Comparison of intraoperative subtraction pachymetry and postoperative anterior segment optical coherence tomography of laser in situ keratomileusis flaps

Accuracy of the Preoperative Predicted Percentage of Tissue Altered Calculation in Refractive Surgery Planning for Myopic LASIK

PURPOSE

To determine the reliability of the percentage of tissue altered (PTA) calculation as part of the planning strategy for myopic laser in situ keratomileusis (LASIK) by comparing the estimated PTA provided by preoperative calculation to the postoperative PTA actually achieved in microkeratome-assisted myopic LASIK.

METHODS

This retrospective study included 3,624 eyes of 3,624 patients who underwent mechanical microkeratome-assisted LASIK surgery for myopic correction. The calculated preoperative PTA values based on the planned flap thickness and ablation depth were compared with the actual achieved postoperative PTA using the difference of corneal central thickness postoperatively for assessing the achieved ablation depth and the intraoperative ultrasound-assisted flap thickness measurement. Regression analysis was performed to reveal preoperative parameters that might influence PTA calculation accuracy.

RESULTS

The mean difference between the estimated and achieved PTA was 0.451 ± 3.45% (P < .001) (95% CI: 0.3708 to 0.5322) with a preoperative and postoperative mean PTA of 31.07 ± 4.07% and 31.52 ± 5.78%, respectively. The differences between the achieved and planned maximum ablation depth and flap thickness were 4.32 ± 13.70 µm (P < .001) and -1.61 ± 13.66 µm (P < .001), respectively.

CONCLUSIONS

Although a statistically significant difference was found between the preoperative calculated PTA and actually achieved PTA, the difference in PTA value (less than 1%) was clinically non-significant and indicated a highly reliable metric for preoperative refractive surgery planning. [J Refract Surg. 2022;38(7):422-427.].

Direct and Indirect Flap Measurements in Femtosecond Laser-Assisted In Situ Keratomileusis

PURPOSE

To compare direct and indirect LASIK flap thickness measurements using ultrasound and Scheimpflug technology.

METHODS

Eighty-two eyes treated with laser-assisted in situ keratomileusis refractive surgery using a femtosecond laser (IntraLase FS150) were prospectively included in the study. Flap thickness was set to 115 μm. Corneal flap thickness was measured using the direct method-ie, ultrasound pachymetry immediately after flap construction in the presence of cavitation bubbles-and indirect methods, with subtraction of intraoperative post-lift corneal thickness measured using ultrasound pachymetry (intrastroma) from preoperative central corneal thickness using ultrasound (Indirect-US) or Scheimpflug thinnest pachymetry (Indirect-Scheimpflug).

RESULTS

Mean flap thickness was overestimated using the indirect methods, Indirect-US and Indirect-Scheimpflug (122.6 ± 24.5 μm and 128.1 ± 26.1 μm, respectively; P < 0.0060 and P < 0.0001, respectively). There were no significant correlations between the direct and indirect methods. Indirect-Scheimpflug was significantly higher (P = 0.0122) than Indirect-US. The closest average flap thickness compared with the set parameter of 115 μm was that of the direct method (115.6 ± 8.6 μm; 95% confidence interval: -1.3 to 2.5; P = 0.5163). The direct method provided the lowest SD of all groups (SD: 8.64).

CONCLUSIONS

The direct method of flap thickness measurement was the most comparable to the set parameter compared with the indirect subtraction methods. Additional studies are needed to determine which method allows for the most accurate measurement of flap thickness.

Comparison of femtosecond and excimer laser platforms available for corneal refractive surgery

PURPOSE OF REVIEW

The evolution of laser technology has left today's refractive surgeon with a choice between multiple laser platforms. The purpose of this review is to compare currently available femtosecond and excimer laser platforms, providing a summary of current evidence.

RECENT FINDINGS

Femtosecond lasers create LASIK flaps with better accuracy, uniformity, and predictability than mechanical microkeratomes. Newer higher-frequency femtosecond platforms elicit less inflammation, producing better visual outcomes. SMILE achieved similar safety, efficacy, and predictability as LASIK with greater preservation of corneal nerves and biomechanical strength. The emergence of wavefront technology has resulted in improved excimer laser treatments. Comparisons of wavefront-guided and wavefront-optimized treatments suggest that there is an advantage to using wavefront-guided platforms in terms of visual acuity and quality of vision. Topography-guided ablations are another well tolerated and effective option, especially in eyes with highly irregular corneas.

SUMMARY

Advances in femtosecond and excimer laser technology have not only improved the safety and efficacy of refractive procedures, but have also led to the development of promising new treatment modalities, such as SMILE and the use of wavefront-guided and topography-guided ablation. Future studies and continued technological progress will help to better define the optimal use of these treatment platforms.

