Applications of corneal topography and ... - Wiley Online Library

Download PDF
0 downloads 0 Views 2MB Size Report
Comment
corneal topography as the most important risk factor for developing ... try measurements obtained by the Aladdin – an ..... trenholm.org/hmmerk/SHBG05.pdf. 4.
Clinical and Experimental Ophthalmology 2018; 46: 133–146 doi: 10.1111/ceo.13136

Review Applications of corneal topography and tomography: a review Rachel Fan MBBS,1 Tommy CY Chan FRCS,2 FRCOphth2,4,5

Gaurav Prakash MD3

and Vishal Jhanji MD

1

Faculty of Medicine, The University of Hong Kong, 2Department of Ophthalmology & Visual Sciences, The Chinese University of Hong Kong, Hong Kong; 3NMC Eye Care, NMC Specialty Hospital, Abu Dhabi, United Arab Emirates; 4Department of Ophthalmology, University of Pittsburgh School of Medicine, Pittsburgh, Pennsylvania, USA; and 5Centre for Eye Research Australia, University of Melbourne, Melbourne, Victoria, Australia

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 analyse 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. Scheimpflug 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. Key words: agreement, cornea, repeatability, tomography, topography.

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, analyse 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 lightand-dark bands over the cornea (Fig. 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

Correspondence: Dr Vishal Jhanji, UPMC Eye Center, Department of Ophthalmology, University of Pittsburgh School of Medicine, 203 Lothrop Street, Pittsburgh, PA 15213, USA. Email: [email protected] Received 16 August 2017; accepted 14 December 2017. Conflict of interest: None declared. Funding sources: None declared. © 2017 Royal Australian and New Zealand College of Ophthalmologists

134

Fan et al. pattern of scanning slits (Fig. 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

Figure 1. Placido disc and representative patterns of corneal shapes.

patterns can be used to create quantitative data and colour-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 (Bausch & Lomb, Orbtek Inc., UT, USA)] 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 threedimensional 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

Figure 2. Overlapping scanning slits to map the cornea in devices with scanning slit technology such as Orbscan.

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 (Fig. 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 (Oculus, Wetzlar, Germany), TMS-5 (Tomey Corp., Nagoya, Japan), Galilei (Ziemer, Port, Switzerland) and Sirius (CSO, Costruzione Strumenti Oftalmici, Florence, Italy). 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 artefacts 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. © 2017 Royal Australian and New Zealand College of Ophthalmologists

Corneal topography and tomography

135

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.

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 (Figs. 4 and 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 OCT) is the most commonly used modality to diagnose keratoconus due to its ability

Figure 4. Swept source optical coherence tomography showing inferior corneal steepening and corneal thinning in keratoconus. © 2017 Royal Australian and New Zealand College of Ophthalmologists

136

Fan et al.

Figure 5. Pentacam depicting inferior corneal steepening and posterior corneal elevation in keratoconus.

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 × I–S × AST × SRAX × 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 and 1 diopter is considered to depict disease progression. Furthermore, © 2017 Royal Australian and New Zealand College of Ophthalmologists

flattening of Kmax is used to gauge treatment effect after interventions such as corneal collagen crosslinking. The Global Consensus on Keratoconus and Ectasic7 defined ectasia progression as a consistent change over time in at least two 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 crosslinking in keratoconus Corneal crosslinking is indicated for slowing down or stopping the progression of keratoconus.16

138

Fan et al.

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

devices such as the Galilei dual-Scheimpflug analyser 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

Refractive surgery Preoperative ectasia risk assessment

LASIK causes changes on the anterior as well as the posterior corneal surface.31 Chan et al. used OCT 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 ingrowth, diffuse lamellar keratitis and central toxic 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, noninflammatory 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

Corneal ectasia is an uncommon but severe sightthreatening 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,

Measurement of surgical outcomes in refractive surgery

© 2017 Royal Australian and New Zealand College of Ophthalmologists

Corneal topography and tomography

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 preexisting astigmatism by combined use of corneal topography and preoperative 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 OCT 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 three 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 analysing 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 © 2017 Royal Australian and New Zealand College of Ophthalmologists

