New devices and clinical implications for measuring corneal thickness

The Evolution of Diagnostics for Keratoconus: From Ophthalmometry to Biomechanics

PURPOSE

To enumerate the various diagnostic modalities used for keratoconus and their evolution over the past century.

METHODS

A comprehensive literature search including articles on diagnosis on keratoconus were searched on PUBMED and summarized in this review.

RESULTS

Initially diagnosed in later stages of the disease process through clinical signs and retinoscopy, the initial introduction of corneal topography devices like Placido disc, photokeratoscopy, keratometry and computer-assisted videokeratography helped in the earlier detection of keratoconus. The evolution of corneal tomography, initially with slit scanning devices and later with Scheimpflug imaging, has vastly improved the accuracy and detection of clinical and sub-clinical disease. Analyzing the alteration in corneal biomechanics further contributed to the earlier detection of keratoconus even before the tomographic changes became evident. Anterior segment optical coherence tomography has proven to be a helpful adjuvant in diagnosing keratoconus, especially with epithelial thickness mapping. Confocal microscopy has helped us understand the alterations at a cellular level in keratoconic corneas.

CONCLUSION

Thus, the collective contribution of the various investigative modalities have greatly enhanced earlier and accurate detection of keratoconus, thus reducing the disease morbidity.

Introduction and Clinical Validation of an Updated Biomechanically Corrected Intraocular Pressure bIOP (v2)

PURPOSE

To improve the stability of the Corvis ST biomechanically-corrected intraocular pressure measurements (bIOP) after refractive surgery and its independence of corneal biomechanics.

METHODS

A parametric study was carried out using numerical models simulating the behavior of the eye globe under the effects of IOP and Corvis ST external air pressure and used to develop a new algorithm for bIOP; bIOP(v2). It was tested on 528 healthy participants to evaluate correlations with CCT and age. Its ability to compensate for the geometrical changes was tested in 60 LASIK and 80 SMILE patients with six months follow up. The uncorrected Corvis ST IOP (CVS-IOP) and the two versions of biomechanically corrected IOP; bIOP(v1) and bIOP(v2), were compared.

RESULTS

In the healthy dataset, bIOP(v2) had weak and non-significant correlation with both CCT (R = -0.048, p = .266) and age (R = 0.085, p = .052). For bIOP(v1), the correlation was non-significant with CCT (R = -0.064, p = .139) but significant with age (R = -0.124, p < .05). In both LASIK and SMILE groups, the median change in bIOP(v2) following surgery was below 1 mmHg at follow-up stages and the interquartile range was smaller than both bIOP(v1) and CVS-IOP.

CONCLUSION

The bIOP(v2) algorithm performs better than bIOP(v1) and CVS-IOP in terms of correlation with CCT and age. The bIOP(v2) also demonstrated the smallest variation after LASIK and SMILE refractive surgeries indicating improved ability to compensate for geometrical changes.

Angle κ and its effect on the corneal elevation maps in refractive surgery candidates

PURPOSE

To assess the associations of angle κ and Pentacam decentration indices with elevation maps in normal refractive surgery candidates.

SETTING

Salouti Cornea Research Center, Salouti Eye Clinic, Shiraz, Iran.

DESIGN

Retrospective observational study.

METHODS

In this research, the right eyes of 173 refractive surgery candidates were assessed. Data of front and back corneal elevation maps, keratometric data, decentration indices, and corneal astigmatism obtained by Pentacam HR system and angle κ obtained by Orbscan IIz were extracted. Maximum elevation (or depression) for each of the 4 quadrants was recorded. Correlations of elevation values with angle κ, Pentacam decentration indices, keratometry, and astigmatism were examined by Pearson correlation coefficient. 148 age- and sex-matched cases with keratoconus grade 1 were selected as a positive control group, and Pentacam variables were compared between the groups.

RESULTS

Overall, data from 173 eyes of 173 normal refractive surgery candidates and 148 eyes of 148 patients with keratoconus were recorded and analyzed. In normal refractive surgery candidates, the mean of angle κ was 5.32 ± 1.36 (SD) degrees. Angle κ had a positive correlation with front and back temporal elevations based on a best-fit sphere (BFS) ( r = 0.339, P = .001; r = 0.300, P < .001, respectively). Front and back keratometric astigmatisms were positively correlated with front and back nasal and temporal elevations ( r ≥ 0.543, P < .001) and negatively correlated with superior and inferior elevations ( r ≤ -0.547, P < .001). These associations no longer existed when using best-fit toric ellipse (BFTE) for calculating elevation data. The thinnest point to vertex decentration was significantly associated with the back temporal elevation (based on the BFTE) in both normal ( r = 0.311, P < .001) and keratoconus ( r = 0.190, P = .021) eyes.

