The predictability of corneal flap thickness and tissue laser ablation in laser in situ keratomileusis

The Accuracy of Flap Thickness and Diameter in LASIK Using a Femtosecond Laser

Purpose

To evaluate the resultant thickness of the WaveLight FS200 (Alcon Vision, LLC) created LASIK flap compared to intended thickness at the surgical visit, using the WaveLight EX500 (Alcon Vision, LLC) pachymeter and the resultant flap diameter compared to intended diameter.

Methods

This single arm, prospective, single surgeon study assessed the accuracy of the intended flap thickness and diameter, after successful bilateral LASIK surgery. The WaveLight FS200 femtosecond laser was used to create all flaps with an intended thickness of 120 μm. Flap thickness was calculated by subtracting the stromal bed thickness after flap lift from the preoperative corneal thickness using the WaveLight EX500 on-board optical pachymeter. Flap diameter was determined using digital analysis.

Results

A total of 58 subjects (116 eyes) completed the study. The calculated mean flap thickness was 120.6 ± 9.0 μm (range 102 to 143 μm) using the EX500 pre- and post-flap pachymetry measurements. There was no statistically significant difference between the planned and achieved flap thickness (p > 0.05). The mean difference in flap diameter between planned and actual was 0.02 ± 0.05 mm. Corneal thickness measured by Pentacam at up to 2 months preoperatively versus EX500 just prior to surgery was similar, with EX500 measuring 2 μm less on average than the Pentacam.

Conclusion

The results suggest that the WaveLight FS200 laser is reliable for LASIK flap thickness and diameter and accurately created flaps at the intended thickness and the intended diameter.

Corneal stromal dehydration and optimal stromal exposure time during corneal refractive surgery measured using a three-dimensional optical profiler

PURPOSE

To quantitatively analyze human corneal stromal dehydration and estimate proper corneal stromal exposure time during corneal refractive surgery.

METHODS

The central thickness changes over time in 34 pieces of human corneal tissue were measured using a white light interferometer. The corneal stromal tissue was obtained by femtosecond laser small incision lenticule extraction. The thickness-time dehydration fitting curves were drawn, and the determination coefficient R² was calculated. The differences in the fitting curve equation coefficients were compared between the thin and thick lenticule groups. The optimal stromal exposure time was calculated under various conditions, including different optical zones and allowable refractive errors.

RESULTS

A water loss variation model was successfully established. Linear and quadratic fitting curves were drawn, and the determination coefficient R² values were significantly close to 1. The average values of R² for quadratic curves and linear phases 1, 2, and 3 were 0.998 ± 0.002, 0.995 ± 0.007, 0.996 ± 0.003, and 0.984 ± 0.035, respectively. The optimal stromal exposure time varied under different optical zones and allowable diopter error conditions. Taking the allowable error of 0.50 D and the optical zone size of 6.5 mm as an example, the optimal time was approximately 24 s.

CONCLUSIONS

The dehydration rate of the human corneal stroma is nonlinear, and the quadratic stromal thickness-time dehydration fitting curve is more in line with the actual water loss trend. The length of the stroma exposure time may affect the postoperative refractive accuracy after corneal refractive surgery.

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.

Very late-onset flap margin corneal ulcer following laser in situ keratomileusis

PURPOSE

To report and characterize cases of very late onset (5 years or more after surgery) flap margin corneal ulcers after laser in situ keratomileusis (LASIK) procedure.

METHODS

A retrospective case series of consecutive patients who were diagnosed with very late flap margin corneal ulcers following LASIK, between January 2014 and July 2017. All patients were treated with topical antibiotics and were followed up until complete resolution.

RESULTS

A total of eight patients, with a mean age of 46.5 ± 11 years, (range 31-64 years), were included in this study. All patients underwent uneventful myopic LASIK 13.3 ± 3 (range 10-20) years before presentation. Patients best corrected visual acuity (BCVA) at presentation was 0.20 ± 0.15 logMAR compared to a final BCVA of 0.10 ± 0.10 logMAR (p = 0.28). The ulcer was located in the bottom two clock hours of the flap margin (5-7 o'clock) in six (75%) patients and superior (11 o'clock) in the remaining two patients (p = 0.048). Seven patients (87.5%) suffered from blepharitis, and only one did not.

CONCLUSIONS

LASIK may be associated with an increased risk of late-onset corneal ulcer occurring years after the procedure. Instability of the flap margin, blepharitis and dry eye are possible causes of epithelial disturbance and may account for this complication.

Predictability of the Achieved Lenticule Thickness in Small Incision Lenticule Extraction for Myopia Correction

PURPOSE

To investigate the predictability of the achieved lenticule thickness in small incision lenticule extraction (SMILE) for the correction of myopia.

METHODS

In this prospective study, a total of 80 eyes of 45 patients who underwent the SMILE procedure were included. A Scheimpflug camera was used to measure the preoperative and postoperative corneal topography. Achieved lenticule thicknesses at the pupil center and corneal vertex were calculated by subtracting the postoperative corneal thickness at the pupil center and corneal vertex from preoperative corresponding values. Predictability between the achieved and VisuMax readout at different locations (the pupil center and corneal vertex) and postoperative visits (postoperative day 1 and 6 months) were evaluated using a general linear model.

RESULTS

Achieved lenticule thickness at the pupil center and corneal vertex showed good correlation at both postoperative day 1 (pupil center: r=0.914, corneal vertex: r=0.903) and 6 months (pupil center: r=0.886, corneal vertex: r=0.866). The achieved lenticule thickness at postoperative day 1 was 8.5-μm thinner at the pupil center and 9.6-μm thinner at the corneal vertex than the VisuMax readout.

CONCLUSIONS

This study suggests that the predictability between the achieved and VisuMax readout is favorable. The lenticule thickness at the pupil center is closer to the VisuMax readout than corneal vertex.

Accuracy of Alcon WaveLight® EX500 optical pachymetry during LASIK

Purpose

To study the accuracy and reliability of optical pachymetry using the Alcon WaveLight EX500 during laser-assisted in situ keratomileusis (LASIK).

Materials and methods

This was a retrospective chart review of 90 eyes from 45 patients who had undergone LASIK (mean age 35.2±8.2 years; 19 males, 26 females). The WaveLight FS200 femtosecond laser was programmed to cut LASIK flaps at a desired depth of 120 μm. Optical low-coherence reflectometry (WaveLight EX500) was used to measure central corneal thickness prior to lifting the flap, and the residual stromal bed immediately after excimer ablation. Flap thickness (FT) was calculated using simple subtraction. Optical coherence tomography (OCT) was used to measure central corneal thickness, flap thickness, and residual stromal bed in the postoperative period and the results compared to intraoperative measurements.

Results

Mean programmed FS200 FT was 119 μm. Mean FT using EX500 optical pachymetry was 109 μm. The difference between FS200- programmed and EX500-measured FT was 9 μm (P<0.001). There was also a significant difference between the EX500 and OCT FT (109 μm vs 119 μm, respectively; P<0.001).

Conclusion

FT values calculated using intraoperative EX500 optical pachymetry were significantly lower than programmed FS200 values or OCT measurements.

The Impact of Flap Creation Methods for Sub-Bowman's Keratomileusis (SBK) on the Central Thickness of Bowman's Layer

Purpose

To determine the impact of flap creation methods for sub-Bowman’s keratomileusis (SBK) on central Bowman’s layer thickness.

Methods

SBK flaps were made by Moria microkeratome for 20 subjects and by femtosecond (FEMTO) laser for 21 subjects. Corneal sublayer thicknesses were measured by ultra-high resolution optical coherence tomography before SBK and at 1 day, 1 week, 2 weeks, and 1 month afterwards. Each subject was imaged twice on each visit. Thicknesses of central epithelium, Bowman’s layer, flap, and total cornea were calculated using a custom-made automated image processing algorithm. The repeatability of sublayer thickness measurements was tested by the intraclass correlation coefficient (ICC) and by the coefficient of repeatability (CoR) at 1 week post-SBK.

