Over the 20-year period, 39,965 of the 4,848,125 pseudophakic patients (0.82%) were diagnosed with IOL dislocation requiring surgical intervention (Fig. 1). The overall incidence of IOL dislocation with surgery was 28,900 per 1,000,000 person-years. The baseline demographic characteristics of the patients are presented in Table 1. The mean age of pseudophakic patients without IOL dislocation was 60.6 ± 13.2 years, while the mean age of patients with IOL dislocation requiring surgical intervention was 68.1 ± 10.7 years. The number of IOL dislocations with surgery in male patients was higher than in female patients during the 20-year period, with 25,297 cases in male and 14,668 cases in female patients (63.3% vs.36.7%, p < 0.001) (Table 1).

The overall 20-year incidence rate of IOL dislocation among pseudophakic patients was 28,900 per 1,000,000 person-years (95% CI, 28,431-29,369) (Table 2). The age-specific incidence was highest in the under 40 years age group at 107,000 per 1,000,000 person-years and gradually decreased as age increased, reaching the lowest at 5,410 per 1,000,000 person-years for those aged 80 years and above (Tables 2, 3). However, the age group with the highest incidence of IOL dislocation was those aged 60-69 years, accounting for 10,662 patients (26.7%) (Table 1).

Between 2002 and 2021, pseudophakic patients experienced a significant increase in the dislocation of intraocular artificial lens 10 years after cataract surgery, and this trend appeared consistently regardless of sex and age (Figs. 2, 3 and Tables 3, 4). The cumulative incidence of IOL dislocation within 1 year after cataract surgery was 3,440 per 1,000,000 person-years. However, it gradually increased, reaching 6,410 per 1,000,000 person-years with- in 5 years, and showed a sharp rise to 10,700 per 1,000,000 person-years after 10 years, marking a significant 3.11-fold increase (Table 4). The cumulative incidence rate over 20 years was always higher in male patients than in female patients (Table 4 and Fig. 3).

In our study, 0.82% of the patients who underwent cataract surgery were diagnosed with IOL dislocation and then underwent surgical treatment in South Korea between 2002 and 2021. The overall incidence rate was 28,900 cases per 1,000,000 person-years, which was notably higher in male than in female patients (male to female ratio, 5.35). The highest incidence was seen in those under 40 years, despite the average age of occurrence being 68.1 ± 10.7 years. IOL dislocation occurred with an average duration of 4.1 ± 4.7 years, with a rapid increase of 3.11-fold after 10 years of cataract surgery in all age groups.

Compared with our study, Pueringer et al. [12] showed a lower incidence rate of 16 of 14,471 patients (0.1%) over a 30-year period in the United States; additionally, 111 of 46,632 patients (0.16%) similarly underwent IOL dislocation in Austria [13]. However, Monestam [22] reported that the 10-year cumulative incidence of IOL dislocation was 1% in Northern Sweden, which is higher than our results in South Korea. The different estimated results between the two countries might be due to the higher prevalence of pseudoexfoliation in the Scandinavian population, ranging between 11% and 42%, which is known as a significant risk factor for late IOL dislocation [23]. Further studies are necessary to evaluate the association between the incidence rate of IOL dislocation and other risk factors, such as the rate of ocular trauma and pseudoexfoliation, in our country. Recently, Bothun et al. [24] reported 80 cases of IOL dislocation among 148,201 patients. However, this study did not suggest whether all patients in the cohort were pseudophakic.

Moreover, changes in the cataract surgical techniques may influence the incidence of IOL dislocation. Some previous studies included phacoemulsification and conventional ECCE [12-14,22]. Therefore, our results were different because we evaluated the incidence of IOL dislocation only after phacoemulsification. Clark et al. [25] reported that the prevalence of IOL dislocation was only 0.17% between 1985 and 1989, which is much lower than our results. This could be due to the shorter study duration and a larger proportion of patients undergoing cataract surgery with ECCE (36.8%), which is uncommon today compared with our study. Phacoemulsification and ECCE might have different zonular stresses. Therefore, the current surgical technique can result in different outcomes of IOL dislocation. Therefore, comparing the incidence rates of cohort groups prior to 2000 was not appropriate.

