Comparison of Javal-Schiøtz keratometer, Orbscan IIz and Pentacam topographers in evaluating anterior corneal topography

CLINICAL RELEVANCE

Inter-instrument variation in anterior corneal shape (ACS) measurement has a consequence for ocular clinical practice.

BACKGROUND

To consider inter-instrument variability in keratometry measurements across the ACS and to explore instrument protocols for determining ACS keratometric analogues (KAs).

METHODS

  Mean keratometry/KAs of the right eye were recorded using Javal-Schiøtz keratometer (J-S), Orbscan IIz and Pentacam from 124 subjects (78 females; mean ± SD age: 24.71 ± 6.61 years). Mean radii of curvature were obtained for 1-mm wide annular zones extending up to 6 mm (horizontally) and 4 mm (vertically) from the apex for Orbscan and Pentacam. Zonal mean radius of curvature was calculated by averaging keratometry values for all measured points within the zone.

RESULTS

KA (mean ± SD): Horizontal: Orbscan (7.80 ± 0.31 mm) and J-S (7.82 ± 0.29 mm) were not significantly different (p = 0.072). Pentacam (7.86 ± 0.29 mm) was significantly flatter than J-S (p < 0.001) and Orbscan (p < 0.001). Vertical: Orbscan (7.64 ± 0.31 mm) was significantly steeper than J-S (7.67 ± 0.29 mm, p < 0.005) and Pentacam (7.70 ± 0.29 mm, p < 0.001). Pentacam was significantly flatter than J-S (p < 0.001) and significant flatter than Orbscan across the entire profile (1-4 mm zones horizontal and vertical, p < 0.001).                        LoAs (CI): J-S/Orbscan: ±0.75 mm (0.05-0.18); J-S/Pentacam: ±0.72 mm (0.01-0.12); Pentacam/Orbscan: ±0.16 mm (0.04-0.08). There was a +0.03 mm positive bias for Orbscan compared to J-S, +0.06 mm positive bias for Orbscan compared to Pentacam and -0.03 mm negative bias for Pentacam compared to J-S.

CONCLUSIONS

Algorithms used by Orbscan and Pentacam to solve the peripheral paraxial ray problem produce significantly different KAs. Instrument-specific KAs cannot be used inter-changeably between instruments. Differences in KA between instruments are not significant for ocular surgery, but may influence rigid contact lens fitting. Pentacam measures flatter than Orbscan and J-S.

Accuracy of the posterior corneal elevation values of Pentacam HR from different reference surfaces in early ectasia diagnosis

PURPOSE

Detecting the accuracy of various posterior elevation (PE) indices of Pentacam HR, and correlating them with some possibly related factors or parameters, in a cohort with early keratoconus (KC).

METHODS

A cross sectional study that was conducted at Eye World Hospital, Egypt. One hundred and two corneas were enrolled, including two groups; group 1 (50 corneas) having forme fruste or early KC, and group 2 (52 corneas) for healthy controls. Corneas were scanned using Pentacam HR (Oculus, Wetzlar, Germany). The investigated PE parameters were: PE from best fit sphere (BFS), PE from best fit toric ellipsoid (BFTE), PE from exclusion map of Belin Ambrosio's display (BAD), and PE from difference map of BAD. The four PE values were correlated to age, thinnest corneal thickness "TCT," posterior aberrations, and posterior Q value.

RESULTS

All the investigated indices were significantly different in group 1 compared to group 2 (p < 0.001). Accuracy of PE parameters revealed the highest AUROC for PE from BFTE (AUROC = 0.989, and best cutoff > 4 um with sensitivity 96.00% and specificity 96.15%). PE from difference map was the least accurate. Correlation coefficients showed a significant correlation between all the studied PE parameters and some of the posterior aberrations (root mean square of higher order aberrations, vertical coma, and spherical aberrations), besides a significant correlation with posterior Q value.

CONCLUSION

PE indices are sensitive detectors of early ectasia. PE from BFTE had the highest deduced AUROC. Alterations in PE values can significantly alter many posterior corneal aberrations and the posterior Q.

OPD scan III accuracy: Topographic and aberrometric indices after accelerated corneal cross-linking

Purpose

To determine topographic and aberrometric changes after accelerated cross-linking (ACXL; 18 mW/cm² for 5 min) as measured with OPD Scan III (Nidek Inc., Tokyo, Japan) and their repeatability in patients with mild and moderate keratoconus (KCN).

Methods

In this prospective study, 25 eyes with mild KCN [Ksteep = 47.24 ± 3.11 diopter (D)] and 20 moderate cases (Ksteep = 52.86 ± 4.39 D) were examined under mesopic conditions (20 lux) twice, 30-45 min apart, at baseline and 6 and 12 months afterwards. Extracted indices were Ksteep, Kflat, ocular and corneal irregularity, ocular and corneal total higher order aberrations (HOAs), coma, trefoil, and spherical aberration (SA). Repeatability index (RI) and intraclass correlation coefficients (ICCs) were determined.