Results

ICCs of the Moria and FEMTO groups were ≥0.959 and ≥0.961 respectively for all sublayer measurements. The segmentation CoRs were less than 6.78% and 5.63% respectively. For both groups, microdistortions were present in the epithelium and Bowman’s layer after SKB. The flap thickness of the Moria group was 9.8 μm (95% confidence interval: 4.8 – 14.8μm) thinner than the FEMTO group one day after SBK (independent samples t-test, P < 0.05). Bowman’s layer became thicker by 1.6 ± 1.1 μm and 1.7 ± 1.6 μm one day post-SBK for the Moria and FEMTO groups (repeated ANOVA, P < 0.05) and then remained stable. Corneal and sublayer thickness were similar between the two groups.

Conclusions

Central Bowman’s layer thickness increased 1 day post-SBK. Flap creation by Moria microkeratome and femtosecond laser did not have significantly different impacts on Bowman’s layer thickness following SBK.

Trial Registration

Chinese Clinical Trial Registry (ChiCTR) NO: ChiCTR-OCH-14004525

Assessment of axial length before and after myopic LASIK with the IOLMaster

PURPOSE

To assess the relationship between theoretical ablation depth and axial length change after LASIK with the IOLMaster (Carl Zeiss Meditec, Dublin, CA).

METHODS

Ninety-nine eyes were examined preoperatively and 1 and 3 months after LASIK.

RESULTS

Mean ± standard deviation (SD) spherical equivalent before LASIK was -4.06 ± 1.91 diopters (D). Mean ± SD ablation depth was 83.13 ± 30.31 μm. Mean ± SD postoperative axial length of 25.11 ± 0.14 mm at 1 month was significantly shorter than mean ± SD preoperative axial length of 25.20 ± 0.14 mm (P < .001) with no subsequent change thereafter (P = .450). An increase in ablation depth of 1 μm led to a decrease in axial length of 0.00118 ± 0.00005 mm. Ablation depth correlated strongly with change in axial length (adjusted R(2) = 0.9039).

CONCLUSIONS

The IOLMaster showed a decrease in axial length after LASIK that correlated well with theoretical ablation depth.

Flap thickness in eyes with ectasia after laser in situ keratomileusis

PURPOSE

To measure central flap thickness in eyes with ectasia after laser in situ keratomileusis (LASIK) and to compare these values with estimated anticipated flap thickness based on mean published values for each device used for flap creation.

SETTING

Emory Vision at Emory University, Atlanta, Georgia, USA.

DESIGN

Retrospective comparative case series, using published data for controls.

METHODS

Confocal microscopic analysis was performed using the Confoscan 3 device to measure central flap thickness in eyes with ectasia after LASIK. Pre-LASIK records were evaluated for information, including basic patient demographics, preoperative corneal topographies, estimated anticipated flap thickness based on the mean average thickness values, and residual stromal bed (RSB) thickness calculations using measured and estimated flap thicknesses.

RESULTS

Fifty eyes of 29 patients were evaluated. The mean measured flap thickness was 138 μm ± 26 (SD) (range 90 to 220 μm). There were no significant differences between measured and estimated flap thicknesses (138 μm versus 135 μm; P=.5) or RSB thickness (329 μm versus 332 μm; P=.7), nor were there differences in flap thickness between eyes developing ectasia with normal corneal topographies and eyes with abnormal corneal topographies. One eye had a measured flap resulting in an unintended RSB thickness less than 250 μm; this eye also had abnormal topography.

CONCLUSIONS

Measured central flap thickness was not thicker than estimated in most eyes developing ectasia after LASIK. Thus, excessively thick flaps do not appear to be a major contributing factor to the pathogenesis of ectasia after LASIK.

FINANCIAL DISCLOSURE

No author has a financial or proprietary interest in any material or method mentioned.

Laser in situ keratomileusis flap-thickness predictability with a pendular microkeratome

PURPOSE

To assess flap-thickness predictability with a pendular microkeratome (130 μm head).

SETTING

Eye Institute of Thrace, Democritus University of Thrace, Alexandroupolis, Greece.

DESIGN

Clinical trials.

METHODS

The study comprised 263 eyes (132 patients). Laser in situ keratomileusis was performed using the 130 μm head of the Carriazo pendular microkeratome; right eyes were treated first. Ultrasound pachymetry and topography were used for central corneal thickness (CCT) and keratometry (K) measurements. Evaluation included flap thickness, flap diameter, and flap shape.

RESULTS

The mean flap thickness was 125 μm ± 22 (SD) (range 74 to 187 μm) in right eyes and 112 ± 21 μm (range 61 to 190 μm) in left eyes. Flap thickness was significantly correlated with preoperative CCT (r = 0.271; P<.001) but not with K values or the manifest refraction spherical equivalent (P>.15). Right eyes had thicker flaps than left eyes (P<.001); both were significantly below the 130 μm head thickness (mean flap thickness 119.2 ± 22.8 μm; P<.001). The mean achieved flap diameter was 9.2 mm using the 9.0 ring and 9.8 mm using the 10.0 mm ring. Flap-thickness stabilization and convergence between right eyes and left eyes occurred after 100 consecutive flap cuts.

CONCLUSIONS

Flap-thickness predictability was influenced by preoperative CCT only. All cuts were significantly thinner than the head thickness regardless of the suction ring size. Second surgical eyes had thinner flaps, possibly from blade deterioration from the first cut. Approximately 100 flaps were required as a learning curve.

Comparison of the predictability, uniformity and stability of a laser in situ keratomileusis corneal flap created with a VisuMax femtosecond laser or a Moria microkeratome

This prospective study compared the predictability, uniformity and stability of laser in situ keratomileusis corneal flap thickness created by a femtosecond laser or a classic microkeratome. Twenty-five femtosecond laser (VisuMax, Carl Zeiss Meditec) flaps and 38 microkeratome (Moria M3) flaps were measured using anterior segment optical coherence tomography at 1 week, 1 month and 6 months postoperatively. Flap thickness at seven points on each of the four meridians was calculated. At 6 months, VisuMax flaps showed better prediction than Moria flaps for central flap thickness. The standard deviation within individual flaps was smaller for VisuMax flaps and their index of symmetry was better. The mean thicknesses among the four eccentricities in the VisuMax flaps were the same, while Moria flaps were thicker at the 3-mm radius compared with the centre. The VisuMax femtosecond laser created corneal flaps with better predictability and uniformity, and similar reproducibility and stability, compared with the microkeratome.

Repeatability of laser in situ keratomileusis flap thickness measurement by Fourier-domain optical coherence tomography

PURPOSE

To evaluate the repeatability of Fourier-domain optical coherence tomography (OCT) measurements of the thickness of femtosecond laser-created laser in situ keratomileusis (LASIK) flaps.

SETTING

Doheny Eye Institute, University of Southern California, Los Angeles, California, USA.

DESIGN

Case series, evaluation of diagnostic technology.

METHODS

In this consecutive series, Fourier-domain OCT (RTVue) was used to measure flap thickness 1 week after LASIK. Flaps were created with a Pulsion 60 kHz femtosecond laser programmed for 110 μm flap thickness. Each eye was scanned 2 times with a radial pachymetry pattern and 1 time with a horizontal line scan. Flap thicknesses were measured at 6 positions across the corneal flap (ie, ± 0.5 mm, ± 1.5 mm, and ± 2.5 mm from the center on the horizontal and vertical meridians). The within-grader flap thickness repeatability and between-grader reproducibility were calculated by pooled standard deviations (SDs).

RESULTS

Twenty-one eyes were measured. The mean flap thickness measurements were highly predictable at all positions. Thickness SDs varied from 5.3 to 9.5 μm and uniformity, from 121.7 to 126.5 μm. The within-grader repeatability was 3.3 to 6.4 μm based on the same image measured at different times and 4.7 to 7.4 μm for different images. The between-grader reproducibility was 4.0 to 9.0 μm. There was no statistically significant asymmetry between the nasal side and the temporal side, the superior side and the inferior side, or the pericentral area and the central area of the corneal flap.

CONCLUSIONS

The femtosecond laser created LASIK flaps with uniform and predictable thicknesses. Fourier-domain OCT gave highly repeatable flap-thickness measurements.

FINANCIAL DISCLOSURE

Drs. Rosas Salaroli, Zhang, Branco Ramos, and Allemann have no financial or proprietary interest in any material or method mentioned. Additional disclosures are found in the footnotes.