Corneal Epithelial Thickness Measured by Manual Electronic Caliper Spectral Domain Optical Coherence Tomography: Distributions and Demographic Correlations in Preoperative Refractive Surgery Patients

PURPOSE

The aim of this study was to report the distributions and demographic correlations of corneal epithelial thickness measured by manual electronic caliper spectral domain optical coherence tomography in preoperative refractive surgery patients.

DESIGN

This was a retrospective review.

METHODS

The charts of 218 consecutive patients (413 eyes) who presented for refractive surgery evaluation from April 2013 through September 2013 were retrospectively reviewed.

RESULTS

The mean corneal epithelial thickness was 51.0 μm with a range of 43 to 61 μm. Corneal epithelial thickness was significantly correlated with sex (P < 0.0001), corneal keratometry (P = 0.01), and underlying corneal thickness excluding the epithelium (P = 0.0268). No significant associations were identified in which corneal epithelial thickness correlated with either age (P = 0.0760) or existing refractive status of the eye (P = 0.5135).

CONCLUSIONS

Corneal epithelial thickness measured by manual electronic caliper optical coherence tomography in preoperative refractive surgery patients is comparable with the findings for the general population using other measurement techniques, the awareness of which may be useful in the preoperative assessment of these patients.

A comparison of LASIK flap thickness and morphology between the Intralase 60- and 150-kHz femtosecond lasers

PURPOSE

To prospectively compare the achieved thickness and consistency of LASIK flaps created with a 60- and 150-kHz femtosecond laser.

METHODS

One hundred twenty eyes of 60 patients with myopia were treated with LASIK. One eye had flap created by the Intralase FS 60-kHz femtosecond laser (Abbott Medical Optics, Abbott Park, IL) and the fellow eye was treated with the IntraLase iFS 150-kHz femtosecond laser (Abbott Medical Optics). Eyes were randomized according to ocular dominance. Flap morphology and measurements were taken with anterior segment optical coherence tomography (AS-OCT) at the 1-month postoperative visit.

RESULTS

AS-OCT showed similar regular planar morphologies in both groups. The mean thickness of the flaps in the FS 60 group was significantly higher than that of the iFS 150 group (105.4 ± 3.9 μm vs 103.9 ± 4.8 μm, P = .009). The mean deviation from targeted flap thickness was not significantly different between the two groups.

CONCLUSIONS

The Intralase iFS 150-kHz femtosecond laser creates flaps of similar thickness and uniformity to the Intralase FS 60-kHz femtosecond laser.

Wavefront-guided versus wavefront-optimized laser in situ keratomileusis for patients with myopia: a prospective randomized contralateral eye study

PURPOSE

To compare the clinical outcomes of wavefront-guided and wavefront-optimized laser in situ keratomileusis (LASIK).

DESIGN

Prospective, randomized, fellow-eye-controlled study.

METHODS

The setting was a single academic institution. The study population included 110 eyes of 55 patients with myopia with and without astigmatism. One eye of each patient was randomized to undergo wavefront-guided LASIK by the AMO Visx CustomVue S4 IR excimer laser system; the fellow eye received wavefront-optimized LASIK by the Alcon Allegretto Wave Eye-Q 400 Hz excimer laser system. Corneal flaps were constructed using the Intralase FS 60 Hz femtosecond laser. Patients were followed at postoperative months 1, 3, 6, and 12. The study's main outcome measures were uncorrected visual acuity, stability of refractive correction, contrast sensitivity, and wavefront aberrometry.

RESULTS

After 12 months, LASIK eyes had achieved visual acuity of 20/12.5 or better (30 eyes, 56%) in the wavefront-guided group compared to those receiving wavefront-optimized treatment (22 eyes, 41%) (P = 0.016). Average spherical equivalent refractions were -0.13 ± 0.46 diopters in wavefront-guided eyes whereas in wavefront-optimized eyes the refractions were -0.41 ± 0.38 diopters at 12 months. Wavefront-guided eyes also achieved better best-corrected visual acuity at both the 5% and 25% contrast levels (P = 0.022 and P = 0.004, respectively). There were no differences in levels of residual astigmatism (P = 0.798) or in higher order aberrations (P = 0.869).

CONCLUSIONS

Both wavefront-guided and wavefront-optimized treatments are able to correct myopia safely and effectively in eyes with and without astigmatism. However, wavefront-guided treatment platforms appear to offer significant advantages in terms of residual refractive error, uncorrected distance acuity and contrast sensitivity.

Prospective contralateral eye study to compare 80- and 120-μm flap LASIK using the VisuMax femtosecond laser

PURPOSE

To compare visual outcomes and flap stability of LASIK with ultrathin 80- and 120-μm flaps created with a VisuMax femtosecond laser (Carl Zeiss Meditec, Jena, Germany) for moderate to high myopia and to evaluate the effect of corneal flap thickness on outcomes.

