This study compares KLEx and FS-LASIK using the VISUMAX 800 and describes Callisto Eye–guided cyclotorsion compensation achieving accurate astigmatic alignment without built-in torsion-tracking technology in myopic astigmatism clinical outcomes analysis.
Abstract
Purpose:
To compare the visual and refractive outcomes of keratorefractive lenticule extraction (KLEx) using the VISUMAX 800 femtosecond laser and femtosecond laser-assisted in situ keratomileusis (FS-LASIK) performed using the VISUMAX 800 femtosecond laser and the EX500 excimer laser in the treatment of myopic astigmatism.
Setting:
Single-center, private ophthalmology center.
Design:
Retrospective comparative study.
Methods:
237 eyes (133 KLEx, 104 FS-LASIK) were evaluated. Cyclotorsion alignment in KLEx was achieved using the Callisto Eye system integrated with VISUMAX 800. Refractive outcomes, vector analysis (Alpins method), and safety and efficacy indices were compared between groups at 1 month and 6 months postoperatively.
Results:
At 6 months, both groups achieved comparable uncorrected distance visual acuity (0.01 ± 0.04 logMAR) and corrected distance visual acuity. Mean spherical equivalent was closer to emmetropia in the KLEx group (−0.04 ± 0.18 diopters [D] vs −0.10 ± 0.15 D, P = .007). Residual cylinder was lower in KLEx (−0.14 ± 0.19 D vs −0.20 ± 0.20 D, P = .010). Vector analysis showed lower difference vector in KLEx (0.14 ± 0.19 D vs 0.21 ± 0.21 D, P = .009), while FS-LASIK had a slightly higher correction index (correction index: 1.02 vs 0.97, P = .005).
Conclusions:
Both KLEx and FS-LASIK performed using the VISUMAX 800 achieved excellent refractive outcomes, with KLEx procedures incorporating an image-guided static cyclotorsion alignment strategy and demonstrating excellent refractive and vector analysis results.
Keratorefractive lenticule extraction (KLEx) and femtosecond laser-assisted in situ keratomileusis (FS-LASIK) are the two of the most widely performed corneal refractive procedures for the correction of myopia and myopic astigmatism. Although both techniques demonstrate high efficacy and safety profiles, differences in their mechanisms of tissue interaction and surgical workflow continue to be participants of investigation. FS-LASIK involves flap creation followed by excimer laser ablation, whereas KLEx achieves refractive correction through a femtosecond laser-created intrastromal lenticule extracted through a small incision, offering potential biomechanical and dry eye advantages due to the absence of a flap.1–4
The VISUMAX 800 femtosecond laser system (Carl Zeiss Meditec AG) offers faster laser pulses, more stable docking, and higher surgical precision compared with its predecessor, the VISUMAX 500. Through optional modules such as CentraLign and OcuLign, the VISUMAX 800 provides software-based cyclotorsion compensation, which represent an important technological development in astigmatism correction. However, these features are not preinstalled by default on all systems. As these modules had not yet been installed on our device at the time of surgery, we used an image-guided alignment method by integrating the Callisto Eye system with the VISUMAX 800 to compensate for static cyclotorsion. This technique, previously described by Köse, involves matching preoperative reference images with limbal landmarks, allowing accurate axis alignment during surgery.5
This study aims to compare the visual and refractive outcomes of KLEx performed using the VISUMAX 800 and FS-LASIK performed with the VISUMAX 800 femtosecond laser and EX500 excimer laser in patients with myopic astigmatism. To our knowledge, this study is among the first to provide a direct comparison of KLEx and FS-LASIK performed using the VISUMAX 800 femtosecond laser platform.
METHODS
Study Design
This retrospective study included patients who underwent either KLEx or FS-LASIK for the correction of myopia and astigmatism at the Ophthalmology Department of Aritmi Osmangazi Hospital in Bursa, Turkey, in 2024. All KLEx procedures were performed using the VISUMAX 800 femtosecond laser (Carl Zeiss Meditec AG), and FS-LASIK procedures used the VISUMAX 800 femtosecond laser followed by wavefront-optimized ablation with the EX500 excimer laser system (Alcon Laboratories, Inc.). The study was conducted in accordance with the tenets of the Declaration of Helsinki and was approved by the Non-Interventional Clinical Research Ethics Committee of Istanbul Medipol University (Decision No: 648, Date: May 22, 2025).