Corneal flap assessment with Rondo microkeratome in laser in situ keratomileusis

PURPOSE

To assess the accuracy of flap thickness in laser in situ keratomileusis (LASIK) with Rondo, Wavelight AG microkeratome and to examine factors that can influence flap thickness (FT).

SETTING

The study took place at the Laser & Ophthalmos Eye Clinic, Thessaloniki, Greece.

MATERIAL AND METHODS

Three hundred and sixty eyes from 180 patients underwent LASIK with Rondo microkeratome. Three surgeons (A, B and C) performed all surgeries with no previous experience of Rondo microkeratome. All patients were treated with the 130 μm plate. Central corneal thickness (CCT) and stromal bed thickness were measured by Scheimpflug and ultrasound pachymetry. Right eye (OD) was treated first.

RESULTS

Mean FT for OD: 120 ± 19 μm (range 69-158 μm); for left eye (OS): 106 ± 17 μm (range 70-147 μm). Flaps in OD were significantly thicker than in OS (p < 0.001). FT was significantly correlated to the keratometric reading (K): r = 0.121; p = 0.02. No correlation was found between FT and CCT or between FT and the attempted refractive correction (SE) (p > 0.14). Mean FT was significantly lower than the manufacturer's 130 μm specification (Mean FT = 113 ± 19 μm; p < 0.001). FT between surgeons A, B and C was significantly different (analysis of variance between surgeons; p < 0.001). Scheimpflug and ultrasound CCT measurements were significantly correlated (r = 0.921; p < 0.001) with ultrasound measuring an average 4.5 μm higher than Scheimpflug (CCT (Oculyzer) = 553.96 ± 27 μm; CCT (Ultrasound) = 558.45 ± 28 μm). Mean flap diameter was 9.2 ± 0.2 mm.

CONCLUSIONS

FT with Rondo microkeratome was significantly influenced by the mean preoperative K reading. First treated eye was significantly thicker than the fellow left eye, while both were significantly lower than the recommended 130 μm thickness. Gaining basic experience of Rondo microkeratome required an average of 90 flaps/surgeon.

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

PURPOSE

To evaluate the accuracy of the ablation depth readout for the MEL 80 excimer laser (Carl Zeiss Meditec).

METHODS

Artemis 1 very high-frequency digital ultrasound measurements were obtained before and at least 3 months after LASIK in 121 eyes (65 patients). The Artemis-measured ablation depth was calculated as the maximum difference in stromal thickness before and after treatment. Laser in situ keratomileusis was performed using the MEL 80 excimer laser and the Hansatome microkeratome (Bausch & Lomb). The Aberration Smart Ablation profile was used in 56 eyes and the Tissue Saving Ablation profile was used in 65 eyes. All ablations were centered on the corneal vertex. Comparative statistics and linear regression analysis were performed between the laser readout ablation depth and Artemis-measured ablation depth.

RESULTS

The mean maximum myopic meridian was -6.66±2.40 diopters (D) (range: -1.50 to -10.00 D) for Aberration Smart Ablation-treated eyes and -6.50±2.56 D (range: -1.34 to -11.50 D) for Tissue Saving Ablation-treated eyes. The MEL 80 readout was found to overestimate the Artemis-measured ablation depth by 20±12 μm for Aberration Smart Ablation and by 21±12 μm for Tissue Saving Ablation profiles.

CONCLUSIONS

The accuracy of ablation depth measurement was improved by using the Artemis stromal thickness profile measurements before and after surgery to exclude epithelial changes. The MEL 80 readout was found to overestimate the achieved ablation depth. The linear regression equations could be used by MEL 80 users to adjust the ablation depth for predicted residual stromal thickness calculations without increasing the risk of ectasia due to excessive keratectomy depth as long as a suitable flap thickness bias is included.

Pentacam Scheimpflug evaluation of corneal volume after LASIK

PURPOSE

To assess the change in corneal volume and laser ablation volume over time after LASIK using a rotating Scheimpflug camera.

METHODS

Twenty-six patients (49 eyes) underwent LASIK. Pentacam Scheimpflug measurements were performed pre- and postoperatively at 1, 3, 7, 15, and 30 days. Central corneal thickness, total corneal volume (10-mm diameter), and partial corneal volumes (3, 5, and 7 mm) based on the apex of the cornea, were measured. Main outcome measures were differences between pre- and postoperative volume measurements of total and partial corneal regions, volume changes over time postoperatively, and comparison between laser ablation volume and corneal volume.

RESULTS

A strong linear relationship was noted between spherical equivalent refraction and laser ablation volume (r = -0.931, P < .0001). Furthermore, 30 days after LASIK, a strong linear relationship was noted between planned thickness and achieved thickness (r = 0.9457, P < .001). Corneal volumes for 3-, 5-, and 7-mm diameter regions significantly decreased over time postoperatively (one-way analysis of variance, P < .05).

CONCLUSIONS

Alterations in corneal volume correlated with laser ablation volume when the diameter of the region considered was 3, 5, or 7 mm from the corneal apex.

Microkeratome blades and corneal flap thickness in LASIK

BACKGROUND AND OBJECTIVE

To determine the corneal flap thickness produced by using two different types of blades on the same microkeratome for LASIK, comparing flap thickness in the first and second eyes.

PATIENTS AND METHODS

The corneal flap thickness was measured in two sets of 100 consecutive patients undergoing bilateral LASIK procedures using the Moria M2 microkeratome with a 110 head (Moria, France) and either Moria or CLB blades (Med-Logics Inc., Laguna Hills, CA). Corneal flap thickness was determined by intraoperative pachymetry using an ultrasonic pachymeter.

RESULTS

The mean corneal flap thickness was 138.95 +/- 21.6 microm (range: 75-205 microm) with Moria blades and 115.07 +/- 16.0 microm (range: 70-153 microm) with CLB blades, which is a significant difference (P < .001). The difference in flap thickness between the first and second eyes was not significant with the Moria blades (141.46 +/- 21.46 microm vs 136.45 +/- 21.55 microm, P = .965), but it was when the CLB blades were used (123.21 +/- 12.07 microm vs 106.93 +/- 15.51 microm; P= .013).

CONCLUSION

In LASIK surgery using a Moria M2 microkeratome, blades from different manufacturers may produce significantly different corneal flap thicknesses, as well as differences between the first and second eyes.

Comparison of central corneal thickness as measured by non-contact specular microscopy and ultrasound pachymetry before and post LASIK

PURPOSE

To compare central corneal thickness (CCT) measurements made by ultrasound pachymetry and non-contact specular microscopy in eyes before and post laser in situ keratomileusis (LASIK).

METHODS

Forty eyes of 20 consecutive refractive surgery candidates were included in this study. The CCT in both eyes was measured before and 3 months after LASIK with a non-contact specular microscope and a ultrasound pachymeter.

RESULT

Both before and after LASIK, the difference of the mean CCT between non-contact specular microscopy and ultrasound pachymetry was statistically significant (P < 0.001 before surgery, and P = 0.02 after surgery). The difference between them was -14.41 microm and -3.69 microm, respectively. In both situations, non-contact specular microscopy and ultrasound pachymetry measurements were highly correlated (r = 0.959, before surgery, and r = 0.979 after LASIK). Bland-Altman analysis showed that either before or post LASIK, the 95% limits of agreement have a span of approximately 30 microm. Preoperatively, the mean standard deviation of repeated measurements was 4.26 microm by ultrasound pachymetry, and 4.02 microm by non-contact specular microscopy, whereas it was 3.83 microm by ultrasound pachymetry, and 3.89 microm by non-contact specular microscopy, postoperatively. No statistically significant differences were found between the mean standard deviations by these two devices in both situations (P = 0.561 before surgery, and P = 0.849 after LASIK).

CONCLUSIONS

Both before and after LASIK, non-contact specular microscopy is not in complete agreement with ultrasound pachymetry in measuring CCT. Both devices provided comparable repeatability of measurements.

Risk Assessment for Ectasia after Corneal Refractive Surgery

PURPOSE

To analyze the epidemiologic features of ectasia after excimer laser corneal refractive surgery, to identify risk factors for its development, and to devise a screening strategy to minimize its occurrence.

DESIGN

Retrospective comparative and case-control study.