CONCLUSIONS

This study showed that elevation maps obtained by the Pentacam system using BFS might be affected by both the large angle κ and corneal astigmatism, confounding the preoperative assessment of refractive surgery candidates. Using BFTE as a reference for calculating elevation data should eliminate most diagnostic issues and thus is encouraged in this context.

The detection of keratoconus using novel metrics derived by anterior segment optical coherence tomography

PURPOSE

To determine through a feasibility study whether anterior and posterior corneal arc length and cross-sectional area measured using anterior segment ocular coherence tomography (AS-OCT) can distinguish between healthy and keratoconic corneas.

METHODS

Patients diagnosed with keratoconus along with healthy controls underwent AS-OCT. ImageJ was used to determine the central 6 mm anterior and posterior corneal arc lengths and cross-sectional areas. Each length and area was then divided into 1-mm segment and relative differences compared.

RESULTS

Twenty-five eyes from 15 patients with keratoconus, along with 25 eyes from 14 healthy controls were enrolled. There was a statistically significant difference in anterior and posterior corneal arc lengths as well as corneal cross-sectional area (p = 0.006, p = 0.005, p = 0.01, respectively). When selecting for the less advanced keratoconus eye, it was noted that posterior corneal arc length was longer in the paracentral temporal segment (1003 vs. 1010 µm, p = 0.04) and that greater change in corneal cross-sectional areas occurred between adjacent segments in less advanced keratoconus eyes.

CONCLUSION

AS-OCT is capable of reliably measuring corneal arc lengths in patients with keratoconus and healthy patients. Both anterior and posterior corneal arc lengths along with central cross-sectional areas are statistically different between healthy and keratoconus eyes.

Keratoconus: Diagnosis and Staging

ABSTRACT

The advent of refractive surgery and corneal cross-linking for ectatic disease has further highlighted the need to recognize early or subclinical ectatic disease. Historical systems depend on anterior corneal changes that occur late in the disease course and are commonly associated with visual loss. Tomographic imaging allows for the acquisition of posterior corneal surface data and corneal thickness distribution. This led to the development of modalities to diagnose early or subclinical keratoconus and to screen patients for refractive surgery.This article reviews the modern methods for assessing changes in posterior corneal surface and pachymetric distribution, now accepted by the major cornea societies to be the hallmarks of ectatic disease. Screening tools utilized by the commonly used tomographic imaging devices are discussed, and the difference between screening for ectasia and diagnosing keratoconus is highlighted. The Belin ABCD staging and classification system and the ABCD Progression Display are reviewed as a new grading and monitoring system that can be used for earlier intervention and prevention of visual loss in keratoconus.

Determination of Corneal Biomechanical Behavior in-vivo for Healthy Eyes Using CorVis ST Tonometry: Stress-Strain Index

Purpose: This study aims to introduce and clinically validate a new algorithm that can determine the biomechanical properties of the human cornea in vivo.

Methods: A parametric study was conducted involving representative finite element models of human ocular globes with wide ranges of geometries and material biomechanical behavior. The models were subjected to different levels of intraocular pressure (IOP) and the action of external air puff produced by a non-contact tonometer. Predictions of dynamic corneal response under air pressure were analyzed to develop an algorithm that can predict the cornea's material behavior. The algorithm was assessed using clinical data obtained from 480 healthy participants where its predictions of material behavior were tested against variations in central corneal thickness (CCT), IOP and age, and compared against those obtained in earlier studies on ex-vivo human ocular tissue.

Results: The algorithm produced a material stiffness parameter (Stress-Strain Index or SSI) that showed no significant correlation with both CCT (p > 0.05) and IOP (p > 0.05), but was significantly correlated with age (p < 0.01). The stiffness estimates and their variation with age were also significantly correlated (p < 0.01) with stiffness estimates obtained earlier in studies on ex-vivo human tissue.

