Relationship between the corneal surface and the anterior segment of the cornea: An Asian perspective

The Impact of Changes in Corneal Back Surface Astigmatism on the Residual Astigmatic Refractive Error following Routine Uncomplicated Phacoemulsification

Purpose

To determine the significance of any association between intersessional changes in ocular residual astigmatism (RA) and astigmatism at corneal front (FSA) and back (BSA) surfaces following uneventful routine phacoemulsification.

Methods

Astigmatism was evaluated by autorefractometry and subjective refraction and at both the corneal surfaces with Orbscan II™ (Bausch & Lomb) over central 3 mm and 5 mm optical zones at 1, 2, and 3 months after routine phacoemulsification in 103 patients implanted with monofocal nontoric intraocular lenses (IOLs, one eye/patient). Data were subjected to vector analysis to determine the actual change (Δ) in astigmatism (power and axis) for the refractive and Orbscan II findings.

Results

The number of cases that attended where ΔRA was ≥0.50 DC between 1 and 2 months was 52 by autorefractometry and 36 by subjective refraction and between 2 and 3 months was 24 by autorefractometry and 19 by subjective refraction. Vector analysis revealed significant correlations between ΔFSA and ΔRA for data obtained by autorefractometry but not by subjective refraction. At all times, ΔBSA was greater than ΔFSA (p < 0.01). Key findings for ΔBSA values over the central 3 mm zone were between (A) the sine of the axis of ΔRA (y) and sine of the axis of ΔBSA (x) for the data obtained by autorefractometry (between 1 and 2 months, y = 0.749 - 0.303x, r = 0.299, n = 52, p=0.031) and subjective refraction (between 2 and 3 months, y = 0.6614 - 0.4755x, r = 0.474, n = 19, p=0.040) and (B) ΔRA (y) and ΔBSA (x) powers between 2 and 3 months postoperatively for the data obtained by autorefractometry (ΔRA = 0.118 ΔBSA + 0.681 r = 0.467, n = 24, p=0.021) and subjective refraction (ΔRA = 0.072 ΔBSA + 0.545 r = 0.510, n = 19, p=0.026).

Conclusion

Changes in the ocular residual refractive astigmatic error after implanting a monofocal nontoric IOL are associated with changes in astigmatism at the back surface of the cornea within the central optical zone.

Three-dimensional non-parametric method for limbus detection

Purpose

To present a novel non-parametric algorithm for detecting the position of the human eye limbus in three dimensions and a new dynamic method for measuring the full 360° visible iris boundary known as white-to-white distance along the eye horizontal line.

Methods

The study included 88 participants aged 23 to 65 years (37.7±9.7), 47 females and 41 males. Clinical characteristics, height data and the apex coordinates and 1024×1280 pixel digital images of the eyes were taken by an Eye Surface Profiler and processed by custom-built MATLAB codes. A dynamic light intensity frequency based white-to-white detection process and a novel three-dimensional method for limbus detection is presented.

Results

Evidence of significant differences (p<0.001) between nasal-temporal and superior-inferior white-to-white distances in both right and left eyes were found (nasal-temporal direction; 11.74±0.42 mm in right eyes and 11.82±0.47 mm in left eyes & superior-inferior direction; 11.52±0.45 mm in right eyes and 11.55±0.46 mm in left eyes). Average limbus nasal-temporal diameters were 13.64±0.55 mm for right eyes, and 13.74±0.40 mm for left eyes, however the superior-inferior diameters were 13.65±0.54 mm, 13.75±0.38 mm for right and left eyes, respectively. No significant difference in limbus contours has been observed either between the nasal-temporal direction (p = 0.91) and the superior-inferior direction (p = 0.83) or between the right (p = 0.18) and left eyes (p = 0.16). Evidence of tilt towards the nasal-temporal side in the three-dimensional shape of the limbus was found. The right eyes mean limbus contour tilt around the X-axis was -0.3±1.35° however, their mean limbus contour tilt around the Y-axis was 1.76±0.9°. Likewise, the left eyes mean limbus contour tilt around the X-axis was 0.77±1.25° and the mean limbus contour tilt around the Y-axis was -1.54±0.89°.