PARTICIPANTS

All cases of ectasia after excimer laser corneal refractive surgery published in the English language with adequate information available through December 2005, unpublished cases seeking treatment at the authors' institution from 1998 through 2005, and a contemporaneous control group who underwent uneventful LASIK and experienced a normal postoperative course.

METHODS

Evaluation of preoperative characteristics, including patient age, gender, spherical equivalent refraction, pachymetry, and topographic patterns; perioperative characteristics, including type of surgery performed, flap thickness, ablation depth, and residual stromal bed (RSB) thickness; and postoperative characteristics including time to onset of ectasia.

MAIN OUTCOME MEASURES

Development of postoperative corneal ectasia.

RESULTS

There were 171 ectasia cases, including 158 published cases and 13 unpublished cases evaluated at the authors' institution. Ectasia occurred after LASIK in 164 cases (95.9%) and after photorefractive keratectomy (PRK) in 7 cases (4.1%). Compared with controls, more ectasia cases had abnormal preoperative topographies (35.7% vs. 0%; P<1.0x10(-15)), were significantly younger (34.4 vs. 40.0 years; P<1.0x10(-7)), were more myopic (-8.53 vs. -5.09 diopters; P<1.0x10(-7)), had thinner corneas before surgery (521.0 vs. 546.5 microm; P<1.0x10(-7)), and had less RSB thickness (256.3 vs. 317.3 microm; P<1.0x10(-10)). Based on subgroup logistic regression analysis, abnormal topography was the most significant factor that discriminated cases from controls, followed by RSB thickness, age, and preoperative corneal thickness, in that order. A risk factor stratification scale was created, taking all recognized risk factors into account in a weighted fashion. This model had a specificity of 91% and a sensitivity of 96% in this series.

CONCLUSIONS

A quantitative method can be used to identify eyes at risk for developing ectasia after LASIK that, if validated, represents a significant improvement over current screening strategies.

Comparison of corneal flap thickness between primary eyes and fellow eyes using the Zyoptix XP microkeratome

PURPOSE

To evaluate corneal flap thickness created in myopic laser in situ keratomileusis (LASIK) using the Zyoptix XP 120 microkeratome (Bausch & Lomb).

SETTING

University-based eye clinic.

METHODS

Corneal thickness was measured preoperatively and intraoperatively after flap creation in 62 consecutive patients (124 eyes) who had LASIK for the correction of myopia. Corneal flap thickness was calculated by subtracting stromal bed thickness from total corneal thickness. In each patient, both corneas (right followed by left) were cut by 1 the same Zyoptix XP 120 microkeratome blade at the same session.

RESULTS

The mean actual flap thickness was 115.34 microm +/- 16.34 (SD) in right eyes and 104.55 +/- 14.34 mum in left eyes. The mean actual flap thickness in right eyes was not statistically significantly different from the 120 microm proposed by the manufacturer (P = .142); however, the mean actual flap thickness in left eyes was statistically significantly different from the 120 microm (P<.001). The mean flap thickness in the second eye was also statistically significantly thinner than in the first eye (P<.001).

CONCLUSIONS

The corneal flaps were thinner than expected when a Zyoptix XP 120 microkeratome was used in LASIK. Although the first cuts produced thinner flaps, this difference was not statistically significant. However, the second cuts produced significantly thinner flaps.

A Longitudinal Study of LASIK Flap and Stromal Thickness with High-speed Optical Coherence Tomography

OBJECTIVE

To assess corneal anatomic changes after LASIK with a high-speed corneal and anterior segment optical coherence tomography (CAS-OCT) system.

DESIGN

Cross-sectional observational study.

PARTICIPANTS

Fifty-one eyes of 26 healthy persons undergoing LASIK.

METHODS

The CAS-OCT prototype operated at a 1.3-mum wavelength and 2000 axial scans/second. The corneas were scanned with a flap profile pattern (horizontal line, 512 axial scans) and a flap map pattern (4 radials, 256 axial scans each). Both patterns are 8 mm long and are centered on the corneal vertex. LASIK flaps were created using either a mechanical microkeratome (Hansatome; Bausch & Lomb, Inc., Rochester, NY) or a femtosecond laser (Pulsion; IntraLase Corp., Irvine, CA). Intraoperative pachymetry was performed using a 50-MHz ultrasound probe. Three OCT scans were obtained on preoperative and post-LASIK visits up to 6 months. An automated algorithm was developed to process the OCT images and to calculate corneal, flap, and stromal bed thickness profiles and maps. The profiles and maps were divided into central (diameter, <2 mm), pericentral (2-5 mm), and transitional (5-7 mm) zones for analysis.

MAIN OUTCOME MEASURES

Corneal, flap, and stromal bed thicknesses as determined by OCT and ultrasound pachymetry.

RESULTS

The flap interface was best detected in the pericentral zone. One week after surgery, the repeatability of OCT flap and stromal bed thickness measurement was 2 to 7 microm by pooled standard deviation for zones inside a 5-mm diameter. The central flap thickness in 24 Hansatome eyes with a 180-microm setting was 143+/-14 microm by OCT and 131+/-17 microm by ultrasound. In the 8 IntraLase cases with a 120-microm setting, it was 156+/-11 microm by OCT and 160+/-19 microm by ultrasound. Eyes with other settings also were analyzed. There were small systematic changes in flap thickness up to 1 week and bed thickness up to 3 months.

CONCLUSIONS

We have developed a method for using high-speed OCT to measure LASIK flap thickness after surgery. The measurement is noncontact, rapid, and repeatable. Profile and map measurements provide more information than point measurements previously demonstrated. This could be valuable for planning LASIK enhancement and characterizing microkeratome performance.

Femtosekundenlaser für die refraktive Hornhautchirurgie

Modern corneal laser surgery for the correction of optical errors of the eye requires a precise, reliable and reproducible creation of corneal lenticels (flaps). The use of ultra-short laser pulses with pulse durations of a few 100 femtoseconds (10(-13) s) allows for non-thermal cuts of ocular tissue. Mean flap thicknesses as small as 100 microm with a reproducibility of 10 microm (standard deviation) can be created by using mechano-optical adaptations through the eye. Thus, the femtosecond laser can be considered a good alternative approach with a safety in clinical use that is comparable with that of mechanical microkeratomes.

Comparison of residual stromal bed and flap thickness in primary and repeat laser in situ keratomileusis in myopic patients

PURPOSE

To compare the change in residual stromal thickness and flap thickness between primary laser in situ keratomileusis (LASIK) and repeat LASIK in myopic patients.

SETTING

Melbourne Excimer Laser Group, East Melbourne, Australia.

METHODS

This retrospective nonrandomized comparative trial comprised 46 eyes of 34 patients who had repeat LASIK. The thickness of the residual stromal bed was calculated by subtracting the calculated stromal ablation from pachymetry of the stromal bed after cutting the flap in primary treatment and directly measuring during retreatment. The thickness of the LASIK flap in primary and repeat LASIK was calculated by subtracting the central pachymetry of the stromal bed after creating the flap from pachymetry before cutting and lifting the flap, respectively. The main outcome measures were comparison of the residual stromal bed and flap thickness between the primary treatment and the retreatment.

RESULTS

The mean thickness of the calculated residual stromal bed after primary treatment was 329.8 microm +/- 40.8 (SD), and the mean measured residual stromal bed at retreatment was 317.3 +/- 42.8 microm. The mean difference in residual stromal bed thickness was 12.5 +/- 13.0 microm (P<.001). Sixteen eyes (34.7%) had a decrease in bed thickness between 11 microm and 20 microm. The mean flap thickness during primary LASIK and repeat LASIK was 145.2 +/- 17.1 microm and 169 +/- 18.3 microm, respectively. The mean interval between primary treatment and retreatment was 7.4 +/- 4.1 months. The mean change in flap thickness was 23.8 +/- 15.2 microm (P<.001). Fifteen eyes (32%) had an increase in flap thickness between 11 microm and 20 microm. There was a negative correlation between refractive error before primary treatment and the difference in flap thickness. No correlation was found between the difference in flap thickness and the interval between the primary treatment and the repeat treatment.

CONCLUSIONS

Intraoperative pachymetry of the stromal bed during retreatment is strongly recommended as the residual stromal bed and flap thickness changes between primary and retreatment. There is a tendency for the measured stromal bed at retreatment to be thinner than the calculated stromal bed and for the flap to be thicker than previously measured.