Conclusions: The study introduced and validated a new method for estimating the in vivo biomechanical behavior of healthy corneal tissue. The method can aid optimization of procedures that interfere mechanically with the cornea such as refractive surgeries and introduction of corneal implants.

Pathogenesis of Keratoconus: The intriguing therapeutic potential of Prolactin-inducible protein

Keratoconus (KC) is the most common ectatic corneal disease, with clinical findings that include discomfort, visual disturbance and possible blindness if left untreated. KC affects approximately 1:400 to 1:2000 people worldwide, including both males and females. The aetiology and onset of KC remains a puzzle and as a result, the ability to treat or reverse the disease is hampered. Sex hormones are known to play a role in the maintenance of the structure and integrity of the human cornea. Hormone levels have been reported to alter corneal thickness, curvature, and sensitivity during different times of menstrual cycle. Surprisingly, the role of sex hormones in corneal diseases and KC has been largely neglected. Prolactin-induced protein, known to be regulated by sex hormones, is a new KC biomarker that has been recently proposed. Studies herein discuss the role of sex hormones as a control mechanism for KC onset and progression and evidence supporting the view that prolactin-induced protein is an important hormonally regulated biomarker in KC is discussed.

Surgical management of peripheral corneal thinning disorders

The peripheral corneal thinning disorders are associated with degenerative, autoimmune, or infective causes. Corneal thinning can subsequently affect the visual acuity either by inducing severe astigmatism or by progressive involvement of the central cornea. In addition to this, the integrity of the eye is at risk. Medical management is necessary to address the underlying inflammatory or infectious causes; however, most of the cases require surgical intervention for tectonic support or for visual rehabilitation in patients with severe astigmatism. Preoperative investigations help in mapping the corneal curvature and thickness, thereby facilitating planning of treatment. Routine corneal transplantation techniques do not yield good results in peripheral corneal thinning disorders. Various surgical modifications have been described to manage these challenging cases. We review the available literature on causes and management of peripheral corneal thinning disorders.

Effect of post crosslinking haze on the repeatability of Scheimpflug-based and slit-scanning imaging devices

Purpose:

The aim of this study was to analyze the effect of postcollagen crosslinking (CXL) haze on the measurement and repeatability of pachymetry and mean keratometry (Km) of four corneal topographers.

Materials and Methods:

Sixty eyes of sixty patients with progressive keratoconus who had undergone accelerated CXL (ACXL) underwent imaging with a scanning slit imaging device (Orbscan II) and three Scheimpflug imaging devices (Pentacam HR, Sirius, and Galilei). Post-ACXL haze was measured using the densitometry software on the Pentacam HR. Readings of the thinnest corneal thickness (TCT) and Km from three scans of each device were analyzed. Effect of haze on the repeatability of TCT and Km measurements was evaluated using regression models. Repeatability was assessed by coefficient of variation.

Results:

Corneal densitometry in different zones affected the repeatability of TCT measurement of Orbscan (P < 0.05) significantly but not the repeatability of TCT with Pentacam HR and Sirius (P = 0.03 and 0.05, respectively). Km values were affected by haze when measured with the Pentacam HR (P < 0.05). The repeatability of Km readings for all devices was unaffected by haze. In the anterior 0–2 mm and 2–6 mm zone, TCT (P = 0.43 and 0.45, respectively), Km values (P = 0.4 and 0.6, respectively), repeatability of TCT (P = 0.1 in both zones), and Km (P = 0.5 and 0.1, respectively) with Galilei were found to be the most reliable.

Conclusion:

Galilei measurements appear to be least affected by post-ACXL haze when compared with other devices. Hence, topography measurements in the presence of haze need to be interpreted with caution.

