Corresponding Author: Young Taek Chung, MD, PhD. Onnuri Eye Hospital, 325 Baekje-daero, Wansan-gu, Jeonju 54972, Korea. Over the past few decades, the global demand for refractive surgery has steadily risen, driven by the increasing prevalence of myopia and the growing desire among patients for a better quality of life [1]. Technological advancements in this field have contributed to improved clinical outcomes and the early resolution of postoperative discomfort [2]. Keratorefractive lenticule extraction (KLEX), a flap-free procedure using a femtosecond laser, has demonstrated good safety and efficacy in the treatment of myopia, with reduced postoperative dry eye symptoms and better biomechanical strength [3,4]. Similarly, the implantable collamer lens (ICL; EVO Visian ICL V4c, STAAR Surgical), a posterior chamber phakic intraocular lens incorporating CentraFLOW technology (STAAR Surgical), has gained widespread acceptance as a highly effective long-term solution for correcting myopia, particularly in cases of high myopia [5,6].
Despite the favorable outcomes of KLEX and ICL implantation in treating myopia, demonstrated in numerous studies, some patients experience postoperative discomfort following either surgery, including blurry vision, focusing difficulty, glare, halo, and dryness [1,7,8]. While these symptoms tend to decrease in severity and disappear over time, there is significant interpatient variability in their severity and resolution rate, with some persisting for an extended period postoperatively [8-10]. However, to date, few studies have examined persistent postoperative discomfort following KLEX and ICL implantation in greater detail.
Thus, the present study aimed to assess the frequency of persistent postoperative discomfort 1 year after KLEX and ICL implantation. Patients were administered a questionnaire allowing an evaluation of the factors associated with symptom persistence. The severities of persistent discomfort symptoms associated with the two surgical procedures were compared as well.
Materials and Methods
Ethics statement
This study was approved by the Public Internal Regulatory Board of the South Korean Ministry of Health and Welfare (No. P01-202501-01-022). All patients provided written informed consent before surgery. All patients were followed up for at least 1 year. The right eye was analyzed in all patients. All procedures were performed in accordance with the tenets of the Declaration of Helsinki.
Study design and setting
This retrospective, comparative study included patients with myopia or myopic astigmatism who underwent KLEX or ICL implantation at Onnuri Smile Eye Clinic (Seoul, Korea), between January 2021 and May 2023. The inclusion criteria for the KLEX group were the following: myopia from −0.5 to −7.0 diopters (D), central corneal thickness of >480 μm, and calculated postoperative residual bed thickness of >300 μm. The inclusion criteria for the ICL group were as follows: myopia from −0.5 to −18.0 D, anterior chamber depth of >2.8 mm, and endothelial cell density of ≥2.000 cells/mm2. The inclusion criteria for both groups were age ≥19 years, corrected distance visual acuity of 20/30 or better, astigmatism up to −3.5 D, and stable refraction of >2 years. The exclusion criteria for both groups were severe dry eye, obvious corneal scarring, active ocular or systemic diseases, glaucoma or retinal diseases, history of ocular surgery, and pregnancy or lactation.
Surgical technique
One experienced surgeon (BKK) performed all the surgeries. KLEX was performed using a VisuMax femtosecond laser (500 kHz, Carl Zeiss Meditec) with a lenticule diameter of 6.0 to 6.7 mm. The cap thickness was 120 μm, the side cut thickness was 15 μm, and an incision of 2.0 mm was created at the 11 o’clock position. During ICL implantation, a 2.8-mm clear corneal incision was made on the steep axis using a Callisto eye system (Carl Zeiss Meditec). A loaded EVO Visian ICL V4c was positioned at the posterior chamber, with the footplates positioned posterior to the iris plane. Emmetropia was targeted in all eyes. After both procedures, topical antibiotics, steroids, and lubricating eyedrops were administered; the eyedrops were used for at least 3 months.