139 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 (Zeiss Humphrey, Dublin, CA, USA), Lenstar (Haag Streit, Köniz, Switzerland) 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 0.10 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

140 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

Fan et al. Pentacam, Galilei, Sirius, Orbscan (with and without acoustic correction), ultrasound pachymetry, Artemis (ArcScan Inc., Morrison, CO, USA), Visante (Carl Zeiss Meditec, Dublin, CA, USA), RTVue (Optovue Inc, Fremont, CA, USA), SL-OCT (Heidelberg Engineering, Dossenheim, Germany), Lenstar, OA-1000 (Tomey, Nagoya, Japan) 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 1.76 μm followed by RTVue (2.56 μm) and Sirius (3.75 μm). 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 (Oculus, Wetzlar, Germany) 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 pair-wise 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 slitPlacido devices are generally reported.52 It has been shown that Orbscan obtained lower central corneal thickness measurements than Pentacam in healthy eyes.72–75 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 (ScheimpflugPlacido) were only found to be in moderate agreement with ultrasound pachymetry.80 Sirius and ultrasound pachymetry were not recommended to © 2017 Royal Australian and New Zealand College of Ophthalmologists

Corneal topography and tomography 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 TDOCT systems.102–104 © 2017 Royal Australian and New Zealand College of Ophthalmologists

141 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 OCT (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 (Nidek Co., Aichi, Japan) 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 et al.105 showed no statistically significant difference in corneal pachymetry assessment between United States and Orbscan measurements 6 months after LASIK. Pentacam and Visante, on the other hand,

142 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), dualScheimpflug 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 followup 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. OCT 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.

REFERENCES 1. Courville CB, Smolek MK, Klyce SD. Contribution of the ocular surface to visual optics. Exp Eye Res 2004; 78: 417–25. 2. Wegener A, Laser H. Imaging of the spatial density distribution on the capsule of the lens with Scheimpflug photography. Ophthalmic Res 1996; 28: 86–91. 3. M HM. The Scheimpflug Principle—Part I 1996. Accessed July 2017. Available from: http://www. trenholm.org/hmmerk/SHBG05.pdf 4. Drexler W, Morgner U, Ghanta RK, Kartner FX, Schuman JS, Fujimoto JG. Ultrahigh-resolution ophthalmic optical coherence tomography. Nat Med 2001; 7: 502–7. 5. Ramos JL, Li Y, Huang D. Clinical and research applications of anterior segment optical coherence tomography - a review. Clin Experiment Ophthalmol 2009; 37: 81–9. 6. Jhanji V, Sharma N, Vajpayee RB. Management of keratoconus: current scenario. Br J Ophthalmol 2011; 95: 1044–50.

