Agreement of non-contact pachymetry after LASIK: comparison of combined scanning-slit/Placido disc topography and specular microscopy

Comparison of central corneal thickness in healthy eyes using ultrasound pachymetry, non-contact specular microscopy and a high-resolution Scheimpflug camera

PURPOSE

To compare and analyze the interchangeability in measuring central corneal thickness (CCT) using ultrasonic pachymetry (USP, PACHMATE 2, DGH, Inc, Exton, PA, USA), non-contact specular microscopy (NCSM, CEM-530, Nidek CO, LTD, Gamagori, Japan) and a high-resolution Scheimpflug Camera (Pentacam HR, OCULUS, Wetzlar, Germany).

METHODS

An observational, cross-sectional study was performed recruiting 216 volunteers, for a sample size of 216 eyes with no ocular abnormalities other than refractive errors. All subjects underwent pachymetric measurements obtained by USP, NCSM and Pentacam HR. Examinations were performed by the same examiner with USP always following the noncontact examinations.

RESULTS

The mean CCT (± SD) was 560.30 ± 38.80 μm, 556.76 ± 36.83 μm and 547.31 ± 35.28 μm for USP, NCSM and Pentacam HR, respectively. The Bland-Altman analysis showed that the highest concordance was found between USP and NCSM, with differences between - 13.18 μm and 20.26 μm. For the differences between measurements obtained with Pentacam HR and USP, the differences at Bland-Altman plot were between - 28.25 and 13.57 μm. The lowest concordance was found for the CCT values measured with Pentacam HR and NCSM, with differences between - 25.67 and 6.86 μm. The intraclass correlation coefficient (ICC) between all pairs of measurements was between 0.979 and 0.987, suggesting that the mean measurements were strongly correlated.

CONCLUSION

Measurements obtained with all three devices had high correlation. USP and NCSM were found in good agreement and high concordance, too. The above results indicate that these two devices are interchangeable in clinical practice. Pentacam HR may be a useful alternative for measuring CCT; however, it significantly underestimates CCT and cannot be used interchangeably with the other devices that we studied.

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.

Evaluating the repeatability of corneal elevation through calculating the misalignment between Successive topography measurements during the follow up of LASIK

The study aims to evaluate, using the Iterative Closest Point (ICP) algorithm, the repeatability of successive corneal elevation measurements taken post-LASIK. Two topography maps of 98 LASIK participants were recorded preoperatively (Pre), 1 month (Pos1M) and 3 months postoperatively (Pos3M). Elevation of the second measurement was fitted to the first measurement by calculating using ICP, and correcting for, both translational and rotational misalignment components. The RMS of elevation differences between anterior corneal measurements were statistically significant post-LASIK compared to preoperation (P < 0.05). A misalignment ratio used to describe the weighting of the elevation difference caused by misalignment relative to the total difference remained stable (0.40 and 0.23 for anterior and posterior corneal surfaces, respectively) in different periods. The study also considered the combined misalignment parameter (CM), which represents the total effect of all individual misalignment components on the repeatability of corneal topography maps. CM was significantly greater post-LASIK relative to pre-LASIK (P < 0.05). Overall, the contribution of misalignment to the total difference between successive corneal measurements remained stable pre and post operation, while the combined effect of refractive error correction and optical diameter appeared to have a significant influence on the elevation repeatability in the early stages of the follow up period.

A comprehensive comparison of central corneal thickness measurement

PURPOSE

To assess the repeatability and reproducibility of central corneal thickness (CCT) measurements by high-resolution rotating Scheimpflug imaging (Pentacam, Oculus) and Fourier-domain optical coherence tomography (RTvue-100, Optovue) after laser in situ keratomileusis (LASIK) and to compare the agreement with ultrasound pachymetry (USP).

METHODS

Forty-seven eyes of 47 patients after LASIK were included in the study. The first examiner took two successive Pentacam and RTvue CCT measurements, and this was repeated once again by the second examiner to assess intraobserver and interobserver repeatability and reproducibility. After performing non-contact examinations, the corneas were measured by USP to compare the level of agreement among the three devices.

RESULTS

All Pentacamcenter, Pentacamapex, Pentacamthinnest, and RTvue CCT measurements demonstrated high intraobserver repeatability, with respective precision (1.96 within-subject standard deviation) and intraclass correlation coefficients of 7.52, 7.43, 7.55, and 3.81 μm and 0.985, 0.986, 0.986, and 0.997; interobserver repeatability results were similar. All coefficients of variation were low: <1% for all measures. Compared with Pentacam and USP measurements, the RTvue measurement significantly underestimated CCT by a mean of 10.52 to 15.28 μm (p < 0.001) and 9.17 μm (p < 0.001), respectively. The agreement of USP with Pentacam and RTvue by Bland-Altman analysis spanned over 30 μm. The agreement of Pentacam with RTvue spanned approximate 20 μm.

CONCLUSIONS

Both Pentacam imaging and RTvue Fourier-domain optical coherence tomography provide reliable and interchangeable measurement of CCT in post-LASIK corneas. However, they cannot be considered to be clinically interchangeable with USP.

Comparison of corneal thickness measurement with the Pentacam, the PARK1 and an ultrasonic pachymeter

PURPOSE

The aim was to determine the agreement between PARK1 pachymetric readings and measurements made with the Pentacam and ultrasonic pachymetry.

METHODS

In this prospective study, we enrolled refractive surgery patients free of other ocular conditions and performed Pentacam and PARK1 acquisitions followed by ultrasonic pachymetry. We recorded pachymetric readings on the centre, apex and thinnest point with the Pentacam, corneal thickness values of the apex and minimum readings from the PARK1, and the central corneal thickness with the ultrasonic system. Data were compared using t-tests, and the Pearson correlations and 95% limits of agreement (LoA) of all pairs were determined.

RESULTS

The mean age of the 47 participants was 32 ± 10.4 years and data from their right eyes were used. Overall, ultrasonic measurements were significantly higher than all readings from the PARK1 and the Pentacam. The smallest difference was between the PARK-apex and Pentacam-apex readings (0.2 ± 11 µm); the 95% LoA of this pair gave a width of 43.7 µm (-21.7 to 22.0 µm). The smallest width pertained to the Pentacam-centre and ultrasound-centre paired readings; the width was 32.5 µm (-21.8 to 11.8 µm), while the mean difference was -5.54 ± 8 µm.

CONCLUSION

The PARK1 measurements of the apical corneal thickness can be a substitute for central corneal pachymetry with an ultrasonic device, provided that the range of agreement is taken into consideration in the interpretation of the results. Similar studies on populations with different corneal conditions such as keratoconus are needed.