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LASIK with optimized ablation profile corrects mixed astigmatism
LASIK with a new-generation excimer laser and optimized ablation profiles corrected high mixed astigmatism, according to a study.
The retrospective study included 52 eyes of 36 patients who had more than 3 D of primary mixed astigmatism.
Investigators used the sixth-generation Amaris excimer laser (Schwind eye-tech solutions) to perform ablation based on optimized aspherical aberration-free profiles calculated with the Ork-Cam (Schwind). Cyclotorsion control and eye tracking software were also used.
LASIK flaps were created with the IntraLase femtosecond laser (Abbott Medical Optics).
At 3 months, there was a statistically significant reduction in refractive sphere and cylinder and improvement in logMAR uncorrected distance visual acuity (both P = .001). Best corrected distance visual acuity was unchanged in 31 eyes; three eyes gained two lines, seven eyes gained one line, eight eyes lost one line, and three eyes lost two lines.
Spherical equivalent was within 0.5 D of targeted refraction in 14 eyes and within 1 D in 34 eyes.
Re-treatment was required in seven eyes.
Postoperative corrected distance visual acuity was 0.3 or better in all eyes.
There was a statistically significant increase in higher-order aberrations attributed to an increase in primary spherical aberrations (both P = .002), the authors said.
Perspective
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This study is a very interesting examination of the treatment effectiveness of high cylinder (more than 3 D) with mixed astigmatism. The value lies in the use of a newly introduced sixth-generation laser, the Amaris Schwind, with cyclotorsional control and high-speed tracking on both pupil and limbus, showing its optimized aspheric profiles and effectiveness.
The treatment is applied at the corneal vertex, offset from the conventional pupil center. The analysis of outcomes at 3 months was vectorial and very useful with a clinically intuitive polar display of the difference vectors. What raises one’s interest are the visual and refractive results compared with what has been published in the past decade and discussed in the paper itself. The difference vectors are aggregated around 70° to 110°, suggesting excess with-the-rule astigmatism. The correction index was 0.67 (corneal) and 0.79 (refractive), showing significant undercorrection. Cyclotorsion control was effective with the angle of error low at 1° arithmetically (no absolute mean given).
Where might the problem be with the outcomes that were not as expected? Certainly nomograms require adjustment. High ocular residual astigmatism, as suggested, is possible, which would result in excess remaining astigmatism on the cornea. Bitoric ablations with smooth transitions without any spherical component in mixed astigmatism should be advantageous in preserving corneal tissue. The novel approach of corneal vertex centration rather than visual axis is a confounding factor, but perhaps something else altogether is at play. All these aspects require further examination.