Lee et al. [9] reported that the cumulative probability of IOL dislocation was 2.73% per person in our country, which is higher compared with our result (0.82%). The incidence rate of IOL dislocation was higher than that in our study because a wide spectrum of cases of IOL dislocation, such as very early dislocation and insignificant IOL subluxation not requiring surgical intervention, were included. Despite the lower number of cases, the incidence rate in the current study was higher indicating 28,900 per 1,000,000 person-years for 20 years. These discrepancies possibly arise from differences in patient demographics and observation periods. First, our study spanned nearly three times longer compared with that study (20 years vs. 8 years). Given that IOL dislocation is a recognized long-term complication of cataract surgery, the extended duration of our study likely contributed to a higher cumulative incidence rate observed over time. Specifically, in our study, we found cumulative rates of 6,410 per 1,000,000 person-years over 5 years and 10,700 per 1,000,000 person-years over 10 years (Table 4), which is comparable to the results in the previous study (7,671 per 1,000,000 person-years for 8 years), indicating a substantial increase with prolonged follow-up. Second, Lee et al. [9] excluded individuals under 40 years of age from their study population, whereas we included this age group. This inclusion is significant as the incidence rate among those under 40 was the highest in the current study (107,000 per 1,000,000 person-years), explaining the higher cumulative incidence rate observed in our comprehensive analysis.

In our study, the age-specific incidence rate of IOL dislocation among the pseudophakic population was the highest in those under 40 years of age in both male and female patients (130,100 and 47,600 per 10,000,000 person-years, respectively), which was lower than that in another Korean population-based study reporting an age range of 50-54 years in male patients and 40-44 years in female patients [9]. However, Lee et al. [9] reported that the incidence rate of IOL dislocation for the 8 years of the study in the pseudophakic population aged 40 years or older was 7,671 per 1,000,000 person-years (95% CI, 7,616-7,727). This disparity in results of our study resulted from the exclusion of nonsurgery-requiring cases, leading to a lower number of IOL dislocation cases compared with the previous study. Pershing et al. [23] reported that the rate of procedure-related revisits after cataract surgery was highest in those in their 20s and 30s and lowest in those in their 70s. They also explained that younger patients are more likely to have other ophthalmologic causes leading to cataracts at an early age, such as ocular trauma or other ocular surgeries, leading to more complicated surgeries with more chances of IOL dislocation.

With an increase in the aging population and advances in surgical devices, the incidence of cataract surgery has increased worldwide, including South Korea [7,26-28]. As the number of cataract surgeries increases with age worldwide, the possibility of postoperative complications, including IOL dislocation, may increase. Previous studies have shown an increased incidence of IOL dislocation due to increased cataract surgery [12,14]. However, our results showed that the rate of IOL dislocation peaked at <40 years of age with male dominance among pseudophakic patients. Hence, the increasing number of cataract surgeries does not simply influence IOL dislocation. Another study reported that the increasing incidence of this complication cannot be explained by the growing pseudophakic population only [14], but by a longer duration of pseudophakia in Sweden, which was also found in the present study.

In addition, male patients had a higher incidence of IOL dislocation than the female patients in all age groups among the pseudophakic population (Table 2); in contrast, cataract surgery was more common in female than male patients (Table 1) [14]. This surprising finding is similar to that of other previous studies showing that male sex is a predisposing factor for readmission after cataract surgery [17,22,23]. As shown in previous studies, there were multiple predisposing factors of IOL dislocation including aging, high myopia, uveitis, trauma, previous vitreoretinal surgery although 90% of cases show zonular weakness [29-32]. Other complications, including postoperative endophthalmitis and retinal detachment after cataract surgery, occurred more frequently in men, as shown in our previous studies [33,34], and these results could be associated the cause of our male dominant result. Another explanation may be that male patients have more tendency to ocular trauma that might have happened a long time ago and was forgotten at the time of cataract surgery so that they might have a history of underreported trauma [22,35]. In another study, some investigators suggested that there may be a sex-related difference that results in weaker zonule in men with pseudoexfoliation (PEX) leading to more prevalent IOL dislocation in spite of higher incidence of PEX in female patients [29]. Further studies on the biological differences such as the different extent of zonular weakness between men and women after cataract surgery need to be more precisely conducted.

