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

The Precision of a New Anterior Segment Optical Coherence Tomographer and Its Comparison With a Swept-Source OCT-Based Optical Biometer in Patients With Cataract

PURPOSE

To assess the precision of a new spectral-domain optical coherence tomographer (SD-OCT)/Placido topographer (MS-39; CSO) and its comparison with a swept-source OCT (SS-OCT) biometer (Argos; Movu, Inc) in patients with cataract.

METHODS

Fifty-three right eyes from 53 patients were examined by two experienced operators three times using both devices randomly. Employing the within-subject standard deviation (S_w), test-retest variability, coefficient of variation, and intraclass correlation coefficient to evaluate intraoperator repeatability and interoperator reproducibility; the double-angle plots to analyze astigmatism; and Bland-Altman plots and 95% limits of agreement to verify the agreement between devices.

RESULTS

The SD-OCT/Placido tomographer showed high precision, with coefficient of variation of 0.44% or less, intraclass correlation coefficient of 0.945 or greater for all parameters, test-retest variability of 4.21 µm or less for central corneal thickness (CCT), 0.03 mm or less for anterior chamber depth (ACD) and aqueous depth (AQD), and 0.25 diopters (D) or less for mean keratometry (Km), J₀, and J₄₅. The inter-device differences in Km, J₀, and J₄₅ were statistically insignificant, whereas the remaining were statistically but not clinically significant. The 95% limits of agreement of CCT, ACD, AQD, Km, J₀, and J₄₅ were -3.70 to 15.25 µm, -0.06 to 0.04 mm, -0.06 to 0.04 mm, -0.28 to 0.35 D, -0.27 to 0.26 D, and -0.27 to 0.21 D, respectively. The double-angle plot confirmed the high agreement in astigmatism.

CONCLUSIONS

For CCT, ACD, AQD, Km, and astigmatism measurements in patients with cataract, the new SD-OCT/Placido tomographer has excellent precision and high agreement with the Argos SS-OCT biometer, and can be used interchangeably. [J Refract Surg. 2021;37(9):616-622.].

Current Developments in Corneal Topography and Tomography

Introduction: Accurate assessment of the corneal shape is important in cataract and refractive surgery, both in screening of candidates as well as for analyzing postoperative outcomes. Although corneal topography and tomography are widely used, it is common that these technologies are confused. The aim of this study was to present the current developments of these technologies and particularly distinguish between corneal topography and tomography. Methods: The PubMed, Web of Science and Embase databases were the main resources used to investigate the medical literature. The following keywords were used in various combinations: cornea, corneal, topography, tomography, Scheimpflug, Pentacam, optical coherence tomography. Results: Topography is the study of the shape of the corneal surface, while tomography allows a three-dimensional section of the cornea to be presented. Corneal topographers can be divided into large- and small-cone Placido-based devices, as well as devices with color-LEDs. For corneal tomography, scanning slit or Scheimpflug imaging and optical coherence tomography may be employed. In several devices, corneal topography and tomography have been successfully combined with tear-film analysis, aberrometry, optical biometry and anterior/posterior segment optical coherence tomography. Conclusion: There is a wide variety of imaging techniques to obtain corneal power maps. As different technologies are used, it is imperative that doctors involved in corneal surgery understand the science and clinical application of devices for corneal evaluation in depth.

The precision and agreement of corneal thickness and keratometry measurements with SS-OCT versus Scheimpflug imaging

Purpose

To assess the repeatability and reproducibility of swept-source optical coherence tomography (SS-OCT) and Scheimpflug system and evaluate the agreement between the two systems in measuring multiple corneal regions in children.

Methods

Pachymetric and keratometric maps for both systems were evaluated. Central, midperipheral and peripheral corneal thickness (CT), keratometry and astigmatism power vectors were recorded. The three outcomes yielded by the same observer were used to assess intraobserver repeatability. The differences in the mean values provided by each observer were used to evaluate interobserver reproducibility. Within-subject standard deviation, test-retest repeatability (TRT) and coefficient of variation (CoV) were used to analyze the intraobserver repeatability and interobserver reproducibility. Paired T-test and Bland-Altman were used to appraise interdevice agreement.

Results

Seventy-eight eyes of 78 children were included. The CoV was ≤2.12 and 1.10%, respectively, for repeatability and reproducibility. TRT and CoV were lower for central and paracentral CT measurements than for peripheral measurements. The SS-OCT device generated higher precision when acquiring CT data, whereas Scheimpflug system showed higher reliability when measuring corneal keratometry. Although the CT readings measured using SS-OCT were significantly thinner than Scheimpflug device (P <  0.001), the central and thinnest CT values were still of high agreement. The interdevice agreement of keratometry measurement was high for the central corneal region and moderate for the paracentral and peripheral areas.

Conclusions

The precision of CT measurements by SS-OCT was higher, while the reliability of keratometry measurements by the Scheimpflug system was higher in children. Apart from the measured values in the central corneal region, the thickness and keratometry readings should not be considered interchangeable between the two systems.

Comparison of refractive outcomes using conventional keratometry or total keratometry for IOL power calculation in cataract surgery

PURPOSE

To compare the refractive outcomes following cataract surgery using conventional keratometry (K) and total keratometry (TK) for intraocular lens (IOL) calculation in the SRK/T, HofferQ, Haigis, and Holladay 1 and 2, as well as Barrett and Barrett TK Universal II formulas.