Preoperative and Postoperative Assessments
All patients underwent a complete ophthalmic evaluation before surgery, including uncorrected distance visual acuity (UDVA) and corrected distance visual acuity (CDVA), manifest and cycloplegic refraction, and corneal tomography with the Pentacam HR (Oculus Optikgeräte GmbH). Intraoperative cyclotorsion alignment was facilitated using limbal vessel reference images obtained with the IOLMaster 700 (Carl Zeiss Meditec AG). Contact lens users were instructed to discontinue soft contact lenses for at least 2 weeks and rigid gas-permeable lenses for 4 weeks before surgery.
Postoperative follow-up visits were scheduled at day 1, week 1, and at 1 month and 6 months postoperatively. Inclusion criteria were age between 20 years and 40 years, CDVA of 20/25 or better, myopia between −0.50 diopters (D) and −9.00 D, astigmatism between −0.25 D and −5.00 D, central corneal thickness (CCT) greater than 500 μm, and stable refraction for a minimum of 2 years. Exclusion criteria included ocular surface disease, corneal ectasia, glaucoma, uveitis, retinal pathology, previous ocular trauma or surgery, systemic disease, pregnancy, or lactation.
Surgical Procedures
KLEx Procedure (Group 1)
All KLEx procedures commonly referred to as small incision lenticule extraction were conducted using a standardized protocol. The VISUMAX 800 femtosecond laser was programmed with a pulse energy of 140 nJ and a 3 μm spot spacing. A small-sized suction cone was used in all cases. The lenticule parameters included an optical zone of 6.5 mm, lenticule diameter of 6.6 mm, cap diameter of 7.5 mm, and a side-cut angle of 2.0 mm. Minimum lenticule thickness was set at 15 μm, ensuring a residual stromal bed of at least 250 μm.
FS-LASIK Procedure (Group 2)
All FS-LASIK procedures were performed bilaterally. Flap creation was achieved using the VISUMAX 800 femtosecond laser, with a targeted flap thickness of 110 to 120 μm and a superior hinge position. Flap diameter ranged from 8.2 to 9.0 mm. Laser parameters included spot spacing of 4.5 μm for the flap bed, 2.0 μm for side cuts, and a pulse energy of 140 nJ. After flap creation, wavefront-optimized stromal ablation was performed using the EX500 excimer laser. The optical zone ranged from 6.5 to 7.0 mm and the ablation zone from 8.5 to 9.0 mm.
Postoperative medical treatment included topical moxifloxacin 0.5% administered 4 times daily for 1 week, and prednisolone acetate 1% 4 times daily for 1 week, tapered over the following 4 weeks. Preservative-free artificial tears were recommended for at least 1 month postoperatively.
Astigmatism Vector Analysis
Astigmatic correction was assessed using the Alpins vector method.6 Spectacle-plane astigmatism values were adjusted to the corneal plane using a vertex distance of 12 mm. Parameters calculated included target-induced astigmatism (TIA), surgically induced astigmatism (SIA), and the difference vector (DV). The correction index (CI) was defined as the ratio of SIA to TIA, and the angle of error (AoE) as the angular difference between SIA and TIA axes. The index of success was calculated as DV divided by TIA.
Statistical Analysis
All statistical analyses were conducted using SPSS software v. 26.0 (IBM Corp.). The Shapiro-Wilk test was used to test for normality. Continuous variables were presented as mean ± SD, and categorical variables as frequencies and percentages. Between-group comparisons were made using the t test or the Mann-Whitney U test, depending on data distribution. Paired t tests were used for within-group comparisons. Categorical variables were analyzed using the chi-square test. A P value less than 0.05 was considered statistically significant.
RESULTS
Baseline Characteristics
Baseline characteristics are presented in Table 1. A total of 237 eyes were evaluated: 133 eyes in the Group 1 and 104 eyes in the Group 2. The mean age was slightly lower in Group 1 (28.53 ± 5.81 years) compared with Group 2 (30.07 ± 6.23 years), but the difference was not statistically significant (P = .054). Gender distribution was similar between the groups (P = .792).
Table 1.