METHODS

In a prospective contralateral eye study, 36 consecutive patients (72 eyes) underwent bilateral LASIK for myopia ranging from -2.00 to -10.00 diopters using the VisuMax femtosecond laser and MEL-80 excimer laser (Carl Zeiss Meditec). One eye of each patient was randomized to have the 80-μm flap and the other to the 120-μm flap created with 200-kHz VisuMax femtosecond laser. Preoperative and postoperative tests included visual acuity, manifest refraction, contrast sensitivity, and flap thickness measured by anterior segment optical coherence tomography. Main outcomes and complications were checked at postoperative 1 week and 1, 3, and 6 months.

RESULTS

There were no differences in visual outcome, residual refractive error, or contrast sensitivity between groups during follow-up, except for better uncorrected visual acuity at postoperative 1 day in the 120-μm group. Mean standard deviations of measured flap thickness during follow-up ranged from 3.16 to 3.80 μm in both groups. Opaque bubble layer, a unique complication in femtosecond LASIK, was more frequent in the 80-μm group (7 of 36: 19%) than in the 120-μm flap group (3 of 36: 8%) without a statistically significant difference (P = .301) and was related to thicker central cornea and steeper keratometric value, although it did not influence clinical results. Comparison of the intended versus achieved correction showed no significant differences between groups.

CONCLUSIONS

LASIK using the VisuMax femtosecond laser supplied good clinical results and flap reproducibility in both the 80- and 120-μm flap groups. Patients with relatively thin cornea may benefit from 80-μm flap LASIK.

The morphology of corneal cap and its relation to refractive outcomes in femtosecond laser small incision lenticule extraction (SMILE) with anterior segment optical coherence tomography observation

Purpose

To investigate the morphology of corneal caps in femtosecond laser small incision lenticule extraction (SMILE) and its relation to the refractive outcomes.

Methods

A prospective study of fifty-four corneal caps created with VisuMax femtosecond laser were examined using an Fourier-domain optical coherence tomography at 1 day, 1 week, 1 month and 6 months after SMILE. The cap thickness at nine points on each of the four meridians (0°, 45°, 90°, 135°) and the diameter were measured. Cap morphology, changes over time and its correlation with refractive outcomes were assessed.

Results

The mean achieved central cap thickness were (108.74±5.06) µm at 6 months and (107.32±4.81 ) µm at 1 month postoperatively, significantly thinner than that at 1 day (110.81±7.95) µm and 1 week (109.58±7.48 ) µm (P<0.05). The mean diameter on 0° meridian was (7.61±0.07) mm, significantly larger than that on 90° meridian (7.57±0.06) mm (P = 0.001). Cap morphology showed good regularity, except that the differences of points in two pairs were significant at 1 day postoperatively. The uniformity was consistent over time and the central cap thickness was thinner than those in the paracentral and peripheral areas. The refractive outcomes stabilized within 1 month. Uncorrected distance visual acuity (UDVA) was correlated to the central cap thickness at 1 day and 1 week (both r_s = 0.33, p<0.05). The uniformity index was correlated with UDVA (r_s = 0.34, p<0.05) and corrected distance visual acuity (r_s = 0.32, p<0.05) at 1 week postoperatively.

Conclusions

Corneal caps of SMILE are predictable with good reproducibility, regularity and uniformity. Cap morphology might have a mild effect on refractive outcomes in the early stage. Further study should focus on the impact on the visual quality.

Corneal stromal ablation with femtosecond ultraviolet pulses in rabbits

PURPOSE

To determine the effectiveness of femtosecond ultraviolet (UV) pulses in ablating corneal stroma in a rabbit model and to compare the healing response between eyes treated with femtosecond UV pulses and eyes treated with standard excimer photorefractive keratectomy.

SETTING

Laser Research Center, Vilnius University, Vilnius, Lithuania.

DESIGN

Experimental study.

METHODS

Myopic photoablation using a femtosecond UV solid-state laser system was applied to corneas of pigmented rabbits. Experiments in 16 eyes were performed for optimization of the laser system parameters (fluence, spot size, pulse repetition rate) and calibration of ablation rate. In 7 rabbits, deep femtosecond UV ablation (∼130 μm) in 1 eye and shallow ablation (∼30 μm) in the contralateral eye were performed. Nine rabbits received an approximately 30 μm ablation with femtosecond UV pulses in 1 eye and with a conventional excimer system in the contralateral eye. Two eyes were used as controls. The ablation process and surface-temperature dynamics were monitored and recorded. Surface quality and haze development were evaluated. Rabbits were humanely killed 0 to 6 months after surgery, and eyes were enucleated for histological examination.

RESULTS

Rabbit corneas ablated with femtosecond UV pulses or excimer laser radiation were similar in terms of the corneal wound-healing process, surface quality, and histology.

