Corneal Sagittal Height Research Articles

A Study on the Relationship between Corneal-Related Value and Ocular Sagittal Height

Purpose : To investigate the effect of corneal related values on the ocular sagittal height and which factors have the greatest influence on the ocular sagittal height. Methods : For 240 healthy eyes in their 20, 30, 40, and 50s, the anterior chamber depth and axial length were measured with IOLMaster 700. The corneal curvature radius and corneal eccentricity were measured with Keratograph 4, and spherical equivalent power was measured using KR-800. The corneal sagittal height was calculated using the formula. Results : The most influential factor in the corneal sagittal height was the corneal diameter, followed by the corneal eccentricity. And the corneal curvature radius had the least effect. A change in corneal saggital height of about 0.10mm corresponds to a change in corneal diameter of 0.20mm, and corneal eccentricity of 0.05 to 0.10 Conclusion : If soft contact lens fitting is greatly affected by the relationship between the sagittal depth of the contact lens and the sagittal height of the cornea, measurements of corneal diameter and corneal eccentricity are considered appropriate methods for selecting parameters of contact lenses. In addition, the numerical relationship between corneal sagittal height, corneal diameter and corneal eccentricity could be used as additional information for soft contact lens fitting.

Change in the geometry of positive- and negative-powered soft contact lenses during wear.

Contact lens wear causes mutual interactions between the ocular surface and the lens, which may affect comfort as well as vision. The aim of this study was to examine deformations in modern positive- and negative-powered silicone hydrogel soft contact lenses (SiH SCLs) after 7 days of continuous wear. This pre-post interventional study included 64 eyes: 42 eyes with myopia of -3.00 D and 22 eyes with hyperopia of +3.00 D. All patients underwent general ophthalmic examination, corneal topography/tomography, total corneal and epithelial thickness mapping, and specular microscopy before and after the wearing period. SiH SCLs made of senofilcon A were worn continuously for 7 days on all eligible eyes. The geometry of the new and used lenses was measured 3 to 6 minutes after removal in two perpendicular planes using a custom-made swept source optical coherence tomography (SS-OCT) system for in vitro measurements. The anterior and posterior radii of curvature decreased in -3.00 D lenses in two perpendicular planes. This effect correlated significantly with average keratometry of the cornea. Sagittal lens height was lower in +3.00 D lens after wear, which correlated moderately with the corneal sagittal height. A significant decrease in central corneal epithelial thickness was observed after wearing +3.0 D lenses. In conclusion, SiH SCLs made of senofilcon A undergo minor deformations after 7-day continuous wear. Geometry modifications are different for -3.00 D and +3.00 D lenses, and they imitate the shape of the anterior eye surface. These geometric changes are accompanied by a decrease in the central thickness of corneal epithelium after +3.00 D lens wear.

The Impact of Scleral Contact Lens Vault on Visual Acuity and Comfort.

To assess how varying degrees of corneal clearance of scleral contact lenses (ScCL) impact visual acuity (VA) and comfort in patients with corneal ectasia. Three ScCL were fitted to 20 subjects with previous diagnosis of either keratoconus (n=18) or pellucid marginal degeneration (n=2). Fitting of ScCL was based on corneal sagittal height (CSH) measured with Visante OCT at a 15-mm chord on the horizontal meridian. To select the ScCL from the diagnostic lens set, values of 325, 375, and 425 μm were randomly added in sequence to CSH. Subjects wore ScCL for 1 hr. Central corneal clearance (CCC) and topographic corneal clearance (TCC) along the vertical meridian were assessed using an ultralong optical coherence tomographer. High-contrast VA (HCVA) and low-contrast VA (LCVA) were measured using a logarithm of the minimum angle of resolution VA chart, and comfort ratings were obtained with a standard comfort scale (0-100). Mean CSH in the horizontal meridian was 3.78±0.53 (range: 3.33-4.17) mm at a 15-mm chord. Mean CCC was 190±100 μm (TCC: 160±94 μm at +3 mm and 180±94 μm at -3 mm), 360±120 μm (TCC: 260±100 μm at +3 mm and 330±110 μm at -3 mm), and 450±170 μm (TCC: 320±120 μm at +3 mm and 400±120 μm at -3 mm) for each lens (P=0.001). Mean HCVA for lenses 1, 2, and 3 were 0.05±0.12, 0.07±0.11, and 0.11±0.08 respectively, which were significantly different (P=0.02). Tukey post hoc analysis showed that this difference was only significant between lenses 1 and 3 (P=0.01). Similar findings were found for LCVA. Comfort ratings for lenses 1, 2, and 3 were 74.9±9.2, 79.7±11.6, and 78.6±10.8, respectively (P=0.24). The CSH is an effective method of determining the appropriate lens/cornea relationship. Lens 2 (+375 μm) gave the best combination of acuity and comfort ratings. Evaluation of the fluorescein pattern must be balanced with VA and comfort ratings for successful fitting in a clinical setting.

