Multifocal intraocular lenses (IOL) were developed to offer patients high optical quality at near distances after cataract surgery and thus provide spectacle independence. Initially, most multifocal IOLs were bifocal (ie, they provided a near and far foci with a gap in between for intermediate vision).1–4 To overcome this limitation, trifocal IOLs were designed to distribute incoming light to three foci,5 improving intermediate vision and achieving higher spectacle independence.6,7
Recently, extended depth of focus (EDOF) IOL designs have become more popular among surgeons. The EDOF theoretical concept of these IOLs is based on achieving good vision at different distances by means of appropriate interactions of higher order spherical aberration and diffractive patterns that elongate the focal zone.8–10 As commonly described by EDOF IOL manufacturers, this optical approach intends to increase the depth of field of patients and minimize the visual disturbances induced by classic diffractive or refractive multifocal designs (eg, halos and/or glare).
Regarding spectacle independence, in the past two decades a large number of patients have undergone corneal refractive surgeries to eliminate myopia worldwide.11,12 Obviously, if these patients are looking for spectacle independence for far distance, they will also look for spectacle independence in near tasks. Myopic corneal refractive surgery induces an increase of positive spherical aberration compared to patients with virgin corneas.13–16 Thus, it is relevant to analyze how the current designs of IOLs perform with corneas with an increased amount of positive spherical aberration.
The current study assessed and compared the optical quality performance of a widespread implanted trifocal IOL and a novel EDOF IOL, as well as the optical performance of both IOLs with a simulated cornea after myopic laser in situ keratomileusis (LASIK).
Patients and Methods
Image Quality Metrics
For the current study, the modulation transfer function (MTF)17–22 was analyzed for an aperture of 4.5 mm. To assess the tolerance to defocus at the far distance focal point, data were recorded for three different focal points from the base power of the lens (+0.50, +0.00, and −0.50 D). To compare the MTF value of the IOLs, we followed a previously described methodology22–24 where the value of each MTF was considered as the average modulation value, which is the modulation averaged across all frequencies within the 0 to 100 cycles/mm range. The average modulation value is proportional to the area under the MTF curve between 0 and 120 cycles/mm.
Through-focus MTF curves comprising 11 different focal points (steps of 0.50 D) were calculated for 4.5 mm and for a discrete spatial frequency of 50 cycles/mm. This spatial frequency could approximately correspond to an optotype for 0.5 Snellen equivalent visual acuity in white light (30 cpd). The higher the MTF value in the curve, the better optical quality the lens has at this focal point.
Optical Quality Analysis
The image quality of the IOLs was performed with the PMTF optical bench (LAMBDA-X) with software version 1.13.6. The device complies with ISO 11979-2 and 11979-9 requirements (ie, it comes with additional lenses for an aberration-free model cornea). It allows MTF measurements at various frequencies and at different focal planes (through-focus). The experimental set-up was assessed according to previous investigations in which other IOLs were analyzed by the same optical bench.5,24,25 Similarly, before each measurement, the optical device was calibrated to guarantee the precision of the MTF values.
The optical quality assessment of a certain IOL is related to ISO 11979-2, which specifies the use of a model eye including an aberration-free model cornea. At the same time, to simulate procedures after myopic LASIK, the aberration-free model cornea was modified and an increment of +0.29 µm was introduced in the optical system. This positive increment of spherical aberration intends to simulate LASIK for low myopia.26 In addition, pupil size modifies the optical performance of multifocal lenses and their depth of focus; the smaller the pupil size, the larger the depth of focus. In the current study, we used a 4.5-mm pupil to be a good compromise between habitual pupil sizes and an attempt to evaluate as much optical surface of the IOL as possible.
IOL Designs Studied
The first lens to be assessed was the XACT Mono-EDoF ME4 (Santen Pharmaceutical). This is a hydrophobic lens with a C-loop platform that presents a biconvex design. The lens shows diffractive rings in the anterior surface and a posterior asphericity designed to induce a negative Zernike 4th order spherical aberration coefficient of −0.17 µm for a 6-mm pupil. The anterior surface shows four diffractive rings inside the 3 mm of the optical zone diffracting light mainly toward far and intermediate foci. This EDOF lens has an overall diameter of 12.5 mm and an optical zone of 6 mm. The lens is available from +10.00 to +30.00 D in 0.50-D increments. The base power of the EDOF lens evaluated in the current study was 18.50 D. At the same time, it incorporates an ultraviolet and blue-light blocker.27
The other IOL analyzed was the FineVision IOL (PhysIOL) in its hydrophilic version. This IOL has a trifocal design that has been described in previous investigations.5,28–30 In summary, for obtaining three foci the lens combines two bifocal diffractive patterns for far/near and far/intermediate vision, respectively. The IOL presents two addition powers: +1.75 D for intermediate and +3.50 D for near vision. Moreover, this lens shows an apodized design in which the step height decreases toward the periphery with an increasing amount of light directed to distance vision. The lens has a biconvex-aspheric optics that generates −0.11 µm of negative spherical aberration for a 6-mm pupil. The diameter of the lens is 10.75 mm and the optic zone diameter is 6.15 mm. The optical power of the lens ranges from +10.00 to +30.00 D in steps of 0.50 D. As in the previous case, the base power of the trifocal lens was 18.50 D. Finally, the FineVision IOL incorporates an ultraviolet and blue-light blocker.
