Applications of corneal topography and tomography: a review

Chan Tommy (Orcid ID: 0000-0003-4228-9482) Prakash Gaurav (Orcid ID: 0000-0002-1505-8024) Jhanji Vishal (Orcid ID: 0000-0002-4429-2004)

Review

Applications of corneal topography and tomography: a review Rachel Fan MBBS,1 Tommy CY Chan FRCS,2 Gaurav Prakash MD3 and Vishal Jhanji MD

FRCOphth2,4,5

1Faculty of Medicine, The University of Hong Kong, Hong Kong 2Department of Ophthalmology & Visual Sciences, The Chinese University of Hong

Kong, Hong Kong 3Ophthalmology, NMC Eye Center, Abu Dhabi 4Department of Ophthalmology, University of Pittsburgh School of Medicine, Pittsburgh,

PA, USA 5Centre for Eye Research Australia, University of Melbourne, Victoria, Australia

Correspondence: Vishal Jhanji, UPMC Eye Center, Department of Ophthalmology,

University of Pittsburgh School of Medicine, 203 Lothrop Street, Pittsburgh, PA, 15213,

USA

Email: [email protected]

Short running title: Applications of corneal topography and tomography

Received 16 August 2017; accepted 14 December 2017

Conflict of interest: None

Funding sources: None

This article is protected by copyright. All rights reserved.

This is the author manuscript accepted for publication and has undergone full peer review buthas not been through the copyediting, typesetting, pagination and proofreading process, whichmay lead to differences between this version and the Version of Record. Please cite this articleas doi: 10.1111/ceo.13136

http://dx.doi.org/10.1111/ceo.13136

http://dx.doi.org/10.1111/ceo.13136

ABSTRACT

Corneal imaging is essential for diagnosing and management of a wide variety of ocular

diseases. Corneal topography is used to characterize the shape of the cornea,

specifically, the anterior surface of the cornea. Most corneal topographical systems are

based on Placido disc that analyze rings that are reflected off the corneal surface. The

posterior corneal surface cannot be characterized using Placido disc technology.

Imaging of the posterior corneal surface is useful for diagnosis of corneal ectasia. Unlike

corneal topographers, tomographers generate a three-dimensional recreation of the

anterior segment and provide information about the corneal thickness. Schiempflug

imaging is one of the most commonly used techniques for corneal tomography. The

cross-sectional images generated by a rotating Scheimpflug camera are used to locate

the anterior and posterior corneal surfaces. The clinical uses of corneal topography

include, diagnosis of corneal ectasia, assessment of corneal astigmatism, and refractive

surgery planning. This review will discuss the applications of corneal topography and

tomography in clinical practice.

Keywords: Topography, tomography, cornea, repeatability, agreement.


INTRODUCTION

‘Topography’ is derived from the Greek words ‘topo’ (meaning ‘to place’) and ‘graphien’

(meaning “to write”). Corneal topography is a non-contact imaging technique that maps

the shape and features of the corneal surface. Corneal topographers such as a Placido

disc, analyze the pattern of light rays reflected off the cornea and tear film-air interface

and reconstruct the corneal shape. Although modern topography devices are able to

map a large part of the anterior segment, a complete pachymetric evaluation is not

possible without information of the posterior corneal surface. Contrary to topography,

corneal tomography (‘tomos’: ’section’; and ‘graphien’: ’to write)’ evaluates the whole

cornea by obtaining information from both anterior and posterior corneal surfaces. The

corneal tomographers are able to reconstruct three-dimensional images of the anterior

segment. A good understanding of corneal imaging techniques is essential for its

successful clinical applications. This review will cover the indications and interpretation

of corneal topography.

PRINCIPLES OF CORNEAL TOPOGRAPHY

Placido disk-based keratoscopy

Placido disk consists of a circular target of alternating concentric light and dark rings

and a central aperture for observing the corneal reflections of these light-and-dark

bands over the cornea (Figure 1).1 Examination of the reflected rings gives information

about the shape of the cornea. The initial use of Placido disc was more qualitative; and

yet with the development of sophisticated software, the reflection patterns can be used

to create quantitative data and color-coded maps as seen in videokeratographs. More


sophisticated Placido disk-based devices combine the Placido disk with other

technologies such as Scheimpflug images and scanning slit technology.

