EDOF IOLs not only offer excellent distance, intermediate, and near visual acuity, but also provide a better objective visual quality compared to monofocal IOLs [13–15]. However, photic phenomena were one of the main complaints in patients with the implantation of EDOF IOLs after cataract surgery, and were related with angle kappa [1, 3]. Furthermore, due to the indeterminacy of the optical axis, angle alpha is rarely applied in clinic work. With the development of high-precision instruments, iTrace has become the common instrument to measure angle alpha. Previous studies have reported that the causes of undesired postoperative outcomes were dry eye, residual lens fragments, posterior capsule opacification, IOL decentration, astigmatism, and MRSE [16–19]. Our study ruled out the interference of these factors. First, we incorporated dry eye, residual lens fragments, and posterior capsule opacification into exclusion criteria. Second, we ruled out the interference about the age, astigmatism, IOL power and target refraction before cataract surgery. Third, we made comparison of subjective and objective visual quality on the basis of no difference with IOL decentration, astigmatism, and MRSE between groups with different angle alpha after cataract surgery. Finally, a correlation analysis was done between angle alpha and the parameters of visual quality. Therefore, based on the above, we evaluated whether angle alpha changes after implantation of EDOF IOLs, and whether magnitude of angle alpha was associated with a deterioration of postoperative visual acuity, MTF, aberration, and photic phenomena outcomes. A cut-off value of 0.3 mm, 0.4 mm, and 0.5 mm in preoperative angle alpha was chosen to divide eyes into groups to determine which cut-off value is the most suitable index for the implantation of EDOF IOLs.
In our study, we evaluated whether angle alpha changes after the implantation of EDOF IOLs, and found the mean postoperative angle alpha were slightly lower than the mean preoperative angle alpha in all eyes. However, no significantly differences were found between preoperative and postoperative angle alpha. In our study, angle alpha was defined as being the distance between the visual axis and the center of cornea. Since the center of cornea is relatively constant, the change of angle alpha might be related to the change of visual axis after the operation, especially in uneven opacity of the crystalline lens, which may cause the line of sight to change, thus resulting in the change of visual axis after the surgery . Besides, previous studies have indicated that the magnitude of angle kappa after the phacoemulsification significantly decreased due to the change of visual axis and the change of diameter, shape, and position of the pupil [9, 10]. This indicated that angle kappa was relatively unstable respect to angle alpha. According to the above, angle alpha may be a more reliable reference compared to angle kappa for the implantation of MIOLs, which was in accordance with a recently published study . Therefore, angle alpha should be more considered in the preoperative assessment of patients implanted MIOLs in clinic work.
In the present study, the mean postoperative logMAR visual acuity in all groups ranged from 0.30 to -0.08. This confirmed that the EDOF IOLs can provide good postoperative far, intermediate, and near vision, in term of visual acuity outcomes. Besides, there were no statistically significant differences in far, intermediate, and near visual acuity between all groups, which showed that magnitude of angle alpha did not negatively impact visual outcomes and had no any influence on postoperative vision.
The MTF is the ratio of contrast between the retinal image and the original scene [1, 18], and this reflects the transmission ability of the optical system to different spatial frequencies. We used 3 mm and 5 mm pupil diameters to simulate the visual quality under photopic and scotopic conditions, respectively. In our study, there were no statistically significant differences in difference spatial frequencies MTF for between all groups with 3 mm and 5 mm diameter, respectively. This showed that MTF was not affected by the magnitude of angle alpha. In a word, the magnitude of angle alpha had no effect on visual image quality of the human eye optical system whether it is under photopic and scotopic conditions.
