Secondary Trifocal Intraocular Lens Implantation in Dense Cataracts: A Promising Alternative to One-Step Surgery

Abstract

Introduction

To compare the benefits of secondary trifocal intraocular lens (IOL) implantations versus one-step IOL implantations in managing dense cataracts.

Methods

Patients with dense cataracts, a preoperative axis length (AL) of 21.00–27.00 mm, and corneal astigmatism ≤ 1.00 D were enrolled at Wuhan Aier Hankou Eye Hospital. Patients unsuitable for femtosecond laser-assisted cataract surgeries (FLACS) were excluded. Patients who underwent one-step monofocal IOL implantations were involved in group A, and those who accepted two-step trifocal IOL implantations entered group B. Pre- and postoperative AL, corneal keratometry (K) value, visual acuity, and endothelial cell density (ECD) were measured. Postoperative spherical equivalent (SE), numerical error (NE), and mean absolute error (MAE) were calculated.

Results

Applying the inclusion/exclusion criteria, 42 eyes were finally included in group A, and 32 eyes were involved in group B. In group B, six eyes (15.79%) were identified as unsuitable for multifocal IOL (MIOL) implantation because of fundus diseases. Patients in group B achieved a greater proportion of SE and NE closer to 0 D. The postoperative MAE of group B was also obviously lower than that of group A. Uncorrected distant visual acuity (UDVA) of patients in group B was superior to that in group A during the 90-day postoperative follow-up period. Postoperative AL, surgically induced astigmatism (SIA), and ECD loss did not show significant differences between the two groups.

Conclusions

Compared to one-step IOL implantation, the two-step approach allowed more accurate IOL power calculation, reduced refractive error, and better UDVA; did not increase SIA or ECD loss; and allowed for fundus disease screening, making it a good choice in dense cataracts for MIOL implantations.

Trial registration

This trial was registered with the Chinese Clinical Trial Registry (http://www.chictr.org.cn/). Trial registration number: ChiCTR2100043570.

Key Summary Points

Why carry out this study?

Dense cataracts severely impair vision. However, inaccurate ocular biometric measurements and intraocular lens (IOL) calculations, as well as the risk of undetectable fundus lesions due to severe lens opacification, discourage the application of multifocal IOLs (MIOLs).

Our study aimed to compare the effectiveness and safety of secondary MIOL implantation and one-step implantation in managing dense cataracts.

What was learned from the study?

Secondary trifocal IOL implantations reduced refractive error and improved uncorrected distant visual acuity compared with one-step monofocal IOL implantations in managing dense cataracts.

Secondary trifocal IOL implantations did not increase surgically induced astigmatism or endothelial cell density loss compared to one-step monofocal IOL implantations.

Secondary MIOL implantation avoided the occurrence of implantation in patients unsuitable for MIOLs by conducting fundus disease screening after cataract extraction.

Introduction

Cataract is the main global cause of blindness, currently affecting approximately 65 million people worldwide [1]. Despite advancements in medical care, a study projected that, by 2020, cataracts remained the leading cause of blindness and visual impairment among those aged ≥ 50 years, especially in developing countries [2]. Dense cataracts, in particular, lie at the extreme of visual impairment. Although this disease is generally curable, its prevalence remains notable in developing countries because of factors such as limited financial resources, high rates of outdoor activities, strained medical resources, and asymmetries in medical information[3, 4].

To date, cataract surgeries are the most effective and common method to restore vision, with most procedures not only aiming to avoid blindness but also offering refractive treatment [5]. Typically, an intraocular lens (IOL) is implanted immediately after cataract extraction to acquire rapid visual recovery and avoid a second surgery and additional costs. This one-step IOL implantation is also mostly the surgical strategy for trifocal IOL implantation in patients with dense cataracts [6]. However, due to the severe opacification of the lens, undetected but potential retinopathy limits the use of multifocal IOLs (MIOLs). Besides, optical biometry (OB) often cannot penetrate dense cataracts and acquire high-quality axial length (AL) measurements [7]. In these cases, ultrasound (US) or intraoperative aberrometry is used as a substitute, but both methods are less accurate than OB in the accuracy of biometry measurements and less adaptable to the new generation IOL formulas [8, 9]. Calculation errors in IOL powers may result in patient dissatisfaction due to suboptimal visual outcomes, additional surgeries for IOL replacement or refractive correction, and out-of-pocket costs after trifocal IOL implantations [10]. Thus, managing patients with dense cataracts who desire a full range of vision presents challenges in MIOL implantation.

