Background
Cataract is the clouding of the normally clear crystalline lens or loss of transparency, which reduces the amount of incoming light and impairs visual perception, and it is the leading cause of vision impairment and blindness worldwide. Data from the World Health Organization (WHO) showed that cataract accounted for approximately 50% of blindness worldwide [
1]. In China, blindness and low vision affects about 5.8% of Chinese aged 50 and above [
2]. Given the rapid population aging and high prevalence of age-related cataract (ARC) in China, especially in rural areas [
3,
4], it is expected that an increasing number of population will suffer from cataract.
It is generally acknowledged that cataract surgery is the most cost-effective way for restoring vision. Harold Ridley performed the first cataract extraction with implantation of intraocular lens (IOLs) in London in 1949. Nowadays, cataract surgery has been modified into a highly specialized procedure, and the IOLs technique has also been developing constantly [
5]. After the monofocal IOLs implementation, other IOLs such as multifocal IOLs (diffractive, refractive) and accommodating type of IOLs, were successively devised to correct not only cloudy lens but also astigmatism and presbyopia. Nowadays, the most commonly used multifocal IOLs in clinical practice are bifocal and trifocal IOLs. Therefore, this systematic review aims to compare the clinical visual performance of bifocal and trifocal IOLs, thereby providing solid evidence for better clinical practice.
Discussion
Admittedly, the level of evidence of RCT study is the highest according to the categories and recommendations of medical studies in the field of Evidence-based medicine (EBM) [
17]
. This study is the systematic review on RCTs comparing the visual performance of bifocal and trifocal IOLs. As the results showed, we found there was a statistically significant difference in intermediate visual acuity between the two groups. However, no statistical significance was found in other variables in our study.
In this systematic review, two studies [
11,
16] reported the process of blinding. In fact, it is difficult to mask the patients and the surgeons about the type of IOLs implanted in the cataract surgery. Moreover, since no study reported the process of allocation concealment, it is possible that these studies had selection bias. Meanwhile two studies [
10,
12] reported reporting bias. Consequently, the pooled effect of this meta-analysis might be affected by these biases.
As indicated by the sensitivity analyses, the study of Mojzis P et al. was the source of heterogeneity of UNVA and DCNVA. The possible explanation might be that the follow-up interval of this study was 3 months while the interval of other studies was 6 mouths. In this study, no statistically significant difference between the two types of IOLs was found for UNVA and DCNVA. There are two possible reasons: 1. despite the inconsistent outcomes of these RCTs, most of them reported no statistical differencein the near vision improvement; 2. multifocal IOLs designs were initially bifocal and were used to improve the postoperative vision acuity and reduce spectacle dependence at near distance. In fact, with the evolution of multifocal IOLs, both bifocal and trifocal IOLs showed excellent near visual performance [
18,
19].
The intermediate vision greatly affects our daily work, exercise and social life, especially the computer uses in daily office work. Hence, it is also an important factor for the postoperative satisfaction of cataract patients [
20,
21]. Our study revealed a better intermediate vision in the trifocal IOL implantation group, which makes sense given the trifocal IOLs was originally devised to overcome the limitation regarding the postoperative intermediate visual function. Since the outcomes of UIVA and DCIVA might be inconvincible due to the heterogeneity (I
2>79%), a subgroup analysis on different types of the bifocal IOLs and follow-up time was conducted. The heterogeneity of these RCTs was eliminated in the subgroup analysis on bifocal IOLs types, but not follow-up time. In the AT LISA subgroup, UIVA and DCIVA were significantly better in the trifocal IOLs group (AT LISA tri 839 M). Many researches have confirmed better intermediate visual performance of AT LISA tri 839 M trifocal IOLs compared with the AT LISA bifocal IOLS with both clinical study and the optical bench [
22‐
25] that proved the AT LISA tri 839 M can provide a third effective focus. Additionally, no difference was found for the UIVA and DCIVA between the bifocal (Re STOR bifocal IOLs) and the trifocal IOLs (Fine Vision trifocal IOLs) in the other subgroup. Plaza-Puche AB [
26] and Ruiz-Alcocer J [
27] have demonstrated that the AT LISA tri IOLs supports better intermediate visual outcome in comparison with Fine Vision trifocal IOLs in both clinical experiment and optic blench. Furthermore, Plaza-Puche AB corroborated that there was no difference in intermediate vision between the Fine Vision IOLs and the AcrySof ReSTOR bifocal IOLs, which is consistent with our results [
11,
15].
Due to the lack of measurement data, no conclusive results could be drawn for contrast sensitivity. In this systematic review, studies that reported the CS suggested no difference of CS was found between bifocal and trifocal IOLs, which is in accordance with previous researches [
26,
28]. Therefore, the addition of a third focal point does not seem to decrease the postoperative optical quality.
Spectacle independence is commonly used for the evaluation of the satisfaction, and life quality of cataract patients in scales like the National Eye Institute Refractive Error Correction Quality of Life Instrument-42 (NEI-RQL 42) questionnaire [
29]
, Visual Function Index-14 (VF-14) [
30]. In our study, the spectacle independence and postoperative satisfaction did not differ between the two IOLs. The lack of statistically significant difference may be attributed to the fact that both IOLs showed excellence performance of spectacle independence, which is in line with the results of previous studies [
31‐
33]. In addition, the version of questionnaire used for spectacle independent and satisfaction in each single study was different, which may affect the result of the pooled effect.
As for the postoperative refraction, no statistically significant difference was found between the two groups. Many researches have proved that both bifocal IOLs and trifocal IOLs showed great clinical performance in terms of the refractive correction after surgery [
34,
35]. Besides the postoperative refraction, there was also no difference in the PCO incidence between bifocal and trifocal groups.
Admittedly, there were some limitations in this study. First, this meta-analysis was not the first review that compares the visual performance of patients receiving bifocal or trifocal IOLs implementation [
36‐
39]. However, this systematic review was the only one that included only RCTs, providing a higher level of evidence. Second, several studies in this meta-analysis did not report sufficient data on risk assessment, especially random sequence generation, allocation concealment and setting blinding, which may lead to bias. Third, given the number of patients in each study is relatively small, we could not draw explicit conclusion on the difference in postoperative visual performance. Last, the studies included used different types of bifocal and trifocal IOLs, thus the pooled effect might not be accurate enough.
Conclusions
Our systematic review revealed the trifocal IOLs provide a better intermediate VA compared with bifocal IOLs, especially for the AT LISA subgroup, while the near and distant visual performance, spectacle independence, contrast sensitivity, postoperative refraction and surgical satisfaction of bifocal IOLs were similar to those of trifocal IOLs. In order to achieve more significant findings, RCTs with larger sample size should be conducted.