Comparative Analysis of Two Clinical Diagnostic Methods of the Corneoscleral Geometry

Agreement between extrapolated corneoscleral topographical data obtained during natural and retracted eyelid positions

PURPOSE

To examine the influence of aperture size (corneoscleral data coverage) on extrapolated scleral sagittal height data generated by the Pentacam HR derived from the Corneo Scleral Profile (CSP) software, by comparing measurements obtained during natural and retracted eyelid positions.

METHODS

Corneoscleral topography of the left eye of 20 young (age: 22 [3] years) healthy adults with normal corneas was measured using the Pentacam HR CSP with the eyelids in their natural primary gaze resting position and during eyelid retraction with a wire speculum. The measured and extrapolated sagittal height data were exported from the instrument and analysed over a range of chord diameters (10.0, 12.5, 15.0 and 17.5 mm) and locations (superior, inferior, nasal and temporal) using customised software.

RESULTS

Eyelid retraction increased data coverage (% of available data points within 360°) for the 12.5 and 15.0 mm chord diameters (by 25% and 35%, respectively, p < 0.001), but by less than 10% for the 10.0 and 17.5 mm chord diameters. Significant differences in extrapolated sagittal height data were observed between the natural and retracted eyelid positions with respect to chord diameter and location (both p < 0.001), with the greatest difference observed superiorly for a 17.5 mm chord diameter (mean extrapolated sagittal height difference, retracted minus natural: -447 ± 401 μm, p < 0.0001).

CONCLUSIONS

Eyelid retraction substantially increased data coverage for the 12.5 and 15.0 mm chord diameters. Significant differences in the extrapolated sagittal height data generated from measurements obtained during natural and retracted eyelid positions were observed for the superior location (12.5, 15.0 and 17.5 mm chord diameters) and inferior and nasal locations (17.5 mm chord diameter). Extrapolated sagittal height values obtained during the natural eyelid position were typically greater than those obtained with eyelid retraction, most likely due to the amount of measured data available for extrapolation.

Orthokeratology effect on the corneoscleral profile: Beyond the bull's eye

PURPOSE

To assess the impact of 3 months of orthokeratology (ortho-k) contact lenses (CLs) for myopia correction on the corneoscleral profile, as changes in scleral geometry could serve as indirect evidence of alteration in the corneal biomechanical properties.

METHODS

Twenty subjects (40 eyes) were recruited to wear ortho-k lenses overnight; however, after discontinuation (two CL fractures, one under-correction and two non-serious adverse events), 16 subjects (31 eyes) finished a 3-month follow-up. Corneoscleral topographies were acquired using the Eye Surface Profiler (ESP) system before and after 3 months of lens wear. Steep (SimKs) and flat (SimKf) simulated keratometry and scleral sagittal height measurements for 13-, 14- and 15-mm chord lengths were automatically calculated by the ESP software. Additionally, sagittal height and slope were calculated in polar format from 21 radii (0-10 mm from the corneal apex) at 12 angles (0-330°). Linear mixed models were fitted to determine the differences between visits.

RESULTS

SimKs and SimKf were increased significantly (p ≤ 0.02). The sagittal height in polar format increased significantly (p = 0.046) at a radius of 2.5 mm for 150°, 180°, 210° and 240° orientations and at a radius of 3.0 mm for 210°. Additionally, the slope in polar format significantly decreased (p ≤ 0.04) at radii ranges of 0.0-0.5, 0.5-1.0 and 1.0-1.5 mm for multiple angles and at a radii range of 5.0-5.5 mm for 90°. It also increased significantly (p ≤ 0.045) at a radii range of 1.5-2.0 mm for 30° and at radii ranges of 2.0-2.5, 2.5-3.0 and 3.0-3.5 mm for multiple angles. No significant changes were found for any parameter measured from the scleral area.

CONCLUSIONS

Three months of overnight ortho-k lens wear changed the central and mid-peripheral corneal geometry as expected, maintaining the peripheral cornea and the surrounding sclera stability.

Reliability and Agreement of Keratometry Measurements Obtained With Eye Surface Profilometry and Partial Coherence Interferometry

PURPOSE

The aim of this study was to assess the repeatability of keratometry parameters obtained using the Eye Surface Profiler (ESP) system and their agreement with the IOL Master 500 device.