Patient-reported satisfaction and discomfort
One year after surgery, a structured questionnaire was administered to assess patient outcomes (Supplementary Material 1). Patients were first asked to report their overall satisfaction with the surgical results. Then, for each of five common postoperative symptoms—blurry vision, focusing difficulty, glare, halos, and dryness—patients were asked whether they had experienced the symptom in the early postoperative period and whether it was still present at the 1-year mark. For each symptom reported in the early postoperative period, patients were asked to rate its frequency, severity, and bothersomeness. At the 1-year follow-up, for persistent symptoms, the same three aspects were evaluated again. For symptoms that had resolved, patients were asked to specify the time to resolution.
Statistical analysis
The data were analyzed using IBM SPSS ver. (IBM Corp). The graphs were drawn using Microsoft Excel 2021 (Microsoft Corp). All data were subjected to a Kolmogorov-Smirnov test and found to be normally distributed. The results are expressed as the mean ± standard deviation. An independent t-test or a chi-square test was used to compare the differences between the KLEX and ICL groups. For comparison of postoperative discomforts from the early postoperative period to 1 year after surgery, the Wilcoxon signed rank test was applied. To identify factors associated with persistent postoperative discomfort at 1 year, univariate logistic regression analyses were first performed on all potential clinical and demographic variables. Variables with a p-value of <0.1 in the univariate analysis were subsequently included in a multivariate logistic regression model to determine independent risk factors. A p-value of <0.05 was regarded as statistically significant.
Results
Baseline characteristics and visual outcomes
Of the 324 eyes (324 patients) analyzed in this study, 227 were treated by KLEX and 97 by ICL implantation. No patient had intraoperative or postoperative complications. Preoperative myopia, astigmatism, and spherical equivalent were significantly smaller in the KLEX group than in the ICL group (all p < 0.001). The differences between the groups in terms of age, sex, intraocular pressure, central corneal thickness, pupil size, Schirmer test, tear break-up time, and ocular surface disease index score were not significant (Table 1), nor were the differences in postoperative visual outcomes (Table 2).
Patient-reported satisfaction and discomfort
The proportion of patients satisfied, neutral, dissatisfied, and very dissatisfied with the surgery was 97.4%, 2.2%, 0.4%, and 0%, respectively, in the KLEX group, and 99%, 1%, 0%, and 0%, respectively, in the ICL group. The differences in the satisfaction rates of the two groups were not significant (p = 0.157).
In the KLEX group, persistent blurry vision, focusing difficulty, glare, halos, and dryness were reported by 18.1%, 15.4%, 50.2%, 48.9%, and 70.5% of patients, respectively, and by 18.6%, 17.5%, 64.9%, 63.9%, and 61.2% of patients, respectively, in the ICL group (Table 3). The incidence of persistent glare and halos was significantly lower in the KLEX group than in the ICL group (p = 0.045 and p = 0.034, respectively); the differences between the groups for other symptoms of discomfort were not significant. In the KLEX and ICL groups, the time until discomfort resolution among patients whose discomfort resolved were 23.81 ± 60.82 and 28.69 ± 37.95 days, respectively, for blurry vision (p = 0.538), 45.30 ± 100.57 and 40.56 ± 50.06 days, respectively, for focusing difficulty (p = 0.730), 41.11 ± 61.25 and 66.41 ± 70.20 days, respectively, for glare (p = 0.031), 39.54 ± 52.52 and 65.88 ± 75.80 days, respectively, for halos (p = 0.024), and 68.68 ± 80.84 and 38.06 ± 54.38 days, respectively, for dryness (p = 0.008) (Table 4).
The most frequent, most bothersome, and most severe persistent symptom in the KLEX group was dryness, while in the ICL group, it was halos. In the ICL group, glare was nearly as frequent, as bothersome, and as severe as halos. In the KLEX group, glare and halos were the second and third most frequent, bothersome, and severe persistent symptoms, and they caused similar levels of discomfort. Dryness was the third most frequent and severe persistent symptom in the ICL group. The differences in the persistent symptoms between the two groups were not significant, except for dryness, which was significantly more frequent, severe, and bothersome in the KLEX group than in the ICL group (all p < 0.001) (Fig. 1,-3). All persistent postoperative discomforts showed statistically significant improvement in frequency, severity, and bother-someness when compared to the corresponding values observed in the early postoperative period (all p < 0.05) (Supplementary Fig. 1-3).
