The advances in multifocal intraocular lens (IOL) technology offer solutions to decreasing spectacle dependence and improving visual function and patients’ quality of life after cataract surgery or refractive lens exchange.1 Because intermediate visual acuity is increasingly important in daily life, new designs providing distance, intermediate, and near visual acuities with trifocal diffraction,2 extended depth of focus,3 or refractive rotational asymmetry4 have been introduced in recent years. The refractive rotationally asymmetric multifocal IOLs have been preferred because they provide better contrast sensitivity than the multifocal IOLs with concentric rings in clinical trials.5,6
The SBL-3 (Lenstec, Inc.), a second-generation refractive rotationally asymmetric multifocal IOL with an inferior surface-embedded near segment, has been clinically used to improve intermediate vision and reduce visual disturbances while restoring good visual outcomes at near and distance.4,7,8 The therapeutic results primarily depend on the accurate biometry and IOL position in the bag,9 and this type of multifocal IOL is sensitive to decentration and tilt.10,11 However, it is difficult to avoid the displacement and differentiate factors with potential influence on the IOL position in clinical practice, and the effects of decentration and tilt on this type of multifocal IOL are unpredictable. There has been no evaluation about the relation between optical quality and the specific magnitude and orientation of the asymmetric multifocal IOL tilt and decentration in vivo and in vitro. Montés-Micó et al.12 demonstrated that tilt and decentration had a prominent effect on optical quality with the rotational asymmetric multifocal IOL compared with the refractive–diffractive IOL in vitro. Bonaque-González et al.13 showed the modulation transfer functions (MTFs) of the rotationally asymmetric IOLs were different between orientations. To achieve better clinical outcomes, it is essential to adequately understand how and how much the multifocal IOL decentration and tilt affect the patients’ visual quality. Herein, the influence of decentration (two levels) and tilt (two angles) of the rotationally asymmetric multifocal SBL-3 IOL at three orientations with two aperture sizes was assessed in vitro.
Characteristics of the Tested Intraocular Lens
The SBL-3 is a biaspheric asymmetrical refractive multifocal IOL manufactured from a hydrophilic acrylic material with two distinct zones. One zone is for distance vision (50% of the optic), and the other is a +3.00 diopter (D) addition near segment in the inferior anterior optic (42% of the optic), which translates to approximately +2.50 D at the spectacle plane. It has a small wedge-shaped transition zone separating the distance from the near add component. The multifocal IOL length is 11.0 mm, with an optic size of 5.75 mm. The refractive power of this type of multifocal IOLs ranges from +10.0 to +36.0 D in 0.50 D increments, whereas the IOLs of +15.0 to +25.0 D available in 0.25 D increments are most commonly used. Venter et al.4 concluded that the shape of the haptic is similar to the accommodating lens from the same manufacturer (Tetraflex, Lenstec, Inc.). However, it is three times wider and 1.5 times thicker. One SBL-3 multifocal IOL with a power of +19.0 D was investigated in this study.
The optical performance of the multifocal IOL was assessed with an optical bench (OptiSpheric IOL Pro, Trioptics GmbH), which included a model eye with an aberration-free model cornea and adhered to the International Organization for Standardization 11979-2 and 11979-9 requirements.A,B The wavelength of the light source was 546.1 nm.14 The tested IOL focused the projected target at its focal plane, which was captured by the measurement detector including an objective microscope lens and a high-resolution charge-coupled device camera with an integrated autofocus. The matrixes of centration, decentration of 0.3 mm and 0.5 mm, and tilt of 3 degrees and 5 degrees with 3.0 mm and 4.5 mm apertures (representing pupil sizes) were used. The IOL was kept within the model eye in saline water with a proper refractive index of 1.334 at ambient temperature (close to that of aqueous and vitreous humors).
Under the apertures of 3.0 and 4.5 mm, the IOL was first centered on the optical axis of the optical bench setup for MTF measurement and the United States Air Force (USAF) 1951 Resolution Target test,C respectively, in accordance with a previous study.15 Then, the IOL was decentered at values of 0.3 mm and 0.5 mm with near–horizontal (near vision zone), distance–horizontal (distance vision zone), and vertical directions of the long axis of the lens, respectively (Figure 1). Last, the multifocal IOL was tilted at angles of 3 degrees and 5 degrees with near–horizontal, distance–horizontal, and vertical directions of the long axis of the multifocal IOL, respectively.
