Corneal ablation depth readout of the MEL 80 excimer laser compared to Artemis three-dimensional very high-frequency digital ultrasound stromal measurements

Predictability of the Achieved Lenticule Thickness in Keratorefractive Lenticule Extraction for Myopia Correction

PURPOSE

To assess the predictability between the SCHWIND ATOS femtosecond laser (SCHWIND eye-tech-solutions) read-out and achieved lenticule thickness measured using MS-39 anterior segment optical coherence tomography (ASOCT; CSO) at the corneal vertex at postoperative 1 day and 1 and 3 months of follow-up.

METHODS

This retrospective case series included 130 eyes of 65 consecutive patients who were treated with SmartSight (SCHWIND eye-tech-solutions) lenticule extraction. Sixty-four percent of patients were women with a mean spherical refraction of -4.98 ± 1.19 diopters (D) and mean astigmatism of 0.53 ± 0.64 D. The measurements were performed using AS-OCT with the Phoenix Software v 4.1.1.5. Lenticule thickness was obtained by the subtraction method between preoperative and postoperative total corneal thickness.

RESULTS

A lower reduction in central corneal thickness (CCT) compared to the laser read-out for all three follow-up visits can be observed. Essentially, the reduction in CCT was identical at 3 months versus 1 month. The reduction in CCT was lowest at postoperative 1 day. Examining the 1- and 3-month data (essentially equivalent), one can see a best fit of y = 0.94 x -7 µm.

CONCLUSIONS

The reduction in CCT was stable from 1 month of follow-up. The stable reduction in CCT was -6% (-1.5% after accounting for design decisions) and -7 µm lower than the respective laser read-out. The findings are predictable, showing a certain level of the lenticules becoming slightly thinner than their respective laser read-outs. [J Refract Surg. 2023;39(11):728-735.].

Agreement Between Predicted and Actual Measured Ablation Depth After FS-LASIK Using Different Rotating Scheimpflug Cameras and OCT

Purpose

To compare the predicted ablation depth (AD) with the postoperatively measured corneal ablation depth (postop-AD) at central, paracentral, and midperipheral locations using two rotating Scheimpflug analyzers and a Fourier-domain optical coherence tomographer in eyes that underwent femtosecond laser-assisted LASIK (FS-LASIK).

Methods

The values of corneal thickness were measured preoperatively and postoperatively at one and three months. The difference between preoperative and postoperative was defined as postop-AD. Measurements were performed at the corneal vertex and mid-peripheral area. The mid-peripheral corneal thickness was measured at the superior, inferior, nasal, and temporal locations at a distance of 1.0 or 2.5 mm from the corneal vertex. The predicted AD was calculated by ORK-CAM software (Schwind eye tech-solutions GmbH, Kleinostheim, Germany), and the difference between the predicted AD and postop-AD was defined as Δ-AD. Paired t-test analysis was employed to evaluate the differences, agreement was assessed by the Bland-Altman method.

Results

Forty-two eyes of 42 patients were investigated. At one month, the predicted AD in the central and paracentral areas was underestimated by the Pentacam HR (Oculus, Wetzlar, Germany), Sirius (Costruzione Strumenti Oftalmici, Florence, Italy) and RTVue OCT (Optovue Inc., Freemont, CA, United States), whereas Δ-AD was negative as established by all devices and predominantly statistically significant. The Δ-AD values approximated zero at three months. The mean difference of Δ-AD at three months at the corneal vertex was 0.67 ± 9.39 mm, −7.92 ± 9.05 mm and −1.36 ± 8.31 mm, respectively. The mid-peripheral measurements had positive values at one month and even more highly positive at three months (with statistically significant differences in most of the cases). The agreement between the predicted and postop-AD was moderate with all devices, but slightly better with RTVue.

Conclusion

The predicted AD seems to be underestimated in the central and paracentral corneal area and overestimated in the mid-periphery.

Translational Relevance

The study could help to partly explain and prevent the refractive errors after FS-LASIK.

