In this prospective study, the clinical outcomes of mix-and-match implantations of ZKB00 and ZLB00 were showed good UCVA and UNVA as well as UIVA and high satisfaction without visual disturbance such as glare and halo. Although there was previous study comparing ZMB00, ZKB00 and ZLB00, we could confirm that depth of focus was increased through contralateral mix-and-match implantation of ZKB00 and ZLB00. Compared with previous studies using the trifocal diffractive IOLs, our results revealed that contrast sensitivity was not reduced and visual disturbance was less. The previous version, ZMB00 Tecnis multifocal IOLs with + 4.0 D add power has the same design as the studied IOLs; however, the study IOLs have a relatively lower add power of + 2.75 D and + 3.25 D. All IOLs of this platform have a refractive zone on the anterior surface to provide distance vision and a full diffractive posterior surface for near vision. The fewer diffractive rings of ZKB00 and ZLB00 compared to ZMB00 are considered to reduce unwanted visual symptoms . Regardless of pupil size, the light is evenly distributed between distance and near foci. Other optical principles of multifocal IOLs are dependent on pupil size .
Other studies with ZKB00 and ZLB00 IOL implantation report that subjects implanted with low add power bifocal IOLs had good intermediate and distance visual acuity with a high level of satisfaction [16, 18, 19]. Kretz et al.  reported 63.3% of the patients implanted with ZKB00 in both eyes achieved a binocular UIVA at 80 cm of 0.1 logMAR or better. In this study, the percentage of patients with binocular logMAR UIVA better than 0.1 logMAR at 80 cm was 68.5%. Kretz et al.  reported that bilaterally implantation of the ZLB00 IOL revealed a binocular UIVA of 0.06 ± 0.09 logMAR at 60 cm. In our study, we found comparable results with 0.09 ± 0.09 logMAR. However, previous studies did not include the defocus curve which makes it difficult to compare the achieved visual acuity at various distances directly. Previous studies of bilateral mix-and-match implantation of diffractive bifocal IOLs reported a better visual acuity over a wider range compared to bilateral implantation of IOLs with the same add power . Our study with mix-and-match implantations of Tecnis ZKB00 and ZLB00 found a 0.1 logMAR or better visual acuity in the 0 to − 2.5 D range of the defocus curve. We could speculate that outcomes of mix-and-match implantation of Tecnis ZKB00 and ZLB00 might be better visual acuity at a broader range than bilateral implantation of IOLs with the same add power (ZKB00 or ZLB00).
This study is the first prospective study applying the bilateral mix-and-match implantation of Tecnis ZKB00 and ZLB00. All previous studies on mix-and-match implantations of diffractive bifocal IOLs used the AcrySof ReSTOR IOL [10, 11]. Nakamura et al.  reported that contralateral implantation of ReSTOR IOLs with + 3.0 and + 4.0 D addition was an effective way to get a broad range of good uncorrected visual acuity in the defocus curve. Mastropasqua R et al.  also reported that patients, implanted with ReSTOR IOLs with contralateral + 2.5 and + 3.0 D additions, had good uncorrected visual acuity over a wide range, and contrast sensitivity and visual quality did not decrease compared to bilateral implantation of diffractive multifocal IOLs with the same additional power. Compared with the defocus curve of Mastropasqua et al., our study revealed 0.1 logMAR or better vision from 0 to − 2.5 D, whereas Mastropasqua et al. report 0.1 logMAR or better in the range from 0D and − 1.5 ~ − 2.5D. In the range of intermediated distance from − 0.5D to − 1.5D, the results of ours study appear better than those of Mastropasqua et al. Although the add power differs slightly between studies, it seems that the IOL design is responsible for the better intermediate vision. And it may be due to differences in clinical characteristics of patients, such as axial length that can affect effective lens position.
Recently, trifocal diffractive IOLs were developed to provide better intermediate visual acuity. So far, no direct comparative study between bilateral implantation of diffractive trifocal IOLs and contralateral implantation of diffractive bifocal IOLs has been published. Ours study shows 0.1 logMAR or better visual acuity in the range from 0 to − 2.5D in the defocus curve and it was comparable to or slightly better than that reported in previous studies on trifocal diffractive IOLs .
Multifocal IOLs had a drawback in decreasing contrast sensitivity However, for Tecnis multifocal IOLs it was known as the prolate anterior surface could improve the mesopic contrast sensitivity [17, 21]. Gierek-Ciaciura et al.  reported that eyes with ZM900 Tecnis multifocal IOLs had better contrast sensitivity than eyes with other diffractive multifocal IOLs or refractive multifocal IOLs. Kim et al.  found that contrast sensitivity was higher in subjects with ZKB00 or ZLB00 than subjects with ZM900. This study, using ZKB00 and ZLB00, also showed improvement of contrast sensitivity compared with preoperative contrast sensitivity, and statistically significant improvement in some spatial frequency.