Optical Quality Parameters
The MTF curve describes the ability of an optical system to transfer the details of an object to the image.16,17 As such, it is routinely used to describe the performance of an optical system. The tangential and sagittal MTF curves expressing the images of horizontal and vertical edges were measured at the best focal plane within the spatial frequency of 0 to 120 LP/mm. Moreover, a through-focus MTF curve at a spatial frequency of 50 LP/mm, equivalent to 20/40 (Snellen),18 was obtained at all possible conditions. The axial scanning in the IOL image space for the through-focus analysis was made from +1.00 to −5.00 D in 0.50 D steps. The aperture sizes of 3.0 mm and 4.5 mm corresponded to the average pupil sizes of humans older than 60 years under photopic and mesopic conditions, respectively.19 The USAF 1951 Resolution Target test images taken at the best far, intermediate, and near vision foci of the multifocal IOL were documented to qualitatively compare the optical performance at different orientations of decentration and tilt.
Figures 2 and 3 show the MTF curves (displacements: 0.5 mm of decentration and 5 degrees of tilt) for the multifocal IOL with a 4.5-mm aperture size. Figures 4 and 5 show the through-focus MTF curves at a frequency of 50 LP/mm with the same displacements as above. Figures S1 and S2 (available at http://jcrsjournal.org) show the image quality of the USAF target at the decentration and tilt of near–horizontal, distance–horizontal, and vertical orientations with 3.0 mm and 4.5 mm aperture sizes, respectively.
Modulation Transfer Function Curves
For both distance and near foci, the MTF curves of the centered multifocal IOL showed higher values at an aperture of 4.5 mm than at 3.0 mm (Figure 2), which was in agreement with the image quality of the USAF target shown in Figure S1 (available at http://jcrsjournal.org).
The characteristics of decentration in the MTF curves for the asymmetric multifocal IOL (Figure 2) were consistent with the image quality of the USAF target shown in Figure S1 (available at http://jcrsjournal.org). Compared with the centration, the curves for near–horizontal decentration were higher in near focus and lower in distance focus, and this finding contrasted with the distance–horizontal decentration. Moreover, the discrepancy between sagittal and tangential MTF curves was increasing in near focus as the multifocal IOL was set to be vertical decentration, and this tendency was more obvious at the 3.0 mm aperture than at 4.5 mm.
The MTF curves of tilt for the asymmetric multifocal IOL displayed similar results (Figure 3) with the USAF target (Figure S2, available at http://jcrsjournal.org). The tilt induced decreased MTF curves in near and distance foci at the three orientations. In contrast to the centration, the MTF curves in near focus appeared to be more declined than distance focus for the near–horizontal tilting, and the distance–horizontal tilting displayed an opposite trend. The MTF curves of near and distance foci were both reduced for the vertical tilting. Contrary to the decentration of the multifocal IOL, this tendency was more distinct at the 4.5 mm aperture than at 3.0 mm.
Through-focus Modulation Transfer Function
The through-focus MTF curves showed that the position of the peak was related to the main focus of the multifocal IOL, but the sagittal and tangential foci did not always coincide for the rotationally asymmetric multifocal IOL. The more overlapping and higher score of the sagittal and tangential MTF values, the better optical quality. Conversely, the wider the separation and the lower the score of sagittal and tangential MTF values, the worse the optical quality. At the central position, the score of MTF peaks was higher at the 4.5 mm aperture than at 3.0 mm at both distance and near foci (Figure 5). In addition, the adverse influence of sagittal and tangential through-focus MTF curves by decentration was found to be different from that by tilt.