Online optical coherence pachymetry to evaluate intraoperative ablation parameters in LASIK

PURPOSE

The ablation of corneal tissue with the excimer laser can be variable and can lead to miscorrections. The purpose of this study was to evaluate intraoperative ablation parameters during laser-assisted in-situ keratomileusis (LASIK) with online optical coherence pachymetry (OCP).

METHODS

In a prospective, nonrandomized, comparative clinical study, the ablation parameters were continuously assessed intraoperatively with online OCP (Heidelberg Engineering, Lübeck, Germany) in 45 myopic and 10 hyperopic LASIK treatments. The central intraoperative ablation values were compared with the calculated values of the excimer laser (ESIRIS, Schwind, Germany) and the postoperative refraction. The ablation process and the ablation rate in mum per layer, time, and dioptric correction were evaluated in myopic corrections.

RESULTS

In myopic LASIK treatments, a linear ablation process was measured with a mean correlation coefficient of -0.968 +/- 0.04. The intraoperative ablation rate was, on average, 0.59 +/- 0.17 microm per layer, 1.45 +/- 0.48 microm per second, and 24.63 +/- 7.81 microm per corrected diopter. These values were 28.7% to 29.6% higher (P < 0.001) than the calculated values. There was a significant correlation (P < 0.001) for the ablation rate per layer (r = 0.823), per second (r = 0.869), and corrected diopter (r = 0.892), but no correlation (r = 0.21, P = 0.239) between the measured linear ablation process and the postoperative refraction. During hyperopic LASIK treatments, without ablation of the corneal center, there was a significant decrease (P = 0.005) of the stromal thickness by 18.34 +/- 14.13 microm, which corresponded to a mean corneal dehydration rate of 0.27 microm per second.

CONCLUSIONS

Online OCP allowed a clinical evaluation of intraoperative ablation parameters in LASIK. Further studies are needed to assess a possible active control of the excimer laser ablation from these continuous values, which could possibly improve current ablation nomograms.

Comparison of Corneal Thickness Measured With Optical Coherence Tomography, Ultrasonic Pachymetry, and a Scanning Slit Method

PURPOSE

To investigate the relationship between 1310-nm optical coherence tomography (OCT), an ultrasound pachymeter, and the Orbscan II (Bausch & Lomb, Rochester, NY) in corneal thickness measurements before and after LASIK.

METHODS

Twenty-eight eyes of 14 patients with a mean age of 39.9 +/- 8.6 years underwent LASIK for correction of myopia with or without astigmatism. Central corneal thickness was measured with real-time 1310-nm OCT and Orbscan preoperatively and 1 day, 1 week, and 1 month postoperatively, and with an ultrasound pachymeter preoperatively and 1 month postoperatively.

RESULTS

Measurements of corneal thickness by all instruments correlated well with one another pre- and postoperatively (r value range: 0.87 to 0.94, P<.05); however, there were significant differences among these methods (ANOVA: P<.005). Baseline readings for ultrasound and Orbscan were similar, as were readings 1 month postoperatively for OCT and Orbscan (P>.05). Thickness measured by OCT was thinner than for other instruments preoperatively (P<.005) and thicker than Orbscan postoperatively (P<.005). Significant corneal thinning (P<.001) was found with all study devices 1 day after laser ablation. Significant corneal thinning (P<.005) was found with OCT and Orbscan 1 week compared to 1 day postoperatively, and thickening (post hoc: P<.001) was evident at 1 month compared to 1 week postoperatively.

CONCLUSIONS

Although the instruments correlated well in measuring corneal thickness after LASIK, there were significant differences. All instruments indicated corneal swelling and recovery during the first postoperative week and thickening at 1 month.

Post-laser in-situ keratomileusis ectasia: current understanding and future directions

PURPOSE OF REVIEW

The aim of this article is to review the causes, risk factors, management, and future research directions for corneal ectasia after laser in situ keratomileusis.

RECENT FINDINGS

Complex corneal biomechanical processes influence the integrity of the normal and postoperative cornea, and developing an understanding of these processes facilitates recognition of risk factors for ectasia after laser in-situ keratomileusis. Currently identified risk factors include keratoconus, high myopia, low residual stromal bed thickness from excessive ablation or thick flap creation, and defined topographic abnormalities such as forme fruste keratoconus and pellucid marginal corneal degeneration. Ectasia can also rarely occur in patients without currently identifiable risk factors, and future identification of at-risk patients may be facilitated by corneal interferometry and corneal hysteresis measurements. Utilization of intraoperative pachymetry measurements at the time of surgery and confocal microscopy prior to enhancement to measure residual stromal bed thickness should avoid unanticipated low residual stromal bed thickness. Management options for ectasia after laser in situ keratomileusis include intraocular pressure reduction, rigid gas permeable contact lenses, and intracorneal ring segments, in addition to corneal transplantation. In the future, collagen cross-linking may reduce corneal steepening and improve refractive error.

SUMMARY

When ectasia develops, early recognition and proper management are essential to prevent progression, to promote visual rehabilitation, and to reduce the need for corneal transplantation for these patients.

The effect of flap thickness on the visual and refractive outcome of myopic laser in situ keratomileusis

AIMS

To study the effect of flap thickness on the visual and refractive outcome of myopic laser in situkeratomileusis (LASIK) surgery.

METHODS

A total of 196 myopic eyes with spherical equivalent (SE) from -2.00 to -5.00 dioptres which underwent LASIK were studied retrospectively. Uncorrected visual acuity (UCVA), best-corrected visual acuity (BCVA) and manifest refraction were measured up to 6 months postoperatively. Logistic and linear regression analyses were used to examine the correlation of flap thickness to visual and refractive outcomes, postoperative complications and enhancement rate.

RESULTS

UCVA at 1 week and 1 month, but not at 1 day and at 6 months (P>0.05), were negatively correlated to flap thickness (P<0.05). BCVA was unrelated to flap thickness (P>0.05). At 1-month post-LASIK sphere and cylinder were not related to flap thickness (P>0.05), but spherical equivalent was negatively correlated (P<0.05). Significant negative correlations with the sphere and SE were noticed at 6 months for the eyes not requiring enhancements (P<0.05). Flap thickness had no significant relationship to postoperative complications (P>0.05) and no significant predictive value on the rate of enhancement procedures or the efficacy, safety and predictability indices (P>0.05).

CONCLUSIONS

In myopic LASIK thinner flaps are associated with faster visual recovery and less myopic SE, but the BCVA and the final UCVA are independent of flap thickness. Postoperative complications are unrelated to flap thickness. Flap thickness does not affect the efficacy, safety and predictability, or the rate of enhancement procedures.

The thin-flap LASIK technique

PURPOSE

To determine if LASIK using a 130-microm microkeratome head (thin-flap LASIK) is as visually effective and safe as when using a 160-microm head.

METHODS

A study was performed comparing postoperative day 1 uncorrected visual acuity (UCVA) and flap complications in eyes undergoing myopic LASIK with a 130-microm head versus a 160-microm head using the BD K-3000 microkeratome (BD Ophthalmic Systems, Waltham, Mass).

RESULTS

The mean preoperative myopia in the 155 eyes of 80 patients in the 130-microm head group was -5.00+/-2.53 diopters (D) compared to -3.78+/-1.73 D in the 279 eyes of 148 patients in the 160-microm head group. The groups were otherwise similar in terms of age, preoperative cylinder, and best spectacle-corrected visual acuity. The geometric mean postoperative day 1 UCVA was 20/25 in the 130-microm head group compared to 20/26 in the 160-microm head group. The only flap complication in the series was a single partial flap in the 160-microm head group.

CONCLUSIONS

LASIK with a 130-microm head (thin-flap LASIK) is as visually effective and safe as when using a 160-microm head with the BD K-3000 microkeratome.

Thin flap laser in situ keratomileusis: flap dimensions with the Moria LSK-One manual microkeratome using the 100-microm head

PURPOSE

To determine the predictability and consistency of corneal flap thickness, flap diameter, and hinge length with the modern 100 microm head of the Moria LSK-One manual microkeratome.

SETTING

Private clinic, office-based practice.

