## Discussion

The perception of our environment presents different challenges to the resolution of our vision. In addition, particularly demanding perceptions are presented such as fine black and white contrasts.19 In this study, we especially focused on high spatial frequencies because they are the most likely to reveal differences in imaging quality in the different constellations of the corneas used and the IOL designs investigated. Even if it can be assumed that high spatial frequencies should be more affected by decentration, the extent of such an effect has to be assessed on combinations of a number of parameters in order to draw conclusions for the lens selection in cataract surgery.

Figure 1 shows the two extremes that are considered to be as similar as possible to the condition range of a pseudophakic eye, namely the cornea with a low spherical aberration of +0.13 µm and a lens that has a low spherical aberration correction, here the aberration neutral CT Asphina 409MP. This constellation leads to a very low to barely detectable decentration sensitivity. The other extreme is the cornea with a high spherical aberration of +0.28 µm and an IOL with a high spherical aberration correction, in this case, the ZCB00 with a spherical aberration correction of −0.27 µm. This combination leads to a high decentration sensitivity (figure 1).

All combinations of the corneas and the three implants examined show that low spatial frequencies are generally transmitted better than high spatial frequencies (exemplarily visible in the curves for 25 lp/mm, 50 lp/mm and 100 lp/mm for the Primus HD in figure 2). This is basically due to the diffraction limits of the optics and the imaging errors even in the best centration.

In our setup, the level of image quality in the best centration is a result of the congruence between the spherical aberration of the cornea and the spherical aberration correction of the lens.

The sensitivity of decentration depends on various factors, starting with the level of spherical aberration of the cornea. This is predetermined and determines the range of useful aberration correction by the IOL. The extent of spherical aberration of the cornea correlates with the decentration sensitivity of an aberration correcting IOL. figures 2 and 3 show an example of this, namely the flatter progression of the 100 lp/mm curve for the ZCB00 for a cornea with a spherical aberration of +0.13 μm compared to a cornea with a spherical aberration of +0.28 μm. µm. If, on the other hand, aberration neutral and not aberration correcting optics are used, almost no decentration sensitivity occurs, regardless of the level of spherical aberration of the cornea. This can be seen in figurefigures 2 and 3 2 with the cornea that exhibits a spherical aberration of +0,13 or +0.28 µm from the almost horizontal progression of the curves for 25 lp/mm, 50 lp/mm and 100 lp/mm for the CT Asphina 409MP.

In addition, the degree of aberration correction of the IOL correlates with the decentration sensitivity. This can be seen in figure 3, for example, in the steeper progression of the curves for 50 lp/mm for the ZCB00 compared with the Primus HD in combination with the cornea that exhibits spherical aberration of +0.28 µm. If the IOL does not correct spherical aberrations of the cornea, as in the case of an aberration neutral IOL, the decentration sensitivity is hardly detectable (see again horizontal progression of curves for 25 lp/mm, 50 lp/mm and 100 lp/mm of the CT Asphina 409MP in figures 2 and 3).

The price for the high decentration stability of aberration neutral optics, however, is the relatively poorer imaging quality in good centration (see, eg, the curves for 50 lp/mm of the ZCB00 and the CT Asphina 409MP in figure 3).

Typical for the progression of a decentration sensitivity curve is the gradual self-limitation. The poorer the image quality, the less it can deteriorate. This can be seen, for example, in the 100 lp/mm curve of the ZCB00 in figure 3. Therefore, image quality at best centration determines the extent of the deterioration in image quality dependent on decentration sensitivity.

For completeness, it should be mentioned that decentration sensitivity is pupil dependent and its role is of lesser importance with small pupil apertures.10 20 Also, the refractive power of the cornea as well as the refractive power of the IOL correlate positively with the decentration sensitivity of the IOL.10

IOLs with a higher aberration correction are more sensitive to decentration (figure 1), while, conversely, lenses with a lower aberration correction will image better above a certain decentration. A certain decentration can be assumed for IOLs in the human eye. Therefore, full aberration correction can only make sense assuming a perfect centration. Consequently, in the case of decentration, undercorrection of the spherical aberration of the cornea provides better results than full correction.

As in reality, a decentration of 0.2–0.4 mm can be assumed3–7 so an under correction of the spherical aberration of the cornea is a prerequisite for good imaging quality in the expected decentration range. This can be seen in figure 3, for example, in the curve for 100 lp/mm for the ZCB00 compared with the Primus HD. Here, the Primus HD, which has a lower spherical aberration correction, performs better than the ZCB00 from a decentration of 0.25 mm. This is all the more true as in reality not only a decentration of the IOL but also a tilt of the IOL of 1°–3° can be assumed.5 6 Thus, the tilt leads to a further deviation from the best IOL position.10 However, there are also rare combinations of decentration and tilt that also allow good imaging qualities.10 In the future, predictability of the IOL position could be improved by using modern OCT (Optical Coherence Tomography) with determination of the preoperative crystalline lens position, as this correlates with the postoperative IOL position.21 22 Additionally, with the knowledge of the respective spherical aberration of the cornea, guidance for lens selection for the specific case can be provided.

The possible extent of decentration sensitivity is determined by the amount of spherical aberration of the cornea. The effective decentration sensitivity is defined by the extent to which the spherical aberration of the cornea is matched by the IOL (figure 1). If the spherical aberration is not compensated, for example, with an aberration neutral lens, there is hardly any decentration sensitivity. If there is a corresponding decentration sensitivity, high spatial frequencies are more affected by the decentration of an IOL than low frequencies (figures 2 and 3). Thus, knowledge of the spherical aberration of the cornea of the eye to be operated on should be a standard in cataract surgery.23–26 Likewise, the IOL selected should undercorrect the spherical aberration of the cornea to ensure the result in the expected decentration range.