Relationship of placido corneal topography data with scleral lens fitting parameters.

To analyze the relationship between corneal sagittal height and asymmetry parameters derived from Placido-videokeratoscopy with the parameters of fitted scleral lenses (ScCLs). Corneal topographies were measured with MedmontE300 in a total of 126 eyes with irregular and regular corneas before ScCL fitting were analyzed. Measurements of sagittal height (OC-SAG) at steep and flat corneal meridians were obtained for 10 mm and 12 mm chords. Estimated Height (EHChord) parameters were taken for a chord equal to the diameter of the lens that each subject was wearing at different semi-meridians. Corneal asymmetry (difference in OC-SAG between steep and flat corneal meridians) was also assessed. These outcomes were correlated to ScCL parameters that subjects were wearing after 1 month. The mean ScCL-SAG was 4696 ± 240 μm, and the mean OC-SAG ranged from 1891 μm (10 mm), 2914 μm (12 mm), and between 4162 μm and 4251 μm for EH0-180º and EH30-210º. Stronger correlations (p < 0.001) between OC-SAG and ScCL-SAG were determined for EH0-180º (r = 0.595) and EH30-210º (r = 0.618). The mean differences between OC-SAG and ScCL-SAG were between 447 ± 290 μm (EH0-180º) and 389 ± 360 μm (EH30-210º). There was no relationship between corneal asymmetry and the need to fit a ScCL with toric haptic design in irregular corneas. Orientation of flat corneal and scleral meridians were similar only in corneas with high regular astigmatism. EHChord attributes were the parameters that best correlated with the ScCL-SAG. The corneal asymmetry was shown to be a poor predictor for the need to fit a ScCL with toricity at landing zone in irregular corneas, but could have some predictive power in regular corneas.

The influence of limbal and scleral shape on scleral lens design.

To summarize the research findings on the ocular surface profile, to provide a definition and a classification of the corneoscleral shape, and to offer guidelines in selecting scleral lens design. The definition of rotational symmetry and rotational asymmetry was inquired and PubMed searches were conducted. The better understanding of the scleral contact lens comportment on the eye and the introduction of new diagnostic instruments to measure the anterior ocular surface have led to improve comprehension of corneoscleral contour formulating new scleral lens designs. The scleral lens landing zone is influenced by corneoscleral profile which may be rotationally symmetric and rotationally asymmetric. Corneal sagittal height, limbal shape, corneoscleral junction profile, corneal, limbal, and conjunctival angles, and scleral shape should be taken in consideration to prevent and manage fitting problems, such as air bubble formation, midday fogging, localized blanching, impingement, conjunctival prolapse, lens decentration, lens flexure, and to increase comfort, wearing time, overall satisfaction, and visual quality. Corneoscleral shape may be considered rotationally symmetric including spherical, aspherical and toric profiles, and rotationally asymmetric including regular and irregular quadrants profiles. Each ocular surface contour requires a different landing zone design for an optimal fitting, vaulting properly over the cornea and limbus, and ideal alignment on the sclera. Further studies are still necessary to clarify many aspects of scleral lenses which are little known yet.

Effect of Time on Scleral Lens Settling and Change in Corneal Clearance.

With the increase in the use of scleral contact lenses among practitioners, questions regarding lens settling are gradually gaining attention. This is because current studies support the notion that scleral lenses settle back over time. More research is needed to understand the exact cause and the factors that underpin such phenomenon. The present study aims to assess the effect of time on topographic corneal clearance of three scleral contact lenses of varying sagittal depths. Three scleral contact lenses were fitted to 20 subjects with previous diagnosis of keratoconus (n = 18) or pellucid marginal degeneration (n = 2). The fit was based on corneal sagittal height measured with the Visante optical coherence tomographer (OCT) at 15 mm along the horizontal meridian. To select an appropriate lens from the diagnostic lens set, values of 325 μm (lens 1), 375 μm (lens 2), and 425 μm (lens 3) were randomly added in sequence to the corneal sagittal height. Subjects wore each lens for 1 hour. Corneal clearance was measured at 10-minute intervals for 1 hour using a custom ultra-long OCT. To assess change in clearance, central point and two mid-peripheral points (+3 mm and -3 mm) along an 8-mm chord were measured by taking differences at each time point up to 1 hour. Measurements were repeated for the two other lenses. Mean central corneal clearance loss for all three lenses was 33.83 ± 48.40 μm. This was 26 ± 27 μm (13 ± 14 μm, +3 mm; 34 ± 37 μm, -3 mm), lens 1; 35 ± 59 μm (38 ± 61 μm, +3 mm; 52 ± 69 μm, -3 mm), lens 2; and 41 ± 54 μm (33 ± 26 μm, +3 mm; 52 ± 48 μm, -3 mm), lens 3, respectively. There was no significant difference (P = 0.06) at central and other locations for lens 1 (location and over time). There were significant differences for both lenses 2 and 3 (P < .001, P = .01, respectively) for all three locations and over time. There is a likelihood of clearance loss after 1 hour of lens wear. This varies between subjects, initial lens-fit relationship, and over time.