Slit-scanning elevation topography

The scanning slit system (e.g. Orbscan) is a projective technique that measures the

triangulation between the reference slit beam surface and the reflected beam captured

by a camera. It combines a three-dimensional scanning slit beam system with an added

Placido attachment. Forty slits are projected sequentially on the cornea (20 nasal, 20

temporal) during image acquisition to create an overlapping pattern of scanning slits

(Figure 2). This data is interpreted using triangulation, and the final image is

represented as a three-dimensional topographic map including curvature, elevation and

pachymetry maps of the entire corneal surface.

Scheimpflug imaging

A problem noted with centrally located scanning slit based cameras was that there was

poor/unreliable capture of the corneal data from the periphery, caused by the non-

planar shape of the cornea. Scheimpflug principle eliminates this problem.2 If the

refracting lens plane and the desired image plane are parallel, an object, which is

parallel to the lens, will form a plane of focus that is also parallel to the lens plane.

However, if some parts of the object to be mapped are not parallel to the prospective

image plane, it will not be possible to focus the entire image on a plane parallel to

image plane. As a result, it may lead to image distortion. The Scheimpflug principle

states that when a planar subject is not parallel to the image plane, an oblique tangent

can be drawn from the image, object and lens planes, and the point of intersection is

called Scheimpflug intersection (Figure 3). A careful manipulation of the image plane

and the lens plane are used to obtain a focused and sharp image of the non-parallel


object.3 The commonly used Scheimpflug devices include Pentacam, TMS-5, Galilei and

Sirius. The Pentacam has a single rotating camera and a static camera. The Galilei and

the Sirius are both Scheimpflug-Placido devices integrating a Placido topographer with a

dual and single rotating Scheimpflug camera respectively.

Optical coherence tomography

Optical coherence tomography (OCT) is based on the principle of low coherence

interferometry.4 It compares the time-delay of infrared light reflected from the anterior

segment structures against a reference reflection. There are currently two types of

OCTs available: time-domain and Fourier-domain OCT. Time-domain OCT produces

cross-sectional images by varying the position of a reference mirror, whereas Fourier-

domain OCT has a fixed mirror. An interference between the sample and the reference

reflections produces cross-sectional images.5 Fourier-domain OCT has a faster

acquisition time compared to Time-domain OCT, therefore it reduces the motion

artifacts due to eye movements. This results in low signal to noise ratios, provides

better resolution and improves the characterization of normal structures as well as that

of ocular pathology.

CLINICAL APPLICATIONS OF CORNEAL TOPOGRAPHY

Keratoconus

Keratoconus is an ectatic corneal dystrophy.6 It is characterized by progressive thinning

of the cornea with resultant irregular astigmatism and loss of visual acuity (Figure 4, 5).

Diagnosis of keratoconus


The Global Consensus on Keratoconus and Ectatic Disease (2015),7 recommended the

following criteria for diagnosis of keratoconus: abnormal posterior elevation, abnormal

corneal thickness distribution and corneal thinning. Corneal tomography (e.g.

Scheimpflug or optical coherence tomography) is the most commonly used modality to

diagnose keratoconus due to its ability to detect posterior corneal elevation

abnormalities even in mild or subclinical disease.8

Several indices such as the inferior-superior index and KISA% index may

facilitate the differentiation of keratoconus from normal corneas.9,10 The central K value,

an expression of central corneal steepening, is the average of the dioptric powers on

rings 2-4 of the Placido disc and a central K value ≥ 47.2 Diopters is indicative of

keratoconus.10 The inferior-superior index (I-S index), an expression of inferior-superior

dioptric asymmetry, is the difference in dioptric power between the inferior and superior

cornea. An I-S value ≥ 1.4 is suggestive of keratoconus.10 The KISA% index, introduced

by Rabinowitz and Rasheed,10 is a topography-based index which is to quantify the

asymmetry of the corneal surface. It is derived from four indices including central K

value (K), I-S index, astigmatism (AST) index and skewed radial axis index (SRAX). The

