Original Research••

IOL decentration sensitivity according to spatial frequencies

Abstract

Background/aims Investigation of the decentration sensitivity of monofocal intraocular implants with a focus on different aberration corrections depending on different spatial frequencies.

Methods Using an optical bench, the decentration sensitivities of an intraocular lens (IOL) with a high spherical aberration correction of −0.27 µm (ZCB00 Johnson & Johnson), an IOL with an aberration correction of −0.20 µm (Primus HD OphthalmoPro) and an IOL with an aberration neutral design (CT Asphina 409MP Carl Zeiss Meditec) were evaluated for Strehl ratio values and for 25, 50 and 100 lp/mm. Two different corneas with +0.13 µm and +0.28 µm were used. The lenses were tested in the best centration and up to a decentration of 0.5 mm.

Results Decentration sensitivity affects high spatial frequencies more than lower ones. The possible decentration sensitivity is determined by the amount of spherical aberration of the cornea. The effective decentration sensitivity is determined by the extent to which these spherical aberrations are compensated. If these are not compensated, there is hardly any decentration sensitivity.

Conclusion High spatial frequencies are more affected by decentration sensitivity. The decentration sensitivity of an IOL is determined by the extent to which the spherical aberration of the cornea is corrected.

What is already known on this topic

  • The imaging quality of intraocular lens (IOL) depends on the centration accuracy. The aim was to examine whether the decentration sensitivity of IOLs has a different effect for different spatial frequencies.

What this study adds

  • The decentration sensitivity has a stronger effect on the high than on the low spatial frequencies.

How this study might affect research, practice or policy

  • This work shows that the choice of an IOL, taking into account the expected decentration and the existing decentration sensitivity for the respective case, has a particular effect on the detection of fine contrast differences and high spatial frequencies.

Introduction

A prolate lens geometry was introduced for the intraocular market in the early 2000s.1 This lens concept aimed to direct peripheral light rays which are too strongly refracted due to the mostly positive spherical aberration of the cornea into the focal point and thus produce a better imaging quality than achieved with spherical lenses.2 The disadvantage of the concept is a possible higher decentration sensitivity. This refers to the extent of the decrease in image quality in correlation with decentration, which can be expected to be between 0.2 and 0.4 mm.3–7The decrease is mainly caused by an increase in coma and higher order aberrations with decentration.8 9 The decentration sensitivity depends on the spherical aberration of the cornea and the spherical aberration correction of the lens.10–12

This study evaluated the decentration sensitivity for an intraocular lens (IOL) with a high aberration correction, a middle spherical aberration and one with an aberration neutral design. The new aspect of this study is an examination of the comparative effect of decentration sensitivity on low, medium and high spatial frequencies.

Materials and methods

Description of the lenses tested

The ZCB00 (Johnson & Johnson, USA) was designed for a cornea that exhibits spherical aberrations of +0.27 µm.1 This concept is intended to excessively reduce the spherical aberration. The lens’ overall diameter is 13.0 mm and its optic diameter is 6.0 mm. It is made of a hydrophobic acrylate and has a circular sharp edge.

The Primus HD (OphthalmoPro, Germany) corresponds to a modern aspheric lens design, with a moderate compensation of the spherical aberration of the human cornea exhibiting a spherical aberration correction of −0.20 µm. This one-piece lens is made of hydrophobic acrylic material with a C-loop haptic. Its optic diameter is 6.0 mm and its overall diameter is 13.0 mm. Furthermore, it has a continuous circular square edge to impede posterior capsule opacification after surgery.

The CT Asphina 409MP (Carl Zeiss Meditec, Germany) follows an aberration-free concept. The objective of this lens design is not to add or compensate any spherical aberration for a determined incidence of light.13 The lens is made of a hydrophilic acrylate with hydrophobic surface properties and has a plate haptic design with a 6.0 mm optical zone and an overall diameter of 11.0 mm.

Experimental setup

The experimental setup with an optical bench (OptiSpheric IOL PRO 2, Trioptics, Germany) consisted of a laser source (546 nm), a collimation system, two artificial corneas, a wet cell with a saline solution containing an artificial iris and a lens holder. Two different types of artificial corneas corresponding to a human corneal refractive power of 43 dioptres were used. One cornea exhibited a low amount of spherical aberration of +0.13 µm, and the other one a high spherical aberration of +0.28 µm (in both cases related to a diameter of 6.0 mm).14–17 All measurements were done with an artificial pupil of 4.5 mm. Measurements were taken with the IOL fixed to the lens holder and immersed in the wet cell. In addition, the lens holder with the lens could be moved by means of a micrometre screw perpendicular to the beam path to generate a lens decentration. The IOL tested focused the projected target at its focal plane, which was captured by an objective microscope lens and a high-resolution charge-coupled device (CCD) camera. The lenses were evaluated in the best centration and in an additional five decentred positions up to 0.5 mm in 100 µm steps.

Optical quality variables

The cross-sectional intensity profile from the line spread function was converted to modulation transfer function (MTF) values using the Fourier transform method.18 In this study, the MTF values at the respective measurement points were calculated for Strehl ratio values and spatial frequencies of 25, 50 and 100 lp/mm. These spatial frequencies correspond in this setup to a visual acuity of Snellen 0.25, 0.5 and 1.0 (logMAR 0.6, 0.3 and 0.0), respectively.