Fan et al. 7. Gomes JA, Tan D, Rapuano CJ et al. Global consensus on keratoconus and ectatic diseases. Cornea 2015; 34: 359–69. 8. Chan TC, Biswas S, Yu M, Jhanji V. Comparison of corneal measurements in keratoconus using sweptsource optical coherence tomography and combined Placido-Scheimpflug imaging. Acta Ophthalmol 2016; 95: e486–94. 9. Maeda N, Klyce SD, Smolek MK, Thompson HW. Automated keratoconus screening with corneal topography analysis. Invest Ophthalmol Vis Sci 1994; 35: 2749–57. 10. Rabinowitz YS, Rasheed K. KISA % index: a quantitative videokeratography algorithm embodying minimal topographic criteria for diagnosing keratoconus. J Cataract Refract Surg 1999; 25: 1327–35. 11. Mas-Tur V, MacGregor C, Jayaswal R, O’Brart DPS, Maycock NJR. A review of keratoconus: diagnosis, pathophysiology and genetics. Surv Ophthalmol 2017; 62: 770–83. 12. Belin MW, Duncan JK. Keratoconus: the ABCD grading system. Klin Monatsbl Augenheilkd 2016; 233: 701–7. 13. Amsler M. The "forme fruste" of keratoconus. Wien Klin Wochenschr 1961; 73: 842–3. 14. Rathi VM, Mandathara PS, Dumpati S. Contact lens in keratoconus. Indian J Ophthalmol 2013; 61: 410–5. 15. Mandathara Sudharman P, Rathi V, Dumapati S, Rose K. Lenses for keratoconus–an Indian experience. Eye Contact Lens 2010; 36: 220–2. 16. Chow VW, Chan TC, Yu M, Wong VW, Jhanji V. One-year outcomes of conventional and accelerated collagen crosslinking in progressive keratoconus. Sci Rep 2015; 5: 14425. 17. Wollensak G, Spoerl E, Seiler T. Riboflavin/ultraviolet-a-induced collagen crosslinking for the treatment of keratoconus. Am J Ophthalmol 2003; 135: 620–7. 18. Wittig-Silva C, Whiting M, Lamoureux E, Lindsay RG, Sullivan LJ, Snibson GR. A randomized controlled trial of corneal collagen cross-linking in progressive keratoconus: preliminary results. J Refract Surg 2008; 24: S720–5. 19. Tong JY, Viswanathan D, Hodge C, Sutton G, Chan C, Males JJ. Corneal collagen crosslinking for post-LASIK ectasia: an Australian study. Asia Pac J Ophthalmol (Phila) 2017; 6: 228–32. 20. Steinberg J, Ahmadiyar M, Rost A et al. Anterior and posterior corneal changes after crosslinking for keratoconus. Optom Vis Sci 2014; 91: 178–86. 21. Randleman JB, Woodward M, Lynn MJ, Stulting RD. Risk assessment for ectasia after corneal refractive surgery. Ophthalmology 2008; 115: 37–50. 22. Santhiago MR, Giacomin NT, Smadja D, Bechara SJ. Ectasia risk factors in refractive surgery. Clin Ophthalmol 2016; 10: 713–20. 23. Klein SR, Epstein RJ, Randleman JB, Stulting RD. Corneal ectasia after laser in situ keratomileusis in patients without apparent preoperative risk factors. Cornea 2006; 25: 388–403. 24. Ambrosio R Jr, Dawson DG, Salomao M, Guerra FP, Caiado AL, Belin MW. Corneal ectasia after LASIK © 2017 Royal Australian and New Zealand College of Ophthalmologists

Corneal topography and tomography

25.

26.

27.

28.

29.

30.

31.

32.

33.

34.

35.

36.

37.

38.

39. 40.

