Eyeworld Supplements

Case study: Corneal aberration cancellation of manifest astigmatism Preop manifest refraction: –1.50 with BCVA of 20/20 Preop auto-refraction: –2.00 Measured Contoura Vision astigmatism: –0.77 x 121 Postop refraction and vision at 1 week: Plano; 20/15 Neurological interpretation of information. Preop topography Postop topography Ablation profile correcting HoA 5 Please see Important Product Information about the products in this supplement on pages 7–8. Example 3 Corneas with corneal coma will depict various refrac- tions for the half meridians and might be positive or nega- tive. • Influence of other HoAs or a combination of aberrations Setting the Modified Refraction for cylinder and sphere to plano will depict the HoA ablation profile: Oval-shaped central ablations and, therefore, irregular refraction of the incoming light might equalize for the existing corneal astigmatism and lead to reduced or increased manifest astigmatism power and changing astigmatism perception by the patient. Likewise, an auto-refractometer device (that usually measures the central ray of light) might get biased by the HoA and deliver deviating astigmatism pow- er and axis results. Example 4 Conceivable reasons for the discrepancy between cor- neal and subjective astigmatism: The lens might accommodate for corneal astigmatism. Wavefront examinations during accommodation showed that the accommodation of the lens can also compensate for astigmatism or generate new HoAs. The astigmatism correction and axis provided by the topography-guided algorithm calculation (at the measured column) are comprised of Zernike polynomials C3 and C5 only and are not biased by HoAs. With the above-men- tioned details and considerations, the ablation profile to be applied is designed and stored. As a side note, there is a specific advantage of topogra- phy-guided ablations when compared to wavefront-guided ablations. The goal of topographic ablations is to target the aberration at their source, which is the anterior sur- face of the cornea. Consequently, ray-tracing issues as shown below are less likely to appear. Performing ablation on the eye Therefore, the ablation sequence does not differ from a standard LASK application. The biggest difference is that the treatment is applied on the corneal apex. Conse- quently, the pupil size during treatment should not differ (too much) from the pupil size during the examination. One needs to consider flap-centration for patients with large angle kappa in order to accommodate the ablation within the flap borders. Registration requires improvements regarding timing during the procedure and how to fit registration into the routine of the surgeon. Lenticular astigmatism might comprise astigmatism, coma, and higher-order astigmatism. Lenticular astigma- tism in conjunction with coma-like aberration might fur- ther complicate the situation. Therefore, it is important to locate where the cylinder is originating and conclude the preferred ablation for such seldom-appearing cases. It is theorized that patients might accept with-the- rule astigmatism better than against-the-rule or oblique astigmatism (neurological interpretation of information). Therefore, it is understandable that some patients may not request full correction for with-the-rule astigmatism but may request against-the-rule astigmatism correction.

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