Minor influence of sub-bowman keratomileusis on the posterior corneal surface at early stage

Correlation between the ablation ratio and posterior corneal stability after small incision lenticule extraction for high myopia

PURPOSE

The aim of this study is to investigate changes in posterior corneal elevation and their correlations with the ablation ratio 3 years after small incision lenticule extraction (SMILE) for high myopia.

METHODS

Eighty eyes underwent SMILE were enrolled in this study. Eyes were classified into two groups based on the ablation ratio (AR, lenticule thickness from SMILE machine/thinnest corneal thickness): group A (< 25%, 40 eyes) and group B (≥ 25%, 40 eyes). Pentacam was used to measure the posterior corneal elevation at the central point, thinnest point, and posterior maximum elevation (PME) and the mean posterior elevation in the central 2-mm area (MPE-2 mm), 4-mm area, and 6-mm area at the 3-year follow-up.

RESULTS

More than 85% of the eyes had an AR of less than 27%, and no cases of iatrogenic keratectasia developed. In both groups, for central region, posterior elevation decreased implying backward displacement; for peripheral region, it increased indicating forward trend. There was no significant difference in changes in all determined parameters between the two groups (P ≥ 0.07). Moreover, no significant correlation was noted between AR and posterior elevation changes. In group A, decreasing changes in PME (r =  - 0.42, P = 0.01) and MPE-2 mm (r = 0.40, P = 0.01) demonstrated negative correlations with residual bed thickness.

CONCLUSION

Region-dependent changes were demonstrated in the eyes that underwent SMILE. The central area showed a subtly declining posterior elevation, and the peripheral area showed a slightly increasing elevation. The limited ablation ratio had no impact on the changes in posterior corneal elevation.

Five-year stability of posterior corneal surface after small incision lenticule extraction for high myopia

Background

To study the 5-year changes in the posterior corneal surface after small incision lenticule extraction (SMILE) for high myopia.

Methods

Eighty eyes received SMILE was included in this prospective study. They were allocated into two groups based on the spherical equivalent: high myopia (40eyes, -7.49 ± 0.70D) and moderate myopia (40eyes, -4.43 ± 0.87D). Certain points of posterior corneal elevation (the central point (PCE), thinnest point (PTE), maximal point (PME), and in various corneal areas) were evaluated using a Scheimpflug camera (Pentacam; Oculus GmbH, Germany) preoperatively and at 6 months and 5 years after surgery.

Results

All surgeries were completed uneventfully and no ectasia was developed throng the observation. The safety index and efficacy index were 1.14 and 1.03 in the high myopia group, and 1.16 and 1.06 in the moderate myopia group, respectively. Most of the calculated values in the high myopia group showed a slight increase at 6 months but decreased at 5 years. At 5 years postoperatively, the value of the PTE was significantly lower than at baseline in both groups (P ≤ 0.047); a statistical difference was also revealed in the PME in the moderate group with slight changes (10.15 ± 3.01 μm vs. 11.60 ± 4.33 μm, P = 0.002); no statistical significance was observed in other calculated values (P ≥ 0.067). Similarly, no significant linear correlation was noted between changes in all values and the residual bed thickness either (P ≥ 0.057).

Conclusions

SMILE causes no protrusion in posterior corneal surface for correction of high myopia at the follow-up visit of 5 years.

Impact of ablation ratio on 5-year postoperative posterior corneal stability after refractive surgery: SMILE and FS-LASIK

BACKGROUND

To investigate the impact of the ablation ratio on 5-year postoperative posterior corneal stability in myopic eyes after small incision lenticule extraction (SMILE) and femtosecond laser-assisted in situ keratomileusis (FS-LASIK) surgery.

METHODS

A prospective, nonrandomized, cohort study: 80 eyes of 43 patients underwent SMILE surgery and 63 eyes of 32 patients underwent FS-LASIK surgery at the EYE & ENT Hospital, Fudan University. Ablation ratio was defined as lenticule thickness (SMILE cases) or ablation depth (FS-LASIK cases) divided by central corneal thickness (CCT). Posterior corneal elevation changes were recorded as posterior central elevation (PCE), posterior corneal surface at thinnest point (PTE) and posterior corneal mean elevation (PME). Patients were followed up at 6-month and 5-year interval to investigate the impact of the ablation ratio on posterior corneal elevation after SMILE and FS-LASIK surgery.

RESULTS

PCE dropped at the 6-month follow-up for both SMILE (decreased by -1.11 ± 2.93 μm, P < 0.05) and FS-LASIK groups (decreased by -0.46 ± 3.72 μm, P < 0.05). PTE also dropped in SMILE (reduced by -2.04 ± 3.02 μm, P < 0.05) and FS-LASIK group (reduced by -1.28 ± 4.21 μm, P < 0.05) at the 6-month follow-up. Stable PCE (elevation change: SMILE -0.28 ± 4.03 μm; FS-LASIK 0.79 ± 4.13 μm, P > 0.05) and PTE (elevation change: SMILE -0.08 ± 4.28 μm; FS-LASIK 1.42 ± 3.85 μm, P > 0.05) for both groups were recorded at the 5-year follow-up compared to the 6-month visit. Ablation ratio was strongly correlated with 5-year postoperative PCE (β = 2.68 ± 1.05, P < 0.01) and PTE (β = 2.35 ± 1.17, P < 0.05). Cut-off value for 5-year postoperative raised PCE and PTE was 27.3 and 27.1%, respectively.

CONCLUSIONS

Ablation ratio was strongly correlated with postoperative posterior corneal elevation in a 5-year follow-up in both SMILE and FS-LASIK groups. PCE and PTE underwent slight backward displacement 6-month postoperatively and remain stable at the 5-year follow-up. Threshold of the ablation ratio for resisting forward displacement of posterior corneal surface was 27.3 and 27.1% for SMILE and FS-LASIK groups, respectively.