Patient and public involvement

No patients or the public were involved in this work.

Results

Figure 1 shows the results of the Strehl ratio curves with ongoing decentration for the ZCB00, the Primus HD and the CT Asphina 409MP for the two corneas used that have a spherical aberration of +0.13 µm and +0.28 µm.

Figure 1
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Figure 1

Strehl ratio in relation to decentration using corneas that exhibit a spherical aberration of +0.13 µm and +0.28 µm. The intraocular lenses examined are the ZCB00, Primus HD and CT Asphina 409MP. All examinations were performed with green light and a pupil opening of 4.5 mm. The grey area from 0.2 mm to 0.4 mm indicates the expandable range of decentration.

Looking at the results with the cornea that has a spherical aberration of +0.13 µm, the Primus HD shows the best result because this IOL only slightly overcompensates the spherical aberration of the cornea. The ZCB00, which achieves the second-best result in the best centration in this overview, overcompensates the spherical aberration of the cornea more than the Primus HD with the spherical aberration correction of −0.27 µm. The CT Asphina 409MP shows the worst result in the best correctioncentration but is equal or superior to the other two lenses with decentration over 0.4 mm.

Regarding the results with the cornea that has a spherical aberration of +0.28 µm, the ZCB00 performs best in the best centration because it only slightly undercorrects the spherical aberration of the cornea.

In the best centration, the Primus HD shows a slight advantage over the CT Asphina 409MP over the entire decentring range because it corrects the spherical aberration of the cornea selected better than the CT Asphina 409MP. However, from a decentration of 0.34 mm, the ZCB00 is inferior to the other two lenses.

Figure 2 displays the results using the +0.13 µm cornea split by spatial frequncies. The results for the ZCB00, for 25, 50 and 100 lp/mm, are shown in green. The ZCB00, which is designed for a +0.27 µm cornea, overcorrects the spherical aberration of the +0.13 µm cornea. The modulation for low spatial frequencies is higher than that for high spatial frequencies. Furthermore, the imaging quality can be seen to deteriorate with ongoing decentration. Considering the results, the Primus HD with its spherical aberration correction of −0.20 µm matches the cornea with a spherical aberration of +0.13 µm better than the ZCB00 with a spherical aberration correction for a cornea with a spherical aberration of +0.27 µm. This leads to better results in relation to all spatial frequencies in the range of decentration examined. The aberration neutral CT Asphina 409MP, which should not influence the spherical aberration of the cornea, performs robustly with respect to decentration. Although the results for this IOL are worse than those of the other two lenses at best centration, it increasingly wins with ongoing decentration due to its robust and de facto horizontal progression for all spatial frequencies.

Figure 2
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Figure 2

Modulation/contrast in relation to decentration using the +0.13 µm cornea. The lenses examined are the ZCB00, Primus HD and CT Asphina 409MP. All examinations were performed with green light and a pupil aperture of 4.5 mm. The expected decentration range of 0.2 mm to 0.4 mm is highlighted in grey.

Comparing the course of the spatial frequency for 25 lp/mm with the course of the spatial frequency for 100 lp/mm of the Primus HD, it becomes obvious that the higher spatial frequencies react more sensitively to decentration.

The increase in decentration sensitivity of the ZCB00 is noticeable when using the cornea with a spherical aberration of +0.28 µm, as shown in figure 3. This IOL shows the best imaging quality in the best centration compared with the other lenses as it almost completely compensates for the spherical aberration of the cornea used. When using this cornea, it is also evident that higher spatial frequencies react more sensitively to decentration than the lower ones with the ZCB00. Considering the significantly lower decentration sensitivity of the comparison lenses and the lower decentration sensitivity of the ZCB00 when using the cornea with a spherical aberration of +0.13 µm (figure 2), the decentration sensitivity is shown to correlate positively both to the amount of spherical aberration of the cornea and the extent to which the spherical aberration of the cornea is corrected as long as the IOL actually has an aberration correction. As the CT Asphina 409MP has no correction of spherical aberration, therefore, there is hardly any detectable sensitivity to decentration (figures 1–3). In addition, decentration sensitivity decreases as decentration progresses. Furthermore, high spatial frequencies are more affected by decentration than low spatial frequencies. This is the case up to a decentration of 0.3 mm (see green line and green dashed lines for the ZCB00 in figure 3). With further decentration, this effect fades because the decentration sensitivity decreases. In figurefigure 3 3the Primus HD, with its under correction of the cornea used here, has a lower decentration sensitivity than the ZCB00. The same applies to the CT Asphina 409MP, which images slightly worse than the Primus HD at low and medium spatial frequencies due to its aberration neutral design. At high spatial frequencies, the Primus HD is better than the CT Asphina 409MP only at the beginning of the curve. This is because the differences are less apparent at low resolutions.

Figure 3
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Figure 3

Modulation/contrast in relation to decentration using the +0.28 µm cornea. The lenses examined are the ZCB00, Primus HD and CT Asphina 409MP. All examinations were performed with green light and a pupil aperture of 4.5 mm. The expected decentration range of 0.2 mm to 0.4 mm is highlighted in grey.

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.

Overview

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