METHODS

Forty-two consecutive eyes with no previous surgery having thin flap laser in situ keratomileusis with the Moria LSK-One manual microkeratome had a new 100 microm (predicted flap thickness) head used for flap creation. Flap thickness was measured intraoperatively by subtraction ultrasound pachymetry (difference between central corneal thickness before flap cutting and residual stromal bed thickness after flap lifting). Vertical flap diameter and nasal hinge length were measured with calipers.

RESULTS

Mean flap thickness was 107 microm +/- 14 (SD) (range 82 to 137 microm). Standard deviation for mean vertical flap diameter was +/-0.24 mm. The cord length of the nasal hinge was variable with a mean of 4.26 +/- 0.63 mm (range 3.12 to 5.75 mm) in length. Postoperatively, there were no slipped flaps, flap striae, diffuse lamellar keratitis, or epithelial defects; there was 1 epithelial slide. At 1 day, the visual acuity was 20/20 or better in 76% of eyes.

CONCLUSIONS

The 100 microm head of the Moria LSK-One manual microkeratome cut a very predictable flap thickness and diameter but with variable length hinges. This flap thickness predictability was superior to that in other series with thicker intended flaps cut with mechanical microkeratomes and is comparable to that reported with the IntraLase FS femtosecond laser. Visual recovery was rapid, epithelial risks minimal, efficiency superior, and cost nominal relative to femtosecond laser technology.

Factors Predictive of LASIK Flap Thickness With the Hansatome Zero Compression Microkeratome

PURPOSE

To determine the explanatory power of preoperative variables and comeal flap thickness in laser in situ keratomileusis (LASIK) using the Hansatome zero compression microkeratome (Bausch & Lomb, Rochester, NY).

METHODS

A prospective, nonrandomized, comparative interventional case study was performed on 250 eyes of 129 consecutive patients who underwent LASIK surgery using the Hansatome zero compression microkeratome. A 160-microm or 180-microm microkeratome head and an 8.5- or 9.5-mm suction ring were used in the procedures. Preoperative measurements included refraction, spherical equivalent, keratometry, intraocular pressure, corneal white-to-white, anterior chamber depth, and corneal eccentricity. Corneal thickness was measured intraoperatively using ultrasonic pachymetry before and after flap creation, and the difference was taken as flap thickness. Flap diameter was measured with a corneal gauge. Data were analyzed using simple, multiple, stepwise linear and non-linear regression analyses and two-tailed t tests.

RESULTS

The mean flap thickness was 124 +/- 17 microm with the nominal 160-microm head and 142 +/- 20 microm with the nominal 180-microm head. One third (33%) of the total variation in flap thickness could be accounted for by three preoperative variables: average corneal thickness, spherical equivalent refraction, and choice of 160- or 180-microm microkeratome head. A simple correlation of 0.114 was noted between corneal eccentricity and flap thickness, but this variable did not add significant explanatory power on multiple regression analysis. Linear regression analysis allowed determination of a flap thickness nomogram with a standard error of the estimate of 16.9 microm and a 95% confidence interval of +/- 33.1.

CONCLUSIONS

Comeal thickness is the most systematic predictor of corneal flap thickness using the Hansatome microkeratome. Because three preoperative variables account for only 33% of the range in flap thickness, future studies should focus on variations in blade extension and corneal biomechanical factors, which may also play an important role in determining flap thickness.

Noninflammatory flap edema after laser in situ keratomileusis associated with asymmetrical preoperative corneal pachymetry

PURPOSE

To report persistent unilateral flap edema following laser in situ keratomileusis (LASIK) in patients with asymmetrical central corneal thickness.

SETTING

Minnesota Eye Consultants, Minneapolis, Minnesota.

METHODS

Retrospective, noncomparative interventional case series.

RESULTS

We examined 6 eyes of 3 patients with asymmetrical preoperative pachymetry who developed persistent unilateral flap edema after uneventful myopic LASIK in the eye with thicker preoperative pachymetry. All cases had asymmetrical preoperative pachymetry with flap edema developing in the eye with higher preoperative mean central corneal thickness (CCT) values, preoperative mean CCT subject eye 622 microm (range 556-664 microm) versus fellow eye 583 microm (range 510-621 microm). There was no associated ocular inflammation or rise in intraocular pressure. Significant flap edema resolved on a combination treatment of topical steroid and hypertonic saline.

CONCLUSIONS

Laser in situ keratomileusis can cause temporary endothelial cell dysfunction or stress, which manifests as temporary flap edema and subclinical corneal thickening. The edema appears to be limited to the actual flap and there was no loss of epithelial integrity in these eyes and no clinically noticeable interface fluid. This new clinical entity appears to occur in patients with asymmetrical preoperative corneal pachymetry and is associated with postoperative specular microscopy abnormalities. In cases with unexplained asymmetrical corneal thickness, preoperative evaluation should include specular microscopy to evaluate for risk features that may increase the chances of a slower postoperative recovery.

LASIK With an Inferior Hinge

PURPOSE

To investigate the refractive results and corneal pachymetric changes after laser in situ keratomileusis (LASIK) with an inferior hinge.

METHODS

Forty-six eyes of 23 patients who underwent LASIK with an inferior hinge were involved in this prospective study. All patients had a complete ophthalmic examination including uncorrected visual acuity (UCVA), best spectacle-corrected visual acuity (BSCVA), manifest refraction, computerized corneal topography (Orbscan II Corneal Topography System; Orbscan Inc, Salt Lake City, Utah), air-puff tonometry, slit-lamp examination of the anterior segment, and fundus examination.

RESULTS

Preoperatively, mean sphere was -4.09+/-0.27 diopters (D) (range: -8.00 to -0.25 D), mean cylinder was -0.60+/-0.11 D (range: -3.25 to 0.00 D), and mean spherical equivalent refraction was -4.39+/-0.27 D (range: -8.63 to -1.50 D). At 1 month postoperatively, mean sphere was -0.04+/-0.01 D (range: -0.25 to 0.00 D), mean cylinder was -0.01+/-0.01 D (range: -0.50 to 0.00 D), and mean spherical equivalent refraction was -0.04+/-0.02 D (range: -0.00 to -0.50 D). At 1 month postoperatively, no eyes lost any lines of BSCVA.

CONCLUSIONS

Laser in situ keratomileusis with an inferior hinge is an alternative technique, which was safe and effective in our series. Further studies with long-term follow-up comparing the incidence of corneal ectasia in eyes with a superior hinge to that in eyes with an inferior hinge are recommended.

The Theoretical vs. Measured Laser Resection for Laser in situ Keratomileusis

PURPOSE

To measure the excimer laser resection during and after laser in situ keratomileusis (LASIK) and to determine the rate of stromal ablation and factors predicting its measurement.

METHODS

A retrospective, comparative, interventional case study of 6010 eyes undergoing LASIK surgery was performed. In vivo ultrasonic pachymetry was performed as a measure of the laser resection (1'MLR). The theoretical laser resection generated by one of three separate excimer lasers (Summit Apex Plus, Alcon LadarVision, VISX) was recorded. The change in central comeal thickness measured at enhancement surgery (2'MLR) and the change in the residual stromal thickness prior to the laser ablation at the primary and enhancement procedure (3'MLR) were recorded as separate measures of the laser resection. Paired sample t test and regression analysis was performed to determine the relationships and to develop a model predictive of laser resection for each laser.

RESULTS

For all lasers, 1'MLR produced the highest estimate compared to 3'MLR. Laser-induced stromal desiccation, microkeratome effects, and change in measured flap thickness were the most likely causes for the differences. Laser resection was overestimated in the Alcon theoretical laser resection group, but underestimated in the Summit and VISX groups. The difference between 2'MLR and 3'MLR was due to epithelial hyperplasia, which measured 8.3, 17.8, and 10.8 microm in the Alcon, Summit, and VISX enhancement groups, respectively. Stromal ablation rates were 11.79, 8.26, and 12.71 microm per spherical equivalent diopter laser setting for the Alcon, Summit, and VISX lasers, respectively, when measured with 3'MLR. Multiple pre- and perioperative factors were associated with laser resection.