AST index quantifies the degree of regular corneal astigmatism (SimK1- SimK2) and the

SRAX index is an expression of irregular astigmatism occurring in keratoconus.10,11 The

KISA index is calculated as: KISA% = (K x I-S x AST x SRAX x 100)/300. The KISA%

index has an excellent clinical correlation.10 A value of 100% is diagnostic of

keratoconus, and it is highly sensitive and specific. A KISA% index range between 60-

100% is considered keratoconus-suspect or subclinical keratoconus whereas KISA% <

60% is considered to be normal.10

The use of displays such as the Belin/Ambrosio Enhanced Ectasia Display (BAD)

on Pentacam can be employed for detection of keratoconus.12 The BAD comprises

deviation of normality of the front elevation, back elevation, pachymetric progression,


corneal thinnest point, and relational thickness. The Pentacam software classifies BAD

value as normal (< 1.6 standard deviation [SD] from the population mean), suspicious

(≥ 1.6 SD and < 2.6 SD), and pathologic (≥ 2.6 SD) (Figure 6).

Classification of keratoconus

The Amsler-Krumeich keratoconus classification (table 1] is the oldest and most

commonly used classification system for keratoconus.13 It relies on anterior surface

topography. The severity of keratoconus is graded from stage 1–4 using refractive error

of patient, central keratometry, presence or absence of scarring, and central corneal

thickness.

A new classification/staging ABCD keratoconus grading system was proposed in

2016 utilizing current tomographic data and it is dependent on corneal tomography.12

The ABCD keratoconus grading system includes the anterior (A) and posterior (B)

average radii of curvature, thinnest pachymetric values (C) and best distance visual

acuity (D) as well as the degree of scarring. The system classifies keratoconus into 5

stages from 0 to 4. Although it is claimed to better reflect the anatomical changes seen

in keratoconus compared to the existing classification systems, further studies are

warranted for its validation on a large number of patients before it can be

recommended for clinical use.

Assessment of ectasia progression

Evaluation of disease progression is important for the formulation of a management

plan. Kmax (maximum anterior sagittal curvature) is one of the most commonly used

parameters to detect or document progression. Since most of the commercially

available corneal tomography devices have a repeatability that does not exceed 0.5 to 1

diopter, a change of >0.5 diopter is considered to depict disease progression.


Furthermore, flattening of Kmax is used to gauge treatment effect after interventions

such as corneal collagen crosslinking. The Global Consensus on Keratoconus and Ectasic 7 defined ectasia progression as a consistent change over time in at least 2 of the

followings – steepening of the anterior corneal surface, steepening of the posterior

corneal surface and thinning and/or an increase in the rate of corneal thickness change

from the periphery to the thinnest point. These changes can be monitored by corneal

tomography.

Contact lens fitting in keratoconus

Contact lens fitting is challenging especially when the corneal apex become steeper in

advanced keratoconus. Furthermore, there is also an increased risk of complications

from a poorly fitted contact lens. Most topographers are equipped with topography

assisted contact lens fitting software enabling more complete data collection and

analysis of eyes with keratoconus. It helps to assess the severity of keratoconus and

provides details of the shape of the cone (nipple, oval or globus).14 The parameters

obtained on corneal topography can reduce contact lens fitting time and help in

achieving a better fit of RGP or Rose K (multicurve lenses with small optical zone)

contact lenses.15

Corneal crosslink ing in keratoconus

Corneal crosslinking is indicated for slowing down or stopping the progression of

keratoconus.16 Wollensak et al. were the first to show clinical effect of crosslinking on

keratoconus in 2003.17 A randomized controlled study by Wittig-Silva et al. reported a

significant decrease in maximal keratometry in keratoconus patients after crosslinking.18

Crosslinking has also shown promising results for post-refractive surgery keratectasia.19