METHODS

Seventy-one eyes of 71 healthy participants were evaluated. Three repeated measurements were performed using the ESP system. Simulated keratometry in the flat (SimKf) and steep (SimKs) meridians, astigmatism, and axis were obtained. The same parameters were measured using the IOL Master 500 device. The J0 and J45 vector components of the astigmatism were calculated. The intrasession repeatability was analyzed using within-subject SD (Sw) and intraclass correlation coefficient (ICC). Agreement was assessed using paired statistical tests and the Bland-Altman method.

RESULTS

The Sw was 0.07 mm, 0.04 mm, 0.51 D, 0.33 D, and 0.22 D, and the ICC was 0.96, 0.98, 0.74, 0.61, and 0.55 for SimKf, SimKs, astigmatism, J0, and J45, respectively. The mean difference and limits of agreement when comparing the ESP system with the IOL Master 500 device were 0.37 mm (0.08/0.66) for SimKf ( P < 0.001), 0.18 mm (0.00/0.35) for SimKs ( P < 0.001), -0.93 D (-2.42/0.56) for astigmatism ( P < 0.001), 0.51 D (-0.22/1.24) for J0 ( P < 0.001), and 0.06 D (-0.48/0.60) for J45 ( P = 0.09).

CONCLUSIONS

The ESP system provides consistent values for simulated keratometry, showing moderate consistency for astigmatism parameters. Contact lens practitioners should be aware that the ESP system and IOL Master 500 device provide different simulated keratometry from a clinically viewpoint.

Metrics of Anterior Sclera in Normal Chinese Adults: Anterior Segment Imaging Using the Swept-Source Optical Coherence Tomography

PURPOSE

To measure the corneoscleral limbus and anterior sclera parameters of normal Chinese adults by swept-source optical coherence tomography (OCT).

MATERIALS AND METHODS

In this cross-sectional study, a total of 56 Chinese subjects with ametropia were evaluated in the Eye Hospital of Wenzhou Medical University from September 2020 to December 2020, including 26 (46.4%) men, with an average age of 24.7±1.8 years old. The OCT SS-1000 (CASIA, Tomey, Tokyo, Japan) was used to measure the sagittal height, corneoscleral junction (CSJ) angle, and scleral angle.

RESULTS

The chord was across the corneal center and the line connecting the center of the cornea and the center of the chord was perpendicular to the chord. The mean sagittal height at chord lengths of 10.0, 12.3, and 15.0 mm were 1,756±72, 2,658±110, and 3,676±155 μm, respectively. The absolute values of the differences between horizontal and vertical meridians at three chord lengths were 54±40, 70±67, and 117±95 μm, respectively. One-way analysis of variance showed that the differences of CSJ angles at 12.3-mm chord and scleral angles at 15.0-mm chord in the four segments were statistically significant ( F values were 32.01 and 13.37, respectively, both P <0.001). The CSJ angles from low to high were 176.53±2.14° (nasal), 178.66±1.84° (inferior), 179.13±1.20° (temporal), and 179.31±1.68° (superior), and 87.5% of the nasal angles were less than 179°. The scleral angles from high to low were 38.35±2.47° (temporal), 38.26±3.37° (superior), 35.37±3.10° (nasal), and 35.30±4.71° (inferior).

CONCLUSIONS

The morphology of corneoscleral limbus and anterior sclera is asymmetrical in normal Chinese adults. The nasal side of the corneoscleral limbus has the largest angle, and the superior and temporal sides of the scleral angle are larger.

Repeatability and Reproducibility of Corneoscleral Topography Measured With Scheimpflug Imaging in Keratoconus and Control Eyes

OBJECTIVES

To determine and compare the repeatability and reproducibility of anterior scleral parameters measured by the corneoscleral profile (CSP) module of Pentacam in keratoconus (KC) and control eyes.

METHODS

This is a prospective observational study. Thirty KC participants (30 eyes) and 24 control participants (24 eyes) were examined three times using the CSP. Sagittal height mean (SHM), sagittal height astigmatism (SHA), and mean bulbar slope (BSM) were measured in 12 mm and 16 mm chord lengths. The repeatability and reproducibility of these measurements were also assessed. Coefficients of variation (CoV), intraclass correlation coefficient (ICC), coefficient of repeatability (CoR1), and coefficient of reproducibility (CoR2) were adopted to assess the reliability.

