Visual Performance of a New Monofocal Aspheric IOL

Assessing the benefits of aspheric IOLs require not only evaluating and comparing their performance with spherical IOLs, but also understanding the various available designs. Aspheric lenses may be classified as either negative or zero aberration. Aberrometric studies^1-8 on the Tecnis (Advanced Medical Optics, Inc., Santa Ana, California) and the SofPort Advanced Optic (AO; Bausch & Lomb, Rochester, New York) suggest that aspheric IOLs produce better results compared with their spherical equivalents. Aspheric IOLs also offer added protection from decentration and tilt when compared with spherical IOLs.

The two major negative aberration aspheric IOLs on today's market are the Tecnis and the AcrySof IQ (Alcon Laboratories, Inc., Fort Worth, Texas). The Tecnis is a first-generation aspheric IOL that generates -0.27 µm of spherical aberration, and the AcrySof IQ generates -0.20 µm of spherical aberration. The SofPort AO is a zero aberration aspheric IOL that aims to cancel out the positive aberration that a comparable spherical IOL may have generated. In comparison with other models, the design of the XL Stabi Zeiss Optic (ZO; Carl Zeiss Meditec AG, Jena, Germany) (Figure 1) is based on a physiological ocular model and not simply from a corneal model centered on the apex. Its design integrates the pupil and the visual axis offset, and the optic profile is based on modular transfer function (MTF) curves with the goal of improving the visual envelope and perfecting its relation to the optic and contrast sensitivity.

On average, the cornea induces 0.25 µm of spherical aberration. The overall spherical aberration of a young phakic patient is approximately 0.10 µm. Any excessive positive or negative spherical aberration will have an adverse effect on the visual envelope.

What differentiates the XL Stabi ZO from other aspheric models is its pattern; it is based on the Liou & Brennan (LBE) eye model, which is widely regarded as one of the most anatomically accurate eye models. LBE considers the physiological decentration of the pupil in nasal and the tilt of the visual axis. Therefore, this aspheric IOL is calculated from a physiological ocular globe model. The XL Stabi ZO compensates for part of the eye's positive corneal spherical aberration and demonstrates a low sensitivity to decentration and tilt. It seems to be a multipurpose implant that is not only less sensitive to decentration and tilt, but it still enhances the visual envelope.

METHODS OF ANALYSIS
The concept of the visual envelope defines how an eye perceives shapes and details under various conditions of contrast and luminance. The MTF is measured with an aberrometer and offers an objective tool to evaluate IOLs because it does not consider retinal and cerebral integration that can distort the analysis. This MTF measurement works as follows: the aberrometric measurement determines the point spread function (PSF); the PSF is used to produce a convoluted image; the MTF is used to define the visual envelope; the perception threshold will be studied for each spatial frequency; and linking the various thresholds produces a curve. In summary, if the image projected onto the retina is kept sharp, the eye is better able to identify it when the size is reduced (by distance) or when the contrast conditions are less favorable.

I was one of seven surgeons from France who participated in a multicenter trial evaluating the XL Stabi ZO. This IOL has an aspheric posterior profile and offers maximum visual quality for the largest range of patients. In total, we implanted 154 XL Stabi ZO lenses, preloaded in a cartridge, using the SkyJet disposable injector (Carl Zeiss Meditec AG). We also used the WASCA analyzer (Carl Zeiss Meditec AG) to conduct MTF measurements.

RESULTS
Patients were aged between 44.5 to 99 years (mean, 75.5 years), and the majority of patients were female (61%). The preoperative IOP was 15.8 ±5.2 mm Hg, and UCVA was 0.2 ±0.3 (0.71 ±0.50 logMAR) for far and Parinaud 4 (0.51 ±0.31 logMAR) for near. BCVA was 0.3 ±0.5 (0.48 ±0.30 logMAR) for far and Parinaud 3 (0.34±0.21 logMAR) for near. The preoperative refraction was 0.00 ±2.50 D for sphere and -0.60 ±0.90 D for cylinder.

We used the SRK/T (65.6%) and SRK II (34.4%) formulas for a target refraction of -0.50 ±0.90 D (minimum, -3.50 D; maximum, 0.25 D). Topical anesthesia was used in 57.1% of cases, and peribulbar was used in the remaining. The size of the capsulorrhexis ranged from 5 to 8 mm (6 ±1 mm), and the IOL power ranged from 11.00 to 20.00 D (21 ±2.50 D). We used between a 2.2- and 3.2-mm incision in all cases (average, 3 mm).

We also measured the spherical aberration MTF on mesopic pupils without pharmacological mydriasis using the WASCA aberrometer. Patients with pupils smaller than 4.5 mm or larger than 6 mm as well as those that had pharmacological mydriasis were excluded from the study to avoid diffracted rays passing outside the IOL optic. We extrapolated the MTF curve for a standard pupil diameter of 4.5 mm.

During postoperative evaluation (range, 7–321 days), the BCVA was 0.9 ±0.08 (0.06 logMAR) for far and Parinaud 2 (0.21 ±0.07 logMAR) for near. The sphere was -0.40 ±1.30 D, cylinder was -0.50 ±0.90 D, and the spherical equivalent was -0.70 ±1.30 D. We measured the spherical equivalent versus the target refraction as 0.22 ±0.77 D (absolute value, 0.51 ±0.61 D).

From this study, we have found that the MTF is an objective tool to evaluate IOLs as well as the visual envelope. We determined that the design of the XL Stabi ZO helps to produce good initial results, which have been confirmed in other ongoing studies.

The search for improved quality of vision in our patients is a joint objective in both refractive and cataract surgery. The aspheric IOL is the product of a shared desire to deliver improved quality of vision that matches the achievements of refractive surgery. In refractive surgery, the first photoablative profiles provided good visual acuity under conditions of maximum contrast, however, they often caused halos in situations with dim light. Occasionally, photoablative profiles also caused a significant drop in visual acuity for mesopic vision. Since the advent of techniques to analyze higher-order aberrations, identification of these troubles, as well as the development of aspheric profiles, is possible.

Aspheric photoablation profiles have eliminated visual disturbances in most cases. In cataract surgery, the general approach is the same, however, patients are always pleased to recover good visual acuity. Here, the main concern is to obtain a good postoperative refraction. With more accurate implantation calculations and ways to prevent astigmatism, the problem now is determining which patients will ultimately have the best visual envelope.

Jean-Pierre Rozenbaum, MD, is a refractive and cataract surgeon at the Centre National d'Ophtalmologie des Quinze-Vingt, in Paris, and works in a private office at Centre Eiffel, in Paris. Dr. Rozenbaum states that he has no financial interest in the products or companies mentioned. He may be reached at jprozen@yahoo.fr.

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