Repeatability and Reproducibility of Posterior Corneal Curvature Measurements by Combined Scanning-Slit and Placido-Disc Topography after LASIK

Long-term evaluation of posterior corneal surface parameters after accelerated corneal cross-linking with a comparison with uncross-linked keratoconic eyes

PURPOSE

To evaluate the 36 months changes in posterior corneal surface parameters in keratoconic eyes after accelerated corneal cross-linking and to compare the data with uncross-linked progressive and non-progressive keratoconic eyes.

METHODS

Thirty five cross-linked, 30 uncross-linked progressive, and 30 uncross-linked non-progressive keratoconic eyes were included. Maximum keratometry (K_max), thinnest pachymetry, minimum radius of curvature back (R_minback), asphericity back, posterior elevation and corneal densitometry, back corneal higher order aberrations (HOAs), back surface deviation (Db), final D, posterior radius of curvature (PRC) and 'B' unit values were recorded at baseline and at the 12, 24, 36 months follow-up. Data were analyzed with repeated measures ANOVA and paired t-tests.

RESULTS

K_max and thinnest pachymetry were significantly changed in the cross-linked and progressive uncross-linked groups. R_minback, asphericity back, and HOAs did not change in either group. Total posterior corneal densitometry improved; posterior elevation, Db and B unit worsened in the cross-linked group and did not change in the uncross-linked groups. PRC and final D worsened in the cross-linked and progressive uncross-linked groups, and did not change in the non-progressive group.

CONCLUSION

Despite a decreased K_max, the posterior corneal surface parameters, posterior elevation values were determined to have significantly worsened in the cross-linked group and this increase was higher than in progressive uncross-linked eyes.

Intrasession repeatability of corneal, limbal and scleral measurements obtained with a fourier transform profilometer

PURPOSE

To evaluate the intrasession repeatability of corneal, limbal and scleral measurements obtained by an experienced operator with a Fourier transform profilometer in healthy eyes.

METHODS

Prospective, single-center study including 35 eyes of 35 participants with ages ranging from 13 to 52 years. All patients underwent three consecutive corneoscleral topographic evaluations with the Eye Surface Profiler (ESP) system (Eaglet Eye b.v.). Intrasession repeatability was analyzed for different geometric and sagittal height variables using the following parameters: the within-subject standard deviation (S_w) of the three consecutive measurements, intrasubject precision (1.96 × S_w), coefficient of variation (CV) and the intraclass correlation coefficient (ICC).

RESULTS

Inner best fit sphere (BFS) showed good repeatability, with ICC of 0.844. Higher variability was observed for the repeated measurements of limbus and outer BFS, with ICCs of 0.636 and 0.739, respectively. For mean corneal and scleral radius, ICCs were 0.933 and 0.888, respectively. The repeatability of all sagittal height data was good for all chords evaluated (11-15 mm), with Sw values from 0.02 to 0.11 mm, and ICCs from 0.568 to 0.909. A significant positive correlation was found between the magnitude of temporal-nasal sagittal height difference for all measured chords and its S_w associated.

CONCLUSION

The ESP system can provide consistent measurements of sagittal height data for different chord diameters as well as for mean corneal and scleral radius in healthy eyes. Best fit approaches for limbal and scleral areas were less repeatable, although within a clinically acceptable range.

Effect of post crosslinking haze on the repeatability of Scheimpflug-based and slit-scanning imaging devices

Purpose:

The aim of this study was to analyze the effect of postcollagen crosslinking (CXL) haze on the measurement and repeatability of pachymetry and mean keratometry (Km) of four corneal topographers.

Materials and Methods:

Sixty eyes of sixty patients with progressive keratoconus who had undergone accelerated CXL (ACXL) underwent imaging with a scanning slit imaging device (Orbscan II) and three Scheimpflug imaging devices (Pentacam HR, Sirius, and Galilei). Post-ACXL haze was measured using the densitometry software on the Pentacam HR. Readings of the thinnest corneal thickness (TCT) and Km from three scans of each device were analyzed. Effect of haze on the repeatability of TCT and Km measurements was evaluated using regression models. Repeatability was assessed by coefficient of variation.

