Aspheric Algorithms and Quality of Vision
Corneal asphericity influences visual outcomes following refractive surgery.1 The surface of the anterior cornea determines—to a great extent—the refractive power of the eye, and it is well documented that conventional laser procedures may induce higher-order aberrations including spherical aberration.2 Patients may suffer from poor results (eg, blurred vision and halos) under mesopic and scotopic lighting conditions. At best, these symptoms are annoying. At worst they are severely debilitating.
According to recent studies, laser treatments for myopia change the corneal shape from prolate (ie, negative asphericity, Q<0) to oblate (ie, positive asphericity, Q>0). Furthermore, positive spherical aberration increases proportionally to the amount of attempted correction following laser ablation for myopic surgery. As a result, recent refractive surgery software developments focus more on ablation profiles that enhance refractive outcomes while minimizing corneal curvature changes.
POSITIVE, NEGATIVE ABERRATION BALANCED
In young eyes, the amount of positive spherical aberration (0.27 µm) in a normal cornea is balanced by an equal amount of negative spherical aberration in the crystalline lens (-0.27 µm). As the eye ages, spherical aberration in the crystalline lens becomes increasingly positive. By age 60 years, there is approximately 0.54 µm of spherical aberration in the entire eye.
It is this relationship between corneal asphericity and spherical aberration along with the K value, that is utilized in the recently released Zyoptix Aspheric Software (Bausch & Lomb, Rochester, New York). When developing the aspheric algorithm, Bausch & Lomb scientists began by examining the US Zyoptix clinical study results to establish exactly what determines the amount of postoperatively observed spherical aberrations. The primary driving factor was spherical equivalent, and the secondary driving factor was preoperative existing spherical aberration.
SPHERICAL ABERRATION MODEL
Based on these findings, the scientists created a mathematical model to calculate the amount of induced spherical aberration. They also examined the ablation strategy and design of an aspheric algorithm and analyzed the effects of each component. The final Zyoptix aspheric algorithm is a combination of two main ablation components: (1) a basic component that corrects the classical refractive error by using 1- and 2-mm Zyoptix laser spots, and (2) an aspheric component that corrects the surgically induced spherical aberration. The total ablation profile is the sum of the two components.
The current clinical results of a Bausch & Lomb randomized, bilateral eye, multicenter study are promising. The Zyoptix aspheric algorithm was compared with a control (ie, current Zyoptix tissue saving algorithm). Patients with myopia and myopic astigmatism were included in the study; higher-order aberrations and high and low contrast visual acuity values were recorded preoperatively and at the time of surgery. They were again recorded at 1 day, 1 week, 1 month, and 3 months postoperatively.
BASELINE CHARACTERISTICS
Mean preoperative manifest refraction was -4.31 D (SD 1.94) for the aspheric group and -4.58 D (SD 1.92) for the control group. Both the test and control algorithm demonstrated good predictability at 1 day, 1 week, and 1 month. After 1 month postoperative, cumulative high contrast visual acuity and BSCVA were positive for both algorithms. By this visit, 98.8% of the aspheric group and 97.6% of the control group had a BSCVA of 20/20 or greater. Cumulative low contrast visual acuity results at 1 month showed a trend at 20/25, favoring the aspheric group. A total of 58.1% aspheric patients achieved a BSCVA of 20/25, compared with 39.3% of control patients. Significantly more aspheric patients gained lines of low contrast BSCVA between the preoperative and 1-month postoperative visit.
When looking at the change in higher-order aberrations at 30 days postoperative, the advantages of the Zyoptix aspheric algorithm are clear. Induced spherical aberrations were statistically significantly reduced (0.038 µm) compared with the control algorithm (0.217 µm). Induction of coma was also further reduced in the aspheric group (0.13 µm versus 0.264 µm). Spherical aberration and coma are the main higher-order aberrations responsible for reducing contrast sensitivity.
Clinical studies on the Zyoptix aspheric algorithm are ongoing, however, these results show potential for the algorithm to significantly reduce surgically induced spherical aberration and coma while causing minimal change to the cornea's natural asphericity. The impact of these results for improving patients' quality of vision—particularly in dim light conditions—is considerable.
Thomas Kohnen, MD, is Professor of Ophthalmology and Deputy Chairman at the Johann Wolfgang Goethe-University Clinic, in Frankfurt, Germany, and he is Visiting Professor at the Baylor College of Medicine, in Houston. Professor Kohnen states that he has no financial interest in the products or companies mentioned. He may be reached at +49 69 6301 3945; kohnen@em.uni-frankfurt.de.
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