Conclusions

The white-to-white distance in the human eye is significantly larger in the nasal-temporal direction than in the superior-inferior direction. The human limbus diameter was found not to vary significantly in these directions. The 3D measures show that the limbus contour does not lay in one plane and tends to be higher on the nasal-inferior side of the eye.

Keratometric index obtained by Fourier-domain optical coherence tomography

PURPOSE

To determine the keratometric indices calculated based on parameters obtained by Fourier-domain optical coherence tomography (FD-OCT).

METHODS

The ratio of anterior corneal curvature to posterior corneal curvature (Ratio) and keratometric index (N) were calculated within central 3 mm zone with the RTVue FD-OCT (RTVue, Optovue, Inc.) in 186 untreated eyes, 60 post-LASIK/PRK eyes, and 39 keratoconus eyes. The total corneal powers were calculated using different keratometric indices: Kcal based on the mean calculated keratometric index, K1.3315 calculated by the keratometric index of 1.3315, and K1.3375 calculated by the keratometric index of 1.3375. In addition, the total corneal powers based on Gaussian optics formula (Kactual) were calculated.

RESULTS

The means for Ratio in untreated controls, post-LASIK/PRK group and keratoconus group were 1.176 ± 0.022 (95% confidence interval (CI), 1.172-1.179), 1.314 ± 0.042 (95%CI, 1.303-1.325) and 1.229 ± 0.118 (95%CI, 1.191-1.267), respectively. And the mean calculated keratometric index in untreated controls, post-LASIK/PRK group and keratoconus group were 1.3299 ± 0.00085 (95%CI, 1.3272-1.3308), 1.3242 ± 0.00171 (95%CI, 1.3238-1.3246) and 1.3277 ± 0.0046 (95%CI, 1.3263-1.3292), respectively. All the parameters were normally distributed. The differences between Kcal and Kactual, K1.3315 and Kactual, and K1.3375 and Kactual were 0.00 ± 0.11 D, 0.21 ± 0.11 D and 0.99 ± 0.12 D, respectively, in untreated controls; -0.01 ± 0.20 D, 0.85 ± 0.18 D and 1.56 ± 0.16 D, respectively, in post-LASIK/PRK group; and 0.03 ± 0.67 D, 0.56 ± 0.70 D and 1.40 ± 0.76 D, respectively, in keratoconus group.

CONCLUSION

The calculated keratometric index is negatively related to the ratio of anterior corneal curvature to posterior corneal curvature in untreated, post-LASIK/PRK, and keratoconus eyes, respectively. Using the calculated keratometric index may improve the prediction accuracies of total corneal powers in untreated controls, but not in post-LASIK/PRK and keratoconus eyes.

Anterior and posterior corneal curvature: normal values in healthy Iranian population obtained with the Orbscan II

The objective of study was to determine the normative values of anterior and posterior best fit sphere (A-BFS and P-BFS) measured with Orbscan II Topography System. In this cross-sectional study, patients (age range: 18-40 years) referred to the Khatam Eye Hospital (Mashhad, Iran) were put in an observational cross-sectional study. The A-BFS and P-BFS were measured with the Orbscan II. The differences between genders, between right and left eyes, and age-related changes were evaluated. A total of 977 healthy participants consisted of 614 female and 363 male subjects aged 18-35 years participated. The average A-BFS in our study population was recorded as 43.060 ± 1.541 D (median: 43.00 D, mode: 43.10 D, range: 38.80-55.80 D). The average P-BFS in our study population was recorded as 52.702 ± 2.190 D (median: 52.60 D, mode: 53.10 D range: 46.9-62.20 D). The A-BFS and P-BFS were respectively 42.753 ± 1.629 and 52.327 ± 2.376 D in males and 43.242 ± 1.457 and 52.924 ± 2.041 D in females, which were statistically different between the genders (P < 0.001). However, A-BFS and P-BFS were not statistically different between right and left eyes (P = 0.649 and P = 0.688 respectively). In addition, A-BFS and P-BFS were not correlated with the age (r = 0.038, P = 0.096 and r = -0.142, P = 0.178 respectively). Considering 95 % confidence interval, A-BFS less than 43.13 D and greater than 42.99 D and P-BFS less than 52.80 D and greater than 52.60 D would be considered abnormal. Detailed description and analysis of A-BFS and P-BFS with Orbscan demonstrated that the obtained average value of BFS were higher in male than female and did not change with increasing age.