despite low preoperative risk: tomographic and biomechanical findings in the unoperated, stable, fellow eye. J Refract Surg 2010; 26: 906–11. Li Y, Chamberlain W, Tan O, Brass R, Weiss JL, Huang D. Subclinical keratoconus detection by pattern analysis of corneal and epithelial thickness maps with optical coherence tomography. J Cataract Refract Surg 2016; 42: 284–95. Steinberg J, Casagrande MK, Frings A et al. Screening for subclinical keratoconus using swept-source Fourier domain anterior segment optical coherence tomography. Cornea 2015; 34: 1413–9. Catalan S, Cadarso L, Esteves F, Salgado-Borges J, Lopez M, Cadarso C. Assessment of corneal epithelial thickness in asymmetric keratoconic eyes and normal eyes using Fourier domain optical coherence tomography. J Ophthalmol 2016; 2016: 1–6. Reinstein DZ, Archer TJ, Gobbe M. Stability of LASIK in topographically suspect keratoconus confirmed non-keratoconic by artemis VHF digital ultrasound epithelial thickness mapping: 1-year follow-up. J Refract Surg 2009; 25: 569–77. Smadja D, Touboul D, Cohen A et al. Detection of subclinical keratoconus using an automated decision tree classification. Am J Ophthalmol 2013; 156: 237–246.e1. Moshirfar M, Smedley JG, Muthappan V, Jarsted A, Ostler EM. Rate of ectasia and incidence of irregular topography in patients with unidentified preoperative risk factors undergoing femtosecond laserassisted LASIK. Clin Ophthalmol 2014; 8: 35–42. Khairat YM, Mohamed YH, Moftah IA, Fouad NN. Evaluation of corneal changes after myopic LASIK using the Pentacam(R). Clin Ophthalmol 2013; 7: 1771–6. Chan TC, Liu D, Yu M, Jhanji V. Longitudinal evaluation of posterior corneal elevation after laser refractive surgery using swept-source optical coherence tomography. Ophthalmology 2015; 122: 687–92. Twa MD, Nichols JJ, Joslin CE et al. Characteristics of corneal ectasia after LASIK for myopia. Cornea 2004; 23: 447–57. Varley GA, Huang D, Rapuano CJ et al. LASIK for hyperopia, hyperopic astigmatism, and mixed astigmatism: a report by the American Academy of Ophthalmology. Ophthalmology 2004; 111: 1604–17. Jain R, Dilraj G, Grewal SP. Repeatability of corneal parameters with pentacam after laser in situ keratomileusis. Indian J Ophthalmol 2007; 55: 341–7. Wang MY, Maloney RK. Epithelial ingrowth after laser in situ keratomileusis. Am J Ophthalmol 2000; 129: 746–51. Randleman JB, Shah RD. Lasik interface complications: etiology, management, and outcomes. J Refract Surg 2012; 28: 575–86. Smith RJ, Maloney RK. Diffuse lamellar keratitis. A new syndrome in lamellar refractive surgery. Ophthalmology 1998; 105: 1721–6. Moshirfar M, Hazin R, Khalifa YM. Central toxic keratopathy. Curr Opin Ophthalmol 2010; 21: 274–9. McQueen BR, Martinez CE, Klyce SD. Corneal topography in cataract surgery. Curr Opin Ophthalmol 1997; 8: 22–8.

© 2017 Royal Australian and New Zealand College of Ophthalmologists

143 41. Savini G, Hoffer KJ, Barboni P, Schiano Lomoriello D, Ducoli P. Corneal asphericity and IOL power calculation in eyes with aspherical IOLs. J Refract Surg 2017; 33: 476–81. 42. Savini G, Hoffer KJ, Barboni P, Balducci N, SchianoLomoriello D, Ducoli P. Accuracy of optical biometry combined with Placido disc corneal topography for intraocular lens power calculation. PLoS One 2017; 12: e0172634. 43. Goto S, Maeda N, Koh S et al. Prediction of postoperative intraocular lens position with angle-to-angle depth using anterior segment optical coherence tomography. Ophthalmology 2016; 123: 2474–80. 44. Seitz B, Langenbucher A, Nguyen NX, Kus MM, Kuchle M. Underestimation of intraocular lens power for cataract surgery after myopic photorefractive keratectomy. Ophthalmology 1999; 106: 693–702. 45. Kalyani SD, Kim A, Ladas JG. Intraocular lens power calculation after corneal refractive surgery. Curr Opin Ophthalmol 2008; 19: 357–62. 46. Zeh WG, Koch DD. Comparison of contact lens overrefraction and standard keratometry for measuring corneal curvature in eyes with lenticular opacity. J Cataract Refract Surg 1999; 25: 898–903. 47. Hamed AM, Wang L, Misra M, Koch DDA. Comparative analysis of five methods of determining corneal refractive power in eyes that have undergone myopic laser in situ keratomileusis. Ophthalmology 2002; 109: 651–8. 48. Cua IY, Qazi MA, Lee SF, Pepose JS. Intraocular lens calculations in patients with corneal scarring and irregular astigmatism. J Cataract Refract Surg 2003; 29: 1352–7. 49. Bland JM, Altman DG. Statistical methods for assessing agreement between two methods of clinical measurement. Lancet 1986; 1: 307–10. 50. Wang Q, Savini G, Hoffer KJ et al. A comprehensive assessment of the precision and agreement of anterior corneal power measurements obtained using 8 different devices. PLoS One 2012; 7: e45607. 51. Reinstein DZ, Gobbe M, Archer TJ. Anterior segment biometry: a study and review of resolution and repeatability data. J Refract Surg 2012; 28: 509–20. 52. Rozema JJ, Wouters K, Mathysen DG, Tassignon MJ. Overview of the repeatability, reproducibility, and agreement of the biometry values provided by various ophthalmic devices. Am J Ophthalmol 2014; 158: 1111–1120.e1. 53. Rao HL, Kumar AU, Kumar A et al. Evaluation of central corneal thickness measurement with RTVue spectral domain optical coherence tomography in normal subjects. Cornea 2011; 30: 121–6. 54. Kumar M, Shetty R, Jayadev C, Rao HL, Dutta D. Repeatability and agreement of five imaging systems for measuring anterior segment parameters in healthy eyes. Indian J Ophthalmol 2017; 65: 288–94. 55. De la Parra-Colin P, Garza-Leon M, BarrientosGutierrez T. Repeatability and comparability of anterior segment biometry obtained by the sirius and the pentacam analyzers. Int Ophthalmol 2014; 34: 27–33. 56. Tajbakhsh Z, Salouti R, Nowroozzadeh MH, Aghazadeh-Amiri M, Tabatabaee S, Zamani M.