Steinberg et al. reported corneal topography to be useful in post-crosslinking follow-up


due to significant changes in the keratometry of the cornea.20 They also reported that

assessment of posterior corneal surface is important in addition to the anterior corneal

surface as increasing posterior elevation values might be a sign of ongoing ectatic

changes despite a stable anterior cornea.20

Refractive surgery

Preoperative ectasia risk assessment

Corneal ectasia is an uncommon but severe sight-threatening complication after

refractive surgery. Randleman et al.21 identified abnormal preoperative corneal

topography as the most important risk factor for developing ectasia after LASIK. The

other risk factors included low residual stromal bed thickness, age of the patient and

preoperative corneal thickness. Santhiago et al.22 recommended that preoperative

screening before refractive surgery should include analysis of intrinsic biomechanical

properties (data obtained from corneal topography/tomography and patient’s age) and

the analysis of alterable biochemical properties (data obtained from the amount of

tissue altered by surgery and the remaining load-bearing tissue). Ectasia could occur

after laser refractive surgery in three scenarios: either in a cornea with intrinsic corneal

disease associated with fragility such as keratoconus or, in a preoperatively weak but

clinically stable cornea with subtle topographic or tomographic signs of abnormality or,

in a relatively normal cornea which is weakened with biomechanical instability after

surgery due to a high percentage of tissue altered.22 It should be noted that the risk of

biomechanical instability could still be increased in eyes that have subtle abnormal

topographic patterns that are not associated with keratoconus even with a low value of

percentage of tissue altered. Cases of ectasia after LASIK without risk factors have also

been reported.23,24


In contrast to a diagnostic test, a screening test for keratoconus requires high

sensitivity. The use of segmental tomography together with epithelial thickness

measurement has been reported to be useful.25-27 The use of epithelial thickness

mapping in addition to corneal topography may pick out false positive ‘at risk’ cases

that would have been otherwise excluded by topography alone.28 Furthermore, devices

such as the Galilei dual-Scheimpflug analyzer have an automated detection program

which includes 56 parameters derived from topography, elevation maps, pachymetry

and wavefront for analysis. It has a sensitivity of 93.7% and a specificity of 97.2%.29

In addition to preoperative evaluation, it may be beneficial to measure flap

thickness and residual bed thickness intraoperatively in order to identify cases that may

be at risk for postoperative ectasia despite a lack of risk preoperatively.30

Measurement of surgical outcomes in refractive surgery

LASIK causes changes on the anterior as well as the posterior corneal surface.31 Chan

et al used optical coherence tomography to depict the fluctuation in posterior corneal

elevation after LASIK and photorefractive keratectomy (PRK).32 Corneal topography is

useful postoperatively to look for increased corneal toricity with topographic

abnormality, progressive corneal thinning and myopic refractive error with increased

astigmatism.33 After hyperopic corrections, the keratometry and the epithelial thickness

may show disagreement. The use of postoperative keratometry together with central

epithelial thickness measurement can determine whether a retreatment is needed in

these patients.34 In post-LASIK patients, Pentacam can be used to study the corneal

thickness, anterior and posterior curvature due to its high repeatability.35

Complications after refractive surgeries: epithelial ingrow th, diffuse lamellar

keratitis and central tox ic keratopathy


The majority of cases with epithelial growth after LASIK can be managed conservatively

until their spontaneous resolution. The decision to intervene surgically is dependent

upon symptoms such as glare and loss of visual acuity.36 Serial corneal topographic

changes in these eyes are an indication for surgical intervention. Majority of the times,

change in corneal thickness and keratometry occurs in parallel to change in manifest

refraction.37

Diffuse lamellar keratitis is the infiltration of white blood cell between the flap

and stromal bed after LASIK.38 Corneal topography shows notable focal flattening

corresponding to the focal haze noted on slit lamp examination.37 Likewise, central toxic

keratopathy is an uncommon, non-inflammatory central corneal opacification that can

be observed after uneventful LASIK or surface ablation surgery.39 Significant focal

flattening can be demonstrated in sagittal curvature map corresponding to focal corneal

haze on slit lamp examination.37

Cataract and intraocular lens power calculation

Similar to its application in refractive surgery, corneal topography and tomography

enable preoperative screening of patients with irregular corneas. Surgeons can attempt

to minimize induced or pre-existing astigmatism by combined use of corneal topography

and pre-operative refraction to plan the placement of corneal incisions.40 In refractive

cataract surgery, the outcomes are influenced by corneal asphericity assessed on

corneal topographers.41 Savini et al. reported that axial length and keratometry

measurements obtained by the Aladdin – an optical biometer combined with a Placido-

ring topographer, can reliably calculate intraocular lens power when using third-

generation power formulas in unoperated eyes undergoing cataract surgery.42

The anterior segment optical coherence tomography has been used to evaluate

the accuracy of a new formula for predicting postoperative anterior chamber depth with


preoperative angle-to-angle depth. 43 The preoperative angle-to-angle depth was found

to be the most effective parameter for predicting postoperative anterior chamber depth.