CONCLUSIONS

To accurately predict residual corneal thickness, the expected laser resection for an attempted refractive change must be known. As the laser resection can be laser-specific, the surgeon needs to establish the mean and range of tissue removal for a given attempted correction.

Continuous monitoring of corneal thickness changes during LASIK with online optical coherence pachymetry

PURPOSE

To assess the continuous intraoperative monitoring of central corneal thickness (CCT) changes during laser in situ keratomileusis (LASIK) using online optical coherence pachymetry (OCP).

SETTING

Department of Ophthalmology, Vivantes Klinikum Neukolln, Berlin, Germany.

METHODS

In this prospective nonrandomized comparative clinical case series of consecutive patients, 32 eyes having LASIK for myopia, myopic astigmatism, or hyperopia were continuously monitored intraoperatively in real time with online OCP integrated into a clinical excimer laser. The intraoperative values were compared to the postoperative flap and residual stromal thicknesses measured with corneal optical coherence tomography (OCT) as well as the calculated myopic ablation depth.

RESULTS

Continuous monitoring with online OCP enabled intraoperative visualization of the CCT changes during LASIK. The CCT, flap thickness after the microkeratome pass, time-resolved ablation, and residual stromal thickness were assessed. Intraoperatively, the mean flap thickness was 135 microm +/- 38 (SD) and the mean residual stromal thickness, 286 +/- 59 microm. The mean intraoperative flap and residual stromal thickness values were 43.7 microm and 15.4 microm lower, respectively, than the postoperative values assessed with corneal OCT (P<.001 and P=.005, respectively). The optically determined myopic ablation depth was 118 +/- 37 microm, which was 28 microm higher than the nominal ablation depth. There was a significant correlation (P<.001) between the postoperative flap (r=0.79) and residual (r=0.88) thickness measured with corneal OCT as well as the calculated myopic ablation depth (r=0.95).

CONCLUSIONS

Intraoperative online OCP could be an important safety feature to monitor the flap and residual stromal thicknesses during LASIK. The individual ablation depth and possible dehydration effects were also monitored continuously.

Monitoring corneal structures with slitlamp-adapted optical coherence tomography in laser in situ keratomileusis

PURPOSE

To monitor corneal structures with slitlamp-adapted optical coherence tomography (OCT) in laser in situ keratomileusis (LASIK).

SETTING

Department of Ophthalmology, Vivantes Klinikum Neukölln, Berlin, Germany.

METHODS

In this prospective, nonrandomized, comparative clinical case series of consecutive patients who had LASIK for myopia and myopic astigmatism, the corneal structures were studied with slitlamp-adapted OCT at a wavelength of 1,310 nm. The central corneal thickness (CCT) and epithelial, flap, and residual stromal thicknesses were assessed preoperatively, immediately after surgery, on postoperative day 1, and then, on average, after 8, 35, and 160 days.

RESULTS

Twenty-five eyes of 13 patients were included. The attempted mean spherical equivalent correction was -6.11 diopters (D) +/- 2.16 (SD) with a mean calculated stromal ablation depth of 92 +/- 24 microm. The CCT was 516 +/- 26 microm preoperatively and 453 +/- 40 microm postoperatively (P<.001). The epithelial thickness increased from 57.0 +/- 7.7 microm preoperatively to 61.0 +/- 7.5 microm postoperatively (P =.04). Imaging of the hyperreflective interface was possible in all patients for up to 15 months. The flap and residual stromal thickness was 211 +/- 28 microm and 344 +/- 48 microm, respectively, immediately after LASIK and 164 +/- 21 microm (P<.001) and 284 +/- 32 microm (P<.001), respectively, on postoperative day 1. There were no further significant changes during the follow-up. The overall mean reproducibility was +/-4.50 microm (coefficient of variation [CV] 0.94%) for CCT, +/-4.99 microm (CV 8.57%) for epithelial thickness, +/-6.25 microm (CV 3.55%) for flap thickness, and +/-7.09 microm (CV 2.42%) for residual stromal thickness.

CONCLUSION

Slitlamp-adapted OCT can be used to longitudinally monitor the variable structures of the cornea, epithelium, flap, and residual stroma in LASIK.

Influence of Flap Thickness on Results of Laser in situ Keratomileusis for Myopia

PURPOSE

To study whether flap thickness influences laser in situ keratomileusis (LASIK) results.

METHODS

This comparative, non-randomized, retrospective study comprised 421 LASIK eyes (233 patients) with spherical equivalent refraction between -0.60 and -6.00 diopters (D). Laser in situ keratomileusis was performed with the Moria One manual microkeratome and the Chiron Technolas 217C laser. Depending on the flap thickness, the eyes were divided into three groups (group 1, flap thickness < or =100 microm; group 2, flap thickness > 100 microm and < 130 microm; and group 3, flap thickness > or = 130 pm). For each group uncorrected visual acuity (UCVA), best spectacle-corrected visual acuity, residual spherical equivalent refraction, and keratometric power (D) were measured 1 day, 1 week, 1 month, and 6 months after LASIK. Complications, retreatment, and correlation between flap thickness and preoperative corneal thickness and keratometric power were also analyzed.

RESULTS

One month postoperatively, UCVA was > or = 20/20 in 76% (96 eyes), 56% (79 eyes), and 61% (93 eyes) of eyes in the three group, respectively (P = .015). One month postoperatively, UCVA was > or = 20/25 in 88% (111 eyes), 76% (108 eyes), and 76% (116 eyes) of eyes in the three groups, respectively (P = .015). Complication rate and retreatment rate were not statistically different among the groups. At 6-month follow-up, no statistically significant differences were noted in UCVA between the groups. Significant correlations between flap thickness and preoperative corneal thickness (P < .0001) and between flap thickness and preoperative keratometric power (P < .05) were found.

CONCLUSION

Patients with flaps < or = 100 pm had better functional results at 1 month than those with thicker flaps. It may be important to perform LASIK with thin flaps to save corneal tissue in the stromal bed.

Corneal flap thickness in laser in situ keratomileusis using the Moria M2 microkeratome

PURPOSE

To determine the predictability of flap thickness in laser in situ keratomileusis (LASIK) using the Moria M2 microkeratome and identify factors that may be related to variations in flap thickness.

SETTING

Laser Vision Correction Center, Bascom Palmer Eye Institute, University of Miami School of Medicine, Miami, Florida, USA.

METHODS

Charts of 208 patients having same-day bilateral LASIK using the Moria M2 microkeratome were reviewed. Intraoperative pachymetry was performed routinely. The right eye was always treated first. The same suction ring, stop, microkeratome head (110 microm or 130 microm), and blade were used in fellow eyes. Subtraction pachymetry was used to calculate flap thickness. Other collected data included age, keratometry, corneal diameter, and preoperative spherical equivalent (SE).

RESULTS

With the 110 microm head and slow translation velocity in both eyes, the mean flap thickness was 151.6 microm +/- 24.0 (SD) and 148.5 +/- 24.3 microm in the right and left eyes, respectively. With the 110 microm head and fast translation velocity in both eyes, the mean thickness was 136.2 +/- 25.5 microm and 132.8 +/- 23.5 microm, respectively. With the 130 microm head and fast translation velocity, the mean flap thickness was 145.8 +/- 25.4 microm and 139.9 +/- 25.5 microm, respectively. Flaps were thinner with fast translation velocity, the 110 microm head, and presumably duller blades used in the left eyes. There was a weak but statistically significant inverse correlation between flap thickness and age and between flap thickness and SE. A stronger correlation was found in flap thickness between right and left eyes.

CONCLUSIONS

Flap thickness with the Moria M2 microkeratome was variable. Fast translation velocity, a used (presumably duller) blade, and the 110 microm head produced thinner flaps. Given the potential variation in flap thickness (SD 23.5 to 25.5 microm), intraoperative pachymetry might be an adjunctive measure to prevent residual stromal beds that are thinner than planned, especially in patients with high myopia and/or thin corneas.

Update on laser subepithelial keratectomy (LASEK)

PURPOSE OF REVIEW

This study reviews current concepts in laser subepithelial keratectomy (LASEK), variations in LASEK techniques, the role of pharmacology in LASEK, and optimizing outcomes in LASEK.