Although the mean timing of artificial IOL dislocation requiring surgical treatment is approximately 4.1 ± 4.7 years, with a rapid increase of 3.11-fold from 1 year to 10 years after cataract surgery in all patients (Fig. 3 and Table 4). The fact that a dramatic increase in this unwanted complication occurred 10 years after cataract surgery is of particular concern. Although a few previous studies investigated a study duration of more than 10 years, different cataract surgery techniques were used, including ECCE, ICCE, and phacoemulsification, which may have different impacts on the incidence of IOL dislocation. However, our findings emphasize the importance of long-term follow-up care after cataract surgery, even in cases where the initial postoperative results seem satisfactory. Additionally, time-dependent increase of IOL dislocation after cataract surgery can be explained based on biological changes after surgery. Zonules become more friable with aging, particularly in eyes with PEX, which have more severe epithelial atrophy compared to non-PEX eyes of patients of the same age [32]. Another biological change after cataract surgery is contraction of capsular bag which happens as early as three months following phacoemulsification. Although not all capsular bag contractions lead to significant IOL displacement, some contraction results in additional stress on potentially weak zonules leading to IOL dislocation [17]. Due to sphincter effect of fibrosis around an intact continuous curvilinear capsulorhexis, especially if the diameter is very small, the progressive weakening of already compromised zonules may make them vulnerable to continuous centripetal forces and cause their rupture resulting in IOL dislocation [10].

However, this study has some limitations. First, compared with data reviewed and obtained from medical charts, the extraction of patients relied solely on health claims, which could be inaccurate and excluded patients without diagnostic codes despite experiencing IOL dislocation after cataract surgery. Patients who did not seek treatment at a hospital were also excluded, leading to underestimating the results. Second, determining whether a surgical procedure could be necessary because of the lack of clinical data is difficult, including patients’ medical history, corneal endothelial status, visual acuity, and extent of IOL dislocation. Third, our study focused only on the residents in South Korea; therefore, our findings cannot be generalized to other ethnicities. Additionally, IOL dislocation could occur as early as less than 3 months or later, and these two types have quite different pathogenesis. Early IOL dislocation can be induced due to tearing of posterior capsule or rupture of equatorial syndrome usually happened during intraoperative or preoperative zonulopathy. Our results only suggested the mean time of IOL dislocation after cataract surgery regardless of cause of IOL dislocation so that a further study should be conducted.

The strengths of our study include the large sample size using a national database covering nearly the entire Korean general population for the long term and the multifaceted implications of our study. Only a few studies have analyzed IOL dislocation, focusing on Asian risk factors [17,36,37]. We targeted the entire population of our country to establish representativeness of the analysis. First, this study will raise awareness among ophthalmologists about the potential for IOL dislocation. In addition, our study highlights the need for long-term monitoring of pseudophakic patients beyond the immediate stable postoperative period. Additionally, it can help identify patients with a higher risk of IOL dislocation, leading to the development of more personalized postoperative care plans.

In conclusion, the incidence rate of IOL dislocation requiring surgery was 0.82% in South Korea, with higher recurrence among younger age groups and in male patients. Although its incidence is relatively low, IOL dislocation is a vision-threatening complication. Given the sharp increase (more than twofold) in the incidence rate of IOL dislocation occurring over 10 years after cataract surgery, long-term follow-up is necessary in high-risk patients.

This study aimed to analyze the frequency of IOL dislocation and patterns of secondary IOL implantation surgery among patients who underwent cataract surgery in South Korea based on large retrospective data of the general population provided by the KNHIS. These findings are expected to serve as an important reference source for predicting complications that can cause severe visual loss, such as dislocation of the intraocular artificial lens following cataract surgery, the most common surgical procedure in South Korea, and assessing associated risk factors.