction to exclude the effects of blur from test results. The ability to achieve superior functional vision with best spectacle correction reflects both the strength and weakness of wavefront-corrected IOLs.

Given the state of the art of biometry and IOL power calculation, it is not possible to achieve precise emmetropia in all eyes. Many pseudophakic patients find that their uncorrected vision is adequate for most daily living tasks, and therefore they do not wear spectacles. The amount of defocus and astigmatism patients accept may negate the pseudophakic correction of their spherical aberration. As Nio et al²⁹ noted, "both spherical and irregular aberrations increase the depth of focus but decrease the modulation transfer at high spatial frequencies at optimum focus. These aberrations, therefore, play an important role in the balance between acuity and depth of focus."# For some patients with adequate uncorrected distance acuity, the advantages of a bit more depth of focus may be worth a little loss of contrast. The ultimate expression of this trend is embodied in the multifocal IOL, which by its design reduces optical quality to enhance spectacle independence. The Tecnis Multifocal IOL, now in FDA-monitored clinical trials, represents a conscious compromise between optical efficiency and functional vision on the one hand and quality of life on the other.

I. Howard Fine, MD, is Clinical Professor of Ophthalmology at the Casey Eye Institute, Oregon Health & Science University, and he is in private practice at Drs. Fine, Hoffman, & Packer LLC, in Eugene, Oregon. Dr. Fine states that he is a paid consultant to Advanced Medical Optics, Inc., and Bausch & Lomb. He receives research and travel support from Alcon Laboratories, Inc., STAAR Surgical Company, Eyeonics, Inc., and Rayner Intraocular Lenses Ltd. He is a member of the CRST Europe Global Advisory Board. Dr. Fine may be reached at +1 541 687 2110; hfine@finemd.com.

Richard S. Hoffman, MD, is Clinical Associate Professor of Ophthalmology at the Casey Eye Institute, Oregon Health & Science University, and he is in private practice at Drs. Fine, Hoffman, & Packer, LLC, in Eugene, Oregon. He states that he has no financial interest in the products or companies mentioned. Dr. Hoffman may be reached at +1 541 687 2110; rshoffman@finemd.com.

Mark Packer, MD, FACS, is Clinical Associate Professor at the Casey Eye Institute, Department of Ophthalmology, Oregon Health, and Science University, and he is in private practice at Drs. Fine, Hoffman & Packer, LLC, in Eugene, Oregon. He states that he is a consultant to Advanced Medical Optics, Inc., and Bausch & Lomb, and he has received travel support and honoraria from Alcon Laboratories, Inc., and STAAR Surgical Company. Dr. Packer may be reached at +1 541 6872110; mpacker@finemd.com.

1. Holladay JT, Piers PA, Koranyi G, van der Mooren M, Norrby S. A new intraocular lens design to reduce spherical aberration of pseudophakic eyes. J Refract Surg. 2002;18:683-691
2. Kurz S, Krummenauer F, Thieme H, Dick HB. Contrast sensitivity after implantation of a spherical versus an aspherical intraocular lens in biaxial microincision cataract surgery. J Cataract Refract Surg. 2007;33:393-400.
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. Initial clinical experience with 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;29:652-660.
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, Morselli S, Chierego C, Criscuoli A, Moretti G, Piers P.
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.
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;82:718-722.
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;80:71-77.
12. Munoz G, Albarran-Diego C, Montes-Mico R, Rodriguez-Galietero A, Alio JL.
Spherical aberration and contrast sensitivity after cataract surgery with the Tecnis Z9000 intraocular lens. J Cataract Refract Surg. 2006;32:1320-1327.
13. Kasper T, Buhren J, Kohnen T. Visual performance of aspherical and spherical intraocular lenses: intraindividual comparison of visual acuity, contrast sensitivity, and higher-order aberrations. J Cataract Refract Surg. 2006;32:2022-2029.
14. Awwad ST, Lehmann JD, McCulley JP, Bowman RW. A comparison of higher order aberrations in eyes implanted with AcrySof IQ SN60WF and AcrySof SN60AT intraocular lenses.
Eur J Ophthalmol. 2007;17:320-326.
15. Sandoval HP, Fernandez 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]
16. Rocha KM, Soriano ES, Chalita MR, Yamada AC, Bottos K, Bottos J, Morimoto L, Nose W.
Wavefront analysis and contrast sensitivity of aspheric and spherical intraocular lenses: a randomized prospective study. Am J Ophthalmol. 2006;142:750-756.
17. Rocha KM, Soriano ES, Chamon W, Chalita MR, Nose W. Spherical Aberration and Depth of Focus in Eyes Implanted with Aspheric and Spherical Intraocular Lenses A Prospective Randomized Study. Ophthalmology. 2007 Apr 18; [Epub ahead of print]
18. Werner L, Olson RJ, Mamalis N. New technology IOL optics. Ophthalmol Clin North Am. 2006;19:469-483.
19. 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.
20. Piers PA, Fernandez EJ, Manzanera S, Norrby S, Artal P. Adaptive optics simulation of intraocular lenses with modified spherical aberration. Invest Ophthalmol Vis Sci. 2004;45:4601-4610.
21. Beiko G. Personalized Correction of Spherical Aberration in Cataract Surgery. Presented at the Annual Meeting of the American Academy of Ophthalmology, 18 October 2006, Chicago, Illinois.
22. Levy Y,Segal O,Avni I,Zadok D. Ocular higher-order aberrations in eyes with supernormal vision. Am J Ophthalmol.2005;139:225-228.
23. Wang L,Koch DD. Ocular higher-order aberrations in individuals screened for refractive surgery. J Cataract Refract Surg.2003;29:1896-1903.
24. Beiko GH, Haigis W, Steinmueller A. Distribution of corneal spherical aberration in a comprehensive ophthalmology practice and whether keratometry can predict aberration values.
J Cataract Refract Surg. 2007;33:848-858.
25. Packer M, Fine IH, Hoffman RS. Customizing selection of aspheric intraocular lenses. XXV Congress of the European Society of Cataract and Refractive Surgeons, Stockholm. September 8, 2007.
26. Guyton DL, Uozato H, Wisnicki HJ. Rapid determination of intraocular lens tilt and decentration through the undilated pupil. Ophthalmology. 1990;97:1259-1264.
27. Wu M, Li H, Cheng W. Determination of intraocular lens tilt and decentration using simple and rapid method. Yan Ke Xue Bao. 1998;14:13-16, 26.
28. Guirao A, Tejedor J, Artal P. Corneal aberrations before and after small-incision cataract
surgery. Invest Ophthalmol Vis Sci. 2004;45:4312-4319.
29. Nio YK, Jansonius NM, Fidler V, Geraghty E, Norrby S, Kooijman AC. Spherical and irregular aberrations are important for the optimal performance of the human eye. Ophthalmic Physiol Opt. 2002;22:103-112.