Rotating Scheimpflug Imaging Indices in Different Grades of Keratoconus

Purpose. To evaluate accuracy of various Keratoconus (KC) screening indices, in relation to Topographic Keratoconus (TKC) grading. Setting. Al Watany Eye Hospital, Cairo, Egypt. Methods. Data of 103 normal (group 1) and 73 KC eyes (group 2), imaged by Pentacam (branded as Allegro Oculyzer), were analysed. Group 2 was divided into 2a: 14 eyes (TKC = 1, early KC), 2b: 25 eyes (TKC = 1 to 2 or 2, moderate KC), and 2c: 34 eyes (TKC = 2 to 3 up to 4, severe KC). Participants were followed up for six years to confirm diagnosis. Area under the receiver operating characteristic curve (AUROC) was calculated for evaluated curvature, elevation, and pachymetry indices with various reference shapes at different diameters. Results. When comparing normal to KC eyes, ten indices had significantly higher AUROC. Only five of them had significantly higher AUROC in early KC compared to normal corneas: Pachymetry Progression Index- (PPI-) Maximum (Max), Ambrósio's Relational Thickness- (ART-) Max, PPI-Max minus PPI-Minimum (Min), central corneal thickness (CCT), and diagonal decentration of thinnest point from the apex (AUROC = 0.690, 0.690, 0.687, 0.683, and 0.674, resp.). Conclusion. Generally, ten pachymetry and elevation-based indices had significantly higher AUROC. Five indices had statistically significant high AUROC when comparing early KC to normal corneas.

Application of corneal tomography before keratorefractive procedure for laser vision correction

Ectasia after refractive surgery represents a major concern among refractive surgeons. Corneal abnormalities and preexisting corneal ectasia are the most important risk factors. Corneal topography and central corneal thickness are the factors traditionally screening for in refractive surgery candidates. Study of the anterior surface by Placido topography allows for identification of keratoconus before biomicroscopy. However, this is insufficient for the evaluation of pre-operative refractive surgery. There are cases of ectasia after laser in situ keratomilusis (LASIK) without identifiable risk factors such that there is a need to go beyond the corneal surface. A key requirement is quantifying susceptibility to corneal biomechanical instability and progression to ectasia. Tomographic indices derived from elevation maps and pachymetry spatial variation produce a Belin Ambrosio display final D index (BAD-D index), which has shown better results compared to surface curvature indices for detecting very mild forms of ectasia. A logistic regression formula, integrating age, residual stromal bed, and BAD-D (Ectasia Susceptibility Score, ESS) resulted in a significant improvement in accuracy, leading to 100% sensitivity and 94% specificity for detecting susceptible cases. A comprehensive corneal structural analysis based on corneal segmental tomography can detect susceptible corneas, which increases safety for refractive surgery patients.

Scheimpflug imaging for keratoconus and ectatic disease

Scheimpflug cross-sectioning anterior segment imaging offers significant advantages over traditional placido based curvature analysis and ultrasound pachymetry. The accurate measurement of both the anterior and posterior corneal surfaces and the anterior and posterior lens allows for the creation of a three-dimensional reconstruction of the anterior segment. Changes on both the posterior cornea and/or corneal thickness map are earlier indicators of ectatic change than would otherwise be identifiable with only anterior curvature and ultrasonic pachymetry. Scheimpflug imaging also covers significantly more of the cornea than was possible with placido based devices. This added coverage is critical in the proper diagnosis of peripheral diseases such as pellucid marginal degeneration (PMD).

Independent Population Validation of the Belin/Ambrósio Enhanced Ectasia Display: Implications for Keratoconus Studies and Screening

With advances in technology and imaging, finding diagnostic criteria that are both sensitive and specific for keratoconus while using the latest corneal imaging modalities is paramount. The Belin/Ambrósio enhanced ectasia display final ‘D’ index, tested on an independent population, illustrated excellent false positive rates for refractive screening while eliminating 99% of keratoconus corneas. A false positive rate of 0% is achieved with a final ‘D’ of 2.69, meeting the more stringent criteria for treatment studies.

How to cite this article

Villavicencio OF, Gilani F, Henriquez MA, Izquierdo L Jr, Ambrósio RR Jr, Belin MW. Independent Population Validation of the Belin/Ambrósio Enhanced Ectasia Display:Implications for Keratoconus Studies and Screening. Int J Kerat Ect Cor Dis 2014;3(1):1-8.

Comprehensive anterior segment normal values generated by rotating Scheimpflug tomography

PURPOSE

To identify normal values for tomographic parameters that are considered useful in screening patients for refractive surgery.

SETTING

Private center, Albany, New York, USA.

DESIGN

Database study.

METHODS

A Pentacam HR Scheimpflug system was used to examine 1 randomly selected eye of patients to determine normal values of 21 parameters considered the most clinically applicable for surgical screening. Normality of data was evaluated using the Kolmogorov-Smirnov test. Statistical analyses were performed using the Student t test to compare means and the 2-paired sample Wilcoxon signed-rank test. Results are displayed in 95.0% and 97.5% confidence intervals (CIs).