Corneal Stroma Thickness Changes after Myopic Laser Corneal Refractive Surgery

PURPOSE

To evaluate the postoperative behavior of the central corneal stroma thickness after myopic femto-LASIK and SMILE by using a combined anterior segment-OCT and placido disc topographer, and to compare the accuracy of both laser machines in predicting the real stromal change .

SETTING

Vissum Miranza, Alicante, Spain.

STUDY DESIGN

Prospective, observational, comparative study.

METHODS

The Visumax-500kHz femtosecond laser (FS), and the Amaris-750 excimer laser were used for the correction of myopia with or without myopic astigmatism. Central and paracentral stromal thicknesses (ST) and 6mm-corneal aberrometry were obtained with the MS39 topographer. Laser predicted stromal consumption was recorded (maximum lenticule thickness for SMILE and central ablation depth for LASIK). Visual and refractive outcomes were also evaluated. Total follow-up was 6 months.

RESULTS

77 LASIK-eyes were matched with 77 SMILE-eyes. Mean preoperative spherical equivalent (SE) was -3.92±1.67D for LASIK and -4.02±1.63D for SMILE;p=0.356. After LASIK, ST parameters showed a significant rethickening between months 1-3 (+4.38µm for central-ST;p<0.001), remaining stable thereafter. After SMILE, all ST parameters remained stable from month-1. Stromal ablation prediction was higher for SMILE compared to LASIK for all SE ranges, although postoperatively such differences were significant only for ametropias≤4D. At 6 months, mean SMILE laser prediction error was -13.21±7.00µm, while LASIK prediction showed better accuracy (+0.92± 8.16µm; p<0.001).

CONCLUSIONS

The accuracy of the Amaris-750 excimer laser in predicting the stromal consumption after LASIK was better than the VisuMax-FS laser for SMILE. While SMILE stromal thicknesses remained stable from month-1, after LASIK a mild stromal rethickening was observed up to the third month.

Evaluation of the Difference between Predicted and Measured Central Corneal Thickness Reduction after SMILE and Femtosecond Laser-assisted LASIK for Myopia

PURPOSE

To evaluate the agreement between the predicted and measured central corneal thickness (CCT) reduction after the small incision lenticule extraction (SMILE) surgery and femtosecond laser-assisted in situ keratomileusis (FS-LASIK) surgery.

METHODS

A total 165 patients were enrolled in this prospective study. Eighty patients with a mean spherical equivalent (SE) of -4.72 ± 1.80 Diopters (D) were treated with the FS-LASIK procedure and Eighty-five patients with a mean SE of -4.78 ± 1.63 D were treated with SMILE procedure. One eye for each patient was randomly selected and included for statistical analysis. Ultrasound pachymetry measurement and Scheimpflug camera corneal topography were performed preoperatively and 3 months postoperatively. The measured CCT reduction was calculated by comparing the preoperative examinations with postoperative examinations. Comparative statistics and linear regression analyses were performed.

RESULTS

The mean predicted CCT reduction was 95.02 ± 21.39 μm in FS-LASIK group and 103.49 ± 22.87 μm in SMILE group (P = .015). The prediction of laser platform was found to overestimate the measured CCT reduction for both FS-LASIK group (ultrasound 13.20 ± 9.34 μm) and SMILE group (ultrasound 13.12 ± 8.68 μm). The prediction of laser platform was found to systematically overestimate the measured CCT reduction in FS-LASIK group. In SMILE group, the difference between predicted and measured CCT reduction were found significantly related with the predicted CCT reduction (P < .001 for ultrasound; and P = .004 for Pentacam).

CONCLUSION

A systematic overestimation of measured CCT reduction in FS-LASIK group did not influence the refractive precision of FS-LASIK. Due to the different biomechanical distributions in post-SMILE cornea, the measured CCT reduction was influenced as the changes in refractive correction. Nomogram adjustment for high myopic correction needs further research.

