Corneal Epithelial Thickness Profile in the Diagnosis of Keratoconus

The role of reference body selection in calculating posterior corneal elevation and prediction of keratoconus using rotating Scheimpflug camera

PURPOSE

To explore the effects of corneal astigmatism and asphericity on posterior elevation values calculated by different reference bodies and to compare their predictive accuracy in the diagnosis of keratoconus.

METHODS

In this prospective observational case series, 44 eyes of 24 patients with keratoconus and 70 eyes of 41 refractive surgery candidates as a control group were measured by the Pentacam Scheimpflug camera. Discriminating ability and predictors of posterior elevation measurements obtained by best fit toric ellipsoid (BFTE) and best fit sphere (BFS) reference surfaces were compared by receiver operator characteristic curves (ROC) and generalized estimating equation (GEE) models. Bland-Altman plots were used to determine the agreement between different reference surfaces.

RESULTS

Receiver operator characteristic curve analysis showed that posterior elevation measured by a BFTE auto had a significantly higher area under ROC curves (0.99) value than BFTE 8-mm or BFS reference surfaces. ROC analysis identified cut-off values for BFTE auto (9.5-μm), for BFTE 8-mm (10.5-μm), for BFS auto (16.5-μm) and for BFS 8-mm (15.5-μm) reference surfaces. According to GEE models, corneal cylinder and posterior asphericity had the least effect in toric ellipsoid models. Bland-Altman plots showed a systematic bias at higher values of average posterior elevation measured BFS reference surfaces.

CONCLUSIONS

Posterior corneal elevation value measured by the Pentacam camera can effectively discriminate keratoconus from normal corneas although measured values and cut-off points depend on the selection of reference body and corneal asphericity. Toric ellipsoid reference surface seems to be the most sensitive method to differentiate keratoconus.

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

PURPOSE

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

Control of femtosecond thin-flap LASIK using OCT in human donor eyes

PURPOSE

Thin-flap keratomileusis is a procedure that minimizes LASIK flap thickness to preserve both the corneal epithelium and the maximum residual stroma. This study investigated the usefulness of optical coherence tomography (OCT) as a tool in guiding the femtosecond laser in the creation of a thin flap in human eyes in a non-randomized case series.

METHODS

In a private research laboratory, an in vitro investigation was performed on human autopsy eyes. Five human cadaver eyes, unsuitable for transplantation, underwent flap creation with a femtosecond laser. The laser procedure was controlled in real-time with an OCT system (Thorlabs HL AG) to ensure that the cut was placed just underneath Bowman's layer. The repetition rate of the femtosecond laser was 10 MHz with a single-pulse duration of <400 femtoseconds (pulse energy in the nJ range). As a control, all eyes underwent histological dissection and were examined using light microscopy.

RESULTS

Video monitoring of the flap creation supported the feasibility of real-time OCT monitoring of the femtosecond laser flap creation process. A clear distinction of the corneal epithelium was possible in all eyes. Bowman's layer was not identified in all donor eyes at the given resolution of the OCT device used in this study. Light microscopy demonstrated flaps approximately 50-microm thick, confirming that the real-time monitoring assured a positioning of the cutting plane at minimum distance underneath Bowman's layer.

CONCLUSIONS

This study of five human cadaver eyes shows that real-time OCT monitoring of the creation of thin-flaps in LASIK using a femtosecond laser is possible, thus ensuring that the flap is created at the desired depth.

Epithelial, stromal, and total corneal thickness in keratoconus: three-dimensional display with artemis very-high frequency digital ultrasound

PURPOSE

To characterize the epithelial, stromal, and total corneal thickness profile in a population of eyes with keratoconus.

METHODS

Epithelial, stromal, and total corneal thickness profiles were measured in vivo by Artemis very high-frequency (VHF) digital ultrasound scanning (ArcScan) across the central 6- to 10-mm diameter of the cornea on 54 keratoconic eyes. Maps of the average, standard deviation, minimum, maximum, and range of epithelial, stromal, and total corneal thickness were plotted. The average location of the thinnest epithelium, stroma, and total cornea were found. The cross-sectional semi-meridional stromal and total corneal thickness profiles were calculated using annular averaging. The absolute stromal and total corneal thickness progressions relative to the thinnest point were calculated using annular averaging as well as for 8 semi-meridians individually.

RESULTS

The mean corneal vertex epithelial, stromal, and total corneal thicknesses were 45.7+/-5.9 microm, 426.4+/-38.5 microm, and 472.2+/-41.4 microm, respectively. The average epithelial thickness profile showed an epithelial doughnut pattern characterized by localized central thinning surrounded by an annulus of thick epithelium. The thinnest epithelium, stroma, and total cornea were displaced on average by 0.48+/-0.66 mm temporally and 0.32+/-0.67 mm inferiorly, 0.31+/-0.45 mm temporally and 0.54+/-0.37 mm inferiorly, and 0.31+/-0.43 mm temporally and 0.50+/-0.35 mm inferiorly, respectively, with reference to the corneal vertex. The increase in semi-meridional absolute stromal and total corneal thickness progressions was greatest inferiorly and lowest temporally.

CONCLUSIONS

Three-dimensional thickness mapping of the epithelial, stromal, and total corneal thickness profiles characterized thickness changes associated with keratoconus and may help in early diagnosis of keratoconus.

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.

Stability of LASIK in topographically suspect keratoconus confirmed non-keratoconic by Artemis VHF digital ultrasound epithelial thickness mapping: 1-year follow-up

PURPOSE

To determine the 1-year stability of LASIK in corneas with topographic suspect keratoconus confirmed as non-keratoconic by epithelial thickness mapping.

METHODS

This was a retrospective case/control comparative study. Eyes suspected of keratoconus using criteria based mainly on Atlas (Carl Zeiss Meditec AG) and Orbscan II (Bausch & Lomb) topography were scanned by Artemis very high-frequency digital ultrasound (ArcScan Inc). Keratoconus was confirmed if the epithelial thickness profile showed relative epithelial thinning coincident with an eccentric posterior elevation best-fit sphere apex. Laser in situ keratomileusis was performed in all eyes where keratoconus was excluded by finding relatively thicker epithelium or not finding localized thinning over the topographically suspected cone. Patients were followed for 1 year after LASIK. A control group was generated matched within 0.50 diopter (D) for sphere, cylinder, and spherical equivalent refraction (SEQ) to compare refractive stability.

RESULTS

The average change in SEQ between 3 and 12 months was -0.10+/-0.30 D for the suspect keratoconus group and -0.10+/-0.28 D for controls. No statistically significant difference in shift from 3 months to 12 months in SEQ or cylinder between groups was noted. No statistically significant change in best spectacle-corrected visual acuity between groups was noted, with no eye losing 2 lines and 5% in the suspect keratoconus group and 2% of controls losing 1 line. No cases of ectasia were observed in either group.

CONCLUSIONS

Suspect keratoconus, confirmed to be non-keratoconic by epithelial thickness profile criteria demonstrated equal stability to control eyes 1 year after LASIK. Epithelial thickness profiles may enable LASIK to be performed in eyes that would otherwise have been excluded due to topographic suspect keratoconus. Further follow-up is being carried out.