Mathematical model for evaluating soft contact lens fit.

To evaluate the effect of varying lens and ocular topography parameters on soft contact lens (SCL) fit, using a novel computer spreadsheet model. Although SCLs are worn by more than 100 million ametropes, the factors governing their fitting characteristics are poorly understood. A spreadsheet-based computer model used a novel ellipto-conical corneal model coupled with population data on corneoscleral topography obtained in a previous clinical study. The model calculated lens edge strain (circumferential elongation) as a predictor of lens tightness. The following parameters were systematically varied: corneal curvature, corneal diameter, corneal shape factor, corneoscleral junction angle, lens base curve (BC), and diameter. The ellipto-conical corneal model showed closer concordance with actual measurements of corneal sagittal height than a simple elliptical model (limits of agreement, ±0.20 vs. ±0.25 mm; p = 0.0015). For an eye with average ocular parameters wearing a typical SCL design (BC, 8.60; diameter, 14.2 mm), the model calculated an edge strain of 2.7%. For the same SCL, the tightest fit (8.5% strain) was found with the eye showing the combination of smallest, flattest, most aspheric cornea. Conversely, the loosest fitting (-2.6%) was found with the eye showing the combination of largest, steepest, least aspheric cornea. A change in BC of 0.4 mm typically resulted in changes in edge strain of less than 2.5%, whereas a change in diameter of 0.5 mm resulted in a change of less than 2%. Using the typical SCL design and average corneal model, wide variations in corneoscleral junction angle did not critically affect lens fit. More extreme combinations of SCL and ocular parameters resulted in edge strain likely to result in a tight (>6%) or loose fit (<0%). A novel ellipto-conical corneal model in conjunction with spreadsheet mathematical modeling proved to be a useful tool for attempting to understand the factors governing SCL fit.

Predictive Value of Corneal Topography for ClearKone Hybrid Contact Lenses

The purpose of this study was to evaluate the predictive value of corneal topography data for fitting ClearKone hybrid contact lenses on eyes with corneal ectasia. Ophthalmic records from eyes with established corneal ectasia that had been successfully fitted with SynergEyes ClearKone lenses (n = 73) were retrospectively reviewed. Correlations between corneal parameters (simulated steep and flat keratometry values, average corneal sagittal height, inferior-superior index for keratoconus, corneal eccentricity, cone displacement, and cone diameter) and ClearKone lens parameters (vault and skirt curvature) were analyzed. Two topographical correlates to ClearKone lens parameters were identified. Mean weighted corneal sagittal height at a 7.4-mm chord was strongly predictive of the optimal ClearKone lens vault; a linear relationship was observed between these parameters (R = 0.96). More significant vertical decentration of the corneal apex was found to be associated with the need for a steeper lens skirt curvature (p < 0.05). This study demonstrates that baseline corneal topography-derived data are valuable for predicting lens parameters for an optimally fit SynergEyes ClearKone hybrid contact lens.

Factors Affecting Corneoscleral Topography

To evaluate factors affecting corneoscleral profile (CSP) using anterior segment optical coherence tomography (AS-OCT) in combination with conventional videokeratoscopy. OCT DATA WERE COLLECTED FROM 204 SUBJECTS OF MEAN AGE 34.9 YEARS (SD: ±15.2 years, range 18-65) using the Zeiss Visante AS-OCT and Medmont M300 corneal topographer. Measurements of corneal diameter (CD), corneal sagittal height (CS), iris diameter (ID), corneoscleral junction angle (CSJ), and scleral radius (SR) were extracted from multiple OCT images. Horizontal visible iris diameter (HVID) and vertical palpebral aperture (PA) were measured using a slit lamp graticule. Subject body height was also measured. Associations were then sought between CSP variables and age, height, ethnicity, sex, and refractive error. Significant correlations were found between age and ocular topography variables of HVID, PA, CSJ, SR, and ID (P < 0.0001), while height correlated with HVID, CD, and ID, and power vector terms with vertical plane keratometry, CD, and CS. Significant differences were noted between ethnicities with respect to CD (P = 0.0046), horizontal and vertical CS (P = 0.0068 and P = 0.0095), and horizontal ID (P = 0.0010). The same variables, with the exception of vertical CS, also varied with sex; horizontal CD (P = 0.0018), horizontal CS (P = 0.0018), and ID (P = 0.0012). Age accounted for the greatest variance in topography variables (36%). Age is the main factor influencing CSP; this should be taken into consideration in contact lens design, IOL selection, and in the optimization of surgical procedures. Ocular topography also varied with height, sex, ethnicity, and refractive error.