Figure 1 illustrates the through-focus MTF curves for the EDOF and trifocal IOLs, both with and without the positive increment of spherical aberration that simulates corneas with prior myopic LASIK. For the analysis of the IOL itself, the curves (black line) show the mean peak values of optical quality of both lenses, showing that the EDOF lens has two peaks with significant overlap and the trifocal lens has three peaks with less overlap (far, intermediate, and near).
Through-focus modulation transfer function (MTF) curves for the (A) extended depth of focus (EDOF) intraocular lens and (B) the trifocal intraocular lens. The curves were calculated for a spatial frequency of 50 cycles per millimeter (cycles/mm). D = diopters; LASIK = laser in situ keratomileusis
When the spherical aberration was introduced in the system (green line of Figures 1A–1B), both lenses showed changes with respect to the case without induced spherical aberration. In this case, the EDOF lens showed a −0.50 D negative shift of the through-focus curve (green line of Figure 1A). For the case of the trifocal IOL, there was also a negative shift of −0.50 D and a drop in the three peak values is shown (green line of Figure 1B).
To show the tolerance to defocus at the far distance focal point, Figures 2–3 show the MTF curves of the EDOF and the trifocal IOL for the 0.00, +0.50, and −0.50 D focal points, respectively. At the same time, it is possible to see the results of both lenses in the two situations considered: without an increment of spherical aberration (Figure 2A) and with an increment of spherical aberration that simulates myopic LASIK ablations (Figure 2B). In each case, the figures show a dotted line that represents the far distance focus (0.00 D) to better illustrate the optical quality of the lenses with a potential defocus of ±0.50 D at far distance.
Modulation transfer function (MTF) curves for the 0.00, +0.50, and −0.50 diopters (D) focal points for the (A) extended depth of focus (EDOF) intraocular lens and (B) EDOF intraocular lens with the positive increment of spherical aberration that simulates previous myopic laser in situ keratomileusis (LASIK).
Modulation transfer function (MTF) curves for the 0.00, +0.50, and −0.50 diopters (D) focal points for the (A) trifocal intraocular lens and (B) trifocal intraocular lens with the positive increment of spherical aberration that simulates previous myopic laser in situ keratomileusis (LASIK).
Finally, Table 1 summarizes the average modulation values (which is proportional to the area under the MTF curve [the higher the area, the better optical quality] between 0 and 120 cycles/mm) at these three focal points for both IOLs and for the situations previously mentioned.
Average Modulation Values of the Two IOLs for 4.5 mm of Aperture at Three Focal Points
In the current study, we assessed and compared the optical performance of the PhysIOL trifocal IOL and the novel XACT Mono-EDoF ME4 IOL in two situations: with and without a positive increment of spherical aberration that simulates a prior low to moderate myopic LASIK ablation.
Analyzing the blue line of Figure 1A (analysis of the IOL itself), it is possible to observe that the EDOF IOL shows two peaks with significant overlap that elongate the far distance focus (0.00 D vergence) to 1.50 D of addition approximately. After the vergence of 1.50 D, the optical quality of the EDOF lens decreases significantly, which could be understood as an IOL with addition just for intermediate vision. This optical quality could be related to clinical outcomes that were observed in several studies with other EDOF IOLs.9,10,31,32 However, no clinical studies have been performed with the IOL in our study and results should not be compared directly.
At the same time, the optical quality of the lens rapidly decreases toward positive vergences. It could be suggested that a negative overrefraction after the surgery will be barely accepted for far distance. However, a slight negative overrefraction could somehow improve the near vision in patients with EDOF IOLs such as the one analyzed in this study. Then, a combination of emmetropia in one eye and a slight myopia in its counterpart, previously called mini-monovision, could offer higher addition ranges without a significant deterioration of distance vision under binocular conditions.33
On the other hand, the green line of Figure 1A, which simulates corneal profiles after LASIK, shows an overall negative shift of −0.50 D of whole trough-focus curve and the elongated distance focus performs 1.00 D of addition. Nonetheless, the profile of through-focus curve in this case was similar to the case of the IOL without an increment of spherical aberration (blue line). It could be suggested that the combination of aberrations (IOL + increased corneal spherical aberration) did not dramatically modify the optical performance at far and intermediate distance in the EDOF lens besides the slight increment of optical power and the consequent displacement of all distances of vision. Related to the myopic shift, there is a previous study in which the clinical outcomes of patients with presbyopia-correcting IOLs after myopic LASIK were analyzed.34 In that study, the authors found that the manifest refraction of these patients after the surgery showed a spherical equivalent of −0.60 D. This means that the effective power of the IOLs was approximately 0.50 D more positive and this agrees with the results of our study. In other words, to reach an effective emmetropia for these patients, the calculated power of the IOL should be less positive (−0.50 D).