144

57.

58.

59.

60.

61.

62.

63.

64.

65.

66.

67.

68.

69.

70.

Fan et al. Comparison of keratometry measurements using the pentacam HR, the orbscan IIz, and the TMS-4 topographer. Ophthalmic Physiol Opt 2012; 32: 539–46. Huang J, Savini G, Chen H et al. Precision and agreement of corneal power measurements obtained using a new corneal topographer ophthaTOP. PLoS One 2015; 10: e109414. Lee YW, Choi CY, Yoon GY. Comparison of dual rotating Scheimpflug-Placido, swept-source optical coherence tomography, and placido-scanning-slit systems. J Cataract Refract Surg 2015; 41: 1018–29. Szalai E, Berta A, Nemeth G, Hassan Z, Modis L Jr. Anterior chamber depth measurements obtained with pentacam HR(R) imaging system and conventional A-scan ultrasound. Ophthalmic Surg Lasers Imaging 2011; 42: 248–53. Ghoreishi SM, Mortazavi SA, Abtahi ZA et al. Comparison of Scheimpflug and swept-source anterior segment optical coherence tomography in normal and keratoconus eyes. Int Ophthalmol 2016; 37: 965–71. McAlinden C, Bao F, Guo Y et al. Agreement of anterior ocular biometric measurements with a new optical biometer and a scheimpflug tomographer. J Cataract Refract Surg 2016; 42: 679–84. Chen W, McAlinden C, Pesudovs K et al. Scheimpflug-Placido topographer and optical low-coherence reflectometry biometer: repeatability and agreement. J Cataract Refract Surg 2012; 38: 1626–32. Shajari M, Cremonese C, Petermann K, Singh P, Muller M, Kohnen T. Comparison of axial length, corneal curvature, and anterior chamber depth measurements of 2 recently introduced devices to a known biometer. Am J Ophthalmol 2017; 178: 58–64. Polat SB, Ugurlu N, Ersoy R, Oguz O, Duru N, Cakir B. Evaluation of central corneal and central retinal thicknesses and intraocular pressure in acromegaly patients. Pituitary 2014; 17: 327–32. de la Fuente L, Segura F, Pinilla I. Comparison of anterior segment measurements obtained by three different devices in healthy eyes. Biomed Res Int 2014; 2014: 498080. Shirayama M, Wang L, Weikert MP, Koch DD. Comparison of corneal powers obtained from 4 different devices. Am J Ophthalmol 2009; 148: 528–535.e1. Rio-Cristobal A, Martin R. Corneal assessment technologies: current status. Surv Ophthalmol 2014; 59: 599–614. Reznicek L, Muth D, Kampik A, Neubauer AS, Hirneiss C. Evaluation of a novel scheimpflug-based non-contact tonometer in healthy subjects and patients with ocular hypertension and glaucoma. Br J Ophthalmol 2013; 97: 1410–4. Shetty R, Arora V, Jayadev C et al. Repeatability and agreement of three scheimpflug-based imaging systems for measuring anterior segment parameters in keratoconus. Invest Ophthalmol Vis Sci 2014; 55: 5263–8. Chen S, Huang J, Wen D, Chen W, Huang D, Wang Q. Measurement of central corneal thickness by high-resolution Scheimpflug imaging, Fourier-

71.

