Assessment of accuracy and repeatability of anterior segment optical coherence tomography and reproducibility of measurements using a customised software program

Anterior and posterior imaging with hyperparallel OCT

Hyperparallel OCT (HP-OCT) is a parallel spectral domain imaging technology particularly well-suited to the anterior segment. It uses a 2-dimensional grid of 1008 beams to simultaneously image across a wide area of the eye. In this paper we demonstrate that sparsely sampled volumes captured at 300 Hz can be registered without the need for active eye tracking to produce 3-dimensional (3D) volumes free from motion artefacts. The anterior volume provides complete 3D biometric information, including lens position, curvature, epithelial thickness, tilt, and axial length. We further demonstrate that, with the change of a detachable lens, we can capture high resolution anterior volumes and importantly, posterior volume images for preoperative assessment of the posterior segment. Advantageously, the retinal volumes have the same 11.2 mm Nyquist range as the anterior imaging mode.

Comparison of corneal flap thickness predictability and architecture between femtosecond laser and sub-Bowman keratomileusis microkeratome in laser in situ keratomileusis

PURPOSE OF THE STUDY

The purpose of the study is to assess and compare the corneal flap thickness predictability and uniformity between Visumax femtosecond laser and Moria sub-Bowman keratomileusis microkeratome in laser in situ keratomileusis procedure to correct myopic and myopic astigmatism refractive errors.

METHODS

One hundred eyes from 100 patients were enrolled in this study. Only one eye (right eye) of each patient was chosen for this study. They were divided into two groups of equal size (50 each). Target flap thickness was 90 μm. Flap thickness was measured by anterior segment optical coherence tomography in 7 specified positions at 3 months postoperative.

RESULTS

For femtosecond laser group, the average central flap thickness (CFT) was 91.35 µm ± 4.97. There was no statistically significant difference between the target flap thickness and the resultant flap thickness (p = 0.12). When the central, nasal and temporal thicknesses in the same flap were compared, there was no statistically significant difference among these measurements (p = 0.9). For the microkeratome group, CFT was 102.18 µm ± 5.63. There was statistically significant difference between the target flap thickness and the resultant flap thickness (p = 0.001). The central, nasal and temporal thicknesses were compared and there was a statistically significant difference (p = 0.000).

CONCLUSIONS

Our study is among few studies in the literature comparing femtosecond laser flap to SBK microkeratome flap. Femtosecond laser creates more predictable flap thickness and more uniform flap architecture. Trial registration Trial registration number: NCT04684888. Trial Registration date: December 2020. Registered at www.

CLINICALTRIALS

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Corneal thickness changes in myopic children during and after short-term orthokeratology lens wear

PURPOSE

To evaluate thickness changes in the central and mid-peripheral cornea (CCT and mPCT), corneal epithelium (CET and mPET) and stroma (CST and mPST) of myopic children during and after short-term orthokeratology (ortho-k) lens wear, with conventional (CCF, 0.75 D) and increased compression factors (ICF, 1.75 D).

METHODS

This was a self-controlled case series study. Subjects wore a CCF lens in one eye and an ICF lens in the other. Anterior segment optical coherence tomography images were captured weekly for 1 month during lens wear and for 2 weeks after discontinuing lens wear. CCT and CET (central 3-mm cornea) and mPCT and mPET (within a 4-6 mm diameter annulus) were measured. Stromal thickness (ST) was determined by subtracting epithelium thickness (ET) from corneal thickness (CT). The repeatability of the analytical software was also investigated on age-matched spectacle-wearing subjects (n = 98).

RESULTS

Excluding three outliers (>3 S.D.s), the coefficient of repeatability and intraclass correlation coefficients of 98 spectacle-wearing subjects ranged from 2.63 to 4.64 μm and from 0.90 to 0.99, respectively. For the weekly-change study, CCT and CET in both eyes were significantly thinner after lens wear (p < 0.001) and CET thinning in the ICF eyes were significantly higher (p < 0.02). CCT changes were mainly contributed by CET. CST, mPCT, mPET and mPST changes were not significant (p > 0.20) in either eye. CT (all sublayers) rebounded to baseline values 2 weeks after discontinuing lens wear (0.99 > p > 0.12).

CONCLUSIONS

Significant reductions in CT and ET, but not ST, were observed within 1 month of ortho-k lens wear. Wearing ICF lenses resulted in a higher reduction in CET. Corneal thickness changes were reversible after discontinuing lens wear.

Thickness profiles of the corneal epithelium along the steep and flat meridians of astigmatic corneas after orthokeratology

Background

The aim of this study was to investigate the changes in corneal epithelial thickness along the principle meridians of astigmatic corneas after six months of overnight spherical myopic orthokeratology (OK) lens wear.