Results

In mild cases, Ksteep and corneal irregularity had lower RI, but Kflat and ocular irregularity had higher RI (all P > 0.050) at 1 year. The RI for ocular total HOAs, coma, and SA decreased and showed no significant change for trefoil (all P > 0.050). Moderate cases showed non-significant increases in RI for Ksteep, Kflat, ocular and corneal irregularity (all P > 0.050), and all aberrometry indices, and significant increases in RI for ocular coma (P = 0.046) and corneal trefoil (P = 0.037). At 1 year, ICC was >0.75 for all indices except ocular and corneal trefoil (ICC = 0.613 and 0.390) in moderate cases.

Conclusions

At one year after ACXL, OPD Scan III showed acceptable repeatability in mild cases. In moderate cases, topographic indices had acceptable repeatability but poorer compared to the mild group. Overall, ocular HOAs showed better repeatability than corneal ones. These changes should be considered in the interpretation of measurements.

The analysis of corneal asphericity (Q value) and its related factors of 1,683 Chinese eyes older than 30 years

Purpose

To determine corneal Q value and its related factors in Chinese subjects older than 30 years.

Design

Cross sectional study.

Methods

1,683 participants (1,683 eyes) from the Handan Eye Study were involved, including 955 female and 728 male with average age of 53.64 years old (range from 30 to 107 years). The corneal Q values of anterior and posterior surfaces were measured at 3.0, 5.0 and 7.0mm aperture diameters using Bausch & Lomb Orbscan IIz (software version 3.12). Age, gender and refractive power were recorded.

Results

The average Q values of the anterior surface at 3.0, 5.0 and 7.0mm aperture diameters were -0.28±0.18, -0.28±0.18, and -0.29±0.18, respectively. The average Q value of the anterior surface at the 5.0mm aperture diameter was negatively correlated with age (B = -0.003, p<0.01) and the refractive power (B = -0.013, p = 0.016). The average Q values of the posterior surface at 3.0, 5.0, and 7.0mm were -0.26±0.216, -0.26±0.214, and -0.26±0.215, respectively. The average Q value of the posterior surface at the 5.0mm aperture diameter was positively correlated with age (B = 0.002, p = 0.036) and the refractive power (B = 0.016, p = 0.043).

Conclusion

The corneal Q value of the elderly Chinese subjects is different from that of previously reported European and American subjects, and the Q value appears to be correlated with age and refractive power.

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.

Assessment of accuracy and repeatability of anterior segment optical coherence tomography and reproducibility of measurements using a customised software program

PURPOSE

The aim was to study the reliability of measurements of the RTVue (Optovue, Fremont, CA, USA) anterior segment optical coherence tomographer (AS-OCT) and assess how results can be improved by analysing raw optical coherence tomography data with customised image analysis software and applying correction factors.

METHODS

Five RTVue AS-OCT instruments (ver. 4.0) were assessed by imaging gauge blocks of three different lengths, single/stepped glass plate (microscope slides) and flat window glass to check for width, depth and linearity of the measurement scans. Five repeats per calibration tool were imaged and averaged. Raw data were exported and loaded into customised image analysis software written in LabWindows/CVI for further analysis. Using two calibration balls with different radii, measurement scans were validated. Repeatability of the optical coherence tomographs and the edge detection procedure were checked and statistical analyses performed.

RESULTS

Variations ranging from 0.01 to 1.93 mm in scan width and 0.1 to 0.17 mm in scan depth were found between the five instruments. Slight curvature distortion of 0.06 ± 0.01 mm (mean and standard deviation) was found in the raw images. By isolating the three sources of image distortion and applying individual correction factors, accuracy for corneal curvature measurements could be improved to better than 0.1 mm. Manual edge detection limited the coefficient of repeatability value to 0.06 and 0.08 mm for anterior and posterior radii of curvature, respectively. The coefficient of repeatability of corneal thickness measurements was less than 8 µm.

CONCLUSIONS

Accuracy of the RTVue AS-OCT varied between instruments. By applying calibration scale factors calculated by customised software, accuracy of thickness and curvature values of the anterior eye was improved. The achievable precision is sufficient to detect clinically relevant corneal curvature variations.

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.

Corneal astigmatism in unilateral Fuchs heterochromic iridocyclitis

PURPOSE

To analyze the results of scanning slit topography in both eyes of patients with unilateral Fuchs heterochromic iridocyclitis (FHI).

METHODS

A noninterventional, case-controlled study was conducted with 30 patients with unilateral FHI. Corneal topographic parameters, obtained with the Orbscan II (Bausch & Lomb), were compared between the two eyes and analyzed statistically. The same number of age-matched normals was enrolled as a control group.

RESULTS

Thirty patients were enrolled in this study. The mean age was 33.3 years ± 10.6 (SD). The mean Sim K astigmatism was 1.65 ± 1.27 diopter in the FHI eyes and 0.88 ± 0.52 diopter in the normal eyes (p = .001).

CONCLUSION

Corneal astigmatism is a common finding in patients with FHI and disparity of corneal astigmatism between the two eyes can be considered a sign of the unilateral form of this disease.