Possible retinopathy and measurement errors may limit the advantages of MIOLs. In such cases, the two-step approach of first removing the cataract and then implanting the IOL in a subsequent surgery can be considered an alternative to address these challenges. Ling et al. reported five cases of white cataracts which accepted optical measurements after cataract extraction followed by secondary bifocal IOL implantations within 2 weeks. Satisfactory postoperative visual acuities were observed, offering a novel surgery option for dense cataracts [11]. However, whether secondary IOL implantations offer more advantages over one-step implantations in managing dense cataracts still needs further investigation.

To evaluate the effectiveness and safety of secondary MIOL implantations in patients with dense cataracts compared to one-step IOL implantations, we compared secondary trifocal IOL implantations with one-step monofocal IOL implantations, aiming to provide a clinical alternative for managing dense cataracts. Our study found that secondary trifocal IOL implantations not only allowed for retinopathy screening, preventing inappropriate MIOL implantation, but also reduced refractive errors and achieved better postoperative uncorrected distant visual acuity (UDVA). These benefits were accomplished without increasing surgically induced astigmatism (SIA) or endothelial cell density (ECD) loss, rendering secondary trifocal IOL implantation as a viable option for patients with dense cataracts pursuing MIOLs.

Methods

Ethics and Informed Consents

This prospective nonrandomized controlled trial was conducted at Wuhan Aier Hankou Eye Hospital, Hubei Province, People's Republic of China, from March 2021 to March 2024. The trial was registered at Chinese Clinical Trial Registry (http://www.chictr.org.cn/) with the registration number ChiCTR2100043570 (registration on 21/02/2021). This study was approved by the Institutional Review Board of Wuhan Aier Hankou Eye Hospital (approval number: HKAIER2020IRB-007-01) and adhered to the Declaration of Helsinki. Written informed consent was obtained from all participants. This trial followed CONSORT 2010 guidelines.

Study Design

Patients with dense cataracts were divided into a one-step monofocal IOL group (group A) and a secondary trifocal IOL implantation group (group B), depending on their willingness to undergo trifocal IOL implantation and two-step surgery. Patients in group A underwent immediate monofocal IOL (AcrySof SN60WF, Alcon, TX, USA) implantation post-phacoemulsification, while group B underwent two surgeries. In group B, phacoemulsification was performed in the first stage, followed by the implantation of a foldable trifocal IOL (TNFT00, Alcon) into the capsular bag through the initial corneal incision in the second stage 1 to 4 days later when the cornea recovered from edema detected by OCT. During the aphakic state, patients undergo close outpatient monitoring to ensure the timely detection and management of any potential issues. Follow-up was conducted at 7, 30, and 90 days after IOL implantations.

Inclusion and Exclusion Criteria

This study enrolled patients over 18 years old with dense cataracts who failed to measure AL or could only obtain low-quality AL measurements by swept-source optical coherence tomography (SS-OCT) (OA2000, Tomey, Nagoya, Japan) and had obscured fundus visualization [12]. The included patients had a preoperative AL of 21.00–27.00 mm measured by US and corneal astigmatism ≤ 1.00 D [13, 14]. Exclusion criteria were unsuitability for femtosecond laser-assisted cataract surgery (FLACS), ECD < 1500.00 cells/mm², postoperative best-corrected distance visual acuity (BCDVA) worse than 0.30/LogMAR, patients with intraoperative or postoperative complications, and additional complexities like glaucoma, retinal detachment, zonulysis, strabismus, and so on. Besides, patients diagnosed with severe diabetic maculopathy or myopic maculopathy after cataract extraction unsuitable for MIOL implantations were also excluded [15].