Small incision lenticule extraction (SMILE) lenticule thickness readout compared to change in axial length measurements with the IOLMaster

PURPOSE

To compare theoretical values from the small incision lenticule extraction (SMILE) lenticule thickness readout with change in axial length measurements taken with the IOLMaster.

METHODS

We prospectively studied 214 eyes from 107 patients undergoing bilateral SMILE surgery for myopia or myopic astigmatism between December 2014 and May 2017 at an ophthalmological practice in Singapore. All eyes were examined pre-operatively and 1 and 3 months post-operatively with the IOLMaster following SMILE surgery. Achieved lenticule thickness was taken as the change in axial length after surgery. A linear mixed-effects model was used to examine changes in axial length, spherical equivalent and acuity over time. The relationships between change in axial length and theoretical lenticule thickness and spherical equivalent were examined with multiple linear regression analyses, and model prediction was assessed with adjusted R² statistics.

RESULTS

Mean (95% confidence interval [CI]) spherical equivalent pre-operatively was - 5.25 (95% CI - 5.38 to - 5.12) diopters (D), at 1 month was 0.04 (95% CI - 0.09 to 0.17) D (p < 0.001), and at 3 months was - 0.02 (95% CI - 0.15 to 0.11) D (p < 0.001). Mean (95% CI) pre-operative axial length was 27,726 (95% CI 25,595 to 25,857) μm. Post-operative axial length at 1 month was significantly shorter at 25,595 (95% CI 25,464 to 25,726) μm (p < 0.001) with no change thereafter (p = 0.647). Pre-operative mean ± standard deviation (SD) refractive target was 0.24 (± 0.3) D, and mean difference between target and post-operative spherical equivalent at 1 month was 0.20 D (95% CI 0.16 to 0.25 D, p < 0.001). Multiple regression analysis showed that change in axial length at 1 month was, on average, 5% lower than theoretical lenticule thickness, indicating an average difference of 5.4 μm (95% CI 5.2 to 5.6 μm). Preoperative spherical equivalent predicted negative association with change in axial length at 1 month (β = - 14.8, 95% CI - 18.2 to - 11.3, adjusted R² = 0.457, p < 0.001).

CONCLUSIONS

Calculated lenticule thickness values were less than expected, and post-operative refractive outcomes at 1 month showed a slight under-correction. Further research in this area is needed to validate these findings.

Comparative study of wave-front aberration and corneal Asphericity after SMILE and LASEK for myopia: a short and long term study

Background

The study compares the wave-front aberration and corneal asphericity from multiple perspectives after Small Incision Lenticule Extraction and Laser-assisted Subepithelial Keratomileusis for mild to moderate myopia in a short and long time period.

Methods

This prospective and comparative study included 32 eyes in the SMILE group, with a mean spherical equivalent (SE) of − 4.1 ± 0.9D and 32 eyes in the LASEK group, with a mean SE of − 3.7 ± 1.0D. Visual acuity, refractive error, wave-front aberration, corneal Q value and corneal refractive power were analyzed pre-, 3 months and 3 years post-operatively.

Results

There was no significant difference in refractive error, wave-front aberration, corneal Q value and corneal refractive power before treatment. Three months postoperative, Q value within 6 mm (SMILE: 0.46 ± 0.27, LASEK: 0.63 ± 0.28, p = 0.02), the relative peripheral corneal power (5-8 mm: p < 0.05), change of higher order aberration (SMILE: 0.10 ± 0.16, LASEK: 0.24 ± 0.20, p = 0.004) and spherical aberration (SA, SMILE: -0.07 ± 0.30, LASEK: -0.41 ± 0.40, p < 0.001) were significantly lower in the SMILE than in LASEK group. The visual acuity, refractive error, coma, peripheral Q value, central corneal power had no significant difference between the two groups. Three years post-operation, the corneal power distribution results and SA were similar to that of 3-month, while the Q value had no significant difference between the two groups.

Conclusion

In the early stage after SMILE, the HOAs was lower, the corneal refractive power from central to periphery was more uniform than after LASEK; and in the long-term run, SMILE still preceded LASEK in the corneal asphericity and aberration.