Methods

This is a prospective study. Fifty-seven subjects with up to 1.50 diopters (D) of corneal toricity wore spherical OK lenses for 6 months. Evaluations of OK lens fit, visual acuity, refractions and corneal toricity (CT) were performed. Fourier-domain optical coherence tomography (FD-OCT) was conducted to measure the corneal epithelial thickness (ET) along the principle meridians of corneal toricity over a diameter of 6 mm. The means of △ET of the same diameter at individual meridians (△ETSm and △ETFm) were calculated and compared.

Results

Visual acuity and refraction improved significantly after OK lens wear. △ETFm thinned more than △ETSm (P = 0.027) at 1.5 mm in radius. △ETSm thickened more than △ETFm at 2.5 mm (P = 0.019) and 3.0 mm (P = 0.036).∣△ETSm - △ETFm∣ were significantly correlated with the baseline central CT at 2.0 mm, 2.5 mm and 3.0 mm. ∣△ETSm - △ETFm∣was significantly correlated with the baseline peripheral CT at 2.5 mm.

Conclusions

Overnight wear of spherical OK lenses resulted in differential changes in the thickness profiles of the corneal epithelium between the steep and flat meridians in eyes with corneal toricity.

Correlation of corneal pigmented arc with wide epithelial thickness map in orthokeratology-treated children using optical coherence tomography measurements

PURPOSE

To determine the intensity of corneal pigmented arc in orthokeratology (ortho-k)-treated children and its correlation with wide epithelial thickness map (ETM) obtained through anterior segment optical coherent tomography (AS-OCT).

METHODS

This retrospective case series reviews medical records of children who received ortho-k treatment for myopia control. Intensity of ortho-k-associated pigmented arc after wearing ortho-k lens more than 12 months and its correlation with each sector/zone of wide ETM obtained by AS-OCT was explored. Pigmented arcs were further divided into apparent and unapparent groups, and the clinical differences between groups were determined.

RESULTS

This study included 57 eyes of 29 children (mean age, 11.4 years, range 9-15); after initiating ortho-k treatment, the incidence of the corneal pigmented arc was 91.2% with mean lens wear duration of 26.1 months. Intensity of pigmented arc was significantly correlated with lens wear duration, target power, baseline degree of myopia, C zone and sectors I2, I3 and IT3 on wide ETM. Comparison between apparent and unapparent groups showed the same significant results except for C zone. After adjusting for lens wear duration and target power, sector I2 has the highest association with pigmented arc severity.

CONCLUSION

Children treated with ortho-k are likely to develop ortho-k-associated pigmented arcs. The new wide ETM of AS-OCT can provide important information regarding the intensity of pigmented arc in these children. This can support customized pigmented arc-free ortho-k treatment for children in the future.

Correlation between pigmented arc and epithelial thickness (COPE) study in orthokeratology-treated patients using OCT measurements

PURPOSE

To determine the intensity of corneal pigmented arc in orthokeratology (ortho-k)-treated children, and its correlation with key epithelial thickness measurements obtained by anterior segment optical coherence tomography (AS-OCT).

METHODS

This study is a retrospective case series. Medical records of children who received ortho-k treatment for myopia control in our hospital were reviewed. Intensity of ortho-k-associated pigmented arc and its correlation with key epithelial thickness parameters in the central 7-mm-diameter zone obtained by AS-OCT was examined. The subjects were further divided into apparent and unapparent pigmented arc groups for severity comparison.

RESULTS

The mean age of children was 11.4 years, and the incidence of corneal pigmented arc was 92.2% after lens wear for a mean duration of 21.2 months. Intensity of pigmented arc was found to be significantly correlated with key epithelial thickness parameters, including maximum and minimum epithelial thickness (Spearman's rank correlation coefficient (rs) = 0.404, P = 0.003; rs = - 0.426, P = 0.002, respectively), the difference between them (Min-Max) (rs = -0.624, P < 0.001) and standard deviation (rs = 0.659, P < 0.001). Significant correlation between intensity of pigmented arc and ortho-k target power (rs = 0.454, P = 0.001) was found. Comparison between the two groups showed significant difference in the same key epithelial thickness parameters.

CONCLUSIONS

Children receiving ortho-k treatment tended to develop pigmented arcs. Significant correlation between intensity of pigmented arc and key epithelial thickness parameters was observed. AS-OCT can be a useful tool for predicting intensity of pigmented arc in ortho-k-treated children.

Repeatability of corneal epithelial thickness measurements using Fourier-domain optical coherence tomography in normal and post-LASIK eyes

PURPOSE

To evaluate the repeatability of corneal epithelial thickness (ET) and corneal thickness (CT) measurements in normal eyes and laser in situ keratomileusis (LASIK)-treated eyes using optical coherence tomography (RTVue system).