Results:

Corneal densitometry in different zones affected the repeatability of TCT measurement of Orbscan (P < 0.05) significantly but not the repeatability of TCT with Pentacam HR and Sirius (P = 0.03 and 0.05, respectively). Km values were affected by haze when measured with the Pentacam HR (P < 0.05). The repeatability of Km readings for all devices was unaffected by haze. In the anterior 0–2 mm and 2–6 mm zone, TCT (P = 0.43 and 0.45, respectively), Km values (P = 0.4 and 0.6, respectively), repeatability of TCT (P = 0.1 in both zones), and Km (P = 0.5 and 0.1, respectively) with Galilei were found to be the most reliable.

Conclusion:

Galilei measurements appear to be least affected by post-ACXL haze when compared with other devices. Hence, topography measurements in the presence of haze need to be interpreted with caution.

Intraocular lens calculations after laser vision correction

PURPOSE OF REVIEW

This article describes different strategies for corneal measurements and/or intraocular lens (IOL) calculations and proposes a systematic approach for IOL selection in patients who have undergone laser corneal refractive surgery.

RECENT FINDINGS

Corneal measurements and IOL calculations cannot be obtained accurately with the standard measuring technologies and formulas in patients with history of laser corneal refractive surgery; therefore a variety of methods and formulas, some of which required prerefractive surgery data, have been proposed to improve the accuracy of measurements and calculations. Formulas that do not rely on prerefractive data seem to be as accurate as those that do; therefore the lack of prerefractive data no longer presents an obstacle for accurate IOL selection in these patients.

SUMMARY

Postrefractive patients undergoing cataract extraction and IOL implantation should have corneal measurements and IOL calculations that take into account and compensate for the limitations in accurate measurements and calculations. IOL selection should also aim to compensate for induced spherical aberration according to the ablation pattern.

Cataract surgery on the previous corneal refractive surgery patient

Cataract surgery in cases with previous corneal refractive surgery may be a major challenge for the ophthalmologist. The refractive outcome of the case deserves special attention in the preoperative planning process, which should be tailored for the type of prior refractive procedure: incisional, ablative under a flap, or on the corneal surface. Avoiding refractive surprise after cataract surgery in these cases is principally dependent on the accuracy of the intraocular lens calculation, together with the selection of the appropriate biometric formula for each case. Modern techniques for cataract surgery help surgeons to move toward the goal of cataract surgery as a refractive procedure free from refractive error. We give practical guidelines for the cataract surgeon in the management of these challenging cases.

Corneal topography in keratoconus: state of the art

The morphological characterization of the cornea using corneal topographers is a widespread clinical practice that is essential for the diagnosis of keratoconus. The current state of this non-invasive exploratory technique has evolved with the possibility of achieving a great number of measuring points of both anterior and posterior corneal surfaces, which is possible due to the development of new and advanced measurement devices. All these data are later used to extract a series of topographic valuation indices that permit to offer the most exact and reliable clinical diagnosis. This paper describes the technologies in which current corneal topographers are based on, being possible to define the main morphological characteristics that the keratoconus pathology produces on corneal surface. Finally, the main valuation indices, which are provided by the corneal topographers and used for the diagnosis of keratoconus, are also defined.

Effect of Misalignment between Successive Corneal Videokeratography Maps on the Repeatability of Topography Data

Purpose

To improve the reliability of corneal topographic data through the development of a method to estimate the magnitude of misalignment between successive corneal videokeratography (VK) maps and eliminate the effect of misalignment on the repeatability of topography data.

Methods

Anterior and posterior topography maps were recorded twice for 124 healthy eyes of 124 participants using a Pentacam, and the repeatability of measurements was assessed by calculating the differences in elevation between each two sets of data. The repeatability of measurements was re-assessed following the determination of the magnitude of misalignment components (translational displacements: x₀, y₀ and z₀, and rotational displacements: α, β and γ) between each two data sets and using them to modify the second data set within each pair based on an Iterative Closest Point (ICP) algorithm. The method simultaneously considered the anterior and posterior maps taken for the same eye since they were assumed to have the same set of misalignment components. A new parameter, named Combined Misalignment parameter (CM), has been developed to combine the effect of all six misalignment components on topography data and so enable study of the association between misalignment and the data repeatability test results.

Results

The repeatability tests resulted in average root mean square (RMS) differences in elevation data of 8.46±2.75 μm before ICP map matching when simultaneously considering anterior and posterior surfaces. With map matching and misalignment correction, the differences decreased to 7.28±2.58 μm (P = 0.00). When applied to only the anterior maps, misalignment correction led to a more pronounced reduction in elevation data differences from 4.58±1.84 μm to 2.97±1.29 μm (P = 0.00). CM was found to be associated with the repeatability error (P = 0.00), with posterior maps being responsible for most of the error due to their relatively lower accuracy compared to anterior maps.

