Cornea Measurement Comparison with Orbscan II and EyeSys Videokeratoscope

Corneal topography with an aberrometry-topography system

PURPOSE

To investigate the agreement between the central corneal radii and corneal eccentricity measurements generated by the new Wave Analyzer 700 Medica (WAV) compared to the Keratograph 4 (KER) and to test the repeatability of the instruments.

METHODS

20 subjects (10 male, mean age 29.1 years, range 21-50 years) were recruited from the students and staff of the Cologne School of Optometry. Central corneal radii for the flat (r_c/fl) and steep (r_c/st) meridian as well as corneal eccentricity for the nasal (e_nas), temporal (e_temp), inferior (e_inf) and superior (e_sup) directions were measured using WAV and KER by one examiner in a randomized order.

RESULTS

Central radii of the flat (r_c/fl) and steep (r_c/st) meridian measured with both instruments were statically significantly correlated (r = 0.945 and r = 0.951; p < 0.001). Comparison between the WAV and KER showed that r_c/fl and r_c/st measured with WAV were significantly steeper than those measured with KER (p < 0.001). Corneal eccentricities were statistically significantly correlated in all meridians (p < 0.05). Compared to KER, e_temp and e_sup measured with WAV were greater (p < 0.05), while there were no statistically significant differences for e_nas and e_inf (p = 0.350 and p = 0.083). For the central radii, repeated measurements were not significantly different for the KER or WAV (p > 0.05). Limits of agreement (LoA) indicate a better repeatability for the KER compared to WAV.

CONCLUSIONS

Corneal topography measurements captured with the WAV were strongly correlated with the KER. However, due to the differences in measured corneal radii and eccentricities, the devices cannot be used interchangeably. For corneal topography the KER demonstrated better repeatability.

Refractive error assessment: influence of different optical elements and current limits of biometric techniques

PURPOSE

To identify and quantify sources of error on refractive assessment using exact ray tracing.

METHODS

The Liou-Brennan eye model was used as a starting point and its parameters were varied individually within a physiological range. The contribution of each parameter to refractive error was assessed using linear regression curve fits and Gaussian error propagation analysis. A MonteCarlo analysis quantified the limits of refractive assessment given by current biometric measurements.

RESULTS

Vitreous and aqueous refractive indices are the elements that influence refractive error the most, with a 1% change of each parameter contributing to a refractive error variation of +1.60 and -1.30 diopters (D), respectively. In the phakic eye, axial length measurements taken by ultrasound (vitreous chamber depth, lens thickness, and anterior chamber depth [ACD]) were the most sensitive to biometric errors, with a contribution to the refractive error of 62.7%, 14.2%, and 10.7%, respectively. In the pseudophakic eye, vitreous chamber depth showed the highest contribution at 53.7%, followed by postoperative ACD at 35.7%. When optic measurements were considered, postoperative ACD was the most important contributor, followed by anterior corneal surface and its asphericity. A MonteCarlo simulation showed that current limits of refractive assessment are 0.26 and 0.28 D for the phakic and pseudophakic eye, respectively.

CONCLUSIONS

The most relevant optical elements either do not have available measurement instruments or the existing instruments still need to improve their accuracy. Ray tracing can be used as an optical assessment technique, and may be the correct path for future personalized refractive assessment.

Anterior corneal asphericity calculated by the tangential radius of curvature

We propose a method of calculating the corneal asphericity (Q) and analyze the characteristics of the anterior corneal shape using the tangential radius. Fifty-eight right eyes of 58 subjects were evaluated using the Orbscan II corneal topographer. The Q-values of the flat principal semi-meridians calculated by the sagittal radius were compared to those by the tangential radius. Variation in the Q-value with semi-meridian in the nasal and temporal cornea calculated by the tangential radius was analyzed. There were significant differences in Q-values (P<0.001) between the two methods. The mean Q-values of the flat principal semi-meridians calculated by tangential radius with -0.33 ± 0.10 in the nasal and -0.22 ± 0.12 in the temporal showed more negative than the corresponding Q-values calculated by the sagittal radius. The Q-values calculated by tangential radius became less negative gradually from horizontal semi-meridians to oblique semi-meridians in both nasal and temporal cornea. Variation in Q-value with semi-meridian was more obvious in the nasal cornea. The method of calculating corneal Q using the tangential radius could provide more reasonable and complete Q-value than that by the sagittal radius. The model of a whole anterior corneal surface could be reconstructed on the basis of the above method.