The new regression formula with 3 variables; angle-to-angle depth, preoperative

anterior chamber depth, and axial length, predicted postoperative anterior chamber

depth more accurately than the SRK/T and Haigis formulas.43

Corneal topography determines the corneal power using the anterior surface

curvature multiplied by an index of refraction which assumes a fixed relationship

between the anterior and posterior curvatures.44,45 Corneal topography has been proven

to be fairly accurate in determining the refractive power of regular and unoperated

corneas by analyzing the anterior corneal surface, but they may be inaccurate in

measuring corneas that have irregular astigmatism and corneas that have undergone

refractive surgery.46,47 It was suggested that the inaccuracy in the default index of

refraction and the corneal power is due to the change in relationship between the

anterior and posterior surfaces after refractive surgery.44,45 However, corneal

tomography such as computerized scanning slit videokeratography, analyses both the

anterior and posterior corneal surfaces and elevation data gives better estimations of

corneal power in patients with irregular corneal astigmatism.48

COMPARISON AMONG DIFFERENT DEVICES: REPEATABILITY AND

AGREEMENT

Repeatability refers to the variation in measurements obtained by the same observer

under same conditions over a short period of time. Agreement quantifies the similarity

between any two measurements using different methods on the same subject. The

limits of agreement, described by Bland and Altman,49 are defined as the mean

difference ± 1.96 SD of differences. Repeatability of an instrument is an important


feature to consider in clinical practice as well as research. It is important to understand

that a large variability in measurements can lead to a false impression in the trend of

postoperative changes after refractive surgeries such as LASIK. Modern devices have an

excellent repeatability in normal as well as postoperative corneas. However, it is

imperative that the agreement between these devices is good enough so that the

readings can be used interchangeably.

Keratometry

Repeatability

Keratometry measures the corneal curvature and determines the corneal power. It also

detects and measures corneal astigmatism. Keratometric measurements are crucial for

refractive surgery, intraocular lens power calculation, and diagnosis of keratoconus.

Good repeatability of corneal power measurements across devices have been

reported.50-52 A meta-analysis comparing the repeatability of multiple topographic

devices including the Pentacam, Galilei, Sirius, Orbscan, Placido, IOLMaster, Lenstar and

Aladdin in terms of keratometric parameters in normal eyes was performed by Rozema

et al.52 For mean anterior and posterior keratometry, the authors reported narrow

ranges of combined measurement errors (from across studies) except an outlier in both

parameters with Orbscan. For steep and flat keratometric parameters, the study

reported measurement error ranging from 0.10D to 0.24D, whilst Sirius and the

IOLMaster had the lowest error values.

Agreement

In a meta-analysis of agreement of biometry values provided by various ophthalmic

devices, significant differences were observed in mean posterior keratometry between

Pentacam and Sirius, and between Pentacam and TMS-5.52 Significant difference in


steep posterior keratometry was also noted between Pentacam and Galilei. Pentacam

was found to be equivalent to Placido-based imaging for anterior keratometry, to Galilei

for selected anterior and posterior keratometry parameters (anterior steep keratometry,

posterior: mean, steep and flat simulated keratometry) and to the Sirius for anterior flat

keratometry and anterior chamber depth measurement. On the other hand, Orbscan

was found to be equivalent to Galilei for anterior flat simulated and steep keratometry

measurements.52

In a comparison between Scheimpflug and Scanning slit-Placido devices, Orbscan

measurements were equivalent to and could be used interchangeably with Galilei for

anterior keratometry measurements (anterior simulated flat and steep keratometry.52 In

other studies, Orbscan consistently underestimated flat keratometry and overestimated