Diffractive multifocal IOLs with fewer diffractive rings and lower add power could theoretically improve the quality of vision after cataract surgery. Trifocal IOLs need it split more light energy to form the third focal point compared to bifocal IOLs and more diffractive rings are used for the trifocal IOLs compared to the IOLs used in our study. This might have an effect on the quality of vision for near and distanace [5,6,7]. Montes-Mico R et al.  used optical bench testing to confirm the quality of the apodized trifocal IOL (Finevision Micro F, PhysIOL, Liege, Belgium), and report a worse quality of vision compared bifocal diffractive IOLs. Kohnen T et al.  reported that halo and glare appeared in 60% and 28% of patients, respectively, after the implantation of AT LISA tri839MP, another trifocal IOL (Carl Zeiss Meditec, Jena, Germany). Our study showed halo and glare in 31.5% and 5.3% of patients, respectively, less visual artifacts compared to the results of Kohnen et al. Jonker et al.  also reported that mesopic contrast sensitivity was slightly decreased in eyes with diffractive trifocal IOLs compared to diffractive bifocal IOLs. Future studies should compare the quality of vision between groups with bilaterally implanted with diffractive trifocal IOLs and contralaterally implanted diffractive bifocal IOLs.
Reading performance, such as reading speed, critical print size, and threshold print size, were significantly improved postoperatively compared to baseline. Alfonso et al.  reported critical print size and threshold size after bilateral implantation of AcrySof + 3.0 toric multifocal IOLs were 0.28 ± 0.12 logRAD and 0.08 ± 0.08 logRAD, respectively. Schmickler et al.  reported that critical print size was 0.27 ± 0.12 logRAD in patients after bilateral implantation of Tecnis ZMB00 + 4.0 diffractive multifocal IOLs. Our results of critical print size and threshold print size were 0.24 ± 0.13 logRAD and 0.14 ± 0.13 logRAD, respectively, and comparable to previous studies [14, 23]. In our study, postoperative reading speed was 86.83 ± 17.45 wpm. Alfonso et al.  reported a reading speed of 132.68 ± 23.69 wpm after the implantation of diffractive multifocal IOLs. Reading speed in our study is slightly lower compared to results from Western regions [14, 24]. One study using the same application as in our study to test reading speed in Koreans reported a reading speed of 129.7 ± 25.9 wpm for adults in their 20s and 30s . Considering that the reading speed of young adults without presbyopia is faster than that of the older adults with presbyopia, it is possible that the difference in the testing method and characteristics of the languages are the reason for the variance between the results [6, 14, 24].
When the overall satisfaction was evaluated on a five-point scale, satisfaction with distance, intermediate and near vision was 4.42 ± 0.51, 4.11 ± 0.81, and 3.89 ± 0.88, respectively. The results showed that most patients were satisfied. When patients were asked if they would choose the same IOLs again and if they would recommend the IOLs to others, 68.4% of the patients (13 of 19) would choose the same IOLs and recommend it to others.
In this study, ocular dominance was tested prior to cataract surgery, and ZKB00 (add power + 2.75D) was implanted in the dominant eyes and ZLB00 (add power + 3.25D) was implanted in the non-dominant eyes. We assumed that the ‘relatively far’ near focus (ZKB00) in the dominant eye and ‘relatively near’ near focus (ZLB00) in the non-dominant eye would benefit according to the classic monovision trial. However, due to conflicting results with cross monovision results, it may be necessary to conduct additional research to compare the results with cross monovision . Although both eyes of each patient were implanted with different add power, visual acuities of the dominant and non-dominant eyes at each distances were not statistically different. This may be due to the fact that difference in add power between the two IOLs was only 0.5 D. When patients were asked whether they could feel differences between eyes, 17 out of 19 patients did not perceive any difference between both eyes and they did not feel uncomfortable with it. It would be interesting to apply the mix-and-match technique using IOLs with an add power of + 2.75 D and + 4.00 D.
The strength of this prospective contralateral study is the first study applying bilateral mix-and-match implantation of Tecnis multifocal IOLs. Second strength is visual acuities were measured at 6 different distances and that an objective measure of the expected vision at different distances was performed with a defocus curve. Previous studies measured intermediate and near visual acuity only at a single distance. And we comprehensively evaluate clinical outcomes including reading performance, contrast sensitivity and questionnaire. The limitation of this study is the missing direct comparison with bilaterally implanted IOLs with the same add power. However, the results of defocus curve of this study were good and not inferior to those of previous studies.