Conclusions

The ICP algorithm can be used to estimate, and effectively correct for, the potential misalignment between successive corneal videokeratography maps.

Geometrical custom modeling of human cornea in vivo and its use for the diagnosis of corneal ectasia

AIM

To establish a new procedure for 3D geometric reconstruction of the human cornea to obtain a solid model that represents a personalized and in vivo morphology of both the anterior and posterior corneal surfaces. This model is later analyzed to obtain geometric variables enabling the characterization of the corneal geometry and establishing a new clinical diagnostic criterion in order to distinguish between healthy corneas and corneas with keratoconus.

METHOD

The method for the geometric reconstruction of the cornea consists of the following steps: capture and preprocessing of the spatial point clouds provided by the Sirius topographer that represent both anterior and posterior corneal surfaces, reconstruction of the corneal geometric surfaces and generation of the solid model. Later, geometric variables are extracted from the model obtained and statistically analyzed to detect deformations of the cornea.

RESULTS

The variables that achieved the best results in the diagnosis of keratoconus were anterior corneal surface area (ROC area: 0.847, p<0.000, std. error: 0.038, 95% CI: 0.777 to 0.925), posterior corneal surface area (ROC area: 0.807, p<0.000, std. error: 0.042, 95% CI: 0,726 to 0,889), anterior apex deviation (ROC area: 0.735, p<0.000, std. error: 0.053, 95% CI: 0.630 to 0.840) and posterior apex deviation (ROC area: 0.891, p<0.000, std. error: 0.039, 95% CI: 0.8146 to 0.9672).

CONCLUSION

Geometric modeling enables accurate characterization of the human cornea. Also, from a clinical point of view, the procedure described has established a new approach for the study of eye-related diseases.

Scheimpflug imaged corneal changes on anterior and posterior surfaces after collagen cross-linking

AIM

To compare the anterior and posterior corneal parameters before and after collagen cross-linking therapy for keratoconus.

METHODS

Collagen cross-linking was performed in 31 eyes of 31 keratoconus patients (mean age 30.6±8.9y). Prior to treatment and an average 7mo after therapy, Scheimpflug analysis was performed using Pentacam HR. In addition to corneal thickness assessments, corneal radius, elevation, and aberrometric measurements were performed both on anterior and posterior corneal surfaces. Data obtained before and after surgery were statistically analyzed.

RESULTS

In terms of horizontal and vertical corneal radius, and central corneal thickness no deviations were observed an average 7mo after operation. Corneal higher order aberration showed no difference neither on anterior nor on posterior corneal surfaces. During follow-up period, no significant deviation was detected regarding elevation values obtained by measurement in mm units between the 3.0-8.0 mm-zones.

CONCLUSION

Corneal stabilization could be observed in terms of anterior and posterior corneal surfaces, elevation and higher order aberration values 7mo after collagen cross-linking therapy for keratoconus.

Technologies for anatomical and geometric characterization of the corneal structure and anterior segment: a review

Corneal and anterior segment imaging techniques have become a crucial tool in the clinical practice of ophthalmology, with a great variety of applications, such as corneal curvature and pachymetric analysis, detection of ectatic corneal conditions, anatomical study of the anterior segment prior to phakic intraocular lens implantation, or densitometric analysis of the crystalline lens. From the Placido-based systems that allow only a characterization of the geometry of the anterior corneal surface to the Scheimpflug photography-based systems that provide a characterization of the cornea, anterior chamber, and crystalline lens, there is a great variety of devices with the capability of analyzing different anatomical parameters with very high precision. To date, Scheimpflug photography-based systems are the devices providing the more complete analysis of the anterior segment in a non-invasive way. More developments are required in anterior segment imaging technologies in order to improve the analysis of the crystalline lens structure as well as the ocular structures behind the iris in a non-invasive way when the pupil is not dilated.

Corneal topography and corneal thickness in children

PURPOSE

To evaluate the corneal topography, thickness, and elevation (anterior and posterior corneal surface) in children.

METHODS

One hundred eyes of 100 patients with a mean ± standard deviation age of 10.6 ± 2.7 years (range: 5 to 15 years) were imaged. The corneal elevation maps were classified into regular ridge, irregular ridge, incomplete ridge, island, and unclassified patterns.