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.

Topography-based assessment of anterior corneal curvature and asphericity as a function of age, sex, and refractive status

PURPOSE

To assess corneal asphericity (Q) and evaluate potential factors influencing the shape of the anterior corneal surface.

SETTING

Medical Optics Research Group, Institute of Medical Physics, University of Erlangen-Nuremberg, Erlangen, Germany.

METHODS

In this cross-sectional consecutive study, 3 topographic measurements were taken. Eyes were grouped by age in years (A: <or=29; B: 30 to 39; C: 40 to 49; D: 50 to 59; E: 60 to 69; F: >or=70), sex, and refraction.

RESULTS

The study comprised 487 eyes (205 men, 288 women; age 17 to 81 years). The mean Q of the anterior corneal surface was -0.22 +/- 0.14 (SD) overall, -0.21 +/- 0.12 in Group A, -0.25 +/- 0.11 in Group B, -0.21 +/- 0.15 in Group C, -0.23 +/- 0.14 in Group D, -0.19 +/- 0.17 in Group E, -0.20 +/- 0.15 in Group F, -0.23 +/- 0.13 in men, -0.21 +/- 0.14 in women, -0.19 +/- 0.14 in hyperopes (n = 166; >+0.50 to +6.50 diopters [D]), -0.23 +/- 0.13 in emmetropes (n = 162; -0.50 to +0.50 D), and -0.23 +/- 0.15 in myopes (n = 165; <-0.50 to -8.00 D). There was no significant correlation between Q and age; Q differed significantly between men and women (P = .005), hyperopes and emmetropes (P<.0001), and hyperopes and myopes (P = .001).

CONCLUSIONS

There were high interindividual variations in the Q value. Thus, proper correction of spherical aberration with intraocular lenses (IOLs) requires sophisticated selection of the asphericity of IOL surfaces based on biometric data and individual corneal Q values.

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.

Comparison of and correlation between anterior and posterior corneal elevation maps in normal eyes and keratoconus-suspect eyes

PURPOSE

To compare the anterior and posterior corneal elevation maps between keratoconus-suspect eyes and normal eyes.

SETTING

Rothschild Foundation, AP-HP, University Paris VII, Hôpital Bichat Claude Bernard, Paris, France.

METHODS

The anterior and posterior corneal surface elevations were analyzed and compared in 60 normal myopic patients and 48 keratoconus-suspect patients. The anterior and posterior best-fit sphere radii, central and thinnest corneal pachymetries, anterior and posterior aconic shape parameters (aconic radius, aconic asphericity, aconic toricity), and anterior and posterior elevation in the 1.0 mm radius zone were analyzed. The correlations between elevation and aconic shape parameters between the anterior and posterior surfaces were compared.

RESULTS

The mean central and thinnest pachymetry values were significantly lower in keratoconus-suspect eyes (P<.0001). Compared with normal eyes, keratoconus-suspect eyes had significantly increased anterior toricity (P = .0002) and posterior toricity (P<.0001), more negative asphericity (P = .042), and higher posterior elevation (P<.0001). The correlation between aconic toricity and the anterior and posterior corneal surfaces was better in keratoconus-suspect eyes than in normal eyes. Aconic asphericity and apical curvature were less correlated in keratoconus-suspect eyes than in normal eyes.

CONCLUSIONS

The posterior corneal elevation and the corneal thickness values were different in keratoconus-suspect eyes. The correlation between the anterior and posterior corneal aconic shapes was between keratoconus-suspect eyes and normal eyes.