Risk factors for persistent postoperative discomfort
A multivariate logistic regression analysis of the data from the KLEX group identified older age as a risk factor for persistent blurry vision (odds ratio [OR], 1.072; p = 0.041), female sex as a risk factor for persistent focusing difficulty (OR, 3.658; p = 0.004) and for dryness (OR, 8.813; p < 0.001), preoperative high myopia as a risk factor for glare (OR, 0.627; p = 0.005) and halos (OR, 0.815; p = 0.013), and large pupil size as an additional risk factor for glare (OR, 1.559; p = 0.022) and halos (OR, 1.800; p = 0.017). In the ICL group, large pupil size (OR, 2.517; p = 0.046), postoperative myopia (OR, 0.118; p = 0.011), and postoperative astigmatism (OR, 0.224; p = 0.042) were identified as risk factors for persistent blurry vision, large pupil size (OR, 2.822; p = 0.010) and female sex (OR, 2.698; p = 0.038) as risk factors for persistent glare, and large pupil size as a risk factor for persistent halos (OR, 3.001, p = 0.007) (Table 5). The full results of the univariate logistic regression analyses for both the KLEX and ICL groups are provided in Supplementary Tables 1 and 2, respectively.
Discussion
This study evaluated persistent discomfort associated with KLEX and ICL implantation 1 year after the procedures. For both groups, the level of patient satisfaction was generally high, with no significant differences in overall satisfaction (p = 0.157). However, postoperative discomfort, such as blurry vision, focusing difficulty, glare, halos, and dryness, persisting for up to 1 year postoperatively, were reported by a subset of patients, with varying frequencies between those in the KLEX and ICL groups.
Glare and halos were common symptoms of persistent discomfort in both groups; they were the most frequent in the ICL group and the second most frequent after dryness in the KLEX group. Reinstein et al. [11] reported that glare and starbursts were the major factors reducing the quality of vision after KLEX. In the study by Siedlecki et al. [12], halos were reported by 80% of patients 2 years after ICL implantation. Glare and halos after refractive surgery can be caused by several general mechanisms, such as a mismatch between pupil size and optical zone diameter as well as changes in the optical system that affect how light is processed [12,13]. Specifically, for KLEX patients, glare and halos are often linked to central corneal flattening, which increases spherical aberrations and surface irregularities, leading to abnormal light scattering [11,12]. For patients who underwent ICL implantation, factors such as CentraFLOW technology and lens toricity contribute to glare and halos [12-14]. These mechanistic differences explain our findings: while persistent glare and halos at 1 year postoperatively were less frequent in the KLEX group compared to the ICL group (p = 0.045 for glare and p = 0.034 for halos), the resolution time of glare and halos, among patients whose symptoms resolved, was significantly shorter in the KLEX group (p = 0.031 for glare and p = 0.024 for halos). Corneal remodeling after KLEX promotes a gradual improvement in optical quality, whereas factors such as lens positioning, residual refractive error, and interaction with the crystalline lens in ICL surgery may lead to a longer persistence of glare and halos [15]. Aruma et al. [16] and Wei et al. [17] reported a significantly higher incidence of postoperative halos after ICL than after KLEX, whereas in our study, the two groups did not significantly differ concerning the frequency, severity, or bothersomeness of residual glare and halos. In the logistic regression analysis, large pupil size was a risk factor for persistent glare and halos in both groups, with preoperative high myopia as an additional risk factor for these symptoms in the KLEX group. Our findings are consistent with those of previous studies, suggesting that a mismatch between pupil size and optical zone diameter is a significant factor contributing to the persistence of glare and halos postoperatively [12-14].