Posterior corneal surface differences between non-laser in situ keratomileusis (LASIK) and 10-year post-LASIK myopic eyes

PURPOSE

To evaluate the posterior corneal surface differences between non-laser in situ keratomileusis (LASIK) and 10-year post-LASIK myopic eyes.

METHODS

The study included 130 eyes from 65 patients, who were treated with myopic LASIK 10 years ago. In addition, 130 eyes from 65 unoperated myopic patients of matching present age and preoperative refraction were divided into control group. Data on the posterior corneal surface and anterior chamber were obtained from Pentacam software and compared between the groups. Postoperative visual acuity (VA) and refractive error were also analysed.

RESULTS

The mean preoperative spherical equivalent (SE) was -6.99 ± 1.78 dioptre (D) in the LASIK group. Ten years after surgery, the mean SE was -0.45 ± 1.22 D, the efficacy index was 0.98, and the safety index was 1.01. The posterior corneal elevations of the LASIK group at 2 mm corneal diameter were significantly lower than those of the control group. However, posterior corneal elevations at 6 mm corneal diameter were higher in the LASIK group than the controls (p < 0.01 for all). The mean Q-values of posterior corneal surface demonstrated significant positive direction compared to that of control eyes at 6 and 7 mm corneal diameters (p < 0.05 for both). At the thinnest point of the cornea, the anterior chamber depths were shallower in the LASIK group than in controls. Meanwhile, the anterior chamber volumes (ACV) were smaller in the LASIK group than in the control group.

CONCLUSION

Our results demonstrated that the posterior corneal surface tends to show signs of central flattening and peripheral steepening 10 years after myopic LASIK surgery compared to that of non-operated myopic eyes.

Longitudinal evaluation of posterior corneal elevation after laser refractive surgery using swept-source optical coherence tomography

PURPOSE

To investigate the change in posterior corneal elevation up to 1 year after myopic femtosecond-assisted LASIK and photorefractive keratectomy (PRK).

DESIGN

Prospective, longitudinal, comparative study.

PARTICIPANTS

Patients undergoing femtosecond-assisted LASIK or PRK.

METHODS

Corneal imaging was performed using swept-source optical coherence tomography at baseline and at each postoperative follow-up. A 2-way analysis of variance model with repeated measures and a linear mixed effect model were used to compare the differences in posterior corneal elevation between LASIK and PRK at different points after adjusting for the preoperative spherical equivalent (SEQ), central corneal thickness (CCT), thinnest corneal thickness (TCT), residual bed thickness (RST), and ablation depth (AD).

MAIN OUTCOME MEASURES

The changes in posterior corneal elevation 1 month, 3 months, 6 months, and 12 months after surgery.

RESULTS

Ninety-eight eyes of 49 patients (mean age 35.2 ± 8.5 years) (62 LASIK, 36 PRK) were included. The mean change in posterior corneal elevation values after LASIK and PRK were 4.88±0.47 μm versus 3.67±0.48 μm (B-1), 2.42±0.56 μm versus 3.00±0.47 μm (B-3), 3.76±0.46 μm versus 2.76±0.46 μm (B-6), and 2.92±0.46 μm versus 2.72±0.46 μm (B-12), respectively. Significant differences in posterior corneal elevation after LASIK were found from month 1, to month 3, to month 6, to month 12 (P ≤ 0.001), whereas posterior corneal elevation did not change significantly from month 3, to month 6, to month 12 (P ≥ 0.373) after PRK. LASIK and PRK eyes showed significant differences at months 3 and 12 (P ≤ 0.023). A similar pattern was observed for the changes in posterior corneal elevation after LASIK and PRK after adjusting for the effect of SEQ, CCT, TCT, RST, and AD. The adjusted forward displacements of the posterior corneal surface were statistically significant throughout the study period after both refractive surgeries (P < 0.05).

CONCLUSIONS

The findings of our study suggested that there was a mild but significant forward protrusion of the posterior cornea after femtosecond laser-assisted LASIK and PRK. The posterior cornea fluctuated during the first postoperative year after LASIK, whereas it stabilized as early as 3 months after PRK.

Effect of anterior corneal surface asphericity modification on fourth-order zernike spherical aberrations

PURPOSE

To evaluate the theoretical influence of the change in corneal asphericity (ΔQ) on the change in fourth-order Zernike spherical aberration coefficient (ΔC(4)0) with customized aspheric refractive correction of myopia and hyperopia.

METHODS

The initial anterior corneal surface profile was modeled as a conic section of apical radius of curvature R0 and asphericity Q₀. The postoperative corneal profile was modeled as a conic section of apical curvature R1 and asphericity Q1, where R1 was computed from defocus D, and Q₁ selected for controlling the postoperative asphericity. The corresponding change in fourth-order spherical aberration (ΔC(0)4) was computed within a 6-mm optical zone using inner products applied to the incurred optical path changes. These calculations were repeated for different values of D, R₀, Q₀, and various intended ΔC(4)0 values.

RESULTS

Increasing negative spherical aberration (ΔC(4)(0) < 0) requires a change toward more negative values of asphericity (increased prolateness; ΔQ < 0) for hyperopic and low myopic corrections, but more positive values (ΔQ < 0) for high myopic correction. The larger the intended change in corneal spherical aberration (ΔC(4)(0)), the more myopic the threshold value for which the required change in asphericity, ΔQ, becomes positive. The influence of the magnitude of paraxial defocus correction is less pronounced when larger changes in C(4)(0) are intended.

CONCLUSIONS

These results provide a basis for controlling the direction (sign) and the magnitude of spherical aberration changes when using customized aspheric profiles of ablation.