METHODS

In 35 right eyes of 35 normal subjects and 45 right eyes of 45 subjects who underwent a myopic LASIK, corneal ET and CT were evaluated in 17 areas: (1) 1 central zone within a 0- to 2.0-mm diameter, (2) 8 paracentral zones from a 2.0- to 5.0-mm diameter, and (3) 8 peripheral zones from a 5.0- to 6.0-mm diameter. The repeatability was assessed using within-subject standard deviation (SD), coefficient of variation, and intraclass correlation coefficient.

RESULTS

At the central and paracentral zones, respectively, the SD values were 0.7 μm and 0.6 to 0.9 μm in normal eyes and 0.7 μm and 0.8 to 1.7 μm in LASIK-treated eyes for ET, and 1.0 μm and 2.8 to 4.6 μm in normal eyes and 1.3 μm and 4.0 to 4.8 μm in LASIK-treated eyes for CT. At the peripheral zones, in normal and LASIK-treated eyes, respectively, the SD values ranged from 0.8 to 1.2 μm and 1.4 to 2.2 μm for ET, and 4.1 to 6.4 μm and 6.0 to 9.1 μm for CT. The coefficient of variation values were low and intraclass correlation coefficient values were high in both groups for both ET and CT measurements.

CONCLUSIONS

The Optical coherence tomography produced excellent repeatability, especially at the central and paracentral zones up to a 5-mm diameter for both corneal ET and CT measurements.

Pachymetry map of corneal epithelium in children wearing orthokeratology contact lenses

PURPOSE

To study the pachymetry map of the corneal epithelium in children wearing orthokeratology lenses automatically generated by a Fourier-domain optical coherence tomography.

MATERIALS AND METHOD

The study was conducted on 60 children who had been fitted with myopic orthokeratology lenses. Patients were divided into two groups according to the duration of OK lens treatment (group 1: ≤14 days, n = 28; group 2: >14 days, n = 32). The control group consisted of 44 children. An FD-OCT device with a pachymetry module was used to map the central 6-mm corneal epithelial thickness. An epithelial thickness map was automatically generated and divided into three zones: central 2 mm, paracentral 2 to 5 mm (P1) and mid-peripheral 5 to 6 mm (P2). The average epithelial thickness of central (C), the temporal (T1), nasal (N1), superior (S1) and inferior (I1) sectors of P1, and the temporal (T2), nasal (N2), superior (S2) and inferior (I2) sectors of P2 were recorded and compared. The minimum and maximum points of epithelial thickness across the map were also recorded. Munnerlyn's formula was used to model the expected change in refractive error based on Δ(Max-Min) (Δ(Max-Min) = (Max-Min)study-(Max-Min) mean of control).

RESULTS

The central epithelial thickness was significantly different between individual groups and a significant difference from the control (Group 0) was seen in each treatment group. Both the epithelial thickness measurements of T1 and I1 were thinnest in Group 1. Both the epithelial thickness measurements of S2 and I2 were thickest in Group 2. The difference between maximum and minimum thickness was significantly different between groups with the largest effect in Group 2. The refractive changes predicted by Munnerlyn's formula were less than the actual refractive changes measured in both study groups.

CONCLUSIONS

The epithelial thickness map automatically generated by FD-OCT can provide regional information about corneal epithelium thickness following overnight wearing of OK lenses.

The effect of inlay implantation on corneal thickness and radius of curvature in rabbit eyes

PURPOSE

To study the effect of inlay implantation on corneal shape, assessing the changes using optical coherence tomography (OCT) and a customized image analysis software.

METHODS

Thirteen rabbit eyes were operated on, with positive-powered corneal inlays implanted into 11 eyes and 2 eyes serving as flap-only controls. Cross-sectional OCT images were obtained using Optovue preoperatively, and 1 hour, 1 week, and 1 month postoperatively. Topography maps were obtained preoperatively using Medmont E300. Image analysis software was built to extract corneal thickness and radius of curvature. Anterior corneal radius of curvature values obtained from the OCT images were compared with the corneal topography maps to assess the validity of the method.

RESULTS

Corneal thickness increased more than predicted immediately after the implantation. However, by 1 month, it matched closer to the added thickness of the corneal inlays. An overall pattern of epithelial thinning was observed of up to 25% at 1 month. The anterior corneal surface had steepened up to 15% at 1 month, whereas inconsistent changes for the posterior corneal surface were observed.

CONCLUSIONS

After corneal inlay implantation, an immediate response was observed in the cornea. Corneal swelling because of surgical trauma was the most likely cause for the observed thickening of the cornea up to 1 week. By 1 month, the epithelial layer measured thinner for most eyes. Most of the added thickness was transferred to the central anterior corneal surface, causing steepening at 1 month. The anterior corneal radius of curvature values obtained from OCT images were in broad agreement with Medmont topography.