RECENT FINDINGS

Recent studies continue to support the use of LASEK over that of LASIK in the correction of refractive error. In addition, the advent of pharmacological/biologic intervention, improved algorithms, and wavefront technology have expanded the armamentarium available to ophthalmologists in the maximization of LASEK outcomes.

SUMMARY

LASEK offers an excellent profile in terms of both final outcome (uncorrected visual acuity) and safety (best corrected visual acuity). Untoward effects of LASEK are readily prevented/treated with a variety of agents. Postoperative pain can be ameliorated using topical and oral analgesia. Infection can be most effectively addressed with the fourth generation of fluoroquinolones. Haze may be treated or prevented using numerous remedies namely autologous serum, steroids, ascorbic acid, mitomycin-c, and NSAIDS. Wavefront combined with LASEK rather than with LASIK may offer the best refractive outcome.

Quality assessment model of 3 different microkeratomes through confocal microscopy

PURPOSE

To study the quality of the cut created by 3 microkeratomes from 2 different generations using corneal confocal microscopy.

SETTING

Department of Refractive Surgery, Instituto Oftalmológico de Alicante, Alicante, Spain.

METHODS

Two different studies were conducted: Study 1 and Study 2. Study 1 was a prospective analysis using confocal microscopy examination data from 2 reference groups: Group A (control) with 20 nonoperated eyes of 20 healthy volunteers, and Group B with 50 eyes of 30 patients operated on with the Bausch & Lomb Automated Corneal Shaper (ACS) microkeratome. Study 2 was a prospective randomized double-masked study in which 40 eyes of 20 patients underwent myopic laser in situ keratomileusis by 1 surgeon. The right and left eyes of each patient were randomly and alternatively assigned to Group C, flap made with the Hansatome microkeratome, or Group D, flap made with the Moria M2. Immediately after surgery, the microscopic appearance of the cut was subjectively evaluated by the surgeon. One month postoperatively, flap thickness, particle density, and the subclinical confocal wound healing opacity (WHO) index were evaluated with the corneal confocal microscope.

RESULTS

Surgeon handling comfort was nearly the same with the Hansatome as with the M2 (P =.540). However, the apparent quality of the resulting flap was better with the M2 microkeratome (P =.041). The depth of the cuts made by the 3 microkeratomes were significantly different (P<.001), with the ACS flaps being thinner than the flaps made with the Hansatome or the M2. Particle density at the interface was significantly poorer in the eyes operated on with the ACS, but in these cases the WHO index was significantly greater (P<.001 in both cases).

CONCLUSIONS

Confocal microscopy is a very useful tool to evaluate the quality of the cut made by different microkeratomes. Overall, the predictability in flap thickness and the apparent quality of the cut made by the Moria M2 microkeratome are better than those obtained with the Hansatome or the ACS. The thin flap made by the ACS microkeratome produced a significantly greater WHO index than the thicker flaps created with the other 2 microkeratomes.

Retrospective analysis of actual LASIK flap diameter compared with microkeratome ring size performed by different surgeons

PURPOSE

To evaluate the difference in the diameter between microkeratome ring and actual laser in situ keratomileusis (LASIK) flap size by 4 different surgeons using 3 different microkeratomes in comparison to preoperative keratometry.

SETTING

Gimbel Eye Centre, Calgary, Alberta, Canada.

METHODS

A retrospective analysis of 6667 consecutive LASIK surgeries performed at the Gimbel Eye Centre between 1998 and 2002 was conducted. The study compared the difference in the actual LASIK flap diameter and microkeratome ring size for 3 Hansatome microkeratomes indicated by serial numbers 2230, 3048, and 3084 in comparison to the preoperative keratometry. The surgeries were performed by 4 surgeons from the Centre.

RESULTS

A statistically significant correlation was found between the actual flap size and the microkeratome ring size. The deviation increased with corneal steepness (#2230: r = 0.45, P<.001; #3048: r = 0.28, P<.001; #3084: r = 0.23, P<.001). The mean difference between actual flap size and the microkeratome ring size was 0.647 mm, 0.808 mm, and 0.916 mm for Hansatome microkeratome #2230, #3048, and #3084, respectively. The actual flap was larger than the microkeratome ring size in 92.8%, 93.8%, and 97.4% of eyes, respectively. The mean difference between microkeratome ring size and measured LASIK flap size was consistently significantly different (P<.0001) per surgeon for all microkeratomes.

CONCLUSIONS

Estimation of the flap size is an important issue while selecting ring size and planning the treatment zone. In this study, the actual flap size was larger than the ring size for all surgeons and all 3 Hansatome microkeratomes. Higher preoperative keratometry values achieved larger flaps than those with lower keratometry values.

Reproducibility of LASIK flap thickness using the Hansatome microkeratome

PURPOSE

To evaluate the actual versus the expected thickness of laser in situ keratomileusis (LASIK) flaps and to determine the factors that affect flap thickness.

SETTING

Centre For Sight, Queen Victoria Hospital, East Grinstead, United Kingdom.

METHODS

A retrospective analysis of LASIK procedures in 757 consecutive eyes was done. The surgery was performed by a single surgeon using 2 Hansatome microkeratomes (Bausch & Lomb) with 160 microm and 180 microm heads. Patient age, preoperative manifest refraction, automated keratometry, preoperative central pachymetry, and intraoperative stromal pachymetry were evaluated to determine whether they influenced the actual flap thickness.

RESULTS

Bilateral LASIK was performed in 343 patients (686 eyes). The 160 microm head was used in 641 eyes (84.6%) (Group 1) and the 180 microm head, in 116 eyes (15.4%) (Group 2). The mean preoperative manifest refraction spherical equivalent (SE) was -3.9 diopters (D) +/- 4.5 (SD) (range +7.4 to -25.0 D) in Group 1 and -4.4 +/- 3.7 D (range +7.1 to -12.9 D) in Group 2. The mean preoperative keratometry reading was 43.6 +/- 1.8 D (range 36.0 to 48.6 D) and 43.6 +/- 1.8 D (range 35.9 to 47.0 D), respectively; the mean preoperative central pachymetry was 543 +/- 35 microm (range 447 to 643 microm) and 548 +/- 31 microm (range 453 to 613 microm), respectively; and the mean flap thickness was 116.4 +/- 19.8 microm and 117.3 +/- 18.0 microm, respectively. The difference between the actual and the expected flap thickness in each group was statistically significant (P<.001). There was no significant difference in the actual flap thickness between the 2 Hansatomes. The preoperative SE and central pachymetry were the only factors that influenced the actual flap thickness (P<.05); thin flaps were more common with increasing myopia and increasing corneal thickness.

CONCLUSIONS

Spherical equivalent and preoperative pachymetry were the principal factors that influenced flap thickness. Preoperative keratometry values and patient age did not influence the actual flap thickness.

Ultrasound Evaluation of Flap Thickness, Ablation Depth, and Corneal Edema After Laser in situ Keratomileusis

PURPOSE

The aim of our study was to evaluate the predictability of flap thickness and changes in flap edema over time after laser in situ keratomileusis (LASIK).

METHODS

LASIK was carried out in 30 eyes. The corneal flap was created with a Moria CB manual microkeratome with a flap thickness of 130 microm. Photoablation was performed with the Zeiss-Meditec MEL 70(G-Scan) flying spot excimer laser. Ultrasound pachymetric measurements were performed with the Humphrey Model 855 pachymeter.

RESULTS

Preoperative mean corneal thickness was 568.43 +/- 34.6 microm. After LASIK, mean flap thickness was 133 +/- 26.4 microm. After excimer laser treatment, mean central corneal thickness decreased to a mean 392.4 +/- 37.4 microm. Five minutes after repositioning the flap, mean central corneal thickness was 572.1 +/- 43.4 microm. On the first postoperative day, it decreased to a mean 501.6 +/- 46.6 microm followed by additional decreases: mean 487.4 microm on day 5, 481.8 microm after 1 month, and 479.6 microm at 6 months. The actual photoablation depth was 10 microm less than the predicted depth (paired sample t-test, no statistically significant difference, P = .018). A significant linear correlation was found (Pearson, R =.725, P = .001) between predicted and measured photoablation depth.

CONCLUSIONS

Corneal flap thickness had greater variability than expected. After flap creation, stromal and flap edema occurred, but decreased during the first five postoperative days and stabilized thereafter.