A novel color-LED corneal topographer to assess astigmatism in pseudophakic eyes

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.

[Early diagnosis of keratoconus]

Keratoconus is morphologically associated with increasing deformation, thinning and scarring of the cornea. This functionally leads to refractive changes and visual deterioration. In the early stages there are often no clear clinical signs in the slit-lamp examination; however, confirming the diagnosis as early as possible is important in order to provide patients with an appropriate treatment. For the early diagnosis of keratoconus, various diagnostic devices have been introduced in recent years and decades. These include keratometry with reflection-based or elevation-based systems and optical coherence tomography. High-frequency ultrasound microscopy and corneal biomechanics can also be used to establish the diagnosis of keratoconus by the measurement of other parameters. The necessity and the available possibilities for early diagnosis of keratoconus are presented in more detail in this article.

The influence of routine uncomplicated phacoemulsification on the orthogonality of the cornea

Purpose:

The aim of this study was to determine the effect of routine uncomplicated phacoemulsification on the orthogonal distribution of mass within the central optical zone of the cornea.

Methods:

Astigmatism at both corneal surfaces was evaluated using Orbscan II (Bausch &and Lomb) before and up to 3 months after routine phacoemulsification (one eye/patient). The data were subjected to vector analysis to estimate the pre-and postoperative total astigmatism of the cornea (TCA).

Results:

Reporting the chief findings in minus cylinder (diopters, DC) over the central 3 mm (A) and 5 mm (B) optical zones. Mean TCA powers (±sd) at pre- and 3-months postop were A) –4.45DC (±2.00) and –5.69DC (±2.69), B) –2.91DC (±2.22) and –2.71DC (±1.60). Change in mean power was significant over 3 mm (P < 0.01, n = 49) but not over 5 mm. Inter-zonal differences were significant (P < 0.01). There was a significant linear relationship between the change in TCA power (y = preoperative-postoperative) and TCA at preoperative stage (x) where, A) y = 0.45x + 3.12 (r = 0.336, n = 49, P = 0.018), B) y = x + 2.65 (r = 0.753, n = 49, P = <0.01). Over the central 3 mm zone only, change (preoperative-postoperative) in axis (°) of TCA (y₁) was significantly associated with TCA axis at preoperative stage (x₁) where y₁ = 1.391x₁-0.008x₁²-0.701 (r = 0.635, n = 49, P < 0.01).

Conclusion:

Changes in TCA power and axis at 3 months postop, determined using Orbscan II, are indicative of orthogonal alterations in the distribution of corneal tissue. Over the central 3 mm zone, the association between y₁ and x₁ shows that a change in TCA axis is more profound when preoperative axis is near 90° i.e., against-the-rule.

Evaluation of posterior and total corneal astigmatism with colour-LED topography

PURPOSE

To characterise the posterior and total corneal astigmatism using colour point-source light-emitting diodes (LED) topography.

METHODS

In a prospective case series 400 eyes from 400 patients were evaluated by colour-LED topography. Only eyes with normal topographies were considered. The following parameters were studied: magnitude and distribution of SimK and posterior corneal astigmatism, correlation between SimK and posterior corneal astigmatism, and differences in magnitude and axis between total and anterior corneal astigmatism.

RESULTS

The mean SimK corneal astigmatism was 1.21 ± 0.94 D. The mean posterior corneal astigmatism was 0.37 ± 0.24 D. Posterior astigmatism was vertically oriented in 68% of eyes. Twenty-two percent of eyes showed a posterior corneal astigmatism ≥ 0.50 D. The correlation coefficients between SimK and posterior corneal astigmatism were: r² = 0.066; p = 0.371 in WTR eyes, r² = 0.112; p = 0.173 in ATR eyes and r² = -0.019; p = 0.879 in oblique eyes. A difference between SimK and total corneal astigmatism ≥ 0.50 D was found in 7% of eyes. A difference in axis between SimK and total corneal astigmatism ≥ 10° was found in 24% of eyes.

CONCLUSIONS

The percentage of eyes with posterior corneal astigmatism ≥ 0.50 D and the differences between anterior and total corneal astigmatism were higher than those previously reported in the literature. Therefore, this study supports the consideration of total corneal astigmatism magnitude and axis is mandatory for a precise surgical correction of astigmatism.