72.

73.

74.

75.

76.

77.

78.

79.

80.

81.

82.

83.

domain optical coherence tomography and ultrasound pachymetry. Acta Ophthalmol 2012; 90: 449–55. Ucakhan OO, Ozkan M, Kanpolat A. Corneal thickness measurements in normal and keratoconic eyes: Pentacam comprehensive eye scanner versus noncontact specular microscopy and ultrasound pachymetry. J Cataract Refract Surg 2006; 32: 970–7. Lanza M, Paolillo E, Gironi Carnevale UA et al. Central corneal thickness evaluation in healthy eyes with three different optical devices. Cont Lens Anterior Eye 2015; 38: 409–13. Mohammadpour M, Mohammad K, Karimi N. Central corneal thickness measurement using ultrasonic pachymetry, rotating Scheimpflug camera, and scanning-slit topography exclusively in thin non-keratoconic corneas. J Ophthalmic Vis Res 2016; 11: 245–51. Doors M, Cruysberg LP, Berendschot TT et al. Comparison of central corneal thickness and anterior chamber depth measurements using three imaging technologies in normal eyes and after phakic intraocular lens implantation. Graefes Arch Clin Experiment Ophthalmol 2009; 247: 1139–46. Rosa N, Lanza M, Borrelli M, Polito B, Filosa ML, De Bernardo M. Comparison of central corneal thickness measured with Orbscan and Pentacam. J Refract Surg 2007; 23: 895–9. Hashemi H, Mehravaran S. Central corneal thickness measurement with pentacam, orbscan II, and ultrasound devices before and after laser refractive surgery for myopia. J Cataract Refract Surg 2007; 33: 1701–7. Hashemi H, Roshani M, Mehravaran S, Parsafar H, Yazdani K. Effect of corneal thickness on the agreement between ultrasound and orbscan II pachymetry. J Cataract Refract Surg 2007; 33: 1694–700. Lackner B, Schmidinger G, Pieh S, Funovics MA, Skorpik C. Repeatability and reproducibility of central corneal thickness measurement with pentacam, orbscan, and ultrasound. Optom Vis Sci 2005; 82: 892–9. McLaren JW, Nau CB, Erie JC, Bourne WM. Corneal thickness measurement by confocal microscopy, ultrasound, and scanning slit methods. Am J Ophthalmol 2004; 137: 1011–20. Huang J, Savini G, Hu L et al. Precision of a new Scheimpflug and Placido-disk analyzer in measuring corneal thickness and agreement with ultrasound pachymetry. J Cataract Refract Surg 2013; 39: 219–24. Jorge J, Rosado J, Diaz-Rey J, Gonzalez-Meijome J. Central corneal thickness and anterior chamber depth measurement by Sirius((R)) Scheimpflug tomography and ultrasound. Clin Ophthalmol 2013; 7: 417–22. Maresca N, Zeri F, Palumbo P, Calossi A. Agreement and reliability in measuring central corneal thickness with a rotating Scheimpflug-Placido system and ultrasound pachymetry. Cont Lens Anterior Eye 2014; 37: 442–6. Bayhan HA, Aslan Bayhan S. Can I. Comparison of central corneal thickness measurements with three new optical devices and a standard ultrasonic pachymeter. Int J Ophthalmol 2014; 7: 302–8.