CONCLUSIONS

The experiments indicate the feasibility of clinical application of femtosecond UV lasers for stromal ablation. The ability to switch between laser harmonics allows fast changeover from infrared to the UV mode, implying that a wide range of ophthalmic procedures can be performed using a single solid-state laser device.

FINANCIAL DISCLOSURE

Ms. Gabryte and Mr. Danielius are paid employees of Light Conversion Ltd. Mr. Danielius is a shareholder of Light Conversion Ltd. No other author has a financial or proprietary interest in any material or method mentioned.

Role of optical coherence tomography on corneal surface laser ablation

This paper focuses on reviewing the roles of optical coherence tomography (OCT) on corneal surface laser ablation procedures. OCT is an optical imaging modality that uses low-coherence interferometry to provide noninvasive cross-sectional imaging of tissue microstructure in vivo. There are two types of OCTs, each with transverse and axial spatial resolutions of a few micrometers: the time-domain and the fourier-domain OCTs. Both have been increasingly used by refractive surgeons and have specific advantages. Which of the current imaging instruments is a better choice depends on the specific application. In laser in situ keratomileusis (LASIK) and in excimer laser phototherapeutic keratectomy (PTK), OCT can be used to assess corneal characteristics and guide treatment decisions. OCT accurately measures central corneal thickness, evaluates the regularity of LASIK flaps, and quantifies flap and residual stromal bed thickness. When evaluating the ablation depth accuracy by subtracting preoperative from postoperative measurements, OCT pachymetry correlates well with laser ablation settings. In addition, OCT can be used to provide precise information on the morphology and depth of corneal pathologic abnormalities, such as corneal degenerations, dystrophies, and opacities, correlating with histopathologic findings.

Comparisons of morphologic characteristics between thin-flap LASIK and SBK

AIM

To compare the morphologic characteristics between thin-flap laser in situ keratomileusis (LASIK) and Sub-Bowman keratomileusis (SBK), and to evaluate the uniformity of flap and to explore the correlative factors of corneal flap thickness.

METHODS

A prospective, randomized, comparative clinical study was performed in Department of Ophthalmology, West Hospital of China, Chengdu, Sichuan Province, China. Totally 59 patients 114 eyes underwent LASIK or SBK to correct myopia, 29 patients 57 eyes underwent SBK, 30 patients 57 eyes underwent LASIK. Anterior optical coherence tomography(OCT) was used to measure corneal flap thickness in all the patients 1 week after surgeries, 16 positions were set to be measured in each eye. Comparisons of flap thickness in each group and between 2 groups were evaluated. Correlative factors of flap thickness were evaluated.

RESULTS

Coefficient of variation (CV, s/×%) in SBK group were lesser than that in LASIK group, t test showed there was significant statistical difference between 2 groups(P=0.000). Comparisons of the difference of 2 paired positions(temporal-nasal; superior-inferior) showed there were no significant differences in each group, but between 2 groups, there were statistical significance of value of difference (D-value) of superior and inferior positions between SBK and LASIK group(P=0.036). Linear regression analysis of correlative factors of flap thickness showed there were no statistic significances related to central corneal thickness(CCT)(P=0.060, t=1.921) and corneal curvature(P=0.083, t=1.766).

CONCLUSION

SBK is better than LASIK in creating much uniform corneal flap. There was no evidence showing correlations between flap thickness and CCT or corneal curvature.

Corneal flap morphological analysis using anterior segment optical coherence tomography in laser in situ keratomileusis with femtosecond lasers versus mechanical microkeratome

AIM

To assess and compare the flap morphology using anterior segment optical coherence tomography (AS-OCT) in laser in situ keratomileusis (LASIK) with Femto LDV femtosecond lasers versus Hansatome mechanical Microkeratome.

METHODS

AS-OCT (Visante) was used to compare 1 month postoperatively the morphology of the flaps created with Femto LDV femtosecond lasers or Hansatome Microkeratome. The intended flap thickness was 110µm and 160µm respectively. The thickness of twenty-five points across each flap, which were 0mm, 1.5mm, 2.5mm, and 3.5mm to the corneal vertex on the horizontal, vertical, 45° and 135° meridian respectively, was evaluated.

RESULTS

One month postoperative, the central flap thickness in the Femto LDV group was 107.43±4.70µm, while 125.90±17.50µm in the Hansatome group. The difference between the actual and the expected flap thickness was 5.61±3.84µm and 31.52±12.27µm, respectively. The Hansatome group had presented a statistically significant result (P<0.001). Flap morphology showed a more regular planar shape in the Femto LDV group and a meniscus shape in the Hansatome group.

CONCLUSION

AS-OCT is a direct and fast procedure to assess the flap morphology. The morphology by AS-OCT showed that the flaps created with Femto LDV femtosecond laser were more accurate and regular than the flaps created with Hansatome microkeratome.