A review of corneal imaging methods for the early diagnosis of pre-clinical Keratoconus

BACKGROUND

Keratoconus (KC) is a corneal ectasia characterised by steepening corneal curvature, changes in refractive error and corneal thickness that result in visual impairment. Early signs of KC include displacement of the thinnest part of the cornea from the central position, changes in the corneal epithelial layer cell distribution, variations in the anterior corneal astigmatism/posterior corneal astigmatism relationship and a variation in corneal thickness. It is important that we review the corneal imaging methods for the diagnosis of preclinical KC.

METHOD

An online literature search was carried out on PubMed. Only publications detailing corneal assessment procedures were considered for this review and any publication on instruments that did not generate KC predictability indices were also excluded from the review. The 308 publications were reviewed.

DISCUSSION

Corneal assessment techniques, with the ability to characterise both the anterior and posterior corneal surfaces, are invaluable in the diagnosis of pre-clinical KC. Reflection based and elevation based corneal imaging systems should be used in conjunction with other assessments such as higher order aberration measuring systems to improve sensitivity and reliability in the diagnosis of pre-clinical KC. Ultra high resolution ultrasound can detect pre-clinical KC. The ability to asses both the epithelium and endothelium makes anterior surface optical coherence tomography a superior technique for pre-clinical KC diagnosis. There is a positive correlation between central corneal thickness and corneal hysteresis. Corneal biomechanics should be considered in conjunction with other corneal assessments in the diagnosis of pre-clinical KC.

How Can We Improve Toric Intraocular Lens Calculation Methods? Current Insights

In this paper, we review current strategies for calculating toric intraocular lenses (IOLs). We discuss the prevalence and clinical relevance of astigmatism and the assessment of toric IOL candidates. We detail recommendations for evaluating astigmatism and current biometry and IOL power calculation techniques. Finally, error sources and results of current toric IOL calculators are discussed.

Comparison of Multicolored Spot Reflection Topographer and Scheimpflug-Placido System in Corneal Power and Astigmatism Measurements With Normal and Post-refractive Patients

PURPOSE

To evaluate the repeatability of corneal power and astigmatism derived by a novel multicolored spot reflection topographer system (Cassini; i-Optics, Hague, Netherlands) and compare its agreement with a Placido-Scheimpflug system (Sirius; Costruzione Strumenti Oftalmici, Florence, Italy) in normal and post-refractive patients.

METHODS

This prospective study comprised patients who underwent myopic excimer laser refractive surgery (96 eyes) and normal patients (102 eyes). Each patient was measured three times with the Cassini and Sirius. The simulated keratometry (SimK), total corneal power (TCP), and astigmatism were recorded. The repeatability was assessed by one-way analysis of variance. The paired t test was used to compare the differences, whereas the agreement was evaluated by Bland-Altman analysis.

RESULTS

All parameters obtained by the Cassini demonstrated high repeatability, except for total corneal astigmatism (TCA) in the post-refractive group. The intraclass correlation coefficients of all parameters were greater than 0.85, the correlation of variation values was less than 0.55%, and the test-retest repeatability was less than 0.85 diopters (D). The paired t test showed significant differences in steep keratometry, astigmatism, TCP, and TCA in the normal group and in J₀, TCP, and TCA in the post-refractive group. The 95% limits of agreement (LoA) in the normal group demonstrated good agreement, except for TCP. Only J₀ and J₄₅ of astigmatism and TCA remained narrow for 95% LoA in the post-refractive group.

CONCLUSIONS

These results suggested that the Cassini provided high repeatable measurements in corneal power and astigmatism, except the TCA of post-refractive patients. The parameters could be used interchangeably in normal patients, except for TCP, whereas only J₀ of astigmatism and J₀, J₄₅ of TCA showed good agreement in post-refractive patients. [J Refract Surg. 2019;35(6):370-376.].

Validation of corneal topographic and aberrometric measurements obtained by color light-emitting diode reflection topography in healthy eyes

PURPOSE

To evaluate the intrasession repeatability of anterior corneal topographic and aberrometric measurements provided by a color-LED topographer as well as their interchangeability with those provided by a Scheimpflug-based system in healthy eyes.