RESULTS

Mean simulated keratometry (Sim K) astigmatism was -0.69 ± 0.35 diopters (D) with mean maximum and minimum keratometry (K) of 44.26 ± 1.55 and 43.56 ± 1.57 D, respectively. Mean astigmatism and refractive power in the 3- and 5-mm zones was 0.64 ± 0.36, 43.85 ± 1.53 and 0.78 ± 0.5, 43.41 ± 1.48, respectively. The thinnest site on the cornea had an average thickness of 540 ± 34.03 μm. This site was most commonly located in the inferotemporal quadrant in 61% of eyes, followed by the superotemporal quadrant in 31%. Among the nine regions of the cornea evaluated, the central cornea had the lowest average thickness of 540 ± 34.03 μm and the superonasal cornea had the greatest average thickness of 628 ± 38.94 μm. The most common anterior corneal elevation pattern was the incomplete ridge (52%), followed by the island pattern. The incomplete ridge was most commonly observed on the posterior corneal elevation map (35%).

CONCLUSIONS

These results provide normal standards for elevation and curvature topography and corneal thickness in children. These data may prove useful as a reference for future comparative studies of different corneal diseases in children.

Posterior corneal curvature assessment after Epi-LASIK for myopia: comparison of Orbscan II and Pentacam imaging

PURPOSE

To compare the changes in posterior corneal curvature using scanning slit topography (Orbscan II) and Scheimpflug imaging (Pentacam) before and after Epi-laser in situ keratomileusis (LASIK) for myopia.

METHODS

In a prospective observational case-series study, 20 myopic patients having undergone Epi-LASIK were examined serially with two different devices, Orbscan II and Pentacam, preoperatively and one month postoperatively. Posterior central elevation (PCE) and posterior maximal elevation (PME) were compared between the two devices, and the changes in parameters after Epi-LASIK were analyzed using a difference map.

RESULTS

All parameters (preoperative and postoperative PCE and preoperative and postoperative PME) that were measured using the Orbscan II were significantly greater compared to those of the Pentacam (for all p < 0.001). PCE and PME were significantly increased one month postoperatively in the Orbscan II measurements (p < 0.05) but were not significantly increased in the Pentacam measurements. Also, ΔPCE and ΔPME, in the difference map obtained by each serial scanning, were significantly greater in the Orbscan II measurements than with the Pentacam (p = 0.012, p = 0.016).

CONCLUSIONS

The Pentacam measurements displayed significantly reduced values in all parameters related to posterior corneal elevation compared to those of the Orbscan II. The Pentacam showed no significant change in posterior corneal curvature after Epi-LASIK, based on the difference map.

Precision analysis of posterior corneal topography measured by Visante Omni: repeatability, reproducibility, and agreement with Orbscan II

PURPOSE

To evaluate the repeatability and reproducibility of posterior corneal curvature and posterior corneal elevation best-fit sphere (BFS) obtained with the Visante Omni (Carl Zeiss Meditec) and to compare the results with the Orbscan II (Bausch & Lomb).

METHODS

Thirty eyes from 30 healthy volunteers were included in this study. All patients were examined 5 times with the Visante Omni and Orbscan II by 2 independent operators. The posterior corneal curvature (3- and 6-mm zone) and posterior corneal elevation BFS (5- and 8-mm zone) were generated for each system. Intraoperator repeatability and interoperator reproducibility and agreement between the systems were evaluated using the intraclass correlation coefficient (ICC) and the Bland-Altman method.

RESULTS

The repeatability of posterior corneal curvature and posterior corneal elevation BFS measured by the Visante Omni was high for all analysis zones (ICC, 0.99 to 1.00). The reproducibility also showed similar results (ICC, 0.99 to 1.00). Agreement between the Visante Omni and Orbscan II was high for posterior corneal curvature (ICC, 0.94 to 0.97) and posterior corneal elevation BFS (ICC, 0.96 to 0.98) with 95% limits of agreement at -0.26 to 0.22 diopters for posterior corneal curvature and 0.11 to 0.69 mm for posterior corneal elevation BFS.

CONCLUSIONS

The Visante Omni provides good repeatability and reproducibility of posterior corneal topography. Overall agreement with the Orbscan II system was high.

Investigation of corneal topography after deep lamellar endothelial keratoplasty

PURPOSE

To analyze the 6-month anterior and posterior topographic changes of the cornea after deep lamellar endothelial keratoplasty.