Patient Examinations

Preoperatively, all eyes underwent examinations including slit-lamp microscopy, UDVA, BCDVA, intraocular pressure (IOP), US, macular OCT, ultrasonography biomicroscopy (UBM), fundus examination after pupil dilation, ECD, and corneal topography. Cataract density was graded preoperatively with intraoperative adjustment using Lens Opacities Classification System III (LOCS III) [16]. Although SS-OCT is more accurate than US in the measurement of AL, it often fails in dense cataracts [8]. Therefore, A-scan US (SW-2100, Souer, Tianjin, China) was used preoperatively to measure AL. Keratometry (K) was obtained by OA-2000 for both groups. After cataract removal, AL and K were re-measured by OA-2000 for group B once the cornea had recovered from edema. Followed up at 7, 30, and 90 days after IOL implantations, eyes were examined for slit-lamp microscope, IOP, macular OCT, ECD, the stability and position of IOLs, posterior capsular opacity, UDVA, BCDVA, and manual manifest refraction.

Surgical Technique

All surgeries were performed by an experienced surgeon (X.R.). Every patient underwent the standard FLACS. Briefly, a 5.20-mm pupil-centered round capsulotomy and nuclear fragmentation using a sextant pattern were created by LenSx software (version 2.23, Alcon LenSx), followed by the creation of a 2.20-mm temporal clear cornea main incision and a 0.80-mm side-port corneal incision. Phacoemulsification was then carried out using the in situ chop technique with the Centurion Vision phacoemulsification system (Centurion, Alcon). Subsequently, patients in group A underwent immediate monofocal IOL implantation. In contrast, patients in group B were secondarily implanted with trifocal IOLs into the capsular bags through the initial surgical incisions 1–4 days later. Postoperatively, both groups received 1.00% prednisolone acetate eye drops four times daily for 1 week, tapering weekly until stopped, and 0.50% levofloxacin eye drops four times daily for 2 weeks.

Intraocular Lens Power Calculation

The IOL power calculation of group A used preoperative AL measured by A-scan US and K measured by OA-2000. Group B used AL and K, both obtained by OA-2000 after cataract extraction. The SRK/T formula was used to calculate the IOL power for both groups with the A constant obtained from User Group for Laser Interference Biometry (ULIB) website (http://ocusoft.de/ulib/) [17]. The target diopter was set to 0 D. For group B, eyes with a large difference (> 1.00 D) in the IOL power prediction before and after cataract extraction, the aphakic refraction procedure reported by Mackool et al. was applied to adjust the IOL power [18].

Refractory Error Calculation

To assess refractive errors, numerical error (NE) was calculated. The actual refractive states of the patients postoperatively were converted into a spherical equivalent (SE). NE was the difference between the preoperative theoretical reserved refractive power and SE. A negative NE represents a myopia drift, and a positive NE indicates a hyperopia shift. The mean numerical error (MNE), mean absolute error (MAE), median absolute error (MedAE), maximum absolute error (MaxAE), and interquartile range (IQR) were also calculated. The MAE was defined as the mean absolute value of NE, and MedAE was defined as the median absolute value of NE. The percentages of operated eyes within ± 0.25 D, ± 0.50 D, ± 0.75D, ± 1.00 D, and ± 2.00 D of postoperative NE were counted.

Statistical Analysis

Statistical analysis was performed using SPSS (version 22.0). Continuous variables were reported as mean ± standard deviation. Data normality was checked by normal probability plots and Kolmogorov-Smirnov test. Age and composition ratios were compared using Pearson χ² or Fisher’s exact test. Student's t-test was conducted for normally distributed and Mann–Whitney U test for skewed data. Differences were considered statistically significant when the P value was < 0.05.

Results

Patient Demographics

Applying the inclusion/exclusion criteria, 38 patients (42 eyes) were included in group A and 32 patients (32 eyes) in group B after excluding six patients (15.79%) with amblyopia or fundus lesions including macular hole, vitreomacular traction, disruption of the foveal ellipsoid zone, and epiretinal membrane after cataract extraction (Fig. 1). The mean age was 64.38 ± 8.87 years old in group A, older than the 54.31 ± 15.27 years old in group B (P < 0.01, Table 1). Younger patients appeared to be more willing to accept the two-step surgery. Preoperative parameters of both groups are compared in Table 1. The preoperative parameters including gender ratio, AL, average K, ACD, LT, cataract density, UDVA, and BCDVA were comparable between groups. All surgeries were uneventful, with no intraoperative or intraocular complications such as infectious endophthalmitis, iris prolapse, and uveitis within 90 days of follow-up. Representative anterior segment photos pre- and post-surgery for one patient in group A (Fig. 2A, B) and group B (Fig. 2C, D, E) are shown in Fig. 2.

