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4 • The contribution of secondary-order astigmatism aber- rations will affect both power and axis across the optical zone. The axis and power of the astigmatism will become pupil size-dependent (e.g., the patient's subjective astig- matism axis might differ in day and night conditions). A prolate astigmatic cornea (Example 2 below) will show curved meridians. The parallel distance of the two meridians depicts no pupil depending astigmatism, similar to Example 1. Fourier display Overview display Fourier Analysis Variating Axis of Astigmatism The result of the spherical compensation could now be added to target spherical correction and to any eventual individual nomogram adjustment applied. Astigmatism management: To understand Contou- ra Vision guided ablations, the forecast of the effect of a Contoura Vision ablation without any correction for sphere and astigmatism would result in a corneal shape that corresponds to the reference body and the select- ed corneal asphericity (within the selected optical zone). Because the astigmatism was not corrected, the expected appearance of the postoperative astigmatism is predicted to be symmetric. This would suggest that the correction of this astigmatism with glasses would now be possible for all categories of preoperative irregular asymmetrical astigmatism. Ideally, the predicted power and axis should in this case (where only sphere and HoA aberrations are correct) confirm the measured astigmatism by the Contoura Vision algorithm. The earlier mentioned projection might lead to the fol- lowing models of understanding astigmatism correction: • The contribution of coma-like aberrations to the visual impression of the patient. Keratoconus is well known to be a mostly horizontal-oriented coma aberration (the Contoura vision system is contraindicated for treatment of eyes with keratoconus). However, because of the lack of coma-correcting lenses, patients will instead accept cylinder correction. Coma aberration of considerable amplitude might interfere with both the power and axis of the subjective astigmatism accepted by the patient. Following is an example of a patient with an obvious predominant coma-like aberration who accepts astigma- tism following the axis of the coma. The patient accepts –0.75 x 88 degrees, while the Fourier display and the Sim Ks don't show any relevant astigmatism. Interestingly, the coma axis (black circles that resemble a pearl neck- lace) at the Fourier display coincides with the axis select- ed by the patient. Example 2 • The following figures demonstrate the influence of dif- ference of asphericity of the two meridians. The below example depicts regular astigmatism power along the corneal diameter for a spherical cornea for the two main meridians. The distance between the two meridians can be translated to corneal astigmatism. For astigmatism example 1, the distance between the two meridians (blue and red line) is parallel and straight (Example 1 be- low). In this or similar cases, the patient will experience no changes in astigmatism power during different light- ing conditions. A prolate astigmatic cornea with dissimilar asphericity (Example Astigmatism 3) of the two main meridians de- picts varying astigmatic power. In this case, the patient's visual impression and the Contoura Vision algorithm cal- culated astigmatism will differ for various lighting condi- tions and optical zones. Example 1

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