METHODS

Orbscan topographies of 22 eyes from 21 patients with corneal decompensation were retrospectively analyzed: 11 eyes received a 9-mm scleral access incision (large-incision group) and 11 eyes received a 5-mm scleral access incision (small-incision group). All the preoperative and postoperative corneal raw data were imported into custom software which computed the average composite corneal maps and difference maps for both study groups in order to evaluate the corneal response to the surgery. The software delineated 2 concentric zones of the cornea to characterize the regional response following the surgery: the central and peripheral regions.

RESULTS

There were no significant differences (analysis of variance, <0.35 D, p>0.05) between 6-month postoperative and preoperative average anterior central corneal topographies in either group. At the end of follow-up, the average posterior curvature tangential map did not significantly differ from before surgery in either group (<0.35 D, p>0.05). However, a significant increase (>0.60 D, p<0.01) in the average posterior central astigmatic power of the cornea was found in both groups.

CONCLUSIONS

Deep lamellar endothelial keratoplasty effectively preserves the preoperative keratometric corneal topography, minimizing changes in curvature and astigmatism of the cornea. The great predictability of corneal topography following deep lamellar endothelial keratoplasty is likely to be attributed to the minimal changes that occur in the anterior stroma, the portion of the cornea that appears to be mainly responsible for maintenance of corneal shape.

Corneal volume, pachymetry, and correlation of anterior and posterior corneal shape in subclinical and different stages of clinical keratoconus

PURPOSE

To evaluate the corneal volume, pachymetry, and correlation of anterior and posterior corneal shape in subclinical and clinical keratoconus.

SETTING

Vissum Corporation, Alicante, Spain.

METHODS

Eyes were placed into 1 of 4 groups as follows: keratoconus 2 (grade II), keratoconus 1 (grade I), subclinical (subclinical keratoconus), and control (normal eyes). All eyes had an ophthalmologic examination including corneal evaluation (curvature, elevation, asphericity, pachymetry, corneal volume) by rotating Scheimpflug imaging (Pentacam). The posterior-anterior corneal power ratio was also calculated.

RESULTS

Seventy-one eyes (51 patients; aged 16 to 64 years) were evaluated. Astigmatism and keratometry of both corneal surfaces were statistically significantly higher in the keratoconus 1 and 2 groups (P<or=.02). Posterior astigmatism was statistically significantly higher in the subclinical group than in the control group (P = .01). A strong correlation (r>or=-0.81) was found between anterior and posterior curvature in the normal and subclinical groups; the correlation was weaker in clinical keratoconus cases (r<or=-0.56). The correlation in astigmatism between the anterior and posterior surface was good in all keratoconus groups (r>or=0.81). The posterior-anterior corneal power ratio was significantly higher in the keratoconus 2 group than in the other groups (P<or=.01). Pachymetric readings were progressively lower in eyes with subclinical, early, or moderate keratoconus (P<.01). The corneal volume was statistically significantly lower in the keratoconus 2 group than in the other groups (P = .04).

CONCLUSION

The correlation between anterior and posterior corneal curvature was lower in keratoconus, although the correlation between anterior and posterior astigmatism was maintained.

Performance in specular reflection and slit-imaging corneal topography

PURPOSE

Assessment of the relative performance in measuring corneal shape and corneal aberrations for two specular reflection topographers: Keratron Placido Ring Topographer, VU Topographer, and two slit-lamp imaging instruments: Orbscan II and Topcon SL-45 Scheimpflug.

METHODS

Corneal height maps of the anterior corneal surface were obtained from a group of 34 subjects with all four instruments; posterior corneal surface height maps were only obtained with the two slit-lamp imaging instruments. Corneal surface shapes are calculated in terms of radius of curvature and asphericity fitting an aspheric model. Wave aberrations for the anterior corneal surface and the total cornea are determined up to and including sixth order Zernike convention by means of ray tracing.

RESULTS

Clinical relevant differences were observed for radius of curvature of the anterior corneal surface, where the slit-imaging instruments measure higher values (mean difference = 0.05 mm, p < 0.05) and anterior corneal astigmatism for which the Orbscan II measures higher values than the VU Topographer [mean difference = 0.174 microm (0.134 Equivalent Diopters), p < 0.01]. Small significant differences were observed for asphericity and spherical aberration of the anterior corneal surface; however, these are not clinically relevant. Clinically relevant differences were also observed for posterior radius (difference = 0.135 mm p < 0.001), total corneal astigmatism (difference = 0.207 microm (0.159 Equivalent Diopters), p = 0.001), and central corneal thickness (CCT) (difference = -18.6 microm, p < 0.001). The differences found for total corneal coma and trefoil were not clinical relevant. Furthermore, the precision of the specular reflection topographers is superior to that of the slit-lamp instruments by at least a factor of two.