Fig. 1.

Patient flow diagram. Patients were conducted trials according to the above flow chart

Table 1.

Comparison of demographics and preoperative parameters between group A and group B

Preoperative parameters	Group A (n = 42)	Group B (n = 32)	P
Mean age (years) ± SD	64.38 ± 8.87	54.31 ± 15.27	< 0.01
Male, n (%)	26 (61.90%)	21 (65.63%)	0.74
Female, n (%)	16 (38.10%)	11 (34.38%)
Density of cataract
Nuclear opacity ≤ 4, n (%)	24 (57.14%)	18 (56.25%)	0.94
Nuclear opacity > 4, n (%)	18 (42.86%)	14 (43.75%)
AL (mm)	23.25 ± 0.88	23.70 ± 1.19	0.08
Average K (D)	43.84 ± 1.73	43.38 ± 1.58	0.24
Anterior chamber depth (mm)	2.76 ± 0.50	2.96 ± 0.56	0.12
Lens thickness (mm)	5.00 ± 0.94	4.54 ± 1.28	0.14
Preoperative BCDVA (LogMAR)	2.30 ± 0.34	2.14 ± 0.52	0.13
Preoperative UDVA (LogMAR)	2.29 ± 0.37	2.13 ± 0.55	0.13

SD standard deviation, AL axial length, K keratometry, D diopters, BCDVA best-corrected distance visual acuity, UDVA uncorrected distance visual acuity

Fig. 2.

Anterior segment photos of group A and group B before and after the surgery. A The left eye of one patient in group A before the cataract surgery. B The same eye was in group A 7 days after one-step mono-intraocular lens (IOL) implantation. C The left eye of one patient in group B before the cataract surgery. D The same eye was in group B 3 days after cataract extraction. E The same eye was in group B 7 days after trifocal IOL implantation

Axial Length Biometry

For IOL power calculation, AL was measured. Preoperative AL was measured by A-scan US, which was 23.25 ± 0.88 mm in group A and 23.70 ± 1.19 mm in group B (P = 0.08, Table 2). The AL after cataract extraction was measured by OA-2000, and it was 23.65 ± 1.16 mm in group B. At the 90-day follow-up, it was 23.19 ± 0.85 mm in group A and 23.66 ± 1.14 mm in group B (P = 0.07, Table 2). The absolute difference between 90 days after IOL implantation and before in Group B was 0.07 ± 0.10 mm, half of the absolute difference between 90 days postoperative and preoperative in group A (P < 0.01, Table 2), suggesting group B’s AL for IOL power calculation might be more optimal.

Table 2.

Comparison of axial length and corneal astigmatism between group A and group B

Parameters	Group A	Group B	P
AL (mm)
Preoperative (ALpre)	23.25 ± 0.88	23.70 ± 1.19	0.06
1–4 days after cataract extraction (ALex)		23.65 ± 1.16
90 days after IOL implantation (AL90d)	23.19 ± 0.85	23.66 ± 1.14	0.07
AL90d–ALpre	− 0.06 ± 0.17	0.05 ± 0.26	0.88
(AL90d–ALpre in group A) VS (AL90d–ALex in group B)	− 0.06 ± 0.17	− 0.02 ± 0.12	0.13
|AL90d–ALpre in group A| VS |AL90d–ALex in group B|	0.14 ± 0.11	0.07 ± 0.10	< 0.01
Corneal astigmatism (D)
Preoperative	− 0.62 ± 0.28	− 0.54 ± 0.25	0.19
After cataract extraction		− 0.65 ± 0.33
90 days after IOL implantation	− 0.75 ± 0.48	− 0.56 ± 0.29	< 0.05
The difference between 90 days  postoperative and preoperative	− 0.12 ± 0.48	− 0.02 ± 0.24	0.28

AL axial length, IOL intraocular lens

Corneal Astigmatism

The preoperative corneal astigmatism measured by OA-2000 was − 0.62 ± 0.28 D in group A and − 0.54 ± 0.25 D in group B (P = 0.19, Table 2). At the 90-day follow-up, astigmatism increased slightly to − 0.75 ± 0.48 D in group A and − 0.56 ± 0.29 D in group B (P < 0.05, Table 2). Astigmatism within 0.50 D in group A and group B at 90 days follow-up was 40.48% and 43.75%, respectively. Alpins vector analysis showed no significant astigmatism changes between pre- and post-surgery in either group (all P > 0.50, Fig. 3), suggesting the stability of corneal astigmatism. Secondary IOL implantation did not increase corneal astigmatism, which created favorable conditions for the two-step method.

