Accuracy and Repeatability of an Anterior Segment Swept-Source Optical Coherence Tomographer

Accuracy comparison of tomography devices for ray tracing-based intraocular lens calculation

PURPOSE

To evaluate the interchangeability of different tomography devices used for ray tracing-based intraocular lens (IOL) calculation.

SETTING

Eye clinic, Castrop-Rauxel, Germany.

DESIGN

Retrospective analysis.

METHOD

Measurements from 3 Placido-Scheimpflug devices and 3 optical coherence tomography (OCT) devices were compared in 83 and 161 other eyes after cataract surgery, respectively. 2-dimensional matrices of anterior local corneal curvature and local corneal thickness are transferred to the ray-tracing software OKULIX. Calculations are performed with the same IOL in the same position of an eye with the same axial length. Differences in spherical equivalent (SE), astigmatism, and spherical aberration are evaluated. Furthermore, the influence of the size of the matrices (optical zone) on the accuracy is quantified.

RESULTS

For the Placido-Scheimpflug devices, the deviations from the average of three measurements taken for each eye in SE (mean ± SD) were 0.17 ± 0.24 diopters (D), -0.26 ± 0.29 D, and 0.08 ± 0.39 D ( P < .001, analysis of variance [ANOVA]), for the centroids of the astigmatic differences 0.04 D/173 degrees, 0.14 D/93 degrees, and 0.10 D/7 degrees, and for the median of the absolute values of the vector differences 0.31 D, 0.33 D, and 0.29 D. For OCT devices, the corresponding results were 0.01 ± 0.21 D, -0.03 ± 0.21 D, and 0.02 ± 0.20 D ( P = .005, ANOVA); 0.18 D/120 degrees, 0.07 D/70 degrees, and 0.22 D/4 degrees; and 0.26 D, 0.30 D, and 0.33 D. The accuracy of the calculated spherical aberrations allows for an individual selection of the best fitting IOL model in most cases.

CONCLUSIONS

The differences are small enough to make the devices interchangeable regarding astigmatism and spherical aberration. Although there are significant differences in SE between Scheimpflug and OCT devices, the differences between OCT devices are also small enough to make them interchangeable, but the differences between Placido-Scheimpflug devices are too large to make these devices interchangeable.

Corneal thickness changes in myopic children during and after short-term orthokeratology lens wear

PURPOSE

To evaluate thickness changes in the central and mid-peripheral cornea (CCT and mPCT), corneal epithelium (CET and mPET) and stroma (CST and mPST) of myopic children during and after short-term orthokeratology (ortho-k) lens wear, with conventional (CCF, 0.75 D) and increased compression factors (ICF, 1.75 D).

METHODS

This was a self-controlled case series study. Subjects wore a CCF lens in one eye and an ICF lens in the other. Anterior segment optical coherence tomography images were captured weekly for 1 month during lens wear and for 2 weeks after discontinuing lens wear. CCT and CET (central 3-mm cornea) and mPCT and mPET (within a 4-6 mm diameter annulus) were measured. Stromal thickness (ST) was determined by subtracting epithelium thickness (ET) from corneal thickness (CT). The repeatability of the analytical software was also investigated on age-matched spectacle-wearing subjects (n = 98).

RESULTS

Excluding three outliers (>3 S.D.s), the coefficient of repeatability and intraclass correlation coefficients of 98 spectacle-wearing subjects ranged from 2.63 to 4.64 μm and from 0.90 to 0.99, respectively. For the weekly-change study, CCT and CET in both eyes were significantly thinner after lens wear (p < 0.001) and CET thinning in the ICF eyes were significantly higher (p < 0.02). CCT changes were mainly contributed by CET. CST, mPCT, mPET and mPST changes were not significant (p > 0.20) in either eye. CT (all sublayers) rebounded to baseline values 2 weeks after discontinuing lens wear (0.99 > p > 0.12).