simulated keratometry compared to Pentacam and Sirius. Sirius was shown to have

better agreement compared to Pentacam in keratometry compared to Orbscan.53-56

Good agreement in anterior keratometry was observed between Pentacam and another

Placido disk device, OphthaTOP.57

A good agreement was noted between Scheimpflug and OCT devices in

unoperated eyes, but most studies only confirmed the high correlations of

measurements among devices without affirming their interchangeability. In other

studies, significant differences were shown in mean keratometry between Pentacam

and different OCT devices.58-60 Good agreement in anterior and posterior keratometric

indices was reported between Scheimpflug (Pentacam and Galilei respectively) and

Swept source OCT (Casia) in normal corneas.58,60 Good agreement for anterior

keratometry measurement was reported between Pentacam and Visante (time-domain

anterior segment OCT).59 High degree of agreement in anterior keratometry but not

posterior keratometry was found between Galilei and Casia Swept source-OCT.58


Studies comparing Scheimpflug topographers and optical biometers have shown

potential interchangeability in keratometry readings between them. Clinically

interchangeable K readings between Pentacam HR and AL-Scan (an optical biometer)

was reported. 61 Good agreement and interchangeable keratometry readings was

reported between Sirius and Lenstar LS900.62 No significant difference in keratometric

measurements was found in Pentacam AXL and biometer IOLMaster 500, but caution

was warranted when using them interchangeably.63 It was shown that Sirius cannot be

used interchangeably with Aladdin optical biometer for flat keratometry readings.64 The

mean corneal power measurements with IOLMaster were significantly higher than the

Galilei as reported in two studies.65,66

Pachymetry

Repeatability

Pachymetry is important in the diagnosis and management of corneal diseases as well

as in preoperative screening of patients before laser refractive surgery. Ultrasound

pachymetry is currently considered as the gold standard for central corneal thickness

measurement.67 In a meta-analysis comparing multiple topographic devices including

the Pentacam, Galilei, Sirius, Orbscan (with and without acoustic correction), ultrasound

pachymetry, Artemis, Visante, RTVue, SL-OCT, Lenstar, OA-1000 and specular

microscopy, the range of combined measurement error across studies in central corneal

thickness among multiple devices was small. The Galilei obtained the lowest

measurement error of 1.76μm followed by RTVue (2.56μm) and Sirius (3.75μm). The

highest measurement errors were obtained in specular microscopy and Arc Scan.52

Agreement


A meta-analysis reported statistically significant differences in pair-wise comparison

between Pentacam and TMS-5, Orbscan with acoustic factor, Visante/Stratus, SL-OCT,

and specular microscopy. Significant differences were noted between Orbscan (with

acoustic factor) and Pentacam, and between Orbscan (without acoustic factor) and

ultrasound. Only Pentacam and ultrasound can be considered clinically equivalent for

central corneal thickness measurements.52

Multiple studies have reported significantly different central corneal thickness

measurements obtained with Pentacam, Sirius, Orbscan, Corvis and ultrasound

pachymetry.53,54,68-71 Recently it was reported that the differences in central corneal

thickness measurements between Sirius-Corvis, Pentacam-Orbscan and Orbscan-

ultrasound pachymetry pairwise comparisons were not statistically significant thereby

suggesting that these devices could be used interchangeably for central corneal

thickness measurements in healthy eyes.54

Significant differences in central corneal thickness measurements between

Scheimpflug and Scanning slit-Placido devices are generally reported.52 It has been

shown that Orbscan obtained lower central corneal thickness measurements than

Pentacam in healthy eyes, 72,73,74,75 Underestimation of central corneal thickness

measurements using Orbscan II persisted even after the acoustic correction factor was

applied.76-79 Therefore, these devices cannot be used interchangeably for central

corneal thickness measurements.