Fig. 3.

The astigmatism vector analysis before and 90 days after surgery. A The preoperative astigmatism of group A. B The astigmatism of group A 90 days after intraocular lens (IOL) implantation. C The surgically induced astigmatism in group A. D The difference vector in group A. E The preoperative astigmatism of group B. F The astigmatism of group B 90 days after IOL implantation. G The surgically induced astigmatism in group B. H The difference vector in group B

Refractive Error

SE reflects the postoperative refractive status. At 90-day follow-up, group A had 33.33%, 19.05%, 26.19%, 16.67%, and 7.14% of eyes with SE within ≤  ± 0.25 D, ± 0.25–0.50 D, ± 0.50–0.75 D, ± 0.75–1.00 D, and ± 1.00–2.00 D. In group B, it was 68.75%, 28.13%, 3.13%, 0.00%, and 0.00%, respectively (Fig. 4A). SE in group B was more concentrated over the range of emmetropia. Group A had a SE of 0.08 ± 0.66 D with hyperopia at 90 days after surgery, while group B had a SE of − 0.11 ± 0.27 D with myopia (P = 0.08).

Fig. 4.

The refractive errors of group A and group B. A The percentages of eyes having spherical equivalent (SE) within the range of ≤  ± 0.25 D, ± 0.25–0.50 D, ± 0.50–0.75 D, ± 0.75–1.00 D, and ± 1.00–2.00 D. B The cumulative percentage of eyes in numeric error at 7-day follow-up. C The cumulative percentage of eyes in numeric error at 30-day follow-up. D The cumulative percentage of eyes in numeric error at 90-day follow-up. **P < 0.01

NE assesses the accuracy of IOL degree calculation. The closer the value is to zero, the more accurate the calculation [19]. At 90-day follow-up, the MNE of group A was 0.11 ± 0.67 D, presenting with a hyperopia drift, while group B’s was − 0.16 ± 0.27 D, presenting with a myopia drift (P < 0.01, Table 3). Moreover, the cumulative percentages of NE at the 7-day follow-up within the range of ± 0.25 D, ± 0.50 D, ± 0.75D, ± 1.00 D, and ± 2.00 D were 28.57%, 50.00%, 76.19%, 83.33%, and 100.00% in group A, while group B was accrued to 100.00% earlier, with 67.65%, 88.24%, 97.06%, 100.00%, and 100.00% in group B, respectively (Fig. 4B). The NEs at 7 days and 30 days after surgery were similar to that at 90 days (Fig. 4C, D). MAE and MedAE, which could not be affected by outliers reflecting the general level of total NE, showed the MAE of group B was about half of that of group A (all P < 0.05, Table 3) at all three visits. MAE was 0.55 ± 0.39 D in group A and 0.21 ± 0.23 D in group B at 90-day follow-up. These results supported that secondary IOL implantation allowed smaller NE and MAE, enabling a more accurate calculation.

Table 3.

Comparison of numerical error between group A and group B

Parameters	Group A	Group B	P
NE at 7-day follow-up (D)
MNE ± SD	0.09 ± 0.67	− 0.11 ± 0.40	0.10
MedAE, IQR	0.48, 0.61	0.23, 0.32	0.01
MAE ± SD	0.54 ± 0.39	0.31 ± 0.28	0.01
MaxAE	1.44	1.20
NE at 30-day follow-up (D)
MNE ± SD	0.10 ± 0.65	− 0.15 ± 0.26	< 0.01
MedAE, IQR	0.52, 0.54	0.11, 0.24	< 0.01
MAE ± SD	0.53 ± 0.38	0.20 ± 0.22	< 0.01
MaxAE	1.44	0.75
NE at 90-day follow-up (D)
MNE ± SD	0.11 ± 0.67	− 0.16 ± 0.27	< 0.01
MedAE, IQR	0.54, 0.64	0.12, 0.26	< 0.01
MAE ± SD	0.55 ± 0.39	0.21 ± 0.23	< 0.01
MaxAE	1.44	0.95