The adverse influence was detected on the tangential MTF curves for the horizontal decentration and on the sagittal MTF curves when it was vertical decentration. A higher score and more overlapping of sagittal and tangential MTF curves at near focus (−3.0 D), and a lower score and wider separation at distance focus (0.0 D) were shown when the decentration was toward the near–horizontal orientation, indicating the optical quality was better at near focus and worse at distance focus. However, the distance–horizontal decentration displayed an opposite trend. Moreover, a wider separation of sagittal and tangential MTF curves at near focus (−3.0 D) was shown when the multifocal IOL was vertical decentration, which suggested a poorer optical quality at near focus. These tendencies were more obvious at the 3.0 mm aperture size than at 4.5 mm.
Tilt was different from the decentration in that horizontal tilt had an adverse influence on the sagittal MTF curves, and vertical tilt affected both the sagittal and tangential MTF curves. Regardless of the tilt orientations, a lower score or wider separation was shown at both distance and near foci compared with the centration. For near–horizontal tilting, the sagittal MTF value of near focus (−3.0 D) was deteriorated more significantly than distance focus (0.0 D), indicating the optical quality was more deteriorated at near focus for near–horizontal tilting, and the optical quality was more deteriorated at distance focus for distance–horizontal tilting. Vertical tilting displayed a lower score or wider separation of the sagittal and tangential MTF curves at near and distance foci. These tendencies were more distinct at the 4.5 mm aperture size than at 3.0 mm.
For the sectorial design of asymmetrical multifocal IOLs, the conditions of centration play an important role in achieving better optical quality,20 but the malposition seems to be inevitable,10–12 and the lack of a gold standard assessment tool has led to a lower quality of vision in some patients.20,21 Therefore, we assessed the qualitative and quantified changes of optical performance of the SBL-3 multifocal IOL under decentration and tilt conditions at three orientations, hoping to provide guidance to clinical practice.
Regarding pupil size, this study showed the optical quality of the 4.5 mm aperture was better than the 3.0 mm both at distance and near foci (the central positions in Figure 2, Figure 4, and Figure S1 [available at http://jcrsjournal.org]), which was distinct from the multifocal IOLs with concentric rings showing decreased optical quality as the pupil enlarged.15,22 However, the characterization was consistent with the design of this rotational asymmetric multifocal IOL. The proportionate exposure of both distance and near components was increased with the increased pupil size, thereby optimizing visual outcomes. The findings were also in agreement with the previously reported clinical outcomes that larger pupils had a significantly positive effect on the quality of vision.23,24 Furthermore, the sagittal and tangential MTF curves were found to be almost identical in multifocal IOLs with concentric rings at the centration,25,26 but the multifocal IOL in the current study had completely inconsistent sagittal and tangential MTF curves for the design of rotational asymmetry.
In our study, the optical quality of the rotationally asymmetric multifocal IOL was observed to be sensitive to decentration and tilt. In comparison with the centration, the MTF value was higher at near focus and lower at distance focus for the near–horizontal decentration (Figures 2 and 4), and the images of the USAF target were consistent with the above results (Figure S1, available at http://jcrsjournal.org), which might be because more near surface area was exposed than the distance surface area. Nonetheless, the situation was reverse at the distance–horizontal decentration. For vertical decentration, the wider separation between the sagittal and tangential MTF curves (Figure 2) corresponded to the misalignments of MTF peaks between the sagittal and tangential through-focus MTF curves (Figure 4) and the decreased image quality of the USAF test at near focus (Figure S1, available at http://jcrsjournal.org). However, the optical quality of distance focus was not influenced, which might be because the exposure of distance surface area was not changed but the near surface area was reduced for the increasing transition region when the multifocal IOL was of vertical decentration. This tendency was more obvious at the 3.0 mm aperture size than at the 4.5 mm aperture size because the exposures of distance and near surface areas were both increased at a large aperture size. There were such different opinions on the rotational symmetric multifocal IOLs with concentric rings because decentration less than 0.5 mm had no effect on or just slightly decreased the quality of vision.12,25 In the current study, however, the rotational asymmetric multifocal IOL was sensitive to and changed dramatically after decentration with increased and decreased optical performance at different orientations; and the results were in agreement with a previous clinical study, in which the multifocal IOL was found to be 0.2 mm near-segmental decentered leading to a myopic state and complaining of blurred vision in brightly light conditions.20 van der Linden et al.27 disclosed that the near visual acuity was poorer because this type of multifocal IOL decentered 0.25 mm toward the distance area. It was also reported that the decentration of the asymmetric multifocal IOL caused by capsule contraction resulted in the degradation of visual acuity.21