Pentacam and ultrasound was shown to have a good agreement for central

corneal thickness measurements in normal eyes.52 However, the interchangeability does

not seem to apply to other Scheimpflug-Placido devices. The central corneal thickness

measurements by TMS-5 (Scheimpflug-Placido) were only found to be in moderate

agreement with ultrasound pachymetry.80 Sirius and ultrasound pachymetry were not

recommended to be used interchangeably for central corneal thickness


measurement.81,82 Multiple studies reported differences in corneal thickness values in

Sirius compared to ultrasound pachymetry.55,82,83 Pachymetry measurements were

thicker when measured with Sirius compared to ultrasound pachymetry.84 However, a

better agreement was reported between Sirius and ultrasound pachymetry compared to

the agreement between Orbscan and ultrasound pachymetry.73 A significant difference

was reported for central corneal thickness measurements between Orbscan (without

acoustic factor) and ultrasound,52 but the difference was not significant once the

acoustic factor was in place for Orbscan. It was suggested that central corneal

thickness measurements with Orbscan (with acoustic factor) and ultrasound are

interchangeable despite the fact that Orbscan reported higher (but not significant)

estimates of central corneal thickness measurements compared to ultrasound.54,85

Overall, it has been established that Orbscan overestimates central corneal thickness as

compared to ultrasound pachymetry.86-88

A comparison between Scheimpflug and OCT devices in normal eyes showed

significant differences in central corneal thickness measurement between Pentacam and

Visante/Stratus OCT and between Pentacam and SL-OCT.89 Multiple studies have shown

that since Scheimpflug devices (Pentacam, Sirius) overestimate and OCT devices

(Visante, RTVue) underestimate central corneal thickness measurements,74,90-92 they

should not be used interchangeably.62

Comparison of OCT devices and ultrasound pachymetry showed that central

corneal thickness measurements with anterior segment OCT were significantly thinner

than ultrasound pachymetry.93,94 Previous retinal OCT studies also showed that

although anterior segment OCT pachymetry correlated well with ultrasound but it tends

to underestimate ultrasound pachymetry values.95-97 However, in one study, it was

showed that retinal OCT overestimated the CCT measured by ultrasound instead.98 The

central corneal thickness values obtained with anterior segment OCT and ultrasound


pachymetry showed no significant difference in some studies.99,100 The difference in

conclusions between studies could be attributed to different study populations. Overall,

CCT measurements should be interpreted in the context of the instrument used.101

In a comparison between Fourier-domain optical coherence tomography (FD-

OCT) and time-domain OCT (TD-OCT) for agreement, mean CCT obtained by FD-OCT

(RTVue) was showed to be significantly higher than that obtained by TD-OCT (Visante).

Fourier domain OCT has better sensitivity than TD-OCT systems.102-104

When comparing Scanning slit-Placido and OCT devices, central corneal thickness

measurements obtained with AS-OCT were thinner compared to Orbscan II. Therefore,

Visante AS-OCT and Orbscan II should not be used interchangeably for assessment of

corneal thickness. 74,105 Similarly, corneal thickness and elevation measurements were

significantly different between swept source optical coherence tomography (Casia) and

slit scanning topography (Orbscan). 106

Previous studies have reported good agreement and possible interchangeability

between Scheimpflug devices and optical biometers. It was reported that IOLMaster

700 (SS-OCT optical biometer) overestimates central corneal thickness measurements

in normal eyes compared to Pentacam but this difference was not significant

statistically.107 Good agreement and interchangeability were reported for central corneal

thickness measurements between Scheimpflug topographers (Sirius and Pentacam

respectively) and Lenstar LS900 OLCR biometer.62,108 Good agreement and clinically

interchangeable measurements in central corneal thickness values were also reported

between Scheimpflug topographers (Pentacam and Galilei) and Nidek AL-Scan (a new

optical biometer).61,109 However, other studies showed that they are not

interchangeable. The central corneal thickness measured with Nidek AL-Scan was

reportedly thinner as compared to Sirius.110


It is noteworthy that not all Scheimpflug devices are interchangeable for central

corneal thickness measurement. Corvis ST and Pentacam are interchangeable for

central corneal thickness measurement. 54,111 Sirius 3D and Galilei G2 can be used

interchangeably with Pentacam for anterior radius of curvature, central corneal

thickness, and anterior chamber depth, but not for maximum anterior and posterior

corneal elevation and total higher-order aberrations.112 Corneal thickness measurements

by Galilei and Pentacam can be considered interchangeable for purposes such as IOL