NE numerical error, MNE mean numerical error, SD standard deviation, MedAE median absolute error, IQR interquartile range, MAE mean absolute error, MaxAE maximum absolute error

Visual Acuity Outcomes

To evaluate the visual acuity outcomes, UDVA and BCDVA were measured after IOL implantation. Visual acuity was tremendously improved after IOL implantation among all the patients. Both groups achieved quite good BCDVA; it was 0.01 ± 0.04/LogMAR in group A and 0.02 ± 0.05/LogMAR in group B 90 days after surgeries (P = 0.46, Table 4). However, the UDVA of group B was superior to that of group A during the three postoperative follow-up visits. At the 90-day follow-up, the UDVA was 0.04 ± 0.05/LogMAR in group B, much better than 0.15 ± 0.12/LogMAR in group A (P < 0.01, Table 4). Group B achieved better naked visual function and comparable BCDVA than group A, supporting that secondary IOL implantation was beneficial for vision.

Table 4.

Comparison of monocular visual acuity and endothelial cell density between group A and group B

Parameters	Group A	Group B	P
UDVA (LogMAR)
At 7-day follow-up	0.17 ± 0.13	0.07 ± 0.06	< 0.01
At 30-day follow-up	0.15 ± 0.12	0.04 ± 0.05	< 0.01
At 90-day follow-up	0.15 ± 0.12	0.04 ± 0.05	< 0.01
BCDVA (LogMAR)
At 7-day follow-up	0.02 ± 0.05	0.04 ± 0.06	0.06
At 30-day follow-up	0.01 ± 0.04	0.02 ± 0.05	0.53
At 90-day follow-up	0.01 ± 0.04	0.02 ± 0.05	0.46
ECD
Preoperative (cells/mm2)	2632.35 ± 356.15	2670.89 ± 284.35	0.62
At 90-day follow-up (cells/mm2)	2286.65 ± 391.33	2347.37 ± 342.45	0.49
ECD loss at 90-day follow-up (%)	13.16 ± 8.20	11.93 ± 10.25	0.57

UDVA uncorrected distant visual acuity, BCDVA best-corrected distant visual acuity, ECD endothelial cell density

Corneal Endothelial Cell Loss

Cataract surgery may decompensate the corneal endothelium [16]. To compare the effectiveness between the one- and two-step methods on the corneal endothelium, preoperative and postoperative ECD were compared. At 90-day follow-up, groups A and B showed no significant ECD loss difference (P = 0.42, Table 4). The percentage of loss in ECD was 13.16 ± 8.22% in group A and 11.93 ± 10.25% in group B, which demonstrated secondary IOL implantation did not elevate the risk of corneal endothelial cell loss.

Discussion

With the rapid development of the social economy and the aging of the population, life expectancy has been extended, and many countries have begun to pay more attention to the quality of life [20]. As a rapidly developing country, People’s Republic of China is no exception. Many people in China, after suffering from dense cataracts, still expect to have MIOLs implanted to achieve better visual outcomes. Trifocal IOLs, designed to provide near, intermediate, and distance focal distances simultaneously, provide satisfactory clinical outcomes and spectacle independence consistent with the changing lifestyle patterns [1, 21]. Residual refractive errors caused by inaccurate measurement and IOL calculation errors might reduce visual quality and increase the rate of dysphotopsias [22]. Dense cataracts, such as mature white cataracts, posterior subcapsular cataracts, and advanced nuclear cataracts, present a unique challenge in accurate ocular biometric measurements and IOL calculation because of the obscure visual axis and impede the fundus visualization, discouraging the application of trifocal IOLs. Our study evaluated the safety and effectiveness of secondary trifocal IOL implantation in patients with dense cataracts who want to have a full range of vision postoperatively. Compared to one-step implantations, we found that secondary trifocal IOL implantations achieved a greater proportion of NE and SE close to 0 D, smaller MAE, and better UDVA. SIA and the percentage of ECD loss did not show significant differences between the two surgical methods, supporting the same safety in secondary trifocal IOL implantation as one-step implantation. Moreover, through the two-step approach, 15.79% of patients with fundus issues unsuitable for MIOL implantation were screened out.