Baseline demographic and preoperative clinical characteristics of KLEx and FS-LASIK groups
| Parameter | Group 1 (n = 133) Mean ± SD |
Group 2 (n = 104) Mean ± SD |
P value |
| Age (y) | 28.53 ± 5.81 | 30.07 ± 6.23 | .054 |
| Sex (F/M) | 77/56 | 58/46 | .792 |
| Sphere (D) | −3.66 ± 1.64 | −2.45 ± 1.29 | <.001* |
| Cylinder (D) | −1.26 ± 0.80 | −1.37 ± 0.79 | .297 |
| Axis (degrees) | 94.33 ± 75.03 | 96.12 ± 73.96 | .855 |
| SE (D) | −4.28 ± 1.62 | −3.14 ± 1.31 | <.001* |
| K1 | 42.87 ± 1.41 | 42.16 ± 1.43 | <.001* |
| K2 | 44.45 ± 1.48 | 43.81 ± 1.60 | .002* |
| CCT (μm) | 553.84 ± 29.15 | 557.40 ± 35.37 | .409 |
| UDVA (logMAR) | 0.94 ± 0.36 | 0.93 ± 0.35 | .991 |
| CDVA (logMAR) | 0.01 ± 0.04 | 0.01 ± 0.05 | .272 |
| Mesopic pupil diameter (mm) | 3.03 ± 0.63 | 3.26 ± 0.61 | .005* |
| Scotopic pupil diameter (mm) | 6.49 ± 0.90 | 6.12 ± 0.84 | .002* |
CCT = central corneal thickness; K1 = flat keratometry; K2 = steep keratometry; KLEx = keratorefractive lenticule extraction; SE = spherical equivalent
Statistically significant
Preoperative spherical equivalent (SE) was significantly higher in Group 1 (−4.28 ± 1.62 D) than in Group 2 (−3.14 ± 1.31 D; P < .001). Similarly, Group 1 had significantly steeper K1 and K2 values (both P < .01). No significant differences were found in preoperative UDVA, CDVA, cylinder, axis, or CCT.
Visual and Refractive Outcomes
As summarized in Table 2, both groups demonstrated significant improvement in UDVA and SE postoperatively. At 1 month, Group 1 showed significantly better UDVA than Group 2 (0.01 ± 0.04 vs 0.03 ± 0.05 logMAR; P = .048), while SE was closer to emmetropia in Group 1 (−0.10 ± 0.24 D) than in Group 2 (−0.20 ± 0.27 D; P = .002). At 6 months, UDVA and CDVA were comparable between groups (P = .303 and P = .254, respectively), with both groups achieving a mean UDVA of 0.01 ± 0.04 logMAR.
Table 2.
Visual acuity and refractive outcomes at 1 month and 6 months postoperatively
| Parameter | Group 1 (n = 133) Mean ± SD |
Group 2 (n = 104) Mean ± SD |
P value |
| 1-mo postop visit | |||
| UDVA (logMAR) | 0.01 ± 0.04 | 0.03 ± 0.05 | .048* |
| CDVA (logMAR) | 0.00 ± 0.03 | 0.01 ± 0.03 | .131 |
| Sphere (D) | 0.01 ± 0.18 | −0.08 ± 0.26 | .001* |
| Cylinder (D) | −0.22 ± 0.28 | −0.25 ± 0.21 | .498 |
| Axis (degrees) | 40.60 ± 63.48 | 59.68 ± 61.42 | .020* |
| SE (D) | −0.10 ± 0.24 | −0.20 ± 0.27 | .002* |
| 6-mo postop visit | |||
| UDVA (logMAR) | 0.01 ± 0.04 | 0.01 ± 0.04 | .303 |
| CDVA (logMAR) | 0.00 ± 0.02 | 0.01 ± 0.03 | .254 |
| Sphere (D) | 0.02 ± 0.16 | 0.00 ± 0.12 | .176 |
| Cylinder (D) | −0.14 ± 0.19 | −0.20 ± 0.20 | .010* |
| Axis (degrees) | 35.75 ± 61.65 | 53.00 ± 62.73 | .035* |
| SE (D) | −0.04 ± 0.18 | −0.10 ± 0.15 | .007* |
SE = spherical equivalent
Statistically significant
Residual cylinder was significantly lower in Group 1 at 6 months (−0.14 ± 0.19 D) compared with Group 2 (−0.20 ± 0.20 D; P = .010), and Group 1 had significantly better axis stability (P = .035). SE was also slightly closer to target in Group 1 (−0.04 ± 0.18 D) than in Group 2 (−0.10 ± 0.15 D; P = .007).
