The goal of cataract surgery is much more than simply restoring functional vision to patients: recent technical advancements have made it possible to offer high quality visual correction, even providing corrections to pre-existing ocular aberrations. The introduction of aspheric intraocular lenses (IOLs), in particular, has widened the potential for correcting corneal spherical aberrations.

"A number of studies examining contrast sensitivity have already established the superiority of aspheric over traditional IOLs in terms of functional vision,"^1–6 asserted Professor George Beiko, of McMaster University, Canada.

Aspheric IOLs provide better visual function because they correct the natural corneal spherical aberration, which, according to Professor Beiko, is 0.27–0.30 µm, on average. Correcting this aberration has been shown to improve contrast sensitivity; different aspheric lenses, however, aim to provide different levels of specific aberrations postoperatively, and there is no literature proving the most desirable degree of postoperative aberration.

To remedy this, Professor Beiko and colleagues conducted a study comparing Alcon's IQ IOL with AMO's Tecnis IOL, to establish which aspheric IOL provides optimal visual function.

How do the lenses compare?

AMO's Tecnis has a wavefront designed aspheric surface, and is a three-piece foldable, UV-blocking hydrophobic acrylic lens with an optimal refractive index of 1.47. It is available, in 0.5 D increments, for +10 to +30 D of correction.

Alcon's IQ lens has a posterior aspheric surface and a square edge; it is a one-piece foldable, UV-blocking and blue light-filtering hydrophobic acrylic lens with an optimal refractive index of 1.55. It is available, in 0.5 D increments, for +6 to +30 D of correction.

Both lenses have an overall lens length of 13.0 mm.

"The AMO lens targets a postop spherical aberration of 0.00 µm, whereas the Alcon lens's target is 0.10 µm," Professor Beiko explained. "We implanted the lenses into patients with corneal spherical aberrations of approximately 0.27 µm and compared the postoperative visual outcomes."

Measuring visual outcomes objectively

Forty patients with similar preoperative profiles were randomly assigned to receive either the Tecnis or the IQ lens. The spherical aberration of the anterior corneal surface had been determined using the Oculus Easygraph. To maintain consistency, a single surgeon using the same technique in each case was used to perform all the implantations. Each patient received the same postoperative care.

"At a minimum of eight weeks after the implantation, distance corrected contrast sensitivity was measured with an Optec 6500, and we used ANOVA analysis to compare all readings," Professor Beiko clarified.

Table 1: Patient profile.

Patients implanted with Tecnis had mean photopic and mesopic pupil measurements of 3.18±0.85 mm and 4.28±0.90 mm, respectively. In patients implanted with the IQ lens, these measurements were 3.00±0.56 mm and 3.98±0.60 mm, respectively (Table 1).

"The study groups were found to be similar in terms of age, pupil size under photopic and mesopic conditions, post-op vision and post-op refractive error. According to our findings, there was a trend for the Tecnis group to outperform the IQ group under photopic conditions, but statistical significance was not reached," explained Professor Beiko (Figure 1/Table 2). However, under mesopic conditions, the Tecnis group had greater contrast sensitivity at 3, 12 and 18 cpd (p = 0.05) and at 6 pcd (p = 0.10) (Figure 2/Table 3).

Figure 1

Figure 2

Table 2: Photopic contrast sensitivity.

Table 3: Mesopic contrast sensitivity.

Tecnis offers superior outcomes

Overall, the results tended to show that implanting an aspheric IOL that aimed to produce a smaller postoperative spherical aberration provided a better mean contrast sensitivity result and sharper vision.

"In terms of provision of functional vision, we therefore found the AMO Tecnis IOL to be the superior lens," Professor Beiko concluded.

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Special Contributor
George H. H. Beiko, BM, BCh, FRCSC is an assistant clinical professor of ophthalmology at McMaster University, Ontario, Canada. He may be reached by E-mail: george.beiko@sympatico.ca
Professor Beiko has disclosed financial relationships with AMO and Alcon Laboratories.

References

1. M. Packer, et al. J. Refract. Surg. 2002;18:692–696.

2. U. Mester, et al. J. Cataract Refract. Surg. 2003;29(4):652–660.

3. R.M. Kershner. J. Cataract Refract. Surg. 2003;29(9):1684–1694.

4. H. Kennis, et al. Bull. Soc. Belge Ophtalmol. 2004;294:49–58.

5. A. Martinez Palmer, et al. Arch. Soc. Esp. Oftalmol. 2005;80(2):71–78.

6. R. Bellucci, et al. J. Cataract Refract. Surg. 2005;31:712–717.