© 2017 Royal Australian and New Zealand College of Ophthalmologists

Corneal topography and tomography 84. Finis D, Ralla B, Karbe M, Borrelli M, Schrader S, Geerling G. Comparison of two different scheimpflug devices in the detection of keratoconus, regular astigmatism, and healthy corneas. J Ophthalmol 2015; 2015: 1–6. 85. Khaja WA, Grover S, Kelmenson AT, Ferguson LR, Sambhav K, Chalam KV. Comparison of central corneal thickness: ultrasound pachymetry versus slit-lamp optical coherence tomography, specular microscopy, and Orbscan. Clin Ophthalmol 2015; 9: 1065–70. 86. Sadoughi MM, Einollahi B, Einollahi N, Rezaei J, Roshandel D, Feizi S. Measurement of central corneal thickness using ultrasound pachymetry and orbscan II in normal eyes. J Ophthalmic Vis Res 2015; 10: 4–9. 87. Radford SW, Lim R, Salmon JF. Comparison of orbscan and ultrasound pachymetry in the measurement of central corneal thickness. Eye (Lond) 2004; 18: 434–6. 88. TH O, Lai IC, Teng MC. Comparison of central corneal thickness measurements by ultrasonic pachymetry, orbscan II, and SP3000P in eyes with glaucoma or glaucoma suspect. Chang Gung Med J 2012; 35: 255–62. 89. Otchere H, Sorbara L. Repeatability of topographic corneal thickness in keratoconus comparing visante OCT and oculus pentacam HR(R) topographer. Cont Lens Anterior Eye 2017; 40: 217–23. 90. Fu J, Wang X, Li S, Wu G, Wang N. Comparative study of anterior segment measurement with pentacam and anterior segment optical coherence tomography. Can J Ophthalmol 2010; 45: 627–31. 91. Huang J, Ding X, Savini G et al. A comparison between Scheimpflug imaging and optical coherence tomography in measuring corneal thickness. Ophthalmology 2013; 120: 1951–8. 92. Mansoori T, Balakrishna N. Repeatability and agreement of central corneal thickness measurement with non-contact methods: a comparative study. Int Ophthalmol 2017. https://doi.org/10.1007/s10792-0170543-1. [Epub ahead of print]. 93. Kim HY, Budenz DL, Lee PS, Feuer WJ, Barton K. Comparison of central corneal thickness using anterior segment optical coherence tomography vs ultrasound pachymetry. Am J Ophthalmol 2008; 145: 228–32. 94. Zhao PS, Wong TY, Wong WL, Saw SM, Aung T. Comparison of central corneal thickness measurements by visante anterior segment optical coherence tomography with ultrasound pachymetry. Am J Ophthalmol 2007; 143: 1047–9. 95. Wirbelauer C, Scholz C, Hoerauf H, Pham DT, Laqua H, Birngruber R. Noncontact corneal pachymetry with slit lamp-adapted optical coherence tomography. Am J Ophthalmol 2002; 133: 444–50. 96. Wong MM, Shukla AN, Munir WM. Correlation of corneal thickness and volume with intraoperative phacoemulsification parameters using Scheimpflug imaging and optical coherence tomography. J Cataract Refract Surg 2014; 40: 2067–75. 97. Bechmann M, Thiel MJ, Neubauer AS et al. Central corneal thickness measurement with a retinal optical coherence tomography device versus standard ultrasonic pachymetry. Cornea 2001; 20: 50–4. © 2017 Royal Australian and New Zealand College of Ophthalmologists