Aberrations lead to defects in image-forming. This caused the image obtained to being imperfect, thus decreasing the visual quality. Previous studies reported that HOAs near the center of the Zernike polynomials, such as coma, SA, and trefoil, tend to more significantly affect visual quality than those at the periphery of the Zernike polynomials [20, 21]. Therefore, in our study, HOAs mainly included coma, SA, and trefoil. Our study found that there were no statistically significant differences of ocular aberration, internal aberration, and corneal aberration between Group A1 and Group B1 in 3 mm and 5 mm pupil diameter, respectively. This showed the cut-off value of 0.3 mm in angle alpha had no influence on aberrations. However, when 0.4 mm in angle alpha was chosen the cut-off value in 3 mm pupil diameter, this caused significant differences of HOAs and SA in internal aberration, whereas there were no significant differences in ocular aberration. The possible reasons were as follows: first, internal aberration represented all aberrations behind the anterior corneal surface. When the implantation of IOL, it mainly caused changes in internal aberration. Second, ocular aberration is composed of internal aberration and corneal aberration, and ocular aberration plays a pivotal role in the visual quality of patients. Under the premise that the corneal aberration was almost constant, the change in internal aberration was not enough to cause a significant change in ocular aberration. This showed the cut-off value of 0.4 mm in angle alpha did not affected ocular aberration in the 3 mm pupil diameter. In a word, the cut-off value of 0.4 mm in angle alpha did not affect the patient's visual quality under photopic conditions. And so was the cut-off value of 0.5 mm in angle alpha in a 3 mm pupil diameter. Therefore, magnitude of angle alpha did not deteriorate visual quality under photopic conditions. More importantly, when 0.4 mm in angle alpha was chosen the cut-off value in a 5 mm pupil diameter, in addition to HOAs and SA in internal aberration, there also were statistically significant differences of HOAs and SA in ocular aberration, thus leading to the decline of visual quality. Moreover, compared to the cut-off value of 0.4 mm, when 0.5 mm in angle alpha was chosen the cut-off value in 5 mm pupil diameter, in addition to HOAs and SA, significant differences of coma in ocular aberration and internal aberration were also added. It was well known that ocular aberrations increase together with pupil diameter under dim light conditions , thus worsening the visual quality of patients. Based on the above, our outcomes indicated that the cut-off value of 0.4 mm or higher in angle alpha mainly affected the visual quality under scotopic conditions. However, magnitude of angle alpha did not affect the visual quality under photopic conditions. To the best of our knowledge, there were few studies evaluating the influence of angle alpha on visual quality, and only one recently published article showed that was no correlation with angle alpha after EDOF IOLs implantation , this is inconsistent with our study. The possible reasons were as follows: first, the parameters of objective visual quality were different. Recently published article showed that objective visual quality included objective scattering index (OSI), MTF cutoff, strehl ratio, and simulated visual acuity at 100%, 20%, and 9% contrast. However, in our study, objective visual quality mainly included MTF, ocular aberration, internal aberration, and corneal aberration. Second, the statistical method between angle alpha and visual quality was not the same. Recently published article calculated their correlation of those visual quality results with angle alpha while our study grouped according to the size of the angle alpha and compared the differences in visual quality between different groups. Therefore, our results might provide a reference for the critical size of the angle alpha necessary for successful implantation of MIOL.
Last but not least, photic phenomena, such as halos and glare, was still the common complaints of patients after the implantation of MIOLs, which seriously affected patient satisfaction [1–3]. In our study, the statistically significant differences in the score of photic phenomena were found in between Group A2 and Group B2 and between Group A3 and Group B3. This showed the cut-off value of 0.4 mm or higher in angle alpha experienced more risk in halos and glare. Previous studies showed photic phenomena were related to be preoperative large angle kappa [23, 24]. Especially when preoperative angle kappa was greater than 0.4 mm, the incidence of glare and halo increased . However, some patients with preoperative large angle kappa had no photic phenomena postoperatively . One possible reason might be that significant reduction in angle kappa was found after cataract surgery compared with pe-operation . Hence, the light enters the eye through the central area of the IOL instead of through other diffraction rings after post-operation, thus not causing photic phenomena. This showed that the magnitude of postoperative angle kappa played a key role in photic phenomena. However, angle alpha, which was similar to angle kappa, played an important role in the implantation of MIOLs , and our study showed that angle alpha almost unchanged before and after surgery (pre-operation vs post-operation: 0.39 mm and 0.37 mm), which is in accordance with recent studies [9, 10]. This indicated angle alpha may a more accurate indicator in the preoperative evaluation of cataract patients compared to angle kappa in clinical work. For patients with a 0.4 mm or higher of angle alpha, the choice to implant the MIOLs should be carefully evaluated.
However, this study has several limitations. There was a relatively small number of patients in the cut-off value of 0.3 mm, 0.4 mm, and 0.5mm in angle alpha, especially the number in between r < 0.3 mm and r ≥ 0.30 mm and between r < 0.5 mm and r ≥ 0.50 mm was relatively unequal, further study with a larger sample size is necessary.