METHODS

Thirty-five healthy eyes of 35 patients (age, 16-66 years) were enrolled. A complete eye examination was performed in all cases including a complete corneal analysis with the Scheimpflug-based system Pentacam (Oculus Optikgeräte) (one measurement) and the Cassini system (i-Optics) (three consecutive measurements). Intrasession repeatability of the Cassini measurements was assessed with the within-subject standard deviation (S_w) and the intraclass correlation coefficient (ICC). The Bland-Altman analysis was used to evaluate the agreement between both devices.

RESULTS

Mean S_w for keratometric readings was 0.02 mm (ICC ≥ 0.992), ranging between 0.16 and 0.05 D (ICC 0.930-0.978) for anterior and total astigmatic measurements. Mean S_w for asphericity and corneal diameter were 0.06 (ICC 0.926) and 0.03 mm (IC 0.997), respectively. Aberrometric parameters showed ICCs ≥ 0.816, except for Z₄² (ICC 0.741) and Z₄⁴ (ICC 0.544). When comparing devices, statistically significant differences were found for most of topographic and aberrometric data (p ≤ 0.044). Likewise, ranges of agreement between devices were clinically relevant (keratometry > 0.06 mm; total astigmatic components > 0.69 D; asphericity 0.35; second-, third-, and fourth-order Zernike terms, more than 0.20, 0.13, and 0.01 μm, respectively).

CONCLUSIONS

Consistent anterior corneal topographic, total corneal astigmatic, and aberrometric measurements are obtained with color-LED topography in healthy eyes, which are not interchangeable with those provided by the Scheimpflug-based topography.

Predictability of different calculators in the minimization of postoperative astigmatism after implantation of a toric intraocular lens

Purpose

To assess the efficacy of five calculators for toric intraocular lenses (IOL).

Methods

Retrospective comparative case series in cataract patients undergoing implantation of trifocal toric IOLs (PhysIOL FineVision POD FT). Inclusion criteria were age-related cataract and a corneal astigmatism between 0.90D and 4.50D. Refractive astigmatism predictability of five different toric calculators or calculation methods were compared. Furthermore, two groups were differentiated according to the type of astigmatism. The mean absolute error and the centroid errors in the predicted residual astigmatism from each calculator were evaluated.

Results

Fifty-one eyes of 43 patients were included in the study. For the standard toric calculator using anterior keratometry values only, the centroid prediction error was 0.39D±0.41@166º, which was reduced by the application of the PhysIOL toric calculator that includes the Abulafia-Koch regression formula and adjustment for the effective lens position (0.05D±0.34@167º), and also by the application of the Barrett toric calculator (0.07D±0.28@160º). Regarding the techniques that directly evaluate posterior corneal surface, the Holladay toric calculator, using total corneal power provided by a color-LED topographer, generated better results (0.10D±0.44@156º) than those using Scheimpflug camera data (0.23D±0.56@158º). Similar results were found for both types of astigmatism.

Conclusion

The PhysIOL and the Barrett toric calculators taking into account the posterior corneal astigmatism by mathematical models, yielded lower astigmatic prediction errors compared to a standard toric calculator based on anterior keratometry data only. When total corneal power measurements were used, prediction errors were lower with color-LED than with Scheimpflug based topography.

Validation of posterior corneal curvature measurements with color light-emitting diode topography

PURPOSE

To evaluate the intrasession repeatability and validity of posterior corneal curvature and astigmatism measurements provided by a color light-emitting diode reflection topography system in healthy eyes.

METHODS

A total of 40 healthy eyes of 40 patients (age, 16-66 years) were enrolled. A complete eye examination was performed in all cases including posterior topographic analysis with two systems: the Scheimpflug-based system (Pentacam; Oculus Optikgeräte GmbH, Wetzlar, Germany) and the Cassini system (i-Optics; Ophthec, The Hague, The Netherlands). With this last system, three consecutive measurements were taken to assess the level of intrasession repeatability (within-subject standard deviation, S_w; intraclass correlation coefficient). The Bland & Altman analysis was used to evaluate the interchangeability of both devices.