Pilot Study for OCT Guided Design and Fit of a Prosthetic Device for Treatment of Corneal Disease.

Purpose. To assess optical coherence tomography (OCT) for guiding design and fit of a prosthetic device for corneal disease. Methods. A prototype time domain OCT scanner was used to image the anterior segment of patients fitted with large diameter (18.5–20 mm) prosthetic devices for corneal disease. OCT images were processed and analyzed to characterize corneal diameter, corneal sagittal height, scleral sagittal height, scleral toricity, and alignment of device. Within-subject variance of OCT-measured parameters was evaluated. OCT-measured parameters were compared with device parameters for each eye fitted. OCT image correspondence with ocular alignment and clinical fit was assessed. Results. Six eyes in 5 patients were studied. OCT measurement of corneal diameter (coefficient of variation, CV = 0.76%), cornea sagittal height (CV = 2.06%), and scleral sagittal height (CV = 3.39%) is highly repeatable within each subject. OCT image-derived measurements reveal strong correlation between corneal sagittal height and device corneal height (r = 0.975) and modest correlation between scleral and on-eye device toricity (r = 0.581). Qualitative assessment of a fitted device on OCT montages reveals correspondence with slit lamp images and clinical assessment of fit. Conclusions. OCT imaging of the anterior segment is suitable for custom design and fit of large diameter (18.5–20 mm) prosthetic devices used in the treatment of corneal disease.

Anterior corneal profile with variable asphericity

We present a corneal profile in which the eccentricity, e(Q=-e(2)), has a nonlinear continuous variation from the center outwards. This nonlinear variation is intended to fit and reproduce our current experimental data in which the anterior corneal surface of the human eye exhibits different values of e at different diameters. According to our clinical data, the variation is similar to an exponential decay. We propose a linear combination of two exponential functions to describe the variation of e. We then calculate the corneal sagittal height by substituting e in the first-order aspherical surface equation to obtain the corneal profile. This corneal profile will be used as a reference to analyze the resultant profiles of the customized corneal ablation in refractive surgery.

Asphericity of the anterior human cornea with different corneal diameters

To measure the anterior corneal asphericity (Q) with different corneal diameters. Department of Physics (Optometry), University of Minho, Braga, Portugal. Thirty-six eyes of 36 patients were evaluated using a videokeratoscope, and the Q-values were recorded. Topographic data were also analyzed using Vol-CT 6.89 software (Sarver & Associates, Inc) to obtain the Q-values with different corneal diameters (3.0 mm, 4.0 mm, 5.0 mm, 6.0 mm, and 7.0 mm). Variable Q models of corneal sagittal height were compared against models assuming constant Q-values obtained with the Medmont E300 videokeratoscope (Medmont Pty. Ltd.) and a standard Q model of -0.26. The peripheral rate of change in corneal Q with different corneal diameters increased as corneal astigmatism increased. As a result, differences in the sagittal height between the constant model and variable model were evident beyond the central 3.0 mm area. There were significant differences between low and high astigmatic corneas in Q-values measured by the Medmont along the flattest meridian (P = .004) and Q-values obtained with Vol-CT software with a 7.0 mm corneal diameter (P = .026). There were differences in sagittal corneal height calculations considering constant or variable models of Q. Concern arises when surgical interventions depend on corneal Q-values to predict the outcomes. Surgeons should be aware which procedure is behind Q computing by different corneal topographers and that a constant Q-value cannot reflect the actual shape of the cornea as significant departures from the actual sagittal height can arise depending on which Q-value is assumed.

The effects of overnight orthokeratology lens wear on corneal thickness.