Corneal refractive power and eccentricity in the 40- to 64-year-old population of Shahroud, Iran

PURPOSE

To determine the normal corneal curvature, power, and eccentricity in an Iranian population and their determinants.

METHODS

This report is part of a population-based study conducted in 2009. Of the 5190 participants of the study, Pentacam data from 8532 eyes of 4266 people who met the inclusion criteria for this analysis were used. For each eye, we extracted minimum and maximum keratometry readings, the average of the 2 readings (mean-K), the difference between these 2 parameters (keratometric astigmatism), and corneal eccentricity.

RESULTS

The average mean-K, keratometric astigmatism, and eccentricity were 43.73 ± 2.47, 0.90 ± 0.93, and 0.27 ± 0.63 diopter, respectively. Mean-K was directly correlated with age; inversely correlated with body mass index, axial length, white-to-white corneal diameter, and anterior chamber depth; increased at higher amounts of myopia; and was higher in women compared with men. Keratometric astigmatism was significantly higher in women, increased at higher amount of refractive error, but showed no association with other variables. Eccentricity was correlated indirectly with age and white-to-white corneal diameter, and directly with axial length. It increased with myopia.

CONCLUSIONS

Compared with other studies, the mean corneal power and eccentricity values were lower in this Iranian population sample. Our findings may have implications for clinical interventions, especially refractive surgery. Further studies can identify the causes of such differences in the shape and size of the cornea, which may also be attributable to the choice of the measuring device.

Corneal elevation topography: best fit sphere, elevation distance, asphericity, toricity, and clinical implications

PURPOSE

To describe the effect of the corneal asphericity and toricity on the map patterns and best fit sphere (BFS) characteristics in elevation topography.

METHODS

The corneal surface was modeled as a biconic surface of principal radii and asphericity values of r1 and r2 and Q1 and Q2, respectively. The apex of the biconic surface corresponded to the origin of a polar coordinates system. Minimization of the squared residuals was used to calculate the values of the radii of the BFSs and apex distance (A-values: z distance between the corneal apex and the BFS) of the modeled corneal surface for various configurations relating to commonly clinically measured values of apical radius, asphericity, and toricity.

RESULTS

Increased apical radius of curvature and increased prolateness (negative asphericity) led to an increase in BFS radius but had opposite effects on the A-value. Increased prolateness resulted in increased BFS radius and A-value. Increasing toricity did not alter these findings. Color-plot elevation maps of the modeled corneal surface showed complete ridge patterns when toricity was increased and showed incomplete ridge and island patterns when prolateness was increased.

CONCLUSIONS

High A-values in patients with corneal astigmatism may result from steep apical curvature and/or high prolateness (negative asphericity). The BFS radius may help in distinguishing between these 2 causes of increased A-values. Increased prolateness and decreased apical radius of curvature (often seen in keratoconus) have opposite effects on the BFS radius but similar effects on the apex distance.

Extended scan depth optical coherence tomography for evaluating ocular surface shape

Spectral domain optical coherence tomography (SD-OCT) with extended scan depth makes it possible for quantitative measurement of the entire ocular surface shape. We proposed a novel method for ocular surface shape measurement using a custom-built anterior segment SD-OCT, which will serve on the contact lens fitting. A crosshair alignment system was applied to reduce the misalignment and tilting of the eye. An algorithm was developed to automatically segment the ocular surface. We also described the correction of the image distortion from the segmented dataset induced by the nontelecentric scanning system and tested the accuracy and repeatability. The results showed high accuracy of SD-OCT in measuring a bicurved test surface with a maximum height error of 17.4 μm. The repeatability of in vivo measurement was also good. The standard deviations of the height measurement within a 14-mm wide range were all less than 35 μm. This work demonstrates the feasibility of using extended depth SD-OCT to perform noninvasive evaluation of the ocular surface shape.

Ethnic differences in higher-order aberrations: Spherical aberration in the South East Asian Chinese eye

PURPOSE

To determine the distribution of higher-order corneal and ocular aberrations in a healthy refractive surgery population.

SETTING

Island Hospital, Penang, Malaysia.