In 2018, Ling et al. reported good visual outcomes of secondary bifocal IOL implantation in five white cataract cases, demonstrating that secondary IOL implantation was a feasible surgical procedure for this purpose [11]. However, whether the refractive results of the two-step method are better than the one-step method remains unclear. When determining the timing for a secondary MIOL implantation, a previous study pointed out that corneal swelling stabilizes 2 weeks after cataract surgery, and automated refraction stabilizes after 1 week [23]. We acknowledge the importance of refractive stability and the potential benefits of waiting 1–3 months after the first surgery, especially in cases with dense cataracts. However, we chose not to delay the secondary surgery excessively because of the risks associated with the aphakic state and the additional burden of care. Instead, we used OCT to measure the central corneal thickness of patients and found that it returned to the preoperative level within 1–4 days after cataract removal in our included patients. At this point, we re-measured AL and K and proceeded with the MIOL secondary implantation.

To compare the refractive outcomes and safety, we conducted one-step monofocal IOL implantation and two-step trifocal IOL implantation. Regarding surgical safety of surgery, no intraocular complications such as infectious endophthalmitis, iris prolapse, or uveitis were observed within 90 days of follow-up. Besides, SIA and the loss of ECD often caused by cataract surgery were evaluated. According to our study, no significant difference in SIA between the two groups was observed, indicating that secondary surgeries did not increase corneal astigmatism. It has been reported that FLACS resulted in a 7.52–17.06% loss of corneal endothelial cells [16, 24]. In our study, factors that can affect endothelial density, including gender [25], ACD, LT [26], and nuclear stiffness [27], were similar preoperatively in both groups. The percentage of ECD loss was 13.16 ± 8.22% in group A and 11.47 ± 10.11% in group B, consistent with reported studies. Moreover, secondary surgery did not increase the loss rate. SIA and the percentage of ECD loss were not raised by secondary IOL implantation, verifying the safety of the two-step method.

Failure to achieve postoperative emmetropia could decrease the optical performance of MIOL [28]. NE was reported to be primarily derived from the accuracy of ocular biometry, prediction of effective lens position (ELP), surgical approach, and technique of cataract surgery [29]. Postoperative NE at ± 0.50 D was defined as emmetropic status after cataract surgery, and National Health Service, UK, recommends that the percentage of NE within ± 0.50 D should be at least 55.00% [30]. In the evidence-based guidelines based on the EUREQUO database in 2020, at least 90.00% of all cases within an absolute error of ± 1.00 D was recommended [31]. Similarly, in our study, 50.00% of eyes in group A and 88.24% of eyes in group B had ± 0.50 D, and 81.81% of eyes in group A and 100.00% of eyes in group B had ± 1.00 D at 90-day follow-up. Compared with group A, group B performed better in SE, NE, MAE, and postoperative UDVA. Group B achieved more accurate calculation of IOL and better visual function.

It has been proven that the accuracy of OB is higher than that of US measurement [32], but OB fails in measuring the AL of eyes with dense cataracts. There was a bias in AL as measured by the two methods. Therefore, the AL measured by US cannot be directly used in the new generation formulas, including the Barrett Universal II, the Kane, the Olsen, and so on, to improve the accuracy of refraction prediction, which may also lead to a poor prediction of ELP [33]. In our study, group A used the AL measured by US, and group B used AL measured by OA-2000 biometry to calculate the IOL power. The AL used in Group B's calculation may be more accurate when using OB. On the other hand, compared to group A, for group B, the IOL power can also be calculate using three different kinds of methods to help elevate the accuracy of ELP [34], including US’s built-in IOL calculator using the SRK/T formula before cataract extraction, a modified aphakic refractive vergence formula by Dr. Mackool, and OA2000’s built-in IOL calculator using SRK/T formula after cataract extraction. IOL constants from the ULIB website were directly used, and they proved to be accurate based on the postoperative NE values.