The through-focus curve of the trifocal IOL is shown in Figure 1B. As in the previous case, the blue line of the figure corresponds to the optical performance of the IOL without the increment of positive spherical aberration. It reveals three peaks corresponding to far, intermediate, and near focal points. These results have been reported in previous investigations that analyzed the optical quality of this IOL.5,28–30,35 At the same time, several clinical studies that assessed the visual performance of this IOL demonstrated that these focal points correspond to three zones with improved visual quality.30,36 It should be mentioned that the trifocal IOL showed a significantly better optical quality for the far distance focal point if compared to the EDOF IOL. However, the trifocal IOL decreases its optical quality rapidly out of the far foci, whereas the EDOF IOL remains more stable. As will be discussed below, it could be suggested that the IOL power calculation is more critical for trifocal IOLs to achieve the best optical performance.
As in the previous case, the green line shows the through-focus curves of the IOL with the mentioned amount of spherical aberration. In this case, the curve of the trifocal IOL also showed an overall negative shift of −0.50 D. These results follow the trend shown in the case of the EDOF IOL. Therefore, it should also be considered that patients after myopic LASIK with this IOL could show a slight myopic refraction that should be compensated for with less positive IOL power.34 Finally, instead of the presence of the three peaks, which means that patients could achieve the attempted trifocality (far, intermediate, and near), the optical quality of the far distance peak is attenuated in comparison to a cornea without previous myopic ablation.
Moreover, positive or negative overrefractions after cataract surgery have important clinical implications for surgeons because they can deteriorate the distance vision and/or modify the distance at which near activities have to be performed by patients. This situation has clinical implications even for monofocal IOLs, but it is crucial for multifocal lenses. Hence, to assess the impact of a small amount of residual refractive error on the optical behavior at far distance of these IOLs, we have analyzed its tolerance to defocus by means of analyzing the MTF curves at 0.00, +0.50, and −0.50 D.
For the case of the EDOF IOL without increased spherical aberration (Figure 2A and Table 1), the MTF was better for the far distance focus, with the +0.50 D focus showing the worst optical quality. Despite a certain tolerance to hyperopic and myopic shifts, surgeons should achieve emmetropia in patients with normal corneas to obtain better performance in distance vision. The simulation of prior low myopic LASIK ablation (Figure 2B and Table 1) showed a significant change in these values and it is possible to see that the better optical quality in this case is achieved for −0.50 D, then leading to a myopic residual refraction and to a worsening at far distance focus. These results agree with the results presented in the through-focus curves. Therefore, surgeons should compensate for this situation in patients with prior myopic ablations to achieve satisfactory far vision results. In addition, it should be mentioned that calculating IOL powers for patients with previous corneal ablations is challenging37 and this issue should be accurately addressed if emmetropia is to be achieved.
Similarly, the trifocal IOL showed a significantly better MTF value for the 0.00 D focal point when a normal cornea is considered (Figure 3A and Table 1). Due to the steep change in the optical quality induced by ±0.50 D of defocus, surgeons should achieve emmetropia with high rates of accuracy for these patients to obtain satisfactory results in far vision. As previously explained, it seems that an accurate IOL power calculation is more important for trifocal than EDOF IOLs. On the other hand, the myopic LASIK ablation situation (Figure 3B and Table 1) showed a change in the optical quality. In this case, better optical quality is achieved with −0.50 D and it should be also taken into consideration at the moment of calculating the IOL power. However, it is interesting to note the differences regarding the tolerance to defocus with and without an increase of spherical aberration. The first difference is that the impact of a small residual refractive error on the optical quality was lower for the increased spherical aberration situation. The second difference is that for the situation of corneas with a significant increase of positive spherical aberration, the IOL power calculation should target a less positive IOL power, whereas the target in the situation with a normal cornea should be emmetropia.
All measurements of the current study were made through centered positions of the IOLs and this may not represent all possible real world scenarios. In future studies, the scope of the study should be extended to account for misalignments between the cornea and the actual placement of the IOL.
The results of the current study show that the trifocal IOL provided three clearly differentiated peaks of vision with a high optical quality for the far distance focal point. Meanwhile, the EDOF IOL showed two peaks with significant overlap that reached an acceptable optical quality from far to intermediate distance. When myopic LASIK ablations were simulated, the EDOF IOL was more robust than the trifocal IOL. Finally, the negative shift in the optical performance of both IOLs suggests that IOL calculations for patients with previous myopic ablations should consider less positive IOL powers to reach an effective emmetropia.
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Average Modulation Values of the Two IOLs for 4.5 mm of Aperture at Three Focal Points
|Focal Point (D)||EDOF||EDOF Myopic LASIK||Trifocal||Trifocal Myopic LASIK|