METHODS

In this prospective observational study, 1 eye of ethnic Chinese refractive surgery patients was evaluated with an Orbscan II corneal topographer and a Zywave Hartmann-Shack aberrometer with a 6.0 mm pupil. Height data were analyzed to derive the higher-order aberrations (HOAs) from the 3rd to 5th Zernike order.

RESULTS

The mean spherical equivalent in the 70 eyes evaluated was -6.46 diopters +/- 3.10 (SD). The mean total corneal HOA was 0.574 +/- 0.218 microm (range 0.269 to 1.249 microm) and the mean total ocular HOA, 0.525 +/- 0.354 microm (range 0.138 to 2.145 microm). There was no statistically significant correlation with age. The mean 3rd-order ocular aberration was 0.399 +/- 0.287 microm; the mean 4th-order, 0.297 +/- 0.223 microm; and the mean 5th-order, 0.108 +/- 0.101 microm. Corneal spherical aberration was greater than ocular spherical aberration (mean 0.312 +/- 0.114 microm versus 0.200 +/- 0.170 microm). Multilinear regression showed that the only dependent that predicted ocular spherical aberration was anterior corneal asphericity (r(2) = 0.227, F = 17.95, P<.001).

CONCLUSION

Corneal and ocular aberrations in South East Asian Chinese eyes were significantly greater than that reported in other populations. Population differences in wavefront errors were significant, and this should be noted in patient management.

Measuring the cornea: the latest developments in corneal topography

PURPOSE OF REVIEW

Corneal measurement uses Placido-disc topographers and tomographers, creating three-dimensional corneal models from cross-sectional images. Technology includes slit scanning, Scheimpflug imaging, very high frequency ultrasound, and high-speed anterior segment optical coherence tomography.

RECENT FINDINGS

Normative data in the Asian population using the Orbscan, and repeatability information for the 3 and 5-mm zone were reported. Best fit sphere and the thinnest point were not significantly different, but posterior surface elevation was higher using the Orbscan in keratoconic eyes. Pachymetry data from the Pentacam show pattern differences used as keratoconus indices. Scheimpflug imaging identified changes in the posterior surface that were not mirrored by the anterior surface with aging, and may be better for surgical planning than conventional keratometry following excimer treatments. Optical coherence tomography mean central corneal thickness measurements were repeatable; however, mean central epithelial thickness measurements were not. Very high frequency ultrasound can be used successfully to create epithelial and flap thickness maps. Studies reveal specular microscopy measures thinner and less reliable readings than Pentacam and Orbscan.

SUMMARY

New corneal topographers, such as Scheimpflug imaging, ultrasound and optical coherence tomography have expanded capabilities and precision in measuring the structure of the cornea.

Validity of the keratometric index: Large population-based study

PURPOSE

To determine the accuracy of the keratometric index of 1.3315 based on the Gullstrand model eye in predicting the power of the posterior cornea, Gullstrand's model was compared to a calculated keratometric index derived from actual measurements of the cornea.

SETTING

Eye Institute, Tan Tock Seng Hospital, Singapore.

METHODS

One eye of 2429 subjects with a mean spherical equivalent of -5.32 diopters (D) +/- 2.88 (SD) was measured with the Orbscan II (Bausch & Lomb). The following variables were analyzed: anterior radius of curvature (r(anterior)), posterior radius of curvature (r(posterior)), radius of keratometry (r(simK)), and central pachymetry.

RESULTS

The r(anterior), r(posterior), and r(simK) were normally distributed, with a mean of 7.87 +/- 0.25 mm (95% confidence interval [CI], 7.38-8.36), 6.46 +/- 0.26 mm (95% CI, 5.95-6.97), and 7.71 +/- 0.27 mm (95% CI, 7.18-8.24), respectively. The mean ratio between the anterior corneal curvature and posterior corneal curvature was 1.22 +/- 0.03 (95% CI, 1.16-1.28). Based on the measurements of each eye, the mean calculated keratometric index, N(calc), was 1.3273 +/- 0.0013 (95% CI, 1.3248-1.3298). Using N(calc), the posterior corneal power was predicted to within +/-0.50 D of the actual posterior power in 98.3% of eyes. The mean absolute error between the actual and calculated posterior power was 0.157 +/- 0.123 D using N(calc) and 0.326 +/- 0.133 D using the Gullstrand model.

CONCLUSION

Modifying the keratometric index increased the accuracy of predicting the posterior corneal power.