RESULTS

The study evaluated 341 adults. High-end outliers at the 97.5% CI were 46.1 diopters (D) for flat keratometry (K), 47.4 D for steep K, 3.4 D for astigmatism, 3.8 μm for anterior chamber depth, 4 μm for front apical elevation, 5 μm for front elevation at the thinnest point, and 12 μm for front elevation in the central 4.0 mm. Respective posterior elevation values were 7 μm, 13 μm, and 25 μm, with a progression index maximum of 1.53 and mean of 1.19, difference between apical and thinnest pachymetric reading of 7 μm, a maximum K of 48.2 D, and an inferior-superior ratio of 1.44 D. Low-end outliers were a maximum Ambrósio relational thickness of 335 and a mean of 425, minimum pachymetry of 479 μm, thickness at the apex of 481 μm, and central 4.0 mm corneal volume of 6.31 mm(3).

CONCLUSION

Scheimpflug-derived corneal tomography identified key refractive surgery parameters that may be useful in screening refractive surgical patients.

Scheimpflug imaging for laser refractive surgery

PURPOSE OF REVIEW

To review the principles and clinical applications of Scheimpflug corneal and anterior segment imaging with special relevance for laser refractive surgery.

RECENT FINDINGS

Computerized Scheimpflug imaging has been used for corneal and anterior segment tomography (CASTm) in different commercially available instruments. Such approach computes the three-dimensional image of the cornea and anterior segment, enabling the characterization of elevation and curvature of the front and back surfaces of the cornea, pachymetric mapping, calculation of the total corneal refractive power and anterior segment biometry. CASTm represents a major evolution for corneal and anterior segment analysis, beyond front surface corneal topography and single point central corneal thickness measurements. This approach enhances the diagnostic abilities for screening ectasia risk as well as for planning, evaluating the results, managing complications of refractive procedures, and selecting intraocular lens power, type, and design. In addition, dynamic Scheimpflug imaging has been recently introduced for in-vivo corneal biomechanical measurements and has also been used for anterior segment imaging of femtocataract surgery.

SUMMARY

Scheimpflug imaging has an important role for laser refractive surgery with different applications, which continuously improve due to advances in technology.

Comparison of artemis 2 ultrasound and Visante optical coherence tomography corneal thickness profiles

PURPOSE

To compare corneal thickness profiles of cross-sections of cornea determined by arc-scanned immersion ultrasound and optical coherence tomography (OCT).

METHODS

Corneas of 28 eyes from 14 participants were scanned in triplicate using the Artemis 2 high-frequency arc-scanned ultrasound system (ArcScan Inc) and the Visante OCT system (Carl Zeiss Meditec). Corneal thickness and reproducibility were compared within 3.5 mm of central cornea in the horizontal plane.

RESULTS

Although highly correlated, Visante central and peripheral corneal thickness values were systematically thinner than Artemis 2 values. Within the central 0.5 mm, the difference was approximately 8 μm, but the difference increased with distance from the center. Reproducibility for each instrument was comparable, measuring <4 μm centrally and increasing peripherally.

CONCLUSIONS

Visante OCT measurements of corneal thickness are thinner than Artemis 2 ultrasound values centrally with an increasing difference with peripheral position. Measurement reproducibility was comparable for the two techniques.

Corneal ectasia after myopic laser in situ keratomileusis: a long-term study

Background

The purpose of this study was to evaluate the long-term postoperative incidence of and key factors in the genesis of corneal ectasia after myopic laser-assisted in situ keratomileusis (LASIK) in a large number of cases.

Methods

A retrospective review of one surgeon’s myopic LASIK database was performed. Patients were stratified into two groups based on date of surgery, ie, group 1 (1313 eyes) from 1999 to 2001 and group 2 (2714 eyes) from 2001 to 2003. Visual acuity, refraction, pachymetry, and corneal topography data were available for each patient from examinations performed both before and after the refractive procedures.