Subgroup Analysis
Subgroup analysis based on astigmatism level (<1.5 D vs ≥ 1.5 D) is summarized in Table 3. In the low astigmatism group, Group 1 showed significantly better SE correction at both 1 month and 6 months (P = .016 and P = .433, respectively). In the high astigmatism group, SE was significantly closer to emmetropia in Group 1 at 6 months (−0.04 ± 0.20 D vs −0.14 ± 0.13 D; P = .003). Cylinder values and UDVA/CDVA were comparable across both subgroups.
Table 3.
Subgroup analysis of visual and refractive results according to preoperative astigmatism magnitude (<1.5 D vs ≥1.5 D)
| Parameter | Low (<1.5 D) (n = 139) Mean ± SD |
P value | High (≥1.5 D) (n = 98) Mean ± SD |
P value | ||
| KLEx (n = 83) | FS-LASIK (n = 56) | KLEx (n = 50) | FS-LASIK (n = 48) | |||
| Preop visit | ||||||
| UDVA (logMAR) | 0.91 ± 3.66 | 0.92 ± 0.37 | .859 | 0.99 ± 0.35 | 0.95 ± 0.33 | 0.529 |
| CDVA (logMAR) | 0.00 ± 0.01 | 0.00 ± 0.02 | .328 | 0.03 ± 0.07 | 0.03 ± 0.06 | 0.774 |
| Sphere (D) | −3.84 ± 1.46 | −2.62 ± 1.30 | <.001* | −3.32 ± 1.87 | −2.25 ± 1.25 | 0.001* |
| Cylinder (D) | −0.75 ± 0.34 | −0.77 ± 0.34 | .746 | −2.11 ± 0.62 | −2.07 ± 0.55 | 0.757 |
| Axis (degrees) | 95.56 ± 72.45 | 86.98 ± 70.03 | .486 | 92.30 ± 79.84 | 106.79 ± 77.67 | 0.365 |
| SE (D) | −4.22 ± 1.51 | −3.01 ± 1.33 | <.001* | −4.37 ± 1.81 | −3.29 ± 1.29 | 0.001* |
| 1-mo postop visit | ||||||
| UDVA (logMAR) | 0.00 ± 0.02 | 0.02 ± 0.04 | .046* | 0.03 ± 0.07 | 0.04 ± 0.06 | 0.439 |
| CDVA (logMAR) | 0.00 ± 0.00 | 0.00 ± 0.02 | .012* | 0.01 ± 0.05 | 0.02 ± 0.04 | 0.784 |
| Sphere (D) | −0.01 ± 0.18 | −0.12 ± 0.28 | .009* | 0.06 ± 0.18 | −0.04 ± 0.23 | 0.014* |
| Cylinder (D) | −0.16 ± 0.23 | −0.18 ± 0.19 | .586 | −0.33 ± 0.31 | −0.32 ± 0.20 | 0.899 |
| Axis (degrees) | 32.65 ± 58.31 | 49.66 ± 65.92 | .121 | 53.82 ± 69.84 | 71.37 ± 54.07 | 0.169 |
| SE (D) | −0.09 ± 0.24 | −0.21 ± 0.31 | .016* | −0.10 ± 0.24 | −0.20 ± 0.21 | 0.039* |
| 6-mo postop visit | ||||||
| UDVA (logMAR) | 0.00 ± 0.01 | 0.00 ± 0.03 | .233 | 0.02 ± 0.06 | 0.03 ± 0.04 | 0.883 |
| CDVA (logMAR) | 0.00 ± 0.00 | 0.00 ± 0.01 | .241 | 0.01 ± 0.04 | 0.01 ± 0.04 | 0.594 |
| Sphere (D) | 0.00 ± 0.15 | 0.01 ± 0.13 | .465 | 0.07 ± 0.17 | −0.02 ± 0.11 | 0.003* |
| Cylinder (D) | −0.09 ± 0.15 | −0.17 ± 0.19 | .007* | −0.22 ± 0.22 | −0.25 ± 0.21 | 0.509 |
| Axis (degrees) | 28.48 ± 56.37 | 49.67 ± 65.94 | .044* | 47.82 ± 68.43 | 56.87 ± 59.23 | 0.485 |
| SE (D) | −0.04 ± 0.17 | −0.06 ± 0.15 | .433 | −0.04 ± 0.20 | −0.14 ± 0.13 | 0.003* |
KLEx = keratorefractive lenticule extraction; SE = spherical equivalent
Statistically significant
Safety
At the 6-month follow-up, no eyes in either group experienced a loss of 2 or more lines in CDVA. CDVA remained within 1 line of baseline in 100% of cases in both groups. The mean safety index was 1.02 ± 0.09 in both Group 1 and Group 2 (P = .810), indicating an excellent safety profile across procedures.