METHODS

Sixty eyes of 60 patients from Siriraj Hospital, Thailand, were prospectively enrolled in this comparative study. Eyes were assessed using a swept-source optical biometer (IOLMaster 700; Carl Zeiss Meditec, Jena, Germany). Posterior keratometry, K, TK, central corneal thickness, anterior chamber depth, lens thickness, axial length, and white-to-white corneal diameter were recorded. Emmetropic IOL power was calculated using K and TK in all formulas. Selected IOL power and predicted refractive outcomes were recorded. Postoperative manifest refraction was measured 3 months postoperatively. Mean absolute errors (MAEs), median absolute errors (MedAEs), and percentage of eyes within ± 0.25, ± 0.50, and ± 1.00 D of predicted refraction were calculated for all formulas in both groups.

RESULTS

Mean difference between K and TK was 0.03 D (44.56 ± 1.18 vs. 44.59 ± 1.22 D), showing excellent agreement (ICC = 0.99, all p < 0.001). Emmetropic IOL powers in all formulas for both groups were very similar, with a trend toward lower MAEs and MedAEs for TK when compared with K. The Barrett TK Universal II formula demonstrated the lowest MAEs. Proportion of eyes within ± 0.25, ± 0.50, and ± 1.00 D of predicted refraction were slightly higher in the TK group.

CONCLUSIONS

Conventional K and TK for IOL calculation showed strong agreement with a trend toward better refractive outcomes using TK. The same IOL constant can be used for both K and TK.

Comparative analysis of corneal measurements obtained from a Scheimpflug camera and an integrated Placido-optical coherence tomography device in normal and keratoconic eyes

PURPOSE

To assess the agreement between a Scheimpflug camera (Pentacam) and a combined Placido-optical coherence tomography device (Visante OMNI) in measuring corneal curvature, thickness and elevation values in normal and keratoconic eyes.

METHODS

Corneal measurements of 110 normal eyes (one eye per subject) and 70 keratoconic eyes were obtained from both devices and compared. Agreement was determined using the Bland-Altman analysis 95% limits of agreement (LoA).

RESULTS

The Pentacam measured significantly greater keratometry readings in the flattest (K1) and steepest meridians (K2) in normal and keratoconic eyes. The 95% LoA in normal eyes were -0.32 to 0.59 dioptres (D) (K1) and -0.41 to 0.74 D (K2). In keratoconic eyes, the 95% LoA were -1.35 to 1.92 D (K1) and -1.38 to 1.99 D (K2). The Pentacam recorded significantly higher central corneal thickness (CCT) values in both groups of eyes. The 95% LoA were -4.31 to 39.89 microns (μ) and -12.92 to 41.35 μ in normal and keratoconic eyes, respectively. Pentacam anterior and posterior corneal elevations were significantly greater in both groups of eyes. The devices demonstrated excellent repeatability and reproducibility for corneal curvature and thickness but not elevation measurements.

CONCLUSIONS

The Pentacam measured significantly greater corneal curvature, thickness and elevation values compared to the Visante OMNI in normal and keratoconic eyes. The devices agree moderately for anterior corneal elevations in normal eyes and do not appear to be interchangeable for corneal measurements in clinical practice.

Repeatability and Reproducibility of Corneal Biometric Measurements Using the Visante Omni and a Rabbit Experimental Model of Post-Surgical Corneal Ectasia

PURPOSE

To investigate the repeatability and reproducibility of the Visante Omni topography in obtaining topography measurements of rabbit corneas and to develop a post-surgical model of corneal ectasia.

METHODS

Eight rabbits were used to study the repeatability and reproducibility by assessing the intra- and interobserver bias and limits of agreement. Another nine rabbits underwent different diopters (D) of laser in situ keratosmileusis (LASIK) were used for the development of ectasia model. All eyes were examined with the Visante Omni, and corneal ultrastructure were evaluated with transmission electron microscopy (TEM).

RESULTS

There was no significant intra- or interobserver difference for mean steep and flat keratometry (K) values of simulated K, anterior, and posterior elevation measurements. Eyes underwent -5 D LASIK had a significant increase in mean amplitude of astigmatism and posterior surface elevation with time (P for trend < 0.05). At 2 and 3 months, the -5 D LASIK group had significant greater mean amplitude of astigmatism (P = 0.036; P = 0.027) and posterior surface elevation (both P < 0.005) compared with control group. On TEM, the mean collagen fibril diameter and interfibril distance in the -5 D LASIK eyes were significantly greater than in controls at 3 months (P = 0.018; P < 0.001).

CONCLUSIONS

The Visante Omni provided imaging of the rabbit cornea with good repeatability and reproducibility. Application of -5 D LASIK treatment produced a rabbit model of corneal ectasia that was gradual in development and simulated the human condition.

TRANSLATIONAL RELEVANCE

The results provide the foundations for the future evaluation of novel treatment modalities for post-surgical ectasia and keratoconus.

Orbscan topography in primary open-angle glaucoma

PURPOSE

To compare anterior and posterior corneal curvatures between eyes with primary open-angle glaucoma (POAG) and healthy eyes.

METHODS

This is a prospective, cross-sectional, observer-masked study. A total of 138 white subjects (one eye per patient) were consecutively recruited; 69 eyes had POAG (study group), and the other 69 comprised a group of healthy control eyes matched for age and central corneal pachymetry with the study ones. Exclusion criteria included any corneal or ocular inflammatory disease, previous ocular surgery, or treatment with carbonic anhydrase inhibitors. The same masked observer performed Goldmann applanation tonometry, ultrasound pachymetry, and Orbscan II topography in all cases. Central corneal thickness, intraocular pressure, and anterior and posterior topographic elevation maps were analyzed and compared between both groups.

RESULTS

Patients with POAG had greater forward shifting of the posterior corneal surface than that in healthy control eyes (p < 0.01). Significant differences in anterior corneal elevation between controls and POAG eyes were also found (p < 0.01).

CONCLUSIONS

Primary open-angle glaucoma eyes have a higher elevation of the posterior corneal surface than that in central corneal thickness-matched nonglaucomatous eyes.