Electronic supplementary material

The online version of this article (10.1186/s12886-019-1084-3) contains supplementary material, which is available to authorized users.

Long-term Visual and Refractive Outcomes After LASIK for High Myopia and Astigmatism From -8.00 to -14.25 D

PURPOSE

To evaluate outcomes of high myopic LASIK using the MEL 80 excimer laser (Carl Zeiss Meditec, Jena, Germany).

METHODS

Retrospective analysis of 479 consecutive high myopic LASIK procedures (318 patients) using the MEL 80 excimer laser and VisuMax femtosecond laser (Carl Zeiss Meditec) in 77% of cases or zero compression Hansatome microkeratome (Bausch & Lomb, Rochester, NY) in 23% of cases. Inclusion criteria were preoperative spherical equivalent refraction (SEQ) of between -8.00 and -14.25 diopters (D) and corrected distance visual acuity (CDVA) of 20/20 or better. Patients were observed for a minimum of 1 year. Flap thickness was between 80 and 160 µm and optical zone was between 5.75 and 6.50 mm. Standard outcomes analysis was performed.

RESULTS

Mean attempted SEQ was -9.39 ± 1.22 D (range: -8.00 to -14.18 D) and mean cylinder was -1.03 ± 0.84 D (range: 0.00 to -4.50 D). Mean age was 37 ± 9 years (range: 21 to 60 years) with 54% female patients. Postoperative SEQ was ±0.50 D in 55% and ±1.00 D in 83% of eyes after primary treatment. After re-treatment, 69% of eyes were within ±0.50 D and 95% were within ±1.00 D. UDVA was 20/20 or better in 89% of eyes after final treatment. One line of CDVA was lost in 3% of eyes and no eyes lost two or more lines. Statistically significant increases (P < .001) were measured in contrast sensitivity (CSV-1000) at 12 and 18 cycles per degree.

CONCLUSIONS

The MEL 80 excimer laser was found to achieve high efficacy and safety for treatment of high myopia between -8.00 and -14.25 D and up to -4.50 D of cylinder. [J Refract Surg. 2016;32(5):290-297.].

Repeatability of layered corneal pachymetry with the artemis very high-frequency digital ultrasound arc-scanner

PURPOSE

To assess the three-dimensional repeatability of thickness measurements for epithelium, stroma, cornea, flap, and residual stromal bed using the Artemis very high-frequency (VHF) digital ultrasound arc-scanner (ArcScan Inc).

METHODS

Five consecutive measurements were obtained for 10 eyes of 10 patients 1 year after LASIK using the Artemis VHF digital ultrasound arc-scanner across the central 10-mm diameter of the cornea. Repeatability analysis was performed for thickness measurements for each corneal layer-epithelium, stroma, cornea, flap, and residual stromal bed. The standard deviation of repeated measurements (point-repeatability) was calculated for each measurement location in 0.1-mm steps for the 10×10-mm matrix. The pooled standard deviation of the point-repeatability for each measurement location within the central 1-, 2-, and 3-mm radius was calculated (region-repeatability). The corneal thickness of the baseline scan set was compared to that of subsequent scan sets within the same session and plotted over time to assess any possible hydration effects of the immersion technique.

RESULTS

The repeatability at the corneal vertex was 0.58 μm for epithelium, 1.78 μm for stroma, 1.68 μm for cornea, 1.68 μm for flap, and 2.27 μm for residual stromal bed. The region-repeatability within the central 1-mm radius was 1.01 μm for epithelium, 3.44 μm for stroma, 3.35 μm for cornea, 2.81 μm for flap, and 3.97 μm for residual stromal bed. The mean difference in corneal thickness from the baseline value was within 1.25 μm for each of the subsequent four scan sets over a 5-minute immersion period.

CONCLUSIONS

Layered pachymetry of the epithelium, stroma, cornea, flap, and residual stromal bed showed high repeatability with the Artemis VHF digital ultrasound arc-scanner. The high repeatability validates the use of the Artemis for in vivo layered pachymetry.