Efficacy
The mean efficacy index was 1.00 ± 0.08 in Group 1 and 1.01 ± 0.11 in Group 2, with no statistically significant difference between the groups (P = .832). These results confirm that both techniques achieved high levels of visual efficacy.
Predictability and Stability
Refractive outcomes remained stable between the 1-month and 6-month postoperative visits in both groups. At 6 months, 86% of eyes in each group were within ±0.50 D of the intended SE, and 100% were within ±1.00 D, demonstrating excellent predictability and long-term refractive stability.
Vector Analysis
Vector analysis results are summarized in Table 4. The TIA was comparable between groups (1.14 ± 0.73 D in Group 1 vs 1.27 ± 0.73 D in Group 2; P = .178). However, Group 2 showed significantly higher SIA (1.30 ± 0.78 D vs 1.10 ± 0.73 D; P = .037), as well as a greater DV (0.21 ± 0.21 D vs 0.14 ± 0.19 D; P = .009), indicating slightly less precise astigmatic correction. Vector analysis of astigmatic correction using the Alpins method demonstrated the distribution of TIA and SIA in both groups (Figure 1).
Table 4.
Vector analysis of astigmatic correction in KLEx and FS-LASIK groups using the Alpins method
| Parameter | Group 1 (n = 133) Mean ± SD |
Group 2 (n = 104) Mean ± SD |
P value |
| Mean efficacy index | 1.00 ± 0.08 | 1.01 ± 0.11 | .832 |
| Mean safety index | 1.02 ± 0.09 | 1.02 ± 0.09 | .810 |
| TIA (D) | 1.14 ± 0.73 | 1.27 ± 0.73 | .178 |
| SIA (D) | 1.10 ± 0.73 | 1.30 ± 0.78 | .037* |
| Difference vector (D) | 0.14 ± 0.19 | 0.21 ± 0.21 | .009* |
| Correction index | 0.97 ± 0.19 | 1.02 ± 0.20 | .005* |
| Index of success | 0.15 ± 0.25 | 0.20 ± 0.25 | .009* |
| Angle of error (degrees) | −0.88 ± 6.49 | 0.05 ± 8.25 | .330 |
| Magnitude of error (degrees) | −0.05 ± 0.18 | 0.03 ± 0.21 | .002* |
KLEx = keratorefractive lenticule extraction; SIA = surgically induced astigmatism; TIA = target induced astigmatism
Statistically significant
Figure 1.
Vector analysis illustrating the relationship between target-induced astigmatism and surgically induced astigmatism in the KLEx and FS-LASIK groups. KLEx = keratorefractive lenticule extraction
The CI was closer to the ideal value of 1.0 in Group 2 (1.02 ± 0.20) compared with Group 1 (0.97 ± 0.19; P = .005), suggesting a mild overcorrection trend in Group 2. The AoE was minimal and similar between the groups (−0.88 ± 6.49 degrees in Group 1 vs 0.05 ± 8.25 degrees in Group 2; P = .330), indicating effective axis alignment in both surgical techniques. The distribution of AoE and DV values further characterized the accuracy of astigmatic correction achieved with both procedures (Figure 2).
Figure 2.