CONCLUSIONS

For traditional spectacle and contact lens applications, the corneal topographers are interchangeable except for measuring anterior radius of curvature. However, for more modern techniques as customized corneal refractive surgery, the subtle differences (e.g., total corneal astigmatism and CCT) between the instruments are clinically relevant.

Advances in technologies for laser-assisted in situ keratomileusis (LASIK) surgery

Laser-assisted in situ keratomileusis (LASIK) has become the most widely used form of refractive surgery today. The objective of this surgical technique is to modify the anterior corneal shape by ablating tissue from the stroma by means of the excimer laser after creating a hinged corneal flap. This way, we are able to change the refractive status of the patient, providing better unaided vision. Continuous improvements in the original technique have made the surgical procedure safer, more accurate and repeatable. These progressions are due to the development of novel technologies that are the responsible for new surgical instrumentation, which makes the surgical procedure easier for the surgeon, and better excimer laser ablation algorithms, which increase the optical quality of the ablation and thus the safety of the vision correction procedure. This article aims to describe the more relevant advances in LASIK that have played an important role in the spread and popularity of this technique.

Analysis of ectasia after laser in situ keratomileusis: Risk factors

PURPOSE

To examine a database of laser in situ keratomileusis (LASIK) procedures for preoperative and operative factors assumed to increase the risk for developing post-LASIK ectasia.

SETTING

Private clinical practice.

METHODS

A computer database was queried for eyes that had LASIK for myopic refractive errors with the following characteristics: preoperative corneal thickness 500 microm or less, mean keratometry greater than 47.0 diopters (D), patient age 25 years or younger, attempted correction greater than -8.0 D, refractive astigmatism not with-the-rule and greater than 2.0 D, and residual stromal bed thickness (RST) 250 microm or less. Flap thickness and RST were measured using ultrasound pachymetry. All recorded information was exported to MS Excel and analyzed for eyes that had ectasia.

RESULTS

Of the 9700 eyes in the database, none with the above characteristics developed ectasia over mean follow-up periods exceeding 2 years. Seven eyes had multiple risk factors without ectasia. Three eyes with abnormal preoperative topography developed ectasia.

CONCLUSIONS

Individual preoperative and operative factors did not in and of themselves increase the risk for ectasia. Unmeasured and unknown factors that affect the individual cornea's biomechanical stability, in combination with some suspected risk factors as well as the current inability to identify corneas at risk for developing ectatic disorders, probably account for most eyes that develop ectasia today.

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.

Intrasession and intersession repeatability of the Pentacam system on posterior corneal assessment in the normal human eye

PURPOSE

To study the intrasession and intersession repeatability of the Pentacam system (Oculus Inc.) in measuring the posterior corneal shape.

SETTING

School of Optometry, Hong Kong Polytechnic University, Hong Kong, China.

METHODS

The posterior corneal shape in 1 eye (randomly selected) was measured with the Pentacam system. Three consecutive readings were taken for intrasession repeatability analysis. Measurements were repeated 1 to 2 weeks later for intersession repeatability analysis. The anterior best-fit sphere (BFS) and posterior BFS at the 5.0 mm and 8.0 mm zones, as well as the elevation of the posterior cornea at these 2 zones, were compared.

RESULTS

The anterior and posterior BFS demonstrated good intrasession repeatability (Cronbach alpha > or = 0.997; intraclass correlation coefficients > or = 0.990). No significant difference was found between the 2 visits. The 95% limits of agreement of posterior BFS were 52.3 to -41.7 microm at the 5.0 mm zone and 66.1 to -60.9 microm at the 8.0 mm zone. The elevation was similar between the 2 visits. The 95% limits of agreement were 4.27 to -5.29 microm at the 5.0 mm zone and 5.33 to -5.77 microm at the 8.0 mm zone.

CONCLUSIONS

Pentacam provided good performance in assessing the posterior cornea. To improve the intersession repeatability, it is suggested that 3 consecutive readings, rather than 1 image, be taken to generate an average BFS for analysis. When comparing the posterior corneal elevation between sessions, an average BFS generated from the first visit should be used for elevation calculation.