Results

Of the 4027 surgically treated eyes, 23 (0.57%) developed keratectasia during the follow-up period, which was a minimum seven years; nine eyes (0.69%) were from group 1 and 14 eyes (0.51%) were from group 2. The onset of corneal ectasia was at 2.57 ± 1.04 (range 1–4) years and 2.64 ± 1.29 (range 0.5–5) years, respectively, for groups 1 and 2. The most important preoperative risk factors using the Randleman Ectasia Risk Score System were manifest refractive spherical error in group 1 and a thin residual stromal bed in group 2. Each of the cases that developed corneal ectasia had risk factors that were identified.

Conclusion

Ectasia was an uncommon outcome after an otherwise uncomplicated laser in situ keratomileusis procedure. The variables present in eyes developing postoperative LASIK ectasia can be better understood using the Randleman Ectasia Risk Score System.

Novel pachymetric parameters based on corneal tomography for diagnosing keratoconus

PURPOSE

To describe pachymetric progression indices (PPI) of the Pentacam HR (Oculus Optikgeräte GmbH) and the concept of relational thickness, and to test their accuracy for differentiating keratoconic and normal corneas compared with single-point thickness values.

METHODS

One hundred thirteen individual eyes randomly selected from 113 normal patients and 44 eyes of 44 patients with keratoconus were studied using the Pentacam HR by acquiring central corneal thickness (CCT), thinnest point (TP), position of the TP and PPI at minimal (PPI Min) and maximal (PPI Max) meridians, and the average (PPI Ave) of all meridians. Relational thickness parameters were calculated as the ratios of TP and CCT and PPI values. Mann-Whitney U test assessed differences in groups for each variable. Receiver operating characteristic (ROC) curves were calculated for all variables and pairwise comparisons were performed.

RESULTS

Statistically significant differences were noted between normal and keratoconic eyes for all parameters (P<.001), except for horizontal position of TP (P=.79). The best parameters, named Ambrósio's Relational Thickness (ART), were ART-Ave (TP/PPI Ave) and ART-Max (TP/PPI Max) with areas under the ROC curves of 0.987 and 0.983, respectively. The best cutoffs were 424 μm and 339 μm for ART-Ave and ART-Max, respectively. Pachymetric progression indices and ART had a greater area under the curve than TP and CCT (P<.001); TP (0.955) had a greater area under the curve than CCT (0.909; P=.002).

CONCLUSIONS

Tomographic-derived pachymetric parameters were better able to differentiate normal and keratoconic corneas than single-point pachymetric measurements. Further studies are needed to evaluate the role of tomography in identifying early forms of ectasia as well as ectasia risk among LASIK candidates.

Evaluation of central corneal thickness measurement with RTVue spectral domain optical coherence tomography in normal subjects

PURPOSE

To determine (1) repeatability of central corneal thickness (CCT) measurements by spectral domain optical coherence tomography (RTVue; Optovue, Inc, Fremont, CA) and (2) agreement between CCT measurements by RTVue and those by ultrasonic pachymetry, Orbscan, and anterior segment optical coherence tomography (ASOCT).

METHODS

In a prospective observational study, 2 cohorts of normal subjects were recruited. In the first cohort (51 subjects), 5 measurements of CCT were obtained by RTVue during the same visit to determine the repeatability. In the second cohort (65 subjects), CCT measurements were obtained by RTVue, ultrasonic pachymetry, Orbscan, and ASOCT during the same visit to determine the agreement among these instruments. Repeatability was assessed by intraclass correlation coefficient (ICC), within-subject standard deviation, coefficient of repeatability, and within-subject coefficient of variation. Agreement was assessed by ICC and Bland and Altman plots.

RESULTS

Repeatability of CCT measurements by RTVue as assessed by ICC, within-subject standard deviation, coefficient of repeatability, and within-subject coefficient of variation was 0.99 (0.99-0.99), 2.2 (1.9-2.5), 4.2 μm (3.6-4.8), and 0.4% (0.3-0.5), respectively. The average CCT by RTVue (529 μm) was comparable to that by ultrasonic pachymetry (539 μm; P = 0.15), Orbscan (536 μm; P = 0.54), and ASOCT (526 μm; P = 0.77). The 95% limits of agreement on Bland and Altman plots ranged from 20 μm (between RTVue and ASOCT) to 33 μm (between RTVue and Orbscan).

CONCLUSIONS

CCT measurements by RTVue have an excellent repeatability. Although CCT measurements by RTVue are comparable to that by ultrasonic pachymetry, Orbscan, and ASOCT, the difference between instruments can be significant depending on the clinical situation considered.