Distribution of angle of error and difference vector values demonstrating the accuracy of astigmatic correction achieved with KLEx and FS-LASIK. KLEx = keratorefractive lenticule extraction
DISCUSSION
This study presents a comparative analysis of KLEx and FS-LASIK for the correction of myopic astigmatism using the VISUMAX 800 femtosecond laser platform. To our knowledge, this study provides early comparative clinical data on these 2 procedures using this advanced system, which offers improved docking stability, faster laser delivery, and higher surgical precision compared with its predecessor, the VISUMAX 500.
KLEx has emerged in recent years as a flapless refractive surgical technique for the correction of myopia and myopic astigmatism.7,8 Compared with FS-LASIK, it offers several advantages, including better preservation of corneal biomechanical stability, a lower risk of postoperative dry eye, and elimination of flap-related complications. However, concerns have been raised regarding potential undercorrection, particularly in the treatment of astigmatism.9 Although these concerns have been addressed over time through various techniques developed by surgeons, the introduction of advanced software modules compatible with the VISUMAX 800 platform has been associated with greater precision in KLEx and has facilitated the treatment of higher degrees of astigmatism.5,10,11 This study demonstrated that both KLEx and FS-LASIK exhibit high efficacy and safety profiles in the correction of high myopic astigmatism.
Numerous studies have compared the visual outcomes of KLEx and FS-LASIK across varying follow-up durations.12–14 In a prospective comparative study conducted by Lin et al., the efficacy index for KLEx was reported as 0.99 and 1.04 at 1 month and 3 months, respectively, while FS-LASIK demonstrated values of 1.02 and 1.10 (n = 111 eyes).12 Liu et al. compared outcomes between KLEx (n = 113 eyes) and FS-LASIK (n = 84 eyes) and found no statistically significant differences in UDVA or SE at 6 months. Some studies have suggested that FS-LASIK may offer faster visual recovery in the early postoperative period, whereas KLEx may exhibit slightly delayed visual rehabilitation. However, these differences tend to resolve by the 6-month follow-up.15,16 Long-term data in the literature, including 5-year follow-up studies, have similarly confirmed that both techniques are safe and effective over time.17 Han et al. also emphasized that KLEx, even in high myopic cases, provides long-term predictability comparable with FS-LASIK.2 Our study does not include long-term outcomes. However, both KLEx and FS-LASIK performed with the VISUMAX 800 femtosecond laser demonstrated substantial visual improvement at 6 months, with no statistically significant difference between the groups. At postoperative month 6, both UDVA and CDVA significantly improved in both groups, with no statistically significant difference between them (P = .303, P = .254). The safety index (1.02 ± 0.09 in both of the groups) and efficacy indices (1.00 ± 0.08 in Group 1 vs 1.01 ± 0.11 in Group 2) were consistent with previous literature, confirming the high safety and effectiveness of both techniques. SE predictability was excellent, with 86% of eyes in both groups within ±0.50 D and 100% within ±1.00 D of the intended target. However, at 6 months, residual astigmatism was significantly higher in Group 2 (−0.20 ± 0.20 D vs −0.14 ± 0.19 D, P = .010). Similarly, SE differed between groups, favoring Group 1 (−0.04 ± 0.18 D vs −0.10 ± 0.15 D, P = .007).
In the literature, KLEx has been reported to be more sensitive to axis misalignment, particularly in cases of high astigmatism. Zhou et al. demonstrated that KLEx yielded less predictable outcomes in patients with high astigmatism. Studies by Hashemi et al. and Chow et al. attributed this limitation to the restricted lenticule manipulation and lack of intraoperative cyclotorsion compensation inherent to the KLEx technique.18–20 Although these techniques are effective for astigmatic correction, there is a prevailing notion that KLEx may result in mild undercorrection, particularly in cases of high astigmatism. Vector analysis has shown more consistent alignment between TIA and SIA in FS-LASIK compared with KLEx, although the differences are relatively small.2 The VISUMAX 800 platform, with its optional integrated cyclotorsion control modules, has been associated with improved astigmatic outcomes in KLEx, primarily through reductions in residual astigmatism. However, software modules such as CentraLign and OcuLign are not included by default and must be added separately. In our study, these modules were not yet activated on the VISUMAX 800 system; therefore, we implemented an image-guided strategy by integrating the Callisto Eye system with the VISUMAX 800 to enable static cyclotorsion compensation intraoperatively. Following the method previously described by Köse, preoperative reference images were aligned with limbal vessel landmarks to ensure accurate axis alignment.5 This integration likely contributed to the improved precision of astigmatic correction observed with KLEx. This study illustrates how cyclotorsion compensation can be achieved using the VISUMAX 800 in conjunction with the Callisto Eye system, highlighting the potential role of technological integration in refractive surgery.