RESULTS

The S_w was ⩽0.06 mm for all posterior corneal radius measurements, with intraclass correlation coefficient of ⩾0.960. The S_w for the magnitude of astigmatism, J₀, and J₄₅ were 0.15, 0.04, and 0.04 D, respectively, with intraclass correlation coefficient values of 0.876, 0.897, and 0.840, respectively. Statistically significant differences between devices were found in all parameters evaluated (p ⩽ 0.025). The interchangeability analysis revealed the presence of clinically relevant limits of agreement for the flattest (0.03 to 0.50 mm) and steepest posterior corneal radii (-0.01 to 0.39 mm). In contrast, limits of agreements were not clinically relevant for the magnitude of posterior astigmatism (-0.17 to 0.27 D) and their power vector components (-0.11 to 0.15 D).

CONCLUSION

The Cassini system provides consistent measures of posterior corneal curvature and astigmatism in healthy eyes, but only measures of posterior astigmatism can be considered as interchangeable with those provided by the Pentacam.

Assessing the Likely Effect of Posterior Corneal Curvature on Toric IOL Calculation for IOLs of 2.50 D or Greater Cylinder Power

PURPOSE

To establish whether average refractive overcorrection or undercorrection of corneal astigmatism based on the orientation (rule) of the astigmatism occurs if toric intraocular lenses (IOLs) are calculated on the basis of anterior corneal measurements in eyes requiring toric IOL cylinder power of 2.50 diopters (D) or greater.

METHODS

One hundred thirteen consecutive eyes with anterior corneal keratometric astigmatism requiring IOL cylinder power of 2.50 D or greater underwent phacoemulsification with IOL powers calculated using anterior corneal curvature data alone. Eyes were grouped as either "with-the-rule" (WTR) or "against-the-rule" (ATR) on the basis of the steep anterior corneal meridian. Targeted and achieved astigmatic outcomes were compared. The main outcome measure was the postoperative refractive astigmatic prediction error.

RESULTS

A mean overcorrection occurred in anterior WTR eyes of 0.16 ± 0.57 D and a mean undercorrection of ATR eyes of -0.14 ± 0.53 D. These were significantly different from the ideal value of zero (WTR: P = .04, ATR: P = .05). Although statistically significant, the effect sizes of these prediction errors were 0.40 for WTR and 0.36 for ATR and the error values fell below a clinically significant value of 0.25 D.

CONCLUSIONS

In eyes requiring toric IOLs of cylinder power 2.50 D or greater, an overcorrection occurs in anterior WTR eyes and an undercorrection in ATR eyes. This probable posterior corneal astigmatism effect is not clinically significant. IOL cylinder powers are sufficiently accurately calculated using unadjusted anterior keratometry values in these eyes. [J Refract Surg. 2017;33(11):730-734.].

Comparability and repeatability of different methods of corneal astigmatism assessment

Purpose

To assess the comparability and repeatability of keratometric and astigmatism values measured by four techniques: Orbscan IIz^® (Bausch and Lomb), Lenstar LS 900^® (Haag-Streit), Cassini^® (i-Optics), and Total Cassini (anterior + posterior surface), in healthy volunteers.

Patients and methods

Fifteen healthy volunteers (30 eyes) were assessed by the four techniques. In each eye, three consecutive measures were performed by the same operator. Keratometric and astigmatism values were recorded. The intraclass correlation coefficient (ICC) was used to assess comparability and repeatability. Agreement between measurement techniques was evaluated with Bland-Altman plots.

Results

Comparability was high between all measurement techniques for minimum keratometry (K1), maximum keratometry (K2), astigmatism magnitude, and astigmatism axis, with ICC >0.900, except for astigmatism magnitude measured by Cassini compared to Lenstar (ICC =0.798) and Orbscan compared to Lenstar (ICC =0.810). However, there were some differences in the median values of K1 and K2 between measurement techniques, and the Bland-Altman plots showed a wide data spread for all variables, except for astigmatism magnitude measured by Cassini and Total Cassini. For J0 and J45, comparability was only high for J0 between Cassini and Orbscan. Repeatability was also high for all measurement techniques except for K2 (ICC =0.814) and J45 (ICC =0.621) measured by Cassini.

Conclusion

All measurement techniques showed high comparability regarding K1, K2, and astigmatism axis. Although posterior corneal surface is known to influence these measurements, comparability was high between Cassini and Total Cassini regarding astigmatism magnitude and axis. However, the wide data spread suggests that none of these devices should be used interchangeably.