CONCLUSIONS

Significant reductions in CT and ET, but not ST, were observed within 1 month of ortho-k lens wear. Wearing ICF lenses resulted in a higher reduction in CET. Corneal thickness changes were reversible after discontinuing lens wear.

Repeatability and agreement of corneal thickness measurements by three methods of pachymetry in small incision lenticule extraction eyes

Background: The accurate evaluation of corneal thickness (CT) post small incision lenticule extraction (SMILE) is clinically relevant to reduce the risk of complications. We aimed to analyze repeatability and agreement of central corneal thickness (CCT), thinnest corneal thickness (TCT), and mid-peripheral corneal thickness (MPCT) measurements using Scheimpflug imaging, anterior segment swept-source optical coherence tomography (AS-SS-OCT, CASIA 1000, Tomey), and ultrasound pachymetry (USP, SP-3000, Tomey) in eyes with previous SMILE. Methods: Ninety-one eyes of 91 patients were included. Within-subject standard deviation (Sw), test-retest repeatability (TRT), intraclass correlation coefficient (ICC), and coefficient of variation (CoV) were used to evaluate repeatability. Agreement was assessed by repeat-measurement analysis of variance and Bland-Altman analysis. Results: The above three instruments revealed that Sw was <5.91 µm, CoV was <1.08%, TRT was <16.38 µm, and ICC was >0.94. The 95% limits of agreement were relatively narrow and Bland-Altman plots were more concentrated at the CCT, at the TCT, and at the CT_2mm. However, it was shown to be wide at the CT_5mm. Conclusions: The three devices provide good repeatability of CT measurements in patients who undergone SMILE. Agreement between measurements at the CCT, TCT, and CT_2mm were high, but measurement agreements among CT_5mm revealed poor agreement.

Corneal Total and Epithelial Thickness Measured by Sonogage Ultrasound Pachometry and High-resolution Optical Coherence Tomography

SIGNIFICANCE

This research questions the validity of using the Sonogage ultrasound (US) pachometer to measure corneal epithelial thickness and coincidentally provides confirmation for the conventional view of the mechanism of orthokeratology (OK) based on central epithelial thinning.

PURPOSE

The Sonogage (Corneo-Gage Plus 1) pachometer uses A-scan US to measure total corneal thickness. It is claimed that this instrument can also measure corneal epithelial thickness. We sought to validate this claim by comparing total and epithelial thickness measurements with the Sonogage with those obtained with high-resolution optical coherence tomography (OCT).

METHODS

Fourteen non-contact lens wearers and 14 subjects who had worn Paragon CRT OK lenses overnight for greater than 1 month were recruited. Three OCT and five US measurements were taken in one eye of each subject. Depending on normality of data, paired t tests or Wilcoxon tests were used to compare total and epithelial thicknesses measured with the Sonogage pachometer and the Tomey Casia OCT. Pearson or Spearman correlation analyses were used to examine relationships between measurements obtained with the two instruments.

RESULTS

There was a significant difference in total corneal thickness measurements between the two instruments. Although a significant correlation was found (r = 0.916, P < .001), the Sonogage consistently measured greater total corneal thickness than did the OCT (+19.5 ± 9.2 μm; P < .001). Epithelial thickness using the Sonogage showed little variation (range, 46.4 to 50.0 μm), whereas epithelial thickness using the OCT ranged from 30.7 to 54.7 μm. There was no significant correlation between epithelial thicknesses obtained with the two instruments (r = -0.135, P = .49). Epithelial thickness measured by OCT was significantly thinner in OK wearers (35.8 ± 2.8 μm) than in nonlens wearers (46.7 ± 4.5 μm, P < .001).

CONCLUSIONS

The Sonogage is not able to measure epithelial thickness in vivo, returning essentially identical measurements over a range of epithelial thicknesses. Optical coherence tomography measurements confirm the conventional view of the mechanism of OK based on central epithelial thinning.