In our study, vector analysis showed that in the KLEx group, the SIA values were slightly lower than the TIA, with a CI of 0.97, falling below unity. This supports the notion that KLEx may have limitations in sensitivity to astigmatic axis alignment. By contrast, the FS-LASIK group demonstrated better alignment between SIA and TIA (CI: 1.02), although the CV was higher. This may suggest that although FS-LASIK provides a more effective magnitude of correction, it may be slightly less precise in axis accuracy. Subgroup analysis revealed that at 6 months, residual astigmatism was significantly higher in FS-LASIK eyes with preoperative astigmatism <1.50 D (P = .007), whereas no significant difference was found in eyes with ≥1.50 D of preoperative astigmatism (P = .509). Interestingly, in eyes with higher astigmatism, a mild myopic shift was observed in the FS-LASIK group (P = .003). This subgroup also showed a significant difference in SE values between groups at 6 months (P = .003). Our findings are consistent with previous reports indicating higher residual myopia after FS-LASIK compared with KLEx.18
Although our study did not assess biomechanical parameters directly, the biomechanical advantages of KLEx due to its flapless design are well documented. KLEx is associated with superior preservation of anterior stromal architecture, fewer corneal nerve disruptions, and a lower risk of iatrogenic ectasia. Although no cases of biomechanical decompensation were observed in our cohort, future studies incorporating biomechanical indices would strengthen these observations.
Limitations of this study include the relatively short follow-up period and the lack of direct biomechanical or ocular surface evaluations. In addition, the retrospective and nonrandomized design may be associated with selection bias and unmeasured confounding factors. Nevertheless, the refractive stability, vector accuracy, and safety profile observed at 6 months support the clinical utility of both procedures using the VISUMAX 800. Further studies with longer follow-up and larger cohorts are warranted to confirm these findings and to evaluate long-term outcomes and higher-order aberrations.
In conclusion, both KLEx and FS-LASIK performed with the VISUMAX 800 femtosecond laser demonstrated excellent safety, efficacy, and predictability in the correction of myopic astigmatism. The integration of image-guided alignment with the VISUMAX 800 platform contributed to precise outcomes even in the absence of built-in cyclotorsion modules. These findings support the expanding role of KLEx in astigmatism correction and highlight the value of technological integration in optimizing refractive surgical results. Further long-term studies with larger cohorts are warranted to validate these results and assess biomechanical and ocular surface outcomes.
WHAT WAS KNOWN
Precise axis alignment is essential for successful astigmatism correction in keratorefractive lenticule extraction (KLEx) and FS-LASIK.
KLEx lacks built-in cyclotorsion tracking, making it vulnerable to axis misalignment, especially in high astigmatism.
Although the VISUMAX 800 platform offers advanced precision, its cyclotorsion compensation modules (CentraLign/OcuLign) are not installed by default on all systems.
WHAT THIS PAPER ADDS
This study provides early comparative data on KLEx and FS-LASIK outcomes using the VISUMAX 800 femtosecond laser platform.
It introduces an image-guided method using the Callisto Eye system to compensate for cyclotorsion during KLEx, enabling precise axis alignment without built-in torsion tracking software.
Vector analysis confirms excellent refractive precision and reduced residual astigmatism, especially in higher astigmatism cases.
These findings demonstrate a practical and effective alternative to software-based compensation for enhancing KLEx outcomes in routine clinical settings.
Footnotes
Disclosures: None of the authors have any financial or proprietary interest in any material or method mentioned.
First author:
Husna Topcu, MD
Department of Ophthalmology, Aritmi Osmangazi Hospital, Bursa, Türkiye
Contributor Information
Mehmet Altun, Email: mehmetaltun1989@hotmail.com.
Mevlut Celal Ocal, Email: mcelal7@hotmail.com.
Dilek Yasa, Email: dilekyasa2@gmail.com.
Gonul Karatas Durusoy, Email: gnlkaratas@gmail.com.
Bulent Kose, Email: drbulentkose@gmail.com.
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