Dryness is another common discomfort after refractive surgery, whether corneal refractive surgery or ICL implantation [7,18]. In our study, dryness was the most common persistent symptom in the KLEX group. Although there was no statistically significant difference in the persistence of dry eye between the two groups at 1 year postoperatively, this symptom was significantly more frequent, severe, and bothersome in the KLEX group than in the ICL group (all p < 0.001). Furthermore, the resolution time among patients whose dryness symptoms resolved was significantly longer in the KLEX group (p = 0.008). Although KLEX involves a smaller incision, there is greater overall damage to corneal sensory nerve branches during the lenticule formation process. By contrast, ICL affects only the corneal incision area, resulting in less ocular surface disruption. This difference in nerve involvement may explain the longer resolution time and more severe dryness associated with KLEX [19]. Female sex was also identified as a risk factor for persistent dryness. This finding is also consistent with previous studies and is likely to be attributable to hormonal effects [20].
Blurry vision and focusing difficulty after KLEX and ICL surgeries are mainly caused by the resulting aberrations of the cornea and by the healing response [1,2]. In general, both symptoms gradually improve as the corneal surface smoothens and the eye adapts to the structural changes. This would explain the shorter persistence of these symptoms than those of postoperative glare and halos [1,11,13]. In our study, the rates of blurry vision and focusing difficulty 1 year postoperatively were relatively low: 18.1% and 15.4%, respectively, in the KLEX group and 18.6% and 17.5%, respectively, in the ICL group. In the remaining patients, in both groups, the frequency, severity, and bothersomeness were lower for these discomforts than for others. Additional risk factors for persistent blurry viKorean sion included old age in the KLEX group, residual refractive errors, as well as large pupil size in the ICL group. In patients undergoing KLEX, age-related changes in ocular structures, such as the cornea and lens, may reduce the overall visual quality postoperatively [21]. Among patients treated by ICL implantation, a large pupil size may result in a mismatch between the optical zone of the ICL, such that peripheral light rays are not properly focused, leading to blurry vision [15].
There are several limitations to this study. First, its retrospective design may have introduced biases in patient selection and data collection. A randomized prospective study would provide stronger evidence. Second, we did not perform statistical adjustments for potential confounding variables such as age, preoperative refraction, or ocular surface status. The lack of adjustment may have introduced bias and affected the interpretation of comparative outcomes between groups. Third, all surgeries were performed by a single surgeon at a single center, which may limit the generalizability of the results to other surgical environments or less experienced surgeons. Fourth, preoperative symptoms such as night vision complaints or ocular surface abnormalities were not comprehensively evaluated using standardized questionnaires. This limits the ability to determine whether preoperative differences between groups influenced postoperative outcomes. Fifth, the classification of “persistent” symptoms in this study was based solely on patients’ subjective perception as assessed through a postoperative questionnaire. We did not evaluate changes in symptom frequency, severity, or bothersomeness compared to the preoperative state. This may have introduced ambiguity in the definition of persistence and limited the precision of symptom characterization. Sixth, our study focused on the first postoperative year, which may not have been sufficient to capture long-term persistent discomforts fully, given that some symptoms may take longer to manifest or resolve. Lastly, factors such as patient-reported outcomes for visual discomfort may be subjective and prone to variability.
In conclusion, both KLEX and ICL implantation resulted in high levels of patient satisfaction and favorable visual outcomes 1 year postoperatively. However, a subset of patients in both groups experienced persistent postoperative discomfort such as glare, halos, blurry vision, focusing difficulty, and dryness. Persistent glare and halos were significantly more common in the ICL group, whereas persistent dryness was more frequent, severe, and bothersome in the KLEX group. Risk factors for persistent blurry vision included old age in the KLEX group, and large pupil size and residual refractive errors in the ICL group. These findings highlight the importance of both a thorough preoperative evaluation and patient counseling regarding the potential for prolonged symptoms. In borderline cases our findings may help inform surgical choice. For example, patients with large pupils may be better suited for KLEX to avoid glare and halos, while those with ocular surface vulnerability may benefit more from ICL implantation to reduce the risk of postoperative dryness. Incorporating individual risk factors into the decision-making process may help optimize patient comfort and outcomes. Further prospective studies with longer follow-up periods and larger sample sizes are needed to understand more fully the long-term persistence of postoperative discomfort following KLEX and ICL implantation and to refine the patient selection criteria for both procedures.
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