The tilt induced decreased optical performance at the three orientations in this study, with the MTF value showing a greater deterioration in near focus than in distance focus for the near–horizontal tilting, and a greater deterioration in distance focus than in near focus for distance–horizontal tilting (Figures 3 and 5), which was similar to the images of the USAF target (Figure S2, available at http://jcrsjournal.org). The monofocal IOL tilt might influence ocular coma-like aberrations, leading to a positive effect on near visual acuity because of the enhanced depth of focus.28 A compensation effect was also demonstrated in that the corneal and internal comas cancelled each other out because of opposite signs.29 The rotationally asymmetric multifocal IOL can result in different levels of coma,30 and the multifocal IOL used in our study has a large primary coma.10,31–33 Clinical studies10,32,33 outlined a higher amount of coma affecting the optical quality when tilt occurred, and in an in vitro study,12 the primary coma values were thought to be partially compensated for by the induced tilt but the optical quality was also poor. In the current study, tilt in different orientations had varied effects on optical performance. Although we did not measure the primary coma of the multifocal IOL and the coma induced by tilt, the coma values were expected to change at different orientations of tilt. As the multifocal IOL has two refractive zones with a difference of +3.0 D, tilt changed not only the effective lens position of the multifocal IOL but also the power at both zones. The coma might be correspondingly influenced. Moreover, the tendency of the multifocal IOL tilt with optical performance was more distinct at the 4.5 mm aperture size than at 3.0 mm, which is similar to the coma-like aberration increase as the pupil enlarges. Therefore, it is necessary to further study the relationship between coma and tilting of this type of multifocal IOLs at different orientations.
The results of this study are somewhat limited. The refractive power of +19.0 D of the assessed multifocal IOL made it not representative for IOLs with significantly larger or lower refractive powers. Another potential issue of the study concerns the aberration-free artificial cornea, in which the potentially adverse effects of the patients’ higher order aberrations, especially spherical-like aberrations, seem to be neglected. However, the SBL-3 multifocal IOL is also aberration-free, which is useful for assessing the influence of decentration and tilt on the optical quality. Moreover, as a rising number of corneal refractive surgeries are performed, the coupling between multifocal IOLs and different corneal profiles deserves further investigations. In clinical practice, the combination of tilt and decentration is more common than tilt or decentration alone, thus further studies are necessary. Because of the neuroadaptation of human eyes, the best optical qualities of distance, intermediate, and near foci were adopted at all possible positions in the current study.
In conclusion, the mild malposition of a rotationally asymmetric multifocal IOL had an effect on image quality. Compared with the central position, the optical quality of the multifocal IOL was better at near focus and worse at distance focus for the near–horizontal decentration, but this situation was reverse for the distance–horizontal decentration, with a more obvious tendency at the 3.0 mm aperture than at 4.5 mm. Tilt just decreased optical performance. For the near–horizontal tilting, the optical quality at near focus was more significantly deteriorated than distance focus, whereas for the distance–horizontal tilting, it was opposite, with a more distinct tendency at the 4.5 mm aperture than at 3.0 mm. The optical performance was better at the aperture of 4.5 mm than at 3.0 mm for the centered multifocal IOL. The effects of decentration and tilt on the rotationally asymmetric multifocal IOLs are unpredictable in vivo. This in vitro study of the SBL-3 multifocal IOL helps to predict the optical behavior in clinical settings.
What Was Known
- The optical quality of an asymmetric multifocal IOL is sensitive to decentration and tilt in clinical practice.
What This Paper Adds
- The optical performance of an asymmetric multifocal IOL was sensitive to and changed dramatically after decentration and tilt, as tested by an optical bench. The decentration induced increased or decreased optical quality, but tilt yielded decreased optical quality at different orientations.
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None of the authors has a financial or proprietary interest in any material or method mentioned.
Supplemental material available at www.jcrsjournal.org.
Other Cited Material
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