RESULTS

In the KC and control groups, SHM showed high repeatability and reproducibility (coefficients of variation [CoVs]≤0.96%, intraclass correlation coefficient [ICCs]≥0.97), and SHM of control eyes showed higher repeatability and reproducibility than that of KC eyes at 12 mm chord length (KC group, CoRs ranged from 35.56 μm to 43.52 μm, control group, ranged from 23.50 μm to 30.31 μm) and 16 mm chord length (KC group, CoRs ranged from 79.54 μm to 81.58 μm, control group, ranged from 48.25 μm to 66.10 μm). Mean bulbar slope also showed high repeatability and reproducibility (CoVs≤3.65%, CoRs≤2.64). Furthermore, the SHA of control eyes showed higher repeatability and reproducibility when compared with KC eyes (control group: CoVs≤29.95%, KC group: CoVs≥32.67%).

CONCLUSIONS

Keratoconus and control eyes demonstrated high repeatability and reproducibility when using CSP measurements, which may prove helpful in fitting contact lenses.

Scheimpflug imaging for grading and measurement of corneo-scleral-profile in different quadrants

PURPOSE

The corneo-scleral-profile (CSP) describes the transition from cornea to sclera, while the corneo-scleral junction angle (CSJ), is the angle formed between the cornea and the sclera. The aims of this study were (i) to analyse the CSP and CSJ in different quadrants and (ii) to test correlation and repeatability of an established observational grading and measurement method, using Scheimpflug images.

METHODS

The nasal, temporal, superior and inferior CSP of 35 healthy eye participants (mean age 25.5 SD ± 3.1 years; 20 female) was imaged using the corneo-scleral-profile module of the Pentacam (Oculus, Wetzlar, Germany). On the captured Scheimpflug images CSP was subjectively graded into five different corneo-scleral transitions, using the Meier grading scale (profile 1 fluid-convex; profile 2 fluid-tangential; profile 3 marked-convex; profile 4 marked-tangential; profile 5 concave). The CSJ was measured on the same images using ImageJ v1.8.0. Grading and measurement was repeated at a second session. Intra-observer reliability for the CSP-grading was analysed by Cohen's Kappa. Differences between repeated CSJ-measurements and different quadrants were analysed by paired-t-test and ANOVA. The eta-coefficient was used to determine the association between subjective CSP-grading and CSJ-measurement.

RESULTS

Intra-observer reliability for the CSP grading system was poor (kappa = 0.098) whereas repeated measurements of CSJ angle showed no statistically significant difference between sessions (0.04°; 95 % CI - 0.21° to 0.29°; p = 0.77). CSJ angles ranged from 172° to 180° with no statistically significant differences between nasal, temporal, superior and inferior (p = 0.24). Eta-coefficient indicated a weak association between CSP-grading and CSJ-measurement (η = 0.27; p = 0.04).

CONCLUSIONS

The subjective CSP-grading showed poorer repeatability than the objective CSJ-measurement, which did not detect any differences in angles between the meridians. The weak association between CSP-grading and CSJ-measurement is likely caused by the limited intra-observer reliability of the Meier grading scale. Furthermore, the CSP-grading scale seems to consider other aspects beside the CSJ angle, such as scleral radius.

Comparing sagittal heights calculated using corneal parameters and those measured with profilometry

PURPOSE

To compare the sagittal height of the anterior eye (OC-SAG) calculated using corneal parameters with the OC-SAG measured by profilometry.

METHOD

Seventy right eyes of soft contact lens wearers measured with the ESP (Eaglet Eye, The Netherlands) after lens removal were retrospectively analyzed for this study. The OC-SAG of the eyes was calculated using mean k-values, eccentricity and the inner (corneal) radius obtained with the ESP for an 11-mm cord diameter. It was then extrapolated to chord diameters of 14, 14.5 and 15 mm. These values were compared with OC-SAG values obtained with the ESP for the same chord diameters. Additionally, the OC-SAG was calculated through the formula used by a lab that manufactures custom soft lenses (mark'ennovy, Madrid, Spain) and compared again with the values obtained using the ESP.