The traditional one-step implantation of monofocal IOLs is cost-effective and time-efficient but cannot cover all the visual requirements. A supplemental trifocal IOL implantation can create trifocal vision. Previous studies by Rocha-de-Lossada et al. demonstrated the efficacy of supplemental MIOLs in correcting presbyopia and residual refractive errors [35, 36]. However, the high incidences of supplementary IOL rotation and pigment deposits limit its application [35, 37, 38]. Excimer laser enhancement after MIOL implantation also provides an efficient method to solve residual ametropia, but it faces the problems of high cost and longer treatment time [39]. In addition, laser surgery can also aggravate surgery-induced dry eyes [40]. In our study, all secondary MIOL implantations were effective and safe, and the two surgeries were completed within 1 week. Moreover, after the removal of dense cataracts, not only can the optical measurement be performed, but the visual function can also be evaluated. It is reported that 10.97–26.68% of patients undergoing cataract surgery have maculopathy [41–43]. A screening OCT for cataract surgery evaluation with an MIOL implantation can be cost-effective and potentially cost-saving from a societal perspective [44]. In group B, 15.79% of patients were screened out for amblyopia or fundus lesions, avoiding the waste of MIOLs and the risk associated with MIOL implantation. The patients in group B were younger, had a greater need for spectacle independence, and were also willing to accept a longer treatment duration and higher costs. All in all, two-step MIOL implantations offer better postoperative UDVA, reduce refractive errors, and identify fundus diseases, providing a feasible option for patients with dense cataracts who have a strong desire for spectacle independence.

Concerning limitations, the relatively small sample size restricts the generalizability of our findings and calls for further validation through larger-scale studies. Besides, the aphakic state during the interval between the two surgeries poses potential risks. To reduce the risk, we performed secondary MIOL implantation as soon as the corneal edema subsided. At that time, the corneal refractive state may not be in the final stable state, which may affect the optimal refractive outcome. Moreover, constant lens optimization can be performed, and the prediction accuracy of the two-step method in patients with long AL, short AL, and high astigmatism needs to be studied further.

Conclusion

The safety, accuracy, and ability to screen for fundus diseases of secondary trifocal IOL implantations make it a good choice for dense cataract surgery although it involves higher treatment costs and longer treatment times. Considering patients’ risks, preferences and resource availability, secondary MIOL implantations can be a safe and effective approach for the surgeon when dealing with dense cataracts.

Author Contributions

Jingyu Qu: Data analysis and interpretation, and manuscript writing and revision; Wei Xiao: Collection and assembly of data, data analysis, and data interpretation; Yue Wang: Collection and assembly of data, data analysis, and data interpretation; Ya Jiao: Collection and assembly of data, data analysis, and data interpretation; Shiqi Dong: Collection and assembly of data, data analysis, and data interpretation; Rong Xu: Conception and design, data analysis and interpretation, and manuscript writing and revision. All authors read and approved the final manuscript.

Funding

This study was supported by National Natural Science Foundation of China (82401216), Youth Project of Natural Science Foundation of Shandong Province (ZR2023QH340), Qingdao Natural Science Foundation Youth Project (23-2-1-145-zyyd-jch), and Qingdao Eye Hospital Foundation for Incoming Doctoral Students (W202210210004). The journal’s Rapid Service Fee was funded by the authors.

Data Availability

Data generated during and analyzed during the current study are available from the corresponding author on reasonable request.

Declarations

Conflict of Interest

Rong Xu was initially affiliated with Wuhan Aier Hankou Eye Hospital during the early stage of the study. She is currently affiliated with Wuhan Fourth Hospital. The study received no funding or financial support from Wuhan Aier Hankou Eye Hospital. There are no conflicts of interest related to these affiliations. Jingyu Qu, Wei Xiao, Yue Wang, Ya Jiao, Shiqi Dong, and Rong Xu declare that they have no conflict of interest.

Ethics Approval

This prospective nonrandomized controlled trial was conducted at Wuhan Aier Hankou Eye Hospital, Hubei Province, People’s Republic of China, from March 2021 to March 2024. The trial was registered at Chinese Clinical Trial Registry (http://www.chictr.org.cn/) with registration number ChiCTR2100043570 (registration on 21/02/2021). This study was approved by the Institutional Review Board of Wuhan Aier Hankou Eye Hospital (approval number: HKAIER2020IRB-007-01) and adhered to the Declaration of Helsinki. Written informed consent was obtained from all participants. This trial followed CONSORT 2010 guidelines.