power calculation with no need for IOL constant adjustment.113 The pachymetry

measured with Sirius was thicker as compared to Pentacam.84,114

Agreement of devices for Post-LASIK corneal measurements

Nassiri et al.115 compared mean CCT measurements with ultrasound, Pentacam and

Orbscan II in high myopic eyes before and after PRK. Both Pentacam and Orbscan II

measurements were lower than those obtained with ultrasound. Ultrasound was

preferred postoperatively. On the contrary, Ho el al.105 showed no statistically significant

difference in corneal pachymetry assessment between US and Orbscan measurements 6

months after LASIK. Pentacam and Visante, on the other hand, showed underestimation

of corneal thickness compared to US measurement.

Park et al.116 compared central corneal thickness measurements using slit

scanning imaging (Orbscan), rotating Scheimpflug imaging (Pentacam), dual

Scheimpflug system (Galilei) to ultrasound pachymetry measurements in normal and

post-LASIK eyes and concluded that refractive surgery had an effect on the agreement

of measurements among devices, with Pentacam or Galilei showing better agreement

than Orbscan II in general, and after surgery in particular. Measurements using

Orbscan were significantly thinner than other modalities. Similar findings were obtained

in other studies comparing after LASIK, and PRK.76 Chan et al.117 reported a


significantly higher repeatability of swept source OCT compared with Scheimpflug

imaging for post-LASIK corneal measurements, and suggested that factors such as

patient’s age, spherical equivalent, and residual bed thickness measurements need to

be considered during follow-up and evaluation of post-LASIK patients for any further

surgical interventions.

CONCLUSIONS

The technological advances in corneal imaging have made precise measurement of both

anterior and posterior corneal curvatures and corneal thickness possible. Corneal

tomography enables imaging of corneal elevation. Optical coherence tomography

provides additional information on corneal pathology making it easier to corroborate the

clinical findings with corneal topographic changes. All together, these devices enable

better diagnosis, classification, and monitoring of progression of corneal diseases

leading to a better understanding of pathophysiology of corneal diseases.


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FIGURE LEGENDS

Figure 1: Placido disc and representative patterns of corneal shapes

Figure 2: Overlapping scanning slits to map the cornea in devices with scanning slit

technology such as Orbscan.

Figure 3: Principle of Scheimpflug imaging; (A) object and image plane are parallel.

Therefore, the image is sharp and focused; (B) object and image plane are not parallel.

therefore, the image is not focused in entirety; (C) Object and image plane are not

parallel; however, the image plane has been rotated in accordance with the

Scheimpflug principle to create an image focused in entirety.

Figure 4: Swept source optical coherence tomography showing inferior corneal

steepening and corneal thinning in keratoconus

Figure 5: Pentacam depicting inferior corneal steepening and posterior corneal

elevation in keratoconus

Figure 6: Belin Ambrosio Detection software showing high D value in a case with

keratoconus


TABLES

Table 1: Amsler-Krumeich classification for keratoconus

Stage I Eccentric steepening Myopia and astigmatism <5.00 D Mean central K readings <48.00 D

Stage II Myopia and astigmatism 5.00-8.00 D Mean central K readings <53.00 D Absence of scarring Minimum corneal thickness >400m

Stage III Myopia and astigmatism 8.00-10.00 D Mean central K readings >53.00 D Absence of scarring Minimum corneal thickness 300-400m

Stage IV Refraction not measurable Mean central K reading >55.00 D Central corneal scarring Minimum corneal thickness 200m


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Minerva Access is the Institutional Repository of The University of Melbourne

Author/s:Fan, R;Chan, TCY;Prakash, G;Jhanji, V

Title:Applications of corneal topography and tomography: a review

Date:2018-03-01

Citation:Fan, R., Chan, T. C. Y., Prakash, G. & Jhanji, V. (2018). Applications of corneal topographyand tomography: a review. CLINICAL AND EXPERIMENTAL OPHTHALMOLOGY, 46 (2),pp.133-146. https://doi.org/10.1111/ceo.13136.

Persistent Link:http://hdl.handle.net/11343/283465

http://hdl.handle.net/11343/283465

Applications of corneal topography and tomography: a review

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