RESULTS

Differences between calculated OC-SAG obviating the shape factor were 121 ± 44, 155 ± 105, 172 ± 117 and 189 ± 129 µm for chord diameters of 11, 14, 14.5 and 15 mm, respectively (p < 0.001). When the shape factor was included in the calculation, differences were 28 ± 48, 62 ± 102, 79 ± 113 and 96 ± 123 µm (p < 0.001). When the inner best fit sphere was used to estimate OC-SAG, differences were 34 ± 11, 0 ± 72, 17 ± 86 and 34 ± 99, respectively, with no significant differences for the 14 and 14.5 mm-chord diameters (p = 0.99 and 0.11, respectively). Correlation coefficients between OC-SAG calculated and measured OC-SAG ranged from 0.53 to 0.90 depending on the chord diameter used. When the mark'ennovy formula was used to calculate the OC-SAG as the lens diameter proposed by the formula, the difference was -47 ± 147 µm (p < 0.01).

CONCLUSIONS

Differences between the OC-SAG calculated using corneal parameters and that measured with a profilometer are statistically and clinically significant, especially for large chord diameters. The impact of this on contact lens fitting should be addressed in future studies.

What we know about the scleral profile and its impact on contact lens fitting

Scleral contact lens fitting has provided practitioners with one of the earliest pieces of evidence that the sclera was more often asymmetric than symmetric. Some preliminary observations such as different haptic alignment patterns over the sclera in 360 degrees, the appearance of fogging in the fluid chamber and excessive tear out-in flow through specific meridians, quadrants, or areas of the haptic provide evidence of some scleral asymmetry. The advent of technologies that allowed measurement of the scleral profile led to formal research confirming that only about 6% of scleras are symmetric, while the rest are toric, quad-specific, or irregular. This has an evident impact on how to focus scleral lens fitting. Measuring the scleral profile also provided us with true ocular sagittal height data for cord diameters beyond the cornea. Although high variability was expected in pathological eyes, healthy eyes also showed a wide range of ocular sagittal heights. Due to this variability among healthy eyes, a discussion has emerged on whether the one-size-fits-all approach to soft lens fitting is a proper strategy to fit the whole spectrum of sagittal heights. The traditional mode of selecting the parameters for custom soft lenses through corneal parameters is also questioned.

Anterior ocular surface sagittal height prediction

PURPOSE

To investigate the validity of Placido-based corneal topography parameters to predict corneoscleral sagittal heights measured by Fourier-based profilometry at various diameters.

METHODS

Minimal (Min_sag ), maximal (Max_sag ) sagittal height, toricity (Max_sag  - Min_sag ) and axis of the flattest meridian (Min_sag ) of 36 subjects (mean age 25.4 SD ± 3.2 years; 21 female) were measured using the Eye Surface Profiler and analysed for diameters (chord length) of 8 to 16 mm (in 2-mm intervals). Furthermore, corneal central radii, corneal astigmatism, eccentricity and diameter were measured using the Keratograph 5 M.

RESULTS

Using multiple linear regression analysis, the best equation for predicting the sagittal heights for 8 mm (r²  = 0.95), and 10 mm (r²  = 0.93) diameters included corneal central radii and eccentricity. The best equation for predicting sagittal heights for 12 mm (r²  = 0.86), 14 mm (r²  = 0.78) and 16 mm (r²  = 0.65) diameters included corneal central radii, eccentricity and corneal diameter. Corneal astigmatism was significantly correlated with sagittal height toricity for 8 and 10 mm diameters (r²  = 0.50 and 0.29; p < 0.01), while no correlation was observed for 12, 14 and 16 mm diameters (p = 0.18 to p = 0.76). The axis of the flattest corneal meridian measured by Placido-based topography was significantly correlated with the axis of the flattest meridian measured by Fourier-based profilometry for 8, 10 and 12 mm diameters (r²  = 0.17 to 0.44; p < 0.05), while there was no correlation for 14 and 16 mm diameters (p = 0.48 and p = 0.75). For a typical soft contact lens diameter of 14 mm, 78% of the variance could be determined with a corneal topographer and 68% with keratometry and corneal diameter measurement.

CONCLUSIONS

The combination of corneal central radii, eccentricity and corneal diameter measured by Placido-based topography is a valid predictor of the corneoscleral sagittal height in healthy eyes. Scleral toricity and axis of the flattest meridian seem to be independent from Placido-based corneal parameters and requires additional measuring tools.