To investigate corneal thickness changes during overnight orthokeratology with reverse-geometry rigid gas-permeable (RGP) contact lenses worn over a 3-month period. Eighteen young adult subjects with low myopia (<or=4.00 D) were fitted with reverse-geometry lenses (BE; UltraVision Pty. Ltd., Brisbane, Queensland, Australia), which were worn for 3 months on an overnight basis and were removed during the day. Another 10 subjects were fitted with conventional RGP lenses (J-Contour; UltraVision) that were worn for 1 month in the right eye on a similar wearing schedule; the left eye acted as a non-lens-wearing control. Refractive error was recorded in the morning and evening, and total, epithelial, and stromal corneal thicknesses were measured across the horizontal meridian with an optical pachometer. The orthokeratology group showed significant reductions in myopia (+1.66 +/- 0.50 D; P < 0.001) from day 1, which stabilized by day 10. Central corneal thinning (-9.3 +/- 5.3 microm, P < 0.001), which was epithelial in origin, was found from day 1; central stromal change was negligible. Midperipheral corneal thickening, which was stromal in origin, was confirmed by day 4 (+10.9 +/- 5.9 microm, P < 0.001). No change was found in peripheral corneal thickness. Analysis of day-90 data by Munnerlyn's formula indicated that corneal sagittal height change resulting from the thickness changes could account for the refractive effect. In the conventional RGP group, there were no significant changes in refractive error or corneal thickness. Overnight orthokeratology causes rapid central corneal epithelial thinning and midperipheral stromal thickening. The consequent change in corneal sagittal height is the primary factor underlying the refractive effect of orthokeratology.

Corneal Response to Orthokeratology

The technique of orthokeratology produces a corneal response to the mechanical pressures exerted by rigid contact lenses. This paper reports a study which investigated the topographic and pachometric corneal changes induced by orthokeratology. Six young myopic subjects (11 eyes) wore "accelerated orthokeratology" lenses (OK-74; Contex Inc., Sherman Oaks, CA) in a high Dk material (AirPerm; Dk = 88) for 28 days. Corneal and epithelial thickness were measured topographically using the Holden-Payor optical micropachometer, and corneal topography was monitored using the EyeSys system. Refractive error change reached 1.71 +/- 0.59 D reduction in myopia after 28 days. After 1 day of lens wear, statistically significant central corneal flattening was noted, which progressed to reach 0.22 +/- 0.07 mm (1.19 +/- 0.38 D) at 28 days. A trend toward central epithelial thinning was apparent, reaching statistical significance on day 28 (7.1 +/- 7.1 microm; 9.6%). Midperipheral corneal thickening was also found approximately 2.5 mm from the corneal center, which was statistically significant by day 14 (13.0 +/- 11.1 microm; 2.4%). Calculations using Munnerlyn's formula indicate that changes in corneal sagittal height based on topographical thickness changes across the flattened central 5.25-mm zone can account for the refractive changes observed. These findings suggest that the initial corneal response to orthokeratology may be explained by redistribution of corneal tissue, rather than by overall bending of the cornea.

Calculation and application of the anterior surface area of a model human cornea

The macroscopic anterior surface area was calculated for three models of the average human cornea. Two models, a general ellipsoid and a rotational ellipse (rotationally symmetric ellipsoid) gave a surface area of 132 mm2, while a spherical model gave 126 mm2. A general ellipsoidal model having the maximum radius horizontal (with-the-rule corneal astigmatism) has less surface area than a rotational ellipse with the same horizontal radius. For a corneal sagittal height of 2.59 mm, the surface area of an ellipsoidal cornea equals -19.2Q + 16.3R -0.476 which specifies a rotational ellipse (radius R, asphericity Q) of equal surface area. In a cornea with the maximum radius vertical (against-the-rule corneal astigmatism), the ellipsoid has slightly more surface area than a rotational ellipse with the same horizontal radius of curvature. For a given horizontal radius of curvature, the sphere has the lowest surface area. For a corneal sagittal height s of 2.59 mm, the sphere underestimates by 8% the surface area of a rotational ellipse with asphericity -0.5. The anterior corneal surface area of a rotational ellipse model, radius R, asphericity Q is given by 2 pi Rs- 19.2Q. In all three models, the surface area increases with horizontal radius of curvature. In the rotational ellipse model, the rate of increase (slope) is independent of asphericity, and the slope found in with-the-rule astigmatism is less than the slope found with against-the-rule astigmatism. The calculated surface area predicts a precorneal tear volume of 0.86 microliter for a 6.5 micron tear thickness. The apparent, or plane projected are of an epithelial lesion underestimates the curved surface area with a percentage error that increases rapidly with lesion diameter. For a 12 mm diameter lesion on a rotational ellipse model, the apparent area underestimates the surface area by 18%. The average posterior corneal surface in human is not spherical but imitates the anterior surface, and has an area of 137 mm2 or 3.8% greater than the anterior area.