145 98. Leung DY, Lam DK, Yeung BY, Lam DS. Comparison between central corneal thickness measurements by ultrasound pachymetry and optical coherence tomography. Clin Experiment Ophthalmol 2006; 34: 751–4. 99. Li H, Leung CK, Wong L et al. Comparative study of central corneal thickness measurement with slit-lamp optical coherence tomography and visante optical coherence tomography. Ophthalmology 2008; 11: 796–801.e2. https://doi.org/10.2147/OPTH.S81376. 100. Beutelspacher SC, Serbecic N, Scheuerle AF. Assessment of central corneal thickness using OCT, ultrasound, optical low coherence reflectometry and Scheimpflug pachymetry. Eur J Ophthalmol 2011; 21: 132–7. 101. Grewal DS, Brar GS, Grewal SP. Assessment of central corneal thickness in normal, keratoconus, and post-laser in situ keratomileusis eyes using Scheimpflug imaging, spectral domain optical coherence tomography, and ultrasound pachymetry. J Cataract Refract Surg 2010; 36: 954–64. 102. Yi K, Mujat M, Park BH et al. Spectral domain optical coherence tomography for quantitative evaluation of drusen and associated structural changes in nonneovascular age-related macular degeneration. Br J Ophthalmol 2009; 93: 176–81. 103. Potsaid B, Gorczynska I, Srinivasan VJ et al. Ultrahigh speed spectral/Fourier domain OCT ophthalmic imaging at 70,000 to 312,500 axial scans per second. Opt Express 2008; 16: 15149–69. 104. Drexler W, Liu M, Kumar A, Kamali T, Unterhuber A, Leitgeb RA. Optical coherence tomography today: speed, contrast, and multimodality. J Biomed Opt 2014; 19: 071412. 105. Ho T, Cheng AC, Rao SK, Lau S, Leung CK, Lam DS. Central corneal thickness measurements using orbscan II, visante, ultrasound, and pentacam pachymetry after laser in situ keratomileusis for myopia. J Cataract Refract Surg 2007; 33: 1177–82. 106. Jhanji V, Yang B, Yu M, Ye C, Leung CK. Corneal thickness and elevation measurements using sweptsource optical coherence tomography and slit scanning topography in normal and keratoconic eyes. Clin Experiment Ophthalmol 2013; 41: 735–45. 107. Ozyol E, Ozyol P. Comparison of central corneal thickness with four noncontact devices: an agreement analysis of swept-source technology. Indian J Ophthalmol 2017; 65: 461–5. 108. Huang J, Lu W, Savini G et al. Comparison between a new optical biometry device and an anterior segment optical coherence tomographer for measuring central corneal thickness and anterior chamber depth. J Ophthalmol 2016; 2016: 1–5. https://doi.org/ 10.1155/2016/634723699. 109. Yagci R, Guler E, Kulak AE, Erdogan BD, Balci M, Hepsen IF. Repeatability and reproducibility of a new optical biometer in normal and keratoconic eyes. J Cataract Refract Surg 2015; 41: 171–7. 110. Caglar C, Kocamis SI, Demir E, Durmus M. Comparison of the measurements of a novel optical biometry: nidek AL-scan with sirius and a ultrasound biometry. Int Ophthalmol 2017; 37: 491–8.

146 111. Yu A, Zhao W, Savini G et al. Evaluation of central corneal thickness using corneal dynamic Scheimpflug analyzer corvis ST and comparison with pentacam rotating Scheimpflug system and ultrasound pachymetry in normal eyes. J Ophthalmol 2015; 2015: 767012. 112. Hernandez-Camarena JC, Chirinos-Saldana P, Navas A et al. Repeatability, reproducibility, and agreement between three different Scheimpflug systems in measuring corneal and anterior segment biometry. J Refract Surg 2014; 30: 616–21. 113. Aramberri J, Araiz L, Garcia A et al. Dual versus single Scheimpflug camera for anterior segment analysis: precision and agreement. J Cataract Refract Surg 2012; 38: 1934–49. 114. Nasser CK, Singer R, Barkana Y, Zadok D, Avni I, Goldich Y. Repeatability of the sirius imaging system

Fan et al. and agreement with the pentacam HR. J Refract Surg 2012; 28: 493–7. 115. Nassiri N, Sheibani K, Safi S et al. Central corneal thickness in highly myopic eyes: inter-device agreement of ultrasonic pachymetry, pentacam and orbscan II before and after photorefractive keratectomy. J Ophthalmic Vis Res 2014; 9: 14–21. 116. Park SH, Choi SK, Lee D, Jun EJ, Kim JH. Corneal thickness measurement using orbscan, pentacam, galilei, and ultrasound in normal and postfemtosecond laser in situ keratomileusis eyes. Cornea 2012; 31: 978–82. 117. Chan TC, Biswas S, Yu M, Jhanji V. Longitudinal evaluation of cornea with swept-source optical coherence tomography and Scheimpflug imaging before and after lasik. Medicine 2015; 94: e1